Submitted to: Avian Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/3/2012
Publication Date: 6/18/2012
Citation: Mays, J.K., Silva, R.F., Kim, T., Fadly, A.M. 2012. Insertion of reticuloendotheliosis virus long terminal repeat into a bacterial artificial chromosome clone of a very virulent Marek's disease virus alters its pathogenicity. Avian Pathology. 41(3):259-265. Available: http://dx.doi.org/10.1080/03079457.2012.675428. Interpretive Summary: Marek’s disease virus (MDV) and reticuloendotheliosis virus (REV) are both avian viruses that belong to two different groups of viruses. MDV is a DNA virus, whereas, REV is an RNA virus; both viruses can cause cancer-like disease in chickens. It has been reported that under certain circumstances part of the genetic material from REV know as long terminal repeat (LTR) can be inserted in and be part of the genome of MDV. The effect of REV-LTR insertion into the genome of MDV on the pathogenicity (disease-inducing potential) of MDV is poorly understood. Recently, using a DNA-based technology termed bacterial artificial chromosome (BAC) we were able to artificially insert REV-LTR into a clone of very virulent MDV. The pathogenicity of this BAC clone of MDV with and without REV-LTR was compared in susceptible chickens. The results confirmed those obtained from our previous experiments indicating that BAC clone of MDV containing REV-LTR was less pathogenic than that without LTR or the wild type MDV. The data also demonstrated the REV-LTR was stable in MDV recovered from inoculated chickens, but only for one week after infection. The information is important, as it adds significantly to the knowledge in the area of retroviral gene insertion into large DNA viruses, an important new area of research in the molecular biology of avian tumor viruses; it should help better understand the molecular basis for pathogenesis and shedding of these two important poultry viruses.
Technical Abstract: Co-cultivation of strain JM/102W of Marek’s disease virus (MDV) with reticuloendotheliosis virus (REV) resulted in the generation of a recombinant MDV containing REV long terminal repeat (LTR) named RM1 strain of MDV; a strain that was highly attenuated for oncogenicity, but induced severe bursal and thymic atrophy (BTA) (Witter et al., 1997). Whether the altered phenotype was as a direct result of insertion of the REV LTR into the MDV genome was not determined. Recently, we artificially inserted REV LTR into a bacterial artificial chromosome (BAC) clone of a very virulent strain of MDV, Md5 that was designated rMd5-RM1-LTR (Kim et al., 2011). In the present study, susceptible chickens with or without MD maternal antibodies were used to study the influence of REV LTR insert on the pathogenicity of strain Md5 of MDV. Chickens were inoculated at hatch with rMd5-RM1-LTR, rMd5 BAC parental virus, wild type of strain Md5, or strain RM1 of MDV. Chickens were observed for MDV-induced tumors and BTA for 8 weeks of age. The rMd5-RM1-LTR virus induced BTA and tumors at passage 10; however was highly attenuated at passage 40 and caused no BTA or tumors. Using PCR analysis, REV LTR insert was detected in MDV isolated from buffy coat cells collected from chickens inoculated with rMd5-RM1-LTR, but only at 1 week post-inoculation. The data suggest that the presence of REV LTR insert within MDV genome for one week PI with virus at hatch is sufficient to cause reduction in pathogenicity of strain Md5 of MDV.