**2.2 Life style factors and DNA adduct formation**

540 DNA Repair

occurring in persons older than 50 years (Schottenfeld & Winawer, 1996) . The mean age at time for diagnosis in Danish colorectal cancer patients is approximately 70 years for men and 72 years for women (Iversen et al., 2005) . The disease develops either sporadically, as a part of a hereditary cancer syndrome, or induced by inflammatory bowel disease. Ten to fifteen percent of colorectal cancer cases are caused by hereditary syndromes (Schottenfeld

Migrant studies and large international variation in incidence rates indicate that life style factors, including dietary, are associated with risk of colorectal cancer, but traditional epidemiological studies based on life style questionnaires and outcome have mostly failed in identifying the exact risk and beneficial factors. Our current knowledge of colorectal carcinogenesis indicates a multi-factorial and multi-step process that involves various genetic alterations and several biological pathways. An understanding of differences in individual susceptibility and better exposure assessment may be crucial in identifying life style risk factors and possible interactions between susceptibility and exposures in relation

Several life style factors and dietary components are suggested to be associated with risk of colorectal cancer, listed in Table 1. The associations may possibly be caused by increased

N-nitroso compounds (NOCs) are present in tobacco smoke and in nitrate- or nitrite-treated meats (Hotchkiss, 1989; Hecht & Hoffmann, 1988). NOCs are alkylating agents able to react with DNA and form adducts. More than 85% of 300 NOCs tested for carcinogenicity in experimental animals were observed to be carcinogenic (Mirvish, 1995), but epidemiologic studies have been inconclusive in finding association between the exposure of NOCs and risk of various cancer forms in humans (Burch et al., 1987; Preston-Martin & Mack, 1991; Carozza et al., 1995), although an increased endogenous production of NOCs, suggested primarily by bacterial catalysis, are proposed associated to the etiology of colorectal cancer

Polycyclic aromatic hydrocarbons (PAHs) and heterocyclic aromatic amines (HCAs) constitute a major class of chemical carcinogens present in the environment. When metabolically activated, these compounds act as mutagens and carcinogens in animal models (Culp et al., 1998; Moller et al., 2002; Dingley et al., 2003) and are able to form bulky DNA adducts in humans (Hecht, 2003), (Phillips, 2002) . Many PAHs and HCAs are found to be tumourigenic in humans or experimental animals (International Agency for Research on Cancer (IARC), 1983). Cooking meat at high temperatures and certain preservation and processing procedures leads to the formation of PAHs and HCAs (Sinha et al., 2005; Guillen et al., 1997) . PAHs are ubiquitous environmental contaminants formed by incomplete combustion of organic matter. They are one of several classes of carcinogenic chemicals present in tobacco smoke (Benhamou et al., 2003; Melikian et al., 1999). PAH compounds may not only be formed by high cooking temperatures but are also found in uncooked food, like sea food and plants, due to contamination of the aquatic environment (Meador et al.,

& Winawer, 1996) .

to risk of colorectal cancer.

formation of DNA adducts.

**2.1 NOC, HCA and PAH** 

(Bingham et al., 1996).

1995) or via atmospheric exposure (Guillen et al., 1997).

**2. DNA adducts** 

Air pollution is not an established risk factor for colorectal cancer in humans, although several studies have shown higher risk among workers exposed to diesel exhaust (Goldberg et al., 2001). Some studies have found an association between ambient air pollution and DNA adduct levels (Poirier et al., 1998; Hemminki et al., 1990b; Binkova et al., 1995; Palli et al., 2001; Nielsen et al., 1996a; Nielsen et al., 1996c), whereas others failed to find such an association (Kyrtopoulos et al., 2001; Peluso et al., 1998). DNA adduct levels are increased following occupational exposure among foundry and coke oven workers and among workers exposed to diesel exhaust (Hemminki et al., 1997; Hemminki et al., 1990a; Hemminki et al., 1994; Perera et al., 1988; Perera et al., 1994; Lewtas et al., 1997; Nielsen et al., 1996a; Nielsen et al., 1996b), while among fire-fighters (Rothman et al., 1993), traffic exposed policemen (Peluso et al., 1998) and aluminium workers (Yang et al., 1998), no associations between occupational exposures and DNA adducts have been found.

