84 |
The Practice of Genetics in Clinical Medicine |
APPLICATIONS OF MOLECULAR GENETICS IN CLINICAL MEDICINE
Genetic testing for inherited abnormalities associated with disease risk is increasingly used in the practice of clinical medicine. Germline alterations include chromosomal abnormalities (Chap. 83e), specific gene mutations with autosomal dominant or recessive patterns of transmission (Chap. 82), and single nucleotide polymorphisms with small relative risks associated with disease. Germline alterations are responsible for disorders beyond classic Mendelian conditions with genetic susceptibility to common adult-onset diseases such as asthma, hypertension, diabetes mellitus, macular degeneration, and many forms of cancer. For many of these diseases, there is a complex interplay of genes (often multiple) and environmental factors that affect lifetime risk, age of onset, disease severity, and treatment options.
The expansion of knowledge related to genetics is changing our understanding of pathophysiology and influencing our classification of diseases. Awareness of genetic etiology can have an impact on clinical management, including prevention and screening for or treatment of a range of diseases. Primary care physicians are relied upon to help patients navigate testing and treatment options. Consequently, they must understand the genetic basis for a large number of genetically influenced diseases, incorporate personal and family history to determine the risk for a specific mutation, and be positioned to provide counseling. Even if patients are seen by genetic specialists who assess genetic risk and coordinate testing, primary care providers should provide information to their patients regarding the indications, limitations, risks, and benefits of genetic counseling and testing. They must also be prepared to offer risk-based management following genetic risk assessment. Given the pace of genetics, this is an increasingly difficult task. The field of clinical genetics is rapidly moving from single gene testing to multigene panel testing, with techniques such as whole-exome and -genome sequencing on the horizon, increasing the complexity of test selection and interpretation, as well as patient education and medical decision making.
COMMON ADULT-ONSET GENETIC DISORDERS
INHERITANCE PATTERNS
Adult-onset hereditary diseases follow multiple patterns of inheritance. Some are autosomal dominant conditions. These include many common cancer susceptibility syndromes such as hereditary breast and ovarian cancer (due to germline BRCA1 and BRCA2 mutations) and Lynch syndrome (caused by germline mutations in the mismatch repair genes MLH1, MSH2, MSH6, and PMS2). In both of these examples, inherited mutations are associated with a high penetrance (lifetime risk) of cancer, although risk is not 100%. In other conditions, although there is autosomal dominant transmission, there is lower penetrance, thereby making the disorders more difficult to recognize. For example, germline mutations in CHEK2 increase the risk of breast cancer, but with a moderate lifetime risk in the range of 20–40%, as opposed to 50–70% for mutations in BRCA1 or BRCA2. Other adult-onset hereditary diseases are transmitted in an autosomal recessive fashion where two mutant alleles are necessary to cause disease. Examples include hemochromatosis and MYH-associated colon cancer. There are more pediatric-onset autosomal recessive disorders, such as lysosomal storage diseases and cystic fibrosis.
The genetic risk for many adult-onset disorders is multifactorial. Risk can be conferred by genetic factors at a number of loci, which individually have very small effects (usually with relative risks of <1.5). These risk loci (generally single nucleotide polymorphisms [SNPs]) combine with other genes and environmental factors in ways that are not well understood. SNP panels are available to assess risk of disease, but the optimal way of using this information in the clinical setting remains uncertain.
Many diseases have multiple patterns of inheritance, adding to the complexity of evaluating patients and families for these conditions. For example, colon cancer can be associated with a single germline mutation in a mismatch repair gene (Lynch syndrome, autosomal dominant), biallelic mutations in MYH (autosomal recessive), or multiple SNPs (polygenic). Many more individuals will have SNP risk alleles than germline mutations in high-penetrance genes, but cumulative lifetime risk of colon cancer related to the former is modest, whereas the risk related to the latter is significant. Personal and family histories provide important insights into the possible mode of inheritance.
FAMILY HISTORY
When two or more first-degree relatives are affected with asthma, cardiovascular disease, type 2 diabetes, breast cancer, colon cancer, or melanoma, the relative risk for disease among close relatives ranges from two- to fivefold, underscoring the importance of family history for these prevalent disorders. In most situations, the key to assessing the inherited risk for common adult-onset diseases is the collection and interpretation of a detailed personal and family medical history in conjunction with a directed physical examination.
Family history should be recorded in the form of a pedigree. Pedigrees should convey health-related data on first- and second-degree relatives. When such pedigrees suggest inherited disease, they should be expanded to include additional family members. The determination of risk for an asymptomatic individual will vary depending on the size of the pedigree, the number of unaffected relatives, the types of diagnoses, and the ages of disease onset. For example, a woman with two first-degree relatives with breast cancer is at greater risk for a specific Mendelian disorder if she has a total of 3 female first-degree relatives (with only 1 unaffected) than if she has a total of 10 female first-degree relatives (with 7 unaffected). Factors such as adoption and limited family structure (few women in a family) should to be taken into consideration in the interpretation of a pedigree. Additional considerations include young age of disease onset (e.g., a 30-year nonsmoking woman with a myocardial infarction), unusual diseases (e.g., male breast cancer or medullary thyroid cancer), and the finding of multiple potentially related diseases in an individual (e.g., a woman with a history of both colon and endometrial cancer). Some adult-onset diseases are more prevalent in certain ethnic groups. For instance, 2.5% of individuals of Ashkenazi Jewish ancestry carry one of three founder mutation in BRCA1 and BRCA2. Factor V Leiden mutations are much more common in Caucasians than in Africans or Asians.
Additional variables that should be documented are nonhereditary risk factors among those with disease (such as cigarette smoking and myocardial infarction; asbestos exposure and lung disease; and mantle radiation and breast cancer). Significant associated environmental exposures or lifestyle factors decrease the likelihood of a specific genetic disorder. In contrast, the absence of nonhereditary risk factors typically associated with a disease raises concern about a genetic association. A personal or family history of deep vein thrombosis in the absence of known environmental or medical risk factors suggests a hereditary thrombotic disorder. The physical examination may also provide important clues about the risk for a specific inherited disorder. A patient presenting with xanthomas at a young age should prompt consideration of familial hypercholesterolemia. The presence of trichilemmomas in a woman with breast cancer raises concern for Cowden syndrome, associated with PTEN mutations.
