Human endogenous retroviruses and cancer

Abstract

Human endogenous retroviruses (HERVs) are retroviruses that infected human genome millions of years ago and have persisted throughout human evolution. About 8% of our genome is composed of HERVs, most of which are nonfunctional because of epigenetic control or deactivating mutations. However, a correlation between HERVs and human cancer has been described and many tumors, such as melanoma, breast cancer, germ cell tumors, renal cancer or ovarian cancer, express HERV proteins, mainly HERV-K (HML6) and HERV-K (HML2). Although the causative role of HERVs in cancer is controversial, data from animal models demonstrated that endogenous retroviruses are potentially oncogenic. HERV protein expression in human cells generates an immune response by activating innate and adaptive immunities. Some HERV-derived peptides have antigenic properties. For example, HERV-K (HML-6) encodes the HER-K MEL peptide recognized by CD8+ lymphocytes. In addition, HERVs are two-edged immunomodulators. HERVs show immunosuppressive activity. The presence of genomic retroviral elements in host-cell cytosol may activate an interferon type I response. Therefore, targeting HERVs through cellular vaccines or immunomodulatory drugs combined with checkpoint inhibitors is attracting interest because they could be active in human tumors.

Keywords: HERVs; cancer; immunotherapy; interferon.

1

Mechanisms of oncogenesis. LTRs recruit transcription factors from the infected cell for retroviral gene transcription. These LTRs can also enhance transcription of the host cell genes, leading to uncontrolled tumor cell proliferation. Some HERVs encode potentially oncogenic proteins like Np9 and Rec that interact with transcription factors and activate immunosuppressor pathways, promoting oncogenesis. HERVs can also induce chromosomal translocations in somatic cells that could lead to tumor proliferation. HERVs also can promote an immunosuppressive response that may lead to cancer formation and spreading, because the Env protein has an immunosuppressive domain (ISD).

2

Retranscription of HERVs would activate the innate response of sensors (pattern-recognition receptors or PRRs) of single RNA strand (RIG1 and MDA5) in cytosol of the cancer cells. This activates the signaling pathways leading to activation of TRAF family member-associated NF-κB activator (TANK)-binding kinase 1 (TBK1) that causes induction of the IFN-regulatory factor-3 and 7 (IRF-3 and IRF-7), NF-KB-dependent gene expression and subsequent production of IFN beta. This results in transcriptional activation of interferon stimulated genes with the production of cytokines, and increased expression of MHC type I on cancer cells.