The DNA damage response - from cell biology to human disease
Abstract
Maintenance of DNA integrity is crucial for faithful transmission of the genetic code from generation to generation. Our genetic code is constantly under attack from both endogenous and exogenous sources of DNA damage. To ensure genome stability, cells have developed elegant DNA damage repair mechanisms. Defects in DNA damage repair have been linked to several human diseases including promoting oncogenesis, heritable neurodegenerative and neuromuscular diseases caused by unstable DNA repeats, neuropathies and myopathies caused by mutations and rearrangements in mitochondrial DNA, neuropsychiatric disorders, and heritable premature aging syndromes. This review will discuss our current understanding of how these underlying errors in DNA repair contribute to the clinical outcomes of patients who present with these diseases.
Keywords
INTRODUCTION
Cells in our body are exposed daily to endogenous and exogenous sources that damage their DNA, which, if left unrepaired, can lead to genome instability as these errors are perpetuated over subsequent cellular divisions[1]. It has been estimated that approximately 105 DNA lesions are generated spontaneously each day in the mammalian genome[2]. Endogenous sources of DNA lesions include the production of reactive oxygen and nitrogen species through oxidative respiration, byproducts of lipid peroxidation, endogenous alkylating agents, estrogen and cholesterol metabolites, reactive carbonyl species, as well as spontaneous disintegration and hydrolysis of DNA under physiologic conditions leaving non-coding apurinic/apyrimidinic abasic sites[3,4]. Exogenous sources include ultraviolet (UV) light from the sun, ionizing radiation (IR), genotoxic chemicals, and carcinogens that are inhaled or ingested[5]. It has been postulated that one day of sun exposure can cause up to 105 UV-induced DNA lesions in each exposed keratinocyte, with the release of oxidative damage locally that can induce inflammation[2].
Unrepaired DNA damage can induce cancer-causing mutations, cell death or senescence, and aging. The type of damage leads to distinct phenotypes and the activation of specific repair pathways. DNA lesions can cause roadblocks for the transcription and replication machinery, errors in gene transcription, changes in the epigenetic landscape, and dysregulation of the cell cycle. Damage caused by double-strand breaks (DSBs) is considered one of the major causes of genomic instability if left unrepaired, as they can lead to recombinatorial replication events that form cancer-prone chromosomal translocations and other chromosomal aberrations including aneuploidy and loss of heterozygosity[5]. Cells have developed sophisticated mechanisms to identify and repair damaged DNA in order to maintain genome stability. These involve multiple signaling pathways focused on the type of DNA lesion [i.e., DSB, single-strand break (SSB), inter-strand crosslink (ICL)], phosphorylation of damage sensing proteins and signaling kinases, recruitment of repair proteins to sites of damage, triggering cell cycle checkpoints and downstream effectors that allow for lesional repair, or activation of apoptotic or senescence pathways to ensure faithful transmission of genetic information to their progeny[6]. Loss of function of these repair pathways leads to characteristic human disease phenotypes, underlying the importance of DNA damage repair in the maintenance of human health and homeostasis.
DNA DAMAGE RESPONSE (DDR) AND REPAIR PATHWAYS
Mammalian cells have developed an evolutionarily conserved and sophisticated DNA damage response (DDR) to identify and repair damaged DNA in order to protect their genetic code for faithful replication and transmission of genetic information to future progeny[6,7]. This DDR repair pathway consists of families of proteins that serve as sensors of damage, mediators of the response, further recruitment of transducers that enable downstream signaling of effectors, which activate cell cycle checkpoints to allow for repair, or trigger apoptosis or senescence, to prevent transmission of erroneous genetic information to daughter cells. Many of these proteins have been implicated in human disease. As an in-depth discussion of the DDR is beyond the scope of this article, a concise review of the different repair pathways will be provided below as a primer to the readers. This will follow with comprehensive sections on the etiopathogenesis of human diseases associated with errors in DNA damage repair (summarized in Table 1).
Human diseases associated with deficient mechanisms of DNA damage repair. List of syndromes caused by mutations in genes involved in DNA damage repair categorized by syndrome, DDR pathways affected, associated mutations in DDR genes, and presenting symptoms.
| Disease | Affected DDR Mechanism | Affected Gene(s) | Disease Prevalence | Phenotype |
| (1) Cancer syndromes | ||||
| Ataxia-Telangiectasia[43,152] | DSB repair | ATM | 1 in 40,000-100,000 live births | Lymphomas, leukemias, breast cancer |
| Ataxia Telangiectasia-Like Disorder[153] | DSB repair | MRE11 | Rare | Leukemias |
| Bloom Syndrome[96,97,154] | HR repair | BLM | Overall prevalence is unknown; estimated 1 per 48,000 births in the Ashkenazi Jewish population | Carcinomas, leukemias, lymphomas |
| Fanconi Anemia[30,32,155] | ICL repair, HR | FANCA-C, FANCD1, D2, FANCE-G, FANCI, J, L-N | 1 in 136,000 newborns, varies from 1 in 100,000 to 250,000 births | Leukemia, myelodysplasia, squamous cell carcinoma |
| Hereditary Breast and Ovarian Cancer Syndrome[156,157] | HR repair | BRCA1, BRCA2 | Estimated 1 in 333 to 500 individuals have a BRCA1/2 mutation | Breast and ovarian cancers |
| Hereditary Nonpolyposis Colorectal Cancer[158-160] | MMR | MSH2, MSH6, MLH1, PMS2 | 2%-5% of the Caucasian population | Colorectal cancer, carcinomas |
| Li-Fraumeni Syndrome[161,162] | DSB repair | TP53 | Not well established, one group reported prevalence at 1:3,555 to 1:5,5476 | Gliomas and breast cancers, sarcoma |
| MYH-Associated Polyposis[163-165] | BER, oxidative damage repair | MYH | Responsible for 7% of attenuated adenomatous polyposis and 6.6% of classic polyposis cases | Colorectal cancer |
| Nijmegen Breakage Syndrome[117,166,167] | DSB repair | NBS1 | 1 in 100,000 newborns worldwide | Lymphomas |
| Rothmund-Thomson Syndrome[101] | BER, HR? | RECQL4 | Prevalence is unknown; about 300 cases have been reported in the literature | Osteosarcoma |
| Werner Syndrome[99,168,169] | BER, HR, telomere maintenance | WRN | 1 in 200,000 in the US, estimated 1 in 30,000 people in Japan and Sardinia | Various cancers (thyroid, melanoma, soft tissue sarcoma, osteosarcoma) |
| Xeroderma Pigmentosum[2,170,171] | TC-NER | XPA-G, POLH | 1 in 1,000,000 in US and Europe, about 1 in 22,000 in Japan | UV-induced skin cancers |
| (2) Neurologic syndromes | ||||
| Aicardi Goutieres Syndrome[172,173] | Damage signaling | RNASEH2, TREX1 | 1 to 5 per 10,000 live births | Cerebral atrophy, intracranial calcifications, microcephaly, neurodegeneration |
| Ataxia Telangiectasia[43,152] | DSB repair | ATM | 1 in 40,000-100,000 live births | Cerebellar ataxia, neurodegeneration, oculomotor apraxia |
| Ataxia Telangiectasia-Like | DSB repair | MRE11 | Rare | Cerebellar ataxia, neurodegeneration, oculomotor apraxia |
| Ataxia with Oculomotor Apraxia Type 1[64,77] | SSB repair | APTX | A rare disease, most frequent in Portugal and Japan | Cerebellar ataxia, neurodegeneration, oculomotor apraxia |
| Ataxia with Oculomotor Apraxia Type 2 | SSB repair, R-loop resolution | SETX | Estimated 1 in 900,000 individuals worldwide | Cerebellar ataxia, neurodegeneration, oculomotor apraxia |
| Cerebro-Ocular Facio-Skeletal syndrome[176,177] | TC-NER | CSB, XPD, XPG, ERCC1 | Rare - fewer than 20 cases confirmed | Brain calcification, hypomyelination, microcephaly, neurodegeneration |
| Cockayne Syndrome[87,178] | TC-NER | CSA, CSB, XPB, XPD, XPG | Less than 1 case per 250,000 live births in the US | Microcephaly, demyelination, neurodegeneration |
| Dyskeratosis Congenita[179] | Telomere maintenance | DKC1, TERC | Prevalence is unknown; more than 400 families were reported in the world | Microcephaly, cognitive impairment, developmental delay |
| Microcephaly, Intractable Seizures, and Developmental Delay | NHEJ, SSB repair | PNKP | Rare, prevalence unknown | Microcephaly, seizures, growth defects |
| Seckel Syndrome[139,183] | DSB repair, replication fork repair | ATR, PCTN, SCKL2, SCKL3 | Less than 1 in 1,000,000 | Microcephaly, cognitive impairment, developmental delay |
| Spinocerebellar Ataxia with Axonal Neuropathy[75] | SSB repair | TDP1 | Rare | Cerebellar ataxia, neurodegeneration |
| (3) Aging syndromes | ||||
| Ataxia-Telangiectasia[43,152] | DSB repair | ATM | 1 in 40,000-100,000 live births | Premature bone marrow exhaustion, early-onset diabetes, neurodegeneration |
| Alpers-Huttenlocher Syndrome[184] | Mitochondrial DNA replication and repair | POLG1 | 1 in 100,000 individuals | Neurodegeneration, liver failure |
| Bloom Syndrome[96,97,154] | HR repair | BLM | Overall prevalence is unknown; estimated 1 per 48,000 births in the Ashkenazi Jewish population | Early-onset diabetes, pulmonary disease, increased cancer risk |
