Double-strand break repair model
[1] In human cells, there are two main DSB repair mechanisms: Homologous recombination (HR) and non-homologous end joining (NHEJ).
HR relies on undamaged template DNA as reference to repair the DSB, resulting in the restoration of the original sequence.
[2] In terms of DSB repair pathway choice, most mammalian cells appear to favor NHEJ rather than HR.
[5] DSB can occur naturally due to the presence of reactive species generated by metabolism, and various external factors (e.g. ionizing radiation or chemotherapeutic drugs).
The blunt ends of the DSB are processed into ssDNA with 3’ extensions, which allows RAD51 recombinase (eukaryotic homologue of prokaryotic RecA) to bind to it to form a nucleoprotein filament.
[9] Although there is little research in regards of break-induced replication, it is known that it is a one-ended recombination mechanism, where only of the one ends of a DSB will be involved in strand invasion.
[14] This means that unlike DSBR, BIR does not link back to the second DSB end after the strand invasion and replication.
This forms a preliminary scaffold which allows the recruitment of various NHEJ factors, such as the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), DNA Ligase IV and X-ray cross complementing protein 4 (XRCC4) to form a bridge and bring both ends of the damaged DNA strands together.
[5] It was found that the selection between MMEJ and NHEJ is mainly dependent on Ku levels and the concurrent cell cycle.
The image in this section illustrates molecular steps in a DNA damage response pathway in which a Fanconi anemia complex is activated during repair of a double-strand break.
[29] The PALB2 protein acts as a hub,[30] bringing together BRCA1, BRCA2 and RAD51 at the site of a DNA double-strand break, and also binds to RAD51C, a member of the RAD51 paralog complex RAD51B-RAD51C-RAD51D-XRCC2 (BCDX2).
[33] As compared to higher eukaryotes, yeast cells have adopted HR as the main repair pathway for DSB.
It was hypothesized that this inefficiency as compared to mammalian cells is due to the lack of three vital NHEJ proteins, including DNA-PKcs, BRCA1, and Artemis.
[36] Research hypothesize that this is due to the higher eukaryote's larger genome size, as it means that more NHEJ related proteins are encoded for NHEJ repair pathways; and a larger genome implies a challenging obstacle to find a homologous template for HR.
As S and G2 phases of the cell cycle generate more chromatids, the increased availability of template access for HR results in the up-regulation of the pathway.
In fact, NHEJ was shown to be active throughout all stages of the cell cycle, and is favoured in G1 phase during low resection action intervals.
(1) Haploid gametes undergo syngamy/fertilisation with the result that chromosome sets of different parental origin come together to share the same nucleus.
(3) Two successive cell divisions (without duplication of chromosomes) result in haploid gametes that can then repeat the meiotic cycle.
[42][43] Some examples of diseases caused by defects of DSB repair mechanisms are listed below: Women tend to live longer than men and the gender gap in life expectancy suggests differences in the ageing process between the sexes.
Sex specific differences in DNA double-strand break repair of cycling human lymphocytes during aging were studied.
[47] Activation of gene transcription during oncogenesis is often associated with the introduction of DNA double-strand breaks and their repair by a process employing RAD51.
