Comparative Full-length Sequence Analysis Of Vaccine (814) Strain Of Marek's Disease Virus

Marek's disease (MD) is characterized by immune suppression, neurological disorder and T-cell lymphoma of chickens which is caused by Marek's disease virus (MDV). MDV is a member of the subfamily Alphaherpesvirinae in the family Herpesviridae. Based on their serological characteristics, MDVs are divided into three serotypes-MDV1,MDV2 and HVT. Only MDV serotypeâ… can cause tumor formation.The disease is largely controlled through mass vaccination with live-attenuated strains of MDV-1. Despite the efforts to control MD through mass vaccination strategies, there is evidence that MDV1 has evolved to greater virulence during the last 40 years. However, there is great concern within the poultry industry that current vaccines will fail to protect against these evolving virulent field isolates. Although both clinical and laboratory data agree that field isolates have evolved to greater virulence, the molecular basis for this evolution is still unknown. In order to understand deeply the pathogenicity of MDV, a better understanding of the genes involved in virulence is needed.MDV1 vaccine strain 814 is naturally attenuated strain. It is used as vaccine to control MD for a long time in China. In order to understand genetic variation of full length genome in virulent and attenuated strains of MDV, in the study, based on the genomic sequence of MDV strain GA; sixty-six pairs of primers were designed and synthesized to amplify the full-length genome. Then amplified fragments were cloned and sequenced. After sequence assembly and analysis,we first obtained the complete DNA sequence of the MDV1 vaccine strain 814,which consists of 172,541 bp with an overall gene organization identical to that of the oncogenic strains. Phylogenetically, the genome of vaccine strain 814 was more similar to that of CVI988. Comparative analysis among the strain 814 and other virulent,attenuated strains logged in Genbank was carried out. The results showed that, A 177 bp insertion was identified in the overlapping genes encoding the Meq,RLORF6 and 23 kDa proteins of strain 814, the same with vaccine strain CVI988; 69bp deletion in the gene encoding RLORF12 of strain 814 was found. The deletion was located in the Origin of replication site (Ori) of MDV. A deletion of 510 bp in the UL36 protein which was the largest structural protein within the tegument of virions was also identified. A total of 24 single nucleotide polymorphisms (SNPs) common to both 814 and CVI988, and not occurring in virulent strains, were identified in the genes encoding UL12,UL38,UL39,UL43,LORF8,LORF11,UL46,UL49,UL50,UL52,UL53,RLORF7,RLORF9,RLORF4,US2,US3,SORF4,US6. Meanwhile, 43 specific amino acid mutations of strain 814 were found in 23 ORFs.In this paper, we first present the sequence of the full-length strain 814 genome. The genomic structure and gene content will be compared with those of three oncogenic strains of MDV: Md5 and Md11, vaccine strain of MDV: CVI988 and mild virulent strain of MDV: CU-2. Comparative analysis of full length of sequence analysis of MDV vaccine strain 814 genome contributes to the research on pathogenicity,immunity and attenuation of MDV.