Effect of Insertion and Deletion in the Meq Protein Encoded by Highly Oncogenic Marek’s Disease Virus on Transactivation Activity and Virulence

Abstract

Marek’s disease virus (MDV) causes malignant lymphoma in chickens (Marek’s disease, MD). Although MD is currently controlled by vaccination, MDV strains have continuously increased in virulence over the recent decades. Polymorphisms in Meq, an MDV-encoded oncoprotein that serves as a transcription factor, have been associated with the enhanced virulence of the virus. In addition, insertions and deletions in Meq have been observed in MDV strains of higher virulence, but their contribution to said virulence remains elusive. In this study, we investigated the contribution of an insertion (L-Meq) and a deletion in the Meq gene (S-Meq) to its functions and MDV pathogenicity. Reporter assays revealed that both insertion and deletion enhanced the transactivation potential of Meq. Additionally, we generated RB-1B-based recombinant MDVs (rMDVs) encoding each Meq isoform and analyzed their pathogenic potential. rMDV encoding L-Meq indueced the highest mortality and tumor incidence in infected animals, whereas the rMDV encoding S-Meq exhibited the lowest pathogenicity. Thus, insertion enhanced the transactivation activity of Meq and MDV pathogenicity, whereas deletion reduced pathogenicity despite having increased transactivation activity. These data suggest that other functions of Meq affect MDV virulence. These data improve our understanding of the mechanisms underlying the evolution of MDV virulence.

Comparative Analysis of Multiple Consensus Genomes of the Same Strain of Marek’s Disease Virus Reveals Intrastrain Variation

“…A large portion of these differences (n = 177 bp) corresponded to a 59 amino acid (AA) insertion in the meq oncogene, which was found in the CVI988 GenBank.BAC and CVI988 USDA.PA.field consensus genomes but absent in the CVI988 Pirbright.lab consensus genome (Figure 2). For meq, previous studies have suggested the existence of up to three isoforms: Meq, Long Meq (L-meq) and Short Meq (S-meq) (43)(44)(45)(46). Meq is the standard isoform and is 339-AA in length.…”

“…The copyright holder for this preprint this version posted September 4, 2023. ; https://doi.org/10.1101/2023.09.04.556264 doi: bioRxiv preprint the CVI988-UK consensus genome (Figure 2). For meq, previous studies have suggested the existence of up to three isoforms: Meq, Long Meq (L-meq) and Short Meq (S-meq) (39)(40)(41)(42).…”

Characterization of a Very Short Meq Protein Isoform in a Marek’s Disease Virus Strain in Japan

“…The transactivation domain includes proline-rich regions and determines the level of transcriptional regulation. Differences in the amino acid sequences of Meq (such as polymorphisms, insertions, and deletions) affect transcriptional regulation and the pathogenicity of MDV [ 15 , 16 , 17 ]. Four consecutive prolines (PPPP) present in the Meq region of classical MDV strains are characteristic of this proline-rich region [ 18 ].…”

“…However, a similar insertion was found in the proline-rich region of Meq of virulent MDV strains circulating in Australia, and PXPP substitutions were observed in the L-Meq proteins of these Australian strains [ 20 ]. Recent studies have shown that insertion into the proline-rich region increases the virulence of MDV [ 16 , 17 ], suggesting that the insertion contributes to the high virulence of MDV, whereas other factors, including polymorphisms in L-Meq, are involved in the low virulence of CVI988 [ 16 , 17 , 21 ].…”

“…In addition to the polymorphisms in Meq and insertions into proline-rich regions, we previously reported a short isoform of Meq (S-Meq) that includes a deletion in the proline-rich region, resulting in a decreased number of PPPP sequences [ 22 ]. Interestingly, the deletion decreases the virulence of MDV, although S-Meq shows a higher transactivation activity on the Meq promoter than on Meq [ 17 ]. These results indicate the contribution of different functions of proline-rich regions, ranging from transcriptional regulation to MDV pathogenicity.…”