Recall of family history is often inaccurate. This is especially so when the history is remote and families lose contact or separate geographically. It can be helpful to ask patients to fill out family history forms before or after their visits, because this provides them with an opportunity to contact relatives. Ideally, this information should be embedded in electronic health records and updated intermittently. Attempts should be made to confirm the illnesses reported in the family history before making important and, in certain circumstances, irreversible management decisions. This process is often labor intensive and ideally involves interviews of additional family members or reviewing medical records, autopsy reports, and death certificates.
Although many inherited disorders will be suggested by the clustering of relatives with the same or related conditions, it is important to note that disease penetrance is incomplete for most genetic disorders. As a result, the pedigree obtained in such families may not exhibit a clear Mendelian inheritance pattern, because not all family members carrying the disease-associated alleles will manifest clinical evidence of the condition. Furthermore, genes associated with some of these disorders often exhibit variable disease expression. For example, the breast cancer–associated gene BRCA2 can predispose to several different malignancies in the same family, including cancers of the breast, ovary, pancreas, skin, and prostate. For common diseases such as breast cancer, some family members without the susceptibility allele (or genotype) may develop breast cancer (or phenotype) sporadically. Such phenocopies represent another confounding variable in the pedigree analysis.
Some of the aforementioned features of the family history are illustrated in Fig. 84-1. In this example, the proband, a 36-year-old woman (IV-1), has a strong history of breast and ovarian cancer on the paternal side of her family. The early age of onset and the co-occurrence of breast and ovarian cancer in this family suggest the possibility of an inherited mutation in BRCA1 or BRCA2. It is unclear however, without genetic testing, whether her father harbors such a mutation and transmitted it to her. After appropriate genetic counseling of the proband and her family, the most informative and cost-effective approach to DNA analysis in this family is to test the cancer-affected 42-year-old living cousin for the presence of a BRCA1 or BRCA2 mutation. If a mutation is found, then it is possible to test for this particular alteration in other family members, if they so desire. In the example shown, if the proband’s father has a BRCA1 mutation, there is a 50:50 probability that the mutation was transmitted to her, and genetic testing can be used to establish the absence or presence of this alteration. In this same example, if a mutation is not detected in the cancer-affected cousin, testing would not be indicated for cancer-unaffected relatives.
FIGURE 84-1 A 36-year-old woman (arrow) seeks consultation because of her family history of cancer. The patient expresses concern that the multiple cancers in her relatives imply an inherited predisposition to develop cancer. The family history is recorded, and records of the patient’s relatives confirm the reported diagnoses.
GENETIC TESTING FOR ADULT-ONSET DISORDERS
A critical first step before initiating genetic testing is to ensure that the correct clinical diagnosis has been made, whether it is based on family history, characteristic physical findings, pathology, or biochemical testing. Such careful clinical assessment can define the phenotype. In the traditional model of genetic testing, testing is directed initially toward the most probable genes (determined by the phenotype), which prevents unnecessary testing. Many disorders exhibit the feature of locus heterogeneity, which refers to the fact that mutations in different genes can cause phenotypically similar disorders. For example, osteogenesis imperfecta (Chap. 427), long QT syndrome (Chap. 277), muscular dystrophy (Chap. 462e), and hereditary predisposition to breast (Chap. 108) or colon (Chap. 110) cancer can each be caused by mutations in a number of distinct genes. The patterns of disease transmission, disease risk, clinical course, and treatment may differ significantly depending on the specific gene affected. Historically, the choice of which gene to test has been determined by unique clinical and family history features and the relative prevalence of candidate genetic disorders. However, rapid changes in genetic testing techniques, as discussed below, may impact this paradigm. It is now technically and financially feasible to sequence many genes (or even the whole exome) at one time. The incorporation of multiplex testing for germline mutations is rapidly evolving.
METHODOLOGIC APPROACHES TO GENETIC TESTING
Genetic testing is regulated and performed in much the same way as other specialized laboratory tests. In the United States, genetic testing laboratories are Clinical Laboratory Improvement Amendments (CLIA) approved to ensure that they meet quality and proficiency standards. A useful information source for various genetic tests is www.genetests.org. It should be noted that many tests need to be ordered through specialized laboratories.
Genetic testing is performed largely by DNA sequence analysis for mutations, although genotype can also be deduced through the study of RNA or protein (e.g., apolipoprotein E, hemoglobin S, and immunohistochemistry). For example, universal screening for Lynch syndrome via immunohistochemical analysis of colorectal cancers for absence of expression of mismatch repair proteins is under way at multiple hospitals throughout the United States. The determination of DNA sequence alterations relies heavily on the use of polymerase chain reaction (PCR), which allows rapid amplification and analysis of the gene of interest. In addition, PCR enables genetic testing on minimal amounts of DNA extracted from a wide range of tissue sources including leukocytes, mucosal epithelial cells (obtained via saliva or buccal swabs), and archival tissues. Amplified DNA can be analyzed directly by DNA sequencing, or it can be hybridized to DNA chips or blots to detect the presence of normal and altered DNA sequences. Direct DNA sequencing is frequently used for determination of hereditary disease susceptibility and prenatal diagnosis. Analyses of large alterations of the genome are possible using cytogenetics, fluorescent in situ hybridization (FISH), Southern blotting, or multiplex ligation-dependent probe amplification (MLPA) (Chap. 83e).
Massively parallel sequencing (also called next-generation sequencing) is significantly altering the approach to genetic testing for adult-onset hereditary susceptibility disorder. This technology encompasses several high-throughput approaches to DNA sequencing, all of which can reliably sequence many genes at one time. Technically, this involves the use of amplified DNA templates in a flow cell, a very different process than traditional Sanger sequencing which is time-consuming and expensive.
Multiplex panels for inherited susceptibility are commercially available and include testing of a number of genes that have been associated with the condition of interest. For example, panels are available for Brugada syndrome, hypertrophic cardiomyopathy, and Charcot-Marie-Tooth neuropathy. For many syndromes, this type of panel testing may make sense. However, in other situations, the utility of panel testing is less certain. Currently available breast cancer susceptibility panels contain six genes or more. Many of the genes included in the larger panels are associated with only a modest risk of breast cancer, and the clinical application is uncertain. An additional problem of sequencing many genes (rather than the genes for which there is most suspicion) is the identification of one or more variants of uncertain significance (VUS), discussed below.
