Non-homologous End Joining (NHEJ) is a process in which eukaryotic cells do not rely on DNA homology, and in order to avoid DNA or chromosomal breaks (Breaks), avoid DNA degradation or the effect of vitality, a special DNA double-strand break repair mechanism that forcibly connects two DNA ends to each other.
Theoretically, non-homologous end joining can be applied to various phases of the cell cycle (Phase), while for cells in the G1 phase, it accounts for a large proportion of cell repair. At the same time, it is a repairing method for DNA double-strand breaks that complements and complements homologous recombination. Compared to homologous recombination repair mechanisms for DNA double-strand breaks, NHEJ does not require stringent DNA homology between recombination ends, and is not a faithful DNA double-strand break repair. In the course of the reaction, it is first necessary to perform "gene silencing" (gene silencing) treatment on the adjacent regions of the two DNA ends (requires methylation of the 9-position lysine residue of the N-terminal tail domain of histones and Sir2/3 /4 and other proteins form heterochromatin) to avoid gene transcription at the end of the gene, etc., and then also require DNA end recognition and binding proteins such as Ku70, Ku86 and other proteins, as well as Mre-11/Rad50-Nbs1 (human A protein complex having a DNase activity such as a cell or a Srs2 (yeast cell) is processed by a DNA fragment involved in binding a protein. The processing of DNase is mainly to remove the proteins covalently linked to the DNA fragment or the damaged nucleotide residues caused by ionizing radiation, etc., and finally to produce a "sticky" end with each other, They are then ligated to each other by DNA ligase ERCCIV. The mechanism of action of NHEJ determines that it is not a faithful means of DNA double-strand break repair. In addition to triggering the disconnection of DNA ends that are not related to each other, resulting in rearrangement changes between chromosomes including shifting (shifting), etc., the processing of DNA ends that are originally connected to each other will also result in a minority. Deletion mutations in nucleotide residues. This is determined by the method of treating a DNA fragment to be ligated to each other using a DNase such as Mre-11/Rad50-Nbs1 (human cell) or Srs2 (yeast cell). NHEJ also causes many human health problems while repairing DNA double-strand breaks.
Some diseases in humans are associated with abnormalities in NHEJ function. These include cell radiosensitivity, microcephaly, and severe combined immunodeficiency (SCID) due to defective V(D)J recombination. Mutations in loss of function in Artemis can also lead to SCID, but these patients did not show neurological deficits associated with LIG4 or XLF mutations. The difference in severity can be explained by the action of the mutant protein. Artemis is a nuclease that is thought to be used only to repair terminally damaged DSB, and all NHEJ events require DNA Ligase IV and XLF. Mutations in genes involved in non-homologous end joining result in ataxia telangiectasia (ATM gene), Fanconi anemia (multiple genes), and hereditary breast and ovarian cancer (BRCA1 gene).
Proteins involved in NHEJ in human cells
In 1990, the researchers discovered a gene directly related to hereditary breast cancer, named breast cancer number 1 gene, referred to as BRCA1. In 1994, another gene related to breast cancer, called BRCA2, was discovered. In fact, BRCA1/2 is a gene that inhibits the development of malignant tumors and plays an important role in regulating the replication of human cells, DNA damage repair of genetic material, and normal cell growth. Families with this genetic mutation tend to have a high incidence of breast cancer, usually at a younger age, where the patient's breasts are both cancerous and have ovarian cancer.
Figure 1. Protein structure of BRCA1.
2. Ku (protein)
Ku is a dimeric protein complex that binds to the ends of DNA double-strand breaks and is required for the non-homologous end joining (NHEJ) pathway of DNA repair. Mutant mice with Ku70 or Ku80 deficiency, or double mutant mice lacking both Ku70 and Ku80 showed early senescence. Ku function is critical to ensure longevity, and the NHEJ pathway of DNA repair (mediated by Ku) plays a key role in repairing DNA double-strand breaks that would otherwise lead to early aging.
Figure 2. Crystal structure of the ku heterodimer.
3. Non-homologous end-joining factor 1
Non-homologous end joining factor 1 (NHEJ1) is a protein encoded by the NHEJ1 gene in humans. X This protein is required for the non-homologous end joining (NHEJ) pathway of DNA repair. Due to V(D)J recombination defects, patients with XLF mutations also have immunodeficiency, which uses NHEJ to produce diversity in the antibody repertoire of the immune system. XLF interacts with DNA ligase IV and XRCC4 and is thought to be involved in the terminal bridging or ligation step of NHEJ.
Figure 3. Structure of protein NHEJ1.
4. DNA polymerase mu
DNA polymerase mu is encoded by the POLM gene. It is involved in the resynthesis of damaged or deleted nucleotides in the non-homologous end joining (NHEJ) pathway of DNA repair. It is structurally and functionally related to polλ, and like polλ, polμ has a BRCT domain that is thought to mediate interactions with other DNA repair proteins. However, unlike polλ, polμ has a unique ability to add a base to the flat-bottom end, which is terminated by the overhang at the other end of the double-strand break. Polμ is also closely related to terminal deoxynucleotidyl transferase (TdT), a special DNA polymerase that adds random nucleotides to the DNA ends during V(D)J recombination. B cell and T cell receptor diversity is produced in cells. Vertebrate immune system. Like TdT, polμ is involved in V(D)J recombination, but only during light chain rearrangement.
Figure 4. Structure of the POLM protein.
1. Li H.; et al. Deletion of Ku70, Ku80, or both causes early aging without substantially increased cancer. Mol. Cell. Biol. 2007,5 (11): a012740.
2. Yoshida K.; et al. Role of BRCA1 and BRCA2 as regulators of DNA repair, transcription, and cell cycle in response to DNA damage. Cancer Science. 2004,95 (11): 866–71.