Targeting the G₂ DNA damage checkpoint in nasopharyngeal carcinoma

Abstract

Nasopharyngeal carcinoma is an aggressive head and neck cancer with high prevalence in southern China, including Hong Kong. Despite the high initial response rate to radiotherapy, treatment failure is often resulted from distant metastases in the advanced disease. As standard radiotherapy and chemoradiotherapy induce DNA damage, I investigated whether uncoupling the DNA damage response pathway could sensitize this cancer type. The potential of sensitizing irradiated cells via inhibition of the G₂ DNA damage checkpoint was first examined. The G₂ DNA damage checkpoint is controlled by the ATM/ATR-CHK1/2- WEE1 axis of protein kinases. I demonstrated that radiosensitization could be achieved by bypassing the ionizing radiation-induced G₂ DNA damage checkpoint using a WEE1 inhibitor (MK-1775). However, bypassing the checkpoint via inhibition of CHK1 or ATR failed to enhance cell death. Intriguingly, I found that ATM was dispensable for both the activation and maintenance of the G₂ DNA damage checkpoint. Further experiments revealed that mitotic catastrophe could be induced with the co-inhibition of WEE1 and CHK1 in unirradiated cells. The capacity of sensitizing cells via inhibition of the major DNA double strand break (DSB) repair pathways was also studied. Eukaryotic cells repair DNA DSBs mainly via non-homologous end-joining (NHEJ), homologous recombination repair (HRR), and a backup pathway called alternative-NHEJ (alt-NHEJ). Inhibition of alt-NHEJ in a HRR-deficient background was proved to be synthetic lethal. I demonstrated that HRR-deficiency could be induced via ATM inhibition. The co-inhibition of HRR and alt-NHEJ via the inhibition of ATM and PARP1 resulted in the accumulation of DNA DSBs, activation of the G₂ DNA damage checkpoint, and reduced cell proliferation. These studies revealed that radiosensitization of nasopharyngeal carcinoma could be achieved by checkpoint bypass in irradiated cells, the co-inhibition of G₂ DNA damage checkpoint components, or the co-inhibition of major DNA DSB repair pathways. (294 words)

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology