Session Item

The utilization of high linear-energy-transfer (LET) ionizing radiation (IR) modalities is rapidly growing worldwide causing excitement but raising also concerns, as our understanding of their biological effects is incomplete. Charged particles such as protons and heavy ions show potential in cancer therapy, owing to their advantageous physical properties over X-rays (photons), but are also present in the space environment, adding to space mission’s health risks. Therapy improvements and protection of humans during space travel will benefit from a better understanding of the mechanisms underpinning the biological effects of high-LET IR. There is evidence that high-LET IR induces DNA double-strand breaks (DSBs) of increasing complexity causing enhanced cell killing by engaging, at least partly, low-fidelity DSB-repair pathways: alternative end-joining (alt-EJ) and single strand annealing (SSA), that are known to alter the genome and to frequently cause structural chromosomal abnormalities (SCAs).

DSB-clusters represent one of the highest levels of DSB complexity that can jeopardize processing by destabilizing chromatin in the vicinity of the cluster. DSB-clusters are generated after exposure of cells to ionizing radiation (IR), particularly high-LET radiation, and have been considered as particularly consequential in several mathematical models of IR action. Yet, experimental demonstration of their relevance to the adverse IR effects, as well as information on the mechanisms underpinning their severity as DNA lesions is lacking. We addressed this void by developing cell lines with especially designed, multiply integrated constructs modeling defined combinations of DSB-clusters through appropriately engineered I-SceI meganuclease recognition sites.

Using this model system, we could demonstrate efficient activation of the DNA damage response, as well as a markedly increased potential of DSB-clusters, as compared to single-DSBs, to kill cells, and cause PARP1-dependent chromosomal translocations. These experiments provide evidence that DSB repair relying on first line DSB-processing pathways (c-NHEJ and HR) is compromised within DSB clusters, presumably through the associated chromatin destabilization, leaving alternative end joining as last option and translocation formation as a natural consequence.

We further show that the increased radiosensitivity of A549 lung adenocarcinoma cells to -particles and 56Fe ions, as well as of HCT116 colorectal cancer cells to -particles correlates with formation of SCAs as detected by mFISH. Furthermore, we find that cells exposed to low doses of -particles and 56Fe ions show an enhanced G2-checkpoint response that is mainly regulated by ATR, rather than the coordinated ATM/ATR-dependent regulation observed after exposure to low doses of X-rays.

These observations advance our understanding of the mechanisms underpinning high-LET IR-effects and suggest a potential utility for ATR inhibitors in high-LET radiation therapy. They will be discussed in the context of “Quality over quantity” that is at the focus of the session.



Funding: This research was funded by grants from the „Bundesministerium für Bildung und Forschung“ (02NUK043B, COLLAR) and the “Bundesministerium für Wirtschaft und Technolo-gie“ (BMWi: ESA-AO-IBER-2017, 50WB1836). “The results presented here are (partially) based on experiments performed at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt (Germany) in the frame of FAIR Phase-0.”a mechanistic explanation for the increased efficacy of high-LET radiation.