Use of Genetics in the Clinical Evaluation of Cardiomyopathy

A 24-year-old man with familial cardiomyopathy sought genetic counseling and cardiac evaluation for family planning purposes. He developed heart failure due to dilated cardiomyopathy early in childhood and underwent orthotopic cardiac transplantation at age 3 years. He has done well since then and is now working as a baker. He came with his fiancée for evaluation regarding their risk of having offspring with early onset cardiomyopathy. He is the third of 6 siblings (II-3, Figure 1). His oldest sister (II-1) died at age 4 years from heart failure. The second person (II-2) in this sibship was a sister who died perinatally of unknown causes. A younger brother (II-4) is well at age 22 years. The fifth person (II-5) in this sibship was a sister who died at age 2 years from heart failure while undergoing evaluation for cardiac transplantation. His youngest sister (II-6) is well at age 18 years. His mother (I-2) is in good health with no known heart problems. He is no longer in communication with his father (I-1), but his father was in good health at least until age 35 years. There is no known history of heart failure, sudden cardiac death, muscular dystrophy, or early onset hearing loss in his 4 grandparents or cousins on both sides.

With so many members of this patient's family having developed dilated cardiomyopathy, he sought advice about the possibility of his offspring having this disorder. To assess this risk, one must consider the possible modes of inheritance. Autosomal dominant is the most commonly recognized pattern. If present in this case, one of the parents would also require a genetic predisposition to dilated cardiomyopathy unless a de novo heterozygous mutation was responsible for disease in the proband. The parents' apparent lack of disease could be explained by reduced penetrance, imprinting, anticipation, or germline mosaicism in either parent. Quiz Ref IDAnticipation refers to a pattern of inheritance with onset at earlier ages in successive generations, often due to expansion of unstable triplet DNA repeats.¹ Another possibility is mitochondrial inheritance. All mitochondrial DNA is inherited from one's mother, although a mutation may be present in only some mitochondria. This is called heteroplasmy. Variable heteroplasmy in the mother and each of her children could be responsible. If so, individuals in the family with a relatively high proportion of mutant mitochondrial DNA would have earlier onset or more severe disease, while those with a relatively low proportion of mutant mitochondrial DNA may have later onset disease or may never develop the condition. An autosomal recessive pattern of inheritance is also possible, with simply bad luck to blame for 3 or perhaps 4 of 6 offspring having familial cardiomyopathy and 2 gene mutations. In the absence of male-to-male transmission, we cannot exclude an X-linked pattern of inheritance, although this seems less likely with similar age of onset in this patient and in 2 or 3 of his sisters because earlier onset and more severe disease is typically present in males with X-linked diseases.

Returning to the original question, what is the risk of this patient having children with a genetic predisposition to cardiomyopathy? At this point, without molecular genetic evaluation, the possibilities range from 0% (if mitochondrial) to 50% (if autosomal dominant). The rest of this article will focus on the role of genetic evaluation to help determine this and other features of inherited forms of cardiomyopathy.

Nearly 8 years ago, through enormous public and private effort, complete sequencing of the human genome was announced.² Despite this profound accomplishment, the affect on clinical practice has so far been limited. However, this is likely to change with expanded clinical genetic testing options and lower costs. Improved understanding of the risks, benefits, options, and possible outcomes of such testing will be described within the context of different forms of inherited cardiomyopathy.

Nonischemic dilated cardiomyopathy is often not recognized as a genetic disease. In a series of 1230 cases referred to Johns Hopkins Hospital for endomyocardial biopsy in the setting of unexplained dilated cardiomyopathy, the final diagnosis was idiopathic in approximately 50%.³ Dilated cardiomyopathy occurs in approximately 1 per 2500 individuals in the United States.⁴ Multiple studies have been performed to look within family members of individuals initially diagnosed with this disorder using echocardiograms to determine the presence of familial dilated cardiomyopathy.^{5 - 7} These studies indicate that 20% to 35% of those individuals have affected family members.^{6 - 7} This has led to the current guidelines recommending that individuals with a diagnosis of idiopathic dilated cardiomyopathy be advised that their siblings, their parents, and their children should also undergo echocardiograms to be assessed for this condition.⁸ If familial disease is recognized, a normal echocardiogram in an individual who is at risk for cardiomyopathy does not adequately determine whether he/she will develop cardiomyopathy later. The frequency of recommendations for recurrent echocardiographic screening varies with age and the underlying disorder, ranging from yearly to every 5 years.⁹

