No association between XRCC1 and XRCC3 gene polymorphisms and breast cancer risk: Iowa Women's Health Study
Introduction
Several established risk factors for breast cancer, such as estrogen exposure and alcohol, generate reactive oxygen radicals that cause a variety of DNA damage lesions, including DNA strand breaks and oxidized DNA bases [1]. Cells repair DNA damage and maintain genomic stability with a variety of DNA repair mechanisms, which may be essential in preventing tumor initiation and delaying tumor progression. A review of epidemiological studies evaluating the association between DNA repair and a variety of cancers revealed that DNA repair is associated with an increased risk of a variety of cancers, including lung, skin, and breast cancers, in a majority of the studies [2], [3], [4].
Oxidative DNA damage and single strand breaks are repaired predominantly by the base excision repair (BER) pathway [5]. Double strand breaks are repaired predominantly by the double strand DNA repair pathway, which consists of two distinct pathways—the non-homologous repair pathway and the homologous recombination (HR) repair pathway [6]. Single nucleotide polymorphisms (SNPs) with high population frequencies have been detected in genes belonging to both, the BER and double strand repair pathways and are thought to contribute to the variation observed in DNA repair in the population thereby contributing to breast cancer susceptibility.
The XRCC1 gene plays an important role in BER, as a scaffold protein that brings together proteins of the DNA repair complex [7], [8], [9]. Three polymorphisms, Arg194Trp, Arg280His, and Arg399Gln have been identified in the coding region of the XRCC1 gene [10]. Arg194Trp polymorphism has been associated with increased bladder and esophageal cancer risk [11], [12], but has no association with breast cancer [13], [14], [15], [16], [17]. On the other hand, the Arg399Gln polymorphism has been associated with increased breast cancer, pancreatic cancer risk, and increased DNA adduct levels [13], [18], [19]. However, not all studies have shown consistently increased risk [14], [15], [16] and further studies are needed to clarify this association. In addition, there is also evidence of gene–gene interactions involving Arg194Trp polymorphism in modulating breast cancer risk [14].
In humans, XRCC3 plays an important role in homologous recombination pathway by interacting with RAD-51 and maintaining chromosomal stability, and repairing DNA damage [20]. Brenneman et al. showed that XRCC3's function was not limited to HR initiation alone, but also extends to later stages in formation and resolution of HR intermediates, possibly by stabilizing heteroduplex DNA [21]. One polymorphism in XRCC3, Thr241Met, has been associated with increased breast cancer risk in one large case control study [22] and the Met/Met genotype has been found to be associated with a significantly higher DNA adduct level as compared to the Thr/Thr genotype in another study [23]. However, other studies have shown no association between Thr241Met polymorphism and breast cancer risk [14], [24], [25]. Thus, further studies are needed to confirm these associations.
We hypothesized that polymorphisms in the XRCC1 and XRCC3 genes will be associated with increased breast cancer risk. The Arg280His polymorphism had a low population frequency among 200 cancer-free women in our study population (2.5%). Since this study did not have statistical power to identify modest associations of low prevalence SNPs the Arg280His polymorphism was not evaluated further in this study. The other two SNPs in XRCC1 (Agr194Trp, Agr399Gln) and the Thr241Met polymorphism in XRCC3 (Thr241Met) were evaluated for their association with breast cancer risk in a nested case control study of 460 breast cancer cases and 324 controls within the Iowa Women's Health Cohort.
Section snippets
Materials and methods
The Iowa Women's Health Study was a prospective cohort study of 41,836 women aged 55–69 years at baseline in 1986. A questionnaire was completed at baseline that focused on anthropometric, dietary, and other major risk factors for cancer including family history of cancer, prior medical conditions, cigarette smoking, reproductive factors, and hormone use [26]. Mortality and cancer incidence data, since 1986, have been collected through a computer linkage of study participants’ identifiers with
Results
DNA was extracted from 326 PET samples (including study samples and blinded controls). Seventy-one PET samples (22%) had low DNA quantity (A260 OD < 0.1) while 78 PET samples (24%) had low DNA quality (A260/A280 < 1.5). Since PET samples were not genotyped for the Arg399Gln polymorphism, genotyping of PET samples will be described for Arg194Trp and Thr241Met polymorphisms. In general, DNA quantity, rather than DNA quality, was correlated with ability to obtain a genotype for a sample. However, the
Discussion
Our study shows that DNA obtained from PET can be reliably used in epidemiological studies to determine SNPs at various loci. Both PCR-INVADER and Sequenom genotyping platforms were successfully used to genotype PET samples and since both PCR-INVADER and Sequenom are more suited to automation as compared to PCR-RFLP they can be more readily adapted to large scale genotyping studies.
No association was found between the Arg194Trp and Arg399Gln polymorphisms in the XRCC1 gene and the Thr241Met
Acknowledgements
This work was supported by NCI (Grant Number: RO1-CA39742).
Contributions. BT designed the PCR-RFLP, PCR-INVADER, and Sequenom assay, assisted in PCR amplification, and performed statistical analysis and manuscript preparation. CFL was involved in study design and collection of paraffin embedded blocks from pathology laboratories while AB and WK were involved in DNA extraction and performing the PCR-INVADER and Sequenom reactions. DJ was involved in statistical analysis. KEA, MDG, and ARF were
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