DNA-damage repair; the good, the bad, and the ugly
2008; Springer Nature; Volume: 27; Issue: 4 Linguagem: Inglês
10.1038/emboj.2008.15
ISSN1460-2075
Autores Tópico(s)Cancer Genomics and Diagnostics
ResumoFocus Quality Control20 February 2008free access DNA-damage repair; the good, the bad, and the ugly Razqallah Hakem Corresponding Author Razqallah Hakem Department of Medical Biophysics, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada Department of Immunology, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Razqallah Hakem Corresponding Author Razqallah Hakem Department of Medical Biophysics, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada Department of Immunology, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada Search for more papers by this author Author Information Razqallah Hakem 1,2 1Department of Medical Biophysics, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada 2Department of Immunology, Ontario Cancer Institute/UHN, University of Toronto, Toronto, Ontario, Canada *Corresponding author. Department of Medical Biophysics, Ontario Cancer Institute/UHN, University of Toronto, 610 University Avenue PMH, Room 10-622, Toronto, Ontario, Canada M5G 2M9. Tel.: +1 416 946 2398/4501; ext: 5661; Fax: +1 416 946 2984; E-mail: [email protected] The EMBO Journal (2008)27:589-605https://doi.org/10.1038/emboj.2008.15 PDFDownload PDF of article text and main figures. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Organisms have developed several DNA-repair pathways as well as DNA-damage checkpoints to cope with the frequent challenge of endogenous and exogenous DNA insults. In the absence or impairment of such repair or checkpoint mechanisms, the genomic integrity of the organism is often compromised. This review will focus on the functional consequences of impaired DNA-repair pathways. Although each pathway is addressed individually, it is essential to note that cross talk exists between repair pathways, and that there are instances in which a DNA-repair protein is involved in more than one pathway. It is also important to integrate DNA-repair process with DNA-damage checkpoints and cell survival, to gain a better understanding of the consequences of compromised DNA repair at both cellular and organismic levels. Functional consequences associated with impaired DNA repair include embryonic lethality, shortened life span, rapid ageing, impaired growth, and a variety of syndromes, including a pronounced manifestation of cancer. Introduction Organisms have evolved to efficiently respond to DNA insults that result from either endogenous sources (cellular metabolic processes) or exogenous sources (environmental factors). Endogenous sources of DNA damage include hydrolysis, oxidation, alkylation, and mismatch of DNA bases; sources for exogenous DNA damage include ionizing radiation (IR), ultraviolet (UV) radiation, and various chemicals agents. At the cellular level, damaged DNA that is not properly repaired can lead to genomic instability, apoptosis, or senescence, which can greatly affect the organism's development and ageing process. More importantly, loss of genomic integrity predisposes the organism to immunodeficiency, neurological disorders, and cancer (O'Driscoll and Jeggo, 2006; Subba Rao, 2007; Thoms et al, 2007). Therefore, it is essential for cells to efficiently respond to DNA damage through coordinated and integrated DNA-damage checkpoints and repair pathways. DNA-damage checkpoints The mechanisms of DNA-damage checkpoints are best understood during their responses to double-strand breaks (DSBs). Initiation of these checkpoints is dependent on the transient recruitment of the MRE11/RAD50/NBS1 (MRN) complex at DSB sites, followed by the recruitment/activation of ataxia–telangiectasia mutated (ATM) a member of the family of phosphoinositide-3-kinase-related kinases (PIKKs) (Su, 2006). In addition, two other PIKKs, DNA-dependent protein kinase (DNA–PK) and ATR (ATM and Rad3 related), are also activated and involved in the response to DSBs. However, the primary function of ATR is the initiation of DNA-damage response to stalled replication forks (RFs) (Su, 2006). ATM, ATR, and DNA–PK phosphorylate various targets that contribute to