Single-molecule techniques in studying the molecular mechanisms of DNA synapsis in non-homologous end-joining repair
image: Three biochemical steps of the non-homologous DNA end-joining (NHEJ) process. This figure outlines the key stages of the NHEJ repair mechanism following DNA double-strand breaks. The process can be divided into three main steps: end synapsis, processing, and ligation. First, the Ku heterodimer recognizes the broken DNA ends, initiating the recruitment of NHEJ proteins that bridge and stabilize the damaged DNA, leading to the formation of a synaptic complex. In most cases, the DNA ends require processing by nucleases (Artemis and APLF), polymerases (pol μ and pol λ), or other modifiers (TDP1 and PNKP) to reveal microhomology between the ends. Once the DNA ends are properly aligned and compatible for ligation, the X4L4 complex completes the repair by sealing the ends
Credit: Yuhao Jiang, Chao Zhao, Chenyang Zhang, Weilin Li, Di Liu, Bailin Zhao
Key Findings:
- Dynamic and Multi-Protein Nature of NHEJ Synapsis: NHEJ synapsis is highly dynamic and involves multi-protein assemblies that facilitate the precise alignment of broken DNA ends. Single-molecule studies, such as smFRET, reveal that this process is tightly regulated, with deficiencies potentially causing chromosomal translocations and genomic instability, as seen in radiation-induced damage models.
- Role of Single-Molecule Techniques: smFRET and other single-molecule methods have enabled real-time observation of synapsis mechanisms, uncovering how proteins mediate end-to-end bridging. These techniques demonstrate that synapsis efficiency depends on molecular interactions that can be disrupted in disease contexts, such as cancer or radiation exposure.
- Coupling with Other NHEJ Steps: The review identifies that synapsis is closely coupled with subsequent steps in NHEJ, such as end processing and ligation. This integration ensures coordinated repair but may introduce vulnerabilities under stress, like genotoxic conditions, where errors accumulate.
Significance:
The insights from single-molecule studies of NHEJ synapsis are crucial for advancing DNA repair research and therapeutic applications. Understanding the molecular mechanisms helps elucidate how DSB repair failures contribute to diseases like cancer, as deficiencies in synapsis are linked to chromosomal abnormalities observed in radiation-induced damage (e.g., from cardiac CT scans or cancer therapies) . This knowledge could inform targeted therapies to enhance repair fidelity, potentially reducing risks from genomic instability. Additionally, the review's focus on techniques like smFRET highlights their role in probing fundamental biological processes, with implications for developing new diagnostic tools or radiation-sensitizing agents in oncology. Overall, this work underscores the importance of synapsis in maintaining genomic integrity and opens avenues for interdisciplinary research in biophysics and biomedicine.
The work entitled “Single-Molecule Techniques in Studying the Molecular Mechanisms of DNA Synapsis in Non-Homologous End-Joining Repair”was published on Biophysics Reports (published on Mar. 2025).