Introduction

High mammographic density for a woman’s age and body mass index (BMI) - meaning large radio-dense fibro-glandular areas that appear white or bright on a mammogram - is considered an established risk factor for breast cancer. The association with mammographic density seems to be present for risk of both estrogen receptor negative and estrogen receptor positive breast cancer [1, 2]. Mammographic density changes over lifetime and generally declines with age [3]. The decline of mammographic density with age may seem paradoxical for a risk factor, as breast cancer risk generally increases with age. This contradiction may be resolved when regarding mammographic density as a risk factor cumulating over time [4, 5]. Heritable factors have been estimated to explain up to 63 % of variation in mammographic density, implicating a strong genetic component [6, 7]. Also, recent evidence suggests that the biological attributes leading to greater mammographic density and development of breast cancer have common predisposing genes [8–11].

The use of tamoxifen is associated with a decrease in mammographic density [12–14], whereas the use of menopausal hormone therapy (MHT) is associated with higher mammographic density [15–17]. To date, the biological mechanisms by which MHT use influences mammographic density are largely unknown [18]. On the other hand, it is established that the use of MHT is associated with increased breast cancer risk and that the health risks of extended MHT use may exceed the benefits [19]. However, approximately 5 % of women aged 40 years or older reported current use of oral MHT in the US National Health and Nutrition Examination Survey in 2009–2010 [20]. In Europe, similar proportions of women aged 45–69 years were reported to currently use MHT in 2010 according to estimations based on sales data [21].

To better understand the relationship between MHT use and mammographic density, several studies investigated whether polymorphisms in candidate genes related to hormone metabolism, nuclear hormone receptors and growth factors are associated with mammographic density and whether these polymorphisms show a statistical interaction with MHT use, i.e., modify the association between MHT and mammographic density [22–28]. Most studies did not identify any significant interaction, but the null findings can also be attributed to the generally small sample sizes. Two studies observed potential gene-environment interactions between MHT use and single nucleotide polymorphisms (SNPs) in AKR1C4, CYP1A1-CYP1A2, CYP1B1, ESR2, PPARG, PRL, SULT1A1-SULT1A2 and TNF, which warrant confirmation by further studies [24, 28]. We therefore conducted a comprehensive replication analysis in the largest study available to date using genotypes and imputed genotypes of SNPs located in or near these genes.

Candidate gene association studies might have missed gene-environment interactions with genes not selected for study. We previously conducted a meta-analysis of four case-only genome-wide gene-environment interaction studies to identify genetic variants that modify the association of MHT use with breast cancer risk [29]. We hypothesized that these common variants may also modify the association between MHT use and mammographic density. Therefore, we selected the most significant SNPs from the genome-wide G×MHT interaction studies of breast cancer risk for assessment of interaction with MHT use on mammographic density. Furthermore, we included ten variants associated with age-adjusted and BMI-adjusted percent density, dense area or non-dense area at the genome-wide significance level (ZNF365-rs10995190 [30], 12q24-rs1265507 [31], and AREG-rs10034692, PRDM6-rs186749, ESR1-rs12665607, 8p11.23-rs7816345, LSP1-rs3817198, IGF1-rs703556, TMEM184B-rs7289126, and SGSM3-rs17001868 [10]).

Materials and methods

Study sample

The analysis was carried out on pooled data from a nested case–control study of the Mayo mammography health study (MMHS), and from five case–control studies (Australian breast cancer family study (ABCFS), Bavarian breast cancer cases and controls (BBCC), Mayo clinic breast cancer study (MCBCS), Ontario familial breast cancer registry (OFBCR), and the Singapore and Sweden breast cancer study (SASBAC)), two nested case–control studies (Melbourne collaborative cohort study (MCCS) and the Multi-ethnic cohort (MEC)), one cohort study (European prospective investigation into cancer and nutrition (EPIC)) and one family study (Sisters in breast screening study (SIBS)), participating in the Breast Cancer Association Consortium (BCAC) [32]; EPIC and SIBS samples are part of SEARCH [33] within BCAC). Details on study design and recruitment are provided in Additional file 1: Table S1. All participants signed informed consent and the studies were approved by the relevant ethics committees. The names of the individual approving ethics committees for each study can be found within the Acknowledgements section.

