API

Event: 155

Key Event Title

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N/A, Inadequate DNA repair

Short name

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N/A, Inadequate DNA repair

Biological Context

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Level of Biological Organization
Cellular

Cell term

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Organ term

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Key Event Components

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Process Object Action
DNA repair deoxyribonucleic acid functional change

Key Event Overview


AOPs Including This Key Event

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Stressors

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Taxonomic Applicability

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Term Scientific Term Evidence Link
mouse Mus musculus High NCBI
rat Rattus norvegicus Moderate NCBI
Syrian golden hamster Mesocricetus auratus Moderate NCBI
Homo sapiens Homo sapiens High NCBI

Life Stages

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Life stage Evidence
All life stages High

Sex Applicability

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Term Evidence
Unspecific High

Key Event Description

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DNA lesions may result from the formation of DNA adducts (i.e., covalent modification of DNA by chemicals), or by the action of agents such as radiation that may produce strand breaks or modified nucleotides within the DNA molecule. These DNA lesions are repaired through several mechanistically distinct pathways that can be categorized as follows:

  1. Damage reversal acts to reverse the damage without breaking any bonds within the sugar phosphate backbone of the DNA. The most prominent enzymes associated with damage reversal are photolyases (Sancar, 2003) that can repair UV dimers in some organisms, and O6-alkylguanine-DNA alkyltransferase (AGT) (Pegg 2011) and oxidative demethylases (Sundheim et al., 2008), which can repair some types of alkylated bases.
     
  2. Excision repair involves the removal of a damaged nucleotide(s) through cleavage of the sugar phosphate backbone followed by re-synthesis of DNA within the resultant gap. Excision repair of DNA lesions can be mechanistically divided into base excision repair (BER) (Dianov and Hübscher, 2013), in which the damaged base is removed by a damage-specific glycosylase prior to incision of the phosphodiester backbone at the resulting abasic site, and nucleotide excision repair (NER) (Schärer, 2013), in which the DNA strand containing the damaged nucleotide is incised at sites several nucleotides 5’ and 3’ to the site of damage, and a polynucleotide containing the damaged nucleotide is removed prior to DNA resynthesis within the resultant gap. The major pathway that removes oxidative DNA damage is base excision repair (BER), which can be either monofunctional or bifunctional; in mammals, a specific DNA glycosylase (OGG1: 8-Oxoguanine glycosylase) is responsible for excision of 8-oxoguanine (8-oxoG) and other oxidative lesions (Hu et al., 2005; Scott et al., 2014; Whitaker et al., 2017). We note that long-patch BER is used for the repair of clustered oxidative lesions, which uses several enzymes from DNA replication pathways (Klungland and Lindahl, 1997). These pathways are described in detail in various reviews e.g., (Whitaker et al., 2017). A third form of excision repair is mismatch repair (MMR), which does not act on DNA lesions but does recognize mispaired bases resulting from replication errors. In MMR the strand containing the misincorporated base is removed prior to DNA resynthesis. The major pathway that removes oxidative DNA damage is base excision repair (BER), which can be either monofunctional or bifunctional; in mammals, a specific DNA glycosylase (OGG1: 8-Oxoguanine glycosylase) is responsible for excision of 8-oxoguanine (8-oxoG) and other oxidative lesions (Hu et al., 2005; Scott et al., 2014; Whitaker et al., 2017). We note that long-patch BER is used for the repair of clustered oxidative lesions, which uses several enzymes from DNA replication pathways (Klungland and Lindahl, 1997). These pathways are described in detail in various reviews (e.g., (Whitaker et al., 2017)).
     
  3. Double strand break repair (DSBR) is necessary to preserve genomic integrity when breaks occur in both strands of a DNA molecule. There are two major pathways for DSBR: homologous recombination (HR), which operates primarily during S phase in dividing cells, and nonhomologous end joining (NHEJ), which can function in both dividing and non-dividing cells (Teruaki Iyama and David M. Wilson III, 2013).

