OBJECTIVE:Experimental brain tumor gene therapy with the herpes simplex virus thymidine kinase (HSV-tk) gene has demonstrated that not only HSV-tk transduced but surrounding non-HSV-tk transduced cells are killed when given ganciclovir. This so-called bystander effect has recently been shown to be dependent on connexin-mediated intercellular communication. To assess potential susceptibility to the bystander effect, we examined levels of connexin-26 and connexin-43 expression in a series of primary brain tumors. Connexin-26 expression has not previously been studied in primary brain tumors and connexin-43 expression has not been studied in nonastrocytic primary brain tumors. We also attempted to enhance the bystander effect in vitro by overexpressing connexin in tumor cells with high basal levels of connexin expression.METHODS:Western blot analysis and immunohistochemistry were used to determine levels of connexin-26 and connexin-43 expression in a series of primary brain tumors. Wild-type 9L gliosarcoma cells were transfected in vitro with the connexin-43 gene and the HSV-tk gene or the HSV-tk gene alone. The bystander effect of each transfectant was then assessed and compared.RESULTS:Most of the primary brain tumors tested, including low-grade astrocytomas, anaplastic astrocytomas, glioblastomas, oligodendrogliomas, gangliogliomas, meningiomas, and medulloblastomas, showed connexin-26 and connexin-43 expression. Bystander experiments revealed a significant enhancement of the bystander effect in the gliosarcoma cells transfected with connexin-43 and HSV-tk, as compared with gliosarcoma cells transfected with HSV-tk alone.CONCLUSION:Most primary brain tumors express connexin-26 and connexin-43. This suggests that most primary brain tumors may be susceptible to the bystander effect of HSV-tk gene therapy. The bystander effect can be enhanced in vitro by overexpression of connexin-43 in a cell line with a high basal level of connexin-43 expression.