Submitted to: Journal of General Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2006
Publication Date: 4/24/2007
Citation: Spatz, S.J., Nair, V. 2007. Comparative full-length sequence analysis of oncogenic and vaccine (Rispens) strains of Marek's disease virus. Journal of General Virology. 88:1080-1096. Interpretive Summary: Marek’s disease is a highly contagious disease of chicken caused by a herpesvirus known as Marek’s disease virus or MDV. Birds infected with this virus develop leukemia of T-lymphocytes, tumor of lymphoid tissues, paralysis and die shortly after infection. Since the late 1960s this disease is prevented through mass vaccination of poultry and represents the first instance in which a vaccine can prevent cancer. Because of this rigorous vaccine program the MD virus found in the field has been evolving and every decade or so newer vaccines have to be introduced to keep the disease in check. Three difference vaccine formulations have be introduced over the last 35 years and most recently the field virus has evolved to greater disease capability or virulence. It is feared that current vaccine programs will fail to protect poultry against these newer “superbugs.” To understand the reason why the current vaccines are becoming less efficacious, SEPRL and the Institute for Animal Health in Compton, UK began a program to determine the genetic differences between the vaccine and disease-causing or virulent field strains. To this end, the complete DNA sequence of the most-utilized vaccine strain, CVI988 was determined using techniques identical to those used in the human genome project. This DNA sequence was then compared to sequences of virulent strains of MDV and the differences were noted. Nine regions in the DNA of the vaccine strain differed from that of the field strains. One major difference was that the vaccine strain had an additional sequence of DNA inserted in a gene (known as meq) proven to be involved in tumor formation. Other differences were in genes involved in assembling the virus particle. This information provided insight into the genes involved in Marek’s disease and will be instrumental in genetic engineering new vaccines that will protect chickens against these ever evolving virulent field viruses.
Technical Abstract: The complete DNA sequence of the Marek’s disease virus serotype 1 vaccine strain CVI988 was determined and consists of 178,311 bp with an overall gene organization identical to that of the oncogenic strains. In examining open reading frames (ORFs), nine ORFs differ between vaccine and oncogenic strains. A 177-base pair insertion was identified in the overlapping genes encoding the Meq, RLORF6 and 23 kD proteins of CVI988. Three ORFs are predicted to encode truncated proteins. One, designated 49.1, overlaps the gene encoding the large tegument protein UL36 and encodes a severely truncated protein of 34 aa. The others, ORF5.5/ORF75.91 and ORF3.0/78.0, located in the repeat regions (diploid), encode a previously unidentified ORF of 52 aa and a truncated version of the virus-encoded chemokine (vIL8), respectively. Subtle genetic changes were identified in two ORFs encoding tegument proteins UL36 and UL49. Only one diploid ORF (ORF6.2/ORF75.6) present in the genomes of the three virulent strains is absent in the CVI988-BAC genome. Seventy non-synonymous amino acid substitutions were identified that could differentiate CVI988-BAC from all three oncogenic strains collectively. Estimates of the non-synonymous to synonymous substitution ratio (omega) indicate that CVI988 ORFs generally are under purifying selection (omega <1), while UL39, UL49, UL50, RLORF6 and RLORF7 (Meq) appear to evolve under relaxed selective constraints. No CVI988 ORF was found to be under positive evolutionary selection (omega >>1). Consistent with its attenuated phenotype, CVI988 contains 14 copies of the 132-bp repeat that has previously been associated with viral attenuation and loss of oncogenicity.