Endogenous retroviruses (ERVs) are fossils of ancient retroviral infection in the germline. In primates they represent around 5% of the genome sequence. During time spent in the genome, being transmitted in a Mendelian fashion, copies of ERVs have accumulated mutations, which rendered them inactive. However, some of them (the most recently integrated ones) are still able to transcribe and produce viral proteins, although few are capable of re-infection. In the past often considered as unharmful ‘junk DNA’, recent evidence link ERVs with cancer and several inflammatory diseases. For example, a few reports demonstrate that ERVs are involved in tumour development using shRNA knock-down and over-expression systems, and their overexpression tends to correlate with inflammation status, generating the hypothesis that they can act as pathogen-associated molecular patterns (PAMPs) and bind to innate sensors. Focusing on the Human (Homo sapiens) and the rhesus macaque (Macaca mulatta), the main aims of this thesis are to look for further evidence linking ERVs to tumour development, with possible implications for therapies, and test the hypothesis that ERVs are PAMPs by seeing if individuals with higher levels of ERV expression exhibit a higher innate immune response. The work on ERVs in cancer involved the human ERV type-K HML2 lineage (HERV-K (HML2)), an ERV lineage found in humans, in Merlin-deficient tumours. These are schwannomas that arise from Schwann cells and for which effective drug therapy is urgently needed. The work on ERVs in inflammation involved the Papio cynocephalus ERV (PcEV), in rhesus macaques infected with simian immunodeficiency virus (SIV) infection. The main outcomes are as follows: regarding HERV-K (HML2) in human schwannomas, (i) HERV-K (HML2) proteins are overexpressed in schwannoma compared to Schwann cells; (ii) these proteins are released from the tumour; (iii) regulation of HERV-K (HML2) expression in the tumour appears to involve the transcription factor TEAD; (iv) schwannomas are potentially treatable using anti-HERV-K (HML2) monoclonal antibodies and antiretroviral drugs since both decreased proliferation in vitro. Regarding PcEV in SIV-infected macaques: (i) PcEV is transcriptionally active; (ii) PcEV can be retrieved at low levels in the blood of some macaque animals; (iii) the levels of PcEV in cells correlates strongly with the strength of the innate response as measured by cellular levels of STAT1 transcripts – an interferon-stimulated gene (ISG). Other recent research has shown that human ERV lineages, namely HERV-W and HERV-H, have been co-opted and are involved in placentation and pluripotency during development, respectively. The present work suggests that ERVs are involved in a wide range of biological process and supports the need for further research into the biological significance of ERVs for their hosts.

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