Autosomal dominant cerebellar ataxia (ADCA) associated with progressive macular degeneration, also designated olivo-ponto-cerebellar-atrophy type III (Konigsmark and Weiner, 1970) or ADCA type II (Harding, 1982, 1993), was initially described by Froment et al. (1937) and is characterized by heterogeneous clinical presentation, disease severity, and age at onset (Benomar et al., 1994; Enevoldson et al., 1994). Because of the retinopathy observed in most patients, this form of ADCA was considered to be a separate entity (OMIM database entry: 164500).
Identification of the SCA7 gene and the responsible mutation
Three independent groups mapped the responsible gene, designated SCA7 (spinocerebellar ataxia 7), to chromosome 3p and postulated that a trinucleotide CAG repeat expansion might be involved in this disease, because the phenomenon of anticipation is particularly marked in SCA7 (_20 years per generation; Gouw et al., 1995; Benomar et al., 1995; Holmberg et al., 1995).
A pure (CAG)10 repeat, located between markers D3S1600 and D3S1287, was found in a subclone of yeast artificial chromosome (YAC) 88-d-9 (David et al., 1997; Del-Favero et al., 1998). Polymerase chain reaction (PCR) analysis with primers flanking this CAG repeat showed the presence of an expansion in SCA7 patients (David et al., 1997; Del-Favero et al., 1998) that was confirmed by a strategy derived from the rapid expansion detection (RED) technique (Koob et al., 1998).