The role of the Marek's disease virus (MDV) Meq oncoprotein in the pathogenesis of Marek's disease and the development of MDV vaccine strains

Marek's disease (MD) is a common disease of chickens involving paralysis and commonly death from the growth of highly malignant T lymphomas (cancers of white blood cells). Unusual for a cancer, MD is caused by a transmissible agent, Marek's disease virus (MDV). MDV is very contagious and is a major threat to the poultry industry worldwide, but it can be controlled by vaccination. More than 5 billion vaccine doses a year are used in an attempt to control the disease. However, there is a continuous evolution of the virus towards greater virulence and consequently new vaccines are required. The mechanisms by which the virus gains virulence and produces tumours remain very poorly understood. Similarly, the factors that determine an effective vaccine strain remain unclear. With financial support from the BBSRC, we have been studying a protein called Meq that is made by MDV in the cancer cells. Our recent results have demonstrated unequivocally that this protein is absolutely essential for the induction of tumours by MDV (it is therefore known as an oncoprotein). Moreover it has been possible to identify specific protein:protein interactions involving Meq that are necessary for its function as an oncoprotein. We have made various defined mutations in the Meq-encoding gene that produce attenuated (non-pathogenic) MDV that we showed can be used as vaccines. This is a proposal to build on this work in order to determine the precise contribution that Meq makes to the development of Marek's disease in its various pathological manifestations / including malignant and paralytic forms / and to try and understand better what constitutes an effective vaccine strain of MDV.

Recently, we identified a binding motif (20PLDLS24) for the cell co-repressor CtBP in the Meq nuclear oncoprotein encoded by MDV. This is similar to binding sites in oncoproteins EBNA3A and EBNA3C of Epstein-Barr virus (EBV). MDV is a lymphotropic alphaherpesvirus of chickens that / like EBV / can induce malignant disease. The T cell hyperplasia it produces is responsible for many of the symptoms of the fatal disease of chickens known as Marek's disease (MD). By constructing mutants of Meq in a bacterial artificial chromosome (BAC) clone of MDV, we showed that this interaction between Meq and CtBP is essential for oncogenicity in chickens. This genetically engineered, attenuated strain of MDV very effectively protects chicks from further challenge with very virulent strains of MDV and has potential as a vaccine. In order to determine the role of the leucine zipper in Meq and of Meq dimerisation in lymphomagenesis, specific point mutations were engineered into the highly oncogenic RB-1B-BAC to produce virus completely lacking a functional Meq leucine zipper and virus encoding Meq that cannot homodimerize, but can still bind to c-Jun. Both of these mutant viruses are non-oncogenic. We conclude that the leucine zipper is necessary for the oncogenic activity of Meq and/or the establishment of long-term MDV latency in T cells. Moreover, it appears that the ability to form repressive homodimeric Meq complexes is an absolute requirement for the cancer associated with virulent MDV. We will determine the roles of Meq in non-malignant, but rapidly fatal CNS-associated MD. We will identify specific target chicken genes that are regulated by Meq and establish the roles of interacting protein partners of Meq. It should also be possible to establish what role Meq plays in viral latency and replication in vivo and explore the molecular characteristics of MDV that constitute an effective vaccine for the prevention of MD.