Novel in-frame duplication variant of SOD1 in a Japanese family with familial amyotrophic lateral sclerosis

Objectives: To analyze the cases of a family with a novel in-frame duplication variant (NM_000454.5:c.357_357 + 2dup, p.Val120dup) of SOD1 and a structural model of the mutated SOD1 protein. Methods: The clinical profiles of three patients in the family were analyzed, including the neuropathological findings of the proband’s mother. Genetic analyses were conducted for three patients. cDNA and in silico structural analyses were performed to evaluate the effects of duplication variants on the structure of SOD1. Results: The clinical features of the patients included predominant involvement of the lower motor neurons, asymmetric onset of motor symptoms in the lower limbs, and a relatively rapid progression of muscular weakness and respiratory insufficiency. Neuropathological findings revealed severe loss of spinal cord motor neurons, and immunohistochemistry using an anti-misfolded SOD1 antibody revealed aggregates in the spinal cord. Genetic analyses revealed a c.357_357 + 2dup at the exon 4–intron 4 boundary of SOD1 in three patients. cDNA analysis of the proband suggested the presence of a valine (p.Val120dup) duplication in the heterozygous state, and the SOD1 transcript level showed no significant differences from those of healthy controls. In silico structural analyses predicted that p.Val120dup could affect the structure of the β-barrels and copper ion binding site of SOD1, suggesting an abnormal conformation of SOD1 that is predicted to interfere with the binding of copper ions. Conclusion: We identified a novel in-frame duplication variant in the C-terminus of β7 of SOD1. This genotype–structure–phenotype study of SOD1 provides valuable insights into disease-causing mechanisms.