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Pathology_Robbins. http://ebooksmedicine.net An increasing number of molecular techniques are being used for the diagnosis of tumors and for predicting their behavior.


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Pathology_Robbins. Diagnosis of malignancy. Because each T cell and B cell has unique antigen receptor gene rearrangements, polymerase chain reaction (PCR)–based detection of rearranged T-cell receptor or immunoglobulin genes allows monoclonal (neoplastic) and polyclonal (reactive) proliferations to be distinguished. Many hematopoietic neoplasms, as well as a few solid tumors, are defined by particular translocations, so the diagnosis can be made by detection of such translocations. For example, fluorescence in situ hybridization (FISH) or PCR analysis (Chapter 7) can be used to detect translocations characteristic of Ewing sarcoma and several leukemias and lymphomas. PCR-based detection of BCR-ABL transcripts can confirm the diagnosis of chronic myeloid leukemia (Chapter 12). Finally, certain hematologic malignancies are now defined by the presence of point mutations in particular oncogenes. For example, as mentioned earlier, the diagnosis of another myeloid neoplasm called polycythemia vera requires


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Pathology_Robbins. malignancies are now defined by the presence of point mutations in particular oncogenes. For example, as mentioned earlier, the diagnosis of another myeloid neoplasm called polycythemia vera requires the identification of specific mutations in JAK2, a gene that encodes a nonreceptor tyrosine kinase.


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Pathology_Robbins. Prognosis and behavior. Certain genetic alterations are associated with a poor prognosis, and thus the presence of these alterations determines the patient’s subsequent therapy. FISH and PCR methods can be used to detect amplification of oncogenes such as HER2 and NMYC, which provide therapeutic and prognostic information for breast cancers and neuroblastomas, respectively. Sequencing of cancer genomes is now routine in some centers, allowing for the identification of point mutations in cancer genes such as TP53 that predict a poor outcome in many different types of cancer. Although not yet standard of care, efforts are ongoing to develop tests that assess the host immune response to tumors, for example, by quantifying the number of infiltrating cytotoxic T cells, as this too is helpful in gauging prognosis.


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Pathology_Robbins. Detection of minimal residual disease. Another emerging use of molecular techniques is for detection of minimal residual disease after treatment. For example, detection of BCR-ABL transcripts by PCR assay gives a measure of residual disease in patients treated for chronic myeloid leukemia. Recognition that virtually all advanced tumors are associated with both intact circulating tumor cells and products derived from tumors (e.g., cell-free circulating tumor DNA) has led to interest in following tumor burden through sensitive blood tests designed to identify tumor-specific nucleic acid sequences.


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Pathology_Robbins. Diagnosis of hereditary predisposition to cancer. Germ line mutation of several tumor suppressor genes, such as BRCA1, increases a patient’s risk for developing certain types of cancer. Thus, detection of these mutated alleles may allow the patient and the physician to devise an aggressive screening protocol, as well as an opportunity for prophylactic surgery. In addition, such detection allows genetic counseling of relatives who are at risk.


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Pathology_Robbins. Therapeutic decision-making. Therapies that directly target specific mutations are increasingly being developed, and thus detection of such mutations in a tumor can guide the development of targeted therapy, as discussed later. It is now becoming evident that certain targetable mutations transgress morphologic categories. One example involves a valine for glutamate substitution in amino acid 600 (V600E) of the serine/threonine kinase BRAF, which you will recall lies downstream of RAS in the growth factor signaling pathway. Melanomas with the V600E BRAF mutation respond well to BRAF inhibitors, whereas melanomas without this mutation show no response. Subsequently, it was realized that the same V600E mutation is also present in a subset of many other diverse cancers, including carcinomas of the colon and thyroid gland, most hairy cell leukemias, and many cases of Langerhans cell histiocytosis (


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Pathology_Robbins. Fig. 6.36 ). These tumors are morphologically diverse and have distinct cells of origin, but they share identical oncogenic lesions in a common pro-growth pathway. Molecular Profiling of Tumors:The Future of Cancer Diagnostics Until recently, molecular studies of tumors involved the analysis of individual genes. However, the past few years have seen the introduction of revolutionary technologies that can rapidly sequence an entire genome; assess http://ebooksmedicine.net Fig. 6.36 Diversetumortypesthatshareacommonmutation,BRAF(V600E),maybecandidatesfortreatmentwiththesameBRAFinhibitors. epigenetic modifications genome-wide (the epigenome); quantify all of the RNAs expressed in a cell population (the transcriptome); measure many proteins simultaneously (the proteome); and take a snapshot of all of the cell’s metabolites (the metabolome). Thus, the diagnosis, management and study of cancer has entered the age of “omics!”


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Pathology_Robbins. The most common method for large-scale analysis of RNA expression in use in research laboratories has been based on DNA microarrays, but newer methods involving RNA sequencing that offer a more comprehensive and quantitative assessment of RNA expression are beginning to supplant older methods. However, RNA is prone to degradation and is a more difficult analyte to work with than DNA in clinical practice. Furthermore, DNA sequencing is technically simpler than RNA sequencing, permitting the development of methods that rely on massively parallel sequencing (so-called “next-generation [NextGen] sequencing”) that can be readily performed on virtually any tissue specimen (Chapter 7). The increases in DNA sequencing capacity and speed that such methods have enabled over the past decade have been breathtaking, and are matched by an equally remarkable decrease in cost. The first reasonably complete draft of the sequence of the human genome, released in 2003, took 12 years of work and cost


Pathology_Robbins

Pathology_Robbins. been breathtaking, and are matched by an equally remarkable decrease in cost. The first reasonably complete draft of the sequence of the human genome, released in 2003, took 12 years of work and cost about $2,700,000,000. The cost of sequencing the whole genome has now decreased to less than $5000 in some commercial laboratories, making the test readily affordable. At present, using NextGen sequencing, the process of whole-genome of individual tumors can be completed in as little as a few weeks, which includes the time required for the extraordinarily complex task of assembling and analyzing the sequencing data.


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Pathology_Robbins. These advances have enabled the systematic sequencing and cataloging of genomic alterations in various human cancers, an effort sponsored by the National Cancer Institute called The Cancer Genome Atlas (TCGA). The main impact of cancer genome sequencing to date has been in the area of research: identification of new mutations that underlie various cancers; description of the full panoply of genetic lesions that are found in individual cancers; and a greater appreciation of the genetic heterogeneity that exists in individual cancers from area to area. While whole-genome sequencing to manage patients can be performed, most efforts in the clinical realm are focused on developing sequencing methods that permit identification of therapeutically “actionable” genetic lesions in a timely fashion at a reasonable cost. Such approaches seem particularly applicable to tumors, such as lung carcinomas, that are genetically diverse and require a “personalized” approach if targeted therapy is to


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Pathology_Robbins. at a reasonable cost. Such approaches seem particularly applicable to tumors, such as lung carcinomas, that are genetically diverse and require a “personalized” approach if targeted therapy is to succeed (


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Pathology_Robbins. Fig. 6.37 ). Thus, the current trend in molecular diagnostic laboratories is to develop methods that permit several hundred exons of key genes to be sequenced simultaneously at sufficient “depth” (fold coverage of the sequence in question) to reliably detect any mutations that might be present in as few as 5% of tumor cells. A second method that is moving into clinical practice involves the use of DNA arrays to identify changes in DNA copy number, such as amplifications and deletions. Arrays containing probes that span the entire genome at some standard spacing can detect all but the smallest copy number aberrations, providing information that is complementary to that obtained from focused DNA sequencing. Other “omics,” such as proteomics and epigenomics, are currently being used mainly in the realm of clinical research, but with many drugs that target the cancer epigenome moving into the clinic, it can be anticipated that tests directed at assessing the state of the epigenome that


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Pathology_Robbins. in the realm of clinical research, but with many drugs that target the cancer epigenome moving into the clinic, it can be anticipated that tests directed at assessing the state of the epigenome that predict response to such agents are soon to follow.


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Pathology_Robbins. The excitement created by the development of new techniques for the global molecular analysis of tumors has led some scientists to predict that the end of histopathology is in sight. However, histopathologic inspection of tumors provides information about important characteristics of cancers, such as anaplasia, invasiveness, and tumor heterogeneity, that cannot be gleaned from DNA sequences. Histopathology coupled with in situ biomarker tests performed http://ebooksmedicine.net From anatomy... ...to molecular target Fig. 6.37 Genetic analysis of cancers is being utilized to identify mutations that can be targeted by drugs.


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Pathology_Robbins. From anatomy... ...to molecular target Fig. 6.37 Genetic analysis of cancers is being utilized to identify mutations that can be targeted by drugs. on tissue sections also remains the best way to assess tumor : stromal cell interactions, such as angiogenesis and host immune responses; the latter may have an increasingly important role in guiding therapeutic interventions that are designed to counteract immune evasion by tumors. Thus, what lies ahead is not the replacement of one set of techniques by another. On the contrary, for the foreseeable future the most accurate diagnosis and assessment of prognosis in cancer patients will be arrived at by a combination of morphologic and molecular techniques.


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Pathology_Robbins. With all of the advances in genomic analyses and targeted therapies, one can safely predict that we are on the cusp of the golden age of tumor diagnosis and treatment. Those of you who are in medical school now can safely assume that the expectations for rapid advances in cancer diagnosis and therapy will be realized while you are still in practice. Get ready! Several sampling approaches exist for the diagnosis of tumors, including excision, biopsy, fine-needle aspiration, and cytologicsmears. Immunohistochemistry and flow cytometry studies help in thediagnosis and classification of tumors, because distinct proteinexpression patterns define different entities. Proteins released by tumors into the serum, such as PSA, canbe used to screen populations for cancer and to monitor forrecurrence after treatment. Molecular analyses are used to determine diagnosis and prognosis, to detect minimal residual disease, and to diagnosepatients with a hereditary predisposition to cancer.


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Pathology_Robbins. Molecular analyses are used to determine diagnosis and prognosis, to detect minimal residual disease, and to diagnosepatients with a hereditary predisposition to cancer. Molecular profiling of tumors by RNA expression profiling, DNA sequencing, and DNA copy number arrays are useful inmolecular stratification of otherwise identical tumors or thoseof distinct histogenesis that share a mutation for the purposeof targeted treatment and prognostication. Assays of circulating tumor cells and of DNA shed into blood, stool, sputum, and urine are under development. Artandi SE, DePinho RA: Telomeres and telomerase in cancer, Carcinogenesis 31:9–18, 2010. [A review discussing the importance of telomeres and telomerase.] Bai L, Wang S: Targeting apoptosis pathways for new cancer therapeutics, Annual Rev Med 65:139–155, 2014. [A review of apoptosis pathways and therapeutic strategies to activate them in cancer cells.]


