Retention of oncogenicity by a Marek's disease virus mutant lacking six unique short region genes

Abstract

We previously reported the construction of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique short (US) region of the viral genome (These strains were constructed by using a high-passage-level serotype 1 MDV strain which grew well in chicken embryo fibroblasts. Despite the growth of the parent and mutant viruses in cell culture, in vivo studies were limited by poor growth of these strains in chickens. One of the mutants studied lacked 4.5 kbp of US region DNA and contained the lacZ gene of Escherichia coli inserted at the site of the deletion. The deletion removed MDV homologs to the US1, US2, and US10 genes of herpes simplex virus type 1 as well as three MDV-specific open reading frames. We now report the construction of a mutant MDV containing a similar deletion in the US region of the highly oncogenic RB1B strain. This mutant, RB1B⌬4.5lac, had a growth impairment in established chicken embryo fibroblasts similar to that described previously for MDVs lacking a functional US1 gene. In chickens, RB1B⌬4.5lac showed decreased early cytolytic infection, mortality, tumor incidence, and horizontal transmission. Several lymphoblastoid cell lines were established from RB1B⌬4.5lacinduced tumors, and virus reactivated from these cell lines was LacZ ؉ . These results indicate that the deleted genes are nonessential for the transformation of chicken T cells or for the establishment and maintenance of latency. On the basis of the growth impairment observed for RB1B⌬4.5lac in cell culture and in vivo, we conclude that deletion of these genes affects the lytic replication of MDV. This is the first MDV mutant constructed in the RB1B oncogenic strain, and the methodology described herein provides for the direct examination of MDV-encoded determinants of oncogenicity.

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

“…The size and the nature of the 3.2-kb transcript are consistent with it being initiated from the LTR promoter and transcribed across the coding sequences of SORF2, US1, and US10, terminating after the US10-proximal poly(A) signal in the U S region. The two transcripts (2.6 and 1.7 kb) detected by the E2.8 probe in both RM1-infected cells and JM-infected cells were identical to the native MDV transcripts previously described (27,28). These transcripts initiate from U S region promoters and terminate near the same poly(A) site ( Fig.…”

“…The presence of a potential zinc finger domain in many US10 homologs has suggested a possible role in gene regulation (10). It thus on July 10, 2020 by guest http://jvi.asm.org/ appears that all three gene products potentially affected by LTR insertion may have transactivation properties, although none of them seems to be essential for in vitro and in vivo growth, as elegantly shown by recent deletion analyses (27,28). This finding notwithstanding, the consequence of overexpression or deregulation of these putative transactivators remains to be analyzed.…”

“…These features make RM1 somewhat unique among the JM isolates. The potential to replicate well in vivo and the potential to induce thymic atrophy usually cosegregate with oncogenic strains (28). The attenuated strains are deficient in all three characteristics.…”

Avian Immunosuppressive Diseases and Immunoevasion

“…It is likely that the SORF2 gene plays a role, based on two studies using opposing experimental strategies. Deletion mutants lacking several ORFS, including SORF2, caused decreased cytolytic infection without preventing tumor formation [132]. Up-regulation of SORF2 expression through the insertion of the REV long terminal repeat (LTR) resulted in enhanced cytolytic infection in the absence of tumor development [133].…”

Avian Immunosuppressive Diseases and Immune Evasion

“…It is likely that the SORF2 gene plays a role, based on two studies using opposing experimental strategies. Deletion mutants lacking several ORF, including SORF2, caused decreased cytolytic infection without preventing tumour formation (Parcells et al, 1995). Upregulation of SORF2 expression through the insertion of the REV long terminal repeat (LTR) resulted in enhanced cytolytic infection in the absence of tumour development .…”