Genome isomerization of Turkey herpesvirus: Identification of inverted unique short genomes and demonstration of protective efficacy against Marek’s disease using BAC clones.

Authors: Kim, Taejoong; Hearn, Cari; Spatz, Stephen

Submitted to: Veterinary Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2025
Publication Date: 12/21/2025
Citation: Kim, T.N., Hearn, C.J., Spatz, S.J. 2025. Genome isomerization of Turkey herpesvirus: Identification of inverted unique short genomes and demonstration of protective efficacy against Marek’s disease using BAC clones.. Veterinary Microbiology. Volume 313. https://doi.org/10.1016/j.vetmic.2025.110848.
DOI: https://doi.org/10.1016/j.vetmic.2025.110848

Interpretive Summary: Marek's Disease (MD), a viral disease caused by the oncogenic avian herpesvirus Marek's disease virus, has a significant economic impact on the poultry industry due to its widespread prevalence and early exposure of chickens. Turkey herpesvirus (HVT) has been used as an MD vaccine to control the disease caused by virulent Marek’s disease virus. The detailed mechanism of HVT genome replication and the variants of the HVT genome that occur during virus replication are poorly understood. The HVT genome can create four different forms by mixing its repeated genetic sections. We found two distinct isomeric forms of the molecularly cloned HVT genome, the prototype and one with a reversed unique short section. No matter the differences in HVT genome structure, the effectiveness of HVT from different genome types was similar to that of the HVT vaccine. These results indicate that HVT genomes form a heterogeneous group and can produce multiple forms during replication.

Technical Abstract: Mardivirus meleagridalpha1 (MeAHV1), better known as turkey herpesvirus (HVT), was developed as a Marek’s disease (MD) vaccine to control the disease. HVT has also been used as a platform for vectored vaccines to prevent viral, bacterial, and parasitic diseases in poultry. To investigate the genome concatemerization of MeAHV1 during replication, the HVT genome was cloned as a bacterial artificial chromosome (BAC) by inserting the mini-F cassette into the genome of the HVT Fc126 strain. The entire genomic sequences of recombinant HVT-BAC were determined using long-read next-generation sequencing (Oxford Nanopore Technology). The comparative alignment of the cloned genomes indicated that HVT-BAC#1 and HVT-BAC#2 have 95.8% and 95.6% identity with the published HVT sequence (GenBank #AF291866), respectively, while HVT-BAC#3 has 99.4% identity. Interestingly, HVT-BAC#1 and HVT-BAC #2 have an inversion of the unique short (US) subgenomic region (IS isomer), while HVT-BAC#3 has a prototype (P) genome arrangement. In silico rearrangement of the inverted US regions of HVT-BAC#1 and #2 increased the sequence identity to 99.4% and 99.5%, respectively. In addition, all three HVT-BACs constructed in this study contained deletions encompassing the HVT071 ORF and the 5' end of the HVT070 ORF. The IS and P genomes of HVT-BAC were re-isolated from the DNA of HVT-BAC#1 reconstituted viruses infected cells. Regardless of the variable genomic structure of HVT-BAC clones, two HVT-BAC-derived viruses showed comparable protective efficacy against virulent Marek’s disease virus challenge, and there were no significant differences with the parental HVT virus. These data have confirmed the variability of the MeAHV1 genome in isomerization during replication.