Intranuclear rod myopathy: Molecular pathogenesis and mechanisms of weakness

Objective: Mutations in the a-skeletal actin gene (ACTAI) result in a variety of inherited muscle disorders characterized by different pathologies and variable clinical phenotypes. Mutations at Val163 in ACTAI result in pure intranuclear rod myopathy; however, the molecular mechanisms by which mutations at Val163 lead to intranuclear rod formation and muscle weakness are unknown. Methods and Results: We investigated the effects of the Val163Met mutation in ACTAI in tissue culture and Drosophila models, and in patient muscle. In cultured cells, the mutant actin tends to aggregate rather than incorporate into cytoplasmic microfilaments, and it affects the dynamics of wild-type actin, causing it to accumulate with the mutant actin in the nucleus. In Drosophila, the Val163Met mutation severely disrupts the structure of the muscle sarcomere. The intranuclear aggregates in patient muscle biopsies impact on nuclear structure and sequester normal Z-disc-associated proteins within the nucleus; however, the sarcomeric structure is relatively well preserved, with evidence of active regeneration. By mass spectrometry, the levels of mutant protein are markedly reduced in patient muscle compared with control. Interpretation: Data from our tissue culture and Drosophila models show that the Val163Met mutation in a-skeletal actin can affect the dynamics of other actin isoforms and severely disrupt sarcomeric structure, processes that can contribute to muscle weakness. However, in human muscle, there is evidence of regeneration, and the mutant protein tends to aggregate rather than incorporate into cytoplasmic microfilaments in cells. These are likely compensatory processes that ameliorate the effects of the mutant actin and contribute to the milder clinical and pathological disease phenotype.