- MRE11 is the catalytic heart of the MRN complex, the master sensor of DNA double‑strand breaks (DSBs) and a central regulator of genome stability. As a nuclease with both exonuclease and endonuclease activity, MRE11 performs the critical first steps of DNA end processing, enabling cells to repair DSBs through homologous recombination or non‑homologous end joining. Because DSBs are among the most lethal forms of DNA damage, MRE11’s functions are indispensable for maintaining chromosomal integrity, preventing genomic instability and safeguarding cellular viability.
- MRE11 forms a stable complex with RAD50 and NBS1, creating a multifunctional scaffold that binds broken DNA ends and coordinates repair pathway choice. Within this complex, MRE11’s nuclease activity is tightly regulated by RAD50’s ATPase cycle and NBS1’s signalling functions. MRE11 binds DNA ends with high affinity, positioning its nuclease domains to initiate end processing. This early action is essential for generating the single‑stranded DNA overhangs required for homologous recombination, the most accurate form of DSB repair.
- One of MRE11’s defining roles is its participation in DNA end resection. Working together with CtIP, MRE11 performs the initial “short‑range” resection that creates short 3′ single‑stranded DNA regions. These regions are subsequently extended by EXO1 and DNA2, coated by RPA and eventually replaced by RAD51 to initiate strand invasion. Through this mechanism, MRE11 ensures that homologous recombination proceeds with high fidelity, particularly during S and G2 phases when sister chromatids are available as templates.
- Although MRE11 is best known for its role in homologous recombination, it also contributes to non‑homologous end joining. In this context, MRE11 trims incompatible DNA ends, preparing them for ligation by the NHEJ machinery. RAD50’s DNA‑tethering function stabilises the broken ends, while MRE11’s nuclease activity shapes the termini to allow efficient repair. This dual functionality allows MRE11 to adapt repair strategy depending on cell‑cycle stage, chromatin context and the nature of the DNA break.
- MRE11 also plays a central role in activating ATM, the major kinase of the DNA‑damage response. NBS1 recruits ATM to sites of damage, and MRE11’s interaction with ATM promotes its autophosphorylation and full activation. Activated ATM phosphorylates numerous substrates, including H2AX, CHK2, BRCA1 and p53, triggering cell‑cycle arrest, chromatin remodelling and repair. If damage is irreparable, ATM signalling promotes apoptosis or other forms of programmed cell death to prevent propagation of genomic instability.
- Beyond double‑strand break repair, MRE11 contributes to replication fork stability. Stalled replication forks are vulnerable to collapse, which can generate lethal DSBs. MRE11 stabilises these forks and coordinates their restart, preventing excessive genomic instability during replication stress. MRE11 also participates in telomere maintenance, helping regulate TRF2 and preventing chromosome end‑to‑end fusions. These functions link MRE11 to broader genome‑maintenance networks, including the proteostasis network and the oxidative stress response.
- Mutations in MRE11 cause severe genomic‑instability disorders. Ataxia‑telangiectasia‑like disorder (ATLD), caused by MRE11 deficiency, resembles ATM deficiency and is characterised by neurodegeneration, radiosensitivity and chromosomal instability. MRE11 dysfunction also contributes to cancer development by promoting chromosomal rearrangements, replication stress and therapy resistance. Because MRE11 is essential for DSB repair, tumours with defective MRE11 show hypersensitivity to radiotherapy, PARP inhibitors and DNA‑damaging chemotherapeutics.
- In summary, MRE11 is the core nuclease of the MRN complex and a master regulator of DNA double‑strand break repair. Through its roles in DNA end processing, homologous recombination, non‑homologous end joining, ATM activation and replication fork stability, MRE11 safeguards genome integrity and prevents catastrophic chromosomal damage. Its central importance in DNA repair makes MRE11 a key determinant of cellular survival and a major focus of research in genome stability and cancer biology.
