Identification of a mutation in the beta cardiac myosin heavy chain gene in a family with hypertrophic cardiomyopathy

Clinical utility of genetic tests for inherited hypertrophic and dilated cardiomyopathies

Genetic testing has become an increasingly important part of medical practice for heritable form of cardiomyopathies. Hypertrophic cardiomyopathy and about 50% of idiopathic dilatative cardiomyopathy are familial diseases, with an autosomal dominant pattern of inheritance.

Some genotype-phenotype correlations can provide important information to target DNA analyses in specific genes. Genetic testing may clarify diagnosis and help the optimal treatment strategies for more malignant phenotypes. In addition, genetic screening of first-degree relatives can help early identification and diagnosis of individuals at greatest risk for developing cardiomyopathy, allowing to focus clinical resources on high-risk family members.

This paper provides a concise overview of the genetic etiology as well as the clinical utilities and limitations of genetic testing for the heritable cardiomyopathies.

Mutation spectrum in a large cohort of unrelated consecutive patients with hypertrophic cardiomyopathy

Defects in nine sarcomeric protein genes are known to cause hypertrophic cardiomyopathy (HCM). Mutation types and frequencies in large cohorts of consecutive and unrelated patients have not yet been determined. We, therefore, screened HCM patients for mutations in six sarcomeric genes: myosin-binding protein C3 (MYBPC3), MYH7, cardiac troponin T (TNNT2), alpha-tropomyosin (TPM1), cardiac troponin I (TNNI3), and cardiac troponin C (TNNC1). HCM was diagnosed in 108 consecutive patients by echocardiography (septum >15 mm, septal/posterior wall >1.3 mm), angiography, or based on a state after myectomy. Single-strand conformation polymorphism analysis was used for mutation screening, followed by DNA-sequencing. A total of 34 different mutations were identified in 108 patients: 18 mutations in MYBPC3 in 20 patients [intervening sequence (intron) 7 + 1G > A and Q1233X were found twice], 13 missense mutations in MYH7 in 14 patients (R807H was found twice), and one amino acid change in TPM1, TNNT2, and TNNI3, respectively. No disease-causing mutation was found in TNNC1. Cosegregation with the HCM phenotype could be demonstrated for 13 mutations (eight mutations in MYBPC3 and five mutations in MYH7). Twenty-eight of the 37 mutation carriers (76%) reported a positive family history with at least one affected first-grade relative; only eight mutations occurred sporadically (22%). MYBPC3 was the gene that most frequently caused HCM in our population. Systematic mutation screening in large samples of HCM patients leads to a genetic diagnosis in about 30% of unrelated index patients and in about 57% of patients with a positive family history.

Mutations in the gene for cardiac myosin-binding protein C and late-onset familial hypertrophic cardiomyopathy

BACKGROUND

Mutations in the gene for cardiac myosin-binding protein C account for approximately 15 percent of cases of familial hypertrophic cardiomyopathy. The spectrum of disease-causing mutations and the associated clinical features of these gene defects are unknown.

METHODS

DNA sequences encoding cardiac myosin-binding protein C were determined in unrelated patients with familial hypertrophic cardiomyopathy. Mutations were found in 16 probands, who had 574 family members at risk of inheriting these defects. The genotypes of these family members were determined, and the clinical status of 212 family members with mutations in the gene for cardiac myosin-binding protein C was assessed.

RESULTS

Twelve novel mutations were identified in probands from 16 families. Four were missense mutations; eight defects (insertions, deletions, and splice mutations) were predicted to truncate cardiac myosin-binding protein C. The clinical expression of either missense or truncation mutations was similar to that observed for other genetic causes of hypertrophic cardiomyopathy, but the age at onset of the disease differed markedly. Only 58 percent of adults under the age of 50 years who had a mutation in the cardiac myosin-binding protein C gene (68 of 117 patients) had cardiac hypertrophy; disease penetrance remained incomplete through the age of 60 years. Survival was generally better than that observed among patients with hypertrophic cardiomyopathy caused by other mutations in the genes for sarcomere proteins. Most deaths due to cardiac causes in these families occurred suddenly.

