The chemokine receptor CCR5 is encoded by the CMKBR5 gene located on the p21.3 region of human chromosome 3, and constitutes the major co-receptor for the macrophage-tropic strains of HIV-1. A mutant allele of the CCR5 gene, Δccr5, was shown to provide to homozygotes with a strong resistance against infection by HIV. The frequency of the Δccr5 allele was investigated in 18 European populations. A North to South gradient was found, with the highest allele frequencies in Finnish and Mordvinian populations (16%), and the lowest in Sardinia (4%). Highly polymorphic microsatellites (IRI3.1, D3S4579 and IRI3.2, D3S4580) located respectively 11 kb upstream and 68 kb downstream of the CCR5 gene deletion were used to determine the haplotype of the chromosomes carrying the Δccr5 variant. A strong linkage disequilibrium was found between Δccr5 and specific alleles of the IRI3.1 and IRI3.2 microsatellites: > 95% of the Δccr5 chromesomes carried the IRI3.1-0 allele, while 88% carried the IRI3.2-0 allele. These alleles were found respectively in only 2 or 1.5% of the chromosomes carrying a wild-type CCR5 gene. From these data, it was inferred that most, if not all Δccr5 alleles originate from a single mutation event, and that this mutation event probably took place a few thousand years ago in Northeastern Europe. The high frequency of the accr5 allele in Caucasian populations cannot be explained easily by random genetic drift, suggesting that a selection advantage is or has been associated with homo- or heterozygous carriers of the Δccr5 allele.
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