Endogenous retroviruses are retroviruses derived from ancient infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome. Retroviruses are viruses that reverse-transcribe their RNA into DNA for integration into the host's genome. Most retroviruses (such as HIV-1) infect somatic cells, but some can also infect germline cells (cells that make eggs and sperm) and once they have done so and have been transmitted to the next generation, they are termed endogenous. Endogenous retroviruses can persist in the genome of their host for long periods. However, they are generally only infectious for a short time after integration as they acquire 'knockout' mutations during host DNA replication. They can also be partially excised from the genome by a process known as recombinational deletion. Many believe that they play a key role in evolution as well.
The human genome project found several thousand ERVs classified into 24 families.
Investigations also suggest possible HERV involvement in the HELLP syndrome and pre-eclampsia. There are many thousands of endogenous retroviruses within human DNA (HERVs comprise 8% of the human genome, with 98,000 elements and fragments). All appear to be defective, containing nonsense mutations or major deletions, and cannot produce infectious virus particles. This is because most are just long-lasting traces of the original virus, having first integrated many millions of years ago. However, there is one family of viruses that have been active since the divergence of humans and chimpanzees. This family, termed HERV-K(HML2), makes up less than 1% of HERV elements but is one of the most studied. There are indications it has even been active in the past few hundred thousand years, as some human individuals carry more copies of the virus family than others. But the absence of known infectious members of the HERV-K(HML2) family, and the lack of elements with a full coding potential within the published human genome sequence, suggests that the family is less likely to be active at present.
Researchers are also looking at a possible link between HERVs and schizophrenia.
In 2007, a collaborative group lead by Doug Nixon and Keith Garrison at the University of California San Francisco, and by Mario Ostrowski and Brad Jones at the University of Toronto, published a study providing evidence for T cell immune responses against HERVs in human immunodeficiency virus (HIV) infected individuals. The group hypothesized that HIV induces HERV expression in HIV infected cells, and that a vaccine targeting HERV antigens could therefore specifically eliminate HIV infected cells. The potential advantage of this novel approach is that, by using HERV antigens as surrogate markers of HIV infected cells, it could circumvent the difficulty inherent in directly targeting notoriously diverse and rapidly mutating HIV antigens.