Induced Expression of the Conditionally Cytotoxic Herpes Simplex Virus thymidine kinase Gene by Means of a Parvoviral Regulatory Circuit

Abstract

As a step toward the achievement of targeted expression of toxic genes, we have established a model system using the selective trans-activation of the late promoter P₃₈ of Minute Virus of Mice (MVMp) by the parvoviral nonstructural protein NS-1. The conditionally toxic herpes simplex virus type 1 thymidine kinase (tk) gene (HSV1-tk) was cloned under the control of the P₃₈ promoter and transfected into NIH-3T3 TK¯ cells. Treatment of the stably transfected cells with acyclovir (ACV) followed by infection with MVMp reduced cell survival by 3.5- to 5-fold compared to the toxic effects of ACV or MVMp alone. These results indicate that it should be possible to combine the genuine cytopathic action of parvoviruses with a specific activation of toxic genes driven by parvoviral promoters, to achieve the targeted destruction of parvovirus-expressing (in particular tumor) cells.

Overview summary

Parvoviruses are oncotropic agents that are considered as potential vectors for the delivery of toxic genes into tumor cells. The herpes simplex virus type 1 thymidine kinase (HSV1-tk) gene, which is toxic when expressed in the presence of the nucleoside analog acyclovir (ACV), was placed under the control of the structural genes' promoter (P₃₈) of parvovirus Minute Virus of Mice (MVMp). Cells that were stably transfected with this construct were killed when treated concomitantly with ACV and MVM. Because P₃₈ is trans-activated by the parvoviral regulatory proteins, these observations suggest that parvovirus-based vectors may be used to target a toxic phenotype in cells that are competent for parvovirus gene expression, which include, more particularly, transformed cells.