the overall DNA damage response. Therefore, within minutes of DSB formation, active ATM phosphorylates different proteins that are essential for DNA-damage response and repair. An example includes the histone H2AX that, following its phosphorylation at the site of DNA damage by ATM, DNA–PK, or ATR (γH2AX), recruits other proteins and initiates the chromatin-remodelling process that is essential for the repair of damaged DNA. Other proteins recruited to sites of DSBs include MDC1, 53BP1, and BRCA1, all of which are ATM substrates and mediators in DNA-damage response. The MRN complex-mediated resection of DSBs is followed by single-stranded DNA coating with replication protein A (RPA), which serves to recruit ATR and its binding partner ATRIP, and subsequent ATR-dependent phosphorylation of clapsin, Rad17, BRCA1, and others (Su, 2006). ATM and ATR are essential for the G1/S, intra-S-phase, and G2/M DNA-damage checkpoints, and are critical for the maintenance of genomic integrity. Defects in either ATM or ATR have been associated with human syndromes. ATM mutations are associated with the human ataxia–telangiectasia (AT), an autosomal recessive disorder characterized by cerebellar ataxia, progressive mental retardation, impaired immune functions, neurological problems, and malignancies (O'Driscoll and Jeggo, 2006). At the cellular level, AT phenotypes include chromosomal breakage and IR sensitivity. Similarly, ATR mutations predispose individuals to Seckel syndrome, a very rare autosomal recessive human disorder characterized by growth and mental retardation, as well as microcephaly (O'Driscoll and Jeggo, 2006). Spontaneous and IR-induced genomic instability and immunological defects have also been observed in Seckel syndrome patients. In contrast to ATM and ATR, no human syndrome has yet been associated with defective DNA–PK. However, studies of mouse models have linked mutations of DNA–PK to severe immunodeficiency (see section Non-homologous end joining repair pathway). Activated ATM and ATR mediate the phosphorylation and subsequent activation of Chk2 and Chk1, respectively; this process is necessary in the induction of phosphorylation of CDC25A, marking it for proteosomal degradation (Su, 2006). The consequential loss of CDC25A results in G1/S arrest, due to the inefficient loading of CDC45 at the origin of replication. In addition, activated ATM, ATR, DNA–PK, Chk2, and Chk1 all aid in the phosphorylation and activation of p53, a key player in DNA-damage checkpoints. Activated p53 transactivates p21, which inhibits two G1/S-promoting cyclin-dependent kinases (CDKs), CDK2 and CDK4. This leads to sustained G1 arrest, which ultimately hampers the replication of damaged DNA. The intra-S-phase checkpoint serves to arrest DNA synthesis during S phase of cells with damaged DNA (Su, 2006). In these cells, CDC25A phosphorylation, mediated by Chk2 or Chk1, leads to its degradation and the subsequent inactivation of the S-phase cyclin E/CDK2 complex. Consequently, these events prevent the loading of CDC45 at the origin of replication and result in intra-S-phase arrest. It has been reported that other proteins, including Nijmegen breakage syndrome 1 (NBS1), BRCA1, SMC1, 53BP1, and MDC1, all contribute to the intra-S-phase checkpoint. The activation of the G2/M DNA-damage checkpoint prevents mitotic entry of the damaged cells (Su, 2006). This checkpoint is mediated by the dual-specificity phosphatase CDC25C, as well as p53. In normal conditions, CDC25C dephosphorylates CDC2, allowing the CDC2–cyclin B kinase to facilitate entry into mitosis. However, phosphorylation of CDC25C by Chk2 or Chk1, initiates its binding with 14-3-3, which leads to its cytoplasmic sequestration away from its substrate, thus preventing mitotic entry. p53 also contributes to the G2/M checkpoint through its transactivation of p21 and 14-3-3. P21 effectively blocks the phosphorylation of CDC2, initiating the onset of the G2/M cell-cycle arrest. 