Whole-exome sequencing (WES) is also now commercially available, although largely used in individuals with syndromes unexplained by traditional genetic testing. As cost declines, WES may be more widely used. Whole-genome sequencing is also commercially available. Although it may be quite feasible to sequence the entire genome, there are many issues in doing so, including the daunting task of analyzing the vast amount of data generated. Other issues include: (1) the optimal way in which to obtain informed consent, (2) interpretation of frequent sequence variation of uncertain significance, (3) interpretation of alterations in genes with unclear relevance to specific human pathology, and (4) management of unexpected but clinically significant genetic findings.
Testing strategies are evolving as a result of these new genetic testing platforms. As the cost of multiple gene panels and WES continue to fall, and as interpretation of such test results improve, there may be a shift from sequential single-gene (or a few genes) testing to multigene testing. For example, presently, a 30-year-old woman with breast cancer but no family history of cancer and no syndromic features would undergo BRCA1/2 testing. If negative, she would subsequently be offered TP53 testing. Notably, a reasonable number of individuals offered TP53 testing for Li-Fraumeni syndrome decline because mutations are associated with extremely high cancer risks (including childhood cancers) in multiple organs and there are no proven interventions to mitigate risk. Without features consistent with Cowden syndrome, the woman would not be routinely offered PTEN testing or testing for CHEK2, ATM, BRIP, BARD, NBN, and PALB2. However, it is now possible to synchronously analyze all of the aforementioned genes, for a nominally higher cost than BRCA1/2 testing alone. Concerns about such panels include appropriate consent strategies related to unexpected findings, VUS, and unclear clinical utility of testing moderate-penetrance genes. Thus, changes from the traditional model of single-gene genetic testing should be done with caution (Fig. 84-2).
FIGURE 84-2 Approach to genetic testing.
Limitations to the accuracy and interpretation of genetic testing exist. In addition to technical errors, genetics tests are sometimes designed to detect only the most common mutations. In addition, genetic testing has evolved over time. For example, it was not possible to obtain commercially available comprehensive large genomic rearrangement testing for BRCA1 and BRCA2 until 2006. Therefore, a negative result must be qualified by the possibility that the individual may have a mutation that was not included in the test. In addition, a negative result does not mean that there is not a mutation in some other gene that causes a similar inherited disorder. A negative result, unless there is known mutation in the family, is typically classified as uninformative.
VUS are another limitation to genetic testing. A VUS (also termed unclassified variant) is a sequence variation in a gene where the effect of the alteration on the function of the protein is not known. Many of these variants are single nucleotide substitutions (also called missense mutations) that result in a single amino acid change. Although many VUSs will ultimately be reclassified as benign polymorphisms, some will prove to be functionally important. As more genes are sequenced (for example, in a multiplex panel or through WES), the percentage of individuals found to have a VUS increases significantly. The finding of a VUS is difficult for patients and providers alike and complicates decisions regarding medical management.
Clinical utility is an important consideration because genetic testing for susceptibility to chronic diseases is increasingly integrated into the practice of medicine. In some situations, there is clear clinical utility to genetic testing with significant evidence-based changes in medical management decisions based on results. However, in many cases, the discovery of disease-associated genes has outpaced studies that assess how such information should be used in the clinical management of the patient and family. This is particularly true for moderate- and low-penetrance gene mutations. Therefore, predictive genetic testing should be approached with caution and only offered to patients who have been adequately counseled and have provided informed consent.
Predictive genetic testing falls into two distinct categories. Presymptomatic testing applies to diseases where a specific genetic alteration is associated with a near 100% likelihood of developing disease. In contrast, predisposition testing predicts a risk for disease that is less than 100%. For example, presymptomatic testing is available for those at risk for Huntington’s disease; whereas, predisposition testing is considered for those at risk for hereditary colon cancer. It is important to note that for the majority of adult-onset disorders, testing is only predictive. Test results cannot reveal with confidence whether, when, or how the disease will manifest itself. For example, not everyone with the apolipoprotein E4 allele will develop Alzheimer’s disease, and individuals without this genetic marker can still develop the disorder.
The optimal testing strategy for a family is to initiate testing in an affected family member first. Identification of a mutation can direct the testing of other at-risk family members (whether symptomatic or not). In the absence of additional familial or environmental risk factors, individuals who test negative for the mutation found in the affected family member can be informed that they are at general population risk for that particular disease. Furthermore, they can be reassured that they are not at risk for passing the mutation on to their children. On the other hand, asymptomatic family members who test positive for the known mutation must be informed that they are at increased risk for disease development and for transmitting the alteration to their children.
Pretest counseling and education are important, as is an assessment of the patient’s ability to understand and cope with test results. Genetic testing has implications for entire families, and thus individuals interested in pursuing genetic testing must consider how test results might impact their relationships with relatives, partners, spouses, and children. In families with a known genetic mutation, those who test positive must consider the impact of their carrier status on their present and future lifestyles; those who test negative may manifest survivor guilt. Parents who are found to have a disease-associated mutation often express considerable anxiety and despair as they address the issue of risk to their children. In addition, some individuals consider options such as preimplantation genetic diagnosis in their reproductive decision making.
When a condition does not manifest until adulthood, clinicians and parents are faced with the question of whether at-risk children should be offered genetic testing and, if so, at what age. Although the matter is debated, several professional organizations have cautioned that genetic testing for adult-onset disorders should not be offered to children. Many of these conditions have no known interventions in childhood to prevent disease; consequently, such information can pose significant psychosocial risk to the child. In addition, there is concern that testing during childhood violates a child’s right to make an informed decision regarding testing upon reaching adulthood. On the other hand, testing should be offered in childhood for disorders that may manifest early in life, especially when management options are available. For example, children with multiple endocrine neoplasia 2 (MEN 2) may develop medullary thyroid cancer early in life and should be considered for prophylactic thyroidectomy (Chap. 408). Similarly, children with familial adenomatous polyposis (FAP) due to a mutation in APC may develop polyps in their teens with progression to invasive cancer in the twenties, and therefore, colonoscopy screening is started between the ages of 10 and 15 years (Chap. 110).
