Several DNA repair genes contain p53 response elements and their transcription is positively regulated by TP53 (p53). TP53-mediated regulation probably ensures increased protein level of DNA repair genes under genotoxic stress.
TP53 directly stimulates transcription of several genes involved in DNA mismatch repair, including MSH2 (Scherer et al. 2000, Warnick et al. 2001), PMS2 and MLH1 (Chen and Sadowski 2005). TP53 also directly stimulates transcription of DDB2, involved in nucleotide excision repair (Tan and Chu 2002), and FANCC, involved in the Fanconi anemia pathway that repairs DNA interstrand crosslinks (Liebetrau et al. 1997). Other p53 targets that can influence DNA repair functions are RRM2B (Kuo et al. 2012), XPC (Fitch et al. 2003), GADD45A (Amundson et al. 2002), CDKN1A (Cazzalini et al. 2010) and PCNA (Xu and Morris 1999). Interestingly, the responsiveness of some of these DNA repair genes to p53 activation has been shown in human cells but not for orthologous mouse genes (Jegga et al. 2008, Tan and Chu 2002). Contrary to the positive modulation of nucleotide excision repair (NER) and mismatch repair (MMR), p53 can negatively modulate base excision repair (BER), by down-regulating the endonuclease APEX1 (APE1), acting in concert with SP1 (Poletto et al. 2016).<p>Expression of several DNA repair genes is under indirect TP53 control, through TP53-mediated stimulation of cyclin K (CCNK) expression (Mori et al. 2002). CCNK is the activating cyclin for CDK12 and CDK13 (Blazek et al. 2013). The complex of CCNK and CDK12 binds and phosphorylates the C-terminal domain of the RNA polymerase II subunit POLR2A, which is necessary for efficient transcription of long DNA repair genes, including BRCA1, ATR, FANCD2, FANCI, ATM, MDC1, CHEK1 and RAD51D. Genes whose transcription is regulated by the complex of CCNK and CDK12 are mainly involved in the repair of DNA double strand breaks and/or the Fanconi anemia pathway (Blazek et al. 2011, Cheng et al. 2012, Bosken et al. 2014, Bartkowiak and Greenleaf 2015, Ekumi et al. 2015).
View original pathway at:Reactome.</div>
Iyer RR, Pluciennik A, Burdett V, Modrich PL.; ''DNA mismatch repair: functions and mechanisms.''; PubMedEurope PMCScholia
Scherer SJ, Maier SM, Seifert M, Hanselmann RG, Zang KD, Muller-Hermelink HK, Angel P, Welter C, Schartl M.; ''p53 and c-Jun functionally synergize in the regulation of the DNA repair gene hMSH2 in response to UV.''; PubMedEurope PMCScholia
Christmann M, Tomicic MT, Roos WP, Kaina B.; ''Mechanisms of human DNA repair: an update.''; PubMedEurope PMCScholia
Amundson SA, Patterson A, Do KT, Fornace AJ.; ''A nucleotide excision repair master-switch: p53 regulated coordinate induction of global genomic repair genes.''; PubMedEurope PMCScholia
Chen J, Sadowski I.; ''Identification of the mismatch repair genes PMS2 and MLH1 as p53 target genes by using serial analysis of binding elements.''; PubMedEurope PMCScholia
Ekumi KM, Paculova H, Lenasi T, Pospichalova V, Bösken CA, Rybarikova J, Bryja V, Geyer M, Blazek D, Barboric M.; ''Ovarian carcinoma CDK12 mutations misregulate expression of DNA repair genes via deficient formation and function of the Cdk12/CycK complex.''; PubMedEurope PMCScholia
Blazek D, Kohoutek J, Bartholomeeusen K, Johansen E, Hulinkova P, Luo Z, Cimermancic P, Ule J, Peterlin BM.; ''The Cyclin K/Cdk12 complex maintains genomic stability via regulation of expression of DNA damage response genes.''; PubMedEurope PMCScholia