14-3-3 sequesters CDC25C in the cytoplasm and promotes the activation of Wee1, a tyrosine kinase that negatively regulates CDC2, thus blocking entry into mitosis. The activation of DNA-damage checkpoints enforces the growth arrest of damaged cells and allows the DNA-repair mechanisms to mend the damaged DNA. Once repair is completed, cells are able to exit the checkpoints and resume their cell-cycle progression and functions. However, unsuccessful DNA repair leads to p53-dependent apoptosis (Chipuk and Green, 2006), in addition to senescence (Collado et al, 2007). Defects of DNA-damage checkpoints, similar to impaired DNA-damage repair, promote genomic instability and predispose individuals to immunodeficiency, neurological defects, and cancer (Niida and Nakanishi, 2006). Although important advances have been made in understanding the cellular mechanisms behind the initiation and maintenance of checkpoints, the mechanisms that control checkpoints exit, as well as how the cell decides survival, death, or senescence, require further investigation. Defects associated with DNA-damage repair pathways Different DNA-repair pathways exist and perform major roles at both cellular and organismic levels. These pathways include (1) the direct reversal pathway, (2) the mismatch repair (MMR) pathway, (3) the nucleotide excision repair (NER) pathway, (4) the base excision repair (BER) pathway, (5) the homologous recombination (HR) pathway, and (6) the non-homologous end joining (NHEJ) pathway (Figure 1). The mechanisms for these pathways will not be discussed in detail in this review; instead we will focus on the functional consequences associated with their defects. Figure 1.DNA-repair pathways. Several DNA-repair pathways exist and deal with various types of DNA insults. These pathways include (1) the direct reversal pathway, (2) the MMR pathway, (3) the NER pathway, (4) the BER pathway, (5) the HR pathway, and (6) the NHEJ pathway. Download figure Download PowerPoint Direct reversal of DNA damage In contrast to other DNA-damage repair pathways, direct reversal of DNA damage is not a multistep process and does not involve multiple proteins (Sedgwick et al, 2007). Furthermore, unlike excision repair, direct reversal of DNA damage does not require the excision of the damaged bases. An example of a DNA lesion that is repaired by direct reversal is the O6-alkylguanine. Alkylating agents can transfer methyl or ethyl groups to a guanine, thereby modifying the base and interfering with its pairing with cytosine during DNA replication. The cytotoxic and mutagenic O6 alkyl adduct in DNA is repaired by direct reversal, which is mediated by the enzyme Ada in Escherichia coli (E. coli) and the mammalian O6-methylguanine-DNA methyltransferase (MGMT). MGMT, also known as AGT, removes the DNA adducts by transferring the alkyl group from the oxygen in the DNA to a cysteine residue in its active site. This reaction leads to the reversal of the base damage; however, the alkylation of MGMT leads to its inactivation and subsequent ubiquitination and proteosomal degradation. MGMT has attracted a great deal of attention, as certain anticancer chemotherapeutic drugs produce O6-alkylguanine, further supporting its role in modulating the therapeutic response of tumors to these drugs. Mouse models for Mgmt inactivation have been generated (Tsuzuki et al, 1996b; Glassner et al, 1999). These mutants were viable and showed no increase in spontaneous tumorigenesis (Table I). However, Mgmt homozygous mice and cells were highly sensitive to chemotherapeutic alkylating agents such as methylnitrosourea. Mgmt homozygous mutant females, but not males, developed larger numbers of dimethylnitrosamine-induced liver and lung tumors compared with controls (Iwakuma et al, 1997). Additionally, transgenic mice over-expressing human MGMT or E. coli Ada have also been generated. In response to alkylating carcinogens that produce O6-alkylguanine in DNA, these transgenic mice demonstrated a significantly reduced susceptibility to developing cancers, including thymomas (Dumenco et al, 1993), liver tumors (Nakatsuru et al, 1993), and skin tumors (Becker et al, 1997). Table 1. Examples of mouse models for direct reversal Genotype Developmental defects Fertility defects Spontaneous tumorigenesis Induced tumorigenesis References Mgmt−/− None None Not affected Increased DMNA- induced lung and liver cancer in females (Tsuzuki et al, 1996a, 1996b; Iwakuma et al, 1997; Glassner et al, 