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

Maternally inherited hypertrophic cardiomyopathy due to a novel T-to-C transition at nucleotide 9997 in the mitochondrial tRNA(glycine) gene

We report a unique heteroplasmic T-to-C transition at nucleotide 9997 in the mitochondrial tRNA(glycine) gene in a multiplex family who manifested nonobstructive cardiomyopathy. The degree of mtDNA heteroplasmy generally correlated with the severity of the symptoms. This T-to-C transition disrupts hydrogen bonding in the region adjacent to the acceptor stem of the tRNA molecule. The thymine residue at position 9997 is highly conserved in mammals, as well as in various vertebrates and invertebrates. A PCR diagnostic test for the presence of the 9997 T-to-C transition revealed that the base change was always present in high proportion in affected family members, not present in unaffected family members, and never present in control subjects from various ethnic groups (25 groups sampled, 42 individuals), thus ruling out the possibility that this change represents a polymorphic variant in the general population. The degree of heteroplasmy in lymphoblast cultures also correlated with the level of enzyme activity present for cytochrome c oxidase (complex IV) and succinate cytochrome c oxidoreductase (complexes II and III). The absence of previously reported mtDNA mutations associated with hypertrophic cardiomyopathy was verified by both PCR diagnostic procedures and sequence analysis. All mitochondrial tRNA genes, as well as genes encoding ATPase subunits 6 and 8, were sequenced and found not to possess base changes consistent with the clinical profile. More detailed biochemical and molecular biological investigations are discussed.

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.

Genetic heterogeneity of familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (FHC) is characterized by an autosomal dominant transmission, a left ventricular hypertrophy and a myocardial disorganization. A first morbid gene coding for the beta-myosin heavy chain (beta-MHC) located on chromosome 14q11-q12 has been identified and genetic heterogeneity has been demonstrated. We performed linkage and haplotype analysis of 17 French families with microsatellite markers located to the beta-MHC locus and we searched for a new locus in a family for which the beta-MHC locus was excluded. Two novel missense mutations were identified in exon 13 of the beta-MHC resulting in the replacement of 403 arginine by a leucine or a tryptophan in two different families. In one other family, systematic screening of the genome with highly informative markers allowed us to find a new locus for FHC on chromosome 11.