Over 8% of the human genome is made up of endogenous retroviruses (ERVs). ERVs are ancient retroviruses that inserted their own DNA into hosts, and over the course of hundreds of thousand of years of mutations they lost the ability to replicate in cells. Most of these insertions do nothing inside our genomes. However, new evidence is emerging that some ERVs “wake up” and are transcribed into RNA by the host. In 2015, researchers were studying HERVK, one of the most recent viral additions to the human genome, and discovered that it can induce viral restriction pathways in early stage embryonic cells and help protect a growing fetus against viral invaders.
HERVK is expressed during days 5-6 of embryonic development post fertilization. Its expression was discovered when researchers noted the presence of viral proteins and virus-like particles (VLPs) in the blastocyst (Fig 1).
When activated, HERVK produced as small accessory protein called Rec, which upregulates binding and exporting of viral mRNAs from the cell. Viral mRNA export likely up-regulates innate antiviral responses, which may stop other viruses from infecting the cells. Although researchers caution that there is not enough evidence to make conclusions yet, they are excited by the possibility that ancient viruses are shaping human development, and that they may even have a positive impact on the embryo as it tries to ward off other invading viruses.
Grow, E. J., Flynn, R. A., Chavez, S. L., Bayless, N. L., Wossidlo, M., Wesche, D. J., . . . Wysocka, J. (2015). Intrinsic retroviral reactivation in human preimplantation embryos and pluripotent cells. Nature, 522(7555), 221-225. doi:10.1038/nature14308
New York Times article:
Elisa Hofmeister ‘18