DSB repair and next generation sequencing Flashcards
What are DNA double-strand (DS) breaks and why are they important?
Double-strand breaks (DSB) occur when the phosphodiester backbone of both strands of a DNA duplex break at the same, or nearly the same, place. DNA double-strand breaks threaten the integrity of the genome, and thus their efficient repair is critical to maintain genome stability. Loss of repair and subsequent genome instability leads to a wide variety of disorders.
Examples of natural DSBs
Meiosis, Generation of immune diversity, both VDJ recombination and class switch recombination, DNA replication, when the replication fork encounters a nick in one template strand or stalls at fragile sites, or sites of topoisomerase failure, On insertion of retroviruses or retrotransposons into genomic DNA
Examples of DSBs due to environmental damage
These include cosmic rays and radiation from soils. The damage from ionizing radiation can be both direct (deposition of energy) and indirect (clustered oxidative free radicals caused by the energy deposition in turn cause DNA breaks). Medical tests and treatments are another significant source of ionizing radiation. Many commonly used imaging techniques including X-rays and CT scans, as well as radiation treatment and some chemotherapeutic agents (e.g. bleomycin) cause DSBs.
How are DS breaks sensed?
Two members of the phosphatidylinositol 3-kinase related kinases (PIKKs) family are principally used to sense DS breaks: 1) ataxia telangiectasia mutated (ATM) and 2) ataxia telangiectasia and RAD3-related (ATR). These are protein kinases used to signal DNA damage more generally than just double-strand breaks. The disease ataxia telangiectasia causes genetic instability, a predisposition to cancer, and neurodegeneration.
phosphatidylinositol 3-kinase related kinases (PIKKs)
a family of Ser/Thr-protein kinases. Include ATM, ATR, and DNA-PKcs
ataxia telangiectasia mutated (ATM)
has an important role in the response to ionizing radiation, by phosphorylating several key proteins such as p53, Mdm2, Chk1, Nbs1 and Brca1 in response to DNA damage. These phosphorylation events are, in part, responsible for the cell cycle arrest that is necessary for DSB repair. Cells in which ATM is mutated are defective in the arrest at both the G1 and G2 phases of the cell cycle. In addition, while normal cells exhibit a dose dependent inhibition of DNA synthesis following exposure to ionizing radiation, A-T cells display almost no alteration in their rates of replication.
ataxia telangiectasia and RAD3-related (ATR)
a serine/threonine-specific protein kinase that is involved in sensing DNA damage and activating the DNA damage checkpoint, leading to cell cycle arrest.[3] ATR is activated in response to persistent single-stranded DNA, which is a common intermediate formed during DNA damage detection and repair.
How are DS breaks repaired?
Two pathways are used: 1) DNA non-homologous end joining (NHEJ) and 2) homologous recombination (HR); multiple proteins are involved in both.
Homologous Recombination (HR)
occurs during and shortly after DNA replication, in S and G2 phases of the cell cycle. It absolutely requires nearly identical DNA strands (homologous) of the unbroken sister chromatid. Using this perfect template, HR repairs the break perfectly, with no gain or loss of nt. Multiple steps are required to repair double-strand breaks by homologous recombination. First the broken ends are resected, i.e., processed to expose single-strands ending in 3’ OHs. The single-strands then invade the homologous DNA, using it as a template for synthesis to bridge the gap caused by the break. The crossover (Holliday junction) then has to be resolved back to two separate DNA double strands. Typically in DNA repair using the identical sister chromatid perfectly restores the break with no exchange.
non-homologous end joining (NHEJ)
NHEJ proteins are required for resistance to ionizing radiation and for VDJ recombination and immunoglobulin class switching in the immune system. NHEJ simply brings together the broken ends, and ligates them, typically (but not necessarily) losing a few nucleotides (nt) in the process. Thus fidelity of this system is imperfect, but its advantages include quick repair at any time in the cell cycle, and the process is not sequence-dependent. Recall that the human genome has a lot of space (introns, intragenic regions) where the loss of a few nucleotides here and there is not likely to cause a problem. The somatic mutations left behind by imperfections in the NHEJ process accumulate over time; a typical somatic cell in a 70 year old human has ~2000 little “footprints” from this imperfect repair system. Unpredictable/imprecise repair by NHEJ contributes to antibody diversity
Holliday junction
a junction between four strands of DNA. Different resolutions of Holliday junctions can lead to exchange of genetic information between the two chromosomes undergoing HR. This exchange is an essential feature of meiosis and can take two forms, gene conversion or crossing over. In some cases of misregulated homologous recombination, using the other chromosomal homolog as a template for double-strand break repair instead of the sister chromatid in somatic cells can lead to loss of heterozygosity (LOH) by these same mechanisms. Holliday junctions must be cleaved to produce recombinant molecules. Depending on the direction of cleavage, the resulting molecules will or will not have exchanged sequences flanking the region of heteroduplex DNA (crossing over).
What determines whether HR or NHEJ is used to repair a given DS break?
Two tumor suppressors, BRCA1 and 53BP1, are pivotal regulators of the choice to repair a DS break using HR or NHEJ, respectively.
BRCA1
The double-strand repair mechanism that BRCA1 participates in is homologous recombination, in which the repair proteins utilize homologous intact sequence from a sister chromatid, from a homologous chromosome, or from the same chromosome (depending on cell cycle phase) as a template. This DNA repair takes place with the DNA in the cell nucleus, wrapped around the histone. Several proteins, including BRCA1, arrive at the histone-DNA complex for this repair. one of the key protein targets of phosphorylation by activated ATM and ATR; phosphorylation recruits BRCA1 to a double-strand break. Proper control of normal BRCA1 activity is required for double-strand break repair by homologous recombination although the exact role(s) of this large (1863 amino acids) protein and its many interactions remain a topic of active investigation. Significantly, in BRCA1 mutant mice that exhibit embryonic lethality, tumorogenesis and chromosomal abnormalities, genetic instability can be phenotypically rescued by a 53BP1 deletion. Importantly, the genetic instability caused by mutations in the core HR machinery, e.g., BRCA2 or XRCC2, are not rescued by a 53BP1 deletion. These findings indicate that, in the absence of HR mediated by BRCA1, 53BP1 promotes deleterious NHEJ; thus, the proteins encoded by BRCA1 and 53BP1 compete to determine the correct DNA DS break repair pathway.
53BP1
a large (1972 amino acids) protein that interacts with a plethora of other proteins. Recently, key findings indicate that 53BP1 plays a crucial role in DS break pathway choice, acting as a positive regulator of NHEJ by promoting the synapsis of distal broken ends and by antagonizing their resection.
XRCC2
This gene encodes a member of the RecA/Rad51-related protein family that participates in homologous recombination to maintain chromosome stability and repair DNA damage. This gene is involved in the repair of DNA double-strand breaks by homologous recombination.
What happens when DS breaks are mis-repaired or not repaired?
Mis-repair is the source of genetic instability with translocations being particularly bad. Loss of HR leads to gross genomic rearrangements and thus genome instability. Translocations most often occur when DNA DSBs are healed between different chromosomes by NHEJ. Failure to repair a DSB will lead to cell death, a fact exploited by cancer therapies (ie radiation and drugs that cause DSBs like bleomycin). DSB repair deficits, which increase cancer susceptibility, can be caused by loss of proteins involved in the signaling (both at the level of sensors and transducers) of the DSBs and in the enzymes that mediate their actual repair