1999) Abh2−/−Abh3−/−Abh2−/−Abh3−/− None None Not affected Not tested (Ringvoll et al, 2006) AlkB is another enzyme that mediates direct DNA damage reversal in E. coli. This dioxygenase is involved in the repair of alkylation damage, particularly 1-methyladenine (1meA) and 3-methylcytosine (3meC). Two mammalian AlkB homologues, ABH2 and ABH3, have been shown to possess DNA-repair functions similar to the bacterial AlkB (Duncan et al, 2002; Sedgwick et al, 2007). Similar to AlkB, ABH2 and ABH3 have the ability to repair 1meA and 3meC residues. However, whereas ABH2 prefers double-stranded DNA, ABH3 and AlkB favour single-stranded DNA and RNA (Aas et al, 2003; Falnes et al, 2004). Further insight into the function of the mammalian ABH2 and ABH3 came from studies of mice carrying targeted mutations of these genes. Mice deficient in Abh2, Abh3, or both, were viable (Ringvoll et al, 2006). Abh2−/−, but not Abh3−/−, mice showed age-dependent accumulation of 1meA in their genomic DNA. As in AlkB mutants in E. coli, mouse embryonic fibroblasts (MEFs) deficient in Abh2 were hypersensitive to methyl methane-sulfonate (MMS) treatment. However, mice deficient in Abh2 or Abh3 did not show increased spontaneous cancer development (Table I). Further studies are required to assess the role of these dioxygenases, and other AlkB homologues, in alkylation damage-induced cancer. These examples of direct DNA-damage reversal mediated by MGMT/Ada or ABH/AlkB demonstrate the conserved role of this mechanism in DNA repair. In addition, increased tumorigenesis of Mgmt mutants, together with the resistance of MGMT transgenic mice to alkylating carcinogens that produce O6-alkylguanine, further demonstrate the important role that direct reversal plays in cancer. The MMR pathway The MMR pathway plays an important role in both prokaryotes and eukaryotes in repairing mismatches, which are small insertions and deletions that take place during DNA replication (Figure 1; Jiricny, 2006). Failure of MMR commonly results in microsatellite instability (MSI). Several homologues of the bacterial MMR genes MutS and MutL have been identified in yeast and mammals. The importance of the MMR pathway became evident upon identification of mutations in certain human MMR genes in hereditary non-polyposis colorectal cancer (HNPCC), a highly penetrant autosomal dominant cancer syndrome (Figure 2; Vasen et al, 2007). HNPCC, also known as Lynch syndrome, is characterized by early-onset colorectal cancer, with elevated levels of MSI in the tumors. Individuals with HNPCC have an approximate 80% lifetime risk for colorectal cancer, and are also predisposed to the development of endometrial, ovarian, gastric, and other types of malignancies. Figure 2.Examples of human syndromes and disorders associated with defective DNA-damage repair. Impaired MMR pathway leads to the hereditary HNPCC. Mutations of certain human NER genes have been associated with syndromes and disorders including the XP, CS, and TTD. MAP, a rare disorder, has been shown associated with mutations of the BER gene MUTYH. Various human syndromes and disorders have been associated with defects of the HR pathway. They include ATLD, NBS, BS, WS and RTS. Mutations of certain human genes involved in NHEJ lead to the SCID or RS-SCID. Download figure Download PowerPoint Approximately 70–80% of germline mutations identified in HNPCC families are mutations in MLH1 or MSH2, whereas mutations in MSH6 are found in approximately 10% of HNPCC families (Peltomaki and Vasen, 1997). Germline mutations in other human MMR genes, including PMS1, PMS2, MLH3, and exonuclease 1 (EXO1), have also been found in HNPCC families; however, they occur at a much lower frequency (Vasen et al, 2007). In addition, inactivation of MLH1 by mutations at the promoter or coding sequences, or by promoter methylation, has been identified in sporadic colorectal tumors (Kane et al, 1997; Veigl et al, 1998). Recent studies, although very limited, have identified rare patients with homozygous germline mutations for MLH1, MSH2, MSH6, or PMS2 (Felton et al, 2007). Typically, these individuals have a reduced life span and, in contrast to heterozygous MMR individuals, tend to develop juvenile haematological malignancies