LGMD: EP.195 The most common CAPN3 disease causing variant in north-eastern Europe population or controversial coincidence

Abstract

CAPN3 gene intronic variant c.1746-20C>G has a relatively high frequency for the disease-causing mutation. It shows a gradual decrease in frequency from the North to South and from the East to West in Europe, having the lowest AF 0.002 in southern European populations, and the highest AF 0.018 in Latvia. GnomAD also reports two homozygous c.1746-20C>G individuals, and two more homozygous individuals identified in his study from BioBank. At least in one homozygous individual CPK level was normal, and the person was classified as healthy. Our study group included calpainopathy patients (n=20) from the Latvia, Lithuania and Myo-Seq project with identified CAPN3 c.1746-20C>G variant in trans position with another CAPN3 disease causing variants. Data was collected retrospectively. Western blot (n=2) and RNA studies (n=4) were done. In all families the CAPN3 c.1746-20C>G variant is found in the biallelic combination, and it segregated with the disease. The second alleles were: c.550delA, p.Arg49Cys, p.Glu234Arg, p.Ser215Pro, p.Gly445Arg, c.598_612del and c.1333G>A. Some of them recently were reported as of having possible autosomal dominant transmission. Family members carriers for those mutations were unaffected. Reduced labeling for Calpain-3 was observed in Western blot analysis, also it coincides with data from the previous publications by Nascimbeni et al. 2010. RNA studies in all of analyzed persons showed some level of abnormal splicing concerning intron 13. We have been able to identify isoforms with complete intron 13 inclusion, and several low yield isoforms with partial intron 13 inclusion in both 5’and 3’end. We can argue that low yield results could be artefacts, or conversely, that it could be caused by nonsense mediated decay of abnormal isoforms. If CAPN3 gene intronic variant c.1746-20C>G is a risk allele for calpainopathy, that would be one of the most common CAPN3 variant in those populations.