DNA Repair r ac

DNA damage is almost constantly occurring, and requires proper repair of the damage in order to prevent apoptosis, senescence or cancer formation. The reliability of these DNA repair mechanisms depends on the cell type, the age of the cell and the environment it is situated in.

Sources of Damage

DNA damage can be endogenous or exogenous, depending on the source of the damage.

Endogenous damage can be related to replication errors causing mismatch of base pairs, or reactive oxygen species oxidating bases and interrupting the strand.

Exogenous damage can occur from radiation, thermal disruption, some toxins, viruses and carcinogens. Radiation causes pyrimidine dimers to form, where adjacent cytosine or thymine bases are cross-linked. It can also produce free radicals or ionizing rays that cause breaks in the DNA strand. Excessive heat causes depurination (loss of purine bases) and single-strand breaks. Chemicals can cause various types of adducts that interrupt normal DNA functions.

Repair Mechanisms

There are several mechanisms that can be used to repair DNA, depending on the nature of the damage. Typically, DNA repair mechanisms are activated when the cell recognizes that the conformation of the DNA helix has been changed, a sign of damage.

Cell Cycle Arrest

An important step that must occur before DNA repair is cell cycle arrest, to allow DNA repair to occur. This pause is mediated by ATM and ATR, which activates the checkpoint effector protein CHK2 by phosphorylation and tumour suppressor p53 by phosphorylation of MDM2. p53 is a transcription factor for p21, a CDK inhibitor. If DNA damage cannot be repaired, it activates transcription of BAX to induce apoptosis.

Single-strand damage

If only one strand is damaged, then the cell can use the other strand as a guide to help direct repair. In these processes, the erroneous area is removed, and the alternate strand is used to replace the area with new, hopefully correct nucleotides. All of these repair mechanisms are considered to be accurate, with a low rate of failure.

• Base excision repair involves removing a single base or nucleotide, then inserting the correct one. This repair pathway begins with removal of the damaged base by glycosylase. Endonucleases then open the DNA strand to allow DNA polymerase attachment and synthesis of a new strand. The new strand is then joined to the remaining DNA by DNA ligaseIII.
• Nucleotide excision repair targets large or bulky areas of damage, such as pyrimidine dimers. A ligase enzyme attaches to the site of damage, along with PARP-1. PARP-1 PARylates (makes a poly-ADP chain) at the ligase attachment site and recruits ALC1, which relaxes the DNA structure to form a bubble. XPC is activated, and the area is resynthesized by DNA polymerase and fused by DNA ligase.
• Mismatch repair targets improper base matching that is not caught by the cell’s proofreading mechanisms. In this process, a protein detects the mismatch and recruits endonuclease to remove the damaged area so DNA polymerase-δ and DNA ligase I can replace the damaged area. In eukaryotes, the proteins that detect the mismatch are most commonly MSH2 or MLH1.

Double-strand damage

With double strand breaks, there is a high incidence of erroneous repair, because there is very little information to tell the cell how the puzzle pieces are supposed to fit back together. This is because the opposing strand, that would normally be used as a template, is also damaged. These repair mechanisms are considered somewhat inaccurate for this reason.

• Nonhomologous end joining uses the short overhangs of DNA at the end of the double-stranded break as a guide. DNA-PKcs and Ku proteins form a complex that attaches to the free ends to tether them for repair. If these overhangs match, DNA ligase IV can attach the ends. However, sometimes nucleotides are lost at the break site. If this occurs, DNA polymerases may insert or remove nucleotides, producing a deletion, insertion or translocation, depending on the nature of the damage.
  • A special instance of NHEJ occurs with V(D)J recombination in B and T cells. In this process, RAG1 or RAG2nuclease produces a double-stranded DNA break, to allow recombination for receptor diversity.
• Homologous recombination occurs only during G₂ or S phases of the cell cycle, because it requires there to be a sister chromatid of the damaged area to be available. It uses this sister chromatid as a template to repair the double stranded break. Because a template is available, this type of repair is considered accurate, however requiring the availability of a sister chromatid limits when this type of repair can occur. BRCA1 and BRCA2 are important proteins involved in this pathway, and are used to mediate strand exchange by interacting with RAD51.

Zachary JF. Pathologic Basis of Veterinary Disease, Sixth Edition.
Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease, Tenth Edition.