The Role of Endogenous Retroviruses in Placental Gene Expression
By Anna Harnsberger
Introduction
The role of integrated or “endogenous” retroviruses (ERVs) is extensive. ERVs contribute both positively and negatively toward the health of humans and other taxonomic groups. The functionality of these viruses relies heavily on the process of retrotransposition. This process involves the reverse-transcription of the retrovirus genome into the DNA of the host cell. Though some ERVs are detrimental to human health, such as HIV-1, others are “domesticated” and do not pose a threat.[1] Domesticated ERVs, likely inactivated by mutations over millions of years, serve in adaptive functions of the host. [2] Specifically, the presence of ERVs are essential during human pregnancy. The placenta, a vital organ developed during gestation, is affected by the presence of ERVs. ERVs, acting as non-coding regulatory elements, are key contributors to placental evolution. Absence or mis-expression of specific ERVs in the placenta during pregnancy, results in pre- and or post- defects such as pre-eclampsia (PE). Despite what is known about the importance of ERVs in placental gene expression, there is still much research to be done to fully understand the impacts of these retroviruses on the placenta.
The Role of Endogenous Viruses in the Placenta
ERVs are an essential component of the development of the placenta, not only for humans but a vast array of animals. The origin of ERVs into the placenta is understood to have occured approximately 100 million years ago (Myr). ERVs parasitized living species even before the viruses began to appear in mammals.[3] As opposed to exogenous viruses which are associated with infection, ERVs are germ-line integrated into the host DNA by vertical gene transfer.[4] ERVs evolve more slowly than their exogenous counterparts and function in a sort of symbiosis with their host, given their normal presence in the genome. The importance of ERVs in the placenta focuses on specific envelope genes that originate from the retrovirus, these will be covered in depth in the following sections. The core components to consider when understanding the importance of ERVs is their role in cell differentiation, cell fusion and their immunosuppressive properties.
References
- ↑ [Haig, D. (2012). Retroviruses and the placenta. Current Biology,22(15), R609-R613.]
- ↑ [Knox, K., & Baker, J. C. (2008). Genomic evolution of the placenta using co-option and duplication and divergence. Genome research, 18(5), 695-705. ]
- ↑ [Lavialle, C., Cornelis, G., Dupressoir, A., Esnault, C., Heidmann, O., Vernochet, C., & Heidmann, T. (2013). Paleovirology of ‘syncytins’, retroviral env genes exapted for a role in placentation. Philosophical Transactions of the Royal Society B: Biological Sciences, 368(1626), 20120507. ]
- ↑ [Harris J. R. (1998). Placental endogenous retrovirus (ERV): structural, functional, and evolutionary significance. BioEssays : news and reviews in molecular, cellular and developmental biology, 20(4), 307–316.]
Authored for BIOL 238 Microbiology, taught by Joan Slonczewski, 2021, Kenyon College.