INFORMED CONSENT
Informed consent for genetic testing begins with education and counseling. The patient should understand the risks, benefits, and limitations of genetic testing, as well as the potential implications of test results. Informed consent should include a written document, drafted clearly and concisely in a language and format that is understandable to the patient. Because molecular genetic testing of an asymptomatic individual often allows prediction of future risk, the patient should understand all potential long-term medical, psychological, and social implications of testing. There have long been concerns about the potential for genetic discrimination. The Genetic Information Nondiscrimination Act (GINA) was passed in 2008 and provides some protections related to job and health insurance discrimination. It is important to explore with patients the potential impact of genetic test results on future health as well as disability and life insurance coverage. Patients should understand that alternatives remain available if they decide not to pursue genetic testing, including the option of delaying testing to a later date. The option of DNA banking should be presented so that samples are readily available for future use by family members, if needed.
FOLLOW-UP CARE AFTER TESTING
Depending on the nature of the genetic disorder, posttest interventions may include: (1) cautious surveillance and awareness; (2) specific medical interventions such as enhanced screening, chemoprevention, or risk-reducing surgery; (3) risk avoidance; and (4) referral to support services. For example, patients with known deleterious mutations in BRCA1 or BRCA2 are strongly encouraged to undergo risk-reducing salpingo-oophorectomy and are offered intensive breast cancer screening as well as the option of risk-reducing mastectomy. In addition, such women may wish to take chemoprevention with tamoxifen, raloxifene, or exemestane. Those with more limited medical management and prevention options, such as patients with Huntington’s disease, should be offered continued follow-up and supportive services, including physical and occupational therapy and social services or support groups as indicated. Specific interventions will change as research continues to enhance our understanding of the medical management of these genetic conditions and more is learned about the functions of the gene products involved.
Individuals who test negative for a mutation in a disease-associated gene identified in an affected family member must be reminded that they may still be at risk for the disease. This is of particular importance for common diseases such as diabetes mellitus, cancer, and coronary artery disease. For example, a woman who finds that she does not carry the disease-associated mutation in BRCA2 previously discovered in the family should be reminded that she still requires the same breast cancer screening recommended for the general population.
GENETIC COUNSELING AND EDUCATION
Genetic counseling should be distinguished from genetic testing and screening, although genetic counselors are often involved in issues related to testing. Genetic counseling refers to a communication process that deals with human problems associated with the occurrence of risk of a genetic disorder in a family. Genetic risk assessment is complex and often involves elements of uncertainty. Counseling, therefore, includes genetic education as well as psychosocial counseling. Genetic counseling can be useful in a wide range of situations (Table 84-1). The role of the genetic counselor includes the following:
1. Gather and document a detailed family history.
2. Educate patients about general genetic principles related to disease risk, both for themselves and for others in the family.
3. Assess and enhance the patient’s ability to cope with the genetic information offered.
4. Discuss how nongenetic factors may relate to the ultimate expression of disease.
5. Address medical management issues.
6. Assist in determining the role of genetic testing for the individual and the family.
7. Ensure the patient is aware of the indications, process, risks, benefits, and limitations of the various genetic testing options.
8. Assist the patient, family, and referring physician in the interpretation of the test results.
9. Refer the patient and other at-risk family members for additional medical and support services, if necessary.
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INDICATIONS FOR GENETIC COUNSELING |
Genetic counseling is generally offered in a nondirective manner, wherein patients learn to understand how their values factor into a particular medical decision. Nondirective counseling is particularly appropriate when there are no data demonstrating a clear benefit associated with a particular intervention or when an intervention is considered experimental. For example, nondirective genetic counseling is used when a person is deciding whether to undergo genetic testing for Huntington’s disease. At this time, there is no clear benefit (in terms of medical outcome) to an at-risk individual undergoing genetic testing for this disease because its course cannot be altered by therapeutic interventions. However, testing can have an important impact on the individual’s perception of advanced care planning and his or her interpersonal relationships and plans for childbearing. Therefore, the decision to pursue testing rests on the individual’s belief system and values. On the other hand, a more directive approach is appropriate when a condition can be treated. In a family with FAP, colon cancer screening and prophylactic colectomy should be recommended for known APC mutation carriers. The counselor and clinician following this family must ensure that the at-risk family members have access to the resources necessary to adhere to these recommendations.
Genetic education is central to an individual’s ability to make an informed decision regarding testing options and treatment. An adequate knowledge of patterns of inheritance will allow patients to understand the probability of disease risk for themselves and other family members. It is also important to impart the concepts of disease penetrance and expression. For most complex adult-onset genetic disorders, asymptomatic patients should be advised that a positive test result does not always translate into future disease development. In addition, the role of nongenetic factors, such as environmental exposures and lifestyle, must be discussed in the context of multifactorial disease risk and disease prevention. Finally, patients should understand the natural history of the disease as well as the potential options for intervention, including screening, prevention, and in certain circumstances, pharmacologic treatment or prophylactic surgery.
THERAPEUTIC INTERVENTIONS BASED ON GENETIC RISK FOR DISEASE
Specific treatments are available for a number of genetic disorders. Strategies for the development of therapeutic interventions have a long history in childhood metabolic diseases; however, these principles have been applied in the diagnosis and management of adult-onset diseases as well (Table 84-2). Hereditary hemochromatosis is usually caused by mutations in HFE (although other genes have been less commonly associated) and manifests as a syndrome of iron overload, which can lead to liver disease, skin pigmentation, diabetes mellitus, arthropathy, impotence in males, and cardiac issues (Chap. 428). When identified early, the disorder can be managed effectively with therapeutic phlebotomy. Therefore, when the diagnosis of hemochromatosis has been made in a proband, it is important to counsel and offer testing to other family members in order to minimize the impact of the disorder.
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EXAMPLE OF GENETIC TESTING AND POSSIBLE INTERVENTIONS |
Preventative measures and therapeutic interventions are not restricted to metabolic disorders. Identification of familial forms of long QT syndrome, associated with ventricular arrhythmias, allows early electrocardiographic testing and the use of prophylactic antiarrhythmic therapy, overdrive pacemakers, or defibrillators. Individuals with familial hypertrophic cardiomyopathy can be screened by ultrasound, treated with beta blockers or other drugs, and counseled about the importance of avoiding strenuous exercise and dehydration. Those with Marfan’s syndrome can be treated with beta blockers or angiotensin II receptor blockers and monitored for the development of aortic aneurysms.
