Endogenous retroviruses

Abstract

Endogenous retroviruses (ERVs) are an integral part of vertebrate genomes. ERVs became inherited genomic components by formation of proviruses of exogenous retroviruses in germ cell genomes. Repeated endogenization of diverse exogenous retroviruses and subsequent increase of provirus numbers resulted in a considerable retroviral load in host genomes. For instance, the human and the mouse genome hold 8 and 10%, respectively, of sequences of retroviral origin. Most ERV sequences accumulated numerous nonsense mutations over time, rendering them defective. Nevertheless, ERVs can affect the host genome in various ways. Long terminal repeats influence transcription of nearby cellular genes. ERV proteins protect from exogenous retrovirus infection. Human ERV sequences may be involved in placenta development and in the formation of germ cell tumors. ERVs are therefore important genome components that contributed significantly to the evolution of host genomes.