By dating the integration of specific ERV loci in the genomes of 40 mammalian species, including the chimpanzee, dolphin, and even the nine-banded armadillo, Magiorkinis and colleagues found that humans and other primates have acquired far fewer ERVs compared to the other mammals surveyed. The authors observed a four-fold decline in ERV insertions in primate genomes over the last 10 million years. In humans, this major decline in ERVs is mainly due to the extinction of the massive HERV-H family, which is responsible for ~80% of ERV integrations in the human genome within the last 30 million years. Where the genomes of other primate species would acquire at least one new ERV lineage during this timeframe, the human genome is curiously absent of new ERVs.
Instead, the ancient HERV-K (HML-2) subfamily had continued to replicate in the human genome until at least a few hundred thousand years ago. HERV-K (HML-2) is potentially medically significant – it is polymorphic in the human population, meaning that some individuals have HERV-K (HML-2) loci that other individuals may not have – and the ERV itself and its gene products are expressed in HIV-1 infection and a number of cancers. It is currently unclear whether HERV-K (HML-2) has any direct role in human disease pathogenesis. The HERV-H family is also of particular interest – specific HERV-H long terminal repeats (LTRs) have recently been identified as having a functional role in maintaining pluripotency in human embryonic stem cells (hESCs). It is unknown if the extinction of HERV-H and the host appropriation of HERV-H sequences in hESC gene regulation is connected or a merely a coincidence.
The authors speculate that the lack of new ERVs in human genomes is partly due to a cultural shift in humans that prevented the acquisition and the transmission of novel retrovirus infections. This could be a result of decreased exposure to retrovirus-infected blood from consumption of infected meat, and from wounds gained during violent male-male conflict. This research also suggests that over the last few million years, the human immune system had become more effective in eradicating infection by exogenous retroviruses, compared to other mammals. The HIV-1 pandemic is relatively recent, and its high pathogenicity in humans contrasts sharply against the many SIVs that endemically infect non-human primates, reflecting the long evolutionary co-existence of non-human primates and retroviruses. This new study is published today in the journal Retrovirology.
Read the abstract here: The decline of human endogenous retroviruses: extinction and survival
University of Oxford press release: Fewer viral relics may be due to a less bloody evolutionary history
Our Inner Viruses: Forty Million Years in the Making by Carl Zimmer
-- Audrey Lin