• Department of Biostatistics and Epidemiology, University of Massachusetts, Amherst, Massachusetts. ssturgeon@schoolph.umass.edu.
• Department of Food Science, University of Massachusetts, Amherst, Massachusetts.
• Department of Veterinary and Animal Sciences, University of Massachusetts, Amherst, Massachusetts.
• Department of Biostatistics and Epidemiology, University of Massachusetts, Amherst, Massachusetts.
• Epidemiology and Biostatistics Program, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Maryland.
• Department of Epidemiology, Department of Pathology, and Department of Laboratory Medicine, Brown University, Providence, Rhode Island.

BACKGROUND: White blood cell (WBC) DNA may be associated with breast cancer risk. Using a high-throughput array and samples collected, on average, 1.3 years prior to diagnosis, a case-cohort analysis nested in the prospective Sister Study identified 250 individual CpG sites that were differentially methylated between breast cancer cases and non-cases. We examined five of the top 40 CpG sites in a case control study nested in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO) Cohort.

METHODS: We investigated the associations between pre-diagnostic WBC DNA methylation in 297 breast cancer cases and 297 frequency-matched controls. Two WBC DNA specimens from each participant were used: a proximate sample collected 1 to 2.9 years and a distant sample collected 4.2-7.3 years prior to diagnosis in cases or the comparable timepoints in controls. Methylation level was measured using targeted bisulfite amplification sequencing. We used logistic regression to obtain odds ratios (OR) and 95% confidence intervals (CI).

RESULTS: A one-unit increase in percent methylation in ERCC1 in proximate WBC DNA was associated with increased breast cancer risk (adjusted OR=1.29; 95%CI: 1.06-1.57). However, a one-unit increase in percent methylation in ERCC1 in distant WBC DNA was inversely associated with breast cancer risk (adjusted OR=0.83; 95%CI:0.69-0.98). None of the other odds ratios met the threshold for statistical significance.

CONCLUSIONS: There was no convincing pattern between DNA methylation in the five CpG sites and breast cancer risk.

IMPACT: The link between prediagnostic WBC DNA methylation and breast cancer risk, if any, is poorly understood.