Fitch ME, Cross IV, Ford JM.; ''p53 responsive nucleotide excision repair gene products p48 and XPC, but not p53, localize to sites of UV-irradiation-induced DNA damage, in vivo.''; PubMedEurope PMCScholia
Jiang Q, Greenberg RA.; ''Deciphering the BRCA1 Tumor Suppressor Network.''; PubMedEurope PMCScholia
Bösken CA, Farnung L, Hintermair C, Merzel Schachter M, Vogel-Bachmayr K, Blazek D, Anand K, Fisher RP, Eick D, Geyer M.; ''The structure and substrate specificity of human Cdk12/Cyclin K.''; PubMedEurope PMCScholia
Kuo ML, Sy AJ, Xue L, Chi M, Lee MT, Yen T, Chiang MI, Chang L, Chu P, Yen Y.; ''RRM2B suppresses activation of the oxidative stress pathway and is up-regulated by p53 during senescence.''; PubMedEurope PMCScholia
Wang W.; ''Emergence of a DNA-damage response network consisting of Fanconi anaemia and BRCA proteins.''; PubMedEurope PMCScholia
Warnick CT, Dabbas B, Ford CD, Strait KA.; ''Identification of a p53 response element in the promoter region of the hMSH2 gene required for expression in A2780 ovarian cancer cells.''; PubMedEurope PMCScholia
Poletto M, Legrand AJ, Fletcher SC, Dianov GL.; ''p53 coordinates base excision repair to prevent genomic instability.''; PubMedEurope PMCScholia
Cohn MA, Kowal P, Yang K, Haas W, Huang TT, Gygi SP, D'Andrea AD.; ''A UAF1-containing multisubunit protein complex regulates the Fanconi anemia pathway.''; PubMedEurope PMCScholia
Bartkowiak B, Greenleaf AL.; ''Expression, purification, and identification of associated proteins of the full-length hCDK12/CyclinK complex.''; PubMedEurope PMCScholia
Cheng SW, Kuzyk MA, Moradian A, Ichu TA, Chang VC, Tien JF, Vollett SE, Griffith M, Marra MA, Morin GB.; ''Interaction of cyclin-dependent kinase 12/CrkRS with cyclin K1 is required for the phosphorylation of the C-terminal domain of RNA polymerase II.''; PubMedEurope PMCScholia
Rothkamm K, Krüger I, Thompson LH, Löbrich M.; ''Pathways of DNA double-strand break repair during the mammalian cell cycle.''; PubMedEurope PMCScholia
Mori T, Anazawa Y, Matsui K, Fukuda S, Nakamura Y, Arakawa H.; ''Cyclin K as a direct transcriptional target of the p53 tumor suppressor.''; PubMedEurope PMCScholia
Liang K, Gao X, Gilmore JM, Florens L, Washburn MP, Smith E, Shilatifard A.; ''Characterization of human cyclin-dependent kinase 12 (CDK12) and CDK13 complexes in C-terminal domain phosphorylation, gene transcription, and RNA processing.''; PubMedEurope PMCScholia
Cohn MA, D'Andrea AD.; ''Chromatin recruitment of DNA repair proteins: lessons from the fanconi anemia and double-strand break repair pathways.''; PubMedEurope PMCScholia
Deans AJ, West SC.; ''DNA interstrand crosslink repair and cancer.''; PubMedEurope PMCScholia
Hanawalt PC, Spivak G.; ''Transcription-coupled DNA repair: two decades of progress and surprises.''; PubMedEurope PMCScholia
Edelbrock MA, Kaliyaperumal S, Williams KJ.; ''DNA mismatch repair efficiency and fidelity are elevated during DNA synthesis in human cells.''; PubMedEurope PMCScholia
Jegga AG, Inga A, Menendez D, Aronow BJ, Resnick MA.; ''Functional evolution of the p53 regulatory network through its target response elements.''; PubMedEurope PMCScholia
Ciccia A, Elledge SJ.; ''The DNA damage response: making it safe to play with knives.''; PubMedEurope PMCScholia
Morris DP, Michelotti GA, Schwinn DA.; ''Evidence that phosphorylation of the RNA polymerase II carboxyl-terminal repeats is similar in yeast and humans.''; PubMedEurope PMCScholia