CONCLUSIONS

The clinical expression of mutations in the gene for cardiac myosin-binding protein C is often delayed until middle age or old age. Delayed expression of cardiac hypertrophy and a favorable clinical course may hinder recognition of the heritable nature of mutations in the cardiac myosin-binding protein C gene. Clinical screening in adult life may be warranted for members of families characterized by hypertrophic cardiomyopathy.

Hypertrophic cardiomyopathy: evaluation and treatment of patients at high risk for sudden death

Hypertrophic cardiomyopathy (HCM) is a heritable disease characterized by LV hypertrophy with markedly variable clinical, morphological, and genetic manifestations. It is the most common cause of sudden death in otherwise healthy young individuals. HCM patients often have disabling symptoms and are prone to arrhythmias. Frequently, there is associated LV systolic and diastolic dysfunction, LV outflow obstruction, and myocardial ischemia. Over the past decade, progress has been made in identifying patients who are at high risk for sudden death, in elucidating potential mechanisms of sudden death, and in defining therapeutic algorithms that may improve prognosis. It has also been possible to determine the genetic defect in some of the patients and to correlate clinical findings with the molecular defects. An exciting development has been the use of dual chamber pacemaker as an alternative to cardiac surgery to improve symptoms and relieve LV outflow obstruction.

Experience from clinical genetics in hypertrophic cardiomyopathy: proposal for new diagnostic criteria in adult members of affected families

The diagnosis of hypertrophic cardiomyopathy has relied on echocardiographic demonstration of unexplained left ventricular hypertrophy. The prevalence of hypertrophic cardiomyopathy defined in this way has been estimated to be 1:500 and experience indicates that these criteria are relatively specific when other causes of left ventricular hypertrophy are absent. In recent years, however, the systematic evaluation of pedigrees performed in the context of molecular genetic studies revealed that in some families with hypertrophic cardiomyopathy up to 20% of adults who carry a disease causing gene defect do not fulfil conventional echocardiographic criteria. None the less, most of these individuals show symptoms, electrocardiographic alterations, and/or minor echocardiographic abnormalities. Revised diagnostic criteria in members of families with hypertrophic cardiomyopathy are proposed, including major and minor criteria based on symptoms, and electrocardiographic and echocardiographic abnormalities. Given that the chance of inheriting the gene defect is 1:2, the likelihood that symptoms plus electrocardiographic or echocardiographic abnormalities are the expression of a disease causing gene is high.

Molecular mechanisms regulating the myofilament response to Ca2+: implications of mutations causal for familial hypertrophic cardiomyopathy

In this chapter we consider a current perception of the molecular mechanisms controlling myofilament activation with emphasis on alterations that may occur in familial hypertrophic cardiomyopathy (FHC). FHC is a sarcomeric disease (100) with an autosomal dominant pattern of heritability (27, 51). There is a substantial body of evidence implicating missense mutations in the beta-MHC gene as causal for the development of this disease. Recently, mutations in genes of two thin filament regulatory proteins, cardiac troponin T(cTnT) and alpha-tropomyosin (alpha-Tm), have also been linked to FHC. The commonality among the functional consequences of these mutations remains an important question. This review discusses how these pathological mutations may impact the activation process by disrupting critical structure function relations in both the thick and thin filaments.

Contractile protein mutations and heart disease

Mutations in several muscle structural proteins (the myosin heavy chain, alpha tropomyosin, cardiac troponin T and myosin binding protein C) result in a genetically dominant heart disease, hypertrophic cardiomyopathy. Biochemical data from studies of mutant myosin suggest a dominant-negative mechanism for inheritance of this disease. The most likely primary defect is sarcomere dysfunction, which is followed by the major clinical symptoms.

The standard electrocardiogram as a screening test for hypertrophic cardiomyopathy