| Cockayne Syndrome[86,87,178] | TC-NER | CSA, CSB, XPB, XPD, XPG | Less than 1 case per 250,000 live | Cataracts, muscle atrophy, neurodegeneration |
| Fanconi Anemia[30,32,155] | ICL repair | FANCA-W | 1 in 136,000 newborns, varies from 1 in 100,000 to 250,000 births | Premature bone marrow exhaustion, increased cancer risk |
| Hutchinson-Guilford Progeria Syndrome[185,186] | DDR, DSB repair, chromatin organization | LMNA | Approximately 1 in 20,000,000 | Alopecia, atherosclerosis |
| Werner Syndrome[99,168,169] | BER, HR, telomere maintenance | WRN | 1 in 200,000 in the US, estimated 1 in 30,000 people in Japan and Sardinia | Growth retardation, short stature, premature graying of hair, alopecia, arteriosclerosis, atrophic skin, bilateral cataracts, type II diabetes |
| (4) Immunodeficiencies | ||||
| Hyper-IgM Syndrome[112] | CSR | AID, UNG | Fewer than 1 in 1,000,000 | Increased IgM levels, lymphoid hyperplasia |
| Immunodeficiency with Microcephaly[1,187] | NHEJ | XLF | Less than 1 in 1,000,000 worldwide | Hypogammaglobulemia, lymphopenia, microcephaly |
| Ligase IV Syndrome[82,188] | NHEJ | LIG4 | Prevalence is unknown; globally only 28 cases are described | Hypogammaglobulemia, lymphopenia[188] |
| Radiosensitive Severe Combined Immunodeficiency[187] | NHEJ | ARTEMIS | Rare | Agammaglobulinemia, lymphopenia |
| Schimke Immuno-Osseous Dysplasia | Replication fork repair | SMARCAL1 | 1 in 1,000,000 to 3,000,000 people in North America | T cell deficiency |
| Severe Combined Immunodeficiency Syndrome[154,187] | NHEJ | Rag1, Rag2 | 1 in 100,000 in US | Agammaglobulinemia, lymphopenia |
Double-strand break (DSB) repair
DSBs are formed following exposure to IR, X-rays, chemotherapies, and free radicals[8]. The presence of DSBs is considered to pose the greatest threat to genome stability in mammalian cells[9]. Repair of DSBs occurs via two pathways: the efficient, but error-prone, non-homologous end-joining (NHEJ) pathway; or the less efficient, but less error-prone, homologous recombination (HR) pathway[10,11]. NHEJ functions during the G1 phase of the cell cycle and repairs breaks by the simple joining of free ends, which increases the risk of mutagenesis. Activation of NHEJ requires the recruitment of the phosphoinositol-kinase-kinase-kinase (PI3K) ataxia telangiectasia mutated (ATM) to sites of DSBs, leading to phosphorylation of the histone variant H2A.X (γH2AX) next to the break sites, which can be propagated over kilobase distances, forming γH2AX DNA damage foci[12]. These foci serve as beacons to recruit downstream repair and effector proteins to sites of damage, leading to activation of the G1 cell cycle checkpoint kinase Chk1 and cell cycle arrest in a p53-dependent manner[13]. HR requires a template sister chromatid for accurate alignment and annealing of DSBs and thus occurs during the G2 and S phases of the cell cycle. Phosphorylation of H2A.X during HR requires the PI3K ataxia telangiectasia related (ATR) and subsequent recruitment of key proteins such as BRCA1 and BRCA2 to the replication fork for DNA template repair. Mutations in BRCA1/2 (breast cancer gene 1/2) have been linked to familial breast cancer syndromes[14,15].
Nucleotide excision repair
Nucleotide excision repair (NER) is the most versatile repair pathway with the ability to repair damaged SSBs and other lesions caused by UV, chemical-induced DNA adducts, crosslinks, and oxidized bases[16]. Two NER pathways have been identified in eukaryotes in response to distinct types of DNA lesions: the general genome NER (GG-NER) pathway responds to genome-wide DNA damage and requires the xeroderma pigmentosum (XP) complementation group C/Human Homolog of Rad23 B (XPC/HHR23B) complex[17]; and transcription-coupled NER (TC-NER), which exclusively repairs lesions identified on single-stranded DNA templates during transcription and requires recruitment of Cockayne Syndrome (CS) A and B (CSA and CSB) proteins to stalled RNA polymerase II at lesion sites[18-20]. Mutations in XPC have been associated with photosensitivity and predisposition to cancers in patients with XP[21], while patients harboring mutations in CSA and CSB present with developmental growth delays and deformities, thought to be due to dysregulated transcription[22].
Base excision repair
Base excision repair (BER) responds to damage caused by byproducts of cellular metabolism including the formation of reactive oxygen species (ROS), methylation, deamination, and hydroxylation[23]. This pathway recognizes and repairs abasic sites, SSBs, and damaged nucleotide bases[24]. There have been no known human disorders associated with BER deficiency to date, suggesting that there are likely redundancies in the BER pathway that allow for safeguards against genome instability. However, loss of function of core BER pathway proteins in mice is embryonic lethal, highlighting the importance of this pathway[5]. Certain polymorphisms in the BER pathway scaffold protein XRCC1 may appear to be associated with lung and other cancers, pointing to the possible involvement of this pathway in regulating oncogenesis[25].
Mismatch repair
Errors in base-pairing or small in-frame deletions (indels) made by DNA polymerase are identified and corrected by the mismatch repair (MMR) system to ensure faithful replication of the genetic code[26]. Mismatches or indels are repaired by a heterodimeric complex requiring the proliferating cell nuclear antigen ring protein[27], which serves as a sliding clamp along a coding strand template of the DNA replication fork to recruit the MMR assembly composed of MSH2, MSH3 and/or MSH6 (MutS Homolog 2/3/6), MLH1 (MutL Homolog 1), EXO1 (Exonuclease 1), and DNA polymerases[28]. Defects in MMR lead to microsatellite instability, as seen in hereditary nonpolyposis colorectal cancer (HNPCC) and other sporadic cancers[29].
Interstrand crosslink repair
The formation of interstrand crosslinks (ICLs) between two strands of DNA causes roadblocks for both the transcription and replication machinery leading to transcriptional and/or replication stress. Due to the complex structure of ICLs, several steps and a multitude of enzymes and protein complexes are required to remove these lesions from damaged DNA and enable repair. Core to this repair machinery is the members of the Fanconi anemia (FA) pathway, 13 of which are mutated in FA syndrome[30,31]. Central to the recognition and repair of ICLs is the ID (FANCI-FANCD2) complex, which accumulates at sites of ICLs and recruits FA enzymes including the FAN1 endo- and exo-nuclease, translesional synthesis polymerases REV1 and Polζ, and FA proteins BRCA2/FANCD1, PALB2/FANCN, and BACH1/FANCJ in an ATR-dependent manner to facilitate HR repair of excised ends[10]. FA patients with mutations in FA proteins present with increased cancer risks and premature bone marrow failure[32].
THE ROLE OF PHYSIOLOGICAL DNA DAMAGE IN BIOLOGICAL PROCESSES
V(D)J recombination, class-switch recombination, and somatic hyper-mutation
Certain biological processes depend on programmed DNA damage. Immature T lymphocytes rely on DSB formation at exons encoding the variable regions used for antigen recognition and binding on T cell receptors (TCR) during V(D)J recombination. Recognition of cleavage sequences flanking these V, D, and J exons by the RAG1-RAG2 complex leads to the formation of blunt-ended DSBs, which are recombined and repaired using NHEJ[33,34]. These recombination events increase the T cell repertoire and TCR diversity, ensuring healthy and active immune surveillance. Subsequently, defects in NHEJ cause severe combined immunodeficiency syndrome (SCID)[35]. Immature B cells have a rearranged immunoglobulin (Ig) heavy-chain variable domain that is initially fused to an Igμ constant region. During antigen-stimulated B-cell differentiation, class-switch recombination (CSR) juxtaposes a V region to any constant region to encode distinct antigenic memory on the resultant differentiated Ig. Antigenic stimulation of B cells also activates somatic hypermutation to increase mutation rates in the heavy- and light-chain variable regions to expand the repertoire of variable segments to allow for the selection of B cells that express Ig molecules with high antigen specificity. CSR and somatic hypermutation require activation-induced deaminase, which targets the variable-region exons and IgH switch regions, causing deamination of cytosine residues to uracil and the formation of U:G mismatches, triggering repair through the MMR or BER pathways to form SSBs. It is thought that SSBs formed during CSR are converted to DSBs and repaired using NHEJ to ligate the variable region to a constant region, while SSBs formed during somatic hyper-mutation are repaired in an error-prone manner to yield mutations within the variable regions[34].
Meiotic recombination
The exchange of genetic information during meiosis is crucial to allow for genetic diversity and gamete formation. Chromosomal alignment and exchange of genetic information between homologous chromosomes during phase I of meiosis requires the formation of DSBs generated by Spo11[36]. The Mre11-Rad50-Nbs1 (MRN) complex is recruited to sites of damage and resects the DSBs to form ssDNA, which can be repaired by HR using the homologous chromosome and requires the meiosis-specific RAD51-like protein DMC1[37]. Spo11 or Dmc1 knockout mice are viable but infertile[38].