Tobacco smoking is an established risk factor for development of adenomas (Ji et al., 2006), and recently an association between tobacco smoking and risk of colorectal cancer has been recognized by IARC. Following tobacco smoking, adducts formed by metabolites of NOCs and PAHs are not only located in airway tissue, but are also found in bladder and cervical tissue from smokers (Benhamou et al., 2003; Melikian et al., 1999).


Table 1. Possible environmental risk and beneficial factors of colorectal cancer and their association with DNA adduct formation. Arrows indicate adverse (↑) or preventive (↓) association with risk of colorectal cancer.

A growing body of evidence supports that avoidance of alcohol is recommended to prevent colorectal cancer (Correa Lima & Gomes-da-Silva, 2005). Acetaldehyde is the primary oxidative metabolite of ethanol. Acetaldehyde and malondialdehyde, the end-product of lipid peroxidation by reactive oxygen species, can combine to form the malondialdehydeacetaldehyde adduct, which is very reactive and avidly binds to DNA (Brooks & Theruvathu, 2005). The level of acetaldehyde DNA adducts in white blood cell DNA in alcohol abusers have been measured up to 13-fold higher than in subjects from the nondrinking control group (Fang & Vaca, 1997).

Polymorphisms in Nucleotide Excision Repair

Genes and Risk of Colorectal Cancer: A Systematic Review 543

Hanawalt, 2002; Volker et al., 2001). The hHR23B (also called Rad23) NER factor co-purifies with XPC (Masutani et al., 1994) and is essential for high XPC activity in NER (Batty et al., 2000; Guzder et al., 1998). XPC-hHR23B complex exhibit a very strong affinity for damaged DNA (Reardon et al., 1996; Batty et al., 2000; Sugasawa et al., 1998), why it is thought to be the initiator in GG-NER. By interaction with the XPC complex XPA and the transcription factor II H (TFIIH) may be recruited to the damaged DNA site (You et al., 2003; Volker et al., 2001). TFIIH is a nine sub-unit protein complex required for opening the DNA helix at the vicinity of the lesion (Schaeffer et al., 1993; Feaver et al., 1993; Drapkin et al., 1994). Biochemical studies have generated conflicting results with regard to association between the XPC-hHR23B complex, XPA and TFIIH. Some have found recruitment of TFIIH to the site of DNA damage to be dependent on XPC (Volker et al., 2001; Yokoi et al., 2000), while others have found XPA to be interacting with TFIIH (Park et al., 1995). Undoubtedly, both XPC and XPA are vital factors in the very early steps of GG-NER, but exactly when

Fig. 1. A proposed molecular mechanism of damage recognition process in the early stage of global genome nucleotide excision repair. Transient steps are indicated with brackets.

Adapted from (You et al., 2003) .

There is some evidence for adverse associations between intake of red and processed meat and risk of colorectal cancer (Johnson & Lund, 2007; Doyle, 2007; Norat et al., 2005). The elevated risk may be due to an increased endogenous production of NOC, which may enhance the colonic formation of the DNA adduct O6-carboxymethyl guanine (Bingham et al., 1996; Lewin et al., 2006). Cooking meat at high temperatures leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and heterocyclic amines (HCAs) (Sinha et al., 2005). Additionally, intake of charbroiled or smoked meat may be associated with increased levels of DNA adducts (Rothman et al., 1990; van Maanen et al., 1994; Georgiadis et al., 2001; Rothman et al., 1993), due to HCAs and PAHs (Bruemmer et al., 1996; Balbi et al., 2001; Peters et al., 2004; Skog et al., 1995). The levels of some HCAs and PAHs are comparable for red meat, fish and poultry smoked or cooked at high temperatures (Sinha et al., 1995; Gomaa et al., 1993). Intake of red meat, but not of fish and poultry, increases the luminal contents of N-nitrosocompounds (NOCs) in colon (Bingham et al., 1996; Lewin et al., 2006). The increase in endogenous *N*-nitrosation can be attributed to heme iron (Cross et al., 2003), which is 10-fold higher in red meat than in white meat (Pierre et al., 2003).