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Pathology_Robbins. Burkhart DL, Sage J: Cellular mechanisms of tumour suppression by the retinoblastoma gene, Nat Rev Cancer 8:671–682, 2008. [A review of Rb function.] Cilloni D, Saglio G: Molecular pathways: BCR-ABL, Clin Cancer Res 18:930–937, 2012. [A discussion of the functional consequences and clinical significance of aberrant tyrosine kinase activity in chronic myeloid leukemia mediated by the constitutive enzyme activity of BCR-ABL.] Coussens LM, Zitvogel L, Palucka AK: Neutralizing tumor-promoting chronic inflammation: a magic bullet?, Science 339:286–291, 2013. [A discussion of clinical and experimental studies describing protumorigenic roles for immune cells that elicit cancer-associated inflammation.] Hanahan D, Coussens L: Accessories to the crime: functions of cell recruited to the tumor microenvironment, Cancer Cell 20:309–322, 2012. [A review discussing the role of stroma in cancer.]


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Pathology_Robbins. Hanahan D, Coussens L: Accessories to the crime: functions of cell recruited to the tumor microenvironment, Cancer Cell 20:309–322, 2012. [A review discussing the role of stroma in cancer.] Hanahan D, Weinberg RA: The hallmarks of cancer (2011): the next generation, Cell 144:646–674, 2011. [A reexamination of the hallmarks of cancer.] Joerger AC, Fersht AR: The p53 pathway: Origins, inactivation in cancer, and emerging therapeutic approaches, Annu Rev Biochem 85:375–404, 2016. [A wide-ranging discussion of p53 functions and dysfunction in cancer, with an eye towards development of new cancer therapies.] Lord C, Ashworth A: BRCAness revisited, Nat Rev Cancer 16:110–120, 2016. [A discussion of how BRCAs function as tumor suppressors and how this informs therapy for tumors with evidence of BRCA dysfunction.]


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Pathology_Robbins. Manning AL, Dyson NJ: RB: mitotic implications of a tumour suppressor, Nat Rev Cancer 12:220–226, 2012. [In addition to the well-established roles of RB in control of cell cycle progression and proliferation, this review describes other emerging tumor suppressive functions, such as maintenance of genomic stability.] Martinez P, Blasco MA: Telomeric and extra-telomeric roles for telomerase and the telomere-binding proteins, Nat Rev Cancer 11:161–176, 2011. [A review discussing the role of telomeric proteins in immortalization of cancer cells and other emerging non-telomeric functions that may also contribute to oncogenesis.] Massagué J, Obenauf AC: Metastatic colonization by circulating tumour cells, Nature 529:298–306, 2016. [A discussion of concepts to explain metastasis.] Munoz-Espin D, Serrano M: Cellular senescence: from physiology to pathology, Nat Rev Mol Cell Biol 15:482–496, 2014. [A discussion of the mechanisms of senescence and their potential therapeutic relevance.]


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Pathology_Robbins. Nieto MA, Huang RY-J, Jackson RA, et al: EMT: 2016, Cell 166:21–45, 2016. [A far-ranging discussion of the causes and consequences of epithelial-mesenchymal transition in cancer.] Pickup M, Novitskly S, Moses HL: The roles of TGF-β in the tumor microenvironment, Nat Rev Cancer 13:788–799, 2013. [A review discussing the tumor-promoting effects of TGF-β.] Stine ZE, Walton ZE, Altman BJ, et al: MYC, metabolism, and cancer, Cancer Discov 5:1024–1039, 2015. [A review of the connections between MYC and cancer cell metabolism.] Stratton MR, Campbell PJ, Futreal PA: The cancer genome, Nature 458:719–724, 2009. [An excellent summary of next-generation sequencing technologies and their application to cancer.] van Roy F: Beyond E-cadherin: roles of other cadherin superfamily members in cancer, Nat Rev Cancer 14:121–134, 2014. [A review discussing the role of members of the cadherin family in cancer.]


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Pathology_Robbins. Ward PS, Thompson C: Metabolic reprogramming: a cancer hallmark even Warburg did not anticipate, Cancer Cell 21:297–308, 2012. [An account of the molecular pathways of reprogramming of energy metabolism in cancer and the emergence of “oncometabolites.”] Welti J, Loges S, Dimmeler S, et al: Recent molecular discoveries in angiogenesis and anti-angiogenesis therapies in cancer, J Clin Invest 123:3190–3200, 2013. [A discussion of angiogenic mechanisms and successes and challenges of anti-angiogenic therapy in cancer.] White E: The role of autophagy in cancer, J Clin Invest 125:42–46, 2016. [A discussion of the diverse possible contributions of autophagy to cancer.] http://ebooksmedicine.net studentconsult.com MendelianDisorders:DiseasesCausedbySingle-GeneDefects Transmission Patterns of Single-Gene Disorders General Features of Chromosomal Diseases Caused by Alterations of Imprinted Regions: Prader-Willi and Angelman Syndromes Molecular Diagnosis of Copy Number Abnormalities


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Pathology_Robbins. General Features of Chromosomal Diseases Caused by Alterations of Imprinted Regions: Prader-Willi and Angelman Syndromes Molecular Diagnosis of Copy Number Abnormalities Direct Detection of DNA Mutations by Polymerase Chain Reaction (PCR) Analysis


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Pathology_Robbins. The completion of the human genome project was a landmark achievement in the study of human diseases. It established that humans have only about 25,000 protein-coding genes, far fewer than the 100,000 previously estimated and almost half the number in the lowly rice plant (Oryza sativa)! The unraveling of this “genetic architecture” is beginning to unlock secrets of inherited as well as acquired human disease. Powerful technologies now allow applications of the human gene sequences to the analysis of human diseases. Current high-throughput “next-generation” sequencing technologies can routinely sequence the human exome (the entire compendium of coding genes) in a couple of days for as little as $1000. The speed and reduced costs of DNA sequencing are increasingly facilitating the application of “personalized medicine” (also known as “precision medicine”) to the treatment of cancer and other diseases with a genetic component.


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Pathology_Robbins. Because disorders of childhood are of genetic origin, developmental and pediatric diseases are discussed along with genetic diseases in this chapter. However, it must be borne in mind that not all genetic disorders manifest in infancy and childhood, and conversely, many pediatric diseases are not of genetic origin. To the latter category belong diseases resulting from immaturity of organ systems. In this context it is helpful to clarify three commonly used terms: hereditary, familial, and congenital. Hereditary disorders, by definition, are derived from one’s parents, are transmitted in the gametes through the generations, and therefore are familial. The term congenital simply implies “present at birth.” Of note, some congenital diseases are not genetic (e.g., congenital syphilis). On the other hand, not all genetic diseases are congenital; the expression of Huntington disease, for example, begins only after the third or fourth decade of life. http://ebooksmedicine.net


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Pathology_Robbins. http://ebooksmedicine.net There are several types of genetic abnormalities that affect the structure and function of proteins, disrupting cellular homeostasis and contributing to disease. The term mutation refers to permanent changes in the DNA. Those that affect germ cells are transmitted to the progeny and may give rise to inherited diseases. Mutations in somatic cells are not transmitted to the progeny but are important in the causation of cancers and some congenital malformations. Details of specific mutations and their effects are discussed along with the relevant disorders throughout this book. Cited here are some common examples of gene mutations and their effects:


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Pathology_Robbins. Details of specific mutations and their effects are discussed along with the relevant disorders throughout this book. Cited here are some common examples of gene mutations and their effects: Point mutations result from the substitution of a single nucleotide base by a different base, resulting in the replacement of one amino acid by another in the protein product. The mutation in the β-globin chain of hemoglobin giving rise to sickle cell anemia is an excellent example of a point mutation that alters the meaning of the genetic code. Such mutations are sometimes called missense mutations. By contrast, certain point mutations may change an amino acid codon to a chain termination codon, or a stop codon. Such “nonsense” mutations interrupt translation, and in most cases RNAs are rapidly degraded, a phenomenon called nonsense mediated decay, such that little or no protein is formed.


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Pathology_Robbins. Frameshift mutations occur when the insertion or deletion of one or two base pairs alters the reading frame of the DNA strand.


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Pathology_Robbins. Trinucleotide repeat mutations belong to a special category, because these mutations are characterized by amplification of a sequence of three nucleotides. Although the specific nucleotide sequence that undergoes amplification varies with different disorders, all affected sequences share the nucleotides guanine (G) and cytosine (C). For example, in fragile X syndrome, prototypical of this category of disorders, there are 200 to 4000 tandem repeats of the sequence CGG within a gene called FMR1. In normal populations, the number of repeats is small, averaging 29. The expansions of the trinucleotide sequences prevent normal expression of the FMR1 gene, thus giving rise to mental retardation. Another distinguishing feature of trinucleotide repeat mutations is that they are dynamic (i.e., the degree of amplification increases during gametogenesis). These features, discussed in greater detail later in this chapter, influence the pattern of inheritance and the phenotypic manifestations of


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Pathology_Robbins. degree of amplification increases during gametogenesis). These features, discussed in greater detail later in this chapter, influence the pattern of inheritance and the phenotypic manifestations of the diseases caused by this class of mutations.


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Pathology_Robbins. In addition to alterations in DNA sequence, coding genes also can undergo structural variations, such as copy number changes—amplifications or deletions—or translocations that result in aberrant gain or loss of protein function. As with mutations, structural changes may occur in the germline, or be acquired in somatic tissues. In many instances, pathogenic germline alterations involve a contiguous portion of a chromosome rather than a single gene, such as in the 22q microdeletion syndrome, discussed later. With the widespread availability of next-generation sequencing technology for assessing DNA copy number variation at very high resolution genome wide, pathogenic structural alterations have now been discovered in common disorders such as autism. Cancers often contain somatically acquired structural alterations, including amplifications, deletions, and translocations. The so-called “Philadelphia chromosome”—translocation t(9;22) between the BCR and ABL genes in chronic myeloid


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Pathology_Robbins. structural alterations, including amplifications, deletions, and translocations. The so-called “Philadelphia chromosome”—translocation t(9;22) between the BCR and ABL genes in chronic myeloid leukemia (Chapter 12)—is a classic example.


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Pathology_Robbins. It is worth noting that until recently the major focus of gene hunting has been discovery of genes that encode proteins. Recent studies indicate, however, that a very large number of genes do not encode proteins. Instead, the non-encoded products of these genes—so-called “non-coding RNAs (ncRNAs)”—play important regulatory functions. Although many distinct families of ncRNAs exist, the two most important examples,—small RNA molecules called microRNAs (miRNAs) and long non-coding RNAs (lncRNAs)—are discussed in Chapter 1. With this brief review of the nature of abnormalities that contribute to the pathogenesis of human diseases, we can turn our attention to the three major categories of genetic disorders:


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Pathology_Robbins. With this brief review of the nature of abnormalities that contribute to the pathogenesis of human diseases, we can turn our attention to the three major categories of genetic disorders: Mendelian disorders resulting from mutations in single genes. The genetic abnormalities show high penetrance, meaning that most individuals who inherit the anomaly show phenotypic effects. These diseases are hereditary and familial. They include many uncommon conditions, such as storage diseases and inborn errors of metabolism. Complex disorders involving multiple genes as well as environmental influences. These are sometimes called multi-factorial diseases. They include some of the most common disorders of mankind, including hypertension, diabetes, and allergic and autoimmune diseases. Diseases arising from chromosomal abnormalities, including changes in the number or structure of chromosomes. Several rare developmental abnormalities are attributable to chromosomal alterations.


