Historic, Demographic, and Genetic Evidence for Increased Population Frequencies of CCR5Δ32 Mutation in Croatian Island Isolates after Lethal 15th Century Epidemics

Aim To assess the frequency of 32 base pair deletion in CCR5 (CCR5 Delta 32), which has been shown to confer resistance to HIV infection in a homozygous form, in 10 isolated island communities of Dalmatia, Croatia, with different histories of exposure to epidemics during and since the medieval period. Methods In 2002, DNA analysis of 100 randomly selected individuals from each of the 10 isolated communities of 5 Croatian islands (Susak, Rab, Vis, Lastovo, and Mljet) showed high levels of 3-generational endogamy, indicating limited gene flow. Five of the communities were decimated by epidemics of unknown cause between 1449-1456, while the other 5 villages remained unaffected. Genotyping of the CCR5 gene was performed using the polymerase chain reaction method with primers flanking the region containing 32-bp deletion. Results The frequency of CCR5 Delta 32 in the 5 villages affected by the epidemic was 6.1-10.0%, and 1.0-3.8% in the 5 unaffected villages. The Delta 32 mutation was found in 71 of 916 alleles among the individuals from the affected villages (7.5%), and in 24 of 968 alleles in unaffected villages (2.5%, chi(2) = 27.3, P < 10(-6)). A previous study in 303 random Croatian blood donors showed the frequency of the CCR5 Delta 32 of 7.1% in the general population. The difference remained significant after correcting for population structure using both STRAT and STRUCTURE software and the genomic control test, to ensure results do not arise from the background genetic differences. Conclusion Our results and historical evidence, suggest that the mid-15th century epidemic could have acted as a selection pressure for the CCR5 Delta 32 mutation.