    In higher eukaryotes such as mammals, NHEJ is usually the preferred pathway for DNA DSBR. Its use, however, is dependent on the cell type, the gene locus, and the nuclease platform (Miyaoka et al., 2016). The use of NHEJ is also dependent on the cell cycle; NHEJ is generally not the pathway of choice when the cell is in the late S or G2 phase of the cell cycle, or in mitotic cells when the sister chromatid is directly adjacent to the double-strand break (DSB) (Lieber et al., 2003). In these cases, the HR pathway is commonly used for repair of DSBs. Despite this, NHEJ is still used more commonly than HR in human cells. Classical NHEJ (C-NHEJ) is the most common NHEJ repair mechanism, but alternative NHEJ (alt-NHEJ) can also occur, especially in the absence of C-NHEJ and HR.

    The process of C-NHEJ in humans requires at least seven core proteins: Ku70, Ku86, DNA-dependent protein kinase complex (DNA-PKcs ), Artemis, X-ray cross-complementing protein 4 (XRCC4), XRCC4-like factor (XLF), and DNA ligase IV (Boboila et al., 2012). When DSBs occur, the Ku proteins, which have a high affinity for DNA ends, will bind to the break site and form a heterodimer. This protects the DNA from exonucleolytic attack and acts to recruit DNA-PKcs, thus forming a trimeric complex on the ends of the DNA strands. The kinase activity of DNA-PKcs is then triggered, causing DNA-PKcs to auto-phosphorylate and thereby lose its kinase activity; the now phosphorylated DNA-PKcs dissociates from the DNA-bound Ku proteins. The free DNA-PKcs phosphorylates Artemis, an enzyme that possesses 5’-3’ exonuclease and endonuclease activity in the presence of DNA-PKcs and ATP. Artemis is responsible for ‘cleaning up’ the ends of the DNA. For 5’ overhangs, Artemis nicks the overhang, generally leaving a blunt duplex end. For 3’ overhangs, Artemis will often leave a four- or five-nucleotide single stranded overhang (Pardo et al., 2009; Fattah et al., 2010; Lieber et al., 2010). Next, the XLF and XRCC4 proteins form a complex which makes a channel to bind DNA and aligns the ends for efficient ligation via DNA ligase IV (Hammel et al., 2011).

    The process of alt-NHEJ is less well understood than C-NHEJ.  Alt-NHEJ is known to involve slightly different core proteins than C-NHEJ, but the steps of the pathway are essentially the same between the two processes (reviewed in Chiruvella et al., 2013). It is established, however, that alt-NHEJ is more error-prone in nature than C-NHEJ, which contributes to incorrect DNA repair. Alt-NHEJ is thus considered primarily to be a backup repair mechanism (reviewed in Chiruvella et al., 2013). 

    In contrast to NHEJ, HR takes advantage of similar or identical DNA sequences to repair DSBs (Sung and Klein, 2006). The initiating step of HR is the creation of a 3’ single strand DNA (ss-DNA) overhang. Combinases such as RecA and Rad51 then bind to the ss-DNA overhang, and other accessory factors, including Rad54, help recognize and invade the homologous region on another DNA strand. From there, DNA polymerases are able to elongate the 3’ invading single strand and resynthesize the broken DNA strand using the corresponding sequence on the homologous strand.

 

Fidelity of DNA Repair


Most DNA repair pathways are extremely efficient. However, in principal, all DNA repair pathways can be overwhelmed when the DNA lesion burden exceeds the capacity of a given DNA repair pathway to recognize and remove the lesion. Exceeded repair capacity may lead to toxicity or mutagenesis following DNA damage. Apart from extremely high DNA lesion burden, inadequate repair may arise through several different specific mechanisms. For example, during repair of DNA containing O6-alkylguanine adducts, AGT irreversibly binds a single O6-alkylguanine lesion and as a result is inactivated (this is termed suicide inactivation, as its own action causes it to become inactivated). Thus, the capacity of AGT to carry out alkylation repair can become rapidly saturated when the DNA repair rate exceeds the de novo synthesis of AGT (Pegg, 2011).

