Enhanced long-term expression from helper virus-free HSV-1 vectors packaged in the presence of deletions in genes that modulate the function of VP16, UL46 and UL47

Herpes simplex virus (HSV-1) gene expression is hypothesized to shut off recombinant gene expression from HSV-1 vectors, but in a helper virus-free HSV-1 vector system, a number of promoters support only short-term expression. Thus paradoxically, recombinant gene expression remains short-term in the absence of almost all (∼99%) of the HSV-1 genome. To resolve this paradox, we hypothesize that specific HSV-1 proteins that affect the virion can shut off recombinant gene expression. In an earlier study, we examined the effects on recombinant gene expression of five different proteins that affect the HSV-1 virion. We found that vectors packaged in the presence of mutated vhs or US11 exhibited minimal changes in gene expression, vectors packaged in the presence of a mutated US3 supported improved gene transfer (numbers of cells at 4 days), and vectors packaged in the presence of mutated UL13 or VP16 supported improved long-term expression. The capability of the VP16 transcriptional complex to reduce gene expression deserves additional study because VP16 is a powerful enhancer that interacts with a number of cellular and viral proteins. In particular, UL46 and UL47 are known to modulate the effects of VP16 on immediate early promoters. In this study, we examined expression from a HSV-1 vector that contains a neuronal-specific promoter and was packaged in the presence of deletions in UL46, or UL47, or both UL46 and UL47. In the rat striatum, each of these vector stocks supported both improved gene transfer (numbers of cells at 4 days) and improved long-term expression (2 months). Vectors packaged in the presence of a deletion in both UL46 and UL47 supported larger improvements in gene expression compared to vectors packaged in the presence of deletions in either gene alone. The implications of these results for strategies to improve long-term expression are discussed.