Vermeulen W, Fousteri M.; ''Mammalian transcription-coupled excision repair.''; PubMedEurope PMCScholia
Marteijn JA, Lans H, Vermeulen W, Hoeijmakers JH.; ''Understanding nucleotide excision repair and its roles in cancer and ageing.''; PubMedEurope PMCScholia
Liebetrau W, Budde A, Savoia A, Grummt F, Hoehn H.; ''p53 activates Fanconi anemia group C gene expression.''; PubMedEurope PMCScholia
Kottemann MC, Smogorzewska A.; ''Fanconi anaemia and the repair of Watson and Crick DNA crosslinks.''; PubMedEurope PMCScholia
Cazzalini O, Scovassi AI, Savio M, Stivala LA, Prosperi E.; ''Multiple roles of the cell cycle inhibitor p21(CDKN1A) in the DNA damage response.''; PubMedEurope PMCScholia
Tan T, Chu G.; ''p53 Binds and activates the xeroderma pigmentosum DDB2 gene in humans but not mice.''; PubMedEurope PMCScholia
Xu J, Morris GF.; ''p53-mediated regulation of proliferating cell nuclear antigen expression in cells exposed to ionizing radiation.''; PubMedEurope PMCScholia
Martín-López JV, Fishel R.; ''The mechanism of mismatch repair and the functional analysis of mismatch repair defects in Lynch syndrome.''; PubMedEurope PMCScholia
Langevin F, Crossan GP, Rosado IV, Arends MJ, Patel KJ.; ''Fancd2 counteracts the toxic effects of naturally produced aldehydes in mice.''; PubMedEurope PMCScholia
Numerous types of DNA damage can occur within a cell due to the endogenous production of oxygen free radicals, normal alkylation reactions, or exposure to exogenous radiations and chemicals. Double-strand breaks (DSBs), one of the most dangerous type of DNA damage along with interstrand crosslinks, are caused by ionizing radiation or certain chemicals such as bleomycin, and occur normally during the processes of DNA replication, meiotic exchange, and V(D)J recombination.
The two most prominent mechanisms for DSB repair are the error-free homologous recombination repair (HRR) pathway and the error-prone nonhomologous end-joining (NHEJ) pathway. The choice of the repair pathway may be determined by whether the DNA region has already replicated and the precise nature of the break. NHEJ functions at all stages of the cell cycle, but plays the predominant role in both the G1 phase and in S-phase regions of DNA that have not yet replicated (Rothkamm et al. 2003). HRR functions primarily in repairing both one-sided DSBs that arise at DNA replication forks and two-sided DSBs arising in S or G2-phase chromatid regions that have replicated. For a recent review, please refer to Ciccia and Elledge 2010.
Fanconi anemia (FA) is a genetic disease of genome instability characterized by congenital skeletal defects, aplastic anemia, susceptibility to leukemias, and cellular sensitivity to DNA damaging agents. Patients with FA have been categorized into at least 15 complementation groups (FA-A, -B, -C, -D1, -D2, -E, -F, -G, -I, -J, -L, -M, -N, -O and -P). These complementation groups correspond to the genes FANCA, FANCB, FANCC, FANCD1/BRCA2, FANCD2, FANCE, FANCF, FANCG, FANCJ/BRIP1, FANCL, FANCM, FANCN/PALB2, FANCO/RAD51C and FANCP/SLX4. Eight of these proteins, FANCA, FANCB, FANCC, FANCE, FANCF, FANCG, FANCL, and FANCM, together with FAAP24, FAAP100, FAAP20, APITD1 and STRA13, form a nuclear complex termed the FA core complex. The FA core complex is an E3 ubiquitin ligase that recognizes and is activated by DNA damage in the form of interstrand crosslinks (ICLs), triggering monoubiquitination of FANCD2 and FANCI, which initiates repair of ICL-DNA.
