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. Overall design: Examination of RUNX1 binding in MCF7 cells

转录因子RUNX1在雌激素受体阳性（ER+）乳腺癌（BCa）中存在频发的功能丧失性突变。体外实验中对其进行基因敲低，会以雌激素依赖的方式下调AXIN1的表达。与之相符的是，在ER+肿瘤而非ER-肿瘤中，RUNX1与AXIN1的mRNA表达水平呈显著相关。RUNX1可在活细胞中结合AXIN1的第二内含子区域，紧邻雌激素受体α（ERα）的结合位点。RUNX1基因敲低后AXIN1表达下调会伴随β-连环蛋白（β-catenin）的上调，而这一效应可被AXIN稳定剂阻断，该现象或可促进BCa进展。然而与结肠癌不同，BCa细胞中β-连环蛋白的调控异常既不会影响c-Myc的表达，也不会影响细胞周期蛋白D1（CCND1）的表达，亦不会影响G1/S细胞周期时相转换。与之相反，细胞周期蛋白B1的表达出现下调，且G2/M检验点功能受损，这一现象可通过微管干扰剂存在时发生的有丝分裂滑移得到证实。综上，通过联合分析RUNX1的转录组、顺式调控组（cistrome）以及RUNX1野生型与突变型肿瘤间的差异mRNA表达谱，本研究证实了RUNX1/AXIN1/β-连环蛋白信号轴在ER+ BCa中的作用。
研究意义：近期三项外显子测序研究证实RUNX1具有BCa抑癌作用。本研究首次揭示了其潜在分子机制，为该抑癌作用仅特异性针对ER+肿瘤提供了解释，并将AXIN1确定为ER+/RUNX1缺陷型BCa的治疗靶点。
实验设计：检测MCF7细胞中RUNX1的结合位点。