Telomere maintenance
The ends of mammalian chromosomes are organized into telomeres - stretches of short tandem DNA repeats terminating in a 3’ ssDNA overhang that is sequestered in a Shelterin complex which prevents access by the NHEJ or HR machinery[39-41]. However, during the G2 phase of the cell cycle, exposed telomeres can be recognized by the MRN complex and ATM to possibly encourage a localized DDR for telomere end-processing and shelterin complex formation[42]. Along these lines, loss of function of several proteins involved in the DDR causes telomere shortening and dysfunction that can trigger chromosomal fusions and chromosomal instability[41,43].
Human cells normally do not express enough telomerase to withstand naturally occurring telomere shortening due to the inability of the DNA replication machinery to fully replicate the ends of chromosomes, leading to open terminal ends of chromosomes that are perceived to be DSBs and are subjected to repeated cycles of breakage and repair[44]. These cycles can lead to apoptosis or senescence, and are also believed to contribute to the aging process and other age-related pathological processes such as atherosclerosis and arthritis[45].
HUMAN DISEASES CAUSED BY ERRORS IN DNA DAMAGE REPAIR
Cancer syndromes
Cancer is considered to be a disease of genome instability and mutation caused by errors in DNA damage repair[5,46,47]. Cytogenetics analysis of cancer cells often reveals chromosomal instability, copy number alterations, and expansions or contractions of repetitive microsatellite sequences[46]. Evidence that errors in DNA damage maintenance are linked to cancer development lies in the fact that mutations in several DDR genes are linked to several cancer syndromes [Table 1].
Point mutations in the HR pathway genes BRCA1 and BRCA2 predispose women to hereditary breast and ovarian cancer (HBOC) syndrome[14,48]. HBOC accounts for up to 10% of breast cancer cases - individuals that are heterozygous carriers of BRCA1/2 mutations have a 40-80% lifetime risk of developing breast cancer[49] while male patients who harbor BRCA1/2 mutations have an increased risk of developing breast, pancreas, and prostate cancers[50-52]. Familial mutations in HR pathway genes BACH1, PALB2, and RAD51C have also been identified in 3% of familial breast cancer patients and confer a 2-fold increased risk of developing breast cancer[53], while mutations in key genes involved in DSB repair including CHK2, ATM, NBS1, and RAD50 have been shown to also confer a 2-fold increased risk of breast cancer[54].
Heterozygous mutations in MMR pathway genes MLH1, MSH2, MSH6, and PMS2 have been linked to HNPCC, or Lynch Syndrome[55]. Patients present primarily with colorectal cancer but also have an increased predisposition to developing endometrial, ovarian, gastric, and renal carcinomas.
FA is a genetic syndrome characterized by mutations in 13 FA genes (FANCA to FANCV) involved in the ICL repair pathway. Monoallelic inactivation of FANCD1(also known as BRCA2) leads to adult HBOC cancer predisposition syndrome with an increased lifetime risk of breast cancer by 50% and a 15% increased lifetime risk of ovarian cancer (reviewed in Nalepa & Clapp)[56]. A review of The Cancer Genome Atlas has identified acquired mutations, epigenetic silencing, and copy number variations in FA genes have been identified in patients with a variety of malignancies[57].
Environmental sources of DNA damage and tumorigenesis
Benzo(α)pyrene in cigarette smoke has been shown to react with the 2-amino position of guanine residues, forming a guanine adduct, which after one cycle of DNA replication, leads to the incorporation of an adenine base instead of cytosine and subsequent guanine to threonine tranversion[58]. This transversion causes a mutational profile in the tumor suppressor gene TP53, which strongly correlates with lung cancer in smokers vs. non-smokers[59]. Environmental sources of IR or X-rays produced by cosmic radiation, soil, rocks, radioactive materials, and medical devices can lead to direct DNA damage or indirect DNA damage by radiolysis of water molecules to form reactive hydroxyl radicals[8]. IR can lead to base damage, DSBs, and SSBs, all of which require prompt repair via the BER and HR pathways[60]. Repair of UV sunlight-induced DNA crosslinks requires the XP family member of proteins involved in the BER pathway[61]. Hereditary mutations in these genes lead to the familial syndrome with their namesake XP, in which patients present with premature skin photoaging and an increased incidence of skin cancer[62]. Lifetime ingestion of carcinogens including polycyclic aromatic hydrocarbons, nitrosamines in cured foods, aldehydes in alcohol, and arsenic in contaminated drinking water, has been implicated in the development of colorectal cancers[63].
Neurological diseases
Until recently, it has been thought that post-mitotic neurons rely solely on error-prone NHEJ for DNA repair, but recent reports now suggest that neurons are able to use a transcription-triggered, replication-independent recombination repair pathway that requires the HR repair protein RAD52 for the repair of DSBs[64,65]. As neurons are obligative aerobes, they rely heavily on oxidative phosphorylation via mitochondrial respiration, which leads to the production of ROS that can cause DNA damage[43]. Defects in DDR repair in neurons lead to neurodegenerative syndromes that primarily affect the oxygen-dependent cells of the cerebellum[66]. Patients present with ataxia (loss of motor coordination), oculomotor apraxia (disco-ordinated eye movements), and dysarthria (speech deficits). Perhaps the most extensively studied central nervous system disease caused by a DDR defect is Ataxia-Telangiectasia (A-T), caused by mutations in ATM[67]. A-T patients have progressive loss of Purkinje and granule cells in their cerebellum, leading to rapid loss of motor coordination with early onset in their childhood years[67]. Similarly, patients with A-T-like Syndrome (A-TLD) have mutations in MRE11, a member of the MRN complex which senses the presence of DSBs and recruits ATM to amplify the DDR, and present with motor deficits similar to patients with A-T, albeit with a delayed onset and slower progression[10].
Patients with DDR syndromes often present with an abnormally small head circumference (microcephaly), thought to be due to defective proliferation of neuroprogenitor cells during fetal development[68]. A hypomorphic mutation in the ATR gene is found in patients with Seckel Syndrome (SS) - these patients present with a constellation of findings including dwarfism and a “bird-like” facies. Loss of function of ATR in cells leads to a decreased ability to repair replication stress-induced DNA damage through HR[68]. Patients with defects in HR, such as Bloom Syndrome (BS) patients (who have mutations in the RECQ helicase BLM), FA patients, and patients with XFE Progeroid Syndrome (who have XPF mutations), also present with microcephaly[68]. Other microcephalic syndromes that are associated with defects in the ATR DDR pathway include Miller-Dieker Lissencephaly and Williams-Beuren Syndrome[69]. Patients with Nijmegen Breakage Syndrome (NBS) and Nijmegen Breakage Syndrome-Like Disorder (NBSLD) have hypomorphic mutations in NBS1 and RAD50, respectively, members of the aforementioned MRN complex that are involved in DSB repair[70,71]. Interestingly, NBS and NBSLD patients do not present with the same findings as patients with A-T despite having mutations in genes that are involved in the ATM DDR pathway; however, NBS mutant neurons exhibit ATM-dependent apoptosis, suggesting that a robust ATM response may decrease the severity of neurodegeneration and microcephaly in these patients[72]. Finally, pregnant survivors of the Hiroshima and Nagasaki atomic bombs were found to deliver fetuses with microcephaly and smaller brain volume, emphasizing the deleterious effects of in utero exposure to large doses of IR[73].
The necessity of non-dividing, post-mitotic neurons to rely on SSB repair (SSBR) machinery has led to the identification of several neurological disorders associated with mutations in members of the SSBR pathway[74]. A histidine to arginine mutation at residue 493 (H493R) in tyrosyl-DNA phosphodiesterase, the enzyme that hydrolyzes the tyrosyl-3’ phosphate linkage in order to resolve stalled topoisomerase 1-DNA complexes during SSBR, was identified in patients with hereditary Spinocerebellar Ataxia Axonal Neuropathy type 1 (SCAN1)[75]. SCAN1 patients present with progressive childhood-onset of cerebellar ataxia followed by areflexia and peripheral neuropathy[76]. Similarly, germline mutations in the SSBR pathway protein APTX have been linked to individuals with Ataxia Ocular Motor Apraxia type 1 (AOA1) , a progressive neurological disorder that resembles other types of cerebellar ataxia including A-T[77,78]. APTX nuclease cleaves AMP from the 5’-terminal of ssDNA breaks, thus allowing for ligation of the 5’-phosphate terminus[79]. Mutant APTX in terminally differentiated cells such as neurons may lead to the accumulation of ssDNA breaks, creating transcriptional blocks and increased transcriptional stress, triggering apoptosis. Indeed, fibroblasts from AOA1 patients are more sensitive to oxidative DNA damage and there is more oxidative DNA damage in the cerebellum of AOA1 patients[80]. Finally, biallelic mutations in exon 12 of the SSBR scaffold protein XRCC1 (K431N) lead to axonal neuropathy and cerebellar ataxia in patients with AOA type 5[81]. Taken together, these findings support the role of dysfunctional DDR in the etiopathogenesis of neurologic diseases. A summary of these neurological diseases is provided in Table 1.
Patients with missense mutations in DNA ligase IV (LIG IV) can present with microcephaly, growth retardation, pancytopenia, a predisposition to lymphomas, combined immunodeficiency, and hypersensitivity to chemotherapies[82]. This is similar to patients with mutations in XLF, who present with milder symptoms of radiosensitivity, pancytopenia, and impaired survival[83]. Loss of function of XRCC4 has been reported in a patient with microcephaly and cerebellar ataxia with an intact immune system, suggesting that this protein is not essential for V(D)J recombination[84].