Phenotypic heterogeneity in hypertrophic cardiomyopathy (HC) makes definitive diagnosis difficult, particularly during family screening. We studied the electrocardiogram (ECG) as a potential initial screening test in patients with HC. Using accepted diagnostic criteria, we examined the ECGs and echocardiograms of 159 patients with a confirmed clinical or genetic diagnosis of HC. An abnormal ECG was found in 154 patients (97%) while only 146 (92%) showed an abnormal echocardiogram. Of the former, 9 patients (6%) had normal echocardiograms and had been diagnosed on the basis of identification of a mutation in the beta myosin heavy chain gene (n = 8) or obligate carrier status (n = 1). Only 1 of these 9 patients was under age 20, the time at which hypertrophy is normally expressed on the echocardiogram. The remaining 5 patients (3%) without ECG abnormality consisted of 1 patient with an echocardiogram clearly diagnostic of HC and 4 clinically normal patients (aged 13, 24, 29, and 33 years) with normal echocardiograms who had been diagnosed by mutation identification (n = 3) or obligate carrier status (n = 1). Thus only these latter 4 patients (3%) would not have been diagnosed as having HC based on an abnormal ECG and/or abnormal echocardiogram. Screening relatives for HC by ECG criteria alone detects all those whom an echocardiogram will diagnose. While echocardiography aids in the specificity of HC diagnosis, the ECG, within the context of a family with a proven case of HC, is a more sensitive marker of the disease. It is therefore both a cost-effective and useful tool for screening those to proceed to echocardiography.

Missense mutation of the beta-cardiac myosin heavy-chain gene in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy occurs as an autosomal dominant familial disorder or as a sporadic disease without familial involvement. We describe a missense mutation of the beta-cardiac myosin heavy chain (MHC) gene, a G to T transversion (741 Gly-->Trp) identified by direct sequencing of exon 20 in four individuals affected with familial hypertrophic cardiomyopathy. Three individuals with sporadic hypertrophic cardiomyopathy, whose parents are clinically and genetically unaffected, had sequence variations of exon 34 of the alpha-cardiac MHC gene (a C to T transversion, 1658 Asp-->Asp, resulting in FokI site polymorphism), of intron 33 of the alpha-cardiac MHC gene (a G to A and an A to T transversion), and also of intron 14 of the beta-cardiac MHC gene (a C to T transversion in a patient with Noonan syndrome). Including our case, 30 missense mutations of the beta-cardiac MHC gene in 49 families have been reported thus far worldwide. Almost all are located in the region of the gene coding for the globular head of the molecule, and only one mutation was found in both Caucasian and Japanese families. Missense mutations of the beta-cardiac MHC gene in hypertrophic cardiomyopathy may therefore differ according to race.

A myosin missense mutation, not a null allele, causes familial hypertrophic cardiomyopathy

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is characterized by myocardial hypertrophy of unknown etiology. Missense mutations of the cardiac beta-myosin-heavy-chain (beta-MHC) gene that may be responsible for cardiac hypertrophy have been detected in patients with HCM. On the other hand, gross structural abnormalities in the cardiac beta-MHC gene, ie, an alpha/beta hybrid gene and partial deletion of the gene, have also been reported. The direct correlation between gross abnormalities and development of HCM is not well understood.

METHODS AND RESULTS

We analyzed the structure of the cardiac beta-MHC gene from patients with HCM by using polymerase chain reaction-DNA conformation polymorphism analysis and found two sequence variations in exons 3 and 22 in one patient. These sequence variations at codon 54 (exon 3; nonsense mutation) and codon 870 (exon 22; Arg-to-His mutation) were identified by direct sequencing and dot-blot hybridization with allele-specific oligonucleotide probes. Relatives of this patient were examined for the mutations. It was revealed that the missense mutation was inherited from the affected father and the nonsense mutation from the unaffected grandmother through the unaffected mother. In addition, the missense mutation was also found in seven other patients from two other unrelated multiplex HCM families.

CONCLUSIONS

The Arg870His mutation was suggested to cause HCM. In contrast, the gene with the nonsense mutation would encode for a cardiac beta-MHC protein of only 53 amino acid residues, which may be too short to be incorporated into the thick filament assembly of cardiac myosin chains and showed no dominant phenotype of heart disease. This is the first report of a nonsense mutation in the human cardiac beta-MHC gene.