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Pathology_Robbins. Diseases arising from chromosomal abnormalities, including changes in the number or structure of chromosomes. Several rare developmental abnormalities are attributable to chromosomal alterations. Other genetic diseases, which involve single gene mutations but do not follow simple mendelian rules of inheritance. These single-gene disorders with nonclassic http://ebooksmedicine.net inheritance patterns include those resulting from triplet repeat mutations or from mutations in mitochondrial DNA, and those in which the transmission is influenced by an epigenetic phenomenon called genomic imprinting. Each of these four categories is discussed separately. MENDELIANDISORDERS:DISEASESCAUSEDBYSINGLE-GENEDEFECTS Single-gene defects (mutations) follow the well-known mendelian patterns of inheritance (


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Pathology_Robbins. MENDELIANDISORDERS:DISEASESCAUSEDBYSINGLE-GENEDEFECTS Single-gene defects (mutations) follow the well-known mendelian patterns of inheritance ( Tables 7.1 7.2 ). Although individually rare, together they account for approximately 1% of all adult admissions to hospitals and about 6% to 8% of all pediatric hospital admissions. Listed next are a few important tenets and caveats when considering mendelian disorders: Mutations involving single genes follow one of three patterns of inheritance: autosomal dominant, autosomal recessive, or X-linked.


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Pathology_Robbins. Mutations involving single genes follow one of three patterns of inheritance: autosomal dominant, autosomal recessive, or X-linked. A single-gene mutation may have many phenotypic effects (pleiotropy) and, conversely, mutations at several genetic loci may produce the same trait (genetic heterogeneity). For example, Marfan syndrome, which results from a basic defect in connective tissue, is associated with widespread effects involving the skeleton, eyes, and cardiovascular system, all of which stem from a mutation in the gene encoding fibrillin, a component of connective tissues. On the other hand, several different types of mutations can cause retinitis pigmentosa, an inherited disorder associated with abnormal retinal pigmentation and consequent visual impairment. Recognition of genetic heterogeneity not only is important in genetic counseling but also facilitates understanding of the pathogenesis of common disorders such as diabetes mellitus (Chapter 20).


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Pathology_Robbins. Recognition of genetic heterogeneity not only is important in genetic counseling but also facilitates understanding of the pathogenesis of common disorders such as diabetes mellitus (Chapter 20). • It is now increasingly recognized that the phenotypic manifestations of mutations affecting a known single Table 7.1 Estimated Prevalence of Selected Mendelian Disorders Among Live-Born Infants Huntingtondisease1in10,000 Sicklecellanemia1in500(U.S.AfricanAmericans) Cysticfibrosis1in3200(U.S.Caucasians) Tay-Sachsdisease1in3500(U.S.AshkenaziJewish;FrenchCanadians) Phenylketonuria1in10,000 MPSs—alltypes1in25,000 Glycogenstoragediseases—all1in50,000types Galactosemia1in60,000 Duchennemusculardystrophy1in3500(U.S.males) Hemophilia1in5000(U.S.males) *Theprevalenceofheterozygoussicklecelltraitis1in12forU.S.AfricanAmericans.MPS, Mucopolysaccharidosis. Table 7.2 Biochemical Basis and Inheritance Pattern for Selected Mendelian Disorders


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Pathology_Robbins. Table 7.2 Biochemical Basis and Inheritance Pattern for Selected Mendelian Disorders MarfansyndromeFibrillinStructuralsupport:extracellularmatrix CollagenStructuralsupport:extracellularmatrix HereditaryspherocytosisSpectrin,ankyrin,orprotein4.1Structuralsupport:redbloodcellmembrane Neurofibromatosis,type1Neurofibromin-1(NF-1)Growthregulation Duchenne/BeckermusculardystrophyDystrophinStructuralsupport:cellmembrane *SomevariantsofEhler-Danlossyndromehaveanautosomalrecessiveinheritancepattern. †Althoughfull-blownsymptomsrequirebiallelicmutations,heterozygotesforthalassemiaandsicklecellanemiamaypresentwithmildclinicaldisease.Thus,thesedisorderssometimesarecategorizedas“autosomalcodominant”entities. http://ebooksmedicine.net gene are influenced by other genetic loci, which are called modifier genes. As discussed later in the section on cystic fibrosis these modifier loci can affect the severity or extent of the disease.


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Pathology_Robbins. • The use of proactive prenatal genetic screening in high-risk populations (e.g., persons of Ashkenazi Jewish descent) has significantly reduced the incidence ( 7.1 ) of certain genetic disorders such as Tay-Sachs disease. Disorders of Autosomal Dominant Inheritance Disorders of autosomal dominant inheritance are manifested in the heterozygous state, so at least one parent in an index case usually is affected. Both males and females can be affected, and both sexes can transmit the condition. When an affected person marries an unaffected one, each child has one chance in two of having the disease. The following features also pertain to autosomal dominant diseases: With any autosomal dominant disorder, some patients do not have affected parents. Such patients owe their disorder to new mutations involving either the egg or the sperm from which they were derived. Their siblings are neither affected nor at increased risk for development of the disease.


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Pathology_Robbins. Clinical features can be modified by reduced penetrance and variable expressivity. Some persons inherit the mutant gene but are phenotypically normal, a phenomenon referred to as reduced penetrance. The variables that affect penetrance are not clearly understood. In contrast with penetrance, if a trait is consistently associated with a mutant gene but is expressed differently among persons carrying the gene, the phenomenon is called variable expressivity. For example, manifestations of neurofibromatosis 1 range from brownish spots on the skin to multiple tumors and skeletal deformities. In many conditions, the age at onset is delayed, and symptoms and signs do not appear until adulthood (as in Huntington disease).


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Pathology_Robbins. In many conditions, the age at onset is delayed, and symptoms and signs do not appear until adulthood (as in Huntington disease). In autosomal dominant disorders, a 50% reduction in the normal gene product is associated with clinical signs and symptoms. Because a 50% loss of enzyme activity can be compensated for, involved genes in autosomal dominant disorders usually do not encode enzyme proteins, but instead fall into two other categories of proteins: Those involved in regulation of complex metabolic pathways, often subject to feedback control (e.g., membrane receptors, transport proteins). An example of this pathogenic mechanism is found in familial hypercholesterolemia, which results from mutation in the low-density lipoprotein (LDL) receptor gene (discussed later). Key structural proteins, such as collagen and components of the red cell membrane skeleton (e.g., spectrin, abnormalities of which result in hereditary spherocytosis).


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Pathology_Robbins. Key structural proteins, such as collagen and components of the red cell membrane skeleton (e.g., spectrin, abnormalities of which result in hereditary spherocytosis). The biochemical mechanisms by which a 50% reduction in the levels of such proteins results in an abnormal phenotype are not fully understood. In some cases, especially when the gene encodes one subunit of a multimeric protein, the product of the mutant allele can interfere with the assembly of a functionally normal multimer. For example, the collagen molecule is a trimer in which the three collagen chains are arranged in a helical configuration. The presence of mutated collagen chains reduces the assembly of the remaining normal chains, producing a marked deficiency of collagen. In this instance the mutant allele is called dominant negative, because it impairs the function of a normal allele. This effect is illustrated in some forms of osteogenesis imperfecta (Chapter 21). Disorders of Autosomal Recessive Inheritance


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Pathology_Robbins. Disorders of Autosomal Recessive Inheritance Disorders of autosomal recessive inheritance are manifested in the homozygous state. They occur when both of the alleles at a given gene locus are mutants. Therefore, such disorders are characterized by the following features: (1) The trait does not usually affect the parents, who are carriers of one diseased allele, but multiple siblings may show the disease; (2) siblings have one chance in four of being affected (i.e., the recurrence risk is 25% for each birth); and (3) if the mutant gene occurs with a low frequency in the population, there is a strong likelihood that the affected patient (the proband) is the product of a consanguineous marriage. They make up the largest group of mendelian disorders. In contrast with the features of autosomal dominant diseases, the following features generally apply to most autosomal recessive disorders: The expression of the defect tends to be more uniform than in autosomal dominant disorders.


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Pathology_Robbins. The expression of the defect tends to be more uniform than in autosomal dominant disorders. Complete penetrance is common. Onset is frequently early in life. Although new mutations for recessive disorders do occur, they are rarely detected clinically. Because the affected person is an asymptomatic heterozygote, several generations may pass before the descendants of such a person mate with other heterozygotes and produce affected offspring. In many cases, enzymes are affected by the mutation. In heterozygotes, equal amounts of normal and defective enzyme are synthesized. Usually the natural “margin of safety” ensures that cells with half of their complement of the enzyme function normally.


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Pathology_Robbins. The Y chromosome is home to the testes-determining gene SRY, which directs male sexual differentiation, but apart from very rare instances of Y-linked familial deafness, no Y chromosome–linked mendelian disorders have ever been reported. Thus, for the most part, sex-linked disorders are X linked. Most X-linked disorders are X-linked recessive and are characterized by the following features: Heterozygous female carriers transmit them only to sons, who of course are hemizygous for the X chromosome. Heterozygous females rarely express the full phenotypic change, because they have the paired normal allele. Although one of the X chromosomes in females is inactivated (see further text), this process of inactivation is random, which typically allows sufficient numbers of cells with the normal expressed allele to emerge.


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Pathology_Robbins. http://ebooksmedicine.net • An affected male does not transmit the disorder to sons, but all daughters are carriers. Sons of heterozygous women have one chance in two of receiving the mutant gene. Autosomaldominantdisordersarecharacterizedbyexpressioninheterozygousstate;theyaffectmalesandfemalesequally,andbothsexescantransmitthedisorder. Autosomaldominantdisordersofteninvovledysfunctionalreceptorsandstructuralproteins. Autosomalrecessivediseasesoccurwhenbothcopiesofagenearemutatedandfrequentlyinvolveenzymes.Malesandfemalesareaffectedequally. X-linkeddisordersaretransmittedbyheterozygousfemalestotheirsons,whomanifestthedisease.FemalecarriersusuallyareunaffectedbecauseofrandominactivationofoneXchromosome.


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Pathology_Robbins. X-linkeddisordersaretransmittedbyheterozygousfemalestotheirsons,whomanifestthedisease.FemalecarriersusuallyareunaffectedbecauseofrandominactivationofoneXchromosome. Marfan syndrome is an autosomal dominant disorder of connective tissues, manifested principally by changes in the skeleton, eyes, and cardiovascular system. It is caused by an inherited defect in an extracellular glycoprotein called fibrillin-1. This glycoprotein, secreted by fibroblasts, is the major component of microfibrils found in the extracellular matrix. Microfibrils serve as scaffolding for the deposition of tropoelastin, an integral component of elastic fibers. Although microfibrils are widely distributed in the body, they are particularly abundant in the aorta, ligaments, and the ciliary zonules that support the ocular lens, precisely the tissues that are prominently affected in Marfan syndrome.


