Background
In Kyrgyzstan, breast cancer (BC) appears to be one of the leading cancer localizations in females, remaining the second most prevalent and the third fatal type of cancer. The advanced disease is diagnosed in 40% of new cases, hampering both treatment and cure [
1]. Therefore, molecular markers of predisposition to BC may be a cornerstone strategy for early detection and primary prevention of this malignant disease.
BC is known to develop from a combined effect of environmental and genetic predictors, whose interplay will determine the individual susceptibility to the negative impact of the environment. To date, series of candidate gene are under study to test the genetic component of such predisposition. Those genes regulating cellular cycle and facilitating DNA reparation as well as inducing apoptosis may have the greatest potential for that [
2,
3].
XRCC1 (X-ray repair cross-complementing group) is one of the leading proteins associated with DNA reparation and coded with
XRCC1 gene of the 19th chromosome in 19q13.2 locus [
4]. A polymorphic marker
Arg399Gln is located in exon 10 of this gene and has been ben tested for an association with a few malignancies, including BC [
5‐
7]. Moreover, BC is known to be linked with apoptosis. Protein p53 is a main driver of apoptosis after cellular genome injury, coded by
TP53 gene, located on a short arm of the 17th chromosome [
8]. This gene
TP53 is known to contain polymorphic marker
Arg72Pro in exon 4 and may play role in carcinogenesis. This marker codes arginine- and proline-containing p53 protein, differing in their capacity to activate transcription of
TP53 target genes and promote p53-mediated apoptosis [
9]. MDM2 protein controls the overall amount of p53 in the cell and performs as a natural p53 inhibitor, coded by gene
MDM2, located on a long arm of the 12th chromosome in locus 12q14.3-12q15 [
10]. The first intron of
MDM2 gene contains mononucleotide polymorphism
T309G, associated with BC in selected ethnic groups [
11‐
13]. The association of genes
XRCC1, TP53 and
MDM2 with BC has never been tested in Kyrgyz population. We, therefore, aimed to identify an association of alleles and genotypes of polymorphic markers
Arg399Gln of gene
XRCC1,
Arg72Pro of gene
TP53, and
T309G of gene
MDM2 with the risk of BC in Kyrgyz women.
Results
Allele and genotype distribution
Table
2 shows the distribution of
Arg399Gln of gene X
RCC1,
Arg72Pro of gene
TP53, and
T309G of gene
MDM2 in the groups of subjects with and without BC. We found that genotype frequency in the control sample corresponded to the expected one with Hardy-Weinberg principle with regard to all the tested markers.
Table 2
The distribution of genotypes and alleles of Arg399Gln of gene XRCC1, Arg72Pro of gene TP53 and T309G of gene MDM2 in BC patients of Kyrgyz ethnicity compared to healthy controls
Arg399Gln
gene XRCC1
rs25487
| Allele Arg399
| 144 (62) | 146 (72) | 4.46 | 0.034 | 0.64 | 0.42–0.95 |
Allele 399Gln
| 90 (38) | 58 (28) | 1.57 | 1.05–2.35 |
Arg399Arg
| 38 (32) | 56 (55) | 13.86 | 0.0010 | 0.39 | 0.22–0.68 |
Arg399Gln
| 68 (58) | 34 (33) | 2.77 | 1.60–4.80 |
Gln399Gln
| 11 (10) | 12 (12) | 0.77 | 0.32–1.84 |
HWE χ2 /р | 6.07/0.01 | 3.33/0.06 | |
Arg72Pro
gene TP53
rs1042522
| Allele Arg72
| 164 (70) | 142 (70) | 0.011 | 0.913 | 1.02 | 0.68–1.54 |
Allele 72Pro
| 70 (30) | 62 (30) | 0.98 | 0.65–1.47 |
Arg72Arg
| 57(49) | 53 (52) | 1.80 | 0.41 | 0.88 | 0.52–1.49 |
Arg72Pro
| 50 (43) | 36 (35) | 1.37 | 0.79–2.36 |
Pro 72Pro
| 10 (8) | 13 (13) | 0.64 | 0.27–1.53 |
HWE χ2 /р | 0.04/0.83 | 2.80/0.09 | |
T309G
gene MDM2
rs2279744
| Allele T309
| 120 (49) | 111(46) | 0.31 | 0.58 | 0.88 | 0.60–1.28 |
Allele 309G
| 114 (51) | 93 (54) | 1.13 | 0.77–1.65 |
G309G
| 29 (24) | 28 (27) | 0.500 | 0.77 | 0.87 | 0.47–1.59 |
T309G
| 62 (53) | 55 (54) | 0.96 | 0.56–1.64 |
T309 T
| 26 (22) | 19 (18) | 1.24 | 0.64–2.42 |
HWE χ2 /р | 0.42/0.51 | 0.77/0.38 | |
Heterozygous genotype
Arg399Gln and
399Gln allele of gene
XRCC1 were associated with BC when compared to controls. This genotype
Arg399Gln resulted in almost 3-fold increase of BC probability (OR 2.77 (95% CI 1.60–4.80)), whereas the
399Gln allele was a marker of BC risk (OR 1.57 (95% CI 1.05–2.35)). With regard to
Arg399 allele, we found its protective effect for BC (OR 0.64 (95% CI 0.42–0.95)) (Table
1).