There is limited evidence for a preventive effect of intake of fruit and vegetables for cancer in colon and rectum (International Agency for Research on Cancer (IARC), 2003). Intake of fruit, vegetables or antioxidant vitamins have been shown to be negatively associated with DNA adduct levels (Palli et al., 2000; Mooney et al., 1997; Palli et al., 2003; Palli et al., 2004), although some studies found no effect (Georgiadis et al., 2001; Nielsen et al., 1996b) and one study found an effect of increased vitamin intake only in females (Mooney et al., 2005).

### **3. Nucleotide excision repair**

The nucleotide excision repair (NER) pathway is the primary mechanism for removal of helix-distorting damages from DNA, including bulky adducts and UV-induced photolesions. The mechanism of NER includes five steps: 1. Damage recognition, 2. Assembly of the repair factors at the site of damage, 3. Dual incisions and excision of the damage-containing oligomers, 4. Resynthesis to fill in the gap, and 5. Ligation of the strands. All these steps involve more than 20 proteins, like recognition factors, replication protein, transcription factor, helicases, endonucleases and polymerases. Steps 1 and 2 are illustrated in Figure 1.

#### **3.1 The NER pathway**

There are two sub-pathways of NER, termed the global genome NER (GG-NER), which corrects lesions in the entire genome including the non-transcribed strands of active genes, and transcription-coupled NER (TC-NER), that only repairs lesions in transcribed strands in active genes. The major differences of the two pathways are the damage recognition step: In GG-NER the proteins Xeroderma Pigmentosum complementation group A and C (XPA/XPC) make the recognition complex (Hanawalt, 2002; Reardon & Sancar, 2002; You et al., 2003; Volker et al., 2001), while in TC-NER a stalled RNA polymerase II (blocked by a lesion) and Cockayne syndrome proteins have this function to act as a signal to recruit NER proteins (Kobayashi et al., 2005; Hanawalt, 2002).

In global genomic NER the XPA and XPC enzymes are involved in the damage recognitioncomplex of NER. Several studies have shown the XPC-hHR23B complex to function at a very early stage of DNA damage recognition (Reardon & Sancar, 2002; You et al., 2003;

There is some evidence for adverse associations between intake of red and processed meat and risk of colorectal cancer (Johnson & Lund, 2007; Doyle, 2007; Norat et al., 2005). The elevated risk may be due to an increased endogenous production of NOC, which may enhance the colonic formation of the DNA adduct O6-carboxymethyl guanine (Bingham et al., 1996; Lewin et al., 2006). Cooking meat at high temperatures leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and heterocyclic amines (HCAs) (Sinha et al., 2005). Additionally, intake of charbroiled or smoked meat may be associated with increased levels of DNA adducts (Rothman et al., 1990; van Maanen et al., 1994; Georgiadis et al., 2001; Rothman et al., 1993), due to HCAs and PAHs (Bruemmer et al., 1996; Balbi et al., 2001; Peters et al., 2004; Skog et al., 1995). The levels of some HCAs and PAHs are comparable for red meat, fish and poultry smoked or cooked at high temperatures (Sinha et al., 1995; Gomaa et al., 1993). Intake of red meat, but not of fish and poultry, increases the luminal contents of N-nitrosocompounds (NOCs) in colon (Bingham et al., 1996; Lewin et al., 2006). The increase in endogenous *N*-nitrosation can be attributed to heme iron (Cross et al., 2003),

There is limited evidence for a preventive effect of intake of fruit and vegetables for cancer in colon and rectum (International Agency for Research on Cancer (IARC), 2003). Intake of fruit, vegetables or antioxidant vitamins have been shown to be negatively associated with DNA adduct levels (Palli et al., 2000; Mooney et al., 1997; Palli et al., 2003; Palli et al., 2004), although some studies found no effect (Georgiadis et al., 2001; Nielsen et al., 1996b) and one study found an effect of increased vitamin intake only in females (Mooney et al., 2005).

