Ajithdoss, Dharani K. (2009-05). Characterization of the Meq oncoproteins of Marek's disease virus vaccine strain CVI988/Rispens. Doctoral Dissertation.
Marek?s disease virus serotype-1 (MDV-1) causes T cell lymphomas in chickens. Vaccines prepared from attenuated CVI988/Rispens MDV-1 strain currently offer the best protection. Although attenuated CVI988 is non-oncogenic, it codes for two forms of the MDV-1 oncoprotein Meq (CVI-Meq and CVI-L Meq). In this study, both CVI-Meq proteins, like the Meq protein of Md5 (a very virulent oncogenic strain), transformed Rat-2 and NIH3T3 cells. Both CVI-Meq and CVI-L Meq proteins activated the meq promoter only in the presence of chicken c-Jun (CK-Jun) whereas Md5-Meq activated the same promoter irrespective of CK-Jun co-expression. However, all three Meq proteins bound the meq promoter regardless of whether CK-Jun was co-expressed. We constructed three chimeric Meq proteins, namely, Md5-CVI-Meq, CVI-Md5-Meq, and Md5-CVI-L by exchanging domains between Md5 meq and CVI meq genes. Although these chimeric Meq proteins transactivated the meq promoter, the activation was significantly less than Md5-Meq. The current study indicated amino acid residues at positions 71 and 320 were important for Md5-Meq increase transcription of its own promoter. All three Meq proteins activated the MDV gB, MMP-3 and Bcl-2 promoters and suppressed transcription from the MDV pp38/pp14 bidirectional promoter. CVI-Meq protein in the context of other Md5 genes caused tumors only in 6% of chickens when compared to parental rMd5 (a very virulent strain), which induced lymphomas in 100% of chickens, (Reddy and Lupiani, unpublished data). Taking advantage of these two different phenotypes, we constructed two chimeric Meq proteins, Md5/CVI-Meq and CVI/Md5-Meq, by exchanging DNA binding and transactivation domains between Md5-Meq and CVI-Meq to understand the role of the DNA binding and the transactivation domains of Meq in transformation. rMd5-Md5/CVI-Meq virus caused 100% mortality in chickens and T lymphomas were found at high frequency in the peripheral nerves and various organs such as the heart, spleen, kidney, and gonads. On the other hand, rMd5-CVI/Md5-Meq induced disease in 36% of chickens on average and lesions were primarily in the nerves. Very rarely, lesions were present in the spleen and heart and no tumors were present in the kidney or gonads. Our results suggest that both the DNA binding domain and transactivation domain of Meq could cooperatively determine the nature of lymphomas in chickens.