Human Endogenous Retroviruses Long Terminal Repeat Methylation, Transcription, and Protein Expression in Human Colon Cancer

Abstract

Colon cancer is the fourth most common malignancy in both incidence and mortality in developed countries. Infectious agents are among the risk factors for colon cancer. Variations in human endogenous retrovirus (HERV) transcript and protein levels are associated with several types of cancers, but few studies address HERV expression in colon cancer. Fifty-eight patients with advanced-stage colon cancer were enrolled in this study. HERV-H,-K (HML-2),-P LTRs, Alu, and LINE-1 methylation levels and transcription of HERV-H,-K (HML-2), and-P env and HERV-K pol genes in normal adjacent and tumor tissues were investigated by pyrosequencing and RT-qPCR, respectively. Expression of the HERV-K (HML-2) Pol and Env proteins in selected tissues was examined by Western blotting. Associations between HERV transcript expression and methylation levels and between clinical characteristics and HERV expression were evaluated. Compared to adjacent normal tissues, LINE-1 was hypomethylated in tumor tissues (p < 0.05), whereas Alu, HERV-K (HML-2), and-H LTRs showed a decreasing trend in tumor tissue compared to normal tissue, though without a significant difference. The transcription levels of HERV env and pol genes were similar. However, the HERV-K (HML-2) Pol protein was more highly expressed (p < 0.01) in surrounding normal tissues, but the HERV-K (HML-2) Env protein was only expressed in tumor tissues. Although HERV LTR methylation and gene expression did not show significant differences between tumor and normal tissues, HERV protein expression differed greatly. Pol protein expression in normal cells may induce reverse transcription and subsequent integration into the host genome, likely favoring cell transformation; in contrast, the Env protein in tumor tissue may contribute to cancer progression through cell-to-cell fusion.

Ancient Adversary – HERV-K (HML-2) in Cancer

“…A follow-up study failed to recapitulate the finding of LTR hypomethylation in tumour samples relative to matched healthy tissue, however, Env protein expression was observed exclusively in malignant samples. Interestingly, and in contrast to most data pertaining to HML-2 expression in cancer, Pol protein expression was significantly higher in healthy tissues versus the matched tumour samples ( 75 ). Elevated HML-2 transcript expression corelated with shorter relapse-free survival in soft tissue sarcoma patients but showed no association with overall survival ( 76 ).…”

Human polyomaviruses genomes in clinical specimens of colon cancer patients

“…Patients' demographic and clinical data are reported in Table 1. Further clinical indications have been previously described by Dolci et al 19,20 …”

“…Patients' demographic and clinical data are reported in Table 1. Further clinical indications have been previously described by Dolci et al 19,20 The study obtained the approval of the Institutional Ethics Committee (Comitato Etico Istituto Clinico Città Studi, Ospedale Maggiore Policlinico, Milan, protocol number 683_2017bis), and was conducted according to the WMA Declaration of Helsinki; all the patients signed the informed consent by the patients. HPyV-6 genomes, the concentration of the primers set and of the probes were: 0.4 mM of JCPyV forward/reverse primers, 0.9 mM of HPyV-6 forward/reverse primers, and 0.15 mM and 0.25 mM probes respectively.…”

Endogenous Retroviral Elements in Human Development and Central Nervous System Embryonal Tumors

“…There are approximately 90 intact HML-2 proviruses which encode one or more viral proteins including a pathogenic envelope (env) protein [19,20]. The promoter of HML-2 is a long terminal repeat (LTR) which sits 5 or upstream of the proviral sequence [21][22][23][24]. HML-2 gene regulation is controlled by several mechanisms, including methylation of its promoter (5 LTR), chromatin remodeling, binding by transcription factors, and repression via transfer RNA derived fragments (tRFs) [21][22][23][24][25].…”

“…Expression of endogenous retroviral elements have been associated with disease progression in a variety of cancers including melanoma, ovarian cancer, and hepatocellular carcinoma [23,[34][35][36]. HML-2 expression upregulation has been found in tumors with marked epigenetic dysregulation [24,30,37]. In a recent study of repetitive element methylation and expression in colon cancer, the HML-2 promoter (5 LTR) was hypomethylated in tumor tissue, resulting in higher transcription compared to normal tissue; further, the env protein was only expressed in the tumor [24].…”