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Loss of the MLL3 tumor suppressor accelerates breast tumor onset via HIF1α-induced CCL2-mediated recruitment of CCR2+ regulatory T cells

Understanding how specific cancer mutations modulate the tumor microenvironment is essential for developing new precision cancer therapies. The histone methyltransferase MLL3 is a tumor suppressor that is frequently mutated together with p53 and PI3-kinase in breast cancer. Herein, using a mouse mammary-stem-cell-based tumor model and CRISPR gene editing, we recapitulated this genetic setting and revealed that Mll3 loss promotes tumorigenesis through enhancing tumor-intrinsic aggressiveness and establishing an immunosuppressive microenvironment. Specifically, HIF1α was upregulated in Mll3-mutant tumors and controlled proliferation and stem cell activity of the tumor cells. HIF1α also transcriptionally upregulated expression of the chemokine CCL2 by tumor cells. CCL2 further contributed to rapid CCR2-dependent accumulation of activated, proliferative KLRG1+Foxp3+ regulatory T (Treg) cells at very early stages of tumor initiation, promoting faster tumor onset. These results link MLL3 loss to HIF signaling, CCL2 and Treg cells, and the establishment of an early immune-suppressive microenvironment, a finding that may open new therapeutic opportunities for MLL3-mutant cancers. One Sentence Summary MLL3 loss promotes tumorigenesis by enhancing recruitment of regulatory T cells.