A heterozygous 21-bp deletion inCAPN3causes dominantly inherited limb girdle muscular dystrophy

Abstract

Limb girdle muscular dystrophy type 2A is the most common limb girdle muscular dystrophy form worldwide. Although strict recessive inheritance is assumed, patients carrying a single mutation in the calpain 3 gene (CAPN3) are reported. Such findings are commonly attributed to incomplete mutation screening. In this investigation, we report 37 individuals (age range: 21-85 years, 21 females and 16 males) from 10 families in whom only one mutation in CAPN3 could be identified; a 21-bp, in-frame deletion (c.643_663del21). This mutation co-segregated with evidence of muscle disease and autosomal dominant transmission in several generations. Evidence of muscle disease was indicated by muscle pain, muscle weakness and wasting, significant fat replacement of muscles on imaging, myopathic changes on muscle biopsy and loss of calpain 3 protein on western blotting. Thirty-one of 34 patients had elevated creatine kinase or myoglobin. Muscle weakness was generally milder than observed in limb girdle muscular dystrophy type 2A, but affected the same muscle groups (proximal leg, lumbar paraspinal and medial gastrocnemius muscles). In some cases, the weakness was severely disabling. The 21-bp deletion did not affect mRNA maturation. Calpain 3 expression in muscle, assessed by western blot, was below 15% of normal levels in the nine mutation carriers in whom this could be tested. Haplotype analysis in four families from three different countries suggests that the 21-bp deletion is a founder mutation. This study provides strong evidence that heterozygosity for the c.643_663del21 deletion in CAPN3 results in a dominantly inherited muscle disease. The normal expression of mutated mRNA and the severe loss of calpain 3 on western blotting, suggest a dominant negative effect with a loss-of-function mechanism affecting the calpain 3 homodimer. This renders patients deficient in calpain 3 as in limb girdle muscular dystrophy type 2A, albeit in a milder form in most cases. Based on findings in 10 families, our study indicates that a dominantly inherited pattern of calpainopathy exists, and should be considered in the diagnostic work-up and genetic counselling of patients with calpainopathy and single-allele aberrations in CAPN3.

Genetic landscape and novel disease mechanisms from a large LGMD cohort of 4656 patients

“…This variant, a 21 bp in‐frame deletion c.643_663del21, cosegregates in 37 individuals from 10 families with muscle disease and dominant‐transmission through several generations. Moreover, it is associated with severe loss of calpain3 expression shown by immunoblotting, suggesting a dominant‐negative effect on calpain3 homodimer 33. In our large‐scale study, the identification of the same 21 bp in‐frame deletion in 17 individuals (Table 2) without any reportable second variant further confirms the c.643_663del21 as a common, dominant mutation, but further deletion‐duplication analysis did not occur.…”

“…This variant occurred in multiple different families with muscle disease, segregating with dominant inheritance 32, 33, 34. A second 15 bp in‐frame deletion (c.598_612del15) was also identified in different 16 unrelated patients.…”

When Recessive Genes Mutate to Dominant Gene Action

“…If, on the other hand, the mutant allele of a gene would suppress the expression of the wild-type allele, as was the case in a patient with an intragenic deletion in the CNTNAP2 gene, significantly less than 50% of wild-type transcript would remain [Lee et al, 2015]. The latter may be the case with the in-frame deletion of CAPN3 [Vissing et al, 2016]. However plausible these explanations for dominant effects of mutations in transmembrane or dimerization domains of proteins may seem, they need corroboration by functional studies.…”

“…Two recent publications challenge this relatively clear-cut picture and may even 'blur the borders of dominant and recessive inheritance' [Lemke et al, 2016;Vissing et al, 2016].…”

“…In 37 patients of 10 independent pedigrees, a 21-bp inframe deletion (c.643_663del21) in the CAPN3 gene cosegregated with signs of muscle disease consistent with autosomal dominant transmission through several generations [Vissing et al, 2016]. The 36 patients who were available for thorough evaluation showed muscle pain, muscle weakness and wasting, fat replacement of muscle tissue, and myopathic changes in muscle biopsies.…”