Abstract P3-05-01: Age-related methylation signals of breast cancer risk in blood
Authors: Hofstatter EW , Levine M , Hatzis C , Pusztai L 
Published: May 2019

Abstract
BACKGROUND: Age is the biggest risk factor for developing breast cancer, which suggests that the biological aging process is a direct driver of cancer etiology. In all normal tissues, DNA methylation status changes systematically with age and is believed to mediate the biological consequences of aging. DNA methylation patterns, commonly referred to as 'Epigenetic Clocks', can be used as measures of aging. However, the chronologic and epigenetic ages can have subtle differences in different individuals. We hypothesize that accelerated epigenetic aging (i.e. DNA methylation indicating an older age than the chronologic age of the individual) is a risk factor for breast cancer development.
METHODS: We used DNA methylation data from blood samples and clinical data from n=2,107 participants in the Women's Health Initiative (WHI) and tested the association between breast cancer risk and two of the most commonly used epigenetic clocks by Horvath (based on 353 CpGs) and Hannum et al. (based on 71 CpGs). DNA methylation in whole blood was measured using the Illumina HumanMethylation450 BeadChip. We used Cox proportional hazard models to assess the association between two epigenetic age predictors (calculated using the algorithms by Horvath and Hannum et al.) and subsequent risk of breast cancer. The model was adjusted for several breast cancer risk factors including: chronologic age at the time of blood sampling, observational vs. clinical trial, clinical trial arm, race/ethnicity, education, BMI, waist-hip ratio, smoking, alcohol, age at menopause, age at menarche, number of pregnancies, age at first birth, previous mastectomy, months breastfed, and cell count estimates. Family history data and BRCA mutation status was incomplete in the WHI and therefore could not be included in our analysis.
RESULTS: Increased epigenetic age determined by the Horvath clock relative to chronological age was associated with increased future incidence of invasive breast cancer, even after adjusting for known risk factors (HR=1.04, P=0.03). Utilizing the Hannum clock, we found no significant association between epigenetic age and breast cancer risk (HR=1.01, P=0.568). When we included both age predictors as independent variables in a single model, the strength of the association between the Horvath epigenetic age and breast cancer risk increased (Horvath HR=1.09, P=6.3e-5; Hannum HR=0.95, p=0.077), such that every one year increase in epigenetic age relative to chronological age was associated with a 9% increased risk of future breast cancer. These results suggest that the aging signal in the Horvath clock that is unique from that captured by Hannum is what drives the specific association with breast cancer.
CONCLUSIONS: Our results support the hypothesis that “accelerated” epigenetic aging measured in the blood increases breast cancer risk. We also demonstrate that the two epigenetic clocks capture different aspects of aging, only some of which have implications for breast cancer risk. Epigenetic clocks may assist in targeting breast cancer screening to higher risk populations in the future, and understanding the biological mechanisms that are altered by the epigenetic changes may lead to new risk reduction strategies.
Citation Format: Hofstatter EW, Levine M, Hatzis C, Pusztai L. Age-related methylation signals of breast cancer risk in blood [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-05-01.