Participants were eligible for this study if information on mammographic density, relevant covariates and SNP genotypes was available and if they were of European descent and postmenopausal at the time of mammography. Ancestry informative principal components were used to define probable ethnic ancestry, with the exception of MMHS, where ethnicity was self-reported or abstracted from clinical records. In total, 6,298 individuals were included in the analysis.

Density measures and exposure definitions

Each study collected radiographs of mammograms from participants, either in the mediolateral oblique or craniocaudal view. The radiographs were digitized and percent density, dense area, and non-dense area measures were obtained using one of two similar semi-automated methods, Cumulus [34] and Madena [35]. Measurements using the two methods have been found to be highly correlated (Pearson correlation coefficient of 0.86) [36].

Information on relevant exposures such as age, MHT use and BMI was collected individually by each study. The date of the mammogram was the reference date used for all exposure definitions. Ever use of MHT was defined as use of any type of MHT. Women were categorized as current users when using MHT at the date of mammography, former users if they had used MHT previously and never users if they had never used MHT. As part of the DENSNP project [9], individual study data were centrally quality checked and harmonized at the Mayo Clinic, Rochester, Minnesota.

SNP selection, genotyping and imputation

Using a meta-analysis of four case-only genome-wide gene-environment interaction studies on the association between MHT use, and overall and lobular breast cancer risk [29], 5,000 SNPs were initially selected based on evidence of interaction with MHT. For each SNP, the lower P value for interaction (P int) from the results for overall and lobular breast cancer was used. After exclusion of SNPs with minor allele frequency (MAF) <0.05, P <0.05 for Cochran’s Q or I 2 ≥30% for study heterogeneity and the availability of the respective SNP data in fewer than two case-only studies, 4,421 SNPs remained. Of these, the 1,391 SNPs showing P int <0.003 were selected for inclusion in a custom Illumina iSelect genotyping array (iCOGS). The iCOGS data were centrally quality controlled after genotyping, which led to the exclusion of 56 SNPs. SNP exclusion criteria were a call rate of <95 %, monomorphism, deviation from Hardy-Weinberg equilibrium with P <1.0 × 10−7 and concordance in duplicate samples <98% [37]. We additionally excluded seven SNPs with MAF <0.05 in our dataset.

We additionally identified 5,457 SNPs located in or 50 kb around AKR1C4, CYP1A1-CYP1A2, CYP1B1, ESR2, PPARG, PRL, SULT1A1-SULT1A2 and TNF for replication analysis, which were available through iCOGS genotyping and imputation. Of these, 2,541 SNPs were available in BCAC studies and MMHS. For MMHS, the 1000 Genomes Phase I version 3 March 2012 release of the reference panel was used for imputation. Imputation was done using SHAPEIT [38] and IMPUTE.V2 [39]. Imputed SNPs were excluded if the imputation accuracy r 2 was <0.3. For BCAC studies, imputation was conducted centrally in Cambridge, using the same methods and reference panel that were used for MMHS. Imputed SNPs with MAF <0.05 or imputation accuracy r 2 <0.3 were excluded from analysis.

In total, we analyzed 1,327 genotyped SNPs selected based on the genome-wide interaction studies, plus 121 genotyped SNPs and 2,420 imputed SNPs located in or near AKR1C4, CYP1A1-CYP1A2, CYP1B1, ESR2, PPARG, PRL, SULT1A1-SULT1A2 and TNF, for replication analysis. In addition, genotypes of AREG-rs10034692, PRDM6-rs186749 (imputed), ESR1-rs12665607, ZNF365-rs10995190, 8p11.23-rs7816345, LSP1-rs3817198, IGF1-rs703556, 12q24-rs1265507, TMEM184B-rs7289126, and SGSM3-rs17001868 were analyzed.