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Pathology_Robbins. Fibrillin is encoded by the FBN1 gene, which maps to chromosomal locus 15q21. Mutations in FBN1 are found in all patients with Marfan syndrome. More than 1800 distinct causative mutations in the very large FBN1 gene have been found, a level of complexity that complicates diagnosis by DNA sequencing. As a result, the diagnosis is mainly based on clinical findings. Because heterozygotes have clinical symptoms, it is thought that the mutant fibrillin protein may act as a dominant negative, preventing the assembly of normal microfibrils. The prevalence of Marfan syndrome is estimated to be 1 in 5000. Approximately 70% to 85% of cases are familial, and the rest are sporadic, arising from de novo FBN1 mutations in the germ cells of parents.


Pathology_Robbins

Pathology_Robbins. Although many of the abnormalities in Marfan syndrome can be explained on the basis of structural failure of connective tissues, some, such as overgrowth of bones, are difficult to relate to simple loss of fibrillin. It is now evident that loss of microfibrils gives rise to abnormal and excessive activation of transforming growth factor-β (TGF-β), because normal microfibrils sequester TGF-β, thereby controlling the bioavailability of this cytokine.


Pathology_Robbins

Pathology_Robbins. Excessive TGF-β signaling has deleterious effects on vascular smooth muscle development and the integrity of extracellular matrix. In support of this hypothesis, mutations in the TGF-β type II receptor give rise to a related syndrome, called Marfan syndrome type 2 (MFS2). Furthermore, patients with germline mutations in one isoform of TGF-β, called TGF-β3, present with an inherited predisposition to aortic aneurysm and other cardiovascular manifestations similar to those found in “classic” Marfan patients (see later). Of note, angiotensin receptor blockers, which inhibit the activity of TGF-β, improve aortic and cardiac function in mouse models of Marfan syndrome. Prevention of cardiovascular disease entails the use of beta-aderenergic blocking agents, which lower blood pressure, as well as angiotensin receptor inhibitors, which not only lower blood pressure but also interfere with TGF-β activity.


Pathology_Robbins


Pathology_Robbins

Pathology_Robbins. PreventionofcardiovasculardisesaeinvolvestheuseofdrugsthatlowerbloodpressureandinhibitTGF-β signaling. Ehlers-Danlos syndromes (EDSs) are a group of diseases characterized by defects in collagen synthesis or structure. All are single-gene disorders, but the mode of inheritance encompasses both autosomal dominant and recessive patterns. There are approximately 30 distinct types of collagen; all have characteristic tissue distributions and are the products of different genes. To some extent, the clinical heterogeneity of EDS can be explained by mutations in different collagen genes. At least six clinical and genetic variants of EDS are recognized. Because defective collagen is the basis for these disorders, certain clinical features are common to all variants.


Pathology_Robbins

Pathology_Robbins. At least six clinical and genetic variants of EDS are recognized. Because defective collagen is the basis for these disorders, certain clinical features are common to all variants. Tissues rich in collagen, such as skin, ligaments, and joints, frequently are affected in most variants of EDS. Because the abnormal collagen fibers lack adequate tensile strength, joints are hypermobile. These features permit grotesque contortions, such as bending the thumb backward to touch the forearm and bending the knee upward to create almost a right angle. Indeed, it is believed that most contortionists have some form of EDS; however, a predisposition to joint dislocation is one of the prices paid for this virtuosity. Skin fragility. The skin is extraordinarily stretchable, extremely fragile, and vulnerable to trauma. Minor injuries produce gaping defects, and surgical repair or any surgical intervention is accomplished only with great difficulty because of the lack of normal tensile strength.


Pathology_Robbins

Pathology_Robbins. Structural failure of organ or tissues. The structural defect in connective tissue may lead to serious internal complications, including rupture of the colon and large arteries (vascular EDS); ocular fragility, with rupture of the cornea and retinal detachment (kyphoscoliotic EDS); and diaphragmatic hernias (classical EDS), among others. The molecular bases for three of the more common variants are as follows: Deficient synthesis of type III collagen resulting from mutations affecting the COL3A1 gene. This variant, vascular EDS, is inherited as an autosomal dominant disorder and is characterized by weakness of tissues rich in type III collagen (e.g., blood vessels, bowel wall), predisposing them to rupture.


Pathology_Robbins

Pathology_Robbins. Deficiency of the enzyme lysyl hydroxylase. Decreased hydroxylation of lysyl residues in types I and III collagen interferes with the formation of crosslinks among collagen molecules. As might be expected, this variant (kyphoscoliotic EDS), resulting from an enzyme deficiency, is inherited as an autosomal recessive disorder. Patients typically manifest with congenital scoliosis and ocular fragility. • Deficient synthesis of type V collagen resulting from mutations in COL5A1 and COL5A2 is inherited as an autosomal dominant disorder and results in classical EDS. TherearesixvariantsofEDS,allcharacterizedbydefectsincollagensynthesisorassembly.Eachofthevariantsiscausedbyadistinctmutation. Clinicalfeaturesmayincludefragile,hyperextensibleskinvulnerabletotrauma,hypermobilejoints,andrupturesinvolvingcolon,cornea,orlargearteries.Woundhealingispoor.


Pathology_Robbins

Pathology_Robbins. Familial hypercholesterolemia is a “receptor disease” caused by loss-of-function mutations in the gene encoding the LDL receptor, which is involved in the transport and metabolism of cholesterol. As a consequence of receptor abnormalities there is a loss of feedback control that normally holds cholesterol synthesis in check. The resulting elevated levels of cholesterol induce premature atherosclerosis and greatly increase the risk of myocardial infarction. Familial hypercholesterolemia is among the most common of mendelian disorders; the frequency of the heterozygous condition is 1 in 500 in the general population. Approximately 7% of the body’s cholesterol circulates in the plasma, predominantly in the form of LDL. As might be expected, the amount of plasma cholesterol is influenced by its synthesis and catabolism, and the liver plays a crucial role in both these processes, as described later. A brief review of the synthesis and transport of cholesterol follows.


Pathology_Robbins

Pathology_Robbins. Normal Cholesterol Metabolism. Cholesterol may be derived from the diet or from endogenous synthesis. Dietary triglycerides and cholesterol are incorporated into chylomicrons in the intestinal mucosa and travel by way of the gut lymphatics to the blood. These chylomicrons are hydrolyzed by an endothelial lipoprotein lipase in the capillaries of muscle and fat. The chylomicron remnants, rich in cholesterol, are then delivered to the liver. Some of the cholesterol enters the metabolic pool (to be described), and some is excreted as free cholesterol or as bile acids into the biliary tract. The endogenous synthesis of cholesterol and LDL begins in the liver (


Pathology_Robbins

Pathology_Robbins. Fig. 7.1 ). The first step in the synthesis is the secretion of triglyceride-rich very-low-density lipoprotein (VLDL) by the liver into the blood. In the capillaries of adipose tissue and muscle, the VLDL particle undergoes lipolysis and is converted to intermediate-density lipoprotein (IDL). In comparison with VLDL, the content of triglyceride is reduced and that of cholesteryl esters is enriched in IDL, but IDL retains on its surface the VLDL-associated apolipoproteins B-100 and E. Further metabolism of IDL occurs along two pathways: Most of the http://ebooksmedicine.net Fig. 7.1 Low-densitylipoprotein(LDL)metabolismandtheroleoftheliverinitssynthesisandclearance.Lipolysisofvery-low-densitylipoprotein(VLDL)bylipoproteinlipaseinthecapillariesreleasestriglycerides,whicharethenstoredinfatcellsandusedasasourceofenergyinskeletalmuscles.IDL(intermediate-densitylipoprotein)remainsinthebloodandistakenupbytheliver.


Pathology_Robbins

Pathology_Robbins. IDL particles are directly taken up by the liver through the LDL receptor described later; others are converted to cholesterol-rich LDL by a further loss of triglycerides and the loss of apolipoprotein E. In the liver cells, IDL is recycled to generate VLDL.


Pathology_Robbins

Pathology_Robbins. The LDL receptor pathway metabolizes two-thirds of the resultant LDL particles, and the rest is metabolized by a receptor for oxidized LDL (scavenger receptor), to be described later. The LDL receptor binds to apolipoproteins B-100 and E and thus is involved in the transport of both LDL and IDL. Although the LDL receptors are widely distributed, approximately 75% are located on hepatocytes, so the liver plays an extremely important role in LDL metabolism. The first step in the receptor-mediated transport of LDL involves binding to the cell surface receptor, followed by endocytotic internalization inside so-called clathrin-coated pits (Fig. 7.2 ). Within the cell, the endocytic vesicles fuse with the lysosomes, and the LDL molecule is enzymatically degraded, resulting ultimately in the release of free cholesterol into the cytoplasm. The exit of cholesterol from the lysosomes requires the action of two proteins, called NPC1 and NPC2 (described later). The cholesterol not only is used by


Pathology_Robbins

Pathology_Robbins. of free cholesterol into the cytoplasm. The exit of cholesterol from the lysosomes requires the action of two proteins, called NPC1 and NPC2 (described later). The cholesterol not only is used by the cell for membrane synthesis but also takes part in intracellular cholesterol homeostasis by a sophisticated system of feedback control:


Pathology_Robbins

Pathology_Robbins. It suppresses cholesterol synthesis by inhibiting the activity of the enzyme 3-hydroxy-3-methylglutaryl– coenzyme A reductase (HMG-CoA reductase), which is the rate-limiting enzyme in the synthetic pathway. It stimulates the formation of cholesterol esters for storage of excess cholesterol. It downregulates the synthesis of cell surface LDL receptors, thus preventing excessive accumulation of cholesterol inside cells. The transport of LDL by the scavenger receptors, alluded to earlier, seems to occur in cells of the mononuclear-phagocyte system and possibly in other cells as well. Monocytes and macrophages have receptors for chemically modified (e.g., acetylated or oxidized) LDLs. The amount catabolized by this scavenger receptor pathway is directly related to the plasma cholesterol level.


Pathology_Robbins

Pathology_Robbins. In familial hypercholesterolemia, mutations in the LDL receptor protein impair the intracellular transport and catabolism of LDL, resulting in accumulation of LDL cholesterol in the plasma. In addition, the absence of LDL receptors on liver cells impairs the transport of IDL into the liver, so a greater proportion of plasma IDL is converted into LDL. Thus, patients with familial hypercholesterolemia develop excessive levels of serum cholesterol as a result of the combined effects of reduced catabolism and excessive biosynthesis ( Fig. 7.1 ). This leads to a marked increase of cholesterol uptake by the monocyte-macrophages and vascular walls through the scavenger receptor. This accounts for the appearance of skin xanthomas and premature atherosclerosis.


Pathology_Robbins

Pathology_Robbins. Familial hypercholesterolemia is an autosomal dominant disease. Heterozygotes have a twofold-to threefold elevation of plasma cholesterol levels, whereas homozygotes may have in excess of a fivefold elevation. Although their cholesterol levels are elevated from birth, heterozygotes remain asymptomatic until adult life, when they develop cholesterol deposits (xanthomas) along tendon sheaths and premature atherosclerosis resulting in coronary artery disease. Homozygotes are much more severely affected, developing cutaneous xanthomas in childhood and often dying of myocardial infarction before the age of 20 years.