A second mechanism relates to cell specific differences in the cellular levels or activity of some DNA repair proteins. For example, XPA is an essential component of the NER complex. The level of XPA that is active in NER is low in the testes, which may reduce the efficiency of NER in testes as compared to other tissues (Köberle et al., 1999). Likewise, both NER and BER have been reported to be deficient in cells lacking functional p53 (Adimoolam and Ford, 2003; Hanawalt et al., 2003; Seo and Jung, 2004). A third mechanism relates to the importance of the DNA sequence context of a lesion in its recognition by DNA repair enzymes. For example, 8-oxoguanine (8-oxoG) is repaired primarily by BER; the lesion is initially acted upon by a bifunctional glycosylase, OGG1, which carries out the initial damage recognition and excision steps of 8-oxoG repair. However, the rate of excision of 8-oxoG is modulated strongly by both chromatin components (Menoni et al., 2012) and DNA sequence context (Allgayer et al., 2013) leading to significant differences in the repair of lesions situated in different chromosomal locations.

DNA repair is also remarkably error-free. However, misrepair can arise during repair under some circumstances. DSBR is notably error prone, particularly when breaks are processed through NHEJ, during which partial loss of genome information is common at the site of the double strand break (Iyama and Wilson, 2013). This is because NHEJ rejoins broken DNA ends without the use of extensive homology; instead, it uses the microhomology present between the two ends of the DNA strand break to ligate the strand back into one. When the overhangs are not compatible, however, indels (insertion or deletion events), duplications, translocations, and inversions in the DNA can occur. These changes in the DNA may lead to significant issues within the cell, including alterations in the gene determinants for cellular fatality (Moore et al., 1996).

Activation of mutagenic DNA repair pathways to withstand cellular or replication stress either from endogenous or exogenous sources can promote cellular viability, albeit at a cost of increased genome instability and mutagenesis (Fitzgerald et al., 2017). These salvage DNA repair pathways including, Break-induced Replication (BIR) and Microhomology-mediated Break-induced Replication (MMBIR). BIR repairs one-ended DSBs and has been extensively studied in yeast as well as in mammalian systems. BIR and MMBIR are linked with heightened levels of mutagenesis, chromosomal rearrangements and ensuing genome instability (Deem et al., 2011; Sakofsky et al., 2015; Saini et al., 2017; Kramara et al., 2018). In mammalian genomes BIR-like synthesis has been proposed to be involved in late stage Mitotic DNA Synthesis (MiDAS) that predominantly occurs at so-called Common Fragile Sites (CFSs) and maintains telomere length under s conditions of replication stress that serve to promote cell viability (Minocherhomji et al., 2015; Bhowmick et al., 2016; Dilley et al., 2016).       

Misrepair may also occur through other repair pathways. Excision repair pathways require the resynthesis of DNA and rare DNA polymerase errors during gap resynthesis will result in mutations (Brown et al., 2011). Errors may also arise during gap resynthesis when the strand that is being used as a template for DNA synthesis contains DNA lesions (Kozmin and Jinks-Robertson, 2013). In addition, it has been shown that sequences that contain tandemly repeated sequences, such as CAG triplet repeats, are subject to expansion during gap resynthesis that occurs during BER of 8-oxoG damage (Liu et al., 2009).


How It Is Measured or Detected

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There is no test guideline for this event. The event is usually inferred from measuring the retention of DNA adducts or the creation of mutations as a measure of lack of repair or incorrect repair. These ‘indirect’ measures of its occurrence are crucial to determining the mechanisms of genotoxic chemicals and for regulatory applications (i.e., determining the best approach for deriving a point of departure). More recently, a fluorescence-based multiplex flow-cytometric host cell reactivation assay (FM-HCR) has been developed to directly measures the ability of human cells to repair plasmid reporters (Nagel et al., 2014).

