Mobility and integration sites of a murine C57BL/6 melanoma endogenous retrovirus involved in tumor progression in vivo

Abstract

Tumor development is a multistep process in which both genetic and epigenetic events cooperate for the emergence of a malignant clone with metastatic properties. The possibility that endogenous retroviruses promote the expansion of a neoplastic clone by subverting immunosurveillance has been proposed and recently demonstrated in the case of the B16 murine melanoma, which spontaneously express the melanoma-associated retrovirus (MelARV). Indeed, knocking down, by RNA interference, this endogenous retrovirus resulted in the rejection of the tumor cells in immunocompetent mice, without any alteration of their transformed phenotype. Here, we characterize the MelARV proviruses present in the B16 melanoma. Complete sequencing of the viral genomic RNA and characterization of the integration sites within both the B16 tumor cells and a subline selected in vivo for increased metastatic activity disclosed mobility of the element with new proviral insertions targeting critical genes and altering their transcriptional profile. The results show that MelARV can act both at the genetic level, inducing mutations by insertion, and at the epigenetic level, promoting immunosuppression of the host. These properties may as well be relevant to human tumors, such as germline tumors and melanoma, where endogenous retroviruses are active. ' 2006 Wiley-Liss, Inc.Key words: endogenous retrovirus; murine melanoma; insertional mutagenesis; metastasis Most cancer deaths result from the invasion of surrounding tissues by tumor cells and widespread metastasis to vital organs. The generation of metastatic tumor cells is a multistep process, which involves a series of genetic alterations, including cell transformation via mutations of oncogenes and/or tumor suppressor genes, inhibition of the host immune response and acquisition of an invasive potential. Tumors can be triggered by infectious oncogenic retroviruses that are able to perform at least some of the abovementioned steps, most often via insertional mutagenesis. [1][2][3] This is for instance the case of a series of slow transforming murine retroviruses such as the Moloney murine leukemia virus, which only possesses the 3 canonical gag, pol and env genes but no transduced oncogene and yet induces leukemia by integrating close to critical cellular genes that are consequently deregulated. Interestingly, all mammalian genomes contain endogenous retroviruses (ERVs) that are the genomic traces of ancestral infections of the germline by active retroviruses. [4][5][6] Most of these elements are defective, but some of them have retained intact open reading frames (ORFs). These elements are normally silent but several of them are found to be transcriptionally activated in some tumors, as revealed by the occurrence of viral-like particles. 7,8 Indeed, in the B16 spontaneous melanoma of C57BL/6 mice, an ecotropic ERV-the melanoma-associated retrovirus (MelARV)-is induced, and has been demonstrated to be involved in tumorigenesis. 9,10 Mangeney et al. 10 have recently shown that MelARV expression is...

Simultaneous Knockdown of Mutant BRAF and Expression of INK4A in Melanoma Cells Leads to Potent Growth Inhibition and Apoptosis

“…RNAi suppresses melanoma cells in vitro and in vivo (Mangeney et al, 2005;Oricchio et al, 2007;Pothlichet et al, 2006;Serafino et al, 2009). There are several potential mechanisms to explain the role of HERV-Ks in melanomagenesis.…”

“…There are several potential mechanisms to explain the role of HERV-Ks in melanomagenesis. First, HERV-K sequences may jump around by retro-transposition leading to mutagenesis and chromosomal abnormalities (Pothlichet et al, 2006;Tchenio & Heidmann, 1991) which may drive the evolution of aggressive clones. Second, HERV-K ENV, which is homologous to syncytin, may have fusogenic activity to mediate melanoma-melanoma and melanoma-target cell intercellular fusions and, therefore, can be the molecular link in the melanoma-normal cell fusion theory of metastasis (Carter, 2008;Pawelek & Chakraborty, 2008).…”

“…A c t i v a t i o n o f B R A F / E R K i s r e c e n t l y s h o w n t o d r i v e chromosome abnormality and aneuploidy in melanocytes (Cui et al, 2010). It is conceivable that the effect may be mediated, at least partly, through HERV-K sequences that are capable of jumping around by retro-transposition leading to mutagenesis and chromosomal abnormalities (Pothlichet et al, 2006;Tchenio & Heidmann, 1991). It is possible that the observed growth promotion and anti-apoptotic effects of activated ERK and CDK4 (Li et al, 2010a;Zhao et al, 2008) can be mediated, at least in part, by HERV-K since HERV-K has been shown to directly affect melanoma cell proliferation, differentiation, and anchorage related survival (Oricchio et al, 2007;Serafino et al, 2009).…”

K-type human endogenous retroviral elements in human melanoma

“…[46][47][48] It has also been reported that, compared to healthy individuals, sera from patients with melanoma were much more likely to harbor HERV-K, ENV, and GAG antibodies. [49][50][51] The antibody concentration varied from 16%-22% depending on the study, but was negligible in healthy controls and correlated with worse prognosis.…”

“…48 Another function of HERV-K in melanoma, proposed by Huang et al, 52 is the blockage of HERV-K by RNA interference and monoclonal antibodies in melanoma cells, resulting in a marked reduction in colonic growth of melanoma cells in vitro. 52 The down-regulation of the HERV-K reverse transcriptase by RNA interference or pharmacological inhibitors resulted in a lower mitotic rate and induced morphological differentiation, [52][53][54] and slowed down melanoma growth and formation of metastasis in vivo.…”