A novel frameshift ACTN2 variant causes a rare adult-onset distal myopathy with multi-minicores.

Introduction: Distal myopathies are a group of rare muscle disorders characterized by selective or predominant weakness in the feet and/or hands. In 2019, ACTN2 gene was firstly identified to be a cause of a new adult-onset distal muscular dystrophy calling actininopathy and another distinctly different myopathy, named multiple structured core disease (MsCD). Thus, the various phenotypes and limited mutations in ACTN2-related myopathy make the genotype-phenotype correlation hard to understand.

Aims: To investigate the clinical features and histological findings in a Chinese family with distal myopathy. Whole exome sequencing and several functional studies were performed to explore the pathogenesis of the disease.

Results: We firstly identified a novel frameshift variant (c.2504delT, p.Phe835Serfs*66) within ACTN2 in a family including three patients. The patients exhibited adult-onset distal myopathy with multi-minicores, which, interestingly, was more like a combination of MsCD and actininopathy. Moreover, functional analysis using muscle samples revealed that the variant significantly increased the expression level of α-actinin-2 and resulted in abnormal Z-line organization of muscle fiber. Vitro studies suggested aggregate formations might be involved in the pathogenesis of the disease.

Conclusion: Our results expanded the phenotypes of ACTN2-related myopathy and provided helpful information to clarify the molecular mechanisms.