Novel splice donor site mutation in the cardiac myosin-binding protein-C gene in familial hypertrophic cardiomyopathy. Characterization Of cardiac transcript and protein.

Abstract

Familial hypertrophic cardiomyopathy is a disease generally believed to be caused by mutations in sarcomeric proteins. In a family with hypertrophic cardiomyopathy linked to polymorphic markers on chromosome 11, we found a new mutation of a splice donor site of the cardiac myosinbinding protein-C gene. This mutation causes the skipping of the associated exon in mRNA from lymphocytes and myocardium. Skipping of the exon with a consecutive reading frame shift leads to premature termination of translation and is thus expected to produce a truncated cardiac myosinbinding protein-C with loss of the myosin-and titin-binding COOH terminus. However, Western blot analysis of endomyocardial biopsies from histologically affected left ventricular myocardium failed to show the expected truncated protein.These data show for the first time that a splice donor site mutation in the myosin-binding protein-C gene is transcribed to cardiac mRNA. Truncated cardiac myosin-binding protein-C does not act as a "poison polypeptide," since it seems not to be incorporated into the sarcomere in significant amounts. The absence of mutant protein and of significantly reduced amounts of wild-type protein in the presence of the mutated mRNA argues against the "poison protein" and the "null allele" hypotheses and suggests yet unknown mechanisms relevant to the genesis of chromosome-11-associated familial hypertrophic cardiomyopathy.

Micro-exons of the cardiac myosin binding protein C gene: flanking introns contain a disproportionately large number of hypertrophic cardiomyopathy mutations

“…In the initial publications of MYBPC3-associated HCM, both normal and mutated transcripts were observed in blood lymphocytes of patients with MYBPC3 mutations 5,6 and comparison of the transcripts present in both blood and cardiac tissue of a patient with an exon 31 donor splice site mutation of MYBPC3 revealed that the same mRNA species could be observed. 4 The c.906À36G4A, c.906À1G4C, c.1224À19G4A and c.1224À2A4G mutations were all demonstrated to cause aberrant transcripts, and no use of alternative splice sites was detected in the examined exons 10 and 14 of the two controls.…”

“…The reason for difference in the type of mutation is not known, but we and others have speculated that these mutations lead to nonsense-mediated decay (NMD) and subsequent haploinsufficiency rather than malfunctioning truncated proteins. 1,4 One way of generating transcripts with frameshifts and ultimately PTCs is through aberrant splicing. Several examples of mutations of both acceptor and donor splice sites in MYBPC3 that can produce aberrant transcripts have already been demonstrated in HCM.…”

Compound heterozygous or homozygous truncating MYBPC3 mutations cause lethal cardiomyopathy with features of noncompaction and septal defects

“…It is thought to lead to reduction in MYBPC3 owing to protein instability and/or loss of the C-terminus of MYBPC3 that binds myosin thick filaments and titin, which is required to incorporate MYBPC3 normally into the A-band of the sarcomere. 19 Consequently, the MYBPC3 protein was not expected to be incorporated into the sarcomere in any of these four patients. This may explain the early and severe presentation of their cardiomyopathy.…”

A Hypertrophic Cardiomyopathy-associated MYBPC3 Mutation Common in Populations of South Asian Descent Causes Contractile Dysfunction

“…Most of the truncation mutations in the C-terminal portion of cMyBP-C lead to haploinsufficiency (6), as the truncated proteins were never found in the heart (7,8). This leads to a reduced cMyBP-C protein level, because the healthy allele can only partly compensate.…”

“…HCM mutations in the MYBPC3 gene were first identified in 1995 (4,5), and, to date, ϳ200 mutations have been published (6). The precise functional and pathological consequences of MYBPC3 mutations are not fully characterized because mutant proteins are typically not found in cardiac tissue from affected patients (7,8).…”