Statistical analysis

The mammographic density variables were square-root-transformed to meet the model assumption of normality of the error distribution. Associations between SNPs and mammographic density measures (first, percent density and second, dense area and non-dense area) were assessed using multiple linear regression models (PROC MIXED, SAS 9.2). All models were adjusted for study, age (continuous), case status (breast cancer case/non-case), BMI (continuous), former MHT use (yes/no), number of pregnancies (continuous) and 15 ancestry informative principal components (continuous) that had been constructed previously [40]. For MMHS, the principal components were not available and have been set to 0. SNP genotypes were coded according to an additive model (0, 1, 2 alleles) and entered as a continuous variable. For imputed SNPs, the estimated allele dosage was entered (values between 0 and 2). To evaluate interactions between SNPs and current MHT use on mammographic density measures, we included a respective interaction term in the models. All statistical tests were two-sided. To account for the number of tests performed, we calculated adjusted P values according to a false discovery rate (FDR) of 10 %, applying the method described by Benjamini and Hochberg [41]. We report estimates from analyses pooling individual study data, and assessed between-study heterogeneity by calculating Cochrane’s Q and I 2 based on the per-study estimates, using the package meta, version 2.1-2 within the R software, version 2.15.2.

For illustration, the association between current use of MHT and mammographic density measures stratified by SNP genotypes was calculated. For imputed SNPs, allele dosages <0.5 of the coding allele were translated into an imputed homozygous reference genotype. Likewise, allele dosages ≥0.5 and <1.5 were translated into an imputed heterozygous genotype and allele dosages ≥1.5 into an imputed homozygous non-reference genotype.

Because SNPs selected for this study were previously found to have a potential multiplicative statistical interaction with use of MHT on breast cancer risk, we evaluated whether interactions on mammographic density differed for cases and non-cases by entering a three-way interaction term (SNP × MHT × case status) into multiple regression models.

Potential functional implications of selected SNPs were assessed using HaploReg v2 [42] and the University of California Santa Cruz (UCSC) genome browser [43]. Linkage disequilibrium (LD) between SNPs of interest was assessed using LD information from the 1000 Genomes Project within HaploReg v2.

Results

The characteristics of the study population according to mammographic density measurements are described in Table 1. The adjusted mean percent mammographic density was higher in younger women, in women with a lower BMI, and in nulliparous women compared to women with one or more pregnancies. Women currently using MHT compared to never users or former users had a higher adjusted mean percent density as did women diagnosed with breast cancer compared to non-cases. Additional file 2: Table S2 shows the characteristics by study.

Table 1 Mammographic density measurements by known breast cancer risk factors, mammographic projection, and case status at time of mammography
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The beta value from fixed effect meta-analysis of the association between current use of MHT and square-root-transformed percent density was 0.43 (95 % CI 0.34, 0.53). There was some indication of between-study heterogeneity (P value for heterogeneity = 0.12, I 2= 35.4 %), which was attributable to one study (OFBCR). The beta value for current use of MHT was very similar when analyzing solely cases (beta = 0.42, 95 % CI 0.26, 0.58) or non-cases (beta = 0.42, 95 % CI 0.30, 0.54). The forest plots for the whole study sample, non-cases and cases, respectively are displayed in Fig. 1.

Fig. 1
figure 1

Meta-analyses of study estimates for association of current use of menopausal hormone therapy with square-root-transformed percent mammographic density analyzing all study subjects (a), non-cases (b) and cases (c). ABCFS Australian breast cancer family study, BBCC Bavarian breast cancer cases and controls, EPIC European prospective investigation into cancer and nutrition, MCBS Mayo clinic breast cancer study, MCCS Melbourne collaborative cohort study, MEC Multi-ethnic cohort, MMHS Mayo mammography health study, OFBCR Ontario familial breast cancer registry, SASBAC Singapore and Sweden breast cancer study, SIBS Sisters in breast screening study