Indirect Measurement

In somatic and spermatogenic cells, measurement of DNA repair is usually inferred by measuring DNA adduct formation/removal. Insufficient repair is inferred from the retention of adducts and from increasing adduct formation with dose. Insufficient DNA repair is also measured by the formation of increased numbers of mutations and alterations in mutation spectrum. The methods will be specific to the type of DNA adduct that is under study.

Some EXAMPLES are given below for alkylated DNA.

DOSE-RESPONSE CURVE FOR ALKYL ADDUCTS/MUTATIONS: It is important to consider that some adducts are not mutagenic at all because they are very effectively repaired. Others are effectively repaired, but if these repair processes become overwhelmed mutations begin to occur. The relationship between exposure to mutagenic agents and the presence of adducts (determined as adducts per nucleotide) provide an indication of whether the removal of adducts occurs, and whether it is more efficient at low doses. A sub-linear DNA adduct curve suggests that less effective repair occurs at higher doses (i.e., repair processes are becoming saturated). A sub-linear shape for the dose-response curves for mutation induction is also suggestive of repair of adducts at low doses, followed by saturation of repair at higher doses. Measurement of a clear point of inflection in the dose-response curve for mutations suggests that repair does occur, at least to some extent, but reduced repair efficiency arises above the breakpoint. A lack of increase in mutation frequencies (i.e., flat line for dose-response) for a compound showing a dose-dependent increase in adducts would imply that the adducts formed are either not mutagenic or are effectively repaired.

RETENTION OF ALKYL ADDUCTS: Alkylated DNA can be found in cells long after exposure has occurred. This indicates that repair has not effectively removed the adducts. For example, DNA adducts have been measured in hamster and rat spermatogonia several days following exposure to alkylating agents, indicating lack of repair (Seiler et al., 1997; Scherer et al., 1987).

MUTATION SPECTRUM: Shifts in mutation spectrum (i.e., the specific changes in the DNA sequence) following a chemical exposure (relative to non-exposed mutation spectrum) indicates that repair was not operating effectively to remove specific types of lesions. The shift in mutation spectrum is indicative of the types of DNA lesions (target nucleotides and DNA sequence context) that were not repaired. For example, if a greater proportion of mutations occur at guanine nucleotides in exposed cells, it can be assumed that the chemical causes DNA adducts on guanine that are not effectively repaired.


Direct Measurement

Nagel et al. (2014) we developed a fluorescence-based multiplex flow-cytometric host cell reactivation assay (FM-HCR) to measures the ability of human cells to repair plasmid reporters. These reporters contain different types and amounts of DNA damage and can be used to measure repair through by NER, MMR, BER, NHEJ, HR and MGMT.

Please refer to the table below for additional details and methodologies for detecting DNA damage and repair.

Assay Name References Description DNA Damage/Repair Being Measured OECD Approved Assay
Dose-Response Curve for Alkyl Adducts/ Mutations

Lutz 1991

 

Clewell 2016

Creation of a curve plotting the stressor dose and the abundance of adducts/mutations; Characteristics of the resulting curve can provide information on the efficiency of DNA repair

Alkylation,

oxidative damage, or DSBs

N/A
Retention of Alkyl Adducts

Seiler 1997

 

Scherer 1987

Examination of DNA for alkylation after exposure to an alkylating agent; Presence of alkylation suggests a lack of repair Alkylation N/A
Mutation Spectrum Wyrick 2015 Shifts in the mutation spectrum after exposure to a chemical/mutagen relative to an unexposed subject can provide an indication of DNA repair efficiency, and can inform as to the type of DNA lesions present

