BRAF activating mutations act as oncogenic drivers and are highly prevalent in thyroid cancer, occurring in about 60% of papillary thyroid cancer (PTC) as well as 30-40% anaplastic thyroid cancer (ATC). MAPK signaling and treatment resistance is driven by BRAF mutations in thyroid cancer. Among the most common of these mutations is BRAFV600E, which can be selectively inhibited by vemurafenib.

Using a panel of PTC and ATC cell lines we assessed how the presence/absence of BRAFV600E impacts radiation sensitivity. We used radiation clonogenics, comet assays, nuclear foci formation, immunoblotting, and mouse xenografts models to determine the effect of vemurafenib on PTC and ATC cells. Analysis of radiation clonogenics implicated higher radioresistance in cell lines containing the BRAFV600E mutation as compared to wild-type BRAF. Additionally, forced expression of BRAFV600E in a wild-type thyroid cancer cell line induced radioresistance. Vemurafenib inhibited MAPK signaling in V600E mutant cell lines, but showed no effect in BRAF wild-type cell lines. Vemurafenib pretreatment selectively radiosensitized BRAFV600E mutants in vitro, as assessed by radiation clonogenic assays. Comet assays revealed impairment of DNA repair in BRAFV600E lines when treated with vemurafenib and radiation. Additionally, γ-H2A.x westerns and nuclear foci staining indicated that vemurafenib pretreatment decreases the tumor cell’s ability to repair DNA double-strand breaks (DSB) in BRAFV600E cell lines. Vemurafenib also appeared to alter the kinetics of formation and resolution of 53BP1 and Rad51 nuclear foci in these cell lines. Evaluation of DSB functional repair using GFP reporter constructs, suggests that BRAFV600E induces NHEJ repair, which can be attenuated by vemurafenib treatment. Finally, vemurafenib effectively increases radiosensitivity of BRAF V600E tumors in vivo.

From our results, BRAF activating mutation appears to be associated with radioresistance. Vemurafenib selectively radiosensitizes both PTC and ATC cells through inhibition of DNA repair mechanisms. Combining vemurafenib and radiation may improve therapeutic control for BRAFV600E mutant thyroid cancers.

Citation Format: Ryan N. Robb, Linlin Yang, Terence Williams. Vemurafenib selectively radiosensitizes BRAF V600E mutant papillary and anaplastic thyroid carcinoma cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5186. doi:10.1158/1538-7445.AM2017-5186