We failed to find similar associations of polymorphic loci Arg72Pro of gene TP53 and T309G of gene MDM2, and the prevalence of these genotypes and alleles in the group of BC patients did not differ from healthy controls (р > 0.05). Therefore, taken separately, polymorphic loci Arg72Pro of gene TP53 and T309G of gene MDM2 were not associated with BC in the population of Kyrgyz women.
Because BC phenotype results from a combination of genotypes and alleles of various genes, rather than one gene only, making BC a genetically heterogeneous disease, we performed the analysis of intergenic (XRCC1/TP53/MDM2) interactions in order to identify the most meaningful gene-gene combinations, which can result in BC in Kyrgyz women.
Gene-gene interaction between XRCC1 and TP53 polymorphisms
When we tested gene-gene interactions of polymorphic loci of
Arg399Gln and
Arg72Pro, we found statistically significant 2-loci combinations of genotypes
XRCC1/TP53 (
Arg399Gln/Arg72Pro), which results in a significant increase of BC probability in Kyrgyz women. Thus,
Arg72Pro heterozygous variant of gene
TP53 combined with
Arg399Gln heterozygous genotype of gene
XRCC1 was associated with almost 4-fold increase in BC probability in the studied sample (OR 3.98 (95% CI 1.57–10.09)) (Table
3).
Table 3
The distribution of combinations of Arg399Gln of gene XRCC1 and Arg72Pro of gene TP53 polymorphic markers in Kyrgyz women with BC and controls
Arg399Arg/Arg72Arg
| 19 (16) | 27 (26) | Ref. | |
Arg399Arg/Arg72Pro
| 18 (15) | 21 (21) | 1.22 (0.52–2.88) | 0.20/0.65 |
Arg399Arg/Pro72Pro
| 1 (1) | 8 (8) | 0.18 (0.02–1.54) | 2.97/0.085 |
Arg399Gln/Arg72Arg
| 32 (27) | 20 (20) | 2.27 (1.01–5.11) | 3.23/0.07 |
Arg399Gln/Arg72Pro
| 28 (24) | 10 (9) | 3.98 (1.57–10.09) | 7.58/0.0059 |
Arg399Gln/Pro72Pro
| 8 (7) | 4 (4) | 2.84 (0.75–10.81) | 2.46/0.116 |
Gln399Gln/Arg72Arg
| 6 (5) | 6 (6) | 1.42 (0.40–5.09) | 0.29/0.588 |
Gln399Gln/Arg72Pro
| 4 (3) | 5 (5) | 1.14 (0.27–4.80) | 0.03/0.861 |
Gln399Gln/Pro72Pro
| 1 (1) | 1 (1) | 1.42 (0.08–24.18) | 0.06/0.807 |
Gene-gene interaction between XRCC1 and MDM2 polymorphisms
Compared to controls (18%), BC women had statistically significant greater prevalence of
Arg399Gln/T309G (38%) genotype (Table
3)
. The combination of
T309G of gene
MDM2 heterozygous genotype with
Arg399Gln of gene
XRCC1 heterozygous genotype was associated with a 3-fold increase of BC probability (OR 3.0 (95% CI 1.18–7.56)) (Table
4), which makes this combination of haplotypes a genetic risk factor of BC in Kyrgyz women.