Alkylation,

oxidative damage, or DSBs

N/A
DSB Repair Assay (Reporter constructs) Mao et al., 2011 Transfection of a GFP reporter construct (and DsRed control) where the GFP signal is only detected if the DSB is repaired; GFP signal  is quantified using fluorescence microscopy or flow cytometry DSBs N/A
Primary Rat Hepatocyte DNA Repair Assay

Jeffrey and Williams, 2000

 

Butterworth et al., 1987

Rat primary hepatocytes are cultured with a 3H-thymidine solution in order to measure DNA synthesis in response to a stressor in non-replicating cells; Autoradiography is used to measure the amount of 3H incorporated in the DNA post-repair Unscheduled DNA synthesis in response to DNA damage N/A
Repair synthesis measurement by 3H-thymine incorporation Iyama and Wilson, 2013 Measure DNA synthesis in non-dividing cells as indication of gap filling during excision repair Excision repair N/A
Comet Assay with Time-Course

Olive et al., 1990

 

Trucco et al., 1998

Comet assay is performed with a time-course; Quantity of DNA in the tail should decrease as DNA repair progresses DSBs  Yes (No. 489)
Pulsed Field Gel Electro-phoresis (PFGE) with Time-Course Biedermann et al., 1991 PFGE assay with a time-course; Quantity of small DNA fragments should decrease as DNA repair  progresses DSBs N/A

Fluorescence -Based Multiplex Flow-Cytometric Host Reactivation Assay

(FM-HCR)

Nagel 2008 Measures the ability of human cells to repair plasma reporters, which contain different types and amounts of DNA damage; Used to measure repair processes including HR, NHEJ, BER, NER, MMR, and MGMT HR, NHEJ, BER, NER, MMR, or MGMT N/A

 


Domain of Applicability

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The retention of adducts has been directly measured in many different types of eukaryotic somatic cells (in vitro and in vivo). In male germ cells, work has been done on hamsters, rats and mice. The accumulation of mutation and changes in mutation spectrum has been measured in mice and human cells in culture. Theoretically, saturation of DNA repair occurs in every species (prokaryotic and eukaryotic). The principles of this work were established in prokaryotic models. Nagel et al. (2014) have produced an assay that directly measures DNA repair in human cells in culture.

NHEJ is primarily used by vertebrate multicellular eukaryotes, but it also been observed in plants. Furthermore, it has recently been discovered that some bacteria (Matthews et al., 2014) and yeast (Emerson et al., 2016) also use NHEJ. In terms of invertebrates, most lack the core DNA-PKcs and Artemis proteins; they accomplish end joining by using the RA50:MRE11:NBS1 complex (Chen et al., 2001).  HR occurs naturally in eukaryotes, bacteria, and some viruses (Bhatti et al., 2016).


Evidence for Perturbation by Stressor



References

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Adimoolam, S. and J.M. Ford (2003), "p53 and regulation of DNA damage recognition during nucleotide excision repair" DNA Repair (Amst), 2(9): 947-54.

Allgayer, J., N. Kitsera, C. von der Lippen, B. Epe and A. Khobta (2013), "Modulation of base excision repair of 8-oxoguanine by the nucleotide sequence", Nucleic Acids Res., 41(18): 8559-8571.

Beranek, D.T. (1990), "Distribution of methyl and ethyl adducts following alkylation with monofunctional alkylating agents", Mutation Research, 231(1): 11-30.

Bhatti, A. et al., (2016), Homologous Recombination Biology. Encyclopedia Britannica.

Bhowmick, R., S. Minocherhomji, and I.D. Hickson (2016), "RAD52 Facilitates Mitotic DNA Synthesis Following Replication Stress", Mol Cell, 64:1117-1126.

Biedermann, A. K. et al., (1991), SCID mutation in mice confers hypersensitivity to ionizing radiation and a deficiency in DNA double-strand break repair. Cell Biology. 88(4): 1394-7. Doi: 10.1073/pnas.88.4.1394.

