Clustered DNA Double-Strand Breaks: Biological Effects and Relevance to Cancer Radiotherapy

Abstract

Cells manage to survive, thrive, and divide with high accuracy despite the constant threat of DNA damage. Cells have evolved with several systems that efficiently repair spontaneous, isolated DNA lesions with a high degree of accuracy. Ionizing radiation and a few radiomimetic chemicals can produce clustered DNA damage comprising complex arrangements of single-strand damage and DNA double-strand breaks (DSBs). There is substantial evidence that clustered DNA damage is more mutagenic and cytotoxic than isolated damage. Radiation-induced clustered DNA damage has proven difficult to study because the spectrum of induced lesions is very complex, and lesions are randomly distributed throughout the genome. Nonetheless, it is fairly well-established that radiation-induced clustered DNA damage, including non-DSB and DSB clustered lesions, are poorly repaired or fail to repair, accounting for the greater mutagenic and cytotoxic effects of clustered lesions compared to isolated lesions. High linear energy transfer (LET) charged particle radiation is more cytotoxic per unit dose than low LET radiation because high LET radiation produces more clustered DNA damage. Studies with I-SceI nuclease demonstrate that nuclease-induced DSB clusters are also cytotoxic, indicating that this cytotoxicity is independent of radiogenic lesions, including single-strand lesions and chemically “dirty” DSB ends. The poor repair of clustered DSBs at least in part reflects inhibition of canonical NHEJ by short DNA fragments. This shifts repair toward HR and perhaps alternative NHEJ, and can result in chromothripsis-mediated genome instability or cell death. These principals are important for cancer treatment by low and high LET radiation.

Micronuclei Formation upon Radioiodine Therapy for Well-Differentiated Thyroid Cancer: The Influence of DNA Repair Genes Variants

“…Significant genotype effects on MN frequency and/or its net balance were observed for HR (NBN) and MMR (MLH1, MSH3, MSH4) repair pathway SNPs across different time points. This was expected because (1) IR exposure results in increased DNA damage, most notably, single-and double-strand breaks, oxidative lesions (e.g., 8-oxoG), DNA-protein crosslinks (DPCs) and clustered DNA lesions [62][63][64][65][66][67]; (2) the HR pathway, acting in the S/G2 stages of the cell cycle, is the major DNA repair pathway involved in the error-free correction of DSBs [11,33,35,68]; (3) MMR proteins, besides their canonical actions on the post-replication repair of mispaired nucleotides and insertion-deletion loops, have also been demonstrated to play an important role on the damage response to IR-induced DSBs, either through cooperation with HR or through signaling for cell-cycle arrest and apoptosis [64,[69][70][71]; (4) DSBs, if left unrepaired, e.g., due to the presence of SNPs that reduce the DNA repair capacity, may give rise to chromosome breakage and MN formation upon replication [28,33,35,72]. The potential influence of functional DSB repair SNPs on 131 I-induced BNMN frequency is, therefore, fully justified.…”

Recruitment of MRE-11 to complex DNA damage is modulated by meiosis-specific chromosome organization

“…The formation of clusters indicates that about half of microirradiation regions contain several sites of DNA damage in very close proximity, indicative of complex DNA damage. Tissue culture cells exposed to high-linear energy transfer (LET) irradiation shift the repair pathway utilized from NHEJ to a combination of NHEJ and HR repair (Nickoloff et al, 2020). It could be interpreted in two ways: clustered DSBs are shifted towards HR, or clustered DSBs are repaired by mixed HR/NHEJ model.…”

“…However, DSBs are more deleterious and require repair prior to cell division. The mechanism of DSB formation is common to all forms of ionizing radiation including heavy particle radiation used in cancer therapy, gamma irradiation, and microirradiation (Nickoloff et al, 2020). However, these forms of radiation differ in the relative abundance of clustered DSBs, with high LET exposure (heavy ion) producing more clustered DSBs than low LET exposure (gamma irradiation).…”

“…The range of damage induced, as well as the close proximity of many different types of DNA damage (creating clusters of damage), makes the type of damage induced by laser microirradiation complex. Studies performed in mammalian tissue culture cells indicate that clustered DSBs are more deleterious than spaced DSBs (Nickoloff et al, 2020). Moreover, in these cells repair pathway choice shifts from NHEJ to HR.…”

Site-specific targeting of a light activated dCas9-KIllerRed fusion protein generates transient, localized regions of oxidative DNA damage

“…As a consequence, these approaches tend to measure bulk repair products, lack the time resolution to monitor events occurring immediately after DNA damage, and may represent responses to persistent damage, rather than acute damage-repair. Further, while endonucleases induce clean, enzymatic DSBs which can be readily resected, they lack the complexity of clustered DNA damage generated by ROS or radiation (30).…”