Abstract 472: Tumor DNA methylation and microRNA expression in breast cancer among American women of African or European ancestry
Authors: Gong Zhihong , Wang Dan , Wang Jie , Buas Matthew F. , Liu Song , Belinsky Steven A. , Higgins Michael J. , Ambrosone Christine B. 
Published: August 2018

American women of African ancestry (AA) are more likely than those of European ancestry (EA) to be diagnosed with aggressive, estrogen receptor (ER) negative breast cancer, and to die of this disease. The reasons remain largely unknown. Epigenetic mechanisms, particularly differential DNA methylation and altered microRNA expression, may contribute to this racial disparity. We previously conducted genome-wide DNA methylation profiling of fresh-frozen breast tumor samples from 58 AA and 80 EA women, with subsequent analyses restricted to CpGs within coding genes. In this study, we focused on CpGs within loci encoding microRNAs, and sought to identify differentially methylated loci (DML) by ER status and by race. We identified a total of 297 DML (Δβ &gt;0.1; FDR&lt;0.05) between ER negative and ER positive breast tumors. Of these, 113 were differentially methylated regardless of race; 65 were specific to EAs; and 119 were specific to AAs. The top DML common to both races were located within miR-155HG, -365-1, -135b, -196b, and -190b, while the top race-specific DML were located within miR-1256, -1224, -1249, -9-3, -657/-338, and miR-25 (EA), or within miR-2053, -526a1, -518B, -489, and -125b1 (AA). We also identified 25 DML (Δβ &gt;0.1; P&lt;0.05) between AA and EA tumors. Among these 25 DML, all of which were hypomethylated in AAs versus EAs, three were specific to ER+ tumors (miR-548G/FILIP1L,-572, -661/PLEC1), 21 were specific to ER- tumors (e.g., miR-2053, -1323, -125b1), and one was common to both tumor subtypes (miR-662). In order to assess how DNA methylation may impact gene expression, we performed miRSeq on tumors from a subset of included women (29 AA and 29 EA). We observed that altered methylation at certain DML was inversely associated with microRNA expression; for example, three DML in the promoter region of miR-190b (Spearman's rho=-0.65, -0.68 and 0.55, respectively; all P&lt;1.2x10-6), and two in miR-135b (rho=-0.56 and -0.59, respectively; both P&lt;1.2x10-07). Interestingly, multiple loci within the promoter region of miR-190b were hypermethylated in ER- compared to ER+ tumors, and we further observed that miR-190b expression was significantly downregulated in ER- versus ER+ tumors (log2FC=-2.45 and -4.92 in EAs and AAs, respectively; FDR&lt;0.05). Lower expression of miR-190 has been linked previously to reduced breast cancer survival, potentially reflecting effects on IGF-related pathways. In summary, based on genome-wide profiling of tumor DNA methylation and microRNA expression, our results suggest that DNA methylation patterns differ by tumor subtype and by race, and that this altered methylation may affect gene expression. These findings will provide the basis for further functional analyses, which are likely to improve our understanding of race-related tumor biological differences and lead to specific targeted preventative and therapeutic strategies.

Citation Format: Zhihong Gong, Dan Wang, Jie Wang, Matthew F. Buas, Song Liu, Steven A. Belinsky, Michael J. Higgins, Christine B. Ambrosone. Tumor DNA methylation and microRNA expression in breast cancer among American women of African or European ancestry [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 472.