No associations between aromatase gene polymorphisms and breast cancer risk in Saudi patients

Abstract

Background: Cytochrome P450 (CYP)19A1 encodes aromatase, the enzyme responsible for the conversion of androgens to estrogens, and may play a role in variation in outcomes among women with breast cancer. The aim of this study was to analyze the genetic association of rs4646 (A > C) and rs700518 (Val > Val) in the CYP19A1 gene with the risk of breast cancer.

Methods: These two single nucleotide polymorphisms (SNPs) were analyzed in a primary study group of breast cancer patients and healthy control subjects. Genotypes were determined by the TaqMan SNP analysis technique. The study data were analyzed using the chi-square or t-test and logistic regression analysis by Statistical Package for the Social Sciences version 16 software.

Results: rs4646 and rs700518 had no association with susceptibility to breast cancer. There was no significant association for either of these SNPs overall in breast cancer samples when compared with healthy control samples. Our data do not support a relationship between the CYP19A1 rs4646 and rs700518 SNPs and risk of breast cancer. It may be that there are ethnic differences with regard to this relationship.

Conclusion: This study demonstrated that CYP19A1 rs4646 and rs700518 SNPs may not be involved in the etiology of breast cancer in the Saudi population. Confirmation of our findings in larger populations of other ethnicities could provide evidence for the role of the CYP19A1 gene in breast carcinomas.

Introduction

Breast cancer is one of the most common types of malignancy and is a leading cause of death worldwide, accounting for nearly one million new cases diagnosed and half a million deaths annually.¹ Incidence rates vary widely according to region, being high in developed countries and lower in developing countries.² This type of cancer is obviously a significant public health problem in Saudi Arabia.

Cytochrome P450 (CYP)19A1 encodes the enzyme aromatase, which catalyzes the conversion of the C19 androgens, androstenedione and testosterone, to estrone and estradiol, respectively.^3,4 Specific single nucleotide polymorphisms (SNPs) in the intronic regions of CYP19A1 have been shown to play a role in altering regulation of transcription and/or splicing of CYP19A1, producing different enzyme products with variable enzymatic activity when compared with the normal gene product.^5,6 Studies have identified SNPs in CYP19A1 that are associated with an increased risk of cancer, primarily in European American, North Indian, and Chinese populations.^7,8 Several studies have been done on polymorphisms in the CYP19 gene in an attempt to identify an association between genetic variations and breast cancer risk, but few studies have found such an association. One report suggested that a tetra-nucleotide repeat polymorphism in intron 4 (TTTA)n was strongly associated with the risk of breast cancer.⁹ However, studies of other genetic variations have not shown a clear association with risk of breast cancer, thus generating a situation of inconsistent results.^10–13 Hence, in the present population-based, case–control study, we investigated the genotype distribution of rs4646 and rs700518 SNPs in patients with breast cancer.

Materials and methods

Study population

The study population comprised 148 females (median age 48 years) suffering from breast cancer and attending the outpatient clinics of the clinical co-investigators at King Fahad Medical City Hospital, Riyadh, Saudi Arabia, and 154 age-matched normal healthy controls also attending King Fahad Medical City Hospital for minor illnesses and recruited following physical examinations after exclusion of a breast cancer diagnosis and history of cancer or cancer-related diseases. The patients and controls were of Saudi Arabian ethnicity. Demographic data, age at diagnosis, tumor grade, and immunohistochemical determination of estrogen receptor (ER), progesterone receptor (PR), and HER2 receptor status were recorded. The study was approved by the institutional review board of King Khalid University Hospital. Written informed consent was obtained from all participants.

Genotyping

Genomic DNA was extracted from blood samples taken from breast cancer cases and controls using a QIAmp DNA blood mini kit (Qiagen, Valencia, CA, USA) following the manufacturer's instructions. The rs1130409 SNP in the APE1 gene was genotyped using the TaqMan allelic discrimination assay as described previously.¹⁴ Ten percent of the samples were subjected to repeated analysis for verification of the genotyping procedures.¹⁵

Statistical analysis

Genotype and allelic frequencies were compared using Fisher's Exact test (two-tailed) as described by Alanazi et al¹⁵ to describe the χ² test and odds ratios, and 95% confidence intervals to know the variation between cancer cases and controls. The statistical analysis was performed using Statistical Package for the Social Sciences version 21.0 (SPSS Inc, Chicago, IL).¹⁶

Results

One hundred and forty-eight breast cancer cases and 154 healthy controls were included in this study. The clinical characteristics of the breast cancer cases and the healthy controls are given in Table 1. Of 148 confirmed cases of breast cancer, 67 were ER-positive, 78 were ER-negative, 89 were PR-positive, 59 were PR-negative, 57 were HER2-positive, and 89 were HER2-negative.

