Myeloid Mir34a suppresses colitis-associated colon cancer: characterization of mediators by single-cell RNA sequencing

We have previously shown that general deletion of the gene encoding the p53-inducible Mir34a microRNA enhances the number and invasion of colitis-associated colorectal cancers (CACs) in mice. Since the p53-pathway has been implicated in tumor-suppression mediated by cells in the tumor microenvironment (TME) we deleted Mir34a in myeloid cells and characterized CACs in these with scRNA-Seq (single cell RNA sequencing). This revealed an increase in specific macrophage subtypes, such as Cdk8⁺ macrophages and Mrc1⁺, M2-like macrophages. The latter displayed elevated expression of 21 known Mir34a target mRNAs, including Csf1r, Axl, Foxp1, Ccr1, Nampt, and Tgfbr2, and 32 predicted Mir34a target mRNAs. Furthermore, Mir34a-deficient BMDMs showed enhanced migration, elevated expression of Csf1r and a shift towards M2-like polarization when compared to Mir34a-proficient BMDMs. Concomitant deletion of Csf1r or treatment with a Csf1r inhibitor reduced the CAC burden and invasion in these mice. Notably, loss of myeloid Mir34a function resulted in a prominent, inflammatory CAC cell subtype, which displayed epithelial and macrophage markers. These cells displayed high levels of the EMT transcription factor Zeb2 and may therefore enhance the invasiveness of CACs. Taken together, our results provide in vivo evidence for a tumor suppressive role of myeloid Mir34a in CACs which is, at least in part, mediated by maintaining macrophages in an M1-like state via repression of Mir34a targets, such as Csf1r. Collectively, these findings may serve to identify new therapeutic targets and approaches for treatment of CAC.