Radiosensitization effect of bromodomain overexpression on thyroid anaplastic tumor using human applicable low iodine-131 dose

Objectives DNA double strand breaks (DSBs) repair can enable tumor cells to overcome DNA damage after radiation therapy and this often leads to radioresistance. Radiation sensitization is required to increase therapeutic effect. Brahma-related gene 1 (BRG1), the catalytic subunit of SWI/SNF chromatin-remodeling complex, involves in DSBs repair. The BRG1 assists DSBs repair by stimulating γ-H2AX formation. DSBs repair is not completed without BRG1 binding to acetylated histones at DSBs via bromodomain (BRD). We previously reported the therapeutic effect of BRD overexpression, resulting inhibition of BRG1 binding, in case of high I-131 dose (500 μCi). However, this dose is compatible to 1,500 mCi in human being (60 kg). In this study, we investigated therapeutic effect of BRD overexpression in case of low I-131 dose (60 μCi), which is converted from clinical therapeutic dose (180 mCi) in human by their weights.

Methods FRO anaplastic thyroid cancer cells which containing a luciferase reporter gene (effluc) were transfected by optimized human sodium iodide symporter (pMSCV-oNIS) and pMX-BRG1-BRD retro-viral vector. The cell lines were divided into three groups: Control, oNIS, and oNIS+BRD expressing cell lines. Clonogenic assay, γ-H2AX foci staining, and Western blotting were conducted. Each cell lines of the three groups were subcutaneously injected into BALB/c nude mice. I-131 was treated when tumor cells reached 108 photon/sec by luminescence signal. Animal SPECT/CT imaging was also acquired to visualize I-131 uptake. To investigate the effect of BRD on I-131 therapy, tumor size was measured with caliper and tumor proliferation was measured with optical signal obtained by IVIS imaging system.

Results The results of clonogenic assay, γ-H2AX foci staining, and Western blotting indicated that radiation sensitizing effect of BRD overexpression. Survival colonies of oNIS+BRD were 12.14%, while oNIS cells survived 28.60%, and control cells survived 64.84% after I-131 therapy. Especially, the luciferase signal of oNIS tumor was 3.4 times higher than oNIS+BRD tumors. Also, the tumor size of oNIS tumor was 2.1 times higher than oNIS+BRD tumors at 6 days after treating 60 μCi of I-131.

Conclusions Radiosensitizing effect of human applicable dose (60 μCi) combined with BRD overexpression was occured. Human application of this system may be possible. We have shown that radio-sensitizing effect induced by BRG1-BRD overexpression on radioiodine therapy, and our results indicating that BRG1-BRD overexpression could be used as a therapeutic gene by combination with NIS for reducing therapeutic dose of radioiodine.

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