SCA27B in Brazil: frequency, phenotype and genotype–phenotype correlations

BACKGROUND: Spinocerebellar Ataxia 27B (SCA27B) is a recently described autosomal dominant ataxia caused by uniallelic GAA intronic expansions at FGF14. It is a frequent SCA subtype in North American/European populations, accounting for > 20% of all SCAs in some series. Despite that, its frequency as well as phenotype in Latin America remains to be established. OBJECTIVES: To determine the frequency and the clinical phenotype of SCA27B in a large Brazilian SCA cohort. METHODS: We recruited 498 SCA patients from 322 unrelated families followed in a reference center. All patients had demographic and clinical data collected. The estimated disease progression rate was computed as the ratio between the Scale for the Assessment and Rating of Ataxia (SARA) score and disease duration (in years). Genetic testing included long-range and triplet-primed PCR-based approaches to diagnose SCA1, 2, 3, 6, 7 and SCA27B. RESULTS: SCA27B was identified in 9 out of the 322 index-patients, totaling 2.8% of all cases. It stands as the fifth most common SCA, surpassed by SCAs 3, 1, 2, and 7, respectively. The typical phenotype in our cases was similar to previous descriptions: late onset (mean age 55.5 years), slow progression (1.0 points/year) and relatively pure ataxic phenotype (11/12). In this cohort, the estimated disease progression rate did correlate with age at onset, but not with (GAA)n. DISCUSSION: SCA27B is a prevalent SCA in Brazil, but the relative frequency seems to be smaller than in Europe/Canada. It should be included in SCA routine diagnostic protocols. Age at onset might be a potential prognostic marker in this condition.