The field of pharmacogenetics identifies genes that alter drug metabolism or confer susceptibility to toxic drug reactions. Pharmacogenetics seeks to individualize drug therapy in an attempt to improve treatment outcomes and reduce toxicity. Examples include thiopurine methyltransferase (TPMT) deficiency, dihydropyrimidine dehydrogenase deficiency, malignant hyperthermia, and glucose-6-phosphate deficiency. Despite successes in this area, it is not always clear how to incorporate pharmacogenetics into clinical care. For example, although there is an association with CYP2C6 and VKORC1 genotypes and warfarin dosing, there is no evidence that incorporating genotyping into clinical practice improves patient outcomes.
The identification of germline abnormalities that increase the risk of specific types of cancer is rapidly changing clinical management. Identifying family members with mutations that predispose to FAP or Lynch syndrome leads to recommendations of early cancer screening and prophylactic surgery, as well as consideration of chemoprevention and attention to healthy lifestyle habits. Similar principles apply to familial forms of melanoma as well as cancers of the breast, ovary, and thyroid. In addition to increased screening and prophylactic surgery, the identification of germline mutations associated with cancer may also lead to the development of targeted therapeutics, for example, the ongoing development of PARP inhibitors in those with BRCA-associated cancers.
Although the role of genetic testing in the clinical setting continues to evolve, such testing holds the promise of allowing early and more targeted interventions that can reduce morbidity and mortality. Rapid technologic advances are changing the ways in which genetic testing is performed. As genetic testing becomes less expensive and technically easier to perform, it is anticipated that there will be an expansion of its use. This will present challenges, but also opportunities. It is critical that physicians and other health care professionals keep current with advances in genetic medicine in order to facilitate appropriate referral for genetic counseling and judicious use of genetic testing, as well as to provide state-of-the-art, evidence-based care for affected or at-risk patients and their relatives.
85e |
Mitochondrial DNA and Heritable Traits and Diseases |
Mitochondria are cytoplasmic organelles whose major function is to generate ATP by the process of oxidative phosphorylation under aerobic conditions. This process is mediated by the respiratory electron transport chain (ETC) multiprotein enzyme complexes I–V and the two electron carriers, coenzyme Q (CoQ) and cytochrome c. Other cellular processes to which mitochondria make a major contribution include apoptosis (programmed cell death) and additional cell type–specific functions (Table 85e-1). The efficiency of the mitochondrial ETC in ATP production is a major determinant of overall body energy balance and thermogenesis. In addition, mitochondria are the predominant source of reactive oxygen species (ROS), whose rate of production also relates to the coupling of ATP production to oxygen consumption. Given the centrality of oxidative phosphorylation to the normal activities of almost all cells, it is not surprising that mitochondrial dysfunction can affect almost any organ system (Fig. 85e-1). Thus, physicians in many disciplines might encounter patients with mitochondrial diseases and should be aware of their existence and characteristics.
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FUNCTIONS OF MITOCHONDRIA |
FIGURE 85e-1 Dual genetic control and multiple organ system manifestations of mitochondrial disease. (Reproduced with permission from DR Johns: Mitochondrial DNA and disease. N Engl J Med 333:638, 1995.)
The integrated activity of an estimated 1500 gene products is required for normal mitochondrial biogenesis, function, and integrity. Almost all of these are encoded by nuclear genes and thus follow the rules and patterns of nuclear genomic inheritance (Chap. 84). These nuclear-encoded proteins are synthesized in the cell cytoplasm and imported to their location of activity within the mitochondria through a complex biochemical process. In addition, the mitochondria contain their own small genome consisting of numerous copies (polyploidy) per mitochondrion of a circular, double-strand mitochondrial DNA (mtDNA) molecule comprising 16,569 nucleotides. This mtDNA sequence (also known as the “mitogenome”) might represent the remnants of endosymbiotic prokaryotes from which mitochondria are thought to have originated. The mtDNA sequence contains a total of 37 genes, of which 13 encode mitochondrial protein components of the ETC (Fig. 85e-2). The remaining 22 tRNA- and 2 rRNA-encoding genes are dedicated to the process of translating the 13 mtDNA-encoded proteins. This dual nuclear and mitochondrial genetic control of mitochondrial function results in unique and diagnostically challenging patterns of inheritance. The current chapter focuses on heritable traits and diseases related to the mtDNA component of the dual genetic control of mitochondrial function. The reader is referred to Chaps. 84 and 462e for consideration of mitochondrial disease originating from mutations in the nuclear genome. The latter include (1) disorders due to mutations in nuclear genes directly encoding structural components or assembly factors of the oxidative phosphorylation complexes, (2) disorders due to mutations in nuclear genes encoding proteins indirectly related to oxidative phosphorylation, and (3) mtDNA depletion syndromes (MDS) characterized by a reduction of mtDNA copy number in affected tissues without mutations or rearrangements in the mtDNA.
FIGURE 85e-2 Maternal inheritance of mitochondrial DNA (mtDNA) disorders and heritable traits. Affected women (filled circles) transmit the trait to their children. Affected men (filled squares) do not transmit the trait to any of their offspring.
MITOCHONDRIAL DNA STRUCTURE AND FUNCTION
As a result of its circular structure and extranuclear location, the replication and transcription mechanisms of mtDNA differ from the corresponding mechanisms in the nuclear genome, whose nucleosomal packaging and structure are more complex. Because each cell contains many copies of mtDNA, and because the number of mitochondria can vary during the lifetime of each cell, mtDNA copy number is not directly coordinated with the cell cycle. Thus, vast differences in mtDNA copy number are observed between different cell types and tissues and during the lifetime of a cell. Another important feature of the mtDNA replication process is a reduced stringency of proofreading and replication error correction, leading to a greater degree of sequence variation compared to the nuclear genome. Some of these sequence variants are silent polymorphisms that do not have the potential for a phenotypic or pathogenic effect, whereas others may be considered pathogenic mutations.