Pathology_Robbins

Pathology_Robbins. Analysis of the cloned LDL receptor gene has shown that more than 900 different mutations can give rise to familial hypercholesterolemia. These can be divided into five categories. Class I mutations are uncommon, and they are associated with complete loss of receptor synthesis. With class II mutations, the most prevalent type, the receptor protein is synthesized, but its transport from the endoplasmic reticulum to the Golgi apparatus is impaired because of defects in protein folding. Class III mutations produce receptors that are transported to the cell surface but fail to bind LDL normally. Class IV mutations give rise to receptors that fail to internalize within clathrin pits after binding to LDL, whereas class V mutations encode receptors that can bind LDL and are internalized but are trapped in endosomes because dissociation of receptor and bound LDL does not occur. http://ebooksmedicine.net


Pathology_Robbins

Pathology_Robbins. http://ebooksmedicine.net Clathrin Coated vesicle Recycling vesicle Endosome Lysosome Cholesterol HMG-CoA reductaseDNA RNA RibosomeReceptor protein Endoplasmic reticulum Receptor Golgi apparatus Plasma membrane PCSK9 StimulatesInhibitsInhibitsAmino acids Coated pit Cell membrane, steroid hormones, and bile acidsStorage of cholesterol esters Oversupply of cholesterol Synthesis of cholesterol Synthesis of LDL receptors Fig. 7.2 TheLDLreceptorpathwayandregulationofcholesterolmetabolism.Theyellow arrows showthreeregulatoryfunctionsoffreeintracellularcholesterol:(1)suppressionofcholesterolsynthesisbyinhibitionofHMG-CoAreductase,(2)stimulatingthestorageofexcesscholesterolasesters,and(3)inhibitionofsynthesisofLDLreceptors.PCSK9causesintracellulardegradationofLDLreceptorsinlivercells,reducingthelevelofLDLreceptorsonthecellmembrane.HMG-CoA reductase, 3-Hydroxy-3-methylglutaryl–coenzymeAreductase;LDL, low-densitylipoprotein;PCSK9, proproteinconvertasesubtilisin/kexintype9.


Pathology_Robbins

Pathology_Robbins. In recent years, a new mechanism of posttranslational regulation of plasma LDL levels has been elucidated, wherein an enzyme referred to as PCSK9 causes intracellular degradation of LDL receptors in hepatocytes, before they reach the cell surface. With fewer LDL receptors on the cell surface, there is reduced uptake of plasma LDL, leading to hypercholesterolemia. Families with activating mutations of the gene PCSK9, which codes for the enzyme, have many features similar to classic familial hypercholesterolemia. The discovery of the critical role of LDL receptors in cholesterol homeostasis has led to the rational design of the statin family of drugs that are now widely used to lower plasma cholesterol. They inhibit the activity of HMG-CoA reductase and thus promote greater synthesis of LDL receptors (


Pathology_Robbins

Pathology_Robbins. Fig. 7.2 ). However, the upregulation of LDL receptors is accompanied by a compensatory increase in PCSK9 levels, which dampens the effects of statins. Therefore, agents that antagonize PCSK9 enzymatic function have been developed, and large clinical trials have shown the benefits using such inhibitors in patients with refractory hypercholesterolemia. FamilialhypercholesterolemiaisanautosomaldominantdisordercausedbymutationsinthegeneencodingtheLDLreceptor.ActivatingmutationsofPCSK9(farlesscommon),whichdegradesLDLreceptors,alsocauseasimilarphenotype. PatientsdevelophypercholesterolemiaasaconsequenceofimpairedtransportofLDLintothecells. Inheterozygotes,elevatedserumcholesterolgreatlyincreasestheriskofatherosclerosisandresultantcoronaryarterydisease;homozygoteshaveanevengreaterincreaseinserumcholesterolandahigherfrequencyofischemicheartdisease.Cholesterolalsodepositsalongtendonsheathstoproducexanthomas.


Pathology_Robbins

Pathology_Robbins. Cystic fibrosis (CF) is a disorder of epithelial ion transport affecting fluid secretion in exocrine glands and the epithelial linings of the respiratory, gastrointestinal, and reproductive tracts. The ion transport defects lead to http://ebooksmedicine.net


Pathology_Robbins

Pathology_Robbins. Mendelian Disorders: Diseases Caused by Single-Gene Defects251 abnormally viscid mucous secretions that block the airways and the pancreatic ducts which in turn are responsible for the two most important clinical manifestations: recurrent and chronic pulmonary infections and pancreatic insufficiency. In addition, although the exocrine sweat glands are structurally normal (and remain so throughout the course of this disease), a high level of sodium chloride in the sweat is a consistent and characteristic biochemical abnormality in CF. At the same time, it must be remembered that CF can present with a bewilderingly variable set of clinical findings. This variation in phenotype results from diverse mutations in the CF-associated gene, tissue-specific effects of loss of the CF gene’s function, and the influence of disease modifier genes.


Pathology_Robbins

Pathology_Robbins. With an incidence of 1 in 2500 live births in the United States, CF is the most common lethal genetic disease that affects white populations. The carrier frequency in the United States is 1 in 20 among Caucasians but significantly lower among African Americans, Asians, and Hispanics. CF follows simple autosomal recessive transmission, although even heterozygous carriers have a predisposition toward pulmonary and pancreatic disease at higher rates than the general population. The primary defect in CF is reduced production, or abnormal function of an epithelial chloride channel protein encoded by the CF transmembrane conductance regulator (CFTR) gene. Disruptive mutations in CFTR render the epithelial membranes relatively impermeable to chloride ions (


Pathology_Robbins

Pathology_Robbins. Fig. 7.3 ). However, the impact of this defect on transport function is tissue specific. The major function of the CFTR protein in the sweat gland ducts is to reabsorb luminal chloride ions and augment sodium reabsorption through the epithelial sodium channel (ENaC). Therefore, in the sweat ducts, loss of CFTR function leads to decreased reabsorption of sodium chloride and production of hyper-tonic (“salty”) sweat ( Fig. 7.3 , top). In contrast to sweat glands, CFTR in the respiratory and intestinal epithelium is one of the most important avenues for active luminal secretion of chloride. At these sites, CFTR mutations result in loss or reduction of chloride secretion into the lumen ( Fig. 7.3 , bottom). Active luminal sodium absorption through ENaCs also is increased, and both of these ion changes increase passive water reabsorption from the lumen, lowering the water content of the surface fluid layer coating mucosal cells. Thus, unlike the sweat ducts, there


Pathology_Robbins

Pathology_Robbins. Fig. 7.3 (Top) In CF, a chloride channel defect in the sweat duct causes increased chloride and sodium concentration in sweat. (Bottom) Patients with CF have decreased chloride secretion and increased sodium and water reabsorption in the airways, leading to dehydration of the mucus layer coating epithelial cells, defective mucociliary action, and mucous plugging. CFTR, Cystic fibrosis transmembrane conductance regulator; ENaC, epithelial sodium channel responsible for intracellular sodium conduction.


Pathology_Robbins

Pathology_Robbins. is no difference in the salt concentration of the surface fluid layer coating the respiratory and intestinal mucosal cells in normal persons and in those with CF. Instead, respiratory and intestinal complications in CF seem to stem from dehydration of surface fluid layer. In the lungs, this dehydration leads to defective mucociliary action and the accumulation of concentrated, viscid secretions that obstruct the air passages and predispose to recurrent pulmonary infections. In addition to chloride, CFTR also regulates secretion of bicarbonate ions. In fact, certain CFTR variants manifest only with abnormal bicarbonate transport while chloride ion transport is largely preserved. Pancreatic insufficiency, a feature of classic CF, is virtually always present in patients who carry CFTR mutations with abnormal bicarbonate conductance.


Pathology_Robbins

Pathology_Robbins. Since the CFTR gene was cloned in 1989, more than 1800 disease-causing mutations have been identified. They can be classified on the basis of clinical course of CF or the nature of the underlying defect. Mechanistically, they may result in reduced quantity of functional CFTR that reaches the cell surface or reduced function of CFTR that reaches the cell surface. As we shall see these differences impact on the development of treatment strategies. They can also be classified as severe or mild, depending on the clinical phenotype. Severe mutations are associated with complete loss of CFTR protein function, whereas mild mutations allow some residual function. The most common severe CFTR mutation is a deletion of three nucleotides coding for phenylalanine at amino acid position 508 (ΔF508). This causes misfolding and total loss of the CFTR. Worldwide, ΔF508 mutation is found in approximately 70% of patients with CF. Since CF is an autosomal recessive disease, affected persons harbor


Pathology_Robbins

Pathology_Robbins. This causes misfolding and total loss of the CFTR. Worldwide, ΔF508 mutation is found in approximately 70% of patients with CF. Since CF is an autosomal recessive disease, affected persons harbor mutations on both alleles. As discussed later, the nature of mutations on the two alleles influences the overall phenotype, as well as organ-specific manifestations. Although CF remains one of the best-known examples of the “one gene–one disease” axiom, there is increasing evidence that other genes modify the frequency and severity of organ-specific manifestations. Examples of modifier genes include mannose-binding lectin 2 (MBL2) and transforming growth factor-β1 (TGFβ1). It is postulated that polymorphisms in these genes influence the ability of the lungs to tolerate infections with virulent microbes (see later), thus modifying the natural history of CF.


Pathology_Robbins

Pathology_Robbins. Fig. 7.4 ). Pancreatic abnormalities are present in 85% to 90% of patients. In the milder cases, there may be only accumulations of mucus in the small ducts, with some dilation of the exocrine glands. In more advanced cases, usually seen in older children or adolescents, the ducts are totally plugged, causing atrophy of the Fig. 7.5 ). The total loss lead to vitamin A deficiency. This may contribute to squamous metaplasia of the lining epithelium of the pancreatic ducts, which tions. Thick viscid plugs of mucus also may be found in the small Azoospermia Infertility Bilateral obstruction of vas deferens Fig. 7.4 Tissues affected in patients with CF. (Adapted from Cutting GR: Cystic fibrosis genetics: from molecular understanding to clinical application. Nat Rev Genet 16:45, 2015.) intestine of infants. Sometimes these cause small bowel obstruction, known as meconium ileus.


Pathology_Robbins

Pathology_Robbins. Fig. 7.6). These changes stem from obstruc tion of the air passages by the viscous mucus secretions of submucosal glands and superimposed infections. The bronchioles often are distended with thick mucus, associated with marked hyperplasia and hypertrophy of the mucus-secreting cells. Superimposed infections give rise to severe chronic bronchitis and Fig. 7.5 Mild to moderate changes of CF in the pancreas. The ducts are dilated and plugged with eosinophilic mucin, and the parenchymal glands are atrophic and replaced by fibrous tissue.