Boboila, C., F. W. Alt & B. Schwer, (2012), Classical and alternative end-joining pathways for repair of lymphocyte-specific and general DNA double-strand breaks. Adv Immunol. 116, 1-49. doi:10.1016/B978-0-12-394300-2.00001-6

Bronstein, S.M., J.E. Cochrane, T.R. Craft, J.A. Swenberg and T.R. Skopek (1991), "Toxicity, mutagenicity, and mutational spectra of N-ethyl-N-nitrosourea in human cell lines with different DNA repair phenotypes", Cancer Research, 51(!9): 5188-5197.

Bronstein, S.M., T.R. Skopek and J.A. Swenberg (1992), "Efficient repair of O6-ethylguanine, but not O4-ethylthymine or O2-ethylthymine, is dependent upon O6-alkylguanine-DNA alkyltransferase and nucleotide excision repair activities in human cells", Cancer Research, 52(7): 2008-2011.

Brown, J.A., L.R. Pack, L.E. Sanman and Z. Suo (2011), "Efficiency and fidelity of human DNA polymerases λ and β during gap-filling DNA synthesis", DNA Repair (Amst)., 10(1):24-33.

Butterworth, E. B. et al., (1987), A protocol and guide for the in vitro rat hepatocyte DNA-repair assay. Mutation Research. 189, 113-21. Doi: 10.1016/0165-1218(87)90017-6.

Chen, L. et al., (2001), Promotion of DNA ligase IV-catalyzed DNA end-joining by the Rad50/Mre11/Xrs2 and Hdf1/Hdf2 complexes. Mol Cell. 8(5), 1105-15.

Chiruvella, K. K., Z. Liang & T. E. Wilson, (2013), Repair of Double-Strand Breaks by End Joining. Cold Spring Harbor Perspectives in Biology, 5(5):127-57. Doi: 10.1101/cshperspect.a012757.

Deem, A., A. Keszthelyi, T. Blackgrove, A. Vayl, B. Coffey, R. Mathur, A. Chabes, and A. Malkova (2011), "Break-Induced Replication Is Highly Inaccurate", PLoS Biol, 9:e1000594.

Dianov, G.L. and U. Hübscher (2013), "Mammalian base excision repair: the forgotten archangel", Nucleic Acids Res., 41(6):3483-90.

Dilley, R.L., P. Verma, N.W. Cho, H.D. Winters, A.R. Wondisford, and R.A. Greenberg (2016), "Break-induced telomere synthesis underlies alternative telomere maintenance", Nature, 539:54-58.

Douglas, G.R., J. Jiao, J.D. Gingerich, J.A. Gossen and L.M. Soper (1995), "Temporal and molecular characteristics of mutations induced by ethylnitrosourea in germ cells isolated from seminiferous tubules and in spermatozoa of lacZ transgenic mice", Proceedings of the National Academy of Sciences of the United States of America, 92(16):7485-7489.

Fattah, F. et al., (2010), Ku regulates the non-homologous end joining pathway choice of DNA double-strand break repair in human somatic cells. PLoS Genet, 6(2), doi:10.1371/journal.pgen.1000855

Fitzgerald, D.M., P.J. Hastings, and S.M. Rosenberg (2017), "Stress-Induced Mutagenesis: Implications in Cancer and Drug Resistance", Ann Rev Cancer Biol, 1:119-140.

Hammel, M. et al., (2011), XRCC4 protein interactions with XRCC4-like factor (XLF) create an extended grooved scaffold for DNA ligation and double strand break repair. J Biol Chem, 286(37), 32638-32650. doi:10.1074/jbc.M111.272641.

Hanawalt, P.C., J.M. Ford and D.R. Lloyd (2003), "Functional characterization of global genomic DNA repair and its implications for cancer", Mutation Research, 544(2-3): 107–114.

Harbach, P. R. et al., (1989), The in vitro unscheduled DNA synthesis (UDS) assay in rat primary hepatocytes. Mutation Research. 216(2):101-10. Doi:10.1016/0165-1161(89)90010-1.

