Canonical DNA non-homologous end-joining; capacity versus fidelity

The significance of canonical DNA non-homologous end-joining (c-NHEJ) for DNA double strand break (DSB) repair has increased from lower organisms to higher eukaryotes, and plays the predominant role in human cells. Ku, the c-NHEJ end-binding component, binds DSBs with high efficiency enabling c-NHEJ to be the first choice DSB repair pathway, although alternative pathways can ensue after regulated steps to remove Ku. Indeed, radiation-induced DSBs are repaired rapidly in human cells. However, an important question is the fidelity with which radiation-induced DSBs are repaired, which is essential for assessing any harmful impacts caused by radiation exposure. Indeed, is compromised fidelity a price we pay for high capacity repair. Two subpathways of c-NHEJ have been revealed; a fast process that does not require nucleases or significant chromatin changes and a slower process that necessitates resection factors, and potentially more significant chromatin changes at the DSB. Recent studies have also shown that DSBs within transcriptionally active regions are repaired by specialised mechanisms, and the response at such DSBs encompasses a process of transcriptional arrest. Here, we consider the limitations of c-NHEJ that might result in DSB misrepair. We consider the common IR-induced misrepair events and discuss how they might arise via the distinct subpathways of c-NHEJ.