Limb-girdle muscular dystrophy (LGMD) type R1/2A, calpain-3-related, is a rare, autosomal recessive disorder caused by pathogenic variants in the CAPN3 gene. LGMDR1/2A causes progressive weakness of upper and lower limbs, leading to difficulty walking and use of arms for overhead activities. The rate of progression of LGMDR1/2A is unknown because of the limited number of published cohorts and the lack of validated outcome measures. Quantifying the natural history of LGMDR1/2A using standardized outcome measures is critical for understanding the rate of disease progression and improving clinical care and clinical trial readiness. The aim of this cross-sectional study was to evaluate the suitability of the North Star Assessment for limb-girdle type muscular dystrophies (NSAD), 100-meter timed test (100MTT), and Performance of Upper Limb 2.0 (PUL) as outcome measures for quantifying disease presentation and progression in individuals with LGMDR1/2A. Ninety-two individuals were evaluated by physiotherapy teams at Nationwide Children's Hospital and the John Walton Muscular Dystrophy Research Centre. Psychometric analysis of NSAD and PUL was completed using Rasch measurement methods. Descriptive statistics and correlation coefficients explored the relationship among all outcomes. Psychometric analyses demonstrated that NSAD and PUL were valid and appropriate functional outcome measures for ambulant and nonambulant individuals with LGMDR1/2A. The NSAD and 100MTT velocity were highly correlated (Spearman rho Rs(88) = 0.89) The 100MTT was particularly useful for the strongest and asymptomatic cohort. In this largest reported cohort of individuals with LGMDR1/2A, NSAD, PUL, and 100MTT were suitable to quantify functional impact of the disease. International harmonization of outcome measures creates meaningful clinical data to inform clinical management and trial design.

The investigated intronic CAPN3 variant NM_000070.3:c.1746-20C>G occurs in the Central and Eastern Europe with a frequency of >1% and there are conflicting interpretations on its pathogenicity. We collected data on 14 patients carrying the CAPN3 c.1746-20C>G variant in trans position with another CAPN3 pathogenic/likely pathogenic variant. The patients compound heterozygous for the CAPN3 c.1746-20C>G variant presented a phenotype consistent with calpainopathy of mild/medium severity. This variant is most frequent in the North/West regions of Russia and may originate from that area. Molecular studies revealed that different splicing isoforms are produced in the muscle. We hypothesize that c.1746-20C>G is a hypomorphic variant with a reduction of RNA and protein expression and only individuals having a higher ratio of abnormal isoforms are affected. Reclassification of the CAPN3 variant c.1746-20C>G from variant with a conflicting interpretation of pathogenicity to hypomorphic variant explains many unidentified cases of limb girdle muscular dystrophy R1 calpain 3-related in Eastern and Central Europe.

Presented here are five members of a family that was ascertained from an isolated, consanguineous, indigenous Amerindian community in Colombia that was affected with calpain 3-related, limb-girdle muscular dystrophy type R1. These patients are homozygous for a unique and novel deletion of four bases (TGCC) in exon 3 of the calpain 3 gene (CAPN3) (NM_000070.2; NP_000061.1) (g.409_412del). The mutation site occurs at the CysPc protein domain, triggering a modified truncated protein structure and affecting motifs within the calpain-like thiol protease family (peptidase family C2) region. The patients reported here developed a very severe phenotype with primary contractures, spinal rigidity in the early stages of the disease, and bilateral talipes equinovarus (clubfoot) in the most affected patients who had the selective involvement of their extremities' distal muscles in a way that resembled Emery-Dreifuss syndrome. We recommend mandatory screening for calpainopathy in all patients with an Emery-Dreifuss-like syndrome or those presenting a non-congenital illness with primary contractures and who, because of other data, are suspected of having muscular dystrophy.