Genetic localization of the Ca²⁺ channel gene CACNG2 near SCA10 on chromosome 22q13

Purpose: Voltage-dependent calcium channel mutations have been associated with spinocerebellar ataxia in humans (SCA6) and with ataxia, progressive cerebellar degeneration, and epilepsy in mice (tottering, lethargic, and stargazer). A novel autosomal dominant spinocerebellar ataxia syndrome with epilepsy (SCA10) was recently mapped to chromosome 22q13. The human ortholog of the mouse stargazer locus, the calcium channel γ subunit gene CACNG2, also is located in this region. Because the phenotypes of stargazer mice and SCA10 patients were similar, consisting of both cerebellar ataxia and seizures, we hypothesized that CACNG2 was a likely candidate for the SCA10 locus. Methods: Polymerase chain reaction (PCR) based assays were developed for two polymorphic microsatellite markers near CACNG2. The location of CACNG2 was determined by linkage and haplotype analysis of the genotypes of 22 individuals from a human pedigree segregating SCA10. Results: SCA10 was previously localized distal to marker D22S1177 on chromosome 22q13. We determined that CACNG2 was linked to D22S283 and D22S1177 with the marker order: centromere-D22S283-bcmDLB1 (CACNG2)-D22S1177-D22S423-telomere. Thus CACNG2 is located proximal to the SCA10 recombinant interval. Conclusions: Here we report the first genetic linkage of CACNG2 on chromosome 22q13 and exclude it as a candidate for SCA10. In addition, our data clarify the relation between the physical and genetic linkage maps of this region and will facilitate isolation of the SCA10 gene.