and brain cancer. In yeast, Msh2 forms heterodimers with Msh3 and Msh6, proteins that bind DNA mismatches and initiate the MMR process. In Saccharomyces cerevisiae (S. cerevisiae), Msh2, Msh3, and Msh6 mutants are viable (Marsischky et al, 1996). Both Msh2 and Msh6 S. cerevisiae mutants show high frequencies of base substitution, whereas only Msh2 mutants exhibit high frameshift mutations. Msh3 mutations in S. cerevisiae result in low rates of frameshift mutations. However, on Msh6-mutant background, synergistic effects of the dual mutations have been observed, including increased MSI and mutability similar to Msh2 mutants. Mutant mice for MutS and MutL MMR homologues have also been generated using gene targeting (Table II). Mutant mice for the MutS homologues include Msh2, Msh3, Msh4, Msh5, and Msh6. Mice carrying homozygous mutations for Msh4 or Msh5 did not exhibit cancer phenotypes; however, males and females were infertile, consistent with the role of MutS homologues in processing meiotic recombination intermediates (de Vries et al, 1999; Edelmann et al, 1999). In contrast, homozygous mutants for Msh2 (de Wind et al, 1995; Reitmair et al, 1995), Msh3 (Edelmann et al, 2000), and Msh6 (Edelmann et al, 1997) have increased risk for developing cancers such as lymphoma, gastrointestinal, and skin cancer. Table 2. Examples of mouse models for the MMR pathway Genotype Developmental defects Fertility defects Spontaneous tumorigenesis References Msh2−/− None None High frequency and early onset of lymphomas, gastrointestinal, and skin cancer (de Wind et al, 1995; Reitmair et al, 1995) Msh3−/− None None Low frequency and late onset of lymphomas, gastrointestinal, and skin cancer (Edelmann et al, 2000) Msh4−/− None Infertile Not affected (Kneitz et al, 2000) Msh5−/− None Infertile Not affected (de Vries et al, 1999; Edelmann et al, 1999) Msh6−/− None Unaffected Lymphoma, gastrointestinal, and skin cancer (Edelmann et al, 1997) Msh3−/−Msh6−/− None None Higher frequency of lymphomas, gastrointestinal, and skin tumours compared to single mutants (Edelmann et al, 2000) Pms1−/− None None Not affected (Prolla et al, 1998) Pms2−/− None Male infertility Lymphomas (Baker et al, 1995; Prolla et al, 1998; Chen et al, 2005) Mlh1−/− None Infertile High frequency and early onset of lymphomas and gastrointestinal tumours (Baker et al, 1996; Edelmann et al, 1996; Prolla et al, 1998; Chen et al, 2005) Mlh3−/− None Infertile Lymphomas and gastrointestinal tumours (Lipkin et al, 2002; Chen et al, 2005) ExoI−/− None Infertile Lymphomas (Wei et al, 2003) Mutant mice for MutL homologues include Pms1−/−, Pms2−/−, Mlh1−/−, and Mlh3−/− mice (Table II). These mutants are viable; however, males and females deficient in Mlh1 (Baker et al, 1996; Edelmann et al, 1996) or Mlh3 (Lipkin et al, 2002) and males deficient in Pms2 (Baker et al, 1995) are sterile, demonstrating a requirement for these proteins during meiosis. In addition, mouse MutL homologues are differentially required for cancer suppression. Pms1−/− mice do not show any increased risk for cancer (Prolla et al, 1998), whereas Mlh1−/− (Prolla et al, 1998; Chen et al, 2005) and Mlh3−/− mice (Chen et al, 2005) are predisposed to developing lymphomas and gastrointestinal tumors. Similarly, Pms2-null mutants (Prolla et al, 1998; Chen et al, 2005) are prone for lymphoma development. EXOI physically interacts with MSH2, MSH3, and MLH1, and is involved in the excision of mismatched bases in DNA (Tishkoff et al, 1997; Schmutte et al, 2001). Mutant mice for ExoI have impaired MMR, accumulate MSI, and exhibit a greater risk for developing lymphomas (Wei et al, 2003). These mutants also have meiotic defects and are sterile, demonstrating the requirement of ExoI in meiosis. Double mutant mice carrying dual mutations of different MMR genes have also been reported. For example, Msh3-mutant mice develop cancer with low frequency and at a later age, whereas Msh3−/−Msh6−/− mice (Edelmann et al, 2000) die prematurely and develop tumors including lymphomas, gastrointestinal, and skin tumors. This phenotypic outcome is similar to that of Msh2−/− or Mlh1−/− mice that are the most cancer-prone MMR mutants, as half of these mutants die around 6 months of age. This