Radiation lays down covering fire for viral shock troops

In a recent study of combined vaccinia virus (VV) and radiation therapy (RT) in BRAF-mutant melanomas, we dissected the precise events by which RT enhances viral cytotoxicity. Our studies revealed that the complex interplay between RT, virus and cell signaling pathways caused a completely novel synthetically lethal form of tumor cell death. We initially sought to assess the effect of combining a VV (GLV-1h68) and RT in melanoma cell lines with different genetic backgrounds (BRAF-mutant, RAS-mutant and BRAF/ RAS-wild-type). We hypothesised that RT would induce stress responses in all tumor genotypes and activate host survival pathways to create an environment favorable to viral replication and subsequent oncolysis. Contrary to our expectations, enhanced tumor cell kill with the combination of virus and RT only occurred in BRAF-mutant melanoma cells. In mechanistic studies, it was clear that VV triggered activation of the c-Jun N-terminal Kinase (JNK) pathway and tumor necrosis factor-α (TNF-α) signaling in all genotypes. In BRAF-mutant cells, virus-induced TNF-α production was associated with anti-apoptotic/pro-survival effects, as previously described with MEK inhibitor-induced cytotoxicity [1]. Remarkably, when radiation was combined with VV, JNK and TNF-α signaling were shut down and this triggered significant apoptotic death – but only in cells with a BRAF mutation. We were able to mimic this ability of RT to enhance virally-mediated cytotoxicity by switching off JNK by small molecule inhibition (SP600125) or siRNA-mediated gene silencing, but again this was only possible in BRAF-mutant cells. This represents an unprecedented synthetically lethal interaction between radiation and VV that is explicable in terms of manipulation of cell survival pathways. Remarkably, the enhanced cell death seen in BRAF-mutant melanoma was associated with significantly reduced viral production, suggesting that " oncolysis " is a complex process that cannot be explained purely in terms of viral replication. It is hoped that similar studies with vaccinia (or other oncolytic viruses) in particular tumor-specific genetic contexts may reveal other clinically translatable synthetically lethal combinations. For now, future studies of VV and radiation (for local disease) or small molecule inhibitors (for systemic disease) should be considered. GLV-1h68 and its clinical equivalent 'GLONC-1' are Lister strain oncolytic VV that have been attenuated by disruption of viral thymidine kinase and hemagglutinin genes and altered with galactosidase, glucuronidase and Renilla luciferase/green fluorescent protein gene insertions. GLONC-1 is currently in Phase I single-agent clinical trials using intravenous, intrapleural or intraperitoneal delivery. Single-agent VV is known to interact with the mitogen-activated …