DSBs

Double-strand breaks (DSBs) are lesions in which both strands of the DNA double helix are severed. They are among the most harmful forms of DNA damage because unrepaired DSBs can lead to cell death, chromosomal rearrangements, or genome instability.

DSBs arise from external sources such as ionizing radiation and certain anticancer drugs, and from internal processes including replication stress, reactive oxygen species, and normal meiotic recombination. They can occur spontaneously but are also produced during programmed genetic events that require recombination.

Cells detect DSBs through sensor complexes, notably the MRN complex (Mre11-Rad50-Nbs1), which activates ATM kinase. This leads to phosphorylation of histone H2AX (gamma-H2AX) and the recruitment of repair and checkpoint proteins, initiating signaling cascades that coordinate repair with cell cycle control.

There are two major repair pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). NHEJ directly ligates broken ends and can be error-prone. HR uses a homologous template, is high fidelity, and predominates in S and G2 phases when a sister chromatid is available. End processing and resection govern the choice between these pathways, with factors such as CtIP, EXO1, and DNA2 contributing to resection for HR.

Repair choice depends on cellular context and chromosomal end processing. Defects in DSB repair genes promote genomic instability and are linked to cancer predisposition syndromes (e.g., BRCA1/2, ATM). Misrepaired DSBs can cause translocations, deletions, or copy-number alterations.

DSBs are measurable by assays such as gamma-H2AX foci formation, the neutral comet assay, or pulse-field gel electrophoresis, which reflect the amount and distribution of breaks.

In medicine, DSB formation is a therapeutic aim; ionizing radiation and many chemotherapeutics kill tumor cells by inducing DSBs, while therapies exploit defects in DSB repair (such as PARP inhibitors in BRCA-mutant cancers) to achieve selective killing.