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We did not identify significant associations between the analyzed SNPs and percent density (all FDR adjusted P values >0.1). The top ten SNPs associated with percent density are shown in Table 2. The association with percent density with the lowest P value was observed for rs181042206 on chromosome 15 (5.2kb 3′ of CYP1A1, beta = −0.10, 95 % CI −0.16, −0.04, P = 0.0006). The association was not heterogeneous between studies (P value for heterogeneity = 0.32, I 2 = 13.6 %). For the components of percent density, rs181042206 was more strongly associated with non-dense area (beta = 0.14, 95 % CI 0.05, 0.23, P = 0.002) than dense area (beta = −0.10, 95 % CI −0.18, −0.02, P = 0.01).

Table 2 Ten SNPs with the lowest P values for association with percent mammographic density
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There was no significant interaction between current use of MHT and the SNPs after correcting for multiple testing (all adjusted P int >0.1). The ten SNPs showing the lowest P int on percent mammographic density are displayed in detail in Additional file 3: Table S3. Results for all investigated SNPs can be found in Additional file 4: Tables S6a to S6c. Table 3 shows associations between current MHT use and mammographic density stratified by these SNPs. The imputed variant rs9358531 on chromosome 6 (6.5kb 5′ of PRL) had the strongest interaction (P int = 0.0004). Current use of MHT was associated with percent mammographic density in women with imputed G/G genotype of rs9358531 with beta = 0.69, 95 % CI 0.49, 0.89, P = 1.4 × 10−11. The association was less strong in women with imputed T/T genotype (beta = 0.23, 95 % CI 0.08, 0.38, P = 2.5 × 10−03). We did not observe study heterogeneity for this interaction (P value for heterogeneity = 0.75, I 2 = 0 %). The interaction between rs9358531 and current use of MHT on percent density was also similar in cases and non-cases (in cases: betaint = 0.28, 95 % CI 0.06, 0.51, P int = 0.01; in non-cases: betaint = 0.21, 95 % CI 0.05, 0.37, P int = 0.01; P int SNP × MHT × case status = 0.60). Six SNPs among the ten SNPs showing the lowest P int are in LD with rs9358531 (rs9356811, rs10946546, rs9393273, rs12525289, rs12199382, rs12524161) with r 2 ranging from 0.50 to 0.80. A genotyped SNP (rs1935007) in moderate LD with rs9356811 (r 2 = 0.42) had a similar but weaker interaction (betaint = 0.20, 95 % CI 0.07, 0.32, P int = 0.002) compared to the imputed SNPs in the region. Furthermore, we observed a potential interaction between MHT and a genotyped SNP on chromosome 13 (rs9542456, 505 kb 3′ of ATXN8OS). In women carrying the G/G genotype, current use of MHT was associated with percent mammographic density (beta = 0.59, 95 % CI 0.46, 0.73, P = 4.7 × 10−18). This association was attenuated in women carrying the A/A genotype (beta = 0.22, 95 % CI −0.01, 0.45, P = 0.06, P int = 0.0009). Also two variants in the proximity of CYP1A1 (rs17861099, rs17861118) had potential interactions, with P int = 0.001. Both variants are in high LD with each other (r 2 = 0.83), but not with the variant rs181042206 identified in the association analysis for percent mammographic density (r 2 <0.20).

Table 3 Association between mammographic density and current use of menopausal hormone therapy stratified by genotypes of ten SNPs showing the lowest P values for interaction
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The ten SNPs showing the lowest P int for percent mammographic density in this study did not overlap with the 14 SNPs recently identified to be potential modifiers of overall/lobular breast cancer risk associated with MHT use [29]. These 14 previously identified SNPs also did not show clear evidence of interaction with current use of MHT on percent mammographic density when analyzing the whole study sample (Additional file 5: Table S4). However, when testing for three-way interactions between SNP, current MHT use and case status, three SNPs in introns of PLCG2 on chromosome 16 (rs7192724, rs4888190, rs17202296) had differential interaction effects for cases and non-cases (P int SNP × MHT × case status = 0.005, 0.007, and 0.019, respectively, Additional file 6: Table S5). Potential interactions with current use of MHT on percent density were observed solely for cases (P int = 0.001, 0.0004, 0.001, respectively), but not in non-cases (P int = 0.57, 0.86, 0.81, respectively).

