Pervasive differential splicing in Marek’s Disease Virus can discriminate CVI-988 vaccine strain from RB-1B virulent strain in chicken embryonic fibroblasts

Abstract

Mareks disease is a major scourge challenging poultry health worldwide. It is caused by the highly contagious Mareks disease virus (MDV), an alphaherpesvirus. Here we show that, similar to other members of its Herpesviridae family, MDV also presents a complex landscape of splicing events, most of which are uncharacterised and/or not annotated. Quite strikingly, and although the biological relevance of this fact is unknown, we found that a number of viral splicing isoforms are strain-specific despite the close sequence similarity of the strains considered, virulent RB-1B and vaccine CVI-988. We validated our findings by devising an assay that discriminates infections caused by the two strains in chicken embryonic fibroblasts based on the presence of some RNA species. To our knowledge, this study is the first ever to accomplish such a result, emphasizing how important a comprehensive knowledge of the viral transcriptome can be to understand viral pathogenesis.\n\nImportanceMareks disease virus (MDV) causes an agro-economically important disease of chickens worldwide. Although commercial poultry are vaccinated against MDV, it is not possible to achieve sterilising immunity, and available vaccines can only protect chickens against the symptoms of the disease. Vaccinated chicken often become superinfected with virulent strains, shedding virus into the environment. The most effective MDV vaccine strain, CVI-988, shares >99% sequence identity with the prototype virulent virus strain RB-1B. Interestingly, our work shows that despite their almost identical sequences MDV strains CVI-988 and RB-1B have significantly different splicing profiles, and hence transcriptomes. We independently validated this discovery by detecting with real-time PCR some splicing isoforms expressed by MDV strain CVI-988 and absent in the transcriptome of the RB-1B strain. These results indicate that the coding potential of MDV might be much larger than previously thought, and suggest a likely underestimation of the role of the viral transcriptome in the pathogenesis and prevention of MDV.