FANCD2 and FANCI form a complex and are mutually dependent on one another for their respective monoubiquitination. After DNA damage and during S phase, FANCD2 localizes to discrete nuclear foci that colocalize with proteins involved in homologous recombination repair, such as BRCA1 and RAD51. The FA pathway is regulated by ubiquitination and phosphorylation of FANCD2 and FANCI. ATR-dependent phosphorylation of FANCI and FANCD2 promotes monoubiquitination of FANCD2, stimulating the FA pathway (Cohn and D'Andrea 2008, Wang 2007). The complex of USP1 and ZBTB32 (UAF1) is responsible for deubiquitination of FANCD2 and negatively regulates the FA pathway (Cohn et al. 2007).
Monoubiquitinated FANCD2 recruits DNA nucleases, including SLX4 (FANCP) and FAN1, which unhook the ICL from one of the two covalently linked DNA strands. The DNA polymerase nu (POLN) performs translesion DNA synthesis using the DNA strand with unhooked ICL as a template, thereby bypassing the unhooked ICL. The unhooked ICL is subsequently removed from the DNA via nucleotide excision repair (NER). Incision of the stalled replication fork during the unhooking step generates a double strand break (DSB). The DSB is repaired via homologous recombination repair (HRR) and involves the FA genes BRCA2 (FANCD1), PALB2 (FANCN) and BRIP1 (FANCJ) (reviewed by Deans and West 2011, Kottemann and Smogorzewska 2013). Homozygous mutations in BRCA2, PALB2 or BRIP1 result in Fanconi anemia, while heterozygous mutations in these genes predispose carriers to primarily breast and ovarian cancer. Well established functions of BRCA2, PALB2 and BRIP1 in DNA repair are BRCA1 dependent, but it is not yet clear whether there are additional roles for these proteins in the Fanconi anemia pathway that do not rely on BRCA1 (Evans and Longo 2014, Jiang and Greenberg 2015). Heterozygous BRCA1 mutations predispose carriers to breast and ovarian cancer with high penetrance. Complete loss of BRCA1 function is embryonic lethal. It has only recently been reported that a partial germline loss of BRCA1 function via mutations that diminish protein binding ability of the BRCT domain of BRCA1 result in a FA-like syndrome. BRCA1 has therefore been designated as the FANCS gene (Jiang and Greenberg 2015).
The FA pathway is involved in repairing DNA ICLs that arise by exposure to endogenous mutagens produced as by-products of normal cellular metabolism, such as aldehyde containing compounds. Disruption of the aldehyde dehydrogenase gene ALDH2 in FANCD2 deficient mice leads to severe developmental defects, early lethality and predisposition to leukemia. In addition to this, the double knockout mice are exceptionally sensitive to ethanol consumption, as ethanol metabolism results in accumulated levels of aldehydes (Langevin et al. 2011).
The mismatch repair (MMR) system corrects single base mismatches and small insertion and deletion loops (IDLs) of unpaired bases. MMR is primarily associated with DNA replication and is highly conserved across prokaryotes and eukaryotes. MMR consists of the following basic steps: a sensor (MutS homologue) detects a mismatch or IDL, the sensor activates a set of proteins (a MutL homologue and an exonuclease) that select the nascent DNA strand to be repaired, nick the strand, exonucleolytically remove a region of nucleotides containing the mismatch, and finally a DNA polymerase resynthesizes the strand and a ligase seals the remaining nick (reviewed in Kolodner and Marsischkny 1999, Iyer et al. 2006, Li 2008, Fukui 2010, Jiricny 2013). Humans have 2 different MutS complexes. The MSH2:MSH6 heterodimer (MutSalpha) recognizes single base mismatches and small loops of one or two unpaired bases. The MSH2:MSH3 heterodimer (MutSbeta) recognizes loops of two or more unpaired bases. Upon binding a mismatch, the MutS complex becomes activated in an ATP-dependent manner allowing for subsequent downstream interactions and movement on the DNA substrate. (There are two mechanisms proposed: a sliding clamp and a switch diffusion model.) Though the order of steps and structural details are not fully known, the activated MutS complex interacts with MLH1:PMS2 (MutLalpha) and PCNA, the sliding clamp present at replication foci. The role of PCNA is multifaceted as it may act as a processivity factor in recruiting MMR proteins to replicating