The nucleotide excision repair (NER) pathway is the primary mechanism for removal of helix-distorting damages from DNA, including bulky adducts and UV-induced photolesions. The mechanism of NER includes five steps: 1. Damage recognition, 2. Assembly of the repair factors at the site of damage, 3. Dual incisions and excision of the damage-containing oligomers, 4. Resynthesis to fill in the gap, and 5. Ligation of the strands. All these steps involve more than 20 proteins, like recognition factors, replication protein, transcription factor, helicases, endonucleases and polymerases. Steps 1 and 2 are illustrated

There are two sub-pathways of NER, termed the global genome NER (GG-NER), which corrects lesions in the entire genome including the non-transcribed strands of active genes, and transcription-coupled NER (TC-NER), that only repairs lesions in transcribed strands in active genes. The major differences of the two pathways are the damage recognition step: In GG-NER the proteins Xeroderma Pigmentosum complementation group A and C (XPA/XPC) make the recognition complex (Hanawalt, 2002; Reardon & Sancar, 2002; You et al., 2003; Volker et al., 2001), while in TC-NER a stalled RNA polymerase II (blocked by a lesion) and Cockayne syndrome proteins have this function to act as a signal to recruit NER

In global genomic NER the XPA and XPC enzymes are involved in the damage recognitioncomplex of NER. Several studies have shown the XPC-hHR23B complex to function at a very early stage of DNA damage recognition (Reardon & Sancar, 2002; You et al., 2003;

which is 10-fold higher in red meat than in white meat (Pierre et al., 2003).

**3. Nucleotide excision repair** 

in Figure 1.

**3.1 The NER pathway** 

proteins (Kobayashi et al., 2005; Hanawalt, 2002).

Hanawalt, 2002; Volker et al., 2001). The hHR23B (also called Rad23) NER factor co-purifies with XPC (Masutani et al., 1994) and is essential for high XPC activity in NER (Batty et al., 2000; Guzder et al., 1998). XPC-hHR23B complex exhibit a very strong affinity for damaged DNA (Reardon et al., 1996; Batty et al., 2000; Sugasawa et al., 1998), why it is thought to be the initiator in GG-NER. By interaction with the XPC complex XPA and the transcription factor II H (TFIIH) may be recruited to the damaged DNA site (You et al., 2003; Volker et al., 2001). TFIIH is a nine sub-unit protein complex required for opening the DNA helix at the vicinity of the lesion (Schaeffer et al., 1993; Feaver et al., 1993; Drapkin et al., 1994). Biochemical studies have generated conflicting results with regard to association between the XPC-hHR23B complex, XPA and TFIIH. Some have found recruitment of TFIIH to the site of DNA damage to be dependent on XPC (Volker et al., 2001; Yokoi et al., 2000), while others have found XPA to be interacting with TFIIH (Park et al., 1995). Undoubtedly, both XPC and XPA are vital factors in the very early steps of GG-NER, but exactly when

Fig. 1. A proposed molecular mechanism of damage recognition process in the early stage of global genome nucleotide excision repair. Transient steps are indicated with brackets. Adapted from (You et al., 2003) .

Polymorphisms in Nucleotide Excision Repair

with defective NER and suggests novel functions for ERCC1.

possibly be associated with development of cancer.

were identified. The studies are listed in Table 2.

the effect of smoking on risk of colorectal cancer.

**3.2.1** *XPD* **Lys751Gln and** *XPD* **Asp312Asn** 

Genes and Risk of Colorectal Cancer: A Systematic Review 545

skeletal syndrome. This discovery represents a novel complementation group of patients

Overall, the above mentioned studies of the polymorphisms in the genes involved in NER, *XPD* Lys751Gln*, XPD* Asp312Asn*, XPA* G23A*, XPC* Lys939Gln, and *ERCC1* Asn118Asn, indicate that the polymorphisms may modulate DNA repair capacity and may thereby

