Clinical and Genetic Analyses of SPG7 in Japanese Patients with Undiagnosed Ataxia

Objective Spastic paraplegia 7 (SPG7) is an autosomal recessive neurodegenerative disorder caused by biallelic pathogenic variants in SPG7. It is predominantly characterized by adult-onset slowly progressive spastic paraparesis. While SPG7 presenting with ataxia with or without spasticity is relatively common in Europe and North America, it is considered rare in Japan. This study aimed to identify SPG7 patients among those with undiagnosed ataxia within the Japanese population.
Methods We retrospectively selected 351 patients with undiagnosed ataxia, excluding those with secondary and common spinocerebellar ataxia. Whole-exome sequence analysis was conducted, and homozygosity of the identified variants was confirmed using droplet digital polymerase chain reaction (ddPCR).
Results Among the 351 patients, 2 were diagnosed with SPG7, and homozygosity was confirmed by ddPCR. Both patients carried homozygous pathogenic variants in SPG7: c.1948G>A, p.Asp650Asn, and c.1192C>T, p.Arg398Ter (NM_003119.4). Clinically, both patients presented with progressive ataxia. In addition, Patient 1 exhibited partial ophthalmoplegia and spastic paraparesis, whereas Patient 2 demonstrated cerebellar ataxia without spasticity.
Conclusion The rarity of SPG7 in Japan may be attributed to variation in the minor allele frequency of the c.1529C>T, p.Ala510Val variant, which is more prevalent in Europe and North America than in other areas.