Does the severity of the LGMD2A phenotype in compound heterozygotes depend on the combination of mutations?

Limb-girdle muscular dystrophy type 2A (LGMD2A) is caused by a deficiency of calpain-3/p94. Although the symptoms in most LGMD2A patients are generally homogeneous, some variation in the severity and progression of the disease has been reported.

We describe 2 patients who carry the same combination of compound heterozygous mutations (pG222R/pR748Q) and whose symptoms are exceptionally benign compared to homozygotes with each missense mutation.

The benign phenotype observed in association with the combined pG222R and pR748Q mutations suggested that it may result from a compensatory effect of compound heterozygosity rather than the individual mutations themselves. Our analyses revealed that these two mutations exert different effects on the protease activity of calpain-3, suggesting "molecular complementation" in these patients.

We propose several hypotheses to explain how this specific combination of mutations may rescue the normal proteolytic activity of calpain-3, resulting in an exceptionally benign phenotype.