# Nonhomologous end joining—the importance of end tethering and beyond

## TL;DR

This review explains how the NHEJ pathway repairs DNA breaks through five phases, emphasizing the role of protein bridges in stabilizing DNA ends.

## Contribution

The paper provides a detailed molecular and structural analysis of the five phases of NHEJ, focusing on end tethering and protein interactions.

## Key findings

- NHEJ operates through five distinct phases involving dynamic protein bridges.
- End tethering is supported by multiple overlapping machineries anchored on Ku.
- Structural insights reveal how NHEJ contributes to genome stability and lymphocyte development.

## Abstract

In this review, Zha and Yi dissect the molecular and structural mechanisms underlying the five discrete phases of nonhomologous end joining (NHEJ); namely, end sensing, end protection, end tethering, end processing, and end ligation. They highlight how the dynamic network of protein bridges (i.e., Ku–protein interactions) stabilizes DNA ends and enables the assembly of NHEJ machinery to facilitate the repair of DNA double-strand breaks independent of sequence complementarity.

The nonhomologous end joining (NHEJ) pathway is a major mechanism for repairing DNA double-strand breaks (DSBs), essential for lymphocyte development and for maintaining genome stability in postmitotic cells, including neurons. Under pathological conditions, NHEJ is also responsible for most oncogenic translocations and the repair of DSBs induced by chemotherapy/radiation. Advances in structural biology and single-molecule imaging have captured NHEJ at various stages of the reaction, revealing a modular, dynamic organization with built-in redundancy and resilience. Here, we describe NHEJ in five phases (end sensing, end protection, end tethering, end processing, and end ligation), highlighting their molecular features and transitions and how structural insights explain genetic interactions and physiological consequences, including lymphocyte development. Unlike other DSB repair mechanisms, NHEJ operates with minimal or no sequence complementarity, relying instead on protein assemblies to bridge and stabilize the two DNA ends. This review highlights the unique end-tethering phase, supported by at least five known overlapping machineries, all anchored on Ku.