There are limited numbers of studies of NER genes in relation to risk of colorectal cancer. A search on the PubMed database of NCBI on January 26th 2011 on the MeSH terms "polymorphism, single nucleotide AND colorectal neoplasms" resulted in 148 hits of which seven studies included polymorphisms in *XPD, XPA, XPC,* and *ERCC1*. In combination with a new search on the PubMed database of NCBI by using different combinations of the words: "XPD XPA XPC ERCC1 polymorphism colorectal colon rectum cancer" 19 studies of SNPs in the four genes in relation to risk of colorectal cancer or prestages to colorectal cancer

The *XPD* Lys751Gln polymorphism is the most frequently studied of the NER polymorphisms in association with risk of cancer. In our Danish prospective study on the Diet, Cancer and Health cohort, we observed no association of the *XPD* Lys751Gln and *XPD* Asp312Asn polymorphisms with risk of colorectal cancer (Hansen et al., 2007). Previously, several studies had similar findings of no association between the *XPD*  Lys751Gln (Moreno et al., 2006; Huang et al., 2006; Berndt et al., 2006; Mort et al., 2003; Starinsky et al., 2005; Skjelbred et al., 2006b; Engin et al., 2010; Stern et al., 2009; Stern et al., 2007; Yeh et al., 2005; Joshi et al., 2009; Wang et al., 2010) and the *XPD* Asp312Asn (Moreno et al., 2006; Huang et al., 2006; Berndt et al., 2006; Goodman et al., 2006; Stern et al., 2009; Stern et al., 2007; Joshi et al., 2009) polymorphisms and risk of colorectal cancer. Additionally, Bigler and colleagues found no association of the two polymorphisms with development of adenomas (Bigler et al., 2005). However, they detected a higher risk of colorectal adenomas among individuals with at least two variant alleles of the *XPD* polymorphisms, with an OR of 1.57 (CI: 1.04-2.38). When stratifying by age the association of the two polymorphisms with risk of adenomatous polyps was restricted to the individuals younger than 60 years when diagnosed (OR=3.77, CI: 1.94-7.35). The risk of adenomatous polyps was higher among smokers carrying the homozygous *XPD* variant alleles (OR=3.93, OR: 1.68-9.21) compared with non-smokers carrying the homozygous wild type. A similar finding could not be detected on risk of hyper-plastic polyps. In our Danish study (Hansen et al., 2007) and in a Singapore Chinese study (Stern et al., 2007) did neither of the two XPD polymorphisms, *XPD* Lys751Gln or *XPD* Asp312Asn, modify

Goodman *et al.*, did not detect any SNP-SNP interaction between the *XPD* Asp312Asn polymorphism and other NER polymorphisms (Goodman et al., 2006). Skjelbred and colleagues detected an association between the *XPD* Lys751Gln polymorphism and development of colorectal adenomas, with an OR of 1.40 (CI: 1.08-1.81), among carriers of the variant allele compared to carriers of the homozygous wild type allele (Skjelbred et al., 2006b). The statistical significance was limited to the low-risk adenoma group (OR: 1.46, CI: 1.11-1.90). The results were contradicted by a large study by Stern *et al.,* including 740 cases with adenomas and 789 controls, where a lower risk of adenomas was observed (OR=0.7, CI:

