[Frequencies of triplet repeat disorders in dominantly inherited spinocerebellar ataxia (SCA) in the Japanese].

A total of 500 SCA patients were examined at the Department of Neurology, Hokkaido University, from 1982 to 1997 (16 years), and 42.2% of them showed positive family history for SCA. Their diagnoses were re-evaluated, based on the current diagnostic criteria including SCA genotyping. A total of 228 patients consisted of MJD (24.6%), SCA6 (11.8%), SCA1 (10.5%), SCA2 (4.4%), DRPLA (0.004%), pure familial spastic paraplegia (FSP, 5.7%), complicated FSP (4.8%), Friedreich ataxia (FRDA)-like recessive SCA (1.8%), "ADCA I" (12.7%), "ADCA III" in which SCA genotypes were not determined (13.6%), and "ADCA III" whose mutations were different from SCA6 (9.6%). Since our "ADCA I" mostly showed MJD phenotype and approximately half of genotyped "ADCA III" was found to be SCA6, both MJD and SCA6 were estimated to be the most prevalent dominant SCA in our subjects. There was no SCA7, or FRDA with unstable GAA repeat expansion. DRPLA has been considered rather prevalent SCA in Japan, however, it was exceedingly rare in our subjects, indicating that the prevalence is different even within Japan.