A heterozygous 9q34 deletion encompassing SPTAN1 as a cause of distal myopathy

We report a family affected with childhood onset distal muscle weakness with a heterozygous chromosome 9q34 deletion encompassing the SPTAN1 gene. The deletion was detected through exome-sequencing based copy number variant (CNV) detection, segregates in four patients and is non-penetrant in two other relatives. Electromyography, muscle MRI and muscle biopsy revealed a myopathic disease phenotype. Cellular consequences of the deletion were investigated using qPCR and western blotting on patient-derived fibroblasts, which revealed a reduction of RNA but not protein levels. Immunocytochemistry was performed on muscle tissue which did not reveal reduction of α-II-spectrin. SPTAN1 loss-of-function variants have previously been reported to cause distal hereditary motor neuropathy and recently distal myopathy. Here, we confirm the role of SPTAN1 haploinsufficiency as a cause of distal myopathy. We propose an age-dependent lack of α-II-spectrin and suggest CNV detection in repurposed exome sequencing as an important diagnostic tool.