Cap disease due to mutation of the beta-tropomyosin gene (TPM2).

Cap disease or cap myopathy is a form of congenital myopathy in which peripheral, well-demarcated 'caps' of disorganised thin filaments are seen in muscle fibres. Mutation of the TPM2 gene, that encodes beta-tropomyosin, is the first reported genetic cause. In this paper, we describe a further case of cap disease due to a mutation in TPM2, confirming the importance of this genetic association. This is the first report of cardiac dysfunction due to a mutation in TPM2. Our patient has an identical TPM2 mutation to the first genetically diagnosed cap disease patient, a denovo heterozygous three base pair deletion that removes glutamic acid 139 from the centre of beta-tropomyosin (p.E139del). 2D-gel electrophoresis studies show that the shortened mutant protein incorporates into sarcomeric structures, where it likely imposes a dominant-negative effect to cause muscle weakness.