Vaccine and oncogenic strains of gallid herpesvirus 2 contain specific subtype variations in the 5′ region of the latency-associated transcript that evolve in vitro and in vivo

Abstract

Gallid herpesvirus 2 (GaHV-2) is the alphaherpesvirus responsible for Marek's disease (MD), a T-cell lymphoma of chickens. The virulence of the GaHV-2 field strain is steadily increasing, but MD is still controlled by the CVI988/Rispens vaccine. We tried to determine distinguishing traits of the CVI988/Rispens vaccine by focusing on the 5' end region of the latency-associated transcript (5'LAT). It includes a variable number of 60-bp tandem repeats depending on the GaHV-2 strain. By analyzing six batches of vaccine, we showed that CVI988/Rispens consisted of a population of 5'LAT molecular subtypes, all with deletions and lacking 60-bp tandem repeat motifs, with two major subtypes that probably constitute CVI988/Rispens markers. Serial passages in cell culture led to a substantial change in the frequency of CVI988/Rispens 5'LAT subtypes, with non-deleted subtypes harboring up to four 60-bp repeats emerging during the last few passages. Dynamic changes in the distribution of 5'LAT-deleted subtypes were also detected after infection of chickens. By contrast, the 5'LAT region of the oncogenic clonal RB-1B strain, which was investigated at every step from the isolation of the clonal bacmid RB-1B DNA to the isolation of the ovarian lymphoma cell line, consisted of non-deleted 5'LAT subtypes harboring at least two 60-bp repeats. Thus, vaccine and oncogenic GaHV-2 strains consist of specific populations of viral genomes that are constantly evolving in vivo and in vitro and providing potential markers for epidemiological surveys.

Marek’s Disease Virus Virulence Genes Encode Circular RNAs

“…The identification of circRNAs encoded from previously characterized genes that play key roles in Marek’s disease pathogenesis challenges our current knowledge and assigned functions of these virulence factors. Indeed, studies addressing the roles of meq ( 35 – 38 , 54 , 55 ), pp14 ( 50 , 56 , 57 ), LATs ( 39 , 45 , 46 ), and ERL ( 44 ), ignored the existence of these original circular transcripts. In that regard, identification of circRNA expression during KSHV and EBV infection from previously characterized loci extended the functions of the related genes.…”

“…In alphaherpesviruses, LATs belong to a family of RNAs that are spliced from a lncRNA antisense to the mRNA, encoding immediate-early transactivator proteins ( 39 , 45 , 46 ). In MDV, LATs were more thoroughly characterized at their 5′ end, with the identification of several miRNA sequences processed from the first intron ( 39 , 67 ) and a polymorphism affecting both the LATs promoter and the first exons according to the virulence level of the MDV strain ( 46 , 68 , 69 ). Functional characterization of this region focused on viral miRNAs involved in Marek’s disease pathogenesis ( 70 – 72 ).…”

“…In addition, the role of numerous linear noncoding RNAs, including the viral microRNAs ( 39 , 40 ), the viral telomerase RNA subunit (vTR) ( 41 – 43 ), and two viral long noncoding RNAs (lncRNAs) have been investigated. These two lncRNAs are encoded by various splicing isoforms from the repeat regions surrounding the unique long regions (IR L /TR L ) for the edited repeat long (ERL) ( 44 ) and the unique short regions (IR S /TR S ) for the latency-associated transcripts (LATs) ( 39 , 45 , 46 ).…”