EXPANDED CAG REPEATS IN SPINOCEREBELLAR ATAXIA (SCA1) SEGREGATE WITH DISTINCT HAPLOTYPES IN SOUTH-AFRICAN FAMILIES

The autosomal dominant late onset spinocerebellar ataxias (SCAs) are g enetically heterogeneous. Three genes, SCA1 on 6p, SCA2 on 12q and MJD 1 on 14q, have been isolated for SCA1, SCA2 and Machado-Joseph disease (MJD), respectively. In these three autosomal dominant disorders the mutation is an expanded CAG repeat. Evidence for heterogeneity in fami lies not linked to the SCA1, SCA2 and MJD loci is provided by the mapp ing of SCA loci to chromosomes 16q, 11cen and 3p. A total of 14 South African kindreds and 22 sporadic individuals with SCA were investigate d for the expanded SCA1 and MJD repeats. None of the families nor the sporadic individuals showed expansion of the MJD repeat. Expanded SCA1 and CAG repeats were found to cosegregate with the disorder in six of the families tested and were also observed in one sporadic individual with a negative family history of SCA. The use of the microsatellite markers D6S260, D6S89 and D6S274 provided evidence that the expanded S CA1 repeats segregated with three distinct haplotypes in the six famil ies. Use of the highly polymorphic tightly linked microsatellite marke rs is still important as this stage, particularly where this coincides with the possibility of a homozygous genotype with the trinucleotide repeat marker. Importantly, our molecular findings indicate: (1) an ab sence of MJD expanded repeats underlying SCA; (2) the major disease in this group is due to mutations in the SCA1 gene; and (3) the familial disorder in the majority population group (i.e. mixed ancestry) in th e Western Cape region of South Africa is most likely to be the result of two distinct founder events.