The CCR5 gene encodes a chemokine receptor (a long name for a protein that sits in the walls of our cells). When the body has been invaded by a pathogen such as a cold virus, CCR5 plays an important role in fighting that virus. Smart viruses such as HIV-1, however, hijack the CCR5 protein and use it to sneak into CD4+ T cells & macrophages.
In some populations the CCR5 gene has experienced a mutation that deleted 32 basepairs in the gene sequence. The mutation prevents the expression of the protein on the cell surface. As a result, people with this mutation show some degree of protection from certain viruses. In fact, homozygosity of the CCR5-D32 allele (meaning BOTH copies of the gene are mutated) leads to “nearly complete resistance to HIV-1 infection.” People with only 1 copy are as much as 70% resistant! Surprisingly, homozygotes do not show any other problems as a result of the mutation.
The CCR5-D32 allele is absent among Amerindians and East Asians but is found in some African populations. The allele is found in high concentration among Eurasians. Indeed, the average frequency of the allele among European populations is 10%. The fact that the allele is found in such a high percentage in one population and not at all in another suggests that there was a strong selective pressure in favor of that allele. The source of that genetic pressure is still being debated by scientists. The early favorite was the Black Death, the Bubonic plague that swept through Europe in the Fourteenth century. The picture above is of Yersinia pestis, the bacterium that causes the plague.
Other researchers have hypothesized that the period of selective pressure created by the Bubonic plague, roughly 400 years, was not long enough for the overwhelming presence of the gene to appear. These researchers suggest that smallpox, which has killed as many people as the plague over a much longer time period, could have provided sufficient selective pressure. To support this conclusion it is proposed that the continued presence of the mutated gene in the face of lowered genetic fitness is due to the fact that smallpox was only eradicated recently while the plague hasn’t been a serious problem in 250 years. Indeed, a cousin of the smallpox virus has been shown to use CCR5 to enter host cells. Thus, CCR5-D32 may provide resistance to smallpox.
So what does this have to do with genetic genealogy? Family Tree DNA offers CCR5 testing through their advanced test options (meaning you have to already have your DNA stored at the company). If a person is tested and has one or two copies of the CCR5-D32 allele, it is very likely that the person has European ancestry.
I would caution that CCR5 testing should NOT be used as a diagnostic test for medical conditions. In my opinion the CCR5 test should be only be used to explore one’s ancestry. The presence of the CCR5-D32 allele implies that a person has European ancestry, NOT that they have a license to engage in risky behavior!