Aging
Aging is a complex process thought to consist of a gradual decline in mitochondrial function, metabolic dysregulation, the loss of stem cell function, and the accumulation of damaged macromolecules within cells[85]. Evidence that the accumulation of DNA damage can lead to premature aging lies in several premature aging syndromes that are linked to defects in DDR pathways, including: progeroid syndromes like Cockayne Syndrome (CS), Trichothiodystrophy (TTD), Cerebro-Oculo-Facio-Skeletal Syndrome (COFS), Dyskeratosis Congenital (DKC); Hutchinson-Gilford Progeroid Syndrome (HGPS); Werner Syndrome (WS); Xeroderma Pigmentosum (XP); and Rothmund-Thomson Syndrome (RTS) [Table 1][1].
CS patients have mutations in CSA (also known as ERCC8) or CSB (also known as ERCC6) and present with early developmental growth retardation, progressive vision loss, sensorineural deafness, early-onset neurodegeneration, and an average life span of 12 years of age[22]. Approximately 70% of CS patients have a mutation in CSA and the remaining 30% have a mutation in CSB[86,87]. There are approximately 78 known CSB mutations and 30 CSA mutations, with patients harboring CSA mutations presenting with a more moderate form of CS[87]. Interestingly, despite defects in TC-NER being linked to mutations in CS genes, patients with CS do not present with UV hypersensitivity, nor have there been any reported cases of cancer in patients with CS, suggesting that repair defects in DNA damage alone are not sufficient to cause cancer but can promote premature aging[1]. This is in contradistinction to XP patients who also harbor defects in TC-NER due to mutations in XP genes, present with increased incidences of skin cancer, and can present with early-onset CS, combined XP/CS, and segmental progeria with features of dwarfism, cachexia, and microcephaly[88].
TTD patients have point mutations in the DNA helicases XPB and XPD, components of the repair and transcription factor IIH (TFIIH), causing them to suffer from progeria with CS features in addition to brittle hair, nails, and ichthyotic skin[88,89]. Mutant NER transgenic mice display signs of progeria, including accelerated aging, bony deformity, neurodegeneration, hearing loss, growth retardation, infertility, cachexia, stem cell depletion, and frailty, further emphasizing the role that NER plays in the aging process[90-92].
Defective GG-NER in XP patients caused by mutations in the NER repair enzymes XPA through to XPG leads to increased DNA damage across the entire genome[93]. Consequently, XP patients with GG-NER suffer from UV-induced skin hyperpigmentation with a 2000x increased risk of skin cancer as well as the development of tumors elsewhere in the body[93]. XP patients who also have defects in TC-NER present with rapid neurodegeneration[2,94].
Patients with mutations in the RECQ family of DNA helicases BLM, RECQL4, and WRN present with progeroid syndromes[95]. The BLM helicase suppresses recombination and maintains genome stability[96,97], RECQL4 participates in DNA replication and repair[98], and the WRN helicase manages replication stress and telomere stability[99]. Mutations in BLM cause BS, in which patients present with a shortened lifespan and early presentation of age-related diseases, including diabetes, chronic obstructive pulmonary disease, and cancer[100]. RECQL4 mutations lead to RTS, presenting with juvenile cataracts, epidermal atrophy, and increased cancer risk[101]. WRN mutations cause WS with premature onset of age-related diseases, growth retardation, and lipodystrophy[102].
Immunodeficiency
Healthy immune surveillance is required to defend the body from foreign pathogens and antigens. Then, lymphoid B and T cells must be able to generate a diverse repertoire of B and T cell receptors (BCR and TCR, respectively). This relies on V(D)J recombination and CSR. To perform V(D)J recombination, cells require the recombinases RAG1 and RAG2[103]. These programmed DSBs are repaired in G0/G1 by NHEJ, requiring the damage sensing PI3K DNA dependent protein kinase to phosphorylate the exonuclease Artemis to process the breaks[104]. Repair by NHEJ requires ATM, the MRN complex, 53BP1 and RNF168[105]. These DSB repair proteins share overlapping functions with the DNA repair protein XLF[105]. End ligation is performed by the DNA LIG IV/XRCC4-XLF complex[106]. B cells undergo CSR to generate multiple Ig subsets and require activation-induced cytidine deaminase (AID) to deaminate cytidine to uracil at the transcriptionally active switch region, which is then modified by UNG (uracil-DNA glycosylase) to allow for BER[107].
Mutations in RAG1 or RAG2 lead to defective V(D)J recombination in T and B cells, resulting in severe SCID[108,109]. RAG1/2 deficient SCID patients are not radiosensitive and do not present with developmental delay but require stem cell transplantation in order to reconstitute their immune systems. In contrast, patients with mutations in the gene DCLRE1C that encodes for Artemis present with radiosensitive SCID and are predisposed to lymphoma[110].
Somatic mutations in AICDA and UNG affect CSR, resulting in hyper IgM gammopathy[111-113]. AID and UNG patients have lymph node hyperplasia due to the presence of giant germinal centers. AID patients may present with signs and symptoms of autoimmunity, inflammation, diabetes, arthritis, hemolytic anemia, immune thrombocytopenia, and chronic uveitis[112]. Defects in the chromatin remodeling and DDR complex INO80 affect CSR with reports of patients presenting with normal IgM but decreased IgG and IgA levels[114]. Constitutional MMR deficiency syndrome (CMMRD) presents with a partial Ig-CSR defect in which patients can present with brain tumors, hematological malignancies, and other solid organ cancers[115]. Somatic hypermutations in the MMR pathway genes PMS2, MSH6, MSH2, or MLH1 have been detected in CMMRD patients[115]. Mutations in RNF168, which is responsible for ubiquitylation of 53BP1 and downstream recruitment of BRCA1 to DSBs[105], lead to defective DSB repair and Radiosensitivity, Immunodeficiency, Dysmorphic features, and Learning difficulties (RIDDLE) Syndrome[116]. Patients with RIDDLE syndrome have decreased IgG and IgA with clinical features of A-T[116]. Finally, patients with mutations in NBS1, a member of the MRN DSB repair sensor complex, present with NBS characterized by microcephaly, growth retardation, cognitive delay, immunodeficiency with a decreased complement of T and B cells, and increased cancer risk[117]. In summary, this diverse group of immunodeficiencies linked to mutations in DDR genes hits home the importance of preprogrammed DNA damage and repair for immune system maintenance (further outlined in Table 1).
USING ANIMAL MODELS TO STUDY HUMAN DISEASES OF DNA DAMAGE
To study the phenotype of deficient DNA repair, mouse models have been used extensively to model the symptoms found in many human diseases of DNA damage repair[118]. These murine models serve as invaluable tools to study the phenotypic effects of defects in DNA damage repair pathways at the cellular and whole organism levels; however, many of these models do not completely recapitulate the complexity of signs and symptoms that are seen in humans, and therefore, one must be aware of these limitations when conducting studies using these animal models. Listed below are several known genetically-engineered mouse models of human diseases of DNA damage:
(1) A-T mice faithfully recapitulate tissue radiosensitivity and immunodeficiency seen in patients with A-T; however, earlier mouse models lacked the typical neurological symptoms and cerebellar dysfunction seen in humans. A-T mice faithfully recapitulate tissue radiosensitivity...cerebellar dysfunction seen in humans[119-124]. A more recent report using the insertion recent report using the insertion of null mutations in both the ATM and APTX genes in a mouse model of A-T, successfully generated mice that developed severe ataxic symptoms and atrophy of the cerebellar molecular layer[125].
(2) Mouse models of HR deficiency require mutations in both alleles of Brca1 or Brca2 in order for mice to develop breast tumors while heterozygous deletions of either BRCA1 or BRCA2 are sufficient to drive tumorigenesis in humans[126-129].
(3) Mouse models of MMR deficiency are also tumor prone; however, unlike the hereditary colon cancer syndrome in human patients (Lynch syndrome), mice with mutations in MMR genes such as Mlh1, Msh6, and Msh2 form tumors in the lymphatic compartment and not in the gut[130,131].
(4) Early mouse models of BS demonstrated that homozygous mutation of the murine BS gene Blm caused developmental delay and embryonic lethality[132].
(5) Several mouse models have attempted to model the complexity of FA, caused by mutations in more than a dozen genes involved in DNA interstrand crosslink repair. However, knockout mice with single-gene knockouts such as FancA-/-mice do not spontaneously display congenital anomalies nor have hematological abnormalities seen in FA patients[133]; however, FancA-/-and FancG-/-mice develop microcephaly due to increased neuronal apoptosis and can be interpreted as accelerated aging of stem cells in FA. Similarly, FancC-/-mice do not develop skeletal abnormalities or hematological abnormalities, but they do have impaired fertility, similar to that seen in FA patients, as well as hypersensitivity to DNA crosslinking agents[134,135].
(6) Mouse models of the progeroid diseases, Hutchinson-Gilford Progeria (HGP) and WS, do not have a marked progeroid phenotype, unlike the premature aging seen in patients. This has been postulated because mice have longer telomere lengths and may lead to relative resistance to the accelerated aging seen in WS patients with mutations in the WRN gene that encodes the BLM-related helicase[136]. Interestingly, mouse models of HGP harboring mutations in LmnA, a gene related to nuclear lamina and not to DNA repair pathways, also do not show signs of early aging, but HGP cells accumulate high levels of DNA damage[137,138].
(7) Seckel Syndrome (SS), a rare genetic condition caused by mutations in several genes, including genes involved in the ATR DNA damage repair pathway, namely ATR and ATRIP, presents with intrauterine growth restriction, dwarfism, microcephaly, and intellectual disability[139]. The mouse model harboring hypomorphic mutations on ATR recapitulates faithfully many of the features of SS patients and mice accumulate high amounts of DNA damage and genome instability in utero, leading to accelerated aging at birth[140].