Pathology_Robbins

Pathology_Robbins. Fig. 7.6 Lungsofapatientwhodiedofcysticfibrosis.Extensivemucousplugginganddilationofthetracheobronchialtreeareapparent.Thepulmonaryparenchymaisconsolidatedbyacombinationofbothsecretionsandpneumonia;thegreenishdiscolorationistheproductofPseudomonas infections.(Courtesy of Dr. Eduardo Yunis: Children’s Hospital of Pittsburgh, Pittsburgh, Pennsylvania.) bronchiectasis.Developmentoflungabscessesiscommon.Staphylococcus aureus, Haemophilus influenzae, andPseudomonas aeruginosa arethethreemostcommonorganismsresponsibleforlunginfections.Evenmoresinisteristheincreasingfrequencyofinfectionwithanotherpseudomonad,Burkholderia cepacia.Thisopportunisticbacteriumisparticularlyhardy,andinfectionwiththisorganismhasbeenassociatedwithfulminantillness(“cepaciasyndrome”).Theliver involvement followsthesamebasicpattern.Bilecanaliculiarepluggedbymucinousmaterial,accompaniedbyductularproliferationandportalinflammation.Hepaticsteatosis


Pathology_Robbins

Pathology_Robbins. involvement followsthesamebasicpattern.Bilecanaliculiarepluggedbymucinousmaterial,accompaniedbyductularproliferationandportalinflammation.Hepaticsteatosis (“fattyliver”)isacommonfindinginliverbiopsies.Withtime,cirrhosisdevelops,resultingindiffusehepaticnodularity.Suchseverehepaticinvolvementisencounteredinlessthan10%ofpatients.Azoospermia andinfertility arefoundin95%oftheaffectedmaleswhosurvivetoadulthood;bilateral absence of the vas deferens isafrequentfindinginthesepatients.Insomemales,thismaybetheonlyfeaturesuggestinganunderlyingCFTRmutation.


Pathology_Robbins

Pathology_Robbins. In few childhood diseases are clinical manifestations as protean as those of CF (


Pathology_Robbins

Pathology_Robbins. Fig. 7.4 ). Approximately 5% to 10% of the cases come to clinical attention at birth or soon after because of an attack of meconium ileus. Exocrine pancreatic insufficiency occurs in a majority (85% to 90%) of patients and is associated with “severe” CFTR mutations on both alleles (e.g., ΔF508/ΔF508). By contrast, 10% to 15% of patients, who have one “severe” and one “mild” CFTR mutation, or two “mild” CFTR mutations, retain sufficient pancreatic exocrine function that enzyme supplementation is not required—the pancreas-sufficient phenotype. Pancreatic insufficiency is associated with malabsorption of protein and fat and increased fecal loss. Manifestations of malabsorption (e.g., large, foul-smelling stools; abdominal distention; poor weight gain) appear during the first year of life. The faulty fat absorption may induce deficiency states of the fat-soluble vitamins, resulting in manifestations of avitaminosis A, D, or K. Hypoproteinemia may be severe enough to cause generalized


Pathology_Robbins

Pathology_Robbins. The faulty fat absorption may induce deficiency states of the fat-soluble vitamins, resulting in manifestations of avitaminosis A, D, or K. Hypoproteinemia may be severe enough to cause generalized edema. Persistent diarrhea may result in rectal prolapse in as many as 10% of children with CF. The pancreas-sufficient phenotype usually is not associated with other gastrointestinal complications, and in general, these patients demonstrate excellent growth and development. In contrast to exocrine insufficiency, endocrine insufficiency (i.e., diabetes) is uncommon in CF and occurs late in the course of the disease especially with improved survival of patients with CF.


Pathology_Robbins

Pathology_Robbins. Cardiorespiratory complications, such as chronic cough, persistent lung infections, obstructive pulmonary disease, and cor pulmonale, constitute the most common cause of death (accounting for approximately 80% of fatalities) in patients who receive follow-up care in most CF centers in the United States. By 18 years of age, 80% of patients with classic CF harbor P. aeruginosa, and 3.5% harbor B. cepacia. With the indiscriminate use of antibiotic prophylaxis, there has been an unfortunate resurgence of resistant strains of Pseudomonas in many patients. Significant liver disease occurs late in the natural history of CF and is foreshadowed by pulmonary and pancreatic involvement; with increasing life expectancy, liver disease is now the third most common cause of death in patients with CF (after cardiopulmonary and transplant-related complications). More extensive DNA sequencing has shown that the spectrum of diseases caused by germline CFTR mutations (either biallelic or in heterozygous


Pathology_Robbins

Pathology_Robbins. cardiopulmonary and transplant-related complications). More extensive DNA sequencing has shown that the spectrum of diseases caused by germline CFTR mutations (either biallelic or in heterozygous carriers) is broader than the “classic” multisystem disease described earlier. For example, some patients suffering from recurrent bouts of abdominal pain and pancreatitis since childhood who were previously classified as having “idiopathic” chronic pancreatitis are now known to harbor biallelic CFTR variants that are distinct from those seen in “classic” CF. Patients with isolated pancreatitis or bilateral absence of vas deferens (see earlier) resulting from CFTR mutations are included under the umbrella of “CFTRopathies.” Similarly, whereas CF carriers were initially thought to be asymptomatic, studies suggest that they have an increased lifetime risk for chronic lung disease (especially bronchiectasis) and recurrent sinonasal polyps. In most cases, the diagnosis of CF is based on


Pathology_Robbins

Pathology_Robbins. studies suggest that they have an increased lifetime risk for chronic lung disease (especially bronchiectasis) and recurrent sinonasal polyps. In most cases, the diagnosis of CF is based on persistently elevated sweat electrolyte concentrations (often the mother makes the diagnosis because her infant “tastes salty”), characteristic clinical findings (sinopulmonary disease and gastrointestinal manifestations), or a family history. Sequencing the CFTR gene is the gold standard for diagnosis of CF.


Pathology_Robbins

Pathology_Robbins. There have been major improvements in the management of acute and chronic complications of CF, including more potent anti-microbial therapies, pancreatic enzyme replacement, and bilateral lung transplantation. Importantly, two new forms of treatment aimed at increasing CFTR protein function in certain CFTR variants have been approved. For example, a group of agents known as CFTR “potentiators” are used in a minority (~3% to 5%) of CF patients who harbor a G155D mutation in the CFTR gene. In this variant, functionally defective CFTR is present in otherwise normal amounts at the cell membrane; the orally bioavailable CFTR “potentiator” partially restores the critical ion transport functions to the defective channel. Another approved drug contains a combination of two agents—one the aforementioned potentiator and the second a drug that increases the transport of the defective Δ508 CFTR protein to the cell membrane. It is too early determine the impact of the emerging molecular therapies


Pathology_Robbins

Pathology_Robbins. potentiator and the second a drug that increases the transport of the defective Δ508 CFTR protein to the cell membrane. It is too early determine the impact of the emerging molecular therapies on prognosis and survival. Overall, improved management of CF have extended the median life expectancy to close to 40 years and, is steadily changing a lethal disease of childhood into a chronic disease of adults.


Pathology_Robbins


Pathology_Robbins

Pathology_Robbins. MoleculartherapiesthatenhancethetransportorstabilityofmutantCFTRproteinareusefulinpatientswhoharborcertainCFTRalleles. PKU results from mutations that cause a severe lack of the enzyme phenylalanine hydroxylase (PAH). It affects 1 in 10,000 live-born white infants and there are several variants of this disease. The most common form, referred to as classic phenylketonuria, is common in persons of Scandinavian descent and is distinctly uncommon in African-American and Jewish populations.


Pathology_Robbins

Pathology_Robbins. Homozygotes with this autosomal recessive disorder classically have a severe lack of PAH, leading to hyperphenylalaninemia and PKU. Affected infants are normal at birth but within a few weeks exhibit a rising plasma phenylalanine level, which impairs brain development. Usually, by 6 months of life, severe mental retardation becomes all too evident; less than 4% of untreated phenylketonuric children have intelligence quotients (IQs) greater than 60. About one-third of these children are never able to walk, and two-thirds cannot talk. Seizures, other neurologic abnormalities, decreased pigmentation of hair and skin, and eczema often accompany the mental retardation in untreated children. Hyperphenylalaninemia and the resultant mental retardation can be avoided by restricting phenylalanine intake early in life. Hence, several screening procedures are routinely performed to detect PKU in the immediate postnatal period.


Pathology_Robbins

Pathology_Robbins. Many female patients with PKU who receive dietary treatment beginning early in life reach childbearing age and are clinically normal. Most of them have marked hyperphenylalaninemia, because dietary treatment is discontinued after they reach adulthood. Between 75% and 90% of children born to such women are mentally retarded and microcephalic, and 15% have congenital heart disease, even though the infants themselves are heterozygotes. This syndrome, termed maternal PKU, results from the teratogenic effects of phenylalanine or its metabolites that cross the placenta and affect specific fetal organs during development. The presence and severity of the fetal anomalies directly correlate with the maternal phenylalanine level, so it is imperative that maternal dietary restriction of phenylalanine be initiated before conception and continued throughout pregnancy.


Pathology_Robbins

Pathology_Robbins. The biochemical abnormality in PKU is an inability to convert phenylalanine into tyrosine. In normal children, less than 50% of the dietary intake of phenylalanine is necessary for protein synthesis. The remainder is converted to tyrosine by the phenylalanine hydroxylase system ( Fig. 7.7 ). When phenylalanine metabolism is blocked because of a lack of PAH enzyme, shunt pathways come into play, yielding several intermediates that are excreted in large amounts in the urine and in the sweat. These impart a strong musty or mousy odor to affected infants. It is believed that excess phenylalanine or its metabolites contribute to the brain damage in PKU. Concomitant lack of tyrosine ( Fig. 7.7 ), a precursor of melanin, is responsible for the light color of hair and skin.


Pathology_Robbins

Pathology_Robbins. At the molecular level, approximately 500 mutant alleles of the PAH gene have been identified, only some of which cause a severe deficiency of the enzyme. Infants with mutations resulting in a severe lack of PAH activity present with the classic features of PKU, whereas those with some residual activity present with milder disease or may be asymptomatic, a condition referred to as benign hyperphenylalaninemia. Because the numerous disease-causing alleles of the PAH gene, complicate molecular diagnosis, measurement of serum phenylalanine levels is used to differentiate benign hyperphenylalaninemia from PKU; the levels in the latter disorder typically are ≥5 times higher than normal. After a biochemical diagnosis is established, the specific mutation causing PKU can be determined. With this information, carrier testing of at-risk family members can be performed. Currently, enzyme replacement therapy is being tried as a method for reducing circulating phenylalanine levels in patients


Pathology_Robbins

Pathology_Robbins. information, carrier testing of at-risk family members can be performed. Currently, enzyme replacement therapy is being tried as a method for reducing circulating phenylalanine levels in patients with classic PKU. The replacement enzyme, known as phenylalanine ammonia lyase (or PAL), converts excess phenylalanine to ammonia and other nontoxic metabolites.


