The wide spectrum of spinocerebellar ataxias (SCAs)

Mario-Ubaldo Manto

Spinocerebellar ataxias (SCAs) are a clinically heterogeneous group of disorders. Current molecular classification corresponds to the order of gene description (SCA1-SCA 25). The prevalence of SCAs is estimated to be 1-4/100,000. Patients exhibit usually a slowly progressive cerebellar syndrome with various combinations of oculomotor disorders, dysarthria, dysmetria/kinetic tremor, and/or ataxic gait. They can present also with pigmentary retinopathy, extrapyramidal movement disorders (parkinsonism, dyskinesias, dystonia, chorea), pyramidal signs, cortical symptoms (seizures, cognitive impairment/behavioral symptoms), peripheral neuropathy. SCAs are also genetically heterogeneous and the clinical diagnosis of subtypes of SCAs is complicated by the salient overlap of the phenotypes between genetic subtypes. The following clinical features have some specific values for predicting a gene defect: slowing of saccades in SCA2, ophthalmoplegia in SCA1, SCA2 and SCA3, pigmentary retinopathy in SCA7, spasticity in SCA3, dyskinesias associated with a mutation in the fibroblast growth factor 14 (FGF 14) gene, cognitive impairment/behavioral symptoms in SCA17 and DRPLA, seizures in SCA10, SCA17 and DRPLA, peripheral neuropathy in SCA1, SCA2, SCA3, SCA4, SCA8, SCA18 and SCA25. Neurophysiological findings are compatible with a dying-back axonopathy and/or a neuronopathy. Three patterns of atrophy can be identified on brain MRI: a pure cerebellar atrophy, a pattern of olivopontocerebellar atrophy, and a pattern of global brain atrophy. A remarkable observation is the presence of dentate nuclei calcifications in SCA20, resulting in a low signal on brain MRI sequences. Several identified mutations correspond to expansions of repeated trinucleotides (CAG repeats in SCA1, SCA2, SCA3, SCA6, SCA7, SCA17 and DRPLA, CTG repeats in SCA8). A pentanucleotide repeat expansion (ATTCT) is associated with SCA10. Missense mutations have also been found recently. Anticipation is a main feature of SCAs, due to instability of expanded alleles. Anticipation may be particularly prominent in SCA7. It is estimated that extensive genetic testing leads to the identification of the causative gene in about 60-75 % of cases. Our knowledge of the molecular mechanisms of SCAs is rapidly growing, and the development of relevant animal models of SCAs is bringing hope for effective therapies in human.