Modiano D, Bancone G, Ciminelli BM, Pompei F, Blot I, Simporé J, Modiano G.
Hum Mol Genet. 2008 Mar 15 ;17(6):789-99. Epub 2007 Nov 28. Erratum in : Hum Mol Genet. 2008 Jul 1 ;17(13):2070.
PMID : 18048408 [PubMed - indexed for MEDLINE]
Haemoglobin S (HbS ; beta6Glu—>Val) and HbC (beta6Glu—>Lys) strongly protect against clinical Plasmodium falciparum malaria. HbS, which is lethal in homozygosity, has a multi-foci origin and a widespread geographic distribution in sub-Saharan Africa and Asia whereas HbC, which has no obvious CC segregational load, occurs only in a small area of central West-Africa. To address this apparent paradox, we adopted two partially independent haplotypic approaches in the Mossi population of Burkina Faso where both the local S (S(Benin)) and the C alleles are common (0.05 and 0.13). Here we show that : both C and S(Benin) are monophyletic ; C has accumulated a 4-fold higher recombinational and DNA slippage haplotypic variability than the S(Benin) allele (P = 0.003) implying higher antiquity ; for a long initial lag period, the C alleles did apparently remain very few. These results, consistent with epidemiological evidences, imply that the C allele has been accumulated mainly through a recessive rather than a semidominant mechanism of selection. This evidence explains the apparent paradox of the uni-epicentric geographic distribution of HbC, representing a ’slow but gratis’ genetic adaptation to malaria through a transient polymorphism, compared to the polycentric ’quick but costly’ adaptation through balanced polymorphism of HbS.