• Department of Biostatistics and Epidemiology, University of Massachusetts-Amherst, 715 North Pleasant Street, Arnold House 407, Amherst, MA, 01003, USA. ssturgeon@schoolph.umass.edu.
• Department of Environmental Health Sciences, University of Massachusetts, Amherst, MA, 01003, USA.
• Department of Veterinary and Animal Sciences, University of Massachusetts, Amherst, MA, 01003, USA.
• Department of Civil and Environmental Engineering, University of Massachusetts, Amherst, MA, 01003, USA.
• Department of Biostatistics and Epidemiology, University of Massachusetts-Amherst, 715 North Pleasant Street, Arnold House 407, Amherst, MA, 01003, USA.
• Eppley Institute and Fred & Pamela Buffett Cancer Center, University of Nebraska Medical Center, Omaha, NE, 68198, USA.
• Epidemiology and Biostatistics Program, Division of Cancer Epidemiology and Genetics, National Cancer institute, Bethesda, MD, 20892, USA.

BACKGROUND: Several studies have suggested that global DNA methylation in circulating white blood cells (WBC) is associated with breast cancer risk.

METHODS: To address conflicting results and concerns that the findings for WBC DNA methylation in some prior studies may reflect disease effects, we evaluated the relationship between global levels of WBC DNA methylation in white blood cells and breast cancer risk in a case-control study nested within the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial (PLCO) cohort. A total of 428 invasive breast cancer cases and 419 controls, frequency matched on age at entry (55-59, 60-64, 65-69, ≥70 years), year of entry (on/before September 30, 1997, on/after October 1, 1997) and period of DNA extraction (previously extracted, newly extracted) were included. The ratio of 5-methyl-2' deoxycytidine [5-mdC] to 2'-deoxyguanine [dG], assuming [dG] = [5-mdC] + [2'-deoxycytidine [dC]] (%5-mdC), was determined by liquid chromatography-electrospray ionization-tandem mass spectrometry, an especially accurate method for assessing total genomic DNA methylation.

RESULTS: Odds ratio (OR) estimates and 95% confidence intervals (CI) for breast cancer risk adjusted for age at entry, year of entry, and period of DNA extraction, were 1.0 (referent), 0.89 (95% CI, 0.6-1.3), 0.88 (95% CI, 0.6-1.3), and 0.84 (95% CI, 0.6-1.2) for women in the highest compared to lowest quartile levels of %5md-C (p for trend = .39). Effects did not meaningfully vary by time elapsed from WBC collection to diagnosis.

DISCUSSION: These results do not support the hypothesis that global DNA hypomethylation in WBC DNA is associated with increased breast cancer risk prior to the appearance of clinical disease.