With respect to transcription, initiation can occur on both strands and proceeds through the production of an intronless polycistronic precursor RNA, which is then processed to produce the 13 individual mRNA and 24 individual tRNA and rRNA products. The 37 mtDNA genes comprise fully 93% of the 16,569 nucleotides of the mtDNA in what is known as the coding region. The control region consists of ~1.1 kilobases (kb) of noncoding DNA, which is thought to have an important role in replication and transcription initiation.
MATERNAL INHERITANCE AND LACK OF RECOMBINATION
In contrast to homologous pair recombination that takes place in the nucleus, mtDNA molecules do not undergo recombination, such that mutational events represent the only source of mtDNA genetic diversification. Moreover, with very rare exceptions, it is only the maternal DNA that is transmitted to the offspring. The fertilized oocyte degrades mtDNA carried from the sperm in a complex process involving the ubiquitin proteasome system. Thus, although mothers transmit their mtDNA to both their sons and daughters, only the daughters are able to transmit the inherited mtDNA to future generations. Accordingly, mtDNA sequence variation and associated phenotypic traits and diseases are inherited exclusively along maternal lines.
As noted below, because of the complex relationship between mtDNA mutations and disease expression, sometimes this maternal inheritance is difficult to recognize at the clinical or pedigree level. However, evidence of paternal transmission can almost certainly rule out an mtDNA genetic origin of phenotypic variation or disease; conversely, a disease affecting both sexes without evidence of paternal transmission strongly suggests a heritable mtDNA disorder (Fig. 85e-2).
MULTIPLE COPY NUMBER (POLYPLOIDY), HIGH MUTATION RATE, HETEROPLASMY, AND MITOTIC SEGREGATION
Each aerobic cell in the body has multiple mitochondria, often numbering many hundreds or more in cells with extensive energy production requirements. Furthermore, the number of copies of mtDNA within each mitochondrion varies from several to hundreds; this is true of both somatic as well as germ cells, including oocytes in females. In the case of somatic cells, this means that the impact of most newly acquired somatic mutations is likely to be very small in terms of total cellular or organ system function; however, because of the manyfold higher mutation rate during mtDNA replication, numerous different mutations may accumulate with aging of the organism. It has been proposed that the total cumulative burden of acquired somatic mtDNA mutations with age may result in an overall perturbation of mitochondrial function, contributing to age-related reduction in the efficiency of oxidative phosphorylation and increased production of damaging ROS. The accumulation of such acquired somatic mtDNA mutations with aging may contribute to age-related diseases, such as metabolic syndrome and diabetes, cancer, and neurodegenerative and cardiovascular disease in any given individual. However, somatic mutations are not carried forward to the next generation, and the hereditary impact of mtDNA mutagenesis requires separate consideration of events in the female germline.
The multiple mtDNA copy number within each cell, including the maternal germ cells, results in the phenomenon of heteroplasmy, in contrast to much greater uniformity (homoplasy) of somatic nuclear DNA sequence. Heteroplasmy for a given mtDNA sequence variant or mutation arises as a result of the coexistence within a cell, tissue, or individual of mtDNA molecules bearing more than one version of the sequence variant (Fig. 85e-3). The importance of the heteroplasmy phenomena to the understanding of mtDNA-related mitochondrial diseases is critical. The coexistence of mutant and nonmutant mtDNA and the variation of the mutant load among individuals from the same maternal sibship, and across organs and tissues within the same individual, play a pivotal role in the manifestation and severity of disease and are crucial to understanding the complexity of inheritance of mtDNA disorders. At the level of the oocyte, the percentage of mtDNA molecules bearing each version of the polymorphic sequence variant or mutation depends on stochastic events related to partitioning of mtDNA molecules during the process of oogenesis itself. Thus, oocytes differ from each other in the degree of heteroplasmy for that sequence variant or mutation. In turn, the heteroplasmic state is carried forward to the zygote and to the organism as a whole, to varying degrees, depending on mitotic segregation of mtDNA molecules during organ system development and maintenance. For this reason, in vitro fertilization, followed by preimplantation genetic diagnosis (PGD), is not as predictive of the genetic health of the offspring in the case of mtDNA mutations as in the case of the nuclear genome. Similarly, the impact of somatic mtDNA mutations acquired during development and subsequently also shows an enormous spectrum of variability.
FIGURE 85e-3 Heteroplasmy and the mitochondrial genetic bottleneck. During the production of primary oocytes, a selected number of mitochondrial DNA (mtDNA) molecules are transferred into each oocyte. Oocyte maturation is associated with the rapid replication of this mtDNA population. This restriction-amplification event can lead to a random shift of mtDNA mutational load between generations and is responsible for the variable levels of mutated mtDNA observed in affected offspring from mothers with pathogenic mtDNA mutations. Mitochondria that contain mutated mtDNA are shown in red, and those with normal mtDNA are shown in green. (Reproduced with permission from R Taylor, D Turnbull: Mitochondrial DNA mutations in human disease. Nat Rev Genetics 6:389, 2005.)
Mitotic segregation refers to the unequal distribution of wild-type and mutant versions of mtDNA molecules during all cell divisions that occur during prenatal development and subsequently throughout the lifetime of an individual. The phenotypic effect or disease impact will, thus, be a function not only of the inherent disruptive effect (pathogenicity) on the mtDNA-encoded gene (coding region mutations) or integrity of the mtDNA molecule (control region mutations), but also of its distribution among the multiple copies of mtDNA in the various mitochondria, cells, and tissues of the affected individual. Thus, one consequence can be the generation of a bottleneck due to the marked decline in given sets of mtDNA variants, consequent to such mitotic segregation. Heterogeneity arises from differences in the degree of heteroplasmy among oocytes of the affected female, together with subsequent mitotic segregation of the pathogenic mutation during tissue and organ development, and throughout the lifetime of the individual offspring. The actual expression of disease might then depend on a threshold percentage of mitochondria whose function is disrupted by mtDNA mutations. This in turn confounds hereditary transmission patterns and hence genetic diagnosis of pathogenic heteroplasmic mutations. Generally, if the proportion of mutant mtDNA is less than 60%, the individual is unlikely to be affected, whereas proportions exceeding 90% cause clinical disease.