cooperation between mutations of Msh2 and Msh6 in mice is reminiscent of their collaboration in the maintenance of genomic integrity of S. cerevisiae. Immunoglobulin (Ig) diversification, an essential process for immunity, involves somatic hypermutation (SHM) of the Ig genes, as well as VDJ recombination and class-switch recombination (CSR), two processes mediated by NHEJ (Maizels, 2005). Interestingly, studies of the various MMR-mutant strains have implicated a role for this pathway in SHM and CSR. Thus, Msh2, Msh6, and ExoI, but not Msh3-mutant mice, have reduced CSR and SHM (Rada et al, 1998; Wiesendanger et al, 2000; Bardwell et al, 2004; Li et al, 2004). Whereas most HNPCC human individuals carry heterozygous germline mutations of MMR genes, which predisposes them to cancer, mice heterozygous for MMR mutations do not appear to have an increased risk for developing cancer. This difference is not specific for MMR mutations, as heterozygous mutations in certain genes involved in other DNA-damage repair pathways are also able to predispose humans, but not mice, to cancer. The reasons for these differences remain unknown, although species differences in the DNA-damage repair pathways, metabolism, or life span could contribute to these observed human–mouse discrepancies. Despite these differences, mouse models have significantly improved our understanding of the MMR and other repair mechanisms, and their roles in preserving genomic integrity and suppressing cancer. The NER pathway The NER pathway is a multistep process that serves to repair a variety of DNA damage, including DNA lesions caused by UV radiation, mutagenic chemicals, or chemotherapeutic drugs that form bulky DNA adducts (Figure 1; Leibeling et al, 2006). Over 30 different proteins are involved in the mammalian NER, whereas only three proteins (UvrA, UvrB, and UvrC) are required by prokaryotes (Truglio et al, 2006). Two NER sub-pathways that have been identified are as follows: the global genome NER (GG-NER) that detects and removes lesions throughout the genome, and the transcription-coupled NER (TC-NER), which repairs actively transcribed genes. NER begins with the recognition of the DNA lesion, followed by incisions at sites flanking the DNA lesion, and culminates in the removal of the oligonucleotide containing the DNA lesion. Ligation of a newly synthesized oligonucleotide, complementary to the pre-existing strand, serves to fill the gap, thus ending the NER process. GG-NER and TC-NER involve several common proteins and proceed through the same repair steps, except during recognition of the DNA lesion. In GG-NER this recognition involves the XPC–RAD23B and DDB1–DDB2/XPE proteins, whereas recognition in TC-NER is mediated by cockayne syndrome group A (CSA) (ERCC8) and CSB (ERCC6). NER has attracted a great deal of attention due to its role in three rare human syndromes characterized by increased cancer frequencies, neurodegeneration and ageing (Figure 2). These syndromes are xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD) (Thoms et al, 2007). XP individuals show extremely severe skin sensitivity to short intervals of sun exposure and most develop freckles at an early age. In addition, XP individuals may exhibit eye damage as they suffer chronic UV-induced conjunctivitis and keratitis as a consequence of continual sun exposure. XP individuals have greater than 1000-fold increased skin cancer risk, which first appears at an average age of 10 years. Approximately 20% of XP individuals also develop neurological abnormalities. XP is caused by mutations of the NER gene XPA, XPB (ERCC3), XPC, XPD (ERCC2), XPE (DDB2), or XPF; XPG. Whereas XP individuals carrying mutations of XPC or XPE (DDB2) are only deficient in GG-NER, the remaining XP individuals are deficient in both GG-NER and TC-NER (Thoms et al, 2007). CS is a very rare human autosomal recessive inherited genetic disease (Thoms et al, 2007). Similar to XP individuals, CS individuals suffer excessive sun sensitivity, but without increased predisposition for skin cancer. Common CS symptoms include growth retardation (dwarfism), progressive cognitive impairment, and ophthalmologic disorders such as cataracts or retinitis pigmentosa. CS