Human Endogenous Retroviruses in Triple-Negative Breast Cancer

Human endogenous retroviruses (HERVs) occupy approximately 8% of the human genome. While most are degraded or quiescent, some are expressed in physiological and pathological states. Among the latter, the expression of HERVs has been documented in germ cell, prostate, and breast cancers. Breast cancer is the most diagnosed cancer in women although it is not a single disease. Rather, it comprises at least four different cancers and other associated diseases. The expression of HERV-K Env and the presence of HERV-K antibodies has been described in breast cancer, with the latter being developed as a possible biomarker. Polymorphisms in HERV loci may also contribute to breast cancer, as observed with the HERV-K HML-2 Xq21.33 locus. More recently, long noncoding RNAs (lncRNAs) have been identified that are associated with breast cancer; at least one of these, TROJAN, contains a HERV and is associated with worsening prognoses in triple-negative breast cancer (TNBC). This chapter will review the evidence for roles for HERVs in breast cancer onset or progression, particularly focusing on more recent evidence involving noncoding RNAs.