Searching for modulating effects of SCA2, SCA6 and DRPLA CAG tracts on the Machado-Joseph disease (SCA3) phenotype

Context – Machado–Joseph disease (MJD/SCA3) is an autosomal dominant cerebellar ataxia of adult onset. The variability in age at onset and the complex and heterogeneous neurologic findings indicate that MJD, caused by a major gene, is modulated by modifier factors. Objective – To study if the polymorphic CAG repeats at other loci (namely, SCA2, SCA6 and DRPLA) thus acted as modifier factors of this disease. Design – Case–control. Setting – Ambulatory care in a referral center. Patients – A convenience sample of 39 unrelated, Brazilian patients with MJD. Main outcome measures: age of onset, anticipation, clinical subtypes and neurological findings. Results – Fasciculations were associated with CAG repeat length of the long SCA2 allele (Mann–Whitney U-test, P < 0.03, after Bonferroni procedure). Other measures (age of onset, anticipation, clinical types and other neurological signs) were not associated with CAG repeat length of SCA2, SCA6 and DRPLA genes. Conclusions – The present results show that the CAG tract of SCA2 gene interferes with MJD phenotype. Further studies, with patients of other origins and with typing of other (CAG)n loci, are necessary.