Structural interpretation of the mutations in the beta-cardiac myosin that have been implicated in familial hypertrophic cardiomyopathy

In 10-30% of hypertrophic cardiomyopathy kindreds, the disease is caused by > 29 missense mutations in the cardiac beta-myosin heavy chain (MYH7) gene. The amino acid sequence similarity between chicken skeletal muscle and human beta-cardiac myosin and the three-dimensional structure of the chicken skeletal muscle myosin head have provided the opportunity to examine the structural consequences of these naturally occurring mutations in human beta-cardiac myosin. This study demonstrates that the mutations are related to distinct structural and functional domains. Twenty-four are clustered around four specific locations in the myosin head that are (i) associated with the actin binding interface, (ii) around the nucleotide binding site, (iii) adjacent to the region that connects the two reactive cysteine residues, and (iv) in close proximity to the interface of the heavy chain with the essential light chain. The remaining five mutations are in the myosin rod. The locations of these mutations provide insight into the way they impair the functioning of this molecular motor and also into the mechanism of energy transduction.

Hereditary dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a common and important cause of morbidity and mortality. Many factors can contribute to the development of this disorder, although most commonly the etiology is unexplained. However, recent studies in individuals with idiopathic DCM now reveal a heritable cause in 20-30% of individuals. Diverse modes of inheritance have been demonstrated, encompassing an autosomal dominant type (by far the most common), together with recessive and X-linked forms, and maternal inheritance through mitochondrial DNA. The hereditary forms of DCM (HDCM) predominantly affect the left ventricle, although inherited abnormalities affecting primarily the right ventricle also are described. HDCM may occur as a primary cardiomyopathy, or secondary to inherited systemic metabolic or neuromuscular disorders. The causative genes for primary HDCM of the autosomal dominant and recessive types have not yet been discovered, but the combination of family pedigree analysis and phenotyping by echocardiography, together with new genetic techniques, should now allow their identification. Knowledge of the gene or genes responsible for HDCM would improve diagnostic accuracy, facilitate genetic counseling, advance understanding of pathogenesis, and provide the starting point for new methods of treatment. Because of the frequently heritable nature of DCM, it is of great importance that a diligent search for all potentially affected family members be undertaken.

The electrocardiogram is a more sensitive indicator than echocardiography of hypertrophic cardiomyopathy in families with a mutation in the MYH7 gene

BACKGROUND

Mutations in the cardiac beta myosin heavy chain gene causing hypertrophic cardiomyopathy have been identified, and to assist both diagnosis and prediction of outcome attempts have been made to correlate phenotype and genotype. Two new mutations in codon 403 of the gene in three unrelated families are described and attention drawn to variable or even absent phenotypic expression in different family members.

METHODS AND RESULTS

The polymerase chain reaction and heteroduplex analysis on Mutation Detection Enhancement gels were used to search for mutations in the globular head of the beta myosin heavy chain gene in families with hypertrophic cardiomyopathy. Two mutations were found in exon 13 (codon 403) of the gene. In two unrelated Polish families the mutation resulted in the conversion of arginine to tryptophan (CGG: >TGG). A second mutation, found in a British family, converted the same arginine to leucine (CGG: >CTG). These mutations were detected in family members who had electrocardiographic and echocardiographic features typical of hypertrophic cardiomyopathy; however, they were also detected in 7 other adult relatives with an abnormal electrocardiogram but a normal echocardiogram. Two unrelated adult relatives had completely normal clinical findings but carried the gene mutation.

CONCLUSIONS

Identification of a specific mutation gives no guide to the clinical phenotype. There is considerable variability in the phenotypic expression of hypertrophic cardiomyopathy. Mutations were detected in adults previously regarded as normal or in whom the diagnosis was questionable. The fact that the clinical significance of the mutation in these people is still unknown emphasises the dilemma facing screening programmes. Isolated, unexplained electrocardiographic abnormalities in first degree relatives in a family with a definitive diagnosis of hypertrophic cardiomyopathy should be regarded as evidence of a carrier state.

Genotype-phenotype correlations in hypertrophic cardiomyopathy. Insights provided by comparisons of kindreds with distinct and identical beta-myosin heavy chain gene mutations

BACKGROUND

We have previously described two distinct mutations in the beta-myosin heavy chain gene with markedly different clinical presentations and outcome: The 908Leu-->Val mutation was associated with a low disease penetrance and a benign prognosis. In contrast, the 403Arg-->Gln mutation in a Caucasian kindred was associated with a 100% disease penetrance and high incidence of sudden cardiac death. Recently, another mutation, 606Val-->Met, has been reported to be associated with "near normal survival" and offered as evidence for the benign nature of neutral charge substitutions.

