Objective: To assess the contribution of noncoding repeat expansions in Japanese patients with amyotrophic lateral sclerosis (ALS).
Methods: Sporadic ALS in Western countries is frequently associated with noncoding repeat expansions in the C9ORF72 gene. Spinocerebellar ataxia type 8 (SCA8) is another noncoding repeat disease caused by expanded CTA/CTG repeats in the ATXN8OS gene. Although the involvement of upper and lower motor neurons in SCA8 has been reported, a positive association between SCA8 and ALS remains unestablished. Spinocerebellar ataxia type 36 is a recently identified disease caused by noncoding repeat expansions in the NOP56 gene and is characterized by motor neuron involvement. We collected blood samples from 102 Japanese patients with sporadic ALS and analyzed the ATXN8OS gene by the PCR-Sanger sequencing method and the C9ORF72 and NOP56 genes by repeat-primed PCR assay.
Results: Three patients with ALS (3%) had mutations in the ATXN8OS gene, whereas no patient had a mutation in the C9ORF72 or NOP56 gene. The mutation-positive patients were clinically characterized by neck weakness or bulbar-predominant symptoms. None of our patients had apparent cerebellar atrophy on MRI, but 2 had nonsymptomatic abnormalities in the white matter or putamen.
Conclusions: Our finding reveals the importance of noncoding repeat expansions in Japanese patients with ALS and extends the clinical phenotype of SCA8. Three percent seems small but is still relatively large for Japan, considering that the most commonly mutated genes, including the SOD1 and SQSTM1 genes, only account for 2%-3% of sporadic patients each.