Deletion of the Marek's disease virus UL41 gene (vhs) has no measurable effect on latency or pathogenesis

Authors: Gimeno, Isabel; Silva, Robert

Submitted to: Virus Genes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2008
Publication Date: 6/4/2008
Citation: Gimeno, I.M., Silva, R.F. 2008. Deletion of the Marek's disease virus UL41 gene (vhs) has no measurable effect on latency or pathogenesis. Virus Genes. 36:499-507.

Interpretive Summary: Marek’s disease is an economically important disease of poultry and is caused by a herpesvirus, Marek’s disease virus (MDV). MDV contains over 50 genes, many of which are similar to genes in other herpesviruses. One of these MDV genes is designated UL41. Many other herpesviruses have a gene similar to the MDV UL41 gene and these genes are responsible for degrading RNA, a nucleic acid responsible for protein generation. Deletion of this gene in other herpesviruses results in a virus that is significantly less pathogenic. We wanted to determine whether the MDV UL41 gene would also degrade RNA and if so, whether deleting the MDV UL41 gene would result in a less pathogenic MDV and consequently be a potential vaccine. Using chicken cells growing in tissue culture, we demonstrated that the MDV UL41 gene degraded RNA and functioned similarly to a related gene in herpes simplex virus. However, when we deleted the UL41 gene from MDV, the virus was just as pathogenic as before. The only detectable difference was an increase in the time course of early virus replication. We hypothesize that the UL41 gene in MDV plays a role in regulating the normal sequence of MDV gene expression. In conclusion, the MDV UL41 gene has some functional similarity with other herpesvirus genes but is not involved in pathogenesis and therefore not a potential mutational target to generate an MDV vaccine.

Technical Abstract: Marek’s disease is an economically important disease of poultry and is caused by an alphaherpesvirus, Marek’s disease virus (MDV). Many MDV genes have homologs in other herpesviruses. The predicted protein product of the MDV UL41 open reading frame has significant protein sequence identity with the varicella-zoster (33%), pseudorabies (42%), and herpes simplex (29%) virion host shutoff (vhs) genes. To determine whether the MDV UL41 gene functions as a virion host shutoff protein, we utilized a transient co-transfection assay and demonstrated that the MDV UL41 protein was as active in degrading RNA as the vhs protein of herpes simplex virus type 1. To evaluate whether the MDV UL41 gene was involved in pathogenesis, we used an overlapping cosmid clone library of MDV and the RecA-assisted restriction endonuclease cleavage procedure to delete the MDV UL41 gene and generate three independent rMd5(deltaUL41) knockout viruses. The MDV UL41 gene was nonessential for replication and all three rMd5(deltaUL41) viruses replicated in cell culture as well as the parental virus (rMd5). The rMd5(deltaUL41) viruses were inoculated into susceptible day-old chicks. The pattern and degree of tumor lesions and neurovirulence produced by rMd5(deltaUL41) inoculation was the same as the pattern of lesions induced by rMd5. In addition, deletion of UL41 did not affect the horizontal spread of the virus or the ability of rMd5(deltaUL41) to induce or reactivate from latency. Unlike the UL41 gene of herpes simplex virus, the MDV UL41 gene has no role in pathogenesis. The only observable difference between the inoculation of rMd5 and rMd5(deltaUL41) was in the time course of the early cytolytic infection. The early cytolytic infection with rMd5 was of short duration, peaking at 6 days post inoculation. In contrast, the early cytolytic infection with rMd5(deltaUL41) began at 4 days post inoculation and continued through 8 days post inoculation. We hypothesize that the deletion of the MDV UL41 gene disrupted the normal coordinate regulation and cascade of alpha, beta and gamma gene expression.