Extended gene diversity at the FMR1 locus and neighbouring CA repeats in a sub-Saharan population.

Abstract

We report on the allele distributions in a normal black African population at two microsatellite loci neighbouring the FRAXA locus and at the CGG repeat in the 5' end of the FMR-1 gene, which causes the fragile X syndrome. The CGG repeat distribution was found to be similar to that of other ethnic groups, as well as to that of other nonhuman primates, possibly predicting a comparable prevalence of fragile X in Africa. Significant linkage disequilibrium has been observed between fragile X mutations and alleles of the DXS548 and FRAXAC1 loci in European and Asian populations, and some founder chromosomes may be extremely old. Those associated with FRAXAC1-A and DXS548-2 alleles are not present in the Asian fragile X samples. We searched for these alleles and their frequency in the well defined Bamileke population of Cameroon. All previously described alleles and some new ones were found in this sample, supporting the hypothesis of their pre-existence and subsequent loss in Asian populations. Finally, the heterozygosity of the Bamileke sample was significantly higher at both marker loci and comparable to that of Europeans at the CGG repeat, confirming the notion that genetic diversity is greater in Africans than in other groups and supporting the view that evolution of modern man started in Africa.