individuals typically die in the first or second decade of life. CS is caused by mutations of either CSA or CSB, two proteins essential for DNA-damage recognition and initiation of TC-NER. Therefore, although CS individuals are deficient in TC-NER, they remain proficient in GG-NER. TTD is a rare human autosomal recessive disorder associated with defective NER; the most severe cases are associated with mutations in the XPB or XPD genes. Clinical characteristics of TTD include brittle hair and nails, dwarfism, and ataxia (Thoms et al, 2007). In addition, half of TTD individuals exhibit sensitivity to sunlight; however, skin cancer predisposition has not been linked to this syndrome. Several mouse models for NER mutations have been generated (Table III). Homozygous mutants for XP genes are viable (Nakane et al, 1995; Sands et al, 1995; Harada et al, 1999; Itoh et al, 2004; Tian et al, 2004a, 2004b; Yoon et al, 2005), with the exception of the pre-implantation embryonic lethality of Xpd mutants (de Boer et al, 1998b). XpdR722W-mutant mice carrying an amino-acid substitution that mimics a human XPD allele associated with TTD have been generated (de Boer et al, 1998a). Similarly, XpdG602D-mutant mice carrying a substitution at amino acid 602 have been generated to mimic the human combined XP/CS (Andressoo et al, 2006). Both XpdR722W and XpdG602D mutants have been proven viable and reproduced some of the characteristics of individuals that carry these XPD mutations. Table 3. Examples of mouse models for the NER pathway Genotype Developmental defects Fertility defects Induced tumorigenesis References Xpd−/− Pre-implantation embryonic lethality NA NA (de Boer et al, 1998b) XpdR722W/R722W Growth retardation None UV- and DMBA-induced skin cancer (de Boer et al, 1998a, 1999) XpdG602D/G602D Growth retardation None Early onset of UV-induced skin and/or eye tumours (Andressoo et al, 2006) Xpe−/− None None UV-induced skin cancer (Itoh et al, 2004; Yoon et al, 2005) Xpa−/− None None UV- and DMBA-induced skin cancer (Nakane et al, 1995) Xpc−/− None None UV-induced skin cancer (Sands et al, 1995) Xpg−/− Growth retardation and premature death NA NA (Harada et al, 1999) Ddb1−/− Early embryonic lethality NA NA (Cang et al, 2006) HR23A−/− Unaffected None NT (Ng et al, 2002) HR23B−/− Intrauterine/neonatal death. 10% viable but growth retarded Male infertility NT (Ng et al, 2002) HR23A−/−; HR23B−/− Embryonic lethality NA NA (Ng et al, 2003) Csa−/− None None UV-induced skin cancer (van der Horst et al, 1997) Csb−/− None None UV-induced skin cancer (van der Horst et al, 2002) Ercc1−/− Growth retardation and death before weaning NA NA (McWhir et al, 1993; Weeda et al, 1997) Xpf−/− Growth retardation and death before weaning NA NA (Tian et al, 2004b) NA, not applicable; NT, not tested. With the exception of Xpe (Ddb2) mutants (Itoh et al, 2004; Yoon et al, 2005), homozygous mutant mouse cells for Xpa (Nakane et al, 1995), Xpc (Sands et al, 1995), Xpf (Tian et al, 2004b), Xpg (Harada et al, 1999; Tian et al, 2004a), XpdR722W (de Boer et al, 1998a), or XpdG602D (Andressoo et al, 2006) were all UV sensitive, correlating to the results obtained from human mutant cells. Increased predisposition for UV-induced skin cancer was observed with Xpa, Xpc, XpdR722W, XpdG602D, and Xpe (Ddb2)-mutant mice (Nakane et al, 1995; Sands et al, 1995; de Boer et al, 1999; Itoh et al, 2004; Yoon et al, 2005; Andressoo et al, 2006). Thus, as seen in humans, XP proteins play a major role in murine NER. DDB1 and HR23B are two proteins involved with XPC in the recognition of DNA damage and the initiation of GG-NER. However, in contrast to Xpc−/− mice (Sands et al, 1995), Ddb1 mutants die during early embryonic development (Cang et al, 2006). Inactivation of Ddb1 in developing CNS and lens resulted in massive p53-dependent apoptosis of dividing cells and lethality just after birth (Cang et al, 2006). MEFs deficient in Ddb1 showed defective proliferation and were UV-sensitive. A total of 90% of HR23B mutants suffer intrauterine or neonatal death (Ng et al, 2002). The surviving HR23B−/− mice were growth retarded and males were sterile; however, their NER and UV sensitivity rem
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