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A genetic variation which evolved to protect people of African descent against malaria has now been shown to increase their susceptibility to HIV infection by up to 40 per cent, according to new research. Conversely, the same variation also appears to prolong survival of those infected with HIV by approximately two years.
HIV affects 25 million people in sub-Saharan Africa today, an HIV burden greater than any other region of the world. Around 90 per cent of people in Africa carry the genetic variation, meaning that it may be responsible for an estimated 11 per cent of the HIV burden there. The authors observe that sexual behaviour and other social factors do not fully explain the large discrepancy in HIV prevalence in populations around the world, which is why genetic factors are a vital field of study.
The full paper is Duffy Antigen Receptor for Chemokines Mediates trans-Infection of HIV-1 from Red Blood Cells to Target Cells and Affects HIV-AIDS Susceptibility. This shouldn’t surprise; there are other genes which can explain HIV susceptibility. It seems plausible that many recessive diseases, such as cystic fibrosis, are evolutionary relics of selection pressures past. Disparate selection pressures and the constant reconfiguration of the adaptive landscape means there are going to be many variations across time and space which we’re going to have to keep track of. Remember the truism that 85% of genetic variation on any given locus is within populations and only 15% between? This is one of those loci where this isn’t true. Almost no Europeans have the nonfunctional variant of DARC, while the majority of Africans lack the functional variant. From Wikipedia:
The Duffy antigen/chemokine receptor gene (DARC) is composed of a single exon. Most Duffy negative blacks carry a silent Fy-b allele with a single T to C substitution at nucleotide -46, impairing the promoter activity in erythroid cells by disrupting a binding site for the GATA1 erythroid transcription factor. The gene is still transcribed in non erythroid cells in the presence of this mutation.
Differences in the racial distribution of the Duffy antigens were discovered in 1954 when it was found that the majority of blacks had the erythrocyte phenotype Fy(a-b-): 68% in African Americans and 88-100% in African blacks (including more than 90% of West African blacks). This phenotype is exceedingly rare in whites.
The silent allele has evolved at least twice in the black population of Africa and evidence for selection for this allele has been found. The selection pressure involved here appears to be more complex than many text books might suggest. An independent evolution of this phenotype occurred in Papua New Guinea has also been documented.