Targeting of BRM Sensitizes BRG1-Mutant Lung Cancer Cell Lines to Radiotherapy

Abstract

Targeting of epigenetic regulators as the chromatin remodeler SWI/SNF is proving to be a promising therapeutic strategy for individualized treatment of cancer patients. Here, we tested whether targeting one of the two mutually exclusive subdomains of the SWI/SNF complex BRM/SMARCA2 can sensitize specifically non-small cell lung carcinoma (NSCLC) cells with mutations in the other subunit BRG1/SMARCA4 toward ionizing radiation (IR). Knockdown of BRM with siRNA or shRNA and its consequences for radiation sensitivity as measured by clonogenic survival and plaque-monolayer control was studied in different NSCLC lines with or without BRG1 mutations and in primary fibroblasts. Furthermore, the effect on double-strand break (DSB) repair markers measured by immunofluorescence staining of 53BP1-, g-H2AX-, and Rad51-foci was investigated. BRG1mutated cell lines showed an increased surviving fraction compared with BRG1 proficient cells. Depletion of BRM (i) leads to a decreased proliferation rate and plating efficiency specifically in BRG1-mutated cells, (ii) specifically sensitized BRG1-mutant NSCLC cells toward IR as characterized by a survival reducing factor of 0.63 [95% confidence interval (CI), 0.57-0.69] in the dose range between 2 and 6 Gy, and (iii) decreased the tumor control doses after daily fractionation at 4 Gy in BRG1-mutant NSCLC cell lines A549 and H1299 in minimonolayers by 9.9% AE 1.3% and 13.6% AE 1.8%, respectively. In addition, an increase of residual Rad51-foci at 24 hours after irradiation in BRG1-mutant cells was demonstrated. Therefore, targeting of BRM in combination with radiotherapy is supposed to improve the therapeutic outcome of lung cancer patients harboring BRG1 mutations.The present study shows that the moderate radioresponsiveness of NSCLC cells with BRG1 mutations can be increased upon BRM depletion that is associated with a prolonged Rad51-foci prevalence at DNA DSBs.

The Genomic Landscape of SMARCA4 Alterations and Associations with Outcomes in Patients with Lung Cancer

“…Although there are no known currently effective targeted treatments for SMARCA4-mutant NSCLCs, our study and others suggest SMARCA4 is a potential target in lung cancer with distinct therapeutic vulnerabilities. For example, CDK4/6, AURKA, ATR, and EZH2 inhibition have recently shown antitumor activity in preclinical models of SMARCA4-deficient tumors (1,16,25,(28)(29)(30)(31)(32)(33). SMARCA2 could be a synthetic lethal vulnerability in SMARCA4mutant cancers.…”

BRM: the core ATPase subunit of SWI/SNF chromatin-remodelling complex—a tumour suppressor or tumour-promoting factor?

“…Loss of BRM protein was more frequent in heavy smokers, supporting the hypothesis that BRM depletion enhances susceptibility to cancer induced by the carcinogen exposure [65, 68]. On the other hand, targeting BRM in the BRG1-deficient lung cancer (NSCLC) sensitised cancer cells (cell lines) to radiotherapy [69].…”

Notable Response of SMARCA4-Deficient Undifferentiated Uterine Sarcoma to Palliative Radiation Therapy

“…16 BRG1 and its combined protein modifications are the targets of research on radiosensitivity and sensitizing agents. 17 , 18 The suppression of ARID1A and ARID1B, which are also components of the SWI/SNF complex, as well as defects in these proteins, which have been found in several malignancies, inhibit the repair of DNA double-strand breaks and cause sensitivity to ionizing radiation. 19 Although these basic findings do not directly explain the remarkable response to radiation therapy observed in the liver metastases in the present case, future research might clarify the relationship between SMARCA4 deficiency and radiosensitivity.…”