X-ray Repair Cross Complementing Protein (XRCC1) encodes a protein which is found to involve in the DNA base excision repair (BER) [1, b]. BER brings up the various constituents like DNA polymerase β, DNA ligase III and PARP of the BER to the location where the DNA has been damaged and improves the efficiency of the BER pathway [1, 3]. It is associated with numerous human cancers. It interacts with various DNA glycosylases such as hOGG1, MPG, hNTH1 and NEIL1 to participate in the first step of the BER. It has the NTD and BRCT1 domains which has the affinity to form the covalent complex with the AP sites with the help of the Schiff base. It is found to have higher affinity when it carries AP-lyase or APE1-incised AP site in its DNA and thus the overall repair process of damaged base pair is accelerated and it can be used as a substrate for the DNA pol β. The repaired DNA strand can be resealed with the help of its role in the multiple protein-protein interaction [9].
Structure of XRCC1
XRCC1 is located on the chromosome 19q13.2 and has 17 exons [1]. It encodes a protein with 633 amino acids [10]. It is a homodimer protein with the molecular weight of about 69526 Da [f]. The N-terminal domain (NTD) region of the gene binds with DNA polymerase beta (pol β) very efficiently. It has been identified from the mutagenic study that the epitopic region of XRCC1 gets involved in the complex formation of pol β. The XRCC1-NTD complexed with pol β creates an unexpected variation in the structure of the XRCC1-NTD. The oxidized form of the XRCC1-NTD has more affinity towards the poly β and the formation of the disulphide bond regulates the repair pathway. Thus XRCC1 has an indirect role in the DNA repair [2].
[c]
Functions of XRCC1
XRCC1 protein is essential for the viability of the human body [10]. The protein product of the XRCC1 gene is used to repair the DNA strand break which is induced by the hydrolysis and the exchange of the sister-chromatids which is formed by the exposure of the ionizing radiation, alkylating agents and the chemicals present in the smoke [1, a, b, 3, 4]. Due to the absence of the DNA repair process, adducts are formed which halts the DNA replication or the cytotoxic mutation and instability of the gene [1]. It also gets involved in the BER pathway by interacting with DNA ligase III, polyadenosine diphosphate-ribose polymerases (PARP) 1 and 2, polynucleotide kinase, apurinic/apyrimidinic endonuclease 1 (APE1) and pol β [b, 2, 10]. It is found to have a minor function in the DNA processing during meiogenesis and recombination in germ cells [b]. It codes for an Mγ 70,000 protein [3]. Absence of this gene leads to the reduction in the level of its partner ligase III [10].
BER pathway
BER pathway is a multi-step process which is used to remove the damaged base pairs in the DNA caused by the oxidation, methylation, deamination or the loss of the DNA base pair. These alterations are known to be mutagenic. It has two subpathways which are initiated by the action of the DNA glycosylase. Barnes and Lindhal reported that eleven different glycosylases were found in the mammalian cells [10]. The DNA glycosylase such as the 8-oxoguanine glycosylase (OGG1) is used to cleave the N-glycosidic bond between the damaged base pair and the sugar phosphate backbone of the DNA and this cleavage creates an apurinic/ apyrimidinic (AP) or abasic sites in the DNA. In humans, eight DNA glycosylases has been found with to have the partially overlapped bases. The AP sites can also be formed by the hydrolysis of the N-glycosidic bond. In both the cases, AP Endonuclease 1 (APE1) cleaves the phosphodiester bond at 5' end of the AP site which results in the 3' hydroxyl group and a transient 5' abasic deoxyribose phosphate (dRP). The dRP can be removed by the action of the DNA pol β which acts by removing the dRP moiety by means of its AP lyase activity and by adding one nucleotide to the 3' end of the nick. Finally, the strand nicks are sealed by the activity of the DNA ligase that restores the integrity of the DNA. The above process of replacing the damaged base pair with the new base pair is known as "short-patch" repair and it represents approximately 80-90% of all BER. The other subpathway of BER is known as the "long-patch" repair pathway which is used only when the modified basepair is resistant to the AP lyase activity of the DNA pol β present in the DNA. It leads to the replacement of 2-10 damaged bases. It also requires some of the factors such as DNA glycosylase, APE1 and DNA pol β which has involved in the "short-patch" repair pathway but the long-patch repair is a PCNA-dependent pathway in which several base pairs can be added to the repair gap by displacing the dRP as a part of a "flap" oligonucleotide with the help of the DNA polymerase. The overhanging oligonucleotides are sealed by the Flap endonuclease FEN-1 whose nick has been formerly sealed by the DNA ligase [d].
BER pathway [d]
Short and long patch pathway
XRCC1 mutation and cancer risk
XRCC1 is one of the genes which get involved in the BER pathways. BER is the main pathway which guards the DNA from the damages caused by UV, ionizing radiation, cellular metabolism, including the reactive oxygen species, methylation, deamination and hydroxylation. XRCC1 is used in the BER by interacting with the DNA repair proteins like PARP, DNA ligase III and DNA pol β. It is found to have eight nonsynonymous SNPs. Among which three of the polymorphism is found to be common and it includes the amino acid substitutions of Arg194 to Trp (C to T) of exon 6, Arg280 to His (G to A) of exon 9 and Arg399 to Gln (G to A) of exon 10. The Arg399 to Gln polymorphism is found in the BRCT-1 interaction domain of XRCC1 within the region of PARP and this polymorphism is related to the cancer risk. Variation in the Gln399 allele decreases the DNA repair capacity of XRCC1 which is due to the formation of the DNA adducts, high level of the sister chromatid exchange, increased RBC glycophorin A, p53 mutations and prolonged delay in the cell cycle. The variations in the residue Arg194 and 280 is known to occur in the proliferating cell nuclear antigen binding region which contains polar Pro, Ser and Arg/Lys rich regions. The function of XRCC1 is altered by the transition of the positively charged amino acid Arg to the hydrophobic Trp within the conserved region. It was reported by Wang et al that the variations in the Trp194 allele had fewer belomycinor benzo[a]pyrene diol epoxides induced chromosomal breaks than that of the wild genotype. Polymorphisms at the codon 288 and 399 didn't have any association with the altered DNA levels and the G2 cell cycle delay.
Substitution of G28152 with A at codon 399 of exon 10. Lunn reported that the XRCC1 28152 polymorphism in codon 399 is related to the high level of both aflatoxin B1-DNA adducts and glycophorin A variants in the normal population [4].
Mutation of XRRC1 related to lung cancer
The substitution Arg280 with His is the polymorphism which is related to that of the lung cancer [5]. The gene product of XRCC1 acts as a scaffold protein which coordinates the action of polymerase β, DNA ligase III and poly (ADP-ribose) polymerase in short-patch BER. Arg399 to Gln polymorphism of the gene which is located in the conserved region alters the function of the gene [6].
Mutation of XRCC1 related to Oral Cancer
XRCC1 plays a vital role in the BER pathway by interacting with DNA pol β, PARP and DNA ligase III. It has the BRCT domain which is responsible for its function in cell cycle checkpoint and is responsible for the DNA damage. XRCC1 has a central role in the carcinogenic pathway of oral cancer in Taiwanese. Shen et al reported five types of polymorphism in the XRCC1 gene [7]. Among the five types of polymorphism, three types of polymorphisms are found in the codon 194, 280 and 399 with the substitution of Arg to Trp, Arg to His on exon 9 and Arg to Gln respectively [7, 8]. Polymorphism in codon 399 of XRCC1 is related to the p53 mutation of the oral cancer caused by the chemical carcinogens [7]
Head and neck cancer
Head and Neck Squamous Cell Carcinoma (HNSCC) encompass 6% of all the malignant neoplasm. Smoking or the consumption of the alcohol is found to be the major risk factor of HNSCC. Two types of the XRCC1 polymorphism have been identified at the codon 194 and 399 which includes the substitution of Arg to Trp (C26304T) of exon 6 and Arg to Gln (G28152A) of exon 10 respectively [4, 9]. Polymorphism of Gln399 is located in the area which codes for the binding site of PARP. PARP detects the strand breaks in the DNA and it is the enzyme which contains the zinc-finger. The higher level of the DNA adducts are found in the individuals who carry polymorphism in the residue Gln399 of XRCC1 [9]. Single Nucleotide Polymorphism (SNP) of the residue Arg194 is mainly related to the lower belomycin and benzo[a]pyrene diol epoxides sensitivity. The residue 280His which is located in the proliferating cell nuclear antigen binding region is sensitive to the higher level of the belomycin. The 399Gln allele which is located at the carboxylic acid terminal side of the polyadenosine diphosphate-ribose polymerase-interacting domain is found to be linked to the higher levels of the aflatoxin B1-DNA adducts and higher belomycin sensitivity [10]. XRCC1 protein gets interacted with the DNA ligase III and DNA pol β. DNA ligase III is used to re-join the strand breaks in the DNA whereas DNA pol β is used in the base excision repair [4]. The variation in Arg 194 and 399 increased the risk to the HNSCC in the smoking individuals [e].
Laryngeal cancer
Polymorphism at the codons Arg 194 and 399 increased the risk of the laryngeal cancer in the individuals who smoke [e].