Pathology_Robbins

Pathology_Robbins. Although 98% of cases of hyperphenylalaninemia are attributable to mutations in PAH, approximately 2% arise from abnormalities in synthesis or recycling of the cofactor tetrahydrobiopterin ( Fig. 7.7 ). Clinical recognition of these variant forms is important because these patients cannot be treated by dietary restriction of phenylalanine alone, but also require supplementation with tetrahydrobiopterin, certain neurotransmitter precursors, and folic acid. http://ebooksmedicine.net Fig. 7.7 Thephenylalaninehydroxylasesystem.NADH, Nicotinamideadeninedinucleotide,reducedform. PKUisanautosomalrecessivedisordercausedbyalackoftheenzymephenylalaninehydroxylaseandaconsequentinabilitytometabolizephenylalanine. ClinicalfeaturesofuntreatedPKUmayincludeseverementalretardation,seizures,anddecreasedpigmentationofskin,whichcanbeavoidedbyrestrictingtheintakeofphenylalanineinthediet.


Pathology_Robbins

Pathology_Robbins. FemalepatientswithPKUwhodiscontinuedietarytreatmentcangivebirthtochildrenwithmalformationsandneurologicimpairmentresultingfromtransplacentalpassageofphenylalaninemetabolites. Galactosemia is an autosomal recessive disorder of galactose metabolism resulting from a mutation in the gene encoding the enzyme galactose-1-phosphate uridyltransferase (GALT). It affects 1 in 60,000 live-born infants. Normally, lactase splits lactose, the major carbohydrate of mammalian milk, into glucose and galactose in the intestinal microvilli. Galactose is then converted to glucose in several steps, in one of which the enzyme GALT is required. As a result of this transferase deficiency, galactose-1-phosphate and other metabolites, including galactitol, accumulate in many tissues, including the liver, spleen, lens of the eye, kidney, and cerebral cortex.


Pathology_Robbins

Pathology_Robbins. The liver, eyes, and brain bear the brunt of the damage. The early-onset hepatomegaly results largely from fatty change, but in time widespread scarring that closely resembles the cirrhosis of alcohol abuse may supervene (Chapter 16). Opacification of the lens (cataract) develops, probably because the lens absorbs water and swells as galactitol, produced by alternative metabolic pathways, accumulates and increases its tonicity. Nonspecific alterations appear in the central nervous system (CNS), including loss of nerve cells, gliosis, and edema. There is still no clear understanding of the mechanism of injury to the brain, although elevated galactitol levels in neuronal tissues suggests that this may contribute to the damage.


Pathology_Robbins

Pathology_Robbins. Almost from birth, affected infants fail to thrive. Vomiting and diarrhea appear within a few days of milk ingestion. Jaundice and hepatomegaly usually become evident during the first week of life. Accumulation of galactose and galactose-1-phosphate in the kidney impairs amino acid transport, resulting in aminoaciduria. Fulminant Escherichia coli septicemia occurs with increased frequency. Newborn screening tests are widely utilized in the United States. They depend on fluorometric assay of GALT enzyme activity on a dried blood spot. A positive screening test must be confirmed by assay of GALT levels in RBC. Antenatal diagnosis is possible by assay of GALT activity in cultured amniotic fluid cells or determination of galactitol level in amniotic fluid supernatant.


Pathology_Robbins

Pathology_Robbins. Many of the clinical and morphologic changes of galactosemia can be prevented or ameliorated by removal of galactose from the diet for at least the first 2 years of life. If instituted soon after birth, this diet prevents cataracts and liver damage and permits almost normal development. Even with dietary restrictions, however, older patients frequently are affected by a speech disorder and gonadal failure (especially premature ovarian failure) and, less commonly, by an ataxic condition. GalactosemiaisanautosomalrecessivedisordercausedbylackoftheGALTenzyme,leadingtotheaccumulationofgalactose-1-phosphateanditsmetabolitesintissues. Clinicalfeaturesmayincludejaundice,liverdamage,cataracts,neuraldamage,vomitinganddiarrhea,andE. coli sepsis.Dietaryrestrictionofgalactosecanpreventatleastsomeofthemoreseverecomplications.


Pathology_Robbins

Pathology_Robbins. Clinicalfeaturesmayincludejaundice,liverdamage,cataracts,neuraldamage,vomitinganddiarrhea,andE. coli sepsis.Dietaryrestrictionofgalactosecanpreventatleastsomeofthemoreseverecomplications. Lysosomes, the digestive system of the cells, contain a variety of hydrolytic enzymes that are involved in the breakdown of complex substrates, such as sphingolipids and mucopolysaccharides, into soluble end products. These large molecules may be derived from the turnover of intracellular organelles that enter the lysosomes by autophagy, or they may be acquired from outside the cell by phagocytosis. With an inherited lack of a lysosomal enzyme, catabolism of its substrate remains incomplete, leading to accumulation of the partially degraded insoluble metabolites within the lysosomes (


Pathology_Robbins

Pathology_Robbins. Fig. 7.8 ). This is called primary storage. Stuffed with incompletely digested macromolecules, lysosomes become large and numerous enough to interfere with normal cell functions. Because lysosomal function is also essential for autophagy, impaired autophagy gives rise to secondary storage of autophagic substrates such as polyubiquinated proteins and old and effete mitochondria. The absence of this quality-control mechanism causes accumulation of dysfunctional mitochondria, which can trigger the generation of free radicals and apoptosis. http://ebooksmedicine.net Approximately 60 lysosomal storage diseases have been identified. These may result from abnormalities of


Pathology_Robbins

Pathology_Robbins. http://ebooksmedicine.net Approximately 60 lysosomal storage diseases have been identified. These may result from abnormalities of Fig. 7.8 Pathogenesisoflysosomalstoragediseases.Inthisexample,acomplexsubstrateisnormallydegradedbyaseriesoflysosomalenzymesintosolubleendproducts(A,BandC).Ifthereisadeficiencyormalfunctionofoneoftheenzymes(e.g.,B),catabolismisincomplete,andinsolubleintermediatesaccumulateinthelysosomes.Inadditiontothisprimarystorage,secondarystorageandtoxiceffectsresultfromdefectiveautophagy. lysosomal enzymes or proteins involved in substrate degradation, endosomal sorting, or lysosomal membrane integrity. Lysosomal storage disorders are divided into categories based on the biochemical nature of the substrates and the accumulated metabolites. ( Table 7.3 ). Within each group are several entities, each resulting from the deficiency of a specific enzyme.


Pathology_Robbins

Pathology_Robbins. Table 7.3 ). Within each group are several entities, each resulting from the deficiency of a specific enzyme. Although the combined frequency of lysosomal storage disorders (LSDs) is about 1 in 5000 live births, lysosomal dysfunction may be involved in the etiology of several more-common diseases. For example, an important genetic risk factor for developing Parkinson disease is the carrier state for Gaucher disease, and virtually all Gaucher disease patients develop Parkinson disease. Niemann Pick C is another LSD connected to the risk for Alzheimer disease. Such interconnectedness stems from the multifunctionality of the lysosome. For example, lysosomes play critical roles in (i) autophagy, resulting from fusion with the autophagosome; (ii) immunity, as they fuse with phagosomes; and (iii) membrane repair, through fusion with the plasma membrane.


Pathology_Robbins

Pathology_Robbins. LSDs are typically fatal, with many displaying profound neurologic impairment that begins in childhood. Studies show that several neurodegenerative LSDs also are associated with a dysregulation of the immune system. For example, immunosuppression occurs in Gaucher disease and mucopolysaccharidoses VII, whereas in Niemann Pick disease-type-C1, hyperactivation of the immune system is seen. The mechanistic basis of linkage to immunologic changes remains to be deciphered. Despite this complexity, certain features are common to most diseases in this group: Patient population consisting of infants and young children Storage of insoluble intermediates in the mononuclear phagocyte system, giving rise to hepatosplenomegaly Frequent CNS involvement with associated neuronal damage Cellular dysfunction caused not only by storage of undigested material, but also by a cascade of secondary events, for example, macrophage activation and release of cytokines


Pathology_Robbins

Pathology_Robbins. Cellular dysfunction caused not only by storage of undigested material, but also by a cascade of secondary events, for example, macrophage activation and release of cytokines Most of these conditions are very rare, and their detailed description is better relegated to specialized texts and reviews. Only a few of the more common conditions are considered here. Tay-SachsDisease(GM2Gangliosidosis:DeficiencyinHexosaminidaseβ Subunit)


Pathology_Robbins

Pathology_Robbins. Tay-SachsDisease(GM2Gangliosidosis:DeficiencyinHexosaminidaseβ Subunit) Gangliosidoses are characterized by accumulation of gangliosides, principally in the brain, as a result of a deficiency of one of the lysosomal enzymes that catabolize these glycolipids. Depending on the ganglioside involved, these disorders are subclassified into GM1 and GM2 categories. Tay-Sachs disease, by far the most common of all gangliosidoses, is caused by loss-of-function mutations of the β subunit of the enzyme hexosaminidase A, which is necessary for the degradation of GM2. More than 100 mutations have been described; most disrupt protein folding or intracellular transport. Tay-Sachs disease, similar to other lipid storage disorders, is most common among http://ebooksmedicine.net Ashkenazi Jews, among whom the frequency of heterozygous carriers is estimated to be 1 in 30. Heterozygote carriers can be detected by measuring the level of hexosaminidase in the serum or by DNA sequencing.


Pathology_Robbins

Pathology_Robbins. In the absence of hexosaminidase A, GM2 ganglioside accumulates in many tissues (e.g., heart, liver, spleen, nervous system), but the involvement of neurons in the central and autonomic nervous systems and retina dominates the clinical picture. The accumulation of GM2 occurs within neurons, axon cylinders of nerves, and glial cells throughout the CNS. Affected cells appear swollen and sometimes foamy ( Fig. 7.9A ). Electron microscopy shows whorled onionskin-like configurations within lysosomes composed of layers of membranes ( Fig. 7.9B ). These pathologic changes are found throughout the CNS (including the spinal cord), peripheral nerves, and autonomic nervous system. The retina usually is involved as well, where the pallor produced by swollen ganglion cells in the peripheral retina results in a contrasting “cherry red” spot in the relatively unaffected central macula.


Pathology_Robbins

Pathology_Robbins. The molecular basis for neuronal injury is not fully understood. Because in many cases the mutant protein is misfolded, it induces the so-called “unfolded protein” response (Chapter 2). If such misfolded enzymes are not stabilized by chaperones, they undergo proteasomal degradation, leading to accumulation of toxic substrates and intermediates within neurons. These findings have spurred clinical trials of molecular chaperone therapy for some variants of later-onset Tay-Sachs and other selected lysosomal storage diseases. Such therapy involves the use of synthetic chaperones that can cross the blood–brain barrier, bind to the mutated protein, and enable its proper folding. Sufficient functional enzyme can then be rescued to ameliorate the effects of the inborn error.


