Endogenous retroviruses (ERVs)

Endogenous retroviruses (ERVs) are LTR (Long Terminal Repeats) retroelements, present mostly in human and other mammal’s genomes, making up, for example, 4,7 % of the human genome. ERVs differ from the conventional LTR retroelements, because they resemble more decayed viral retroelements rather than true transposons.

ERVs are very similar to conventional retrotransposons, because in both cases there’s the involvement of a RNA intermediate. The difference is that in transcription mechanisms in retrotransposition the RNA molecule has an endogenous origin, being transcribed from its own genome, while in retroviral replication mechanisms, transcription occurs from an exogenous viral genome.

ERVs are permanent constituents of the host’s genome, and are inherited from generation to generation, like any other constituent of the host’s genome. Retroviral infections normally attack somatic cells, but in the ERV’s case, it is thought that this genomic integration is the result of retroviral infection of germ cells, which resulted in its integration and transmission to subsequent generations. That’s why the term “endogenous retroviruses” is used.

In the human genome, ERVs possess copies of the genes gag, pol and env, but however most ERVs suffered mutations or deletions which inactivated one or more of these genes, which resulted in the loss of its viral replication activity. The majority of ERVs are therefore inactive sequences that are not capable of additional proliferation as retrotransposons.