Distinct polymorphisms in a single herpesvirus gene are capable of enhancing virulence and mediating vaccinal resistance

Abstract

Modified-live herpesvirus vaccines are widely used in humans and animals, but field strains can emerge that have a higher virulence and break vaccinal protection. Since the introduction of the first vaccine in the 1970s, Marek’s disease virus overcame the vaccine barrier by the acquisition of numerous genomic mutations. However, the evolutionary adaptations in the herpesvirus genome responsible for the vaccine breaks have remained elusive. Here, we demonstrate that point mutations in the multifunctional meq gene acquired during evolution can significantly alter virulence. Defined mutations found in highly virulent strains also allowed the virus to overcome innate cellular responses and vaccinal protection. Concomitantly, the adaptations in meq enhanced virus shedding into the environment, likely providing a selective advantage for the virus. Our study provides the first experimental evidence that few point mutations in a single herpesviral gene result in drastically increased virulence, enhanced shedding, and escape from vaccinal protection.

Marek’s Disease Virus Virulence Genes Encode Circular RNAs

“…Without reconsidering the roles of the Meq protein as a functional transcriptional factor controlling a large set of viral and cellular genes, and without questioning data showing the impact of meq sequence polymorphisms associated with various levels of MDV virulence ( 62 , 63 ), the phenotype of meq deletion mutants ( 38 , 55 , 64 – 66 ) might be challenged by the present study. Mechanistic studies on the circRNAs produced from the meq transcriptional unit might reveal additional functions or another kind of regulation linked to the major MDV oncogene.…”

Characterization of a Novel Viral Interleukin 8 (vIL-8) Splice Variant Encoded by Marek’s Disease Virus

“…The following viruses were generated in our study: vE3′-FLAG contains a FLAG tag with a glycine-serine (GS) linker at 89 bp of the vIL-8 intron II, which is thereby only encoded in the novel vIL-8-E3′ and not the previously known vIL-8 exons; vΔE3′ and vΔE3′-FLAG contain a point mutation (G to A) in the novel splice acceptor site at 82 bp of intron II to abrogate splicing and expression of this putative novel protein ( Figure 1 A). All recombinant viruses were confirmed by restriction fragment length polymorphism (RFLP), PCR, Sanger sequencing, and Illumina MiSeq sequencing with more than 1000-fold coverage to ensure that the entire viral genome is correct [ 19 ]. In addition, we used a previously generated mutant virus that lacks the expression of the vIL-8 chemokine (vΔMetvIL-8) due to the mutation of its start codon [ 9 ].…”

“…DNA was isolated from all blood samples using the NucleoSpin 96 Blood Core Kit (Macherey-Nagel; Düren, Germany) according to the manufacturer’s instructions. To evaluate the efficiency of the virus delivery to and replication in the feather follicle epithelium (FFE), proximal ends of each feather containing the feather pulp were collected from infected birds at 14, 21, 28, 35, and 42 dpi as described previously [ 19 , 23 ]. DNA was extracted through treatment of the feather pulp with proteinase K at 55 °C overnight, followed by phenol:chloroform:isoamyl alcohol extraction and ethanol precipitation as described previously [ 24 ].…”

Identification and Validation of Ikaros (IKZF1) as a Cancer Driver Gene for Marek’s Disease Virus-Induced Lymphomas

“…Additional experiments in which Meq was deleted from the MDV genome validated that Meq is necessary for transformation [12]. More recently, recombinant viruses with alleles from MDVs that vary in pathogenicity have clearly demonstrated that key Meq polymorphisms are associated with virulence and evading vaccinal resistance [13]. However, other viral genes in the ~175 Kb genome are also likely to be relevant.…”