Table 1 Genotype frequencies of Aromatase gene polymorphism in breast cancer cases and controls Abbreviations: CI, confidence interval; OR, odds ratio; Ref, reference; SNP, single nuclear polymorphism.

All the genotypic distributions were consistent with that expected in the Hardy–Weinberg model. The homozygous wild-type allele was used as a reference to calculate the odds of acquiring breast cancer in comparison with the other two genotypes. The genotype frequencies of the analyzed SNPs along with the resulting odds ratio and significance levels are shown in Table 1. We did not find any significant association between the two SNPs and risk of breast cancer.

In the present study, we found no association with CYP19A1 rs4646 and rs700518 genotypes between breast cancer cases and matched healthy controls. The frequencies of rs4646 (A > C) genotypes in breast cancer cases were 8 (0.05), 46 (0.31), and 94 (0.64), respectively, whereas in healthy controls the frequencies were 8 (0.05), 47 (0.30), and 99 (0.65), respectively. Breast cancer patients did not show any risk when compared with healthy individuals (Table 1). As shown in Table 1, the frequency of the rs700518, (A > G) A/A, A/G, and G/G genotypes were 60 (0.41), 66 (0.45), and 22 (0.15), respectively, in breast cancer patients and 55 (0. 36), 71 (0.46), and 28 (0.18), respectively, in controls. Breast cancer patients did not show any risk when compared with healthy individuals (Table 1).

The correlation between CYP19A1 rs4646 and rs700518 SNP status and clinicopathological characteristics was also analyzed, but interestingly none of these parameters showed an association with breast cancer in this Saudi population. Breast cancer patients did not show any association between age and ER-positive, ER-negative, PR-positive, HER2-positive, and HER2-negative status (Tables 2–5).

Table 2 Genotype frequencies of Aromatase gene polymorphism in breast cancer cases (above 48 years old versus below 48 years old) Abbreviations: CI, confidence interval; OR, odds ratio; Ref, reference; SNP, single nuclear polymorphism.

Table 3 Genotype frequencies of Aromatase gene polymorphism in breast cancer cases (ER-positive versus ER-negative) Abbreviations: CI, confidence interval; ER, estrogen receptor; OR, odds ratio; Ref, reference; SNP, single nuclear polymorphism.

Table 4 Genotype frequencies of Aromatase gene polymorphism in breast cancer cases (PR-positive versus PR-negative) Abbreviations: CI, confidence interval; PR, progesterone receptor; OR, odds ratio; Ref, reference; SNP, single nuclear polymorphism.

Table 5 Genotype frequencies of Aromatase gene polymorphism in breast cancer cases (HER2-positive versus HER2-negative) Abbreviations: CI, confidence interval; OR, odds ratio; Ref, reference; SNP, single nuclear polymorphism.

We also generated a regional linkage disequilibrium (LD) plot using SNAP (SNP Annotation and Proxy Search, http://www.broadinstitute.org/mpg/snap/ldplot.php) for CYP19A1 rs4646 and rs700518. The LD plot indicated that there are multiple loci near rs4646 and rs700518 with high LD (r²>0.8), which suggests that fine mapping is necessary to evaluate the genetic effect of CYP19A1 on cancer as well as functional studies (Figures 1 and 2).

Figure 1 Regional linkage disequilibrium plot for the single nuclear polymorphism rs4646.

Figure 2 Regional linkage disequilibrium plot for the rs700518 single nuclear polymorphism.

Discussion

Numerous genetic variations of CYP19A1 have been reported, even though the possible functional significance of most of these polymorphisms is still undefined. Various studies conducted in specific breast cancer populations have analyzed a few common CYP19A1 polymorphisms and generated inconsistent results with regard to their possible association with cancer risk, sex hormone levels, HER2 status, and survival.^9,17–20 In the present study, our data do not support any appreciable association between CYP19A1 rs4646 and rs700518 genotypes and breast cancer risk in the Saudi population (Table 1). We analyzed the association of both these SNPs with various clinical parameters and found no evidence of heterogeneity for either rs4646 or rs700518 in Saudi breast cancer patients, and found no association except with rs4646 in PR-negative patients (Tables 2–5).

Some strengths and limitations of our study should be noted. Its strengths include the population-based and prospective study design in the central region of Saudi Arabia, thereby minimizing selection bias, and the detailed review of the cancer diagnosis, thereby minimizing disease misclassification. However, our nested case–control sample was relatively small in size, which hampered our ability to evaluate specific gene-disease association. Confirmation of our findings in larger populations of women of different ethnicities could provide evidence for the role of the CYP19A1 gene in breast carcinoma.

Acknowledgment

The authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding this work through research group project RGPVPP-081.

Disclosure

The authors declare that they have no competing interests in this work.

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