Homozygosity mapping of spinocerebellar ataxia with cerebellar atrophy and peripheral neuropathy to 9q33-34, and with hearing impairment and optic atrophy to 6p21-23.

With the availability of a simple molecular test that distinguishes Friedreich ataxia, the most frequent form of inherited ataxia, from other recessive ataxias, it now becomes possible to unravel the genetic heterogeneity of the latter. We have now localised two genes causing autosomal recessive spinocerebellar ataxia in two consanguineous families. In the first family, the four affected Japanese sibs had spinocerebellar ataxia associated with elevated levels of serum creatine kinase, gamma-globulin, and alpha-foetoprotein. Homozygosity over a 20 cM region allowed to demonstrate linkage at 9q33.3-34.3 with a lod score of 3.0. Genotyping two unrelated Japanese patients from first degree consanguineous parents revealed that one was homozygous for the same region but did not share the biochemical features. In the second family, an Israeli uncle and a niece were affected by an early-onset recessive ataxia and subsequently developed hearing impairment and optic atrophy. Homozygosity over a 17 cM region allowed demonstration of linkage at 6p21-23 with a lod score of 3.25. These two localisations of autosomal recessive ataxia genes represent a first step toward the identification of genetically homogenous, non-Friedreich, ataxic patients and subsequent cloning of the genes.