Glycogen storage diseases are important causes of myopathy and cardiomyopathy. This study describes ten patients from eight families with childhood or juvenile onset of myopathy, eight of whom also had rapidly progressive cardiomyopathy requiring heart transplant in four. The patients were homozygous or compound heterozygous for missense or truncating mutations in the ubiquitin ligase RBCK1 and had extensive polyglucosan accumulation in skeletal muscle and in the heart in cases of cardiomyopathy. RBCK1 deficiency is a frequent cause of polyglucosan storage myopathy associated with progressive muscle weakness and cardiomyopathy.
