Adult onset spinocerebellar ataxia linked to HLA in a South African kindred of mixed ancestry

Hereditary spinocerebellar ataxia (SCA) is a relatively common disorder in the Western Cape region of South Africa. At present there are no genetic markers available for prenatal or presymptomatic diagnosis. A large kindred of mixed ancestry with late onset SCA was studied in which the disorder segregated in an autosomal dominant fashion. HLA typing was undertaken on 44 family members, and the HLA haplotypes were assigned on the basis of segregation. The LIPED computer program, with a correction factor allowing for the age of onset, was used to analyze the pedigree for linkage to HLA. Of 22 individuals in whom disease status could be definitely assessed, only one recombinant between HLA and the SCA locus occurred. The lod score reached a maximum of 4.13 at a recombination fraction of 0.05, indicating the odds to be approximately 13,500 to 1 in favor of linkage between HLA and the putative disease allele for SCA. A possible recombination within the HLA region suggested that the disease allele lies telomeric of the HLA region. In view of the recent demonstration of tight linkage between SCA1 and D6S89, however, HLA should not be used for presymptomatic diagnosis or genetic counselling.