Table 4
The distribution of combinations of Arg399Gln of gene XRCC1 and T309G of gene MDM2 polymorphic markers in Kyrgyz women with BC and controls
Arg399Arg/G309G
| 12 (10) | 17 (17) | Reference | |
Arg399Arg/T309G
| 18 (15) | 31 (30) | 0.82 (0.32–2.11) | 0.17/0.684 |
Arg399Arg/T309T
| 8 (7) | 8 (8) | 1.42 (0.42–4.84) | 0.31/0.578 |
Arg399Gln/ G309G
| 14 (12) | 8 (8) | 2.48 (0.79–7.76) | 2.48/0.115 |
Arg399Gln/ T309G
| 38 (32) | 18 (18) | 3.00 (1.18–7.56) | 4.49/0.034 |
Arg399Gln/ T309T
| 16 (14) | 8 (8) | 2.83 (0.92–8.73) | 3.37/0.066 |
Gln399Gln/ G309G
| 3 (3) | 3 (3) | 1.42 (0.24–8.26) | 0.15/0.697 |
Gln399Gln/ T309G
| 6 (5) | 6 (5) | 1.42 (0.37–5.48) | 0.26/0.613 |
Gln399Gln/ T309T
| 2 (2) | 3 (3) | 1.42 (0.17–11.51) | 0.11/0.74 |
Gene-gene interaction between TP53 and MDM2 polymorphisms
When comparing genotype distribution of
Arg72Pro polymorphic loci of
TP53 gene and
T309G of
MDM2 gene, no statistical differences between BC and control groups were identified (Table
5). Of note,
Pro72Pro/G309G genotype combination was only found in control group, but not in BC group.
Table 5
The distribution of combinations of Arg72Pro of gene TP53 and T309G of gene MDM2 polymorphic markers in Kyrgyz women with BC and controls
Arg72Arg/G309G
| 8 (7) | 11(11) | Reference | |
Arg72Arg/G309T
| 36 (31) | 31 (30) | 1.60 (0.57–4.47) | 0.80/0.37 |
Arg72Arg/T309T
| 13 (11) | 11 (11) | 1.63 (0.48–5.47) | 0.62/0.43 |
Arg72Pro/ G309G
| 21 (18) | 11 (11) | 2.63 (0.82–8.43) | 2.69/0.10 |
Arg72Pro/ G309T
| 18 (15) | 19 (19) | 1.30 (0.43–3.98) | 0.22/0.64 |
Arg72Pro/ T309T
| 11 (9) | 6 (6) | 2.52 (0.65–9.71) | 1.84/0.18 |
Pro72Pro/ G309G
| 0 (0) | 6 (6) | 0.10 (0.005–2.11) | 3.72/0.05 |
Pro72Pro / G309T
| 8 (7) | 5 (5) | 2.20 (0.52–9.30) | 1.17/0.28 |
Pro72Pro / T309T
| 2 (2) | 2(2) | 1.38 (0.16–11.94) | 0.08/0.77 |
Gene-gene interaction between XRCC1, TP53 and MDM2 polymorphisms
We tested 27 different combinations (XRCC1, TP53, MDM2) and found that the interaction of Arg399Gln/Arg72Pro/T309G of genes XRCC1/TP53/MDM2 heterozygous genotypes was associated with BC (χ2 = 5.04; р = 0.025) and increased its likelihood with an OR of 6.40 (95% CI 1.18–34.63). Additionally, we tested whether the selected polymorphic loci were associated with cancer histologic type, tumor size, N or M stage or even degree of differentiation. We found no association of these markers with any of these attributes of cancer in our patients.
Discussion
In this case-control study, we have identified a number of genetic associations with BC in Kyrgyz women. These exposures included allele 399Gln (OR 1.57; p = 0.034), Arg399Gln of gene XRCC1 heterozygous genotype (OR 2.77; p = 0.001), as well as the combination of Arg399Gln/Arg72Pro of genes XRCC1/TP53 heterozygous genotype (OR 3.98; p = 0.0059), Arg399Gln/T309G of genes XRCC1/MDM2 (OR 3.0; p = 0.034), and Arg399Gln/Arg72Pro/T309G of genes XRCC1/TP53/MDM2 (OR 6.40; p = 0.025).
Arg399Gln polymorphism of gene XRCC1, Arg72Pro of TP53 gene and T309G of MDM2 gene, coding enzyme synthesis with a variety of reparative and apoptosis activity, may shift the balance of reparation and injury both ways. Our findings confirm the association of heterozygous genotypes of XRCC1/TP53/MDM2 genes with the elevated risk of BC. The strongest association of heterozygous carriage of XRCC1/TP53/MDM2 genes with the disease calls for further analysis and more studies.
