Association Between Serum Iron Biomarkers and Breast Cancer

Iron is essential to life, but potentially carcinogenic at high levels, through mechanisms related to tumor initiation and growth (1). In epidemiologic studies, evidence for an association between cancer and iron status has been mixed, with different biomarkers examined in different populations across varying durations of time. A recent meta-analysis of breast cancer incidence found substantial heterogeneity in associations with higher levels of circulating iron (2). Our aim was to comprehensively assess three common iron biomarkers separately and jointly in relation to breast cancer incidence using data from a large cohort of women.

The Sister Study (3) recruited a cohort of 50,884 women (2003–2009), each of whom had at least one sister who was diagnosed with breast cancer. Participants provided written informed consent and the study was overseen by the Institutional Review Board of the NIH (Bethesda, MD). This analysis uses a case–cohort sample of 6,008 women, including 2,808 incident cases and a randomly selected subcohort of 3,200 women (of whom 222 developed breast cancer during follow-up), through September 2017, data release 7.1. Of the 6,008 women, 5,926 women remained after excluding women with all iron measures missing (n = 56), uncertain event status (n = 9), follow-up time <1 month (n = 14), or unknown event timing (n = 3).

Cases were identified as ductal carcinoma in situ (DCIS) or invasive incident breast cancer. Tumor hormone status was based on medical records and self-report, in that order of availability (4).

We used three biomarkers of iron status, serum iron, ferritin, and percent transferrin saturation, assessed in samples provided at cohort enrollment. Ferritin is a circulating iron-binding protein representing iron stores. Percent transferrin saturation indicates the percentage of bound plasma transferrin iron-binding sites, calculated as a function of measured serum iron and unsaturated iron binding capacity (UIBC) as 100[iron/(iron + UIBC)]. The mean intrabatch coefficients of variation were 3.1, 1.4, and 2.7 for iron, ferritin, and transferrin saturation, respectively. A description of laboratory measures is included in the Supplementary Data. We also performed principal components (PC) analysis for the three biomarkers and used the first PC as a summary measure. The first PC explained 72% of the variance for the 3-dimensional marker distribution.

We used R (5) software with the survival package (6) to fit Cox models with age as the primary time scale to estimate breast cancer HRs for serum iron biomarkers, using the Prentice case–cohort method (7). Iron variables were analyzed as both continuous and divided into quartiles in separate models. We adjusted for baseline smoking, alcohol, education, hormone replacement therapy, age at menarche, age at first birth, oral contraceptive use, body mass index (BMI), menopause status, and a product term between the last two variables. This product term allowed the association with BMI to change after menopause.

Sensitivity and/or post hoc analyses included: (i) restricting the analyses to the first 4 years of follow-up, as more proximate to the baseline iron measures; (ii) restricting by excluding the first 2 years of follow-up, to avoid iron sequestration responses to early carcinogenesis; (iii) estimating breast cancer HRs for particularly low values of ferritin or high values of transferrin saturation, often used as cut-off points for iron deficiency or overload, respectively; (iv) assessing etiologic heterogeneity related to iron levels for invasive versus DCIS, tumor subtypes, and stage, based on case-only logistic analyses; (v) stratifying Cox models based on race/ethnicity and BMI; and (iv) stratifying on pre- and postmenopausal status.

Sample characteristics are shown in Table 1. Cases were more educated and less likely to have experienced early menopause than noncases.

There was little evidence of an association between the three serum iron biomarkers, or the first PC of those three measures, and breast cancer risk (Table 2). Adjusted HRs [95% confidence interval (CI)] for fourth versus first quartile of exposure and breast cancer were: iron [1.06, (0.90–1.25)], ferritin [1.03 (0.87–1.23)], transferrin saturation [0.94 (0.80–1.12)], and the first PC [1.06 (0.90–1.25)].

Sensitivity analyses were consistent with the primary findings, further supporting the absence of any strong association with breast cancer. The post hoc analyses (Supplementary Tables S1–S8) largely aligned with the primary findings. Case-only analyses did not provide evidence for an association between higher iron levels and cancer subtype or stage (Supplementary Tables S7 and S8). However, analyses by BMI subgroups suggested overall decreased risk for the lowest quartile compared with others (Supplementary Table S2). If we compare the lowest with the grouped top three quartiles, the adjusted HRs (95% CI) were 0.72 (0.56–0.92) for iron and 0.84 (0.66–1.09) for ferritin among women with a BMI ≥ 30 kg/m² (Supplementary Table S3). Similarly, stratified analyses for the premenopausal group indicated decreased risk for those with low iron stores (Supplementary Table S4). Comparing the lowest quartile with the top three quartiles, the adjusted HRs (95% CI) were 0.70 (0.51–0.97) for iron, 0.73 (0.54–0.99) for ferritin, 0.71 (0.52–0.96) for transferrin saturation, and 0.64 (0.47–0.88) for the first PC (Supplementary Table S4). Iron stores tend to increase postmenopausally, but there was also evidence for protection for those who still had low ferritin postmenopausally (Supplementary Tables S5 and S6).

There was little evidence of association between iron overload and breast cancer incidence in the Sister Study. These findings, from one of the largest studies to date, are similar to those of another U.S. cohort (8), but contrast with studies from other countries, some of which found either strong positive or inverse associations between serum ferritin levels and breast cancer (2). Our exploratory analyses suggested reduced risk at the lowest levels of ferritin and iron, particularly among obese women, a group with increasing numbers worldwide. However, little evidence exists in this large U.S. sample to support an association between iron overload and breast cancer.

No disclosures were reported.

A. Von Holle: Conceptualization, formal analysis, visualization, writing–original draft, writing–review and editing. K.M. O'Brien: Writing–review and editing. D.P. Sandler: Supervision, funding acquisition, investigation, writing–review and editing. R. Janicek: Writing–review and editing, responsible for the sample handling and laboratory assessments. C.R. Weinberg: Conceptualization, supervision, funding acquisition, investigation, methodology, writing–original draft, project administration, writing–review and editing.

This work was supported by the Division of Intramural Research at NIEHS, in the NIH, under projects Z01-ES044005 (to D.P. Sandler) and Z01-ES102245 (to C.R. Weinberg).

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