DNA Damage Clustering after Ionizing Radiation and Consequences in the Processing of Chromatin Breaks

Abstract

Charged-particle radiotherapy (CPRT) utilizing low and high linear energy transfer (low-/high-LET) ionizing radiation (IR) is a promising cancer treatment modality having unique physical energy deposition properties. CPRT enables focused delivery of a desired dose to the tumor, thus achieving a better tumor control and reduced normal tissue toxicity. It increases the overall radiation tolerance and the chances of survival for the patient. Further improvements in CPRT are expected from a better understanding of the mechanisms governing the biological effects of IR and their dependence on LET. There is increasing evidence that high-LET IR induces more complex and even clustered DNA double-strand breaks (DSBs) that are extremely consequential to cellular homeostasis, and which represent a considerable threat to genomic integrity. However, from the perspective of cancer management, the same DSB characteristics underpin the expected therapeutic benefit and are central to the rationale guiding current efforts for increased implementation of heavy ions (HI) in radiotherapy. Here, we review the specific cellular DNA damage responses (DDR) elicited by high-LET IR and compare them to those of low-LET IR. We emphasize differences in the forms of DSBs induced and their impact on DDR. Moreover, we analyze how the distinct initial forms of DSBs modulate the interplay between DSB repair pathways through the activation of DNA end resection. We postulate that at complex DSBs and DSB clusters, increased DNA end resection orchestrates an increased engagement of resection-dependent repair pathways. Furthermore, we summarize evidence that after exposure to high-LET IR, error-prone processes outcompete high fidelity homologous recombination (HR) through mechanisms that remain to be elucidated. Finally, we review the high-LET dependence of specific DDR-related post-translational modifications and the induction of apoptosis in cancer cells. We believe that in-depth characterization of the biological effects that are specific to high-LET IR will help to establish predictive and prognostic signatures for use in future individualized therapeutic strategies, and will enhance the prospects for the development of effective countermeasures for improved radiation protection during space travel.

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation

“…DNA damage, especially DSB, rapidly induces the DNA damage response (DDR) pathway [ 133 , 142 ]. The primary pathways of DBS DNA repair are homologous recombination repair (HRR), non-homologous end-joining (NHEJ), and alternative end-joining (alt-EJ) [ 133 , 142 , 143 , 144 ]. These three pathways involve different initiating proteins that recognize DSB and different downstream enzymes ( Table 4 ).…”

The high toxicity of DSB-clusters modelling high-LET-DNA damage derives from inhibition of c-NHEJ and promotion of alt-EJ and SSA despite increases in HR

“…Alt-EJ operates with slower kinetics and lower efficiency than c-NHEJ, and is error-prone causing deletions and other modifications at the junction, more than c-NHEJ. Alt-EJ also can join unrelated DNA-ends and is therefore considered a dominant source of SCAs ( Zhang et al, 2010 ; Soni et al, 2015 ; Mladenova et al, 2022 ). Single-strand annealing (SSA) is also error-prone owing to the large deletions it generates between the homologous DNA segments it requires for normal function, and can be promiscuous in partner selection and form SCAs ( Bhargava et al, 2016 ; Iliakis et al, 2019 ).…”

“…HR, alt-EJ and SSA are classified as DNA end-resection resection (henceforth simply resection) -dependent DSB-repair pathways, because they share this initial processing step ( Ceccaldi et al, 2016 ). During this step, the MRN/CtIP complex orchestrates short-range resection that is followed by BLM-DNA2/EXO1-mediated long-range resection ( Sartori et al, 2007 ; Mladenova et al, 2022 ). The resulting ssDNA is a prerequisite for the engagement of HR and SSA, and benefits also alt-EJ ( Paiano et al, 2021 ) by exposing microhomologies (4-6bp).…”

NBN, RAD51 and XRCC3 Polymorphisms as Potential Predictive Biomarkers of Adjuvant Radiotherapy Toxicity in Early HER2-Positive Breast Cancer

“…These kinases enable the activation and recruitment of BRCA1, PALB2 and BRCA2 that exchange RPA with recombinase RAD51, a key protein of HRR that forms a nucleoprotein filament [ 12 ]. Several RAD51 paralogs, including XRCC3, facilitate this process [ 13 ]. Then, the RAD51 nucleoprotein filament searches for a homologous template in the sister chromatid, leading to strand invasion and elongation.…”

“…Then, the RAD51 nucleoprotein filament searches for a homologous template in the sister chromatid, leading to strand invasion and elongation. This results in the formation of Holliday junction intermediates, which can be resolved differently, resulting in completely repaired DSBs [ 13 ].…”