Around 11% of HIV infections in Africa may be due to a genetic variant common in people of African descent that makes them more vulnerable to the virus. The genetic change, which is less prevalent in other ethnic groups, increases the likelihood of infection with the most common strain of the virus (HIV-1) by 40%.
Once infection has occurred, though, the genetic variant slows the progression of the disease, prolonging the patient's life by around two years.
The newly discovered genetic factor may go some way to explaining why AIDS is so prevalent in sub-Saharan Africa. According to the World Health Organisation, there were 4.3m people newly infected with HIV worldwide in 2006 and 2.9m deaths from AIDS-related illnesses. Around a third of all new infections and AIDS-related deaths occur in sub-Saharan Africa, where there are eight countries in which adult HIV prevalence exceeds 15% of the population.
Ironically, scientists believe that the genetic variant is at such high levels in Africa because it conferred resistance to a now extinct form of malaria.
A team of British and US researchers studied a group of 3,484 people in the US Air Force, of whom 1,266 were infected with HIV. They tested each for a gene variant called Duffy Antigen Receptor for Chemokines (DARC), which has been extensively studied in the past because of its ability to confer resistance to one form of the malaria parasite. The gene is known to be common among people of African descent.
The team report in the journal Cell, Host and Microbe that subjects who were DARC-negative were more likely to be infected with HIV. The gene variant appeared to make them 40% more susceptible to infection.
By extrapolating this figure to the number of people in Africa with the same genetic variation, the researchers estimate that 11% of all HIV infections in the continent are due to this increased susceptibility.
"The mystery of variable infection and progression was originally thought to be mainly the result of viral characteristics, but in recent years it has become evident that there is a strong host genetic component," said team member Dr Sunil Ahuja of the University of Texas Health Science Center in San Antonio.
"The big message of this paper is that something that protected people against malaria in the past is now leaving them more susceptible to HIV," said Robin Weiss of University College London, who also worked on the study.
