Muscle weakness in TPM3-myopathy is due to reduced Ca2+-sensitivity and impaired acto-myosin cross-bridge cycling in slow fibres

Hum Mol Genet. 2015 Nov 15;24(22):6278-92. doi: 10.1093/hmg/ddv334. Epub 2015 Aug 24.


Dominant mutations in TPM3, encoding α-tropomyosinslow, cause a congenital myopathy characterized by generalized muscle weakness. Here, we used a multidisciplinary approach to investigate the mechanism of muscle dysfunction in 12 TPM3-myopathy patients. We confirm that slow myofibre hypotrophy is a diagnostic hallmark of TPM3-myopathy, and is commonly accompanied by skewing of fibre-type ratios (either slow or fast fibre predominance). Patient muscle contained normal ratios of the three tropomyosin isoforms and normal fibre-type expression of myosins and troponins. Using 2D-PAGE, we demonstrate that mutant α-tropomyosinslow was expressed, suggesting muscle dysfunction is due to a dominant-negative effect of mutant protein on muscle contraction. Molecular modelling suggested mutant α-tropomyosinslow likely impacts actin-tropomyosin interactions and, indeed, co-sedimentation assays showed reduced binding of mutant α-tropomyosinslow (R168C) to filamentous actin. Single fibre contractility studies of patient myofibres revealed marked slow myofibre specific abnormalities. At saturating [Ca(2+)] (pCa 4.5), patient slow fibres produced only 63% of the contractile force produced in control slow fibres and had reduced acto-myosin cross-bridge cycling kinetics. Importantly, due to reduced Ca(2+)-sensitivity, at sub-saturating [Ca(2+)] (pCa 6, levels typically released during in vivo contraction) patient slow fibres produced only 26% of the force generated by control slow fibres. Thus, weakness in TPM3-myopathy patients can be directly attributed to reduced slow fibre force at physiological [Ca(2+)], and impaired acto-myosin cross-bridge cycling kinetics. Fast myofibres are spared; however, they appear to be unable to compensate for slow fibre dysfunction. Abnormal Ca(2+)-sensitivity in TPM3-myopathy patients suggests Ca(2+)-sensitizing drugs may represent a useful treatment for this condition.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Actins / genetics
  • Actins / metabolism
  • Adolescent
  • Adult
  • Calcium / metabolism
  • Child
  • Child, Preschool
  • Female
  • Humans
  • Infant
  • Male
  • Middle Aged
  • Muscle Contraction / physiology
  • Muscle Fibers, Slow-Twitch / metabolism*
  • Muscle Weakness / genetics
  • Muscle Weakness / metabolism
  • Muscular Atrophy / genetics
  • Muscular Atrophy / metabolism*
  • Muscular Diseases / genetics
  • Muscular Diseases / metabolism*
  • Mutation
  • Myosins / genetics
  • Myosins / metabolism*
  • Protein Isoforms
  • Tropomyosin / genetics*
  • Tropomyosin / metabolism


  • Actins
  • Protein Isoforms
  • TPM3 protein, human
  • Tropomyosin
  • Myosins
  • Calcium