NG.O.10 - CASQ1 mutations impair calsequestrin polymerization and cause tubular aggregate myopathy

Abstract

Tubular aggregate myopathy (TAM) is a rare muscle disorder associated with progressive muscle weakness, cramps, myalgia, and exercise intolerance. Muscle biopsies from TAM patients typically show regular arrays of membranes tubules appearing as honeycomb-like structures. To date, two causative TAM gene are known: STIM1 and ORAI1. Both encode key factors in the regulation of Ca2+ homeostasis, and mutations in either gene result in excessive Ca2+ entry. Despite the discovery of two TAM genes, many TAM patients still await molecular diagnosis. In an attempt to identify additional TAM genes, we performed linkage analysis and exome sequencing on molecularly undiagnosed TAM cases. We identified CASQ1 as the third TAM gene, and we evidence this discovery by clinical, histological, and functional data. CASQ1 encodes calsequestrin, the major Ca2+ storage protein in the sarcoplasmic reticulum. Calsequestrin binds Ca2+ with moderate affinity and high capacity, and forms needle-like polymers with increasing Ca2+-binding capacities. A single CASQ1 mutation has previously been associated with vacuolar myopathy, differing from TAM at both the clinical and the histopathological level. Immunolocalization studies on muscle biopsies from our CASQ1 patients revealed that the tubular aggregates contain calsequestrin and STIM1, and thereby provide a histopathological link between CASQ1 and STIM1-related TAM. We assessed the impact of the CASQ1 mutations on protein function in transfected muscle cells and using a biophysical assay, and we functionally demonstrate that the TAM-related CASQ1 mutations delay calsequestrin polymerization, whereas the vacuolar myopathy mutation induces the formation of insoluble polymers at low Ca2+ concentrations. These results illustrate that CASQ1 mutations cause either TAM or vacuolar myopathy, and that both disorders involve different pathomechanisms.