Spinocerebellar ataxia type 7 associated with pigmentary retinal dystrophy

Spinocerebellar ataxia type 7 (SCA7) is an autosomal-dominant, late-onset, slowly progressive disorder, primarily characterized by gradual loss of motor coordination, resulting from dysfunction and degeneration of the cerebellum and its connecting pathways. The disease is caused by expansion of a CAG trinucleotide repeat within the SCA7 gene, which encodes a polyglutamine tract within a novel protein, termed ataxin-7. The expansion of polyglutamine-encoding CAG repeats in dissimilar genes underlies eight neurodegenerative conditions besides SCA7, including a number of dominant ataxias related to SCA7. Although elongated polyglutamine itself can initiate neuronal dysfunction and death, its toxicity is modulated by the context of the disease proteins, as evidenced by the differing clinical and pathological presentation of the various disorders. In this respect, it is exciting that SCA7 constitutes the only polyglutamine disorder, in which the photoreceptors of the retina are also severely affected, leading to retinal degeneration and blindness. Since the discovery of the SCA7 mutation, numerous studies attempted to pinpoint the molecular mechanisms underlying the unique features of SCA7, particularly the retinal involvement. Here we summarize the clinical, pathological, and genetic aspects of SCA7, and review the current understanding of the pathogenesis of this disorder.