Background
Breast cancer is the most frequent cancer in women accounting for 23% of all cancers in 2008 [
1]. It shows strong familial aggregation being about twice as common in first-degree relatives of individuals with the disease than in the general population [
2]. Familial linkage studies have identified high penetrance, low frequency mutations in genes such as the breast cancer susceptibility genes
BRCA1 and
BRCA2 but these mutations only account for approximately 20% of the familial risk (reviewed in [
3]). In unselected breast cancer patients, genome-wide association studies have identified low penetrance, high frequency SNPs that are associated with breast cancer risk (reviewed in [
4]). SNP rs3803662, at 16q12, is one of them. It was demonstrated that the minor allele of rs3803662 conferred increased risk of breast cancer in European women [
5,
6], a finding that has been validated in large studies of unselected patients and patients with a familial history of breast cancer and meta-analyses [
7‐
13]. This finding was also observed in women of East Asian descent [
14‐
16] but not in women of African American descent [
17‐
19]. The association was confined to oestrogen receptor positive cancer [
6,
20] but studies in large consortia have shown associations in ER positive and negative breast cancer, albeit stronger in ER positive disease [
21,
22]. The minor rs3803662 allele also increased the risk of breast cancer in BRCA1 and BRCA2 carriers [
21,
23,
24] as well as in a population-based study of men [
25].
The genes located closest to rs3803662 are
TOX3 and
LOC643714. The SNP is in the last exon (exon 4) of
LOC643714 and near the 5’ end of
TOX3. The chromosomal region spanning the 5’ end of
TOX3, the intergenic region between
TOX3 and
LOC643714 and the entire coding part of
LOC643714 are located in a 133 kb linkage disequilibrium (LD) block [
13]. Other SNPs within this LD block show association with breast cancer but rs3803662 shows the strongest effect [
5,
13].
LOC643714 is an uncharacterised gene whose function is unknown [
26]. The
TOX3 gene, also known as
TNRC9 and
CAGF9, has a tri-nucleotide repeat motive [
27] and a high mobility group (HMG)-box [
28]. It is involved in the regulation of calcium dependent transcription and interacts with cAMP-response-element-binding protein CREB and CBP (CREB-binding protein) [
29]. Additionally, TOX3 can interact with CITED1 and increase transcription [
29,
30]. CITED1 is a transcription co-regulator that enhances the activity of transcription factors such as the ER [
31] and SMAD4 [
32].
A correlation of the rs3803662 genotype with lower
TOX3 mRNA has been observed in breast tumours where
TOX3 mRNA expression decreased in an allele-dependent manner [
33]. So far, one study showed an effect of
TOX3 expression on breast cancer in that increased level of
TOX3 mRNA predicted breast cancer metastasis to bone [
34]. In the present study we report the results of association analysis between the rs3803662 genotype and expression of its closest genes,
TOX3 and
LOC643714, in a cohort of breast cancer patients as well as the correlation analyses between rs3803662, and expression of
TOX3 and
LOC643714, with known clinical and pathological characteristics.
Discussion
In summary, the minor allele of SNP rs3803662 was associated with shorter survival in breast cancer patients with luminal A tumours, with lower mRNA expression of
TOX3 in ER positive tumours as well as being more frequently observed in lobular tumours (Additional file
1: Figure S5). Also, low expression of
TOX3 and the neighbouring gene
LOC643714 was associated with high Ki67 and the basal tumour subtype while high mRNA expression was associated with ER status, positive lymph nodes at diagnosis and shorter survival (Additional file
1: Figure S6).
The risk allele of rs3803662 has been shown to associate with ER and PgR status [
6,
11,
20‐
22,
40], grade of tumours [
21,
22,
40] and diagnosis before the age of 60 [
9]. In our study cohort, that was smaller than the aforementioned studies, we did not observe a correlation between genotype at rs3803662 and age or tumour characteristics except that the risk allele was more frequent in patients with lobular tumours, which is in agreement with published data [
6,
21]. The Breast Cancer Association Consortium (BCAC) recently reported that ER positive patients, homozygous for the risk allele, had poorer overall and breast cancer specific survival than patients, homo- and heterozygous for the non-risk allele [
40]. The BCAC study did not include information on the molecular subtypes of the tumours. In our cohort, the risk allele was significantly associated with poor overall survival only in patients with luminal A tumours. The rs3803662 genotype has been shown to associate most strongly with the ER/PgR positive and ERBB2 negative subgroups [
21], which is the expression pattern of tumours of the luminal A subtype, further supporting that the effect seen in the BCAC study may be connected to luminal A tumours.
A decrease in
TOX3 mRNA according to the rs3803662 genotype was observed in a large Dutch study of 1401 breast tumours [
33] but analysis according to ER status was not reported. In our study, the allele-dependent decrease of
TOX3 or
LOC643714 mRNA level was only observed in ER positive tumours. The
TOX3 mRNA levels were quantified by microarrays and confirmed by qRT-PCR. A significant association was not seen using the latter method, possibly due to the lower number of tumours that were available for qRT-PCR. Expression of
LOC643714 was only measured by qRT-PCR and a positive correlation was found between
TOX3 and
LOC643714 mRNA levels. The closely located genes are potentially co-regulated.
LOC643714 mRNA expression in breast tumours was very low and near the detection limit of the assay. This was previously observed by Udler et al. who reported negligible mRNA levels of
LOC643714 in normal breast and breast tumours [
13]. A larger cohort will be necessary to confirm the association between rs3803662 and
LOC643714 expression.