(8) Mouse models of XP, defective in NER genes (XPA, XPB, XPC, and others), present with hypersensitivity to UV light and a high cancer incidence, similar to that seen in XP patients[141]. Interestingly, mouse models of CS, which are caused by mutations in the NER genes ERCC6 and ERCC8, also present with accelerated aging and a predisposition to cancer[142].
(9) Mouse models of NBS have hypomorphic mutations in Nbn1, and null mutations of Nbn are embryonically lethal[143]. Mice harboring hypomorphic mutations in Nbn are viable, but they do not demonstrate any neurological phenotypes as that seen in the human NBS patients[144]. The cerebellum of Nbn mice has agenesis and decreased proliferation of neuronal progenitor cells accompanied by marked death of cerebellar neurons[145]. Specific deletion of Nbn in the mouse CNS (NbnCNS-del) leads to growth retardation and early onset of cataracts[146], as well as microcephaly, decreased white matter integrity, retina and astrocyte functionality, recapitulating the microcephaly seen in human patients with NBS[147-149].
DIFFERENCES IN THE DNA DAMAGE AND REPAIR RESPONSE BETWEEN HUMAN AND MOUSE NEURONS AND ASTROCYTES
It is interesting that many mouse models of DNA damage repair do not faithfully recapitulate the neurological symptoms seen in their human counterparts, suggesting that there are inherent differences in DNA damage repair between mouse and human neurons. Indeed, a recent study comparing mouse and human neurons identified distinct differences in their response to different forms of DNA damage[150]. Human and mouse neurons treated with identical doses of the DNA damaging agent camptothecin (CPT) for the same period of time demonstrated markedly different alterations in the nuclei of degenerating neurons. CPT-treated differentiated human neurons showed homogeneous and uniform chromatin condensation within the nucleus but did not form discrete round chromatin clumps or crescentic marginations at the nuclear envelope nor form discrete round chromatin clumps at the nuclear envelope as would be seen during neuronal apoptosis; instead, they demonstrated signs of pyknosis. In contrast, mouse neurons demonstrated signs of apoptosis which was demonstrated consistently regardless of where neurons were harvested in the mouse brain. Mouse neurons underwent apoptosis through activation of caspase-3, while human neurons only had mild activation of caspase-3 but instead showed robust activation of caspase-6. Interrogation of the mitochondria cell death pathway also demonstrated different response rates between human and mouse neurons. Mouse neurons had early activation of Bax while Noxa levels remained at baseline, and Puma levels also had an early spike in response to CPT. In contrast, human neurons had a much later spike of Bax levels, while Noxa and Puma levels exhibited a significant and progressive increase over the course of many hours throughout the treatment course[150]. Finally, an investigation of the individual components of the DDR pathway in mouse and human neurons during CPT treatment demonstrated that mouse neurons did not mount a robust activation of the MRN complex, which senses DSBs caused by CPT, while human neurons mounted a rapid activation of MRN. Human neurons showed sustained activation of ATR, p53 and p73, while mouse neurons showed rapid suppression of ATR and had sustained activation of p53 and p73[150].
Another study identified differences in the response of mouse and human astrocytes to oxidative stress, hypoxia, inflammatory cytokine treatment, and simulated viral infections[151]. This study helps shed light on why many mouse models of neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease often have milder phenotypes compared to human patients, possibly owing to the inherent increased resilience of mouse astrocytes to the effects of oxidative stress. The authors also showed that mouse astrocytes activated a pro-growth program in response to hypoxia but not in human astrocytes, possibly explaining the greater functional recovery observed in mouse models of ischemic stroke compared to human stroke patients. Taken together, these studies demonstrate differences in the ways that different populations of mouse and human brain cells process DNA damage and other forms of cellular stress and insult, and further address the potential limitations in interpreting studies performed using animal models of DNA damage and neurological diseases.
CONCLUSION
In summary, the ability of human cells to respond effectively to daily endogenous and exogenous sources of DNA damage ensures longevity and health. As we continue to uncover a further mechanistic understanding of these inherited syndromes, the hope is to be able to discover targeted therapies that will effectively reverse the devastating effects of these diseases and improve the quality of life and survival for these patients.
DECLARATIONS
Authors’ contributionsThe author contributed solely to the article.
Availability of data and materialsNot applicable.
Financial support and sponsorshipNone.
Conflicts of interestThe author declared that there are no conflicts of interest.
Ethical approval and consent to participateNot applicable
Consent for publicationNot applicable.
Copyright© The Author(s) 2022.
REFERENCES
1. Tiwari V, Wilson DM 3rd. DNA damage and associated DNA repair defects in disease and premature aging. Am J Hum Genet 2019;105:237-57.
3. Bont R, van Larebeke N. Endogenous DNA damage in humans: a review of quantitative data. Mutagenesis 2004;19:169-85.
8. Chatterjee N, Walker GC. Mechanisms of DNA damage, repair, and mutagenesis. Environ Mol Mutagen 2017;58:235-63.
9. Pilié PG, Tang C, Mills GB, Yap TA. State-of-the-art strategies for targeting the DNA damage response in cancer. Nat Rev Clin Oncol 2019;16:81-104.
10. Ciccia A, Elledge SJ. The DNA damage response: making it safe to play with knives. Mol Cell 2010;40:179-204.
11. Guirouilh-Barbat J, Lambert S, Bertrand P, Lopez BS. Is homologous recombination really an error-free process? Front Genet 2014;5:175.
12. Paull TT, Rogakou EP, Yamazaki V, Kirchgessner CU, Gellert M, Bonner WM. A critical role for histone H2AX in recruitment of repair factors to nuclear foci after DNA damage. Current Biol 2000;10:886-95.
13. Lanz MC, Dibitetto D, Smolka MB. DNA damage kinase signaling: checkpoint and repair at 30 years. EMBO J 2019;38:e101801.
14. Futreal PA, Liu Q, Shattuck-Eidens D, et al. BRCA1 mutations in primary breast and ovarian carcinomas. Science 1994;266:120-2.
15. Powell SN, Kachnic LA. Roles of BRCA1 and BRCA2 in homologous recombination, DNA replication fidelity and the cellular response to ionizing radiation. Oncogene 2003;22:5784-91.
16. Garfinkel DJ, Bailis AM. Nucleotide excision repair, genome stability, and human disease: new insight from model systems. J Biomed Biotechnol 2002;2:55-60.
17. Sugasawa K, Ng JM, Masutani C, et al. Xeroderma pigmentosum group C protein complex is the initiator of global genome nucleotide excision repair. Mol Cell 1998;2:223-32.
18. Henning KA, Li L, Iyer N, et al. The cockayne syndrome group a gene encodes a WD repeat protein that interacts with CSB protein and a subunit of RNA polymerase II TFIIH. Cell 1995;82:555-64.
19. Mellon I. Selective removal of transcription-blocking DNA damage from the transcribed strand of the mammalian DHFR gene. Cell 1987;51:241-9.
20. van Gool AJ, Citterio E, Rademakers S, et al. The Cockayne syndrome B protein, involved in transcription-coupled DNA repair, resides in an RNA polymerase II-containing complex. EMBO J 1997;16:5955-65.
21. Kraemer KH, Lee MM, Scotto J. DNA repair protects against cutaneous and internal neoplasia: evidence from xeroderma pigmentosum. Carcinogenesis 1984;5:511-4.
22. Wilson BT, Stark Z, Sutton RE, et al. The Cockayne Syndrome Natural History (CoSyNH) study: clinical findings in 102 individuals and recommendations for care. Genet Med 2016;18:483-93.
24. Mol CD, Parikh SS, Putnam CD, Lo TP, Tainer JA. DNA repair mechanisms for the recognition and removal of damaged DNA bases. Annu Rev Biophys Biomol Struct 1999;28:101-28.
25. Divine KK, Gilliland FD, Crowell RE, et al. The XRCC1 399 glutamine allele is a risk factor for adenocarcinoma of the lung. Mutat Res/DNA Rep 2001;461:273-8.
27. Umar A, Buermeyer AB, Simon JA, et al. Requirement for PCNA in DNA mismatch repair at a step preceding DNA resynthesis. Cell 1996;87:65-73.
29. Jiricny J, Nyström-lahti M. Mismatch repair defects in cancer. Curr Opin Genet Dev 2000;10:157-61.
30. Knipscheer P, Räschle M, Smogorzewska A, et al. The Fanconi anemia pathway promotes replication-dependent DNA interstrand cross-link repair. Science 2009;326:1698-701.
31. Moldovan GL, D’Andrea AD. How the fanconi anemia pathway guards the genome. Annu Rev Genet 2009;43:223-49.
32. Ceccaldi R, Sarangi P, D’Andrea AD. The Fanconi anaemia pathway: new players and new functions. Nat Rev Mol Cell Biol 2016;17:337-49.
33. Chen HT, Bhandoola A, Difilippantonio MJ, et al. Response to RAG-mediated VDJ cleavage by NBS1 and gamma-H2AX. Science 2000;290:1962-5.
34. Bassing CH, Alt FW. The cellular response to general and programmed DNA double strand breaks. DNA Repair (Amst) 2004;3:781-96.
35. Moshous D, Callebaut I, de Chasseval R, et al. Artemis, a novel DNA Double-Strand Break Repair/V(D)J recombination protein, is mutated in human severe combined immune deficiency. Cell 2001;105:177-86.
36. Keeney S, Giroux CN, Kleckner N. Meiosis-Specific DNA double-strand breaks are catalyzed by spo11, a member of a widely conserved protein family. Cell 1997;88:375-84.
37. Neale MJ, Keeney S. Clarifying the mechanics of DNA strand exchange in meiotic recombination. Nature 2006;442:153-8.
38. Richardson C, Horikoshi N, Pandita TK. The role of the DNA double-strand break response network in meiosis. DNA Repair (Amst) 2004;3:1149-64.
40. Lange T. Shelterin: the protein complex that shapes and safeguards human telomeres. Genes Dev 2005;19:2100-10.
41. di Fagagna F, Teo SH, Jackson SP. Functional links between telomeres and proteins of the DNA-damage response. Genes Dev 2004;18:1781-99.
43. Jackson SP, Bartek J. The DNA-damage response in human biology and disease. Nature 2009;461:1071-8.
44. Longhese MP. DNA damage response at functional and dysfunctional telomeres. Genes Dev 2008;22:125-40.
45. Sedelnikova OA, Horikawa I, Zimonjic DB, Popescu NC, Bonner WM, Barrett JC. Senescing human cells and ageing mice accumulate DNA lesions with unrepairable double-strand breaks. Nat Cell Biol 2004;6:168-70.
46. Negrini S, Gorgoulis VG, Halazonetis TD. Genomic instability - an evolving hallmark of cancer. Nat Rev Mol Cell Biol 2010;11:220-8.
48. Chen S, Parmigiani G. Meta-analysis of BRCA1 and BRCA2 penetrance. J Clin Oncol 2007;25:1329-33.
49. Fackenthal JD, Olopade OI. Breast cancer risk associated with BRCA1 and BRCA2 in diverse populations. Nat Rev Cancer 2007;7:937-48.
50. Kote-Jarai Z, Leongamornlert D, Saunders E, et al. UKGPCS Collaborators. BRCA2 is a moderate penetrance gene contributing to young-onset prostate cancer: implications for genetic testing in prostate cancer patients. Br J Cancer 2011;105:1230-4.
51. Tai YC, Domchek S, Parmigiani G, Chen S. Breast cancer risk among male BRCA1 and BRCA2 mutation carriers. J Natl Cancer Inst 2007;99:1811-4.
52. Ibrahim M, Yadav S, Ogunleye F, Zakalik D. Male BRCA mutation carriers: clinical characteristics and cancer spectrum. BMC Cancer 2018;18:179.
53. Levy-Lahad E. Fanconi anemia and breast cancer susceptibility meet again. Nat Genet 2010;42:368-9.
55. Spry M, Scott T, Pierce H, D’Orazio JA. DNA repair pathways and hereditary cancer susceptibility syndromes. Front Biosci 2007;12:4191-207.
56. Nalepa G, Clapp DW. Fanconi anaemia and cancer: an intricate relationship. Nat Rev Cancer 2018;18:168-85.
57. Chan SH, Ni Y, Li S, et al. Spectrum of germline mutations within fanconi anemia–associated genes across populations of varying ancestry. JNCI Cancer Spectrum 2021;5:pkaa117.
58. Ruggeri B, DiRado M, Zhang SY, Bauer B, Goodrow T, Klein-Szanto AJ. Benzo[a]pyrene-induced murine skin tumors exhibit frequent and characteristic G to T mutations in the p53 gene. Proc Natl Acad Sci USA 1993;90:1013-7.
60. Lomax ME, Folkes LK, O’Neill P. Biological consequences of radiation-induced DNA damage: relevance to radiotherapy. Clin Oncol (R Coll Radiol) 2013;25:578-85.
61. D’Errico M, Parlanti E, Teson M, et al. New functions of XPC in the protection of human skin cells from oxidative damage. EMBO J 2006;25:4305-15.
62. Kraemer KH, DiGiovanna JJ. Forty years of research on xeroderma pigmentosum at the US National Institutes of Health. Photochem Photobiol 2015;91:452-9.
63. Carpenter DO, Bushkin-Bedient S. Exposure to chemicals and radiation during childhood and risk for cancer later in life. J Adolesc Health 2013;52:S21-9.
64. Rass U, Ahel I, West SC. Defective DNA repair and neurodegenerative disease. Cell 2007;130:991-1004.
65. Welty S, Teng Y, Liang Z, et al. RAD52 is required for RNA-templated recombination repair in post-mitotic neurons. J Biol Chem 2018;293:1353-62.
66. Madabhushi R, Pan L, Tsai LH. DNA damage and its links to neurodegeneration. Neuron 2014;83:266-82.
67. Biton S, Barzilai A, Shiloh Y. The neurological phenotype of ataxia-telangiectasia: solving a persistent puzzle. DNA Repair (Amst) 2008;7:1028-38.
68. O’Driscoll M, Jeggo PA. The role of the DNA damage response pathways in brain development and microcephaly: insight from human disorders. DNA Repair (Amst) 2008;7:1039-50.
69. Kerzendorfer C, O’Driscoll M. Human DNA damage response and repair deficiency syndromes: linking genomic instability and cell cycle checkpoint proficiency. DNA Repair (Amst) 2009;8:1139-52.
70. Katyal S, McKinnon PJ. DNA strand breaks, neurodegeneration and aging in the brain. Mech Ageing Dev 2008;129:483-91.
71. Waltes R, Kalb R, Gatei M, et al. Human RAD50 deficiency in a Nijmegen breakage syndrome-like disorder. Am J Hum Genet 2009;84:605-16.
72. Shull ER, Lee Y, Nakane H, et al. Differential DNA damage signaling accounts for distinct neural apoptotic responses in ATLD and NBS. Genes Dev 2009;23:171-80.
73. Otake M, Schull WJ. Radiation-related brain damage and growth retardation among the prenatally exposed atomic bomb survivors. Int J Radiat Biol 1998;74:159-71.
74. Reynolds JJ, Stewart GS. A single strand that links multiple neuropathologies in human disease. Brain 2013;136:14-27.
75. Takashima H, Boerkoel CF, John J, et al. Mutation of TDP1, encoding a topoisomerase I-dependent DNA damage repair enzyme, in spinocerebellar ataxia with axonal neuropathy. Nat Genet 2002;32:267-72.
76. El-Khamisy SF, Saifi GM, Weinfeld M, et al. Defective DNA single-strand break repair in spinocerebellar ataxia with axonal neuropathy-1. Nature 2005;434:108-13.
77. Moreira MC, Barbot C, Tachi N, et al. The gene mutated in ataxia-ocular apraxia 1 encodes the new HIT/Zn-finger protein aprataxin. Nat Genet 2001;29:189-93.
78. Inlora J, Sailani MR, Khodadadi H, et al. Identification of a novel mutation in the. APTX ;3:a002014.
79. Harris JL, Jakob B, Taucher-Scholz G, Dianov GL, Becherel OJ, Lavin MF. Aprataxin, poly-ADP ribose polymerase 1 (PARP-1) and apurinic endonuclease 1 (APE1) function together to protect the genome against oxidative damage. Hum Mol Genet 2009;18:4102-17.
80. Seidle HF, Bieganowski P, Brenner C. Disease-associated mutations inactivate AMP-lysine hydrolase activity of Aprataxin. J Biol Chem 2005;280:20927-31.
81. Hoch NC, Hanzlikova H, Rulten SL, et al. Care4Rare Canada Consortium. XRCC1 mutation is associated with PARP1 hyperactivation and cerebellar ataxia. Nature 2017;541:87-91.
83. Tilgner K, Neganova I, Singhapol C, et al. Brief report: a human induced pluripotent stem cell model of cernunnos deficiency reveals an important role for XLF in the survival of the primitive hematopoietic progenitors. Stem Cells 2013;31:2015-23.
84. Guo C, Nakazawa Y, Woodbine L, et al. XRCC4 deficiency in human subjects causes a marked neurological phenotype but no overt immunodeficiency. J Allergy Clin Immunol 2015;136:1007-17.
85. Kennedy BK, Berger SL, Brunet A, et al. Geroscience: linking aging to chronic disease. Cell 2014;159:709-13.
86. Natale V. A comprehensive description of the severity groups in Cockayne syndrome. Am J Med Genet A 2011;155A:1081-95.
87. Laugel V. Cockayne syndrome: the expanding clinical and mutational spectrum. Mech Ageing Dev 2013;134:161-70.
88. Andressoo JO, Hoeijmakers JH. Transcription-coupled repair and premature ageing. Mutat Res 2005;577:179-94.
89. Vermeulen W, Rademakers S, Jaspers NG, et al. A temperature-sensitive disorder in basal transcription and DNA repair in humans. Nat Genet 2001;27:299-303.
90. de Boer J, Andressoo JO, de Wit J, et al. Premature aging in mice deficient in DNA repair and transcription. Science 2002;296:1276-9.
91. Wijnhoven SW, Beems RB, Roodbergen M, et al. Accelerated aging pathology in ad libitum fed Xpd(TTD) mice is accompanied by features suggestive of caloric restriction. DNA Repair (Amst) 2005;4:1314-24.
92. van der Pluijm I, Garinis GA, Brandt RM, et al. Impaired genome maintenance suppresses the growth hormone - insulin-like growth factor 1 axis in mice with Cockayne syndrome. PLoS Biol 2007;5:e2.
93. Kraemer KH, Lee MM, Scotto J. Xeroderma pigmentosum. Cutaneous, ocular, and neurologic abnormalities in 830 published cases. Arch Dermatol 1987;123:241-50.
