Abstract B51: Exploring the role of reproductive factors and DNA methylation in ethnic disparities in breast cancer tumor aggressiveness
Authors: Dookeran Keith A. , Mahmoud Abeer M. , Poulin Matthew , Yan Liying , Ehrlich Melanie , Kresovich Jacob K. , Macias Virgilia , Kajdacsy-Balla Andre , Wiley Elizabeth , Rauscher Garth H. 
Published: October 2015

Purpose: Non-Hispanic (nH) Black and Hispanic (or minority) breast cancer patients tend to be diagnosed with more aggressive forms of breast cancer compared to their nH White counterparts. Prior research as well as analyses from the current study has identified hormonal and reproductive factors associated with breast cancer aggressiveness subtypes. We explored the potential role of hormonal and reproductive factors, and the potential contribution of DNA methylation, in explaining the racial/ethnic disparity in tumor aggressiveness in a population based study of breast cancer disparities.

Methods: The breast Cancer Care in Chicago (BCCC) study included 989 recently diagnosed nH White, nH Black and Hispanic patients with first primary breast cancer. Analyses include a subset of 286 patients with available tumor immunohistochemistry (IHC) data on estrogen and progesterone receptor (ER/PR), HER2, p53 and Ki67 status. A tumor aggressiveness score (TAS) with a high internal reliability coefficient (Chronbach's alpha=0.76) was created from tumor grade and IHC data on ER, PR, HER2, p53 and Ki67. Values were standardized to have a mean of 0 and standard deviation of 1, then summed together and re-standardized to create the score. Pyrosequencing assays for DNA methylation were conducted on a set of DNA sequences identified based on prior literature, ENCODE data for DNA methylation, and transcription for normal vs. cancer cell lines from the UCSC genome browser. Of the 286 patients with tumor aggressiveness data, 214 had available methylation data for BRCA1, GSTM2, EGFR, RASSF1, Sat2 and TFF1 genes. Multivariable linear regression models were estimated with standardized aggressiveness score as the dependent variable and using nested models to conduct likelihood ratio tests for both forward (type 1) and backwards (type 3) analyses. A method of rescaled-coefficients was then used to estimate an average controlled direct effect representing the ethnic disparity in breast cancer tumor aggressiveness and the extent to which the disparity might be explained by patient reproductive factors and tumor DNA methylation. Because stage at diagnosis is downstream of, and strongly influenced by, tumor aggressiveness, it was excluded from our analyses.

Results: Factors significantly associated with having a higher TAS (p&lt; 0.05) included: nH black or Hispanic race/ethnicity; younger age at first birth; nulliparity; positive family history (FH) of breast cancer, longer use of oral contraceptives; higher pathologic stage; and higher mean BRCA1, GSTM2 and TFF2 methylation levels. In multivariable modeling, variables retained for further analysis included: FH, nulliparity, and methylation of BRCA1, GSTM2, TFF1 and Sat2. In forwards (type 1) analysis of nested models, both nulliparity (p= 0.034) and DNA methylation variables (p&lt; 0.001) were retained. In backwards (type 3) analyses, DNA methylation variables (p&lt; 0.001) but not nulliparity (p= 0.261) were retained. As independent domains, DNA methylation variables explained 46% of the ethnic disparity in TAS (p= 0.054), whereas nulliparity explained 38% (p= 0.058), and family history did not explain any of the disparity. Further, the combination of DNA methylation variables and nulliparity together explained 63% of the disparity (p= 0.033).

Conclusions: Our findings suggest that DNA methylation of specific genes may influence breast cancer tumor aggressiveness and may help to explain the preponderance of aggressive subtypes diagnosed in ethnic minority women. DNA methylation may represent a promising avenue for biomarker development for early detection for biologically aggressive tumor types in vulnerable populations. These findings require replication and validation in other studies.

Citation Format: Keith A. Dookeran, Abeer M. Mahmoud, Matthew Poulin, Liying Yan, Melanie Ehrlich, Jacob K. Kresovich, Virgilia Macias, Andre Kajdacsy-Balla, Elizabeth Wiley, Garth H. Rauscher. Exploring the role of reproductive factors and DNA methylation in ethnic disparities in breast cancer tumor aggressiveness. [abstract]. In: Proceedings of the Seventh AACR Conference on The Science of Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; Nov 9-12, 2014; San Antonio, TX. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2015;24(10 Suppl):Abstract nr B51.