DNA Double-Strand Break Repair by Non-Homologous End Joining

Double-strand breaks (DSBs) are severe forms of DNA damage in which both strands of the DNA double helix are broken. They can result from external factors like ionizing radiation (from medical imaging, environmental sources, or cosmic rays), exposure to chemical mutagens (such as chemotherapeutic drugs and environmental pollutants), and lifestyle factors like smoking. DSBs can also result from internal sources such as replication stress, reactive oxygen species generated during cellular metabolism, and mechanical stress during cell division. In the immune system, DSBs are intentionally created during processes like V(D)J recombination for antibody diversity.

Repairing double-strand breaks is crucial for maintaining genomic stability. Unrepaired DSBs can lead to chromosomal rearrangements, mutations, and even cell death. These genomic alterations can disrupt vital genetic information and potentially lead to diseases like cancer. Efficient DSB repair is essential for preventing these adverse outcomes and ensuring the proper functioning of cells.

Non-Homologous End Joining (NHEJ) is a primary pathway for repairing DSBs in cells. It is crucial because it operates throughout the cell cycle, unlike other repair mechanisms limited to specific phases. NHEJ is particularly important in repairing DSBs caused by ionizing radiation and other random DNA breaks. It works by directly joining the broken DNA ends, a process that is essential for maintaining genomic integrity and preventing severe consequences like carcinogenesis.

• Ku70/Ku80 Heterodimer: This protein complex is the first to respond to a DSB. It initiates the repair process by recognizing and binding to the DNA break. The Ku70/Ku80 heterodimer also helps to protect the DNA ends from degradation and aligns them for subsequent repair steps.
• DNA-Dependent Protein Kinase (DNA-PK): Once the Ku70/Ku80 complex is bound to the DNA, DNA-PK is recruited and activated. This kinase is central in coordinating the repair process, primarily by phosphorylating various substrates. These phosphorylation events are crucial for facilitating the next stages of the repair pathway.
• Artemis Nuclease: Artemis nuclease is involved in processing the DNA ends to make them suitable for ligation. It plays a key role in opening hairpin structures and trimming damaged nucleotides, ensuring the DNA ends are properly prepared for the final ligation step.
• DNA Polymerases Mu (Pol μ) and Lambda (Pol λ): Both Pol μ and Pol λ share the ability to perform template-independent DNA synthesis, which is crucial in NHEJ where a complementary DNA strand may not be available. These polymerases are particularly important for gap-filling during the repair process, ensuring that any gaps created during DNA end-processing are accurately filled. Their activity contributes significantly to the efficiency and fidelity of the NHEJ repair mechanism.
• XRCC4-DNA Ligase IV Complex: This complex is responsible for the actual ligation of the DNA ends, a critical step in the repair process. DNA Ligase IV, in association with XRCC4, performs the final rejoining of the broken DNA strands, effectively restoring the integrity of the DNA molecule.

If NHEJ is dysfunctional, cells can experience genomic instability, which increases the risk of chromosomal aberrations, translocations, and mutations. This genomic instability is a hallmark of many cancers. Additionally, defects in NHEJ components are linked to genetic disorders such as severe combined immunodeficiency (SCID). Therefore, proper functioning of NHEJ is crucial for preventing these adverse health outcomes.