Neurovirulent factor ICP34.5 uniquely expressed in the herpes simplex virus type 1 Δγ134.5 mutant 1716

The herpes simplex virus type 1 (HSV-1) diploid gene γ 1 34.5 encodes a neurovirulent factor, infected cell protein 34.5 (ICP34.5). The promoter to γ134.5 is located within the HSV-1 genome where there are repeated sequences. This region of the genome also contains important overlapping transcripts involved with the virus’s ability to establish lytic and latent infections and reactivation. These transcripts include the latency-associated transcripts and regulator proteins ICP0 and ICP4. This study aimed to separate ICP34.5 from these overlapping transcripts and test if its expression from a single gene could restore wild-type HSV-1 strain 17+ virulence. To address these aims, different recombinant viruses were constructed using the Δγ 1 34.5 mutant 1716. Immunoblots probed with different ICP34.5 antisera demonstrated that one of the newly generated recombinant viruses, 1622, overexpresses ICP34.5 relative to a panel of wild-type viruses. Interestingly, the overexpression of ICP34.5 does not yield a more virulent virus. The onset of ICP34.5 expression from 1622-infected cells in vitro matched that of 17+, and its expression restored the function of maintaining protein synthesis in human neuroblastoma cells. Replication of 1622, however, was only partially restored to 17+ levels in vivo . Additionally, plaque morphology from 1622-infected cells indicates there is an additional defect. The authors report that the mutant virus 1622 can express ICP34.5 from a single γ 1 34.5 gene and restore most (but not all) wild-type function. These findings are discussed with respect to the use of the γ 1 34.5 deleted mutant, 1716, in oncolytic viral vector therapies and future studies for ICP34.5.