Interactive effect of genetic susceptibility with height, body mass index, and hormone replacement therapy on the risk of breast cancer

Genome wide association studies (GWAS), have discovered several new genetic polymorphisms affecting breast cancer risk [1‐3]. Even though these individual risk-factors each confer quite small increases in risk, a positive association is seen between the number of risk alleles carried and risk for breast cancer [4, 5].

The phenotypic variables height, body mass index (BMI) and use of hormone replacement therapy (HRT) reflect to varying degrees genetic background and environmental exposure. Both height and BMI have previously been shown to associate with breast cancer [6, 7]. Increase in height has been shown to yield a proportional increase in breast cancer risk and obese women have a greater risk to contract postmenopausal breast cancer. Increased risk is also established for users of HRT [7], which has been speculated to interact with low-risk polymorphisms in the FGFR2 gene [8, 9].

Although there have been investigations on gene-environment interactions in breast cancer [10], this area remains to a large extent unexplored.

The aim of this study was to investigate if height, BMI and HRT modify the genetic predisposition to breast cancer conferred by reported low-risk polymorphisms. For this purpose we had access to two well defined Swedish population based cohorts as well as an Icelandic hospital based case control study, altogether 7738 samples (3016 cases and 4722 controls).

Of the initial 7738 samples selected for the project 7392 (95.5%) were successfully retrieved and genotyped for ≥ 80% of the SNPs. All SNPs had a genotyping success rate > 94%, with an average of 98.0%. Results of all 200 analyses performed on duplicate samples were in 100% concordance.

Per allele OR for each independent SNP is presented in Figure 1. Ten of the SNPs were significant (p < 0.05) in our material with rs2981852 (FGFR2), rs889312 (MAP3K1) and rs3803662 (TOX3) exhibiting the highest ORs. Two of the SNPs had p-values >0.1 (rs1045485 [CASP8] and rs30099 [5q11]) and were excluded from further analysis.

Three of the SNPs in TOX3 (rs3803662, rs12443621 and rs8051542) exhibited linkage (results not shown) as has previously been reported [1, 4]. Rs12443621 and rs80515442 were therefore excluded from further analysis.

Independent analysis of risk association with each phenotypic variable (height, BMI, HRT) within the entire study population revealed a significantly increased risk of breast cancer for individuals >162 cm compared to shorter women, this association was weakened following age adjustment. No statistical significant correlation between BMI and risk for breast cancer was found in this population. For current use vs. non-use of HRT, a significantly increased risk was seen for users, OR (95% CI) 1.24 (1.08-1.42), which remained after adjustment for age (Table 1).

In this study we have explored interactions between reported genetic risk factors for breast cancer and the three additional established risk factors; height, BMI and HRT in 2884 cases and 4508 controls. The strongest tendency for interaction found was that between height and rs851987 in ESR1, although it did not pass the threshold for multiple comparisons. Taller women carrying the T-allele appeared to have reduced breast cancer risk (p for interaction = 0.007) (Table 2). Rs851987 was described by Harlid et al. [11] and is situated in the far end of the extended promoter region of ESR1, about 3.7 kb 5′ of exon F. Exon F and its promoter were originally described by Thompson et al. [17] and have later been shown to affect the level of ESR1 expression in osteoblastic cells [18, 19]. A potential association between ESR1 and height has been described in another study comprising adult males from two Swedish population cohorts [20]. Mutations in ESR1 have been reported to delay fusion of the epiphyseal plates at puberty [21], and one may speculate that rs851987 either participates in this biological effect or is linked to other causal variants.

One SNP (rs13281615 in 8q4) first described by Easton et al. [1] showed a weak tendency for interaction with use of HRT. The minor allele seems to confer increased breast cancer risk in HRT non-users but no excess risk in current users. The association in non-users is strong with a per-allele OR (95%CI) of 1.20 (1.10-1.31) (p-trend = 6.1 × 10^-5) compared to a per-allele OR (95%CI) of 1.08 (1.01-1.15) (p-trend = 0.03) in all users. The SNP is situated in region 8q24 that contains no known genes but is in close proximity to FAM84B (coding for a breast cancer membrane associated protein) and the proto-oncogene MYC. The 8q24 locus has previously been reported to associate with other types of cancer in addition to breast cancer [22] and to be more strongly associated with ER + than ER- tumours [23].

Since the first GWAS on breast cancer was published in 2007 several replication and interaction studies of varying sizes have been published [24‐27]. In 2010, a large interaction study comprising 7610 breast cancer cases from the Million Women Study in UK was undertaken and potential interactions between 12 different SNPs and 10 different variables (including height, BMI and HRT) were tested [10]. This study did not find (contrary to previous suggestions) any significant gene-environment interactions. Our study originally included ten of the same polymorphisms as in the Million Women Study (excluding rs1982073 in TGFB1 and rs1800054 in ATM), but also included one additional SNP from Easton et al. [1] and two additional SNPs from our own candidate CpG study [11] (rs7766585 and rs851987 both in ESR1). Although our material is not as large, our study is comprised of three well described study-populations, two of which were prospectively followed for breast cancer incidence using the comprehensive, population-based Swedish Cancer Registry [28]. Thus, our complete case ascertainment and ability to select matched controls from the same study base is likely to have resulted in low risk for selection biases. However, the intervals between data collection, blood sampling, and diagnosis differ substantially between the three different study populations, something that might be considered a limitation of the study.

Considering demographic traits, participants in the MDCS have a slightly higher socioeconomic status than the general population, but as this selection is the same for the study base from which cases and controls are derived, it should not affect the validity of our study [13]. MDCS participants were recruited at age 45–65 years. The exclusion of prevalent cases removes early breast cancer cases from this population. While the NHSDS participants were primarily included from age 40 and upwards, mammography screening had identified some cases as young as 27 years. In Iceland prevalent cases of breast cancer were recruited at varying times after diagnosis, resulting in an exclusion of early lethal cases and older women with other causes of death. As the Icelandic controls were collected later and from the same sample population as the cases there is the possibility of selection bias. Another limitation of our study is the fact that HRT is reported only once (at recruitment) without information about duration. We also lacked information about other risk factors than age, height, BMI, HRT and therefore could not adjust our results for other potential confounders.

Our evaluation of genetic predisposition for breast cancer in relation to three different environmental risk factors found no significant gene-environment interactions. We did find tendencies for certain SNPs to exert an effect on breast cancer risk only in women with certain phenotypes. In particular the potential interaction between height and rs851987 in ESR1 in relation to breast cancer risk could merit further investigation. However, independent studies with many more cases would be needed to verify this finding.