Attenuation of Marek's disease virus by codon pair deoptimization of a core gene

Authors: CONRAD, STEVEN - US Department Of Agriculture (USDA); SILVA, ROBERT - US Department Of Agriculture (USDA); HEARN, CARL - US Department Of Agriculture (USDA); CLIMANS, MEGAN - Michigan State University; DUNN, JOHN - US Department Of Agriculture (USDA)

Submitted to: Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2018
Publication Date: 2/22/2018
Citation: Conrad, S.J., Silva, R.F., Hearn, C.J., Climans, M., Dunn, J.R. 2018. Attenuation of Marek's disease virus by codon pair deoptimization of a core gene. Virology. 516:219-226. https://doi.org/10.1016/j.virol.2018.01.020.
DOI: https://doi.org/10.1016/j.virol.2018.01.020

Interpretive Summary: Gallid herpesvirus 2 (GaHV2), also known as Marek’s disease virus (MDV) is the virus which causes Marek’s disease, an economically-important disease of farmed chickens and a major cause of chicken mortality. It is necessary to vaccinate each individual chick at the time of hatch to prevent outbreaks of this disease, but this universal vaccination with live attenuated virus (LAV) vaccines (and other factors as well) provide a constant selective pressure for the evolution of new and more deadly strains of Marek’s disease. To combat these new strains it is necessary to develop new vaccines. The established way to develop new LAVs is to culture virulent strains of MDV in the laboratory until they lose their virulence and to then test these attenuated strains in chickens. This can be a long process and there is no guarantee of success, and even successfully attenuated strains of MDV will need to be characterized before use. Recently several research groups have shown that it is possible to re-write virus genes such that they express the same protein but do so in a less efficient way because of changes in the codon structure of the gene (a practice called “synonymous deoptimization”). This reduction in protein production from targeted genes is thought to cripple the virus just enough that the organism has a chance to mount an immune response to curtail the infection and develop immunity against future challenges from this same virus. We used this approach to reduce protein production from two genes in MDV. Changes from one of the genes successfully reduced protein production and when tested in chickens, caused fewer cases of disease and a less-severe form Marek’s disease. We believe that synonymous deoptimization may be successfully used in the future and are currently investigating other genes and ways of deoptimization to further attenuate Marek’s disease virus.

Technical Abstract: Marek’s disease virus (MDV) is an oncogenic alphaherpesvirus of Gallus gallus, the domesticated chicken. Control strategies rely upon comprehensive vaccination in ovo with live attenuated virus vaccines consisting of antigenically similar avian herpesviruses or attenuated strains of MDV. Recent studies in other viruses have shown that recoding certain viral genes to employ synonymous but rarely-used codon pairs resulted in viral attenuation. We deoptimized two MDV proteins, UL54/ICP27 and UL49/VP22, and demonstrate that the more severely deoptimized variant of UL54 accumulates significantly less gene product in vitro. Using these UL54 deoptimized mutants, we further demonstrate that in vivo, animals infected with the UL54-recoded recombinant virus exhibited decreased viral genome copy number in lymphocytes, reduced lymphoid atrophy and reduced visceral tumor incidence. This study demonstrates that codon pair deoptimization of a single viral gene can produce attenuated strains of MDV. This approach may be useful as a rational way of making novel live attenuated virus vaccines for Marek’s disease virus.