Myopathic changes caused by protein aggregates in adult-onset spinal muscular atrophy

Spinal muscular atrophy (SMA), an autosomal-recessive lower motor neuron disease, causes progressive proximal muscle waste and weakness. It remains unclear whether myopathic changes are involved in pathogenesis. We encountered a patient with adult-onset SMA caused by a homozygous deletion in exon 7 of the survival motor neuron 1 (SMN1) gene who had had four copies of SMN2 exon 7. Muscle biopsy showed neurogenic features of groups of atrophic fibers, fiber-type grouping, and pyknotic nuclear clumps associated with fibers with rimmed vacuoles. Immunohistochemistry revealed sarcoplasmic aggregates of phosphorylated TDP-43 and p62 but not SMN. This study demonstrated myopathic changes with the accumulation of phosphorylated p62 and TDP-43 in the muscles of a patient with SMA, suggesting that abnormal protein aggregation may be involved in myopathic pathology.