DNA, interact with MLH1:PMS2 and Exonuclease 1 (EXO1) to initiate excision of the recently replicated strand and direct DNA polymerase delta to initiate replacement of bases. MLH1:PMS2 makes an incision in the strand to be repaired and EXO1 extends the incision to make a single-stranded gap of up to 1 kb that removes the mismatched base(s). (Based on assays of purified human proteins, there is also a variant of the mismatch repair pathway that does not require EXO1, however the mechanism is not clear. EXO1 is almost always required, it is possible that the exonuclease activity of DNA polymerase delta may compensate in some situations and it has been proposed that other endonucleases may perform redundant functions in the absence of EXO1.) RPA binds the single-stranded region and a new strand is synthesized across the gap by DNA polymerase delta. The remaining nick is sealed by DNA ligase I (LIG1). Concentrations of MMR proteins MSH2:MSH6 and MLH1:PMS2 increase in human cells during S phase and are at their highest level and activity during this phase of the cell cycle (Edelbrock et al. 2009). Defects in MSH2, MSH6, MLH1, and PMS2 cause hereditary nonpolyposis colorectal cancer (HNPCC, also known as Lynch syndrome) (reviewed in Martin-Lopez and Fishel 2013).
Nucleotide excision repair (NER) was first described in the model organism E. coli in the early 1960s as a process whereby bulky base damage is enzymatically removed from DNA, facilitating the recovery of DNA synthesis and cell survival. Deficient NER processes have been identified from the cells of cancer-prone patients with different variants of xeroderma pigmentosum (XP), trichothiodystrophy (TTD), and Cockayne's syndrome. The XP cells exhibit an ultraviolet radiation hypersensitivity that leads to a hypermutability response to UV, offering a direct connection between deficient NER, increased mutation rate, and cancer. While the NER pathway in prokaryotes is unique, the pathway utilized in yeast and higher eukaryotes is highly conserved. NER is involved in the repair of bulky adducts in DNA, such as UV-induced photo lesions (both 6-4 photoproducts (6-4 PPDs) and cyclobutane pyrimidine dimers (CPDs)), as well as chemical adducts formed from exposure to aflatoxin, benzopyrene and other genotoxic agents. Specific proteins have been identified that participate in base damage recognition, cleavage of the damaged strand on both sides of the lesion, and excision of the oligonucleotide bearing the lesion. Reparative DNA synthesis and ligation restore the strand to its original state. NER consists of two related pathways called global genome nucleotide excision repair (GG-NER) and transcription-coupled nucleotide excision repair (TC-NER). The pathways differ in the way in which DNA damage is initially recognized, but the majority of the participating molecules are shared between these two branches of NER. GG-NER is transcription-independent, removing lesions from non-coding DNA strands, as well as coding DNA strands that are not being actively transcribed. TC-NER repairs damage in transcribed strands of active genes. Several of the proteins involved in NER are key components of the basal transcription complex TFIIH. An ubiquitin ligase complex composed of DDB1, CUL4A or CUL4B and RBX1 participates in both GG-NER and TC-NER, implying an important role of ubiquitination in NER regulation. The establishment of mutant mouse models for NER genes and other DNA repair-related genes has been useful in demonstrating the associations between NER defects and cancer. For past and recent reviews of nucleotide excision repair, please refer to Lindahl and Wood 1998, Friedberg et al. 2002, Christmann et al. 2003, Hanawalt and Spivak 2008, Marteijn et al. 2014).
DSIF is a heterodimer consisting of hSPT4 (human homolog of yeast Spt4- p14) and hSPT5 (human homolog of yeast Spt5-p160). DSIF association with Pol II may be enabled by Spt5 binding to Pol II creating a scaffold for NELF binding (Wada et al.,1998). Spt5 subunit of DSIF can be phosphorylated by P-TEFb.
The C-terminal domain (CTD) of POLR2A contains about 52 repeats of the consensus heptad YSPTSPS. Serines-2 and 5 of the heptads are phosphorylated in RNA polymerase II initiating transcription of protein coding genes. The exact repeats that are phosphorylated are not known.
