Impeding the NHEJ Pathway for Overcoming Radioresistance in the Context of Precision Radiotherapy of Cancer

Non-homologous end joining (NHEJ) is a critical DNA double-strand break (DSB) repair pathway that operates throughout the cell cycle to maintain the genomic stability of the cell. Unlike homologous recombination (HR), NHEJ is capable of repairing DSBs without the need for a homologous template, making it a rapid response mechanism, but potentially prone to errors. Central to NHEJ function and essential for the ligation through the recruitment and activation of additional repair factors, such as Artemis, XRCC4, and DNA ligase IV, is the DNA-dependent protein kinase (DNA-PK) complex. Dysregulation in the NHEJ pathway contributes to genomic instability, oncogenesis, and resistance to genotoxic therapies. Consequently, inhibitors of DNA-PK have emerged as promising therapeutic agents to sensitize tumor cells to radiation and DNA-damaging chemotherapeutics. Inhibiting the DNA-PK ability to recruit the protein complex needed for successful DSB repair promotes cell death through apoptosis or mitotic catastrophe. While inhibitors of DNA-PK can be used to enhance the effects of genotoxic therapies, the field still struggles to address critical problems: how to best exploit the differential DNA repair capacities among tumor subtypes, how to maximize radiosensitization of cancerous cells while sparing normal tissues, and how to translate preclinical studies into clinical benefits. Given that NHEJ constitutes the primary line of defense against radiation-induced damage, rapidly repairing the majority of double-strand breaks throughout the cell cycle, this review concentrates on targeting the DNA-PK complex, as the master regulator of this rapid-response mechanism, highlighting why its inhibition represents a strategic action to overcome intrinsic radioresistance. The implementation of DNA-PK inhibitors into medical practice can enable the stratification of oncologic patients into two categories, based on the tumors' vulnerability to NHEJ disruptions. Thus, the therapeutic pathways of patients with NHEJ tumors could branch, combining traditional genotoxic therapies (radiation and DNA-damaging chemotherapeutics) with DNA-PK inhibitors to achieve an enhanced effect and improved survival outcomes.