In vitro transposon mutagenesis of an equine herpesvirus 1 genome cloned as a bacterial artificial chromosome

Impact of equine herpesvirus-1 ORF15 (EUL45) on viral replication and neurovirulence

Equine herpesvirus 1 (EHV-1) causes respiratory illness, fetal loss, perinatal mortality, and myeloencephalopathy. This study investigated ORF15's impact on virus infectivity and neurovirulence. The Ab4p neurovirulent strain of EHV1 was used as a backbone to create Ab4p attB, Ab4p∆ORF15, and Ab4p∆ORF15R chimeras via BAC DNA transfection into RK-13 cells. Viral growth kinetics, plaque size, transcription, and growth were assessed in MDBK cells, mouse neurons, and fetal equine brain cells. Neurovirulence was evaluated post-intranasal inoculation into male CBA/N1 SPF mice, measuring signs, virus titers, and histopathological changes. Deletion of EUL45 (Ab4p-∆EUL45) reduced viral replication efficiency, resulting in decreased release and smaller plaques. EUL45 deletion also upregulated neighbouring genes (EUL46 and EUL44). Ab4p-∆EUL45 exhibited reduced virulence and poor growth in neural cells compared to wild-type viruses. This study sheds light on EUL45's role in EHV-1, viral replication, and regulation of EUL46 and EUL44 expression, suggesting potential as a vaccine candidate.

Identification of non-essential loci within the Meleagrid herpesvirus 1 genome

Background

Meleagrid herpesvirus 1 (MeHV-1) infectious bacterial artificial chromosomes (iBACs) are ideal vectors for the development of recombinant vaccines for the poultry industry. However, the full potential of iBACS as vectors can only be realised after thorough genetic characterisation, including identification of those genetic locations that are non-essential for virus replication. Generally, transposition has proven to be a highly effective strategy for rapid and efficient mutagenesis of iBAC clones. The current study describes the characterisation of 34 MeHV-1 mutants containing transposon insertions within the pMeHV1-C18 iBAC genome.

Methods

Tn5 and MuA transposition methods were used to generate a library of 76 MeHV-1 insertion mutants. The capacity of each mutant to facilitate the recovery of infectious MeHV-1 was determined by the transfection of clone DNA into chicken embryo fibroblasts.

Results

Attempts to recover infectious virus from the modified clones identified 14 genetic locations that were essential for MeHV-1 replication in cell culture. Infectious MeHV-1 was recovered from the remaining 14 intragenic insertion mutants and six intergenic insertion mutants, suggesting that the respective insertion locations are non-essential for MeHV-1 replication in cell culture.

Conclusions

The essential and non-essential designations for those MeHV-1 genes characterised in this study were generally in agreement with previous reports for other herpesviruses homologues. However, the requirement for the mardivirus-specific genes LORF4A and LORF5 are reported for the first time. These findings will help direct future work on the development of recombinant poultry vaccines using MeHV-1 as a vector by identifying potential transgene insertion sites within the viral genome.

Electronic supplementary material

The online version of this article (doi10.1186/s12985-015-0362-9) contains supplementary material, which is available to authorized users.

Herpesvirus mutagenesis facilitated by infectious bacterial artificial chromosomes (iBACs)

A critical factor in the study of herpesviruses, their genes and gene functions is the capacity to derive mutants that harbor deletions, truncations, or insertions within the genetic elements of interest. Once constructed the impact of the introduced mutation on the phenotypic properties of the rescued virus can be determined in either in vitro or in vivo systems. However, the construction of such mutants by traditional virological mutagenesis techniques can be a difficult and laborious undertaking. The maintenance of a viral genome as an infectious bacterial artificial chromosome (iBAC), however, endows the capacity to manipulate the viral genome for mutagenesis studies with relative ease. Here, the construction and characterization of two gene deletion mutants of an alphaherpesvirus maintained as iBAC in combination with an inducible homologous recombination system in Escherichia coli is detailed. The methodology is generally applicable to any iBAC and is demonstrated to be a highly efficient and informative approach for mutant virus construction.

Equine herpesvirus 4: recent advances using BAC technology

The equine herpesviruses are major infectious pathogens that threaten equine health. Equine herpesvirus 4 (EHV-4) is an important equine pathogen that causes respiratory tract disease, known as rhinopneumonitis, among horses worldwide. EHV-4 genome manipulation with subsequent understanding of the viral gene functions has always been difficult due to the limited number of susceptible cell lines and the absence of small-animal models of the infection. Efficient generation of mutants of EHV-4 would significantly contribute to the rapid and accurate characterization of the viral genes. This problem has been solved recently by the cloning of the genome of EHV-4 as a stable and infectious bacterial artificial chromosome (BAC) without any deletions of the viral genes. Very low copy BAC vectors are the mainstay of present genomic research because of the high stability of inserted clones and the possibility of mutating any gene target in a relatively short time. Manipulation of EHV-4 genome is now feasible using the power of BAC technology, and should aid greatly in assessing the role of viral genes in the virus-host interaction.

Herpesvirus BACs: past, present, and future

The herpesviridae are a large family of DNA viruses with large and complicated genomes. Genetic manipulation and the generation of recombinant viruses have been extremely difficult. However, herpesvirus bacterial artificial chromosomes (BACs) that were developed approximately 10 years ago have become useful and powerful genetic tools for generating recombinant viruses to study the biology and pathogenesis of herpesviruses. For example, BAC-directed deletion mutants are commonly used to determine the function and essentiality of viral genes. In this paper, we discuss the creation of herpesvirus BACs, functional analyses of herpesvirus mutants, and future applications for studies of herpesviruses. We describe commonly used methods to create and mutate herpesvirus BACs (such as site-directed mutagenesis and transposon mutagenesis). We also evaluate the potential future uses of viral BACs, including vaccine development and gene therapy.

Complete genomic sequence and an infectious BAC clone of feline herpesvirus-1 (FHV-1)

Infection with feline herpesvirus-1 (FHV-1) is a major cause of upper respiratory and ocular diseases in Felidae. We report the first complete genomic sequence of FHV-1, as well as the construction and characterization of a bacterial artificial chromosome (BAC) clone of FHV-1, which contains the entire FHV-1 genome and has the BAC vector inserted at the left end of U(L). Complete genomic sequences were derived from both the FHV-1 BAC clone and purified virion DNA. The FHV-1 genome is 135,797bp in size with an overall G+C content of 45%. A total of 78 open reading frames were predicted, encoding 74 distinct proteins. The gene arrangement is collinear with that of most sequenced varicelloviruses. The virus regenerated from the BAC was very similar to the parental C-27 strain in vitro in terms of plaque morphology and growth characteristics and highly virulent in cats in a preliminary in vivo study.