CDK12, in complex with CCNK (cyclin K), phosphorylates heptapeptide repeats in the C-terminal domain (CTD) of the RNA polymerase II (RNA Pol II) subunit POLR2A. CDK12 may require phosphorylation of its threonine residue T893 to achieve full catalytic activity, but the activating kinase is not known. CDK12-mediated phosphorylation of the CTD of POLR2A occurs after the heptapeptide repeats in the CTD of POLR2A undergo phosphorylation by the CDK9-containing P-TEFb complex. It is unclear whether CDK12 acts on the second serine or the fifth serine or both in the YSPTSPS repeats. The mammalian POLR2A contains 52 heptapeptide repeats that start at amino acid position 1615. The exact localization of CDK9 and CDK12 target sites relative to the full-length POLR2A is not known. CDK12-mediated phosphorylation of the pre-phosphorylated RNA Pol II complex is important for the transcription of a group of genes with long and complex structures, involved in DNA repair (Blazek et al. 2011, Cheng et al. 2012, Bosken et al. 2014, Bartkowiak and Greenleaf 2015, Liang et al. 2015).
The complex of CDK12 and CCNK (cyclin K) associates with the RNA polymerase II (RNA Pol II) elongation complex at DNA repair genes encoding long primary transcripts, such as BRCA1, ATR, FANCI, FANCD2, ATM, MDC1, CHEK1, RAD51D and APEX1 (Blazek et al. 2011, Bartkowiak and Greenleaf 2015, Ekumi et al. 2015, Liang et al. 2015).
CDK12-mediated phosphorylation of the C-terminal domain (CTD) of the RNA polymerase II (RNA Pol II) subunit POLR2A (Rpb1) positively regulates transcription and, hence, expression of a set of DNA repair genes that encode long primary transcripts. CDK12, in complex with the TP53-regulated CCNK (cyclin K), phosphorylates POLR2A that was pre-phosphorylated by the CDK9-containing complex P-TEFb. CDK12-mediated phosphorylation of POLR2A is thought to increase the processivity of the RNA Pol II, enabling efficient transcription of long DNA repair genes (Blazek et al. 2011, Cheng et al. 2012, Bosken et al. 2014, Bartkowiak and Greenleaf 2015). CDK12 was shown to colocalize with the RNA Pol II complex at FANCD2, FANCI, ATM, CHEK1, MDC1, RAD51D and ATR gene loci (Ekumi et al. 2015) and to be necessary for achieving sufficient BRCA1 expression (Blazek et al. 2011). CDK12 positively regulates the expression of APEX1, involved in base excision repair (Liang et al. 2015). Recurrent CDK12 mutations are found in ovarian cancer. These mutations affect either the catalytic cleft of CDK12 or disable the interaction of CDK12 with CCNK, resulting in the loss of CDK12 function. Ovarian tumors that harbour inactivating CDK12 mutations exhibit decreased BRCA1 levels, defective homologous recombination repair, increased sensitivity to DNA crosslinking agents, and sensitivity to PARP inhibitors (Joshi et al. 2014, Ekumi et al. 2015).
TP53 (p53) stimulates transcription of the MLH1 gene, involved in DNA mismatch repair, by binding to the p53 response element in the first intron of the MLH1 gene (Chen and Sadowski 2005).
TP53 and the AP-1 transcription factor complex cooperatively stimulate transcription of the MSH2 gene, involved in DNA mismatch repair, by binding to adjacent sites in the MSH2 promoter (Scherer et al. 2001). TP53 may stimulate transcription of MSH2 independently of the AP-1 complex when bound to a different p53 response element in the MSH2 promoter (Warnick et al. 2001).
TP53 (p53) stimulates transcription of PMS2, involved in DNA mismatch repair, by binding to the p53 response element in the first intron of the PMS2 gene (Chen and Sadowski 2005).
