HIV-Like Viruses Found in Africa for 16 Million Years?
The lentiviruses are a small group of retroviruses to which HIV and its primate equivalent (SIV) belong. Since they can have such a profound effect on human health, the lentiviruses have attracted a great deal of research attention and earlier this month (August 2015) a new paper was published revealing the unexpectedly long evolutionary history of these viruses.
It is usually near impossible to track the evolution of disease over geological time scales. Viruses don’t fossilise and the bones of their victims are usually untouched by their effects. This can make it very difficult to study the antiquity of disease but luckily for researchers there is another way to investigate the problem. Viruses and their hosts are locked in a perpetual arms race of attack and defence and this process leaves its mark on the DNA of the hosts. In the case of the lentiviruses, there is a particular gene called TRIM5 which produces the TRIM5 protein to protect host cells. TRIM5 works by bonding to the outer surface of the virus particle once it enters a host cell, which prevents it from replicating itself. In the case of HIV, the human TRIM5 protein can’t bond to the virus’s surface which is why we aren’t immune to the disease, but in primates different TRIM5 variants give immunity against different SIVs. Since TRIM5 only works on lentiviruses, unlike other molecules in the immune system which can have multiple functions, the researchers realised that by tracking changes to the TRIM5 gene sequences across the primates they could track the evolution of the SIVs it protects against. In order to do this they collected sequences from 22 Old World primate species and compared them. The analysis revealed a sudden cluster of unique adaptations to the TRIM5 gene in a subset of African primates called the Cercopithecine, to which the macaques and baboons belong, but not in their close relatives, the Colobinae. This strongly suggested that the lentiviruses first began colonising Africa around 11-16 million years ago, when the Cercopithecine first diverged from the other Old World monkeys.
Further lab work helped confirm this discovery. The researchers were able to resurrect the ancient TRIM5 gene, and its associated protein, and showed that it did indeed confer an advantage against the Cercopithecine strain of SIV but had no effect at all on other modern lentiviruses. This is exactly what the genetic results had predicted and proves that these viruses have been endemic to primates for far longer then previously suspected.
Reference: McCarthy, K.R., et al. 2015. Evolutionary and Functional Analysis of Old World Primate TRIM5 Reveals the Ancient Emergence of Primate Lentiviruses and Convergent Evolution Targeting a Conserved Capsid Interface. PLoS One. DOI: 10.1371/journal.ppat.1005085
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