Our results highlight the role of
Arg399Gln polymorphic locus of gene
XRCC1 in BC origin in Kyrgyz women. Our findings confirm the earlier data from Chinese [
4], Polish [
6], American [
5], and Egyptian [
7] populations, which altogether showed that
399Gln allele and
Arg399Gln genotype carriers had a greater BC risk compared to
Arg399 allele and
Arg399Arg genotype. The association of
399Gln allele and
Arg399Gln genotype with BC may sound plausible, because published reports have shown that XRCC1 protein, having glutamine in its 399th position, has a smaller potency to repair damaged DNA, and that results in the accumulation of genetically unstable cells and may promote malignancy [
2].
Arg72Pro of gene
TP53 polymorphic marker is located in the high proline concentration domain [
8], and this domain is responsible for apoptotic functioning of р53 protein. After mutation, р53 is no more capable of activating transcription of pro-apoptotic genes, resulting in disrupted apoptosis, which altogether leads to a greater number of cells of various DNA alterations with subsequent cellular proliferation. Arginine-containing variant of р53 (
Аrg72) protein is more potent to induce apoptosis that it’s proline-containing variant (
Pro72) [
10].
Literature data on the association of
Arg72Pro of gene
TP53 polymorphic versions with BC are not homogenous and are somewhat contrasting. Some studies have demonstrated that
72Pro of gene
TP53 allele has a significant association with BC [
18,
19]. Other studies, in contrast, have confirmed
Arg72 allele may be more relevant [
20,
21] to promote BC. Moreover, in newer studies and even meta-analyses, the associations of
Arg72Pro of gene
TP53 marker with BC was not statistically significant [
22,
23]. Such contradicting findings are likely explained by the ethnic differences in the molecular and genetic mechanisms of BC initiation and progression.
The concentration and activity of р53 cancer suppressing protein in a cell is controlled by MDM2 protein, which inactivates and accelerates degrading of р53 [
10] cancer suppressing protein, thus, hampering DNA reparation and, therefore, promotes, carcinogenesis.
With regard to T309G of gene MDM2 polymorphic marker, we failed to demonstrate its statistically significant association with BC in a group of BC females compared to controls. However, Arg72Pro heterozygous variant in combination with Arg399Gln of gene XRCC1 heterozygous genotype was associated with a 4-fold increase in the probability of BC (OR 3.98 (95% CI 1.57–10.09)).
A combination of risk genotypes of a number of candidate genes, producing additive effect, may result in simultaneous DNA reparation disorder and apoptosis, leaving some potential for a new phenotype formation [
11].
The latest meta-analysis [
24] of 19 publications with a total of 9788 BC cases and 11,195 controls has shown that
T309G of gene
MDM2 polymorphic locus is associated with BC both in Asian and Caucasian populations. The magnitude of such association was most pronounced when
T309G of gene
MDM2 heterozygous genotype was present with the greatest effect in Asians (OR 1.21 (95% CI 1.03–1.41));
p = 0.02), compared to Caucasians (OR 1.09 (95% CI 1.00–1.18);
p = 0.04). Of note, GG genotype of polymorphic locus of
MDM2 gene is considered a risk factor for BC in Taiwanese women (OR 3.05 (95% CI 1.04–8.95)); p = 0.04) [
12].
Therefore, combined with the findings of other cohorts, our data confirmed the individual susceptibility to BC resulting from polymorphic markers of DNA repair genes (XRCC1), apoptosis genes (TP53), as well as of apoptosis inhibition genes (MDM2).
Conclusions
Our study has enabled to identify the inter-loci interaction and to find molecular markers of individual risk of BC in Kyrgyz women. The list of potential risk factors for BC in Kyrgyz females may include 399Gln allele and Arg399Gln of gene XRCC1 heterozygous genotype, as well a combination of heterozygous genotypes of Arg399Gln/Arg72Pro of genes XRCC1/TP53, Arg399Gln/T309G of genes XRCC1/MDM2 and Arg399Gln/Arg72Pro/T309G of genes XRCC1/TP53/MDM2.
Identification of risk combinations of genes XRCC1, TP53 and MDM2 with BC may increase the study validity and determine groups of women with high individual risk of BC, which may help in the prevention, early detection and effective cure of this condition.