HOMOPLASMIC VARIANTS AND HUMAN MTDNA PHYLOGENY
In contrast to classic mtDNA diseases, most of which begin in childhood and are the result of heteroplasmic mutations as noted above, during the course of human evolution, certain mtDNA sequence variants have drifted to a state of homoplasmy, wherein all of the mtDNA molecules in the organism contain the new sequence variant. This arises due to a “bottleneck” effect followed by genetic drift during the very process of oogenesis itself (Fig. 85e-3). In other words, during certain stages of oogenesis, the mtDNA copy number becomes so substantially reduced that the particular mtDNA species bearing the novel or derived sequence variant may become the increasingly predominant, and eventually exclusive, version of the mtDNA for that particular nucleotide site. All of the offspring of a woman bearing an mtDNA sequence variant or mutation that has become homoplasmic will also be homoplasmic for that variant and will transmit the sequence variant forward in subsequent generations.
Considerations of reproductive fitness limit the evolutionary or population emergence of homoplasmic mutations that are lethal or cause severe disease in infancy or childhood. Thus, with a number of notable exceptions (e.g., mtDNA mutations causing Leber’s hereditary optic neuropathy; see below), most homoplasmic mutations are considered to be neutral markers of human evolution, which are useful and interesting in the population genetics analysis of shared maternal ancestry but which have little significance in human phenotypic variation or disease predisposition.
More importantly is the understanding that this accumulation of homoplastic mutations occurs at a genetic locus that is transmitted only through the female germline and that lacks recombination. In turn, this enables reconstruction of the sequential topology and radiating phylogeny of mutations accumulated through the course of human evolution since the time of the most recent common mtDNA ancestor of all contemporary mtDNA sequences, some 200,000 years ago. The term haplogroup is usually used to define major branching points in the human mtDNA phylogeny, nested one within the other, which often demonstrate striking continental geographic ancestral partitioning. At the level of the complete mtDNA sequence, the term haplotype is usually used to describe the sum of mutations observed for a given mtDNA sequence and as compared to a reference sequence, such that all haplotypes falling within a given haplogroup share the total sum of mutations that have accumulated since the most recent common ancestor and the bifurcation point they mark. The remaining observed variants are private to each haplotype. Consequentially, human mtDNA sequence is an almost perfect molecular prototype for a nonrecombining locus, and its variation has been extensively used in phylogenetic studies. Moreover, the mtDNA mutation rate is considerably higher than the rate observed for the nuclear genome, especially in the control region, which contains the displacement, or D-loop, in turn comprising two adjacent hypervariable regions (HVR-I and HVR-II). Together with the absence of recombination, this amplifies drift to high frequencies of novel haplotypes. As a result, mtDNA haplotypes are more highly partitioned across geographically defined populations than sequence variants in other parts of the genome. Despite extensive research, it has not been well established that such haplotype-based partitioning has a significant influence on human health conditions. However, mtDNA-based phylogenetic analysis can be used both as a quality assurance tool and as a filter in distinguishing neutral mtDNA variants comprising human mtDNA phylogeny from potentially deleterious mutations.
MITOCHONDRIAL DNA DISEASE
The true prevalence of mtDNA disease is difficult to estimate because of the phenotypic heterogeneity that occurs as a function of heteroplasmy, the challenge of detecting and assessing heteroplasmy in different affected tissues, and the other unique features of mtDNA function and inheritance described above. It is estimated that at least 1 in 200 healthy humans harbors a pathogenic mtDNA mutation with the potential to causes disease, but that heteroplasmic germline pathogenic mtDNA mutations actually affect up to approximately 1 in 8500 individuals.
The true disease burden relating to mtDNA sequence variation will only be known when the following capabilities become available: (1) ability to distinguish a completely neutral sequence variant from a true phenotype-modifying or pathogenic mutation, (2) accurate assessment of heteroplasmy that can be determined with fidelity, and (3) a systems biology approach (Chap. 87e) to determine the network of epistatic interactions of mtDNA sequence variations with mutations in the nuclear genome.
OVERVIEW OF CLINICAL AND PATHOLOGIC FEATURES OF HUMAN MTDNA DISEASE
Given the vital roles of mitochondria in all nucleated cells, it is not surprising that mtDNA mutations can affect numerous tissues with pleiotropic effects. More than 200 different disease-causing, mostly heteroplasmic mtDNA mutations have been described affecting ETC function. Figure 85e-4 provides a partial mtDNA map of some of the better characterized of these disorders. A number of clinical clues can increase the index of suspicion for a heteroplasmic mtDNA mutation as an etiology of a heritable trait or disease, including (1) familial clustering with absence of paternal transmission; (2) adherence to one of the classic syndromes (see below) or paradigmatic combinations of disease phenotypes involving several organ systems that normally do not fit together within a single nuclear genomic mutation category; (3) a complex of laboratory and pathologic abnormalities that reflect disruption in cellular energetics (e.g., lactic acidosis and neurodegenerative and myodegenerative symptoms with the finding of ragged red fibers, reflecting the accumulation of abnormal mitochondria under the muscle sarcolemmal membrane); or (4) a mosaic pattern reflecting a heteroplasmic state.
FIGURE 85e-4 Mutations in the human mitochondrial genome known to cause disease. Disorders that are frequently or prominently associated with mutations in a particular gene are shown in boldface. Diseases due to mutations that impair mitochondrial protein synthesis are shown in blue. Diseases due to mutations in protein-coding genes are shown in red. ECM, encephalomyopathy; FBSN, familial bilateral striatal necrosis; LHON, Leber’s hereditary optic neuropathy; LS, Leigh syndrome; MELAS, mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes; MERRF, myoclonic epilepsy with ragged red fibers; MILS, maternally inherited Leigh syndrome; NARP, neuropathy, ataxia, and retinitis pigmentosa; PEO, progressive external ophthalmoplegia; PPK, palmoplantar keratoderma; SIDS, sudden infant death syndrome. (Reproduced with permission from S DiMauro, E Schon: Mitochondrial respiratory-chain diseases. N Engl J Med 348:2656, 2003.)