TP53 (p53) binds to the p53 response element in the promoter of the MSH2 gene. The p53 response element is flanked by two AP-1 sites. The AP-1 transcription factor complex binds the MSH2 promoter cooperatively with TP53 (Scherer et al. 2000). An additional p53 response element in closer proximity to the MSH2 transcription start site has been reported that does not involve a nearby AP-1 binding site (Warnick et al. 2001).
TP53 (p53) binds the p53 response element in the 5'UTR-encoding region of the DDB2 gene. The p53 response element is present in the human DDB2 gene, but absent from the mouse Ddb2 gene (Tan and Chu 2002).
TP53 (p53) stimulates transcription of the DDB2 gene, involved in nucleotide excision repair, by binding to the p53 response element in the 5'UTR-encoding region of the DDB2 gene (Tan and Chu 2002).
TP53 (p53) stimulates transcription of the FANCC (Fanconi anemia group C) gene, involved in the Fanconi anemia pathway that repairs DNA interstrand crosslinks, by binding to the p53 response element in the FANCC promoter (Liebetrau et al. 1997).
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FANCI, ATM, MDC1,
CHEK1, RAD51DThe two most prominent mechanisms for DSB repair are the error-free homologous recombination repair (HRR) pathway and the error-prone nonhomologous end-joining (NHEJ) pathway. The choice of the repair pathway may be determined by whether the DNA region has already replicated and the precise nature of the break. NHEJ functions at all stages of the cell cycle, but plays the predominant role in both the G1 phase and in S-phase regions of DNA that have not yet replicated (Rothkamm et al. 2003). HRR functions primarily in repairing both one-sided DSBs that arise at DNA replication forks and two-sided DSBs arising in S or G2-phase chromatid regions that have replicated. For a recent review, please refer to Ciccia and Elledge 2010.
FANCD2 and FANCI form a complex and are mutually dependent on one another for their respective monoubiquitination. After DNA damage and during S phase, FANCD2 localizes to discrete nuclear foci that colocalize with proteins involved in homologous recombination repair, such as BRCA1 and RAD51. The FA pathway is regulated by ubiquitination and phosphorylation of FANCD2 and FANCI. ATR-dependent phosphorylation of FANCI and FANCD2 promotes monoubiquitination of FANCD2, stimulating the FA pathway (Cohn and D'Andrea 2008, Wang 2007). The complex of USP1 and ZBTB32 (UAF1) is responsible for deubiquitination of FANCD2 and negatively regulates the FA pathway (Cohn et al. 2007).
Monoubiquitinated FANCD2 recruits DNA nucleases, including SLX4 (FANCP) and FAN1, which unhook the ICL from one of the two covalently linked DNA strands. The DNA polymerase nu (POLN) performs translesion DNA synthesis using the DNA strand with unhooked ICL as a template, thereby bypassing the unhooked ICL. The unhooked ICL is subsequently removed from the DNA via nucleotide excision repair (NER). Incision of the stalled replication fork during the unhooking step generates a double strand break (DSB). The DSB is repaired via homologous recombination repair (HRR) and involves the FA genes BRCA2 (FANCD1), PALB2 (FANCN) and BRIP1 (FANCJ) (reviewed by Deans and West 2011, Kottemann and Smogorzewska 2013). Homozygous mutations in BRCA2, PALB2 or BRIP1 result in Fanconi anemia, while heterozygous mutations in these genes predispose carriers to primarily breast and ovarian cancer. Well established functions of BRCA2, PALB2 and BRIP1 in DNA repair are BRCA1 dependent, but it is not yet clear whether there are additional roles for these proteins in the Fanconi anemia pathway that do not rely on BRCA1 (Evans and Longo 2014, Jiang and Greenberg 2015). Heterozygous BRCA1 mutations predispose carriers to breast and ovarian cancer with high penetrance. Complete loss of BRCA1 function is embryonic lethal. It has only recently been reported that a partial germline loss of BRCA1 function via mutations that diminish protein binding ability of the BRCT domain of BRCA1 result in a FA-like syndrome. BRCA1 has therefore been designated as the FANCS gene (Jiang and Greenberg 2015).