Pathology_Robbins

Pathology_Robbins. In the most common acute infantile variant of Tay-Sachs disease, infants appear normal at birth, but motor weakness begins at 3 to 6 months of age, followed by neurologic impairment, onset of blindness, and progressively more severe neurologic dysfunctions. Death occurs within 2 to 3 years. Type A and type B Niemann-Pick diseases are related entities characterized by a primary deficiency of acid http://ebooksmedicine.net


Pathology_Robbins

Pathology_Robbins. Type A and type B Niemann-Pick diseases are related entities characterized by a primary deficiency of acid http://ebooksmedicine.net Fig. 7.9 GanglioncellsinTay-Sachsdisease.(A)Underthelightmicroscope,alargeneuronhasobviouslipidvacuolation.(B)Aportionofaneuronundertheelectronmicroscopeshowsprominentlysosomeswithwhorledconfigurationsjustbelowpartofthenucleus.(A, Courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas Southwestern Medical Center, Dallas, Texas. B, Courtesy of Dr. Joe Rutledge, Children’s Regional Medical Center, Seattle, Washington.) sphingomyelinase and the resultant accumulation of sphingomyelin. As with Tay-Sachs disease, Niemann-Pick disease types A and B are common in Ashkenazi Jews. The gene for acid sphingomyelinase maps to chromosome 11p15.4 and is one of the imprinted genes that is preferentially expressed from the maternal chromosome as a result of epigenetic silencing of the paternal gene (discussed later).


Pathology_Robbins

Pathology_Robbins. In type A, characterized by a severe deficiency of sphingomyelinase, the breakdown of sphingomyelin into ceramide and phosphorylcholine is impaired, and excess sphingomyelin accumulates in phagocytic cells and in neurons. Macrophages become stuffed with droplets or particles of the complex lipid, imparting a fine vacuolation or foaminess to the cytoplasm (


Pathology_Robbins

Pathology_Robbins. Fig. 7.10 ). Electron microscopy shows engorged secondary lysosomes that often contain membranous cytoplasmic bodies resembling concentric lamellated myelin figures, sometimes called “zebra” bodies. Because of their high content of phagocytic cells, the organs most severely affected are the spleen, liver, bone marrow, lymph nodes, and lungs. The splenic enlargement may be striking. In addition, the entire CNS, including the spinal cord and ganglia, is involved in this inexorable process. The affected neurons are enlarged and vacuolated as a result of the accumulation of lipids. This variant manifests in infancy with massive organomegaly and severe neurologic deterioration. Death usually occurs within the first 3 years of life. By comparison, patients with the type B variant, which is associated with mutant sphigomyelinase with some residual activity, have organomegaly but no neurologic manifestations. Estimation of sphingomyelinase activity in the leukocytes can be used for diagnosis


Pathology_Robbins

Pathology_Robbins. with mutant sphigomyelinase with some residual activity, have organomegaly but no neurologic manifestations. Estimation of sphingomyelinase activity in the leukocytes can be used for diagnosis of suspected cases, as well as for detection of carriers. Molecular genetic tests are also available for diagnosis at specialized centers.


Pathology_Robbins

Pathology_Robbins. Although previously considered to be related to type A and type B Niemann-Pick disease type C (NPC) is quite distinct at the biochemical and molecular levels and is more common than types A and B combined. Mutations in two related genes, NPC1 and NPC2, can give rise to this disorder, with NPC1 being responsible for a majority of cases. Unlike most other lysosomal storage diseases, NPC results from a primary defect in lipid transport. Affected cells accumulate cholesterol as well as gangliosides such as GM1 and GM2. Both NPC1 and NPC2 are involved in the transport of free cholesterol from the lysosomes to the cytoplasm. NPC is clinically heterogeneous. The most common form manifests in childhood and is marked by ataxia, vertical supranuclear gaze palsy, dystonia, dysarthria, and psychomotor regression.


Pathology_Robbins

Pathology_Robbins. Gaucher disease results from mutation in the gene that encodes glucocerebrosidase, and the resultant deficiency of this enzyme leads to an accumulation of glucocerebroside, an intermediate in glycolipid metabolism, in the mononuclear phagocytic cells. There are three autosomal recessive variants of Gaucher disease resulting from distinct allelic mutations. Common to all is deficient activity of a glucocerebrosidase that cleaves the glucose residue from ceramide. Normally, macrophages, particularly in the liver, spleen, and bone marrow, sequentially degrade glycolipids derived from the breakdown of senescent blood cells. In Gaucher disease, the degradation stops at the level of glucocerebrosides, which accumulate in macrophages. These cells—the so-called “Gaucher cells”—become enlarged, with some reaching a diameter as great as 100 µm, because of the presence of distended lysosomes, and they acquire a pathognomonic cytoplasmic appearance characterized as “wrinkled tissue paper” (


Pathology_Robbins

Pathology_Robbins. Fig. 7.11 ). It is evident now that Gaucher disease is caused not just by the burden of storage material but also by activation of the macrophages. High levels of macrophage-derived cytokines, such as interleukins (IL-1, IL-6) and tumor necrosis factor (TNF), are found in affected tissues. Fig. 7.10 Niemann-Pickdiseaseinliver.ThehepatocytesandKupffercellshaveafoamy,vacuolatedappearanceresultingfromdepositionoflipids. (Courtesy of Dr. Arthur Weinberg, Department of Pathology, University of Texas Southwestern Medical Center, Dallas,Texas.) http://ebooksmedicine.net


Pathology_Robbins

Pathology_Robbins. One variant, type 1, also called the chronic nonneuronopathic form, accounts for 99% of cases of Gaucher disease. It is characterized by clinical or radiographic bone involvement (osteopenia, focal lytic lesions, and osteonecrosis) in 70% to 100% of cases. Additional features are hepatosplenomegaly and the absence of CNS involvement. The spleen often enlarges to massive proportions, filling the entire abdomen. Gaucher cells are found in the liver, spleen, lymph nodes, and bone marrow. Marrow replacement and cortical erosion may produce radiographically visible skeletal lesions, and peripheral blood cytopenias. It is believed that bone changes are caused by the aforementioned macrophage-derived cytokines. Type 1 is most common in Ashkenazi Jews; unlike other variants, it is compatible with long life. Neurologic signs and symptoms characterize types 2 and 3 variants. In type 2, these manifestations appear during infancy (acute infantile neuronopathic form) and are more severe, whereas


Pathology_Robbins

Pathology_Robbins. long life. Neurologic signs and symptoms characterize types 2 and 3 variants. In type 2, these manifestations appear during infancy (acute infantile neuronopathic form) and are more severe, whereas in type 3, they emerge later and are milder (chronic neuronopathic form). Although the liver and spleen also are involved, the clinical features in types 2 and 3 are dominated by neurologic disturbances, including convulsions and progressive mental deterioration. As mentioned earlier mutation of the glucocerebroside gene is a very important risk factor for Parkinson disease. Patients with Gaucher disease have a 20 fold higher risk of developing Parkinson disease (compared to controls) and 5–10% of patients with Parkinson disease have mutations in the gene encoding glucocerebrosidase.


Pathology_Robbins

Pathology_Robbins. There is a reciprocal relationship between the level of this enzyme and alpha synuclein. The latter is involved in the pathogenesis of Parkinson disease (Chapter 24). The level of glucocerebrosides in leukocytes or cultured fibroblasts is helpful in diagnosis and in the detection of heterozygote carriers. DNA testing is also available in select populations.


Pathology_Robbins

Pathology_Robbins. Currently, there are two approved therapies for type I Gaucher disease. The first is lifelong enzyme replacement therapy via infusion of recombinant glucocerebrosidase. The second, known as substrate reduction therapy, involves oral intake of an inhibitor of the enzyme glucosylceramide synthase. This leads to reduced systemic levels of glucocerebroside, the substrate for the defective enzyme in Gaucher disease. Clinical trials have confirmed that substrate reduction therapy leads to reduced spleen and liver sizes, improved blood counts, and enhanced skeletal function. Other emerging treatments include gene therapy through transplant of hematopoietic stem cells containing the corrected enzyme.


Pathology_Robbins

Pathology_Robbins. Mucopolysaccharidoses (MPSs) are characterized by defective degradation and excessive storage of mucopolysaccharides in various tissues. Recall that mucopolysaccharides are part of the extracellular matrix and are synthesized by connective tissue fibroblasts. Most mucopolysaccharide is secreted, but a certain fraction is degraded within lysosome through a catabolic pathway involving multiple enzymes. Several clinical variants of MPS, classified numerically from MPS I to MPS VII, have been described, each resulting from the deficiency of one specific enzyme in this pathway. The mucopolysaccharides that accumulate within the tissues include dermatan sulfate, heparan sulfate, keratan sulfate, and (in some cases) chondroitin sulfate.


Pathology_Robbins

Pathology_Robbins. Hepatosplenomegaly, skeletal deformities, lesions of heart valves, subendothelial arterial deposits, particularly in the coronary arteries, and lesions in the brain, are features that are seen in all of the MPSs. In many of the more protracted syndromes, coronary subendothelial lesions lead to myocardial ischemia. Thus, myocardial infarction and cardiac decompensation are important causes of death. Most cases are associated


Pathology_Robbins

Pathology_Robbins. Fig. 7.11 Gaucherdiseaseinvolvingthebonemarrow.(A)Gauchercellswithabundantlipid-laden“wrinkled”cytoplasm.(B)ElectronmicrographofGauchercellswithelongateddistendedlysosomes.(Courtesy of Dr. Matthew Fries,Department of Pathology,University of Texas Southwestern Medical Center,Dallas,Texas.) with coarse facial features, clouding of the cornea, joint stiffness, and mental retardation. Urinary excretion of the accumulated mucopolysaccharides often is increased. With all of these disorders except one, the mode of inheritance is autosomal recessive; the exception, Hunter syndrome, is an X-linked recessive disease. Of the seven recognized variants, only two well-characterized syndromes are discussed briefly here.


Pathology_Robbins

Pathology_Robbins. MPS type I, also known as Hurler syndrome, is caused by a deficiency of α-L-iduronidase. In Hurler syndrome, affected children have a life expectancy of 6 to 10 years, and death is often a result of cardiac complications. Accumulation of dermatan sulfate and heparan sulfate is seen in cells of the mononuclear phagocyte system, in fibroblasts, and within endothelium and smooth muscle cells of the vascular wall. The affected cells are swollen and have clear cytoplasm, resulting from the accumulation of material positive for periodic acid–Schiff staining within engorged, vacuolated lysosomes. Lysosomal inclusions also are found in neurons, accounting for the mental retardation.


Pathology_Robbins

Pathology_Robbins. MPS type II or Hunter syndrome, differs from Hurler syndrome in its mode of inheritance (X-linked), the absence of corneal clouding, and often its milder clinical course. As in Hurler syndrome, the accumulated mucopolysaccharides in Hunter syndrome are heparan sulfate and dermatan sulfate, but this results from a deficiency of L-iduronate sulfatase. Despite the difference in enzyme deficiency, an accumulation of identical substrates occurs because breakdown of heparan sulfate and dermatan sulfate requires both α-L-iduronidase and the sulfatase; if either one is missing, further degradation is blocked. Diagnosis is made by measuring the level of enzyme in leukocytes. DNA diagnosis is not routinely employed because of the large number of alellic mutations. Tay-SachsdiseaseiscausedbyaninabilitytometabolizeGM2gangliosidesbecauseofalackoftheβ subunitoflysosomalhexosaminidase.GM2gangliosidesaccumulateintheCNSandcauseseverementalretardation,blindness,motorweakness,anddeathby2to3yearsofage.


Pathology_Robbins