Enhancer recruitment of transcription repressors RUNX1 and TLE3 by mis-expressed FOXC1 blocks differentiation in acute myeloid leukemia

Despite absent expression in normal hematopoiesis, the Forkhead factor FOXC1, a critical mesenchymal differentiation regulator, is highly expressed in ~30% of HOXAhigh AML to confer blocked monocyte/macrophage differentiation. Through integrated proteomics and bioinformatics, we discovered that FOXC1 and RUNX1 interact through Forkhead and Runt domains respectively and cooccupy primed and active enhancers distributed close to differentiation genes. FOXC1 stabilises association of RUNX1, HDAC1 and Groucho repressor TLE3 to limit enhancer activity: FOXC1 knockdown induced loss of repressor proteins, gain of CEBPA binding, enhancer acetylation and upregulation of nearby genes, including KLF2. Furthermore, it triggered genome-wide redistribution of RUNX1, TLE3 and HDAC1 from enhancers to promoters leading to repression of self-renewal genes including MYC and MYB. Our studies highlight RUNX1 and CEBPA transcription factor swapping as a feature of leukemia cell differentiation, and reveal that FOXC1 prevents this by stabilising enhancer binding of a RUNX1/HDAC1/TLE3 transcription repressor complex, to oncogenic effect.