Inhibiting BRAF Oncogene-Mediated Radioresistance Effectively Radiosensitizes BRAF-Mutant Thyroid Cancer Cells by Constraining DNA Double-Strand Break Repair.

Purpose: Activating BRAF mutations, most commonly BRAF, are a major oncogenic driver of many cancers. We explored whether BRAF promotes radiation resistance and whether selectively targeting BRAF with a BRAF inhibitor (vemurafenib, BRAFi) sensitizes BRAF thyroid cancer cells to radiotherapy.

Experimental Design: Immunoblotting, neutral comet, immunocytochemistry, functional reporter, and clonogenic assays were used to analyze the outcome and molecular characteristics following radiotherapy with or without BRAF or vemurafenib in thyroid cancer cells.

Results: BRAF thyroid cancer cell lines were associated with resistance to ionizing radiation (IR), and expression of BRAF into wild-type BRAF thyroid cancer cells led to IR resistance. BRAFi inhibited ERK signaling in BRAF mutants, but not BRAF wild-type thyroid cancer cell lines. BRAFi selectively radiosensitized and delayed resolution of IR-induced γH2AX nuclear foci in BRAF cells. Moreover, BRAFi impaired global DNA repair and altered the resolution of 53BP1 and RAD51 nuclear foci in BRAF cells following IR. BRAF mutants displayed enhanced nonhomologous end-joining (NHEJ) repair activity, which was abolished by BRAFi. Intriguingly, BRAF mutation led to upregulation of XLF, a component of NHEJ, which was prevented by BRAFi. Importantly, BRAFi in combination with radiotherapy resulted in marked and sustained tumor regression of BRAF thyroid tumor xenografts.

Conclusions: BRAF mutation promotes NHEJ activity leading to radioresistance and BRAFi selectively radiosensitizes BRAF thyroid cancer cells through inhibiting NHEJ. Our findings suggest that combining BRAFi and radiation may improve the therapeutic outcome of patients with BRAF-mutant thyroid cancer.