Retroviral insertional activation in a herpesvirus: transcriptional activation of US genes by an integrated long terminal repeat in a Marek's disease virus clone

Abstract

Insertional activation of host proto-oncogenes has been recognized as a basic mechanism by which nonacute retroviruses induce cancer. Our previous work has demonstrated that retroviruses can efficiently integrate into DNA virus genomes. Specifically, coinfection of cultured fibroblasts with a chicken herpesvirus, Marek's disease virus (MDV), and a chicken retrovirus results in frequent stable retroviral insertions into the herpesvirus genome. Such insertions could alter the expression of herpesvirus genes, possibly resulting in novel phenotypic properties. In this article, we report the characterization of a replication-competent clone of MDV with integrated retroviral sequences. This virus was isolated from a chicken following injection of fibroblasts coinfected with MDV and the retrovirus, reticuloendotheliosis virus. Transcripts originating from the reticuloendotheliosis virus long terminal repeat promoters were found to encode the adjoining MDV genes, SORF2, US1, and US10. This virus replicates well in culture but has an unusual phenotype in chickens, characterized by an attenuated virulence which produces no nerve lesions but, rather, severe thymic atrophy. While the causal relationship between the insertion and the observed phenotypes remains to be established, our data provide the first evidence of retroviral insertional activation of herpesvirus genes.

Protective efficacy of a recombinant bacterial artificial chromosome clone of a very virulent Marek’s disease virus containing a reticuloendotheliosis virus long terminal repeat

“…The LTR insertion with flanking MDV regions was assembled using Velvet version 1.2.10 (Zerbino & Birney, 2008) with a Columbus extension with reads blasting (Altschul et al, 1997) against the reference sequences, Gallid herpesvirus 2 (|NC_002229.3; Tulman et al, 2000) or REV (|NC_006934.1; Wang & Cui, 2006), with approximately 440,000 and 75,000 reads, respectively. The assemblies for both early and late passages at k values ranging from 21 to 71 step size 10 matched the LTR reference (Jones et al, 1996) exactly, with flanking sequence matching insertion sites at 152,216 and 171,754 within both early and late passages. A strain-specific genome was generated by inserting the LTR sequence into both regions of the parent reference and validated by mapping.…”

“…Co-cultivation of REV with virulent MDV (strain JM/102W) resulted in the insertion and duplication of the REV LTR into the MDV genome. The resultant virus, RM-1 (Isfort et al, 1992;Jones et al, 1993;Kost et al, 1993;Jones et al, 1996), was non-oncogenic and highly protective against challenge with vv+MDV; it however caused bursa and thymus atrophy and thus was unsuitable for commercial use (Witter et al, 1997). Lupiani et al, (2013) inserted the REV LTR into the CVI988 cosmid genome and the resultant virus, denoted as CVRM, exhibited enhanced growth in cell culture, and superior protection against challenge with vv+MDV.…”

Increased Horizontal Transmission of Recombinant Marek’s Disease Virus Due to Reticuloendotheliosis Virus Long Terminal Repeat Is the Major Competitive Advantage of the Virus Being a Prevalent Strain

“…The REV LTR possesses promoter and enhancer activity (Jones et al, 1996). Most of the viral genes were significantly upregulated in GX0101 with an REV LTR insert as compared to GX0101 LTR, among which SORF2 was maximally upregulated.…”

Horizontal Gene Transfers with or without Cell Fusions in All Categories of the Living Matter

“…The phenomena of retroviral genomic segmental insertions into the genome of DNA viruses were observed naturally and induced artificially. The two avian T cell lymphoma viruses, MDV and REV, synergized their pathogenicity as the MDV genome accepts inserted segments of the REV genome [217] Transcripts of the REV's LTR promoters enhanced the expression of MDV US (unique short) genes [223,224]. Insertions of the retroviral genome occur within one or two passages in MDVinfected cells.…”