94. Brooks PJ. The 8,5’-cyclopurine-2’-deoxynucleosides: candidate neurodegenerative DNA lesions in xeroderma pigmentosum, and unique probes of transcription and nucleotide excision repair. DNA Repair (Amst) 2008;7:1168-79.
95. Sidorova JM, Monnat RJ. Human RECQ helicases: roles in cancer, aging, and inherited disease. Adv Genomics Genet 2014;5:19-33.
96. Root H, Larsen A, Komosa M, et al. FANCD2 limits BLM-dependent telomere instability in the alternative lengthening of telomeres pathway. Hum Mol Genet 2016;25:3255-68.
97. Wietmarschen N, Merzouk S, Halsema N, Spierings DCJ, Guryev V, Lansdorp PM. BLM helicase suppresses recombination at G-quadruplex motifs in transcribed genes. Nat Commun 2018;9:271.
98. Luong TT, Bernstein KA. Role and regulation of the RECQL4 family during genomic integrity maintenance. Genes (Basel) 2021;12:1919.
99. Liu FJ, Barchowsky A, Opresko PL. The Werner syndrome protein suppresses telomeric instability caused by chromium (VI) induced DNA replication stress. PLoS ONE 2010;5:e11152.
100. Hanada K, Hickson ID. Molecular genetics of RecQ helicase disorders. Cell Mol Life Sci 2007;64:2306-22.
101. Croteau DL, Singh DK, Hoh Ferrarelli L, Lu H, Bohr VA. RECQL4 in genomic instability and aging. Trends Genet 2012;28:624-31.
102. Kudlow BA, Kennedy BK, Monnat RJ Jr. Werner and Hutchinson-Gilford progeria syndromes: mechanistic basis of human progeroid diseases. Nat Rev Mol Cell Biol 2007;8:394-404.
103. Schatz DG, Ji Y. Recombination centres and the orchestration of V(D)J recombination. Nat Rev Immunol 2011;11:251-63.
104. Villartay JP, Fischer A, Durandy A. The mechanisms of immune diversification and their disorders. Nat Rev Immunol 2003;3:962-72.
105. Bohgaki M, Bohgaki T, El Ghamrasni S, et al. RNF168 ubiquitylates 53BP1 and controls its response to DNA double-strand breaks. Proc Natl Acad Sci USA 2013;110:20982-7.
106. Lieber MR. The mechanism of double-strand DNA break repair by the nonhomologous DNA end-joining pathway. Annu Rev Biochem 2010;79:181-211.
107. Keim C, Kazadi D, Rothschild G, Basu U. Regulation of AID, the B-cell genome mutator. Genes Dev 2013;27:1-17.
108. Villartay J, Poinsignon C, de Chasseval R, Buck D, Le Guyader G, Villey I. Human and animal models of V(D)J recombination deficiency. Curr Opin Immunol 2003;15:592-8.
110. Felgentreff K, Lee YN, Frugoni F, et al. Functional analysis of naturally occurring DCLRE1C mutations and correlation with the clinical phenotype of ARTEMIS deficiency. J Allergy Clin Immunol 2015;136:140-150.e7.
111. Durandy A, Taubenheim N, Peron S, Fischer A. Pathophysiology of B-cell intrinsic immunoglobulin class switch recombination deficiencies. Adv Immunol 2007;94:275-306.
113. Casellas R, Basu U, Yewdell WT, Chaudhuri J, Robbiani DF, Di Noia JM. Mutations, kataegis and translocations in B cells: understanding AID promiscuous activity. Nat Rev Immunol 2016;16:164-76.
114. Kracker S, Gardës P, Durandy A. Inherited defects of immunoglobulin class switch recombination. Adv Exp Med Biol 2010;685:166-74.
115. Wimmer K, Kratz CP, Vasen HF, et al. Diagnostic criteria for constitutional mismatch repair deficiency syndrome: suggestions of the European consortium ‘care for CMMRD’ (C4CMMRD). J Med Genet 2014;51:355-65.
116. Stewart GS, Panier S, Townsend K, et al. The RIDDLE syndrome protein mediates a ubiquitin-dependent signaling cascade at sites of DNA damage. Cell 2009;136:420-34.
117. Chrzanowska KH, Gregorek H, Dembowska-Bagińska B, Kalina MA, Digweed M. Nijmegen breakage syndrome (NBS). Orphanet J Rare Dis 2012;7:13.
118. Specks J, Nieto-soler M, Lopez-contreras AJ, Fernandez-capetillo O. Modeling the study of DNA damage responses in mice. Methods Mol Biol 2015;1267:413-37.
119. Elson A, Wang Y, Daugherty CJ, et al. Pleiotropic defects in ataxia-telangiectasia protein-deficient mice. Proc Natl Acad Sci USA 1996;93:13084-9.
120. Herzog KH, Chong MJ, Kapsetaki M, Morgan JI, McKinnon PJ. Requirement for Atm in ionizing radiation-induced cell death in the developing central nervous system. Science 1998;280:1089-91.
121. Xu Y, Baltimore D. Dual roles of ATM in the cellular response to radiation and in cell growth control. Genes Dev 1996;10:2401-10.
122. Spring K, Cross S, Li C, et al. Atm knock-in mice harboring an in-frame deletion corresponding to the human ATM 7636del9 common mutation exhibit a variant phenotype. Cancer Res 2001;61:4561-68.
123. Tal E, Alfo M, Zha S, et al. Inactive Atm abrogates DSB repair in mouse cerebellum more than does Atm loss, without causing a neurological phenotype. DNA Repair (Amst) 2018;72:10-7.
124. Lavin MF. The appropriateness of the mouse model for ataxia-telangiectasia: neurological defects but no neurodegeneration. DNA Repair (Amst) 2013;12:612-9.
125. Perez H, Abdallah MF, Chavira JI, et al. A novel, ataxic mouse model of ataxia telangiectasia caused by a clinically relevant nonsense mutation. Elife 2021;10:e64695.
126. Hakem R, de la Pompa JL, Sirard C, et al. The Tumor Suppressor Gene Brca1 Is Required for Embryonic Cellular Proliferation in the Mouse. Cell 1996;85:1009-23.
127. Liu X, Holstege H, van der Gulden H, et al. Somatic loss of BRCA1 and p53 in mice induces mammary tumors with features of human BRCA1-mutated basal-like breast cancer. Proc Natl Acad Sci USA 2007;104:12111-6.
128. Xu X, Wagner KU, Larson D, et al. Conditional mutation of Brca1 in mammary epithelial cells results in blunted ductal morphogenesis and tumour formation. Nat Genet 1999;22:37-43.
129. Drost R, Bouwman P, Rottenberg S, et al. BRCA1 RING function is essential for tumor suppression but dispensable for therapy resistance. Cancer Cell 2011;20:797-809.
130. Wind N, Dekker M, Berns A, Radman M, te Riele H. Inactivation of the mouse Msh2 gene results in mismatch repair deficiency, methylation tolerance, hyperrecombination, and predisposition to cancer. Cell 1995;82:321-30.
131. Wind N, Dekker M, van Rossum A, van der Valk M, Te Riele H. Mouse models for hereditary nonpolyposis colorectal cancer. Cancer Res 1998;58:248-255.
132. Chester N, Kuo F, Kozak C, O’Hara CD, Leder P. Stage-specific apoptosis, developmental delay, and embryonic lethality in mice homozygous for a targeted disruption in the murine Bloom’s syndrome gene. Genes Dev 1998;12:3382-93.
133. Cheng NC, van de Vrugt HJ, van der Valk MA, et al. Mice with a targeted disruption of the Fanconi anemia homolog Fanca. Hum Mol Genet 2000;9:1805-11.
134. Whitney MA, Royle G, Low MJ, et al. Germ cell defects and hematopoietic hypersensitivity to gamma-interferon in mice with a targeted disruption of the Fanconi anemia C gene. Blood 1996;88:49-58.
135. Carreau M, Gan OI, Liu L, Doedens M, McKerlie C, Dick JE. Bone marrow failure in the Fanconi anemia group C mouse model after DNA damage. Blood 1998;91:2737-44.
136. Chang S, Multani AS, Cabrera NG, et al. Essential role of limiting telomeres in the pathogenesis of Werner syndrome. Nat Genet 2004;36:877-82.
137. Eriksson M, Brown WT, Gordon LB, et al. Recurrent de novo point mutations in lamin A cause Hutchinson-Gilford progeria syndrome. Nature 2003;423:293-8.
138. Liu B, Wang J, Chan KM, et al. Genomic instability in laminopathy-based premature aging. Nat Med 2005;11:780-5.
139. Alderton GK, Joenje H, Varon R, Børglum AD, Jeggo PA, O’Driscoll M. Seckel syndrome exhibits cellular features demonstrating defects in the ATR-signalling pathway. Hum Mol Genet 2004;13:3127-38.
140. Murga M, Bunting S, Montaña MF, et al. A mouse model of ATR-Seckel shows embryonic replicative stress and accelerated aging. Nat Genet 2009;41:891-8.
141. Murai M, Enokido Y, Inamura N, et al. Early postnatal ataxia and abnormal cerebellar development in mice lacking Xeroderma pigmentosum Group A and Cockayne syndrome Group B DNA repair genes. Proc Natl Acad Sci USA 2001;98:13379-84.
142. Van der Horst GT, van Steeg H, Berg RJ, et al. Defective Transcription-Coupled Repair in Cockayne Syndrome B Mice Is Associated with Skin Cancer Predisposition. Cell 1997;89:425-35.
