Critical Role of the Virus-Encoded MicroRNA-155 Ortholog in the Induction of Marek's Disease Lymphomas

Abstract

Notwithstanding the well-characterised roles of a number of oncogenes in neoplastic transformation, microRNAs (miRNAs) are increasingly implicated in several human cancers. Discovery of miRNAs in several oncogenic herpesviruses such as KSHV has further highlighted the potential of virus-encoded miRNAs to contribute to their oncogenic capabilities. Nevertheless, despite the identification of several possible cancer-related genes as their targets, the direct in vivo role of virus-encoded miRNAs in neoplastic diseases such as those induced by KSHV is difficult to demonstrate in the absence of suitable models. However, excellent natural disease models of rapid-onset Marek's disease (MD) lymphomas in chickens allow examination of the oncogenic potential of virus-encoded miRNAs. Using viruses modified by reverse genetics of the infectious BAC clone of the oncogenic RB-1B strain of MDV, we show that the deletion of the six-miRNA cluster 1 from the viral genome abolished the oncogenicity of the virus. This loss of oncogenicity appeared to be primarily due to the single miRNA within the cluster, miR-M4, the ortholog of cellular miR-155, since its deletion or a 2-nucleotide mutation within its seed region was sufficient to inhibit the induction of lymphomas. The definitive role of this miR-155 ortholog in oncogenicity was further confirmed by the rescue of oncogenic phenotype by revertant viruses that expressed either the miR-M4 or the cellular homolog gga-miR-155. This is the first demonstration of the direct in vivo role of a virus-encoded miRNA in inducing tumors in a natural infection model. Furthermore, the use of viruses deleted in miRNAs as effective vaccines against virulent MDV challenge, enables the prospects of generating genetically defined attenuated vaccines.

Marek's disease virus undergoes complete morphogenesis after reactivation in a T-lymphoblastoid cell line transformed by recombinant fluorescent marker virus

“…A large number of MDCC lines have been established in several laboratories since 1973, from various chicken lines and various MDV strains, including a few recombinant tagged viruses (Akiyama & Kato, 1974b;Frazier & Powell, 1975;Nazerian, 1987;Nazerian et al, 1977;Parcells et al, 1999;Powell et al, 1974;Schat et al, 1991;Zhao et al, 2011). The 3867K cell line showed numerous characteristics shared by other MDCCs, including cell morphology, cell surface markers, reactivation rate and infectivity, and therefore can be considered as a good MDCC model.…”

MicroRNA expression by an oncogenic retrovirus

“…The 7-nt miRNA seed sequence, extending from position 2 to 7 or 8 from the miRNA 5′ end, is the major determinant of mRNA target recognition in miRNAs (12) and, indeed, the authors are COMMENTARY able to show that miR-B4 and miR-29 could down-regulate a shared set of mRNA targets (3). This result is reminiscent of another pro-oncogenic cellular miRNA, miR-155, which is mimicked by viral miRNAs encoded by two oncogenic herpesviruses, human Kaposi's sarcomaassociated herpesvirus (KSHV) and the avian herpesvirus Marek's disease virus (MDV) (13)(14)(15). Overexpression of miR-155 is associated with the development of B-cell lymphomas (16).…”

“…Overexpression of miR-155 is associated with the development of B-cell lymphomas (16). KSHV infection is also associated with B-cell lymphomas, whereas MDV infection largely induces Tcell lymphomas (13)(14)(15). Importantly, an MDV mutant lacking the viral miR-155 analog grows normally in culture, yet is no longer capable of causing lymphomas in experimentally infected chickens (15).…”

“…KSHV infection is also associated with B-cell lymphomas, whereas MDV infection largely induces Tcell lymphomas (13)(14)(15). Importantly, an MDV mutant lacking the viral miR-155 analog grows normally in culture, yet is no longer capable of causing lymphomas in experimentally infected chickens (15). Similarly, miR-29 overexpression is causatively linked to CLL in humans and mice (4), and BLV causes B-cell tumors that closely resemble CLL (5).…”

MicroRNAs as B-cell lymphoma biomarkers

“…18,53 Moreover, CD10, indicative of GC-type DLBCL, 61 and the constitutive expression of NF-kB, a hallmark of ABC-type DLBCL, 62 are linked via the miR-155/PU.1 regulatory loop. 41 Interestingly, the lymphoma-inducing herpes viruses, Kaposi's sarcomaassociated herpes virus and Marek's disease virus, encode miR-155 orthologs, 63,64 whereas Epstein-Barr virus -also intimately involved in lymphomagenesis -can deregulate miR-155 via LMP1 transactivation. 65 miR-34a is a well-described tumor suppressor miRNA that forms part of the p53 network, 66 with a positive feedback loop between the p53 induction of miR-34a expression and activation of p53 by miR-34a through SIRT1 inhibition.…”