UNRAVELING THE ROLE OF THE HUMAN ENDOGENOUS RETROVIRUSES IN THE PATHOGENESIS OF COLON CANCER

Background: Human endogenous retroviruses (HERV) are remnants of exogenous retroviral infections, representing 8% of the human genome. Their regulation is based on the DNA methylation of promoters, the long terminal repeats (LTRs). Transcripts from HERV have been associated with cancers, but reports concerning HERV expression in colorectal cancer remain sporadic. Methods: 63 Italian patients and 58 Tunisian patients with advanced stages of colorectal cancer were enrolled in this study. HERV-H, -K, -R, -P LTRs, and Alu, LINE-1 methylation levels, and the expressions of HERV env and pol gene were investigated by pyrosequencing and by RT qPCR, respectively, in the tumor, normal adjacent tissues, and, when possible, in the blood and plasmatic extracellular vesicles (EVs). Associations among clinical characteristics and HERV expression and methylation levels were also evaluated. The expression of the HERV-K Pol/Env proteins was also evaluated by Western Blot. Results: As for the Italians patients, Alu, LINE-1, HERV-H and -K LTRs were demethylated in the tumor tissues compared to the normal adjacent tissues (p<0.05), while no differences were observed in HERV env gene expression levels, among the clinical specimens. The env gene was expressed in the EVs (p<0.01) of 54% (-H), 38% (-K), 31% (-R) patients. Associations were found between HERV expression and right tumor colon location and between HERV methylation and vascular invasion (p<0.05). HERV K Pol protein was more expressed (p<0.01) in the adjacent normal tissues compared to the tumor tissues. The Env protein was only expressed in the tumor tissue. As for the Tunisian population, LINE-1 was less methylated in the tumor tissue compared to the adjacent normal tissue (p<0.05), while Alu, HERV-K and -H LTRs showed the same trends, but the difference was not significant. The expression of the HERV env and pol genes were similar in the biological samples. No association was found between the HERV expression/methylation and the clinical characteristics of the Tunisian patients. Conclusions: The changes in DNA methylation of retroelements are specific in colorectal cancer but do not correlate with viral overexpression. The Pol protein expression in the normal cells may induce the retrotrascription and the subsequent transfer of HERV sequences into other cells, possibly through EVs. HERV genome insertion might cause cells transformation. In the cancer cells, the Env protein may contribute to the cancer progression through cell to cell fusion.