Herpes simplex virus type 2 mutants with deletions in the intergenic region between ICP4 and ICP22/47: identification of nonessential cis-acting elements in the context of the viral genome

Transcriptional targeting of herpes simplex virus for cell-specific replication

Tissue- or cell-specific targeting of vectors is critical to the success of gene therapy. We describe a novel approach to virus-mediated gene therapy, where viral replication and associated cytotoxicity are limited to a specific cell type by the regulated expression of an essential immediate-early viral gene product. This is illustrated with a herpes simplex virus type 1 (HSV-1) vector (G92A) whose growth is restricted to albumin-expressing cells. G92A was constructed by inserting an albumin enhancer/promoter-ICP4 transgene into the thymidine kinase gene of mutant HSV-1 d120, deleted for both copies of the ICP4 gene. This vector also contains the Escherichia coli lacZ gene under control of the thymidine kinase promoter, a viral early promoter, to permit easy detection of infected cells containing replicating vector. In the adult, albumin is expressed uniquely in the liver and in hepatocellular carcinoma and is transcriptionally regulated. The plaquing efficiency of G92A is > 10(3) times higher on human hepatoma cells than on non-albumin-expressing human cells. The growth kinetics of G92A in albumin-expressing cells is delayed compared with that of wild-type HSV-1, likely due to aberrant expression of ICP4 protein. Cells undergoing a productive infection expressed detectable levels of ICP4 protein, as well as the reporter gene product beta-galactosidase. Confining a productive, cytotoxic viral infection to a specific cell type should be useful for tumor therapy and the ablation of specific cell types for the generation of animal models of disease.

Construction and properties of a recombinant herpes simplex virus 1 lacking both S-component origins of DNA synthesis

The herpes simplex virus 1 (HSV-1) genome contains three origins of DNA synthesis (Ori) utilized by viral DNA synthesis proteins. One sequence (OriI) maps in the L component, whereas two sequences (OriS) map in the S component. We report the construction of a recombinant virus, R7711, from which both OriS sequences have been deleted, and show that the OriS sequences are not essential for the replication of HSV-1 in cultured cells. In addition to the deletions of OriS in R7711, the alpha 47 gene and the 5' untranscribed and transcribed noncoding regions of the U(S)11 gene were deleted, one of the alpha 4 promoter-regulatory regions was replaced with the simian virus 40 promoter, and the alpha 22 promoter was substituted with the alpha 27 promoter. The total amount of viral DNA synthesized in Vero cells infected with the OriS-negative (OriS-) virus was approximately that seen in cells infected with the OriS-positive virus. However, cells infected with the OriS- virus accumulated viral DNA more slowly than those infected with the wild-type virus during the first few hours after the onset of DNA synthesis. In single-step growth experiments, the yield of OriS- progeny virus was reduced at most fourfold. Although a single OriS (R. Longnecker and B. Roizman, J. Virol. 58:583-591, 1986) and the single OriL (M. Polvino-Bodnar, P. K. Orberg, and P. A. Schaffer, J. Virol. 61:3528-3535, 1987) have been shown to be dispensable, this is the first indication that both copies of OriS are dispensable and that one copy of an Ori sequence may suffice for the replication of HSV-1.

Elements in the transcriptional regulatory region flanking herpes simplex virus type 1 oriS stimulate origin function

Like other DNA-containing viruses, the three origins of herpes simplex virus type 1 (HSV-1) DNA replication are flanked by sequences containing transcriptional regulatory elements. In a transient plasmid replication assay, deletion of sequences comprising the transcriptional regulatory elements of ICP4 and ICP22/47, which flank oriS, resulted in a greater than 80-fold decrease in origin function compared with a plasmid, pOS-822, which retains these sequences. In an effort to identify specific cis-acting elements responsible for this effect, we conducted systematic deletion analysis of the flanking region with plasmid pOS-822 and tested the resulting mutant plasmids for origin function. Stimulation by cis-acting elements was shown to be both distance and orientation dependent, as changes in either parameter resulted in a decrease in oriS function. Additional evidence for the stimulatory effect of flanking sequences on origin function was demonstrated by replacement of these sequences with the cytomegalovirus immediate-early promoter, resulting in nearly wild-type levels of oriS function. In competition experiments, cotransfection of cells with the test plasmid, pOS-822, and increasing molar concentrations of a competitor plasmid which contained the ICP4 and ICP22/47 transcriptional regulatory regions but lacked core origin sequences resulted in a significant reduction in the replication efficiency of pOS-822, demonstrating that factors which bind specifically to the oriS-flanking sequences are likely involved as auxiliary proteins in oriS function. Together, these studies demonstrate that trans-acting factors and the sites to which they bind play a critical role in the efficiency of HSV-1 DNA replication from oriS in transient-replication assays.