Heteroplasmy can sometimes be elegantly demonstrated at the tissue level using histochemical staining for enzymes in the oxidative phosphorylation pathway, with a mosaic pattern indicating heterogeneity of the genotype for the coding region for the mtDNA-encoded enzyme. Complex II, CoQ, and cytochrome c are exclusively encoded by nuclear DNA. In contrast, complexes I, III, IV, and V contain at least some subunits encoded by mtDNA. Just 3 of the 13 subunits of the ETC complex IV enzyme, cytochrome c oxidase, are encoded by mtDNA, and, therefore, this enzyme has the lowest threshold for dysfunction when a threshold level of mutated mtDNA is reached. Histochemical staining for cytochrome c oxidase activity in tissues of patients affected with heteroplasmic inherited mtDNA mutations (or with the somatic accumulation of mtDNA mutations, see below) can show a mosaic pattern of reduced histochemical staining in comparison with histochemical staining for the complex II enzyme, succinate dehydrogenase (Fig. 85e-5). Heteroplasmy can also be detected at the genetic level through direct Sanger-type mtDNA genotyping under special conditions, although clinically significant low levels of heteroplasmy can escape detection in genomic samples extracted from whole blood using conventional genotyping and sequencing techniques.
FIGURE 85e-5 Cytochrome c oxidase (COX) deficiency in mitochondrial DNA (mtDNA)–associated disease. Transverse tissue sections that have been stained for COX and succinate dehydrogenase (SDH) activities sequentially, with COX-positive cells shown in brown and COX-deficient cells shown in blue. A. Skeletal muscle from a patient with a heteroplasmic mitochondrial tRNA point mutation. The section shows a typical “mosaic” pattern of COX activity, with many muscle fibers harboring levels of mutated mtDNA that are above the crucial threshold to produce a functional enzyme complex. B. Cardiac tissue (left ventricle) from a patient with a homoplasmic tRNA mutation that causes hypertrophic cardiomyopathy, which demonstrates an absence of COX in most cells. C. A section of cerebellum from a patient with mtDNA rearrangement that highlights the presence of COX-deficient neurons. D, E. Tissues that show COX deficiency due to clonal expansion of somatic mtDNA mutations within single cells—a phenomenon that is seen in both postmitotic cells (D; extraocular muscles) and rapidly dividing cells (E; colonic crypt) in aging humans. (Reproduced with permission from R Taylor, D Turnbull: Mitochondrial DNA mutations in human disease. Nat Rev Genetics 6:389, 2005.)
The emerging next-generation sequencing (NGS) techniques and their rapid penetration and recognition as useful clinical diagnostic tools are expected to also dramatically improve the clinical genetic diagnostic evaluation of mitochondrial diseases at the level of both the nuclear genome and mtDNA. In the context of the larger nuclear genome, the ability of NGS techniques to dramatically increase the speed at which DNA can be sequenced at a fraction of the cost of conventional Sanger-type sequencing technology is particularly beneficial. Low sequencing costs and short turnaround time expedite “first-tier” screening of panels of hundreds of previously known or suspected mitochondrial disease genes or screening for the entire exome or genome in an attempt to identify novel genes and mutations affecting different patients or families. In the context of the mtDNA, NGS approaches hold the particular promise for rapid and reliable detection of heteroplasmy in different affected tissues. Although Sanger sequencing allows for complete coverage of the mtDNA, it is limited by the lack of deep coverage and low sensitivity for heteroplasmy detection when it is much less than 50%. In contrast, NGS technology is an excellent tool for rapidly and accurately obtaining a patient’s predominant mtDNA sequence and also lower frequency heteroplasmic variants. This is enabled by deep coverage of the genome through multiple independent sequence reads. Accordingly, recent studies making use of NGS techniques have demonstrated sequence accuracy equivalent to Sanger-type sequencing, but also have uncovered heretofore unappreciated heteroplasmy rates ranging between 10 and 50% and detection of single-nucleotide heteroplasmy down to levels of <10%.
Clinically, the most striking overall characteristic of mitochondrial genetic disease is the phenotypic heterogeneity associated with mtDNA mutations. This extends to intrafamilial phenotypic heterogeneity for the same mtDNA pathogenic mutation and, conversely, to the overlap of phenotypic disease manifestations with distinct mutations. Thus, although fairly consistent and well-defined “classic” syndromes have been attributed to specific mutations, frequently “nonclassic” combinations of disease phenotypes ranging from isolated myopathy to extensive multisystem disease are often encountered, rendering genotype-phenotype correlation challenging. In both classical and nonclassical mtDNA disorders, there is often a clustering of some combination of abnormalities affecting the neurologic system (including optic nerve atrophy, pigment retinopathy, and sensorineural hearing loss), cardiac and skeletal muscle (including extraocular muscles), and endocrine and metabolic systems (including diabetes mellitus). Additional organ systems that may be affected include the hematopoietic, renal, hepatic, and gastrointestinal systems, although these are more frequently involved in infants and children. Disease-causing mtDNA coding region mutations can affect either one of the 13 protein encoding genes or one of the 24 protein synthetic genes. Clinical manifestations do not readily distinguish these two categories, although lactic acidosis and muscle pathologic findings tend to be more prominent in the latter. In all cases, either defective ATP production due to disturbances in the ETC or enhanced generation of ROS has been invoked as the mediating biochemical mechanism between mtDNA mutation and disease manifestation.
MTDNA DISEASE PRESENTATIONS
The clinical presentation of adult patients with mtDNA disease can be divided into three categories: (1) clinical features suggestive of mitochondrial disease (Table 85e-2), but not a well-defined classic syndrome; (2) classic mtDNA syndromes; and (3) clinical presentation confined to one organ system (e.g., isolated sensorineural deafness, cardiomyopathy, or diabetes mellitus).
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COMMON FEATURES OF MTDNA-ASSOCIATED DISEASES IN ADULTS |
Table 85e-3 provides a summary of eight illustrative classic mtDNA syndromes or disorders that affect adult patients and highlights some of the most interesting features of mtDNA disease in terms of molecular pathogenesis, inheritance, and clinical presentation. The first five of these syndromes result from heritable point mutations in either protein-encoding or protein synthetic mtDNA genes; the other three result from rearrangements or deletions that usually do not involve the germline.
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MITOCHONDRIAL DISEASES DUE TO MTDNA POINT MUTATIONS AND LARGE-SCALE REARRANGEMENTS |
