RETENTION OF ONCOGENICITY BY A MAREKS-DISEASE VIRUS MUTANT LACKING 6 UNIQUE SHORT REGION GENES

We previously reported the construction of Marek's disease virus (MDV) strains having mutations in various genes that map to the unique shor t (US) region of the viral genome (J. L. Cantello, A. S. Anderson, A. Francesconi, and R. W. Morgan, J. Virol. 65:1584-1588, 1991; M. S, Par cells, A S. Anderson, and R W. Morgan, Virus Genes 9:5-13, 1994; M. S. Parcells, A. S. Anderson, and R. W. Morgan, J. Virol. 68:8239-8253, 1 994). These strains were constructed by using a high-passage-level ser otype 1 MDV strain which grew well in chicken embryo fibroblasts. Desp ite the growth of the parent and mutant viruses in cell culture, in vi vo studies were limited by poor growth of these strains in chickens. O ne of the mutants studied lacked 4.5 kbp of US region DNA and containe d the lacZ gene of Escherichia coli inserted at the site of the deleti on. 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 read ing frames. We now report the construction of a mutant MDV containing a similar deletion in the US region of the highly oncogenic RB1B strai n. This mutant, RB1B Delta 4.5lac, had a growth impairment in establis hed chicken embryo fibroblasts similar to that described previously fo r MDVs lacking a functional US1 gene. In chickens, RB1B Delta 4.5lac s howed decreased early cytolytic infection, mortality, tumor incidence, and horizontal transmission. Several lymphoblastoid cell lines were e stablished from RB1B Delta 4.5lac-induced tumors, and virus reactivate d from these cell lines was LacZ(+). These results indicate that the d eleted genes are nonessential for the transformation of chicken T cell s or for the establishment and maintenance of latency. On the basis of the growth impairment observed for RB1B Delta 4.5lac in cell culture and in vivo, we conclude that deletion of these genes affects the lyti c replication of MDV. This is the first MDV mutant constructed in the RB1B oncogenic strain, and the methodology described herein provides f or the direct examination of MDV-encoded determinants of oncogenicity.