In our study sample, the ten variants known from genome-wide association studies (GWASs) were associated with percent mammographic density, dense area or non-dense area as expected, also due to overlap between the discovery samples and the studies included in this work (Additional file 7: Table S7). None of them showed a significant interaction with current use of MHT on percent mammographic density (Additional file 8: Table S8).

Discussion

We assessed whether 3,878 SNPs selected based on a meta-analysis of four genome-wide case-only gene-environment interaction studies, candidate gene studies and GWAS of percent mammographic density are differentially associated with mammographic density according to current use of MHT. After accounting for multiple testing, there were no significant interactions for mammographic density between the investigated SNPs and current use of MHT. However, three SNPs in PLCG2 showing potential interaction with MHT use for overall breast cancer risk in our previous study also showed potential interactions with MHT use for mammographic density in this study, but solely among cases. Thus, the results of this study may indicate a potential common pathway between the biological mechanisms underlying higher mammographic density and increased risk of breast cancer associated with MHT use.

The variants rs7192724, rs17202296, and rs4888190 are located within PLCG2, although in two different intronic regions. All three SNPs are located within 6 kb and are in relatively strong LD (D’ >0.85, R 2 >0.71). As none of them are in LD with a coding variant, the effect of the causal SNP may be exerted through a regulatory mechanism. Using HaploReg and the UCSC genome browser, we identified rs12448089 in strong LD with rs4888190 (D’ = 0.98, R 2 = 0.87) as a potential functional variant. SNP rs12448089 is located in a DNase I hypersensitive site and binding motifs of several transcription factors (Additional file 9: Figure S1). Genotypes for rs12448089 were not available in this study.

PLCG2 encodes phospholipase C-gamma 2, an enzyme involved in the transmission of activation signals across the cell membranes predominantly of B cells [44] as well as natural killer cells [45]. PLCG2 plays an important role in immune response regulation [46] and aberrant functioning of PLCG2 due to exon deletions [47] or a missense mutation [46] causes autoimmunity diseases. With regard to breast cancer, PLCG2 has been identified as an irradiation-responsive gene and a potential modifier of breast cancer risk in BRCA2-mutation carriers [48]. The functional role of PLCG2 in breast cancer has to be further elucidated before it is possible to derive a model that would explain our findings with PLCG2 variants.

Two studies reported significant interactions between MHT use and genetic variants in hormone-related genes on mammographic density [24, 28]. The first study of 232 postmenopausal women in clinical trials of estrogen therapy and combined estrogen-progesterone therapy investigated the change in percent mammographic density between a baseline mammogram and a mammogram taken at about one year after randomization [24]. Significant interactions between the assignment to the estrogen therapy/combined estrogen-progesterone therapy treatment arm and the CYP1B1 Val432Leu (rs1056836) polymorphism and the AKR1C4 Leu311Val (rs17134592) polymorphism were reported (P int = 0.0004 and 0.001, respectively). The CYP1B1 Val432Leu polymorphism was previously evaluated for interaction with MHT use in 538 women and no interaction for percent mammographic density was found (P int = 0.70) [22]. In this study, neither the CYP1B1 Val432Leu nor the AKR1C4 Leu311Val polymorphism showed an interaction with current use of MHT on percent mammographic density (P int = 0.19 and 0.47, respectively). The second and larger study of 2,036 postmenopausal women who attended the Norwegian Breast Cancer Screening Program reported a significant interaction between SNP rs10946545 located 1.4kb 3′ of PRL and current use of estrogen-progesterone therapy for percent mammographic density (P int = 0.0008) [28]. This SNP did not show a potential interaction with current MHT use here (P int = 0.07), however, the non-significant interaction was in the same direction as that previously reported [28]. Several other variants located in the 5′ region of PRL had the lowest P int values (Table 3), however, they were not in LD with rs10946545. The function of those SNPs near PRL is unknown, the SNP with the strongest interaction (rs9358531) is not located in an obvious regulatory element (Additional file 10: Figure S2). PRL encoding the hormone prolactin may be an interesting candidate for further studies of interactions between genetic variants and use of MHT with respect to mammographic density. In humans, prolactin is primarily produced in the anterior pituitary gland, but it is also expressed in the mammary gland itself and adipose tissue, among others [49]. In postmenopausal women, higher prolactin levels have been associated with higher mammographic density in three studies [50–52], two other studies found no association [53, 54].