XPA enters the site of damage is not clear. XPA physically interacts with replication factor A (RPA) and is essential to efficient NER (Stigger et al., 1998) by stabilizing the interaction between XPA and the damaged DNA. XPA is capable of binding to the XPF-ERCC1 complex with very high affinity (Park & Sancar, 1994). The XPF-ERCC1 is a specific 5´ endonuclease complex, and thus must be located near the site of 5´ incision (Niedernhofer et al., 2001). XPG, a 3´ endonuclease, seems to be the next factor recruited to the site, and is probably positioned at the 3´ incision site (Reardon & Sancar, 2002). Previous studies have observed XPG to co-purify with TFIIH, like XPC, and that XPG exclude XPC when binding to TFIIH (Wakasugi & Sancar, 1999; Wakasugi & Sancar, 1998), which may suggest that the binding of XPG to the NER complex displaces XPC. Hence, XPA is thought to be crucial to the subsequent positioning of the involved NER enzymes by binding to XPF-ERCC1 complex and possibly recruit XPG to the site of DNA damage. XPD and XPB are helicases and parts of the large TFIIH complex. They participate in the unwinding of helix in opposite directions of the region of damaged DNA (Reardon & Sancar, 2002; Schaeffer et al., 1993). When the DNA around the DNA lesion is unwound, the endonucleases XPG and XPF-ERCC1 complex excises an oligonucleotide of 24-32 bases including the damaged site (Mu et al., 1996). The two endonucleases require an opening of approximately 5-8 bases (Evans et al., 1997; de Laat et al., 1998). The final steps of NER are re-synthesis of the strand to fill in the gap and ligation of the new strand with the remaining strand. In mammals the synthesis requires the DNA polymerases δ and/or ε (Hunting et al., 1991; Coverley et al., 1992), the replication protein A (RPA) and replication factor C (RFC) (Shivji et al., 1995) and proliferating cell nuclear antigen (PCNA) (Shivji et al., 1992). The XPF-ERCC1 5´ incision leaves a hydroxyl-group at the 3´ terminus of the gap. This terminus may act as a DNA primer for DNA polymerases (Sijbers et al., 1996). RPA is required for the gap-filling DNA synthesis (Shivji et al., 1995), possibly to protect the template strand against nucleases, and RFC and PCNA as a complex that facilitates the assembly of the polymerases (Shivji et al., 1992). The new fragment of DNA is synthesized and the final step is ligation of the new patch to the original sequence, which possibly may be performed by DNA ligase I (Tomkinson & Levin, 1997).

#### **3.2 SNPs in NER genes and colorectal cancer risk**

The variant alleles of *XPA* G23A (Wu et al., 2003), *XPD* Asp312Asn and *XPD* Lys751Gln (Spitz et al., 2001; Qiao et al., 2002) polymorphisms and a polymorphism in *XPC* (Qiao et al., 2002), in full linkage disequilibrium with the *XPC* Lys939Gln polymorphism (Khan et al., 2000), have been associated with a lowered DNA repair capacity compared to the wild type allele. *ERCC1* gene polymorphism is a predictor for clinical outcome in advanced colorectal cancer patients treated with platinum-based chemotherapy (Viguier et al., 2005). Furthermore, the variant alleles of the polymorphisms *XPD* Asp312Asn and *XPD* Lys751Gln have been associated with higher DNA adduct levels (Hou et al., 2002; Matullo et al., 2001; Palli et al., 2001) than the wild type alleles.

Mutations in the NER gene *XPD* are associated with the rare, autosomal-recessive inherited disorder Xeroderma Pigmentosum, where patients suffer from severe photosensitivity and actinic changes leading to early onset of skin cancers induced by sunlight (Cleaver, 2005). Recently the first case of human inherited ERCC1 deficiency was reported (Jaspers et al., 2007). Cells from the patient showed moderate hypersensitivity to ultraviolet rays, but the clinical features were very severe and compatible with a diagnosis of cerebro-oculo-facio-