The rs3803662 genotype may only affect expression of
TOX3 and
LOC643714 in certain tumour subtypes. Although we observed a reduction in gene expression in ER positive tumours, our preliminary analysis suggests that this is limited to luminal A tumours (Additional file
1: Figure S3) and another mechanism may be responsible for controlling expression of these genes in luminal B tumours. Although a higher number of tumours are required to confirm this observation, the majority of the adverse effect of high
TOX3 expression in ER positive tumours was related to luminal B tumours (Additional file
1: Figure S4). Furthermore, higher
TOX3 mRNA was observed in patients with luminal B tumours with metastasis (p = 0.056, data not shown) but more tumours are necessary to observe a significant association of metastasis to bone as has been reported with high
TOX3 expression [
34]. The risk allele of rs3803662 was highly correlated with a subgroup of basal tumours that expresses
EGFR and
CK5/6 but not ER, PgR and ERBB2 [
40]. Expression of
TOX3 and
LOC643714 was significantly lower in basal tumours in our cohort but we did not observe differences in expression according to genotype or a difference in allele distribution compared to the other tumour subtypes as has been reported for rs9940048, which is located within
TOX3[
41].
The rs3803662 SNP may regulate expression of genes more distantly located than
TOX3 and
LOC643714. It lies within an evolutionary conserved region that has an open chromatin conformation suggestive of an active transcription regulatory region [
13]. Our preliminary analysis of the association of the rs3803662 risk allele with the expression of genes located within 1 Mb of the SNP revealed an association with the retinoblastoma-like 2 gene (
RBL2) only, where an increase in expression was seen with the number of risk alleles (p = 0.02, data not shown). This result is in agreement with Udler et al., who reported a dose-dependent association between the rs3803662 risk allele with increased expression of
RBL2 mRNA expression in lymphocytes; however, an association between the rs3803662 genotype and
RBL2 mRNA expression in 77 breast tumours was not seen [
13]. Expression of
RBL2 was recently reported to be associated with prognosis [
40]. It is a member of the retinoblastoma gene family [
42] and has been shown to be involved in cell cycle regulation, and it is frequently deleted in breast tumours [
43].
Very low expression of
TOX3 and
LOC643714 was observed in the basal subtype, an observation that has been reported for
TOX3[
41]. The significantly lower
TOX3 and
LOC643714 expression in tumours expressing high Ki67 was due to the relatively high number of basal tumours expressing high Ki67. Expression correlated significantly with metastasis in ER negative tumours but not with metastasis to bone [
34]. High expression of both
TOX3 and
LOC643714 mRNA was observed in tumours of lymph node positive patients but only
TOX3 mRNA correlated with survival where high levels were seen in patients with shorter overall and distant metastasis free survival in the entire group and for patients with ER positive tumours. This discrepancy between
TOX3 and
LOC643714 may possibly be due to fewer
LOC643714 tumours than for
TOX3, different distribution of available samples into subtypes, or the fact that there was not a correlation between
TOX3 and
LOC643714 in ER positive tumours. It was recently reported that
TOX3 expression was not associated with prognosis [
40]. The study was large but the data came from 9 separate studies in publicly available databases. Thus, we suggest that the effect potentially vanished due to variability in study design and data acquisition.
The biological mechanism that potentially explains our results is the implication of
TOX3 in proliferation and apoptosis. Knockdown of
TOX3 expression with siRNA in the luminal A breast cancer cell line ZR-75-1 increased cellular proliferation [
44]. The same study described a FOXA1 binding site at breast cancer SNP rs4784227, whose risk allele segregates with the risk allele of rs3803662; FOXA1 preferentially bound the site that included the rs4784227 risk allele resulting in a 5-fold decrease in
TOX3 expression [
44]. Therefore, the correlation of FOXA1 expression with luminal A tumours [
45] could explain lower expression of TOX3 in our study and poorer survival of carriers of the rs3803662 risk allele and luminal A tumours. In these tumours,
TOX3 would appear to be a tumour suppressor. However, a different scenario has been described in neuronal cells that potentially may explain the effect of high
TOX3 mRNA in ER positive tumours which we ascribe to luminal B tumours. Increased expression of
TOX3 protected neuronal cells from cell death by altering expression levels of genes that affect apoptosis [
30]. Such function in ER positive breast cancer cells, where
TOX3 expression is high, could promote tumour enlargement, invasion of nearby tissues, seeding of lymph nodes which would subsequently shorten the time until death or distant metastasis. The effect of
TOX3 on apoptosis in neuronal cells was mediated via its interaction with CITED1 and ERE [
30] and may be the case for proliferation as well. However, the TOX3 ERE-mediated transcription appeared independent of ER and oestrogen [
30] unlike that of CITED1, a co-activator of ER that modulates ER responsive genes [
31]. We can only speculate that
LOC643714 has a role in tumour development based on results similar to
TOX3 but its mRNA is not translated and its function is unknown.
Although we assume that mRNA expression of our candidate genes may differently affect breast tumour subtypes, the study is small and the analyses according to ER status or molecular subtypes further reduced the number of tumours. As such, the study is exploratory and the results should be confirmed with a larger number of tumours. Our findings provide a suitable basis for further validation studies.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
ETG performed the RT-PCR experiments, contributed to data analysis as well as the writing of the manuscript. RBB conceived the study, interpreted the data and contributed to the writing of the manuscript. AA participated in the microarray experiments and critically revised the manuscript. LTA interpreted the data and critically reviewed the manuscript. HG performed the genotyping, participated in the microarray experiments and analyzed the data. BAA participated in the acquisition and interpretation of the pathologic data and critical reviewing of the manuscript. OTJ participated in acquisition of the clinical data and critical reviewing of the manuscript. IR analysed and interpreted the data, and led the writing of the manuscript. All authors read and approved the final manuscript.