Iyama, T. and D.M. Wilson III (2013), "DNA repair mechanisms in dividing and non-dividing cells", DNA Repair, 12(8): 620– 636.

Jeffrey, M. A., M. G. Williams, (2000), Lack of DNA-damaging Activity of Five Non-nutritive Sweeteners in the Rat Hepatocyte/DNA Repair Assay. Food and Chemical Toxicology. 38: 335-338. Doi: 10.1016/S0278-6915(99)00163-5.

Köberle, B., J.R. Masters, J.A. Hartley and R.D. Wood (1999), "Defective repair of cisplatin-induced DNA damage caused by reduced XPA protein in testicular germ cell tumours", Curr. Biol., 9(5):273-6.

Kozmin, S.G. and S. Jinks-Robertson S. 2013. The mechanism of nucleotide excision repair-mediated UV-induced mutagenesis in nonproliferating cells. Genetics. 193(3): 803-17.

Kramara, J., B. Osia, and A. Malkova (2018), "Break-Induced Replication: The Where, The Why, and The How", Trends Genet, 34:518-531.

Lieber, M. R., (2010), The mechanism of double-strand DNA break repair by the nonhomologous DNA end-joining pathway. Annu Rev Biochem. 79:181-211. doi:10.1146/annurev.biochem.052308.093131.

Lieber, M. R. et al., (2003), Mechanism and regulation of human non-homologous DNA end-joining. Nat Rev Mol Cell Biol. 4(9):712-720. doi:10.1038/nrm1202.

Liu, Y., R. Prasad, W.A. Beard, E.W. Hou, J.K. Horton, C.T. McMurray and S.H. Wilson (2009), "Coordination between polymerase beta and FEN1 can modulate CAG repeat expansion", J. Biol. Chem., 284(41): 28352-28366.

Mao, Z. et al., (2011), SIRT6 promotes DNA repair under stress by activating PARP1. Science. 332(6036): 1443-1446. doi:10.1126/science.1202723.

Matthews, L. A., & L. A. Simmons, (2014), Bacterial nonhomologous end joining requires teamwork. J Bacteriol. 196(19): 3363-3365. doi:10.1128/JB.02042-14.

Menoni, H., M.S. Shukla, V. Gerson, S. Dimitrov and D. Angelov (2012), "Base excision repair of 8-oxoG in dinucleosomes", Nucleic Acids Res. , 40(2): 692-700.

Minocherhomji, S., S. Ying, V.A. Bjerregaard, S. Bursomanno, A. Aleliunaite, W. Wu, H.W. Mankouri, H. Shen, Y. Liu, and I.D. Hickson (2015), "Replication stress activates DNA repair synthesis in mitosis", Nature, 528:286-290.

Miyaoka, Y. et al., (2016), Systematic quantification of HDR and NHEJ reveals effects of locus, nuclease, and cell type on genome-editing. Sci Rep, 6, 23549. doi:10.1038/srep23549/.

Moore, J. K., & J. E. Haber, (1996), Cell cycle and genetic requirements of two pathways of nonhomologous end-joining repair of double-strand breaks in Saccharomyces cerevisiae. Molecular and Cellular Biology. 16(5), 2164–73. Doi: 10.1128/MCB.16.5.2164.

Nagel, Z.D., C.M. Margulies, I.A. Chaim, S.K. McRee, P. Mazzucato, A.A. Ahmad, R.P. Abo, V.L. Butty, A.L. Forget and L.D. Samson (2014), "Multiplexed DNA repair assays for multiple lesions and multiple doses via transcription inhibition and transcriptional mutagenesis", Proc. Natl. Acad. Sci. USA. 111(18):E1823-32.

