The Case for Selection at CCR5-Δ32

The C-C chemokine receptor 5, 32 base-pair deletion (CCR5- Δ 32) allele confers strong resistance to infection by the AIDS virus HIV. Previous studies have suggested that CCR5- Δ 32 arose within the past 1,000 y and rose to its present high frequency (5%–14%) in Europe as a result of strong positive selection, perhaps by such selective agents as the bubonic plague or smallpox during the Middle Ages. This hypothesis was based on several lines of evidence, including the absence of the allele outside of Europe and long-range linkage disequilibrium at the locus. We reevaluated this evidence with the benefit of much denser genetic maps and extensive control data. We find that the pattern of genetic variation at CCR5- Δ 32 does not stand out as exceptional relative to other loci across the genome. Moreover using newer genetic maps, we estimated that the CCR5- Δ 32 allele is likely to have arisen more than 5,000 y ago. While such results can not rule out the possibility that some selection may have occurred at C-C chemokine receptor 5 (CCR5), they imply that the pattern of genetic variation seen at CCR5- Δ 32 is consistent with neutral evolution. More broadly, the results have general implications for the design of future studies to detect the signs of positive selection in the human genome. Sabeti and colleagues use dense genetic maps to show that the HIV-resistance CCR5- Δ 32 allele is more than 5,000 years old and is likely to have been under mainly neutral selection.