Variants in CAPN3 Causing Autosomal Dominant Limb-Girdle Muscular Dystrophy Combined With Calpain-3 Deficiency

Abstract

Limb-girdle muscular dystrophy Type 2A/R1 or calpain-3 deficiency is the most common autosomal recessive limb-girdle muscular dystrophy. However, in recent years, autosomal dominant cases and families with calpain-3 deficiency have been reported, and there is an emerging interest in looking for single variants in the calpain-3 gene in mildly to moderately affected patients with limb-girdle muscular dystrophy without biallelic gene variants in CAPN3. Here, we report four cases with creatine kinase levels above 1500 U/L, mild-to-moderate proximal weakness, waddling gait, and scapular winging. Two patients, a son and his father, are heterozygous for the CAPN3 variant c.304C>T; p.(Pro102Ser), which has previously been reported in patients with compound heterozygous variants in CAPN3. The third and fourth patients were heterozygous for c.1371C>G; p.(Asn457Lys) and c.1490C>T; p.Ala497_Glu508del, respectively, neither of which has been reported before. All four patients had a near-complete loss of calpain-3 as determined by western blotting. While inherited autosomal dominant calpainopathy has now been firmly established, additional single cases of dominant calpainopathy are likely to emerge; some will be associated with clinical findings from parents or siblings, while others will arise from spontaneous mutations, but nevertheless with similar clinical findings of mild-to-moderate proximal weakness, increased level of creatine kinase, and near-complete loss of calpain-3 protein in affected individuals. This report expands the known number of variants causing dominant calpainopathy from 8 to 11 that appears to exclusively reside in two out of four domains that make up calpain-3. This information could aid in determining whether a CAPN3 variant of unknown significance is pathological.

Keywords: autosomal dominant calpainopathy; calpain-3; limb–girdle muscular dystrophy Type D4.

Figure 1

Calpain-3, patient calpain-3 levels, and calpain-3 AD variant distribution. (a) The dimerized calpain-3 consists of four functional domains each. The proteolytic Cores 1 and 2 (PC1/PC2), calpain-type beta-sandwich (CBSW), and the PEF(L) dimerization domain. (b) Western blot of calpain-3 (FL full length) shows that this is barely detectable in Patients 1 and 2 compared to controls; α-sarcoglycan (α-SG) is used as a loading control. (c) Western blot of calpain-3 from Patient 3 also shows barely detectable calpain-3 compared to controls; glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as a loading control. (d) Western blot of calpain-3 from Patient 4 also shows barely detectable calpain-3 compared to controls; myosin heavy chain (myosin) was used as a loading control. Western blotting was carried out at two sites (Paris and Copenhagen) in three separate runs as shown over a period of 3 years.