O’Brien, J.M., M. Walker, A. Sivathayalan, G.R. Douglas, C.L. Yauk, and F. Marchetti (2015), "Sublinear response in lacZ mutant frequency of Muta™ Mouse spermatogonial stem cells after low dose subchronic exposure to N-ethyl-N-nitrosourea", Environ. Mol. Mutagen., 56(4): 347-55.

Olive, L. P., J. P. Bnath & E. R. Durand, (1990), Heterogeneity in Radiation-Induced DNA Damage and Repairing Tumor and Normal Cells Measured Using the "Comet" Assay. Radiation Research. 122: 86-94.

Pardo, B., B. Gomez-Gonzalez & A. Aguilera, (2009), DNA repair in mammalian cells: DNA double-strand break repair: how to fix a broken relationship. Cell Mol Life Sci, 66(6), 1039-1056. doi:10.1007/s00018-009-8740-3.

Pegg, A.E. (2011), "Multifaceted roles of alkyltransferase and related proteins in DNA repair, DNA damage, resistance to chemotherapy, and research tools", Chem. Res. Toxicol., 4(5): 618-39.

Sancar, A. (2003), "Structure and function of DNA photolyase and cryptochrome blue-light photoreceptors", Chem Rev., 103(6): 2203-37.

Saini, N., S. Ramakrishnan, R. Elango, S. Ayyar, Y. Zhang, A. Deem, G. Ira, J. Haber, K.S. Lobachev, and A. Malkova (2017), "Migrating bubble during break-induced replication drives conservative DNA synthesis", Nature, 502:389-392.

Sakofsky, C.J., S. Ayyar, A. Deem, W.H. Chung, G. Ira, and A. Malkova (2015), "Translesion Polymerases Drive Microhomology-Mediated Break-Induced Replication Leading to Complex Chromosomal Rearrangements", Mol Cell, 60:860-872.

Schärer, O.D. (2013), "Nucleotide excision repair in eukaryotes", Cold Spring Harb. Perspect. Biol., 5(10): a012609.

Scherer, E., A.A. Jenner and L. den Engelse (1987), "Immunocytochemical studies on the formation and repair of O6-alkylguanine in rat tissues", IARC Sci Publ., 84: 55-8.

Seiler, F., K. Kamino, M. Emura, U. Mohr and J. Thomale (1997), "Formation and persistence of the miscoding DNA alkylation product O6-ethylguanine in male germ cells of the hamster", Mutat Res., 385(3): 205-211.

Shelby, M.D. and K.R. Tindall (1997), "Mammalian germ cell mutagenicity of ENU, IPMS and MMS, chemicals selected for a transgenic mouse collaborative study", Mutation Research, 388(2-3): 99-109.

Seo, Y.R. and H.J. Jung (2004), "The potential roles of p53 tumor suppressor in nucleotide excision repair (NER) and base excision repair (BER)", Exp. Mol. Med., 36(6): 505-509.

Sundheim, O., V.A. Talstad, C.B. Vågbø, G. Slupphaug and H.E. Krokan (2008), "AlkB demethylases flip out in different ways", DNA Repair (Amst)., 7(11): 1916-1923.

Sung, P., & H. Klein, (2006), Mechanism of homologous recombination: mediators and helicases take on regulatory functions. Nat Rev Mol Cell Biol, 7(10), 739-750. doi:10. 1038/nrm2008.

Trucco, C., et al., (1998), DNA repair defect i poly(ADP-ribose) polymerase-deficient cell lines. Nucleic Acids Research. 26(11): 2644–2649.

Wyrick, J.J. & S. A. Roberts, (2015), Genomic approaches to DNA repair and mutagenesis. DNA Repair (Amst). Dec;36:146-155. doi: 10.1016/j.dnarep.2015.09.018. Epub 2015, Sep 15.

van Zeeland, A.A., A. de Groot and A. Neuhäuser-Klaus (1990), "DNA adduct formation in mouse testis by ethylating agents: a comparison with germ-cell mutagenesis", Mutat. Res., 231(1): 55-62.