Single-strand break repair pathway disruption impacts radiosensitivity to both low- and high-LET radiation.

Differences in individual radiosensitivity are not currently reflected in radiotherapy treatment planning, and identifying genetic drivers of this variation is a key step in personalising future treatments. While the role of DNA double-strand break (DSB) repair in radiosensitivity is well-studied, the impacts of single-strand break (SSB) repair defects are relatively unexplored. Through targeted SSB repair gene knockout, we show that disrupted SSB repair increases radiosensitivity, with stronger effects for high Linear Energy Transfer (LET) radiation. Mechanistic studies identified both an increase in DSB production at low LETs, due to delayed SSB repair, and higher levels of unrepaired complex damage at high LETs. This altered sensitivity is also reflected by differences in disease-specific survival for patients with SSB repair gene alterations in a retrospective pan-cancer clinical analysis. SSB repair pathway disruption may therefore be clinically relevant and important to consider for radiotherapy personalisation, particularly for high-LET radiation treatments.