Connexins are expressed in primary brain tumors and enhance the bystander effect in gene therapy

OBJECTIVE: Experimental brain tumor gene therapy with the herpes simplex vi rus thymidine kinase (HSV-tk) gene has demonstrated that not only HSV-tk tr ansduced 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 poten tial susceptibility to the bystander effect, we examined levels of connexin -26 and connexin-43 expression in a series of primary brain tumors. Connexi n-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 over expressing connexin in tumor cells with high basal levels of connexin expre ssion. METHODS: Western blot analysis and immunohistochemistry were used to determ ine levels of connexin-26 and connexin-43 expression in a series of primary brain tumors. Wild-type 9L gliosarcoma cells were transfected in vitro wit h the connexin-43 gene and the HSV-tk gene or the HSV-tk gene alone. The by stander effect of each transfectant was then assessed and compared. RESULTS: Most of the primary brain tumors tested, including low-grade astro cytomas, anaplastic astrocytomas, glioblastomas, oligodendrogliomas, gangli ogliomas, meningiomas, and medulloblastomas, showed connexin-26 and connexi n-43 expression. Bystander experiments revealed a significant enhancement o f the bystander effect in the gliosarcoma cells transfected with connexin-4 3 and HSV-tk, as compared with gliosarcoma cells transfected with HSV-tk al one. CONCLUSION: Most primary brain tumors express connexin-26 and connexin-43. This suggests that most primary brain tumors may be susceptible to the byst ander effect of HSV-tk gene therapy. The bystander effect can be enhanced i n vitro by overexpression of connexin-43 in a cell line with a high basal l evel of connexin-43 expression.