Quiz Ref IDAlthough clinicians may be tempted to attribute idiopathic dilated cardiomyopathy to a viral etiology, every patient who presents with this disorder should have a detailed family history taken. The family history obtained in such cases should include at least 3 generations, including information about cardiomyopathy, sudden cardiac death, and syndromic features. Dilated cardiomyopathy in a single additional family member establishes the diagnosis of familial dilated cardiomyopathy for both, in the absence of exclusion factors such as severe hypertension, obstructive coronary disease, or chronic excessive alcohol ingestion.¹⁰ Isolated individuals with idiopathic dilated cardiomyopathy may reflect de novo gene mutations, recessive inheritance, or reduced penetrance. The age dependence of this phenotype, small families, and inability to track down accurate antecedent family history in many cases limit the recognition of familial dilated cardiomyopathy. This suggests that 20% to 35% is the bare minimum of individuals with idiopathic dilated cardiomyopathy that are genetic in origin.^{5 - 7}

There are treatment implications for recognizing those predisposed to dilated cardiomyopathy. The Studies of Left Ventricular Dysfunction (SOLVD) prevention trial showed that the risk of developing congestive heart failure was reduced by early treatment with enalapril among asymptomatic individuals with an ejection fraction of less than or equal to 35%.¹¹ Quiz Ref IDThere are only limited data on use of medications to prevent cardiomyopathy in those genetically predisposed. One study looked at 57 boys with Duchenne muscular dystrophy and normal echocardiograms, treating half with perindopril and half with placebo for 3 years.¹² After that, both groups received angiotensin-converting enzyme (ACE) inhibitor treatment. At the end of 5 years, those who received perindopril in the beginning had a lower incidence of cardiomyopathy than those who received placebo for 3 years.¹² At 10 years of follow-up, survival was improved in those who were treated with ACE inhibitor treatment earlier.¹³ Thus, it is likely that recognition of asymptomatic cardiomyopathy can lead to early treatment and hopefully save lives.

Genetic testing for dilated cardiomyopathy is complicated by marked genetic heterogeneity. Today there are at least 35 different genes in which mutations have been reported to cause dilated cardiomyopathy.¹⁴ At this point, clinical genetic testing is available for about half of those. Research testing is certainly available for many others. Testing options can be identified through resources such as http://www.genetests.org. New genes with mutations resulting in familial dilated cardiomyopathy will undoubtedly be recognized, and the pace of translation to clinical testing will be rapid.

Studies indicate that mutations in some genes are more common than others in the setting of familial dilated cardiomyopathy. For instance, the lamin A/C gene (LMNA; GenBank accession number XM_044163) is abnormal in 5% to 20% of individuals with familial dilated cardiomyopathy.^{15 - 16} Mutations in this gene appear to result in a worse prognosis compared with other forms of dilated cardiomyopathy.^{17 - 18} Another 10% of individuals with familial dilated cardiomyopathy have a mutation in 1 of the sarcomere genes.¹⁹

Returning to the original case presentation, the patient and his fiancée met with a board-certified genetic counselor. After genetic counseling, he opted for clinical genetic testing. His health insurance covered the testing, and analysis of the sarcomere genes was performed. This identified 2 mutations in MYBPC3 (GenBank accession number U91629), which encodes cardiac myosin binding protein C. Although they have not previously been reported, these mutations (p.Gln404Ter and p.Arg1271Ter) each result in premature termination codons; they are each presumed to be pathogenic. Although DNA from his deceased siblings was not available, those who had early dilated cardiomyopathy also likely had these 2 MYBPC3 mutations. Further testing determined that they are on opposite alleles, indicating that all of the proband's future offspring will have 1 mutation but that inheritance of both mutations is very unlikely. In addition, the proband's adult siblings, parents, and extended family members are all at risk of having 1 mutation, which would likely result in later onset cardiomyopathy. In fact, heterozygous nonsense mutations in this gene, which are likely present in each of the proband's parents, typically result in hypertrophic cardiomyopathy.^{20 - 21}

Hypertrophic cardiomyopathy is perhaps better recognized as a genetic disease than dilated cardiomyopathy. Unexplained increase in the thickness of the left ventricular heart wall occurs in about 1 in 500 individuals in the United States.²² Quiz Ref IDIt is the most common cause of sudden cardiac death in teens and young adults, especially among athletes.²³ Typically, this disorder is caused by a mutation in a gene encoding an element of the cardiac sarcomere.²⁴ An abnormality in any of the genes encoding the components of the sarcomere can cause either hypertrophic cardiomyopathy or dilated cardiomyopathy.^{14 ,24}

There are several reasons why one might consider genetic testing for a patient with hypertrophic cardiomyopathy. One is confirmation of a clinical diagnosis, although thorough phenotypic assessment is usually sufficient to accomplish this goal without genetic analysis. However, there are examples in which a genetic diagnosis helps to recognize related disorders with syndromic features, such as Danon or Fabry disease.^{25 - 26} When I talk to families about genetic testing for cardiomyopathy, the most common reason that they request genetic analysis is to determine who else within the family is at risk for the condition, particularly when the family history suggests that the initial presentation is heart failure or sudden cardiac death. Prenatal family planning is another reason to consider genetic testing, as I described in the initial case presentation. If a mutation is recognized in an affected family member, preimplantation genetic diagnosis may be considered to avoid passing on an inherited disease to future generations.^{27 - 28}

