Bacterial-Artificial-Chromosome-Based Genome Editing Methods and the Applications in Herpesvirus Research

A simple method for rapid cloning of complete herpesvirus genomes

Herpesviruses are large DNA viruses and include important human and veterinary pathogens. Their genomes can be cloned as bacterial artificial chromosomes (BACs) and genetically engineered in Escherichia coli using BAC recombineering methods. While the recombineering methods are efficient, the initial BAC-cloning step remains laborious. To overcome this limitation, we have developed a simple, rapid, and efficient BAC-cloning method based on single-step transformation-associated recombination (STAR) in Saccharomyces cerevisiae. The linear viral genome is directly integrated into a vector comprising a yeast centromeric plasmid and a BAC replicon. Following transfer into E. coli, the viral genome can be modified using standard BAC recombineering techniques. We demonstrate the speed, fidelity, and broad applicability of STAR by cloning two strains of both rat cytomegalovirus (a betaherpesvirus) and Kaposi's sarcoma-associated herpesvirus (a gammaherpesvirus). STAR cloning facilitates the functional genetic analysis of herpesviruses and other large DNA viruses and their use as vaccines and therapeutic vectors.

Genome editing of pseudorabies virus in the CRISPR/Cas9 era: a mini-review

Pseudorabies virus (PRV) is an important swine virus that has a significant impact on the global swine industry. PRV is a member of the herpesvirus family, specifically the alphaherpesvirus subfamily, and has been extensively utilized as a prototype herpesvirus. Notably, recent studies have reported that PRV sporadically spills over into humans. The PRV genome is approximately 150 kb in size and is difficult to manipulate at the genomic level. The development of clustered regularly interspaced short palindromic repeat-associated protein (CRISPR/Cas9) technology has revolutionized PRV genome editing. CRISPR/Cas9 has been widely used in the construction of reporter viruses, knock-out/knock-in of genes of interest, single virus tracking and antiviral strategies. Most importantly, for vaccine development, virulence gene knockout PRV vaccine candidates can be obtained within 2 weeks using CRISPR/Cas9. In this mini-review, we provide a concise overview of the application of CRISPR/Cas9 in PRV research and mainly share our experience with methods for efficiently editing the PRV genome. Through this review, we hope to give researchers better insight into the genome editing of pseudorabies virus.