Sequencing of hypermutated Marek's Disease Virus Y547S genomes isolated from different passages, replicates in CEC or T7 chicken intestinal epithelial cells

Mareks disease virus (MDV) is a ubiquitous and highly oncogenic alphaherpesvirus of chickens that causes significant economic losses in the poultry industry. In contrast to most RNA viruses, large DNA viruses feature high-fidelity genome replication, mainly achieved through a proofreading function of the viral DNA polymerase. We have generated one mutant Y547S within the conserved exonuclease domain (Exo) III, by replacing a conserved amino acid tyrosine with serine. The Y547S mutant proved to be a slow hypermutator, exhibiting a mutation frequency about three times higher than wild type virus (WT) in chicken embryo cell (CEC) culture, which makes this particular mutant an interesting model to study the correlation between mutation frequency and adaptation. To address this question, we adapted both WT and Y547S not only to long-term propagation in permissive CEC culture, but also to growth in a naturally non-permissive cell line. We then determined the phenotype of adapted viruses both in vitro and in vivo using chickens, the natural host of MDV.