The Marek’s disease virus (MDV) is an alphaherpesvirus of chickens, causing various clinical symptoms including: immunosuppression, paralysis, neurological disorders and terminal lymphomas. Interestingly, in contrast to most herpesviruses, MDV integrates its genome into the telomeres of latently infected host cells. While it’s known that telomere sequences in the virus genome are essential for this process, the actual mechanism and factors influencing this process remain poorly defined. MDV is relatively closely related to the human virus herpes simplex virus 1 (HSV-1) and they possess genes that are functionally conserved. Both virus genomes contain two genes called UL12 and UL29, which encode for a 5’-3’ exonuclease and a single stranded DNA binding protein (ICP8), respectively. For HSV-1, pUL12 and ICP8 interact with each other and promote DNA recombination, a process likely involved in virus replication. Studies have showed that HSV-1’s UL12 when abrogated leads to a severe virus replication defect of 100-1000 fold, which makes it very important for virus replication. On the other hand, UL29 has been shown to be an essential gene for HSV-1 replication. Moreover, the 5’-3’ exonuclease activity of pUL12 has been shown to enhance single strand annealing during HSV-1 replication, whilst ICP8 is dispensable. Despite so many advances in understanding the HSV-1 UL12 and UL29 genes, little is known about their orthologues in MDV. The first aims of this project were to address the role of UL12 and UL29 in MDV replication, verify if these two genes are involved in DNA recombination, and if the recombination complex formed by UL12 and UL29 could help MDV integration into telomeres. To address the roles of UL12 and UL29 in lytic MDV replication, aa 6 and 7 in UL12 and aa 1 in UL29 were replaced for stop codons, abrogating both proteins’ expression individually. After transfecting these UL12 mutant viruses, we could confirm that the stop mutations in UL12 did not completely abrogate virus replication, although reduced the size and number of plaques formed. Next, we mutated the second methionine in UL12 located at aa 136 to a stop codon. Transfection of this mutant virus showed that it could not replicate, indicating that the c-terminus of UL12 is important for replication. In accordance with the data on HSV-1, mutation of UL29 completely blocked virus replication. After addressing the importance of UL12 and UL29 in MDV replication, we assayed their recombination activity in the four main DNA repair recombination pathways in eukaryotic cells: single strand annealing (SSA), homologous recombination (HR), non-homologous end-joining (NHEJ) and alternative non-homologous end-joining (A-NHEJ). Using a cell-based assay and viral protein expression vectors, we could show that UL12 can aid in SSA DNA repair, whilst UL29 was not active for SSA. As UL12 can aid repair of homologous DNA, we investigated the role of UL12 and UL29 in MDV integration using an integration assay with an immortalized chicken T-cell line. We generated shRNAs to knockdown UL12 and UL29 and tested the effect on virus integration. This analysis showed that the absence of these two genes did not interrupt virus integration into T cells. The second main aim of this work was to address whether MDV DNA replication is important for virus integration in T cells. We focused on the UL30 gene, which encodes the MDV DNA polymerase, and generated a mutant virus containing a destabilization domain-UL30 fusion (vUL30DHFR). This destabilization domain fusion enabled inducible control of the viral DNA polymerase. Our results showed that when the DNA polymerase is blocked and virus DNA replication is abrogated, there was no virus genome maintenance in T cells at 14 days post-infection. Moreover, this defect was of a similar magnitude to MDV mutants lacking the viral telomere repeats, which are essential for integration. This evidence is the first to suggest that virus replication is involved in the establishment of the latent MDV genome. Taken together, the work presented in this thesis has shown that MDV encodes essential viral factors for virus replication and that are active in SSA DNA repair but are not essential for MDV telomere integration. Lastly this work has built a foundation for further studies on the links between virus DNA replication and MDV integration into telomeres.
