A novel autosomal dominant spinocerebellar ataxia (SCA22) linked to chromosome 1p21-q23.

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Abstract
The autosomal dominant cerebellar ataxias (ADCA) are a clinically, pathologically and genetically heterogeneous group of disorders. Ten responsible genes have been identified for spinocerebellar ataxia types SCA1, SCA2, SCA3, SCA6, SCA7, SCA8, SCA10, SCA12 and SCA17, and dentatorubral pallidoluysian atrophy (DRPLA). The mutation is caused by an expansion of a CAG, CTG or ATTCT repeat sequence of these genes. Six additional loci, SCA4, SCA5, SCA11, SCA13, SCA14 and SCA16 have also been mapped. The growing heterogeneity of the autosomal dominant forms of these diseases shows that the genetic aetiologies of at least 20% of ADCA have yet to be elucidated. We ascertained and clinically characterized a four‐generation Chinese pedigree segregating an autosomal dominant phenotype for cerebellar ataxia. Direct mutation analysis, linkage analysis for all known SCA loci and a genome‐wide linkage study were performed. Direct mutation analysis excluded SCA1, 2, 3, 6, 7, 8, 10, 12, 17 and DRPLA, and genetic linkage analysis excluded SCA4, 5, 11, 13, 14 and 16. The genome‐wide linkage study suggested linkage to a locus on chromosome 1p21‐q23, with the highest two‐point LOD score at D1S1167 (Zmax = 3.46 at θ = 0.00). Multipoint analysis and haplotype reconstruction traced this novel SCA locus (SCA22) to a 43.7‐cM interval flanked by D1S206 and D1S2878 (Zmax = 3.78 under four liability classes, and 2.67 using affected‐only method). The age at onset ranged from 10 to 46 years. All affected members had gait ataxia with variable features of dysarthria and hyporeflexia. Head MRI showed homogeneous atrophy of the cerebellum without involvement of the brainstem. In six parent–child pairs, median onset occurred 10 years earlier in offspring than in their parents, suggesting anticipation. This family is distinct from other families with SCA and is characterized by a slowly progressive, pure cerebellar ataxia.