Genetic evaluation for hypertrophic cardiomyopathy can influence the assessment of arrhythmia susceptibility in selected cases, as demonstrated in this family (Figure 2).²⁹ The proband, a 55-year-old woman (III-3), presented to the Johns Hopkins Center for Inherited Heart Disease asking, “Will my 2 daughters develop hypertrophic cardiomyopathy?” She had moderate apical hypertrophy with normal systolic left ventricular function. Her father (II-3) died suddenly around her age (58 years) with a preceding history of congestive heart failure. Her sister (III-5) is affected, although she has more concentric left ventricular hypertrophy with maximal wall thickness of 1.6 cm. Genetic testing resulted in recognition of a cardiac troponin T gene (TNNT2; GenBank accession number BC002653) mutation that disrupts a highly conserved glycine in a functional domain of the protein (p.Gly82Arg). It was classified as probably pathogenic and had not been seen in more than 500 individuals who have had sequencing of this gene. The same mutation is shared by her sister, further supporting its pathogenicity. It was around this point that her cousin (III-2) presented with only a mild increase in his left ventricular wall thickness (1.3 cm compared with a maximum normal of 1.2 cm). His blood pressure was normal and he had no preceding history of hypertension. His mother (II-2), the sibling of the proband's father (II-3), was without symptoms of heart failure. The male cousin described that his mother had sudden cardiac death without any preceding symptoms at the age of 49 years, younger than his age (which was 55 years at the time of evaluation). He asked about his risk of life-threatening cardiac arrhythmia. Targeted genetic testing showed that he does have this same TNNT2 gene mutation. He is now taking a β-blocker and is undergoing further phenotypic investigation regarding his risk of arrhythmia. Both of his sons (IV-1 and IV-2) had normal echocardiograms; they received genetic counseling and elected to proceed with presymptomatic-targeted genetic testing. Neither one shares this troponin T gene mutation. The proband's 2 daughters (IV-3 and IV-4) also received genetic counseling for hypertrophic cardiomyopathy, and one of them elected to proceed with targeted genetic testing, which was negative. The other decided not to pursue genetic testing, and she undergoes echocardiography every 2 years looking for hypertrophic cardiomyopathy.

A decision not to undergo genetic testing may occur for several reasons, such as concern about establishing a preexisting condition, difficulty obtaining life insurance, and the possibility of employment discrimination. Fortunately, some (but not all) of these concerns are addressed by the Genetic Information Nondiscrimination Act (GINA). This bill became law in 2009 in the United States, providing federal protection from discrimination in employment and health insurance on the basis of a genetic test. Further information about GINA is available at http://www.genome.gov/24519851.

Clinicians and patients should be aware that a DNA variant of uncertain significance may be found with clinical genetic testing. When this occurs, additional affected members of these families may undergo targeted testing for the variant of uncertain significance. If the DNA variant of uncertain significance does not segregate with cardiomyopathy, it is not solely responsible. If the variant of uncertain significance does segregate with cardiomyopathy in the extended family, then it is more likely that it is pathogenic. Despite this, it is sometimes not possible to identify the significance of a rare missense variant, particularly in populations or ethnicities in which DNA testing has not commonly been performed in the past.^{30 - 31}

Sometimes a genetic test will help to identify additional features of a disorder for which the patient is at risk. Fabry disease is an important one to recognize.³² Affected individuals have left ventricular hypertrophy with an appearance that may be indistinguishable from more common forms of hypertrophic cardiomyopathy by echocardiography and electrocardiography. This X-linked disorder can also result in skin manifestations known as angiokeratomas that may be relatively subtle, or sometimes are more profound red spots on the skin. Corneal clouding is common, and the diagnosis is often made by an ophthalmologist.³³ The corneal clouding is not readily distinguishable from that induced by amiodarone.³⁴ Peripheral neuropathy, renal failure, and anhidrosis are also typical manifestations of Fabry disease.³² Unfortunately, this disorder is poorly recognized. A survey of affected patients indicates that the average age of first symptom was 10½ years, but the average age of diagnosis was 28½ years, 18 years later.

