Loss of calpain-3 autocatalytic activity in LGMD2A patients with normal protein expression

The diagnosis of limb girdle muscular dystrophy (LGMD) type 2A (due to mutations in the gene encoding for calpain-3) is currently based on protein analysis, but mutant patients with normal protein expression have also been identified. In this study we investigated 150 LGMD patients with normal calpain-3 protein expression, identified gene mutations by an allele-specific polymerase chain reaction test, and analyzed the mutant calpain-3 catalytic activity. Four different mutations were found in eight patients (5.5%): a frame-shifting deletion (550 A del) and three missense (R490Q, R489Q, R490W). Patients with normal calpain-3 protein expression on Western blot are a considerable proportion (20%) of our total LGMD2A population. While in control muscle the calpain-3 Ca++-dependent autocatalytic activity was evident within 5 minutes and was prevented by ethylene diaminetetraacetic acid, in all mutant patient samples the protein was not degraded, indicating that the normal autocatalytic function had been lost. By this new functional test, we show that conventional protein diagnosis fails to detect some mutant proteins, and prove the pathogenetic role of R490Q, R489Q, R490W missense mutations. We suggest that these mutations impair protein activity by affecting interdomain protein interaction, or reduce autocatalytic activity by lowering the Ca++ sensitivity.