Ten loci have been identified by GWAS to be associated with percent mammographic density, ZNF365-rs10995190 [10, 30], 12q24-rs1265507 [31], and AREG-rs10034692, PRDM6-rs186749, ESR1-rs12665607, 8p11.23-rs7816345, LSP1-rs3817198, IGF1-rs703556, TMEM184B-rs7289126, and SGSM3-rs17001868 [10]. The ZNF365, ESR1, LSP1 and SGSM3 loci are also associated with breast cancer risk [10, 30], implicating a shared genetic basis of mammographic density and breast cancer. This is also supported by the findings of a study, which assessed a polygenic score based on mammographic density GWAS with respect to breast cancer risk [8]. Women in the top 10 % of the score had an associated 31 % increased risk of breast cancer compared to women in the bottom 10 % of the score. The locus on 12q24 on the other hand, seems not to be associated with breast cancer risk [31]. Our data indicated that associations of the SNPs identified by GWAS with mammographic density are unlikely to be modified by MHT use.

Unlike in previous studies investigating interactions between genetic variants and MHT use with regard to mammographic density, the comprehensive set of SNPs investigated here was not only based on candidate genes, but selected also based on genome-wide case-only gene-environment interaction studies on breast cancer risk. GWAS have been generally more successful at identifying novel associations with complex diseases than candidate gene or linkage studies [55]. Thus, this approach to selecting SNPs can be considered as more promising. On the other hand, in doing so we made the fairly strong assumption that the biological mechanisms involved in the increase in breast cancer risk associated with MHT use at least in part overlap with the mechanisms involved in the increase in mammographic density associated with MHT use. This does not necessarily have to be true, although we identified polymorphisms in PLCG2 that had interactions with MHT use for both breast cancer risk and mammographic density.

We were able to account for potential confounders of the association between current use of MHT and mammographic density such as age, BMI, case status, and number of pregnancies. All included studies were genotyped using the same genotyping platform. The study design and methods used to measure mammographic density varied between studies included in this analysis. However, the estimates for association and interaction were consistent across studies, supporting the robustness of our results. The sample size of our study was fairly large and the power was sufficient to detect strong SNP-MHT interactions on mammographic density with a beta value of 0.40. However, the power to detect weak to moderate interactions, in the range we observed in this dataset with betas of about 0.20, was limited and would require a sample at least four times larger. It is therefore possible that we missed potentially relevant interactions. Another limitation is the lack of type-specific information on MHT use. We could therefore not investigate interactions with use of combined estrogen-progesterone therapies, which seem to be more strongly associated with an increased mammographic density than estrogen-only therapies [16, 17]. Studies with type-specific information on MHT use could therefore also observe interactions of larger magnitudes compared to the magnitudes of the interactions observed here. However, the four genome-wide gene-environment interaction studies on which we partly based our SNP selection also investigated solely interactions with use of any MHT.

Conclusion

We observed little evidence for strong interactions between the investigated SNPs and MHT use on mammographic density. The study identified variants near PRL and also in PLCG2 for cases only, as potential modifiers of the association between MHT use and mammographic density. These findings will need to be confirmed in larger independent studies. The identification of additional gene-MHT interactions is likely to require very large (genome-wide) studies with type-specific information on MHT use, given the likely moderate/weak magnitude of such interactions.