Fabry disease is due to mutations in the gene encoding the enzyme α-galactosidase A.³⁵ This enzyme converts globotriacylceramide or GL-3 to GL-2. Quiz Ref IDReplacement enzyme infusion therapy is now approved by the US Food and Drug Administration for treatment of this condition. In the study that led to its approval, 58 individuals with Fabry disease were randomized to either intravenous infusions of recombinant enzyme every 2 weeks vs placebo for 6 months, and then everyone received open-label treatment.³⁶ At 6 months, two-thirds of those treated had clearance of pathological GL-3 deposits in their kidneys and none of the placebo cohort had clearance of their GL-3 deposits. At 12 months, 99% of those treated had clearance of their GL-3 deposits.³⁶

Familial amyloidosis is another syndromic disorder with left ventricular hypertrophy that may present quite similarly to more common forms of hypertrophic cardiomyopathy. However, there is typically discordance between the echocardiogram showing left ventricular hypertrophy and the electrocardiogram showing normal or low voltage.³⁷ One should look for the presence of monoclonal light chain overproduction in serum and urine because isolated cardiac amyloidosis may be due to AL-type amyloidosis or light-chain amyloid. A cardiac biopsy is typically diagnostic, and additional staining should be performed if amyloid is present, looking for abnormal light chain and transthyretin staining.³⁷

Familial cardiac amyloid is often caused by a mutation in the transthyretin gene (TTR; GenBank accession number DQ839490).³⁷ Familial amyloid may associate with neuropathy (peripheral or autonomic), as well as chronic diarrhea in some families. A study investigated isolated cardiac amyloid on autopsies of individuals older than age 60 years, comparing whites with blacks.³⁸ They found that 23% of the blacks with late-onset cardiac amyloid carried a TTR gene mutation, Val122Ile.³⁸ This allele is present among 3% to 4% of blacks, although the penetrance of this mutation resulting in familial amyloid is not known. Liver transplantation may be performed for familial transthyretin amyloidosis because the liver is the site in which most transthyretin is made.³⁹

Today, standard DNA analysis is usually performed by dideoxy sequencing, which is relatively expensive, and not a high-throughput technique. The development of genomic DNA sequencing chips is affecting the cost and the ability of genetic testing to become more widely applied in clinical practice.⁴⁰ A few years ago, investigators reported a novel method of DNA sequencing by synthesis that combined short segments of DNA with massively parallel analysis, resulting in markedly improved capacity.⁴¹ In April 2008, this technology was used to perform complete sequencing of an individual.⁴² The authors estimated their cost for this analysis to be less than $1 million compared with about $100 million for complete sequencing of the first person.⁴² More recently, 2 articles in Nature reported full genome sequencing of the third and fourth individuals by this same technique.^{30 ,43} The cost for each of these was estimated to be less than $500 000.⁴⁴ Inevitably, improvements in technology are leading to lower cost and higher throughput testing. If a patient showed up in a physician's office today with his or her full genome sequence, it would be nearly impossible to interpret such an enormous amount of data. This emphasizes the need for other technologies to handle this type of information to translate such findings into clinical practice.

Genetic counseling should be provided prior to genetic testing and in coordination with communication of genetic test results. This is typically performed by a genetic counselor. A standard appointment with a genetic counselor involves discussion of family history to discern the pattern of inheritance of the condition, helping to figure out who within the family is at risk for inherited heart disease. They review the benefits and the limitations of genetic testing, as well as coordinate and interpret genetic test results along with a physician who specializes in this area. They also address the psychological and social implications of a genetic test result. Individuals with a newly recognized gene mutation may ask, “How do I tell my children that I have a gene mutation that they might have inherited?” Genetic counseling can help to address this issue.

The concept of genetic malpractice was something that was first discussed in the 1970s revolving around the tendency of physicians to focus attention solely on the patient in front of them without considering who else within the family is at risk of an inherited disease. An article in 2004 emphasized the duty to warn a patient's family members about hereditary disease risks.⁴⁵ Because cardiomyopathy is often a genetic disorder, physicians should consider not only the patient in front of them, but also the other family members who are at risk for developing cardiomyopathy or sudden cardiac death.

Cardiomyopathy is frequently an inherited disorder. This applies to hypertrophic, dilated, restrictive, noncompaction, and right ventricular forms of cardiomyopathy. Recognition of a familial form of cardiomyopathy helps to identify others within the family who are at risk of developing this condition. In addition, syndromic disorders may occur with familial cardiomyopathy and associated manifestations may be anticipated by appropriate genetic evaluation.

Genetic counseling is an important part of the care of individuals and families with inherited conditions like cardiomyopathy. Referral to a tertiary care center with specific expertise in the genetic evaluation of individuals with familial cardiomyopathy should be considered to ensure proper counseling and interpretation of clinical genetic tests. Each of the inherited forms of cardiomyopathy is genetically heterogeneous, which complicates genetic testing.

With the rapid pace of technological development in the field of DNA sequencing, lower costs and larger panels of appropriate genetic tests are becoming widely available. Such testing may help to confirm a diagnosis, to recognize syndromic features, to identify presymptomatic individuals within a family who are at risk of cardiomyopathy or sudden cardiac death, and may be useful for family planning purposes. Ultimately, improved understanding of disease pathogenesis through genetic research should lead to better therapies.