Background: The beta-myosin heavy chain (beta-MHC) gene has been identified as a major locus for familial hypertrophic cardiomyopathy (FHCM). We recently showed that one of the common mutations associated with FHCM is expressed in the cardiac muscle messenger RNA (mRNA) of an affected individual. Since beta-MHC is a major sarcomeric protein of cardiac and skeletal muscle, studies were performed to determine whether the mutation is also expressed in skeletal muscle.
Methods and results: Biopsies were obtained of skeletal muscle (biceps brachii) from a proband with FHCM known to have the missense mutation in exon 13 of the beta-MHC gene. RNA was extracted from skeletal muscle and lymphocytes by the RNAzol method. First-strand complementary DNA was synthesized by reverse transcription using an antisense primer to exon 16. Polymerase chain reaction (PCR) was performed using primers to exons 12 and 14 to amplify the segment encompassing exon 13. The PCR products were digested with Ddel restriction endonuclease. Undigested PCR product in the control and the proband was 321 base-pairs (bp). Ddel digestion of the PCR product from normal skeletal and lymphocytes showed two DNA fragments of 181 and 140 bp as expected, whereas digestion of the PCR product from the proband's skeletal muscle and lymphocytes showed four DNA fragments of 181, 149, 140, and 32 bp due to the mutation in exon 13. This indicates that the mutation in affected individuals is also expressed in the mRNA of skeletal muscle and lymphocytes.
Conclusions: To our knowledge, this is the first documentation of a beta-MHC gene mutation expressed in skeletal muscle. This finding is provocative. Does it impair skeletal muscle function? If so, how? If not, why not? Is the impairment, or lack of it, a clue to the molecular defect of cardiac muscle? Furthermore, skeletal muscle provides a readily accessible source of mRNA for expression studies and for purification of the beta-MHC protein, which is probably essential to future investigation designed to unravel the molecular basis of this disorder.