143. Zhu J, Petersen S, Tessarollo L, Nussenzweig A. Targeted disruption of the Nijmegen breakage syndrome gene NBS1 leads to early embryonic lethality in mice. Curr Biol 2001;11:105-9.
144. Kang J, Bronson RT, Xu Y. Targeted disruption of NBS1 reveals its roles in mouse development and DNA repair. EMBO J 2002;21:1447-55.
145. Frappart PO, Tong WM, Demuth I, et al. An essential function for NBS1 in the prevention of ataxia and cerebellar defects. Nat Med 2005;11:538-44.
146. Yang YG, Frappart PO, Frappart L, Wang ZQ, Tong WM. A novel function of DNA repair molecule Nbs1 in terminal differentiation of the lens fibre cells and cataractogenesis. DNA Repair (Amst) 2006;5:885-93.
147. Assaf Y, Galron R, Shapira I, et al. MRI evidence of white matter damage in a mouse model of Nijmegen breakage syndrome. Exp Neurol 2008;209:181-91.
148. Baranes K, Raz-Prag D, Nitzan A, et al. Conditional inactivation of the NBS1 gene in the mouse central nervous system leads to neurodegeneration and disorganization of the visual system. Exp Neurol 2009;218:24-32.
149. Galron R, Gruber R, Lifshitz V, et al. Astrocyte dysfunction associated with cerebellar attrition in a Nijmegen breakage syndrome animal model. J Mol Neurosci 2011;45:202-11.
150. Martin LJ, Chang Q. DNA damage response and repair, DNA methylation, and cell death in human neurons and experimental animal neurons are different. J Neuropathol Exp Neurol 2018;77:636-55.
151. Li J, Pan L, Pembroke WG, et al. Conservation and divergence of vulnerability and responses to stressors between human and mouse astrocytes. Nat Commun 2021;12:3958.
152. Gatti RA. Ataxia Telangiectasia. 600-606 (Lippincott Williams & Wilkins, 2003). Available from: https://www.derm.theclinics.com/article/S0733-8635(18)30100-1/pdf [Last accessed on 9 May 2022].
153. Stewart GS, Maser RS, Stankovic T, et al. The DNA double-strand break repair gene hMRE11 is mutated in individuals with an ataxia-telangiectasia-like disorder. Cell 1999;99:577-87.
154. Gennery AR, Cant AJ, Jeggo PA. Immunodeficiency associated with DNA repair defects. Clin Exp Immunol 2000;121:1-7.
155. Niraj J, Färkkilä A, D’Andrea AD. The Fanconi anemia pathway in cancer. Annu Rev Cancer Biol 2019;3:457-78.
156. Katsuki Y, Takata M. Defects in homologous recombination repair behind the human diseases: FA and HBOC. Endocr Relat Cancer 2016;23:T19-37.
157. Prakash R, Zhang Y, Feng W, Jasin M. Homologous recombination and human health: the roles of BRCA1, BRCA2, and associated proteins. Cold Spring Harb Perspect Biol 2015;7:a016600.
158. Fishel R, Lescoe MK, Rao M, et al. The human mutator gene homolog MSH2 and its association with hereditary nonpolyposis colon cancer. Cell 1993;75:1027-38.
159. Parsons R, Li G, Longley MJ, et al. Hypermutability and mismatch repair deficiency in RER+ tumor cells. Cell 1993;75:1227-36.
160. Ryan E, Sheahan K, Creavin B, Mohan HM, Winter DC. The current value of determining the mismatch repair status of colorectal cancer: A rationale for routine testing. Crit Rev Oncol Hematol 2017;116:38-57.
161. de Andrade KC, Frone MN, Wegman-Ostrosky T, et al. Variable population prevalence estimates of germline TP53 variants: a gnomAD-based analysis. Hum Mutat 2019;40:97-105.
162. Wiesmüller L, Ford JM, Schiestl RH. DNA damage, repair, and diseases. J Biomed Biotechnol 2002;2:45.
163. Al-Tassan N, Chmiel NH, Maynard J, et al. Inherited variants of MYH associated with somatic G:C-->T:A mutations in colorectal tumors. Nat Genet 2002;30:227-32.
164. Weren RD, Ligtenberg MJ, Kets CM, et al. A germline homozygous mutation in the base-excision repair gene NTHL1 causes adenomatous polyposis and colorectal cancer. Nat Genet 2015;47:668-71.
165. Grover S, Kastrinos F, Steyerberg EW, et al. Prevalence and phenotypes of APC and MUTYH mutations in patients with multiple colorectal adenomas. JAMA 2012;308:485-92.
166. Carney JP, Maser RS, Olivares H, et al. The hMre11/hRad50 protein complex and nijmegen breakage syndrome: linkage of double-strand break repair to the cellular DNA damage response. Cell 1998;93:477-86.
167. Matsuura S, Tauchi H, Nakamura A, et al. Positional cloning of the gene for Nijmegen breakage syndrome. Nat Genet 1998;19:179-81.
168. Oshima J, Sidorova JM, Monnat RJ Jr. Werner syndrome: clinical features, pathogenesis and potential therapeutic interventions. Ageing Res Rev 2017;33:105-14.
170. Lehmann J, Seebode C, Martens MC, Emmert S. Xeroderma pigmentosum - facts and perspectives. Anticancer Res 2018;38:1159-64.
171. Sharma R, Lewis S, Wlodarski MW. DNA repair syndromes and cancer: insights Into genetics and phenotype patterns. Front Pediatr 2020;8:570084.
173. Livingston JH, Crow YJ. Neurologic phenotypes associated with mutations in TREX1, RNASEH2A, RNASEH2B, RNASEH2C, SAMHD1, ADAR1, and IFIH1: aicardi-goutières syndrome and beyond. Neuropediatrics 2016;47:355-60.
175. Anheim M, Monga B, Fleury M, et al. Ataxia with oculomotor apraxia type 2: clinical, biological and genotype/phenotype correlation study of a cohort of 90 patients. Brain 2009;132:2688-98.
176. Laugel V, Dalloz C, Tobias ES, et al. Cerebro-oculo-facio-skeletal syndrome: three additional cases with CSB mutations, new diagnostic criteria and an approach to investigation. J Med Genet 2008;45:564-71.
177. Le Van Quyen P, Calmels N, Bonnière M, et al. Prenatal diagnosis of cerebro-oculo-facio-skeletal syndrome: Report of three fetuses and review of the literature. Am J Med Genet A 2020;182:1236-42.
178. Karikkineth AC, Scheibye-Knudsen M, Fivenson E, Croteau DL, Bohr VA. Cockayne syndrome: clinical features, model systems and pathways. Ageing Res Rev 2017;33:3-17.
180. Dumitrache LC, McKinnon PJ. Polynucleotide kinase-phosphatase (PNKP) mutations and neurologic disease. Mech Ageing Dev 2017;161:121-9.
181. Reynolds JJ, Walker AK, Gilmore EC, Walsh CA, Caldecott KW. Impact of PNKP mutations associated with microcephaly, seizures and developmental delay on enzyme activity and DNA strand break repair. Nucleic Acids Res 2012;40:6608-19.
182. Shen J, Gilmore EC, Marshall CA, et al. Mutations in PNKP cause microcephaly, seizures and defects in DNA repair. Nat Genet 2010;42:245-9.
183. O’Driscoll M, Ruiz-Perez VL, Woods CG, Jeggo PA, Goodship JA. A splicing mutation affecting expression of ataxia-telangiectasia and Rad3-related protein (ATR) results in Seckel syndrome. Nat Genet 2003;33:497-501.
184. Saneto RP, Cohen BH, Copeland WC, Naviaux RK. Alpers-Huttenlocher syndrome. Pediatr Neurol 2013;48:167-78.
185. Harhouri K, Frankel D, Bartoli C, Roll P, De Sandre-Giovannoli A, Lévy N. An overview of treatment strategies for Hutchinson-Gilford Progeria syndrome. Nucleus 2018;9:246-57.
186. Hennekam RC. Hutchinson-Gilford progeria syndrome: review of the phenotype. Am J Med Genet A 2006;140:2603-24.
187. Morio T. Recent advances in the study of immunodeficiency and DNA damage response. Int J Hematol 2017;106:357-65.
Cite This Article
Export citation file: BibTeX | RIS
OAE Style
Lam FC. The DNA damage response - from cell biology to human disease. J Transl Genet Genom 2022;6:204-22. http://dx.doi.org/10.20517/jtgg.2021.61
AMA Style
Lam FC. The DNA damage response - from cell biology to human disease. Journal of Translational Genetics and Genomics. 2022; 6(2): 204-22. http://dx.doi.org/10.20517/jtgg.2021.61
Chicago/Turabian Style
Lam, Fred C.. 2022. "The DNA damage response - from cell biology to human disease" Journal of Translational Genetics and Genomics. 6, no.2: 204-22. http://dx.doi.org/10.20517/jtgg.2021.61
ACS Style
Lam, FC. The DNA damage response - from cell biology to human disease. J. Transl. Genet. Genom. 2022, 6, 204-22. http://dx.doi.org/10.20517/jtgg.2021.61
About This Article
Copyright
Data & Comments
Data
Cite This Article 71 clicks
Like This Article 5
likes
Comments
Comments must be written in English. Spam, offensive content, impersonation, and private information will not be permitted. If any comment is reported and identified as inappropriate content by OAE staff, the comment will be removed without notice. If you have any queries or need any help, please contact us at support@oaepublish.com.