Background Myosin binding protein C (MyBPC) is essential for the structure of the sarcomeres in striated muscle. There is one cardiac specific isoform and two skeletal muscle specific isoforms. Mutations in MYBPC3 encoding the cardiac isoform cause cardiomyopathy.

Methods and results We have identified an infant with fatal cardiomyopathy due to a homozygous mutation, p.R943X, in MYBPC3. The patient also had an unexpected skeletal myopathy. The patient expressed the cardiac specific MyBPC isoform in skeletal muscle at transcript and protein levels. Numerous muscle fibres expressing the mutant cardiac isoform showed structural abnormalities with disorganisation of sarcomeres and depletion of myosin thick filaments.

Conclusions The surprising identification of a skeletal myopathy in this patient was due to aberrant expression of mutant cardiac MyBPC in skeletal muscle.