The FA pathway is involved in repairing DNA ICLs that arise by exposure to endogenous mutagens produced as by-products of normal cellular metabolism, such as aldehyde containing compounds. Disruption of the aldehyde dehydrogenase gene ALDH2 in FANCD2 deficient mice leads to severe developmental defects, early lethality and predisposition to leukemia. In addition to this, the double knockout mice are exceptionally sensitive to ethanol consumption, as ethanol metabolism results in accumulated levels of aldehydes (Langevin et al. 2011).
Humans have 2 different MutS complexes. The MSH2:MSH6 heterodimer (MutSalpha) recognizes single base mismatches and small loops of one or two unpaired bases. The MSH2:MSH3 heterodimer (MutSbeta) recognizes loops of two or more unpaired bases. Upon binding a mismatch, the MutS complex becomes activated in an ATP-dependent manner allowing for subsequent downstream interactions and movement on the DNA substrate. (There are two mechanisms proposed: a sliding clamp and a switch diffusion model.) Though the order of steps and structural details are not fully known, the activated MutS complex interacts with MLH1:PMS2 (MutLalpha) and PCNA, the sliding clamp present at replication foci. The role of PCNA is multifaceted as it may act as a processivity factor in recruiting MMR proteins to replicating DNA, interact with MLH1:PMS2 and Exonuclease 1 (EXO1) to initiate excision of the recently replicated strand and direct DNA polymerase delta to initiate replacement of bases. MLH1:PMS2 makes an incision in the strand to be repaired and EXO1 extends the incision to make a single-stranded gap of up to 1 kb that removes the mismatched base(s). (Based on assays of purified human proteins, there is also a variant of the mismatch repair pathway that does not require EXO1, however the mechanism is not clear. EXO1 is almost always required, it is possible that the exonuclease activity of DNA polymerase delta may compensate in some situations and it has been proposed that other endonucleases may perform redundant functions in the absence of EXO1.) RPA binds the single-stranded region and a new strand is synthesized across the gap by DNA polymerase delta. The remaining nick is sealed by DNA ligase I (LIG1).
Concentrations of MMR proteins MSH2:MSH6 and MLH1:PMS2 increase in human cells during S phase and are at their highest level and activity during this phase of the cell cycle (Edelbrock et al. 2009). Defects in MSH2, MSH6, MLH1, and PMS2 cause hereditary nonpolyposis colorectal cancer (HNPCC, also known as Lynch syndrome) (reviewed in Martin-Lopez and Fishel 2013).
NER is involved in the repair of bulky adducts in DNA, such as UV-induced photo lesions (both 6-4 photoproducts (6-4 PPDs) and cyclobutane pyrimidine dimers (CPDs)), as well as chemical adducts formed from exposure to aflatoxin, benzopyrene and other genotoxic agents. Specific proteins have been identified that participate in base damage recognition, cleavage of the damaged strand on both sides of the lesion, and excision of the oligonucleotide bearing the lesion. Reparative DNA synthesis and ligation restore the strand to its original state.
NER consists of two related pathways called global genome nucleotide excision repair (GG-NER) and transcription-coupled nucleotide excision repair (TC-NER). The pathways differ in the way in which DNA damage is initially recognized, but the majority of the participating molecules are shared between these two branches of NER. GG-NER is transcription-independent, removing lesions from non-coding DNA strands, as well as coding DNA strands that are not being actively transcribed. TC-NER repairs damage in transcribed strands of active genes.
Several of the proteins involved in NER are key components of the basal transcription complex TFIIH. An ubiquitin ligase complex composed of DDB1, CUL4A or CUL4B and RBX1 participates in both GG-NER and TC-NER, implying an important role of ubiquitination in NER regulation. The establishment of mutant mouse models for NER genes and other DNA repair-related genes has been useful in demonstrating the associations between NER defects and cancer.
For past and recent reviews of nucleotide excision repair, please refer to Lindahl and Wood 1998, Friedberg et al. 2002, Christmann et al. 2003, Hanawalt and Spivak 2008, Marteijn et al. 2014).
Annotated Interactions
FANCI, ATM, MDC1,
CHEK1, RAD51D