METHODS AND RESULTS

We report (1) a large kindred (245 family members at risk of inheriting the disease gene) with a 256Gly-->Glu mutation characterized by a similar disease penetrance in adults and in children (56% and 60%, respectively) and a cumulative sudden cardiac death rate of only 2% at 50 years of age, (2) a kindred with the 606Val-->Met mutation with four sudden cardiac deaths in eight affected individuals, and (3) a Korean kindred with the 403Arg-->Gln mutation. Although the disease occurred early and was associated with a high prevalence of myocardial ischemia in both of our kindreds with the 403Arg-->Gln mutation, no sudden cardiac death or syncope has occurred in the Korean kindred. Furthermore, in the Caucasian kindred, all patients had nonobstructive hypertrophic cardiomyopathy, but most of the patients in the Korean kindred had left ventricular outflow obstruction.

CONCLUSIONS

The conclusions are as follows: (1) Although several sudden cardiac deaths are sufficient to establish that a mutation is malignant, study of a large kindred is necessary to be certain that a mutation is benign. To date, only the 908Leu-->Val and the 256Gly-->Glu mutations satisfy this requirement. (2) The 256Gly-->Glu mutation demonstrates that not all mutations that result in a charge change are malignant. (3) Conversely, the 606Val-->Met mutation is malignant in some kindreds; hence, despite the absence of a charge change, minor substitutions in critical regions of beta-myosin heavy chain protein may also have serious consequences. (4) The diverse ethnic origins of the two 403Arg-->Gln kindreds provide evidence suggesting that the identical mutation occurred independently and was associated with different genetic backgrounds. Their distinct phenotypes underline the importance of modifying genes and nongenetic factors.

Prognostic implications of novel beta cardiac myosin heavy chain gene mutations that cause familial hypertrophic cardiomyopathy

Three novel beta cardiac myosin heavy chain (MHC) gene missense mutations, Phe513Cys, Gly716Arg, and Arg719Trp, which cause familial hypertrophic cardiomyopathy (FHC) are described. One mutation in exon 15 (Phe513Cys) does not alter the charge of the encoded amino acid, and affected family members have a near normal life expectancy. The Gly716Arg mutation (exon 19; charge change of +1) causes FHC in three family members, one of whom underwent transplantation for heart failure. The Arg719Trp mutation (exon 19; charge change of -1) was found in four unrelated FHC families with a high incidence of premature death and an average life expectancy in affected individuals of 38 yr. A comparable high frequency of disease-related deaths in four families with the Arg719Trp mutation suggests that this specific gene defect directly accounts for the observed malignant phenotype. Further, the significantly different life expectancies associated with the Arg719Trp vs. Phe513Cys mutation (P < 0.001) support the hypothesis that mutations which alter the charge of the encoded amino acid affect survival more significantly than those that produce a conservative amino acid change.

Familial hypertrophic cardiomyopathy. Microsatellite haplotyping and identification of a hot spot for mutations in the beta-myosin heavy chain gene

Familial hypertrophic cardiomyopathy (FHC) is a clinically and genetically heterogeneous disease. The first identified disease gene, located on chromosome 14q11-q12, encodes the beta-myosin heavy chain. We have performed linkage analysis of two French FHC pedigrees, 720 and 730, with two microsatellite markers located in the beta-myosin heavy chain gene (MYO I and MYO II) and with four highly informative markers, recently mapped to chromosome 14q11-q12. Significant linkage was found with MYO I and MYO II in pedigree 720, but results were not conclusive for pedigree 730. Haplotype analysis of the six markers allowed identification of affected individuals and of some unaffected subjects carrying the disease gene. Two novel missense mutations were identified in exon 13 by direct sequencing, 403Arg-->Leu and 403Arg-->Trp in families 720 and 730, respectively. The 403Arg-->Leu mutation was associated with incomplete penetrance, a high incidence of sudden deaths and severe cardiac events, whereas the consequences of the 403Arg-->Trp mutation appeared less severe. Haplotyping of polymorphic markers in close linkage to the beta-myosin heavy chain gene can, thus, provide rapid analysis of non informative pedigrees and rapid detection of carrier status. Our results also indicate that codon 403 of the beta-myosin heavy chain gene is a hot spot for mutations causing FHC.