P01.10 Dock4 heterozygous deletion normalizes the glioblastoma microenvironment & improves response to radiation

Abstract

Introduction: Invasion and growth of brain tumours require cooperation of the tumour cell and endothelial cell compartments. Limited efficacy of anti-angiogenic therapy in glioblastoma is associated with activation of VEGF independent mechanisms, and the development of hypoxia. Those can be overcome by targeting intracellular signaling pathways, and designing strategies for blood vessel normalization. Here we investigate the role of the Rac1 exchange factor Dock4, a regulator of the small GTPase Rac1, in cancer cell invasion and the microenvironment of glioblastoma. Materials and Methods: We assessed Dock4 expression in glioblastoma patient samples. The invasive potential of cancer cells with Dock4 knockdown was tested in spheroid assays in collagen matrix. To investigate the role of Dock4 in the microenvironment, the murine glioma cell line CT2A was injected intracranially into Dock4 heterozygous mice and wild type mice. Mice of each genotype were randomly allocated into two groups: control or treatment with radiation therapy, 2Gy daily x 5 days or 5 Gy daily x 3 days, delivered using an image guided small animal irradiation platform (SARRP). Results: We demonstrate that Dock4 expression in patient samples was associated with high nestin positivity. Knocking down Dock4 suppressed invasion of patient derived neurospheres in collagen. Heterozygous Dock4 genetic deletion in the microenvironment reduced blood vessel size, and reversed morphological abnormalities including the lining of blood vessels by cancer cells in syngeneic tumours. In this pilot study, tumours growing in Dock4 heterozygous mice showed improved response to radiation compared to tumours growing in wild type mice. Conclusions: These findings ­suggest that targeting the Dock4 pathway may provide a means of improving radiotherapy by normalizing the blood vessels, which may provide a therapeutic opportunity in parallel with inhibiting glioblastoma invasion.