Role of the Leucine Zipper of Marek's Disease Virus Oncoprotein Meq in Pathogenesis

Marek's disease virus (MDV), the etiologic agent of Marek's disease, is a potent oncogenic herpesvirus. MDV is highly contagious and elicits a rapid onset of malignant T-cell lymphomas in chickens within several weeks after infection. The MDV genome encodes an oncoprotein, Meq, which shares resemblance with the Jun/Fos family of bZIP transcription factors. Similar to c-Jun, the leucine zipper region of Meq allows the formation of homo- and heterodimers. Meq homo- and heterodimers have different DNA binding affinities and transcriptional activity; therefore, they may differentially regulate transcription of viral and cellular genes. Previous in vitro data has suggested that Meq homodimers may be involved in regulating viral latency/reactivation, while Meq/c-Jun heterodimers are involved in transformation. Therefore, this research investigates the role of Meq homodimers and Meq-Jun heterodimers in the pathogenicity of MDV, by generating chimeric meq genes, which contain the leucine zipper region of the yeast transcription factor GCN4 (meqGCN) or leucine zipper region of c-Fos (meqFos). Thus producing Meq proteins that exclusively homodimerize (MeqGCN) or heterodimerize with Jun family members (MeqFos). Recombinant viruses (rMd5-MeqGCN and rMd5- MeqFos) containing the chimeric genes meqGCN or meqFos, respectively, in place of parental meq were generated with overlapping cosmid clones of Md5, a very virulent MDV strain. The chimeric genes were evaluated in vitro and retained DNA binding and transactivation/repressive functions, however, selected cells expressing MeqGCN and MeqFos had reduced colony formation in soft agar. Both the rMd5-MeqGCN and rMd5- MeqFos viruses replicated in vitro and in vivo, but rMd5-MeqGCN was unable to transform T-cells in infected chickens, while rMd5-MeqFos induced preneoplastic nerve lesions in 50% of infected birds. However, a third virus rMd5-MeqFos/GCN, which contains one copy of each meqGCN and meqFos, induced preneoplastic nerve lesions in 60% of infected chickens and neoplastic lesions in 20% of infected chickens. These data provide the first in vivo evidence that both Meq homodimers and heterodimers are necessary for MDV induced transformation.