Breast Cancer Suppressor Role of RUNX1: Estrogen-dependent Regulation of AXIN1 and b-catenin

The transcription factor RUNX1 exhibits recurrent loss-of-function mutations in estrogen receptor-positive (ER+) breast cancer (BCa). Its knockdown in vitro decreased AXIN1 expression in estrogen-dependent manner. Consistently, RUNX1 and AXIN1 mRNA levels are strongly correlated in ER+, not ER- tumors. RUNX1 occupies AXIN1’s second intron in living cells, abutting an ERa-binding site. Potentially promoting BCa progression, decreased AXIN1 expression after RUNX1 knockdown associated with upregulation of b-catenin, and this was preventable by AXIN stabilizers. Unlike in colon cancer, however, deregulation of b-catenin in BCa cells affect neither c-Myc, nor CCND1, nor G1/S cell cycle phase transition. Instead, cyclin B1 was decreased and the G2/M checkpoint was compromised as indicated by mitotic slippage in the presence of microtubule disruptors. Thus, combined analysis of the RUNX1 transcriptome, its cistrome, and differential mRNA expression in tumors with wild type versus mutant RUNX1, altogether highlight a role for the RUNX1/AXIN1/b-catenin axis in ER+ BCa. Significance: Three recent exome sequencing studies assigned to RUNX1 a BCa suppressor role. The present study begins to uncover the underlying molecular mechanisms, offers an explanation for the specificity to ER+ tumors, and marks AXIN1 as a therapeutic target for ER+/RUNX1- BCa. Examination of RUNX1 binding in MCF7 cells