Mutations in Sarcomere Protein Genes as a Cause of Dilated Cardiomyopathy

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Abstract
The molecular basis of idiopathic dilated cardiomyopathy, a primary myocardial disorder that results in reduced contractile function, is largely unknown. Some cases of familial dilated cardiomyopathy are caused by mutations in cardiac cytoskeletal proteins; this finding implicates defects in contractile-force transmission as one mechanism underlying this disorder. To elucidate this important cause of heart failure, we investigated other genetic causes of dilated cardiomyopathy. Clinical evaluations were performed in 21 kindreds with familial dilated cardiomyopathy. A genome-wide linkage study prompted a search of the genes encoding β-myosin heavy chain, troponin T, troponin I, and α-tropomyosin for disease-causing mutations. A genetic locus for mutations associated with dilated cardiomyopathy was identified at chromosome 14q11.2–13 (maximal lod score, 5.11; θ=0), where the gene for cardiac β-myosin heavy chain is encoded. Analyses of this and other genes for sarcomere proteins identified disease-causing dominant mutations in four kindreds. Cardiac β-myosin heavy-chain missense mutations (Ser532Pro and Phe764Leu) and a deletion in cardiac troponin T (ΔLys210) caused early-onset ventricular dilatation (average age at diagnosis, 24 years) and diminished contractile function and frequently resulted in heart failure. Affected persons had neither antecedent cardiac hypertrophy (average maximal left-ventricular-wall thickness, 8.5 mm) nor histopathological findings characteristic of hypertrophy. Mutations in sarcomere protein genes account for approximately 10 percent of cases of familial dilated cardiomyopathy and are particularly prevalent in families with early-onset ventricular dilatation and dysfunction. Because distinct mutations in sarcomere proteins cause either dilated or hypertrophic cardiomyopathy, the effects of mutant sarcomere proteins on muscle mechanics must trigger two different series of events that remodel the heart.