BRCC36 Is Essential for Radioactive Iodine Treatment Induced DNA Double-Strand Break Repair

Background: Thyroid cancer is the most common endocrine malignancy ^[1]. The effect of radioactive iodine (RAI) treatment in differentiated thyroid cancer (DTC) is positive, however, the RAI uptake in some of tumors reduce or disappear in the treatment process. The radioactive iodine-refractory differentiated thyroid cancer (RR-DTC) is almost poorly differentiated, and not sensitive to the traditional treatment methods ^[2]. Ionizing radiation (IR) can injure cells by induce DNA double-strand break (DSB) ^[3]. The deubiquitinating enzyme BRCC36 not only participates in the homologous recombination repair (HR) of DSB ^[4], but also contributes to the proper mitotic spindle assembly in mammalian cells ^[5]. [Objectives] To further elucidate the functional of BRCC36 in RAI treatment of DTC. [Methods] Silencing studies were used by small interfering RNA targeting BRCC36 in DTC cell lines BCPAP and TPC-1. Cell cycle and apoptosis assays were used by the flow cytometry. The DSB level was determined by γH2AX immunofluorescence and Western Blotting. [Results] The flow cytometry assay results showed that knock-down of BRCC36 alone induced S phase arrest and apoptosis of DTC cells, suggesting BRCC36 was participated in DNA synthesis and cell proliferation. Both dose gradient (50, 100, 200 μCi/mL, 24 h) and time gradient (2, 8, 24 h, 100 μCi/mL) RAI treatment alone induced G2 phase arrest and apoptosis of DTC cells. Immunofluorescence images showed that RAI treatment leaded to γH2AX foci counts increased, which was confirmed by Western Blotting. These results implied that RAI treatment caused DSB and then induced G2 phase arrest and apoptosis. Compared with small interfering RNA control group, BRCC36 depletion increased γH2AX foci counts, as well as the apoptosis ratio. Interestingly, RAI treatment of BRCC36 depletion DTC cells leaded to G2 phase arrest rather than S phase arrest. [Conclusions] These results suggested that down-regulation of BRCC36 expression impairs the DNA repair activated in response to RAI treatment, thereby sensitizing DTC cells to RAI-induced cell cycle arrest and apoptosis.