XPA enters the site of damage is not clear. XPA physically interacts with replication factor A (RPA) and is essential to efficient NER (Stigger et al., 1998) by stabilizing the interaction between XPA and the damaged DNA. XPA is capable of binding to the XPF-ERCC1 complex with very high affinity (Park & Sancar, 1994). The XPF-ERCC1 is a specific 5´ endonuclease complex, and thus must be located near the site of 5´ incision (Niedernhofer et al., 2001). XPG, a 3´ endonuclease, seems to be the next factor recruited to the site, and is probably positioned at the 3´ incision site (Reardon & Sancar, 2002). Previous studies have observed XPG to co-purify with TFIIH, like XPC, and that XPG exclude XPC when binding to TFIIH (Wakasugi & Sancar, 1999; Wakasugi & Sancar, 1998), which may suggest that the binding of XPG to the NER complex displaces XPC. Hence, XPA is thought to be crucial to the subsequent positioning of the involved NER enzymes by binding to XPF-ERCC1 complex and possibly recruit XPG to the site of DNA damage. XPD and XPB are helicases and parts of the large TFIIH complex. They participate in the unwinding of helix in opposite directions of the region of damaged DNA (Reardon & Sancar, 2002; Schaeffer et al., 1993). When the DNA around the DNA lesion is unwound, the endonucleases XPG and XPF-ERCC1 complex excises an oligonucleotide of 24-32 bases including the damaged site (Mu et al., 1996). The two endonucleases require an opening of approximately 5-8 bases (Evans et al., 1997; de Laat et al., 1998). The final steps of NER are re-synthesis of the strand to fill in the gap and ligation of the new strand with the remaining strand. In mammals the synthesis requires the DNA polymerases δ and/or ε (Hunting et al., 1991; Coverley et al., 1992), the replication protein A (RPA) and replication factor C (RFC) (Shivji et al., 1995) and proliferating cell nuclear antigen (PCNA) (Shivji et al., 1992). The XPF-ERCC1 5´ incision leaves a hydroxyl-group at the 3´ terminus of the gap. This terminus may act as a DNA primer for DNA polymerases (Sijbers et al., 1996). RPA is required for the gap-filling DNA synthesis (Shivji et al., 1995), possibly to protect the template strand against nucleases, and RFC and PCNA as a complex that facilitates the assembly of the polymerases (Shivji et al., 1992). The new fragment of DNA is synthesized and the final step is ligation of the new patch to the original sequence, which possibly may be

The variant alleles of *XPA* G23A (Wu et al., 2003), *XPD* Asp312Asn and *XPD* Lys751Gln (Spitz et al., 2001; Qiao et al., 2002) polymorphisms and a polymorphism in *XPC* (Qiao et al., 2002), in full linkage disequilibrium with the *XPC* Lys939Gln polymorphism (Khan et al., 2000), have been associated with a lowered DNA repair capacity compared to the wild type allele. *ERCC1* gene polymorphism is a predictor for clinical outcome in advanced colorectal cancer patients treated with platinum-based chemotherapy (Viguier et al., 2005). Furthermore, the variant alleles of the polymorphisms *XPD* Asp312Asn and *XPD* Lys751Gln have been associated with higher DNA adduct levels (Hou et al., 2002; Matullo et al., 2001; Palli et al., 2001) than the

Mutations in the NER gene *XPD* are associated with the rare, autosomal-recessive inherited disorder Xeroderma Pigmentosum, where patients suffer from severe photosensitivity and actinic changes leading to early onset of skin cancers induced by sunlight (Cleaver, 2005). Recently the first case of human inherited ERCC1 deficiency was reported (Jaspers et al., 2007). Cells from the patient showed moderate hypersensitivity to ultraviolet rays, but the clinical features were very severe and compatible with a diagnosis of cerebro-oculo-facio-

performed by DNA ligase I (Tomkinson & Levin, 1997).

**3.2 SNPs in NER genes and colorectal cancer risk** 

wild type alleles.

skeletal syndrome. This discovery represents a novel complementation group of patients with defective NER and suggests novel functions for ERCC1.

Overall, the above mentioned studies of the polymorphisms in the genes involved in NER, *XPD* Lys751Gln*, XPD* Asp312Asn*, XPA* G23A*, XPC* Lys939Gln, and *ERCC1* Asn118Asn, indicate that the polymorphisms may modulate DNA repair capacity and may thereby possibly be associated with development of cancer.

There are limited numbers of studies of NER genes in relation to risk of colorectal cancer. A search on the PubMed database of NCBI on January 26th 2011 on the MeSH terms "polymorphism, single nucleotide AND colorectal neoplasms" resulted in 148 hits of which seven studies included polymorphisms in *XPD, XPA, XPC,* and *ERCC1*. In combination with a new search on the PubMed database of NCBI by using different combinations of the words: "XPD XPA XPC ERCC1 polymorphism colorectal colon rectum cancer" 19 studies of SNPs in the four genes in relation to risk of colorectal cancer or prestages to colorectal cancer were identified. The studies are listed in Table 2.
