Human endogenous retrovirus K and cancer: Innocent bystander or tumorigenic accomplice?

Harbored as relics of ancient germline infections, human endogenous retroviruses (HERVs) now constitute up to 8% of our genome. A proportion of this sequence has been co‐opted for molecular and cellular processes, beneficial to human physiology, such as the fusogenic activity of the envelope protein, a vital component of placentogenesis. However, the discovery of high levels of HERV‐K mRNA and protein and even virions in a wide array of cancers has revealed that HERV‐K may be playing a more sinister role—a role as an etiological agent in cancer itself. Whether the presence of this retroviral material is simply an epiphenomenon, or an actual causative factor, is a hotly debated topic. This review will summarize the current state of knowledge regarding HERV‐K and cancer and attempt to outline the potential mechanisms by which HERV‐K could be involved in the onset and promotion of carcinogenesis.