β-Catenin associates with a Wnt signaling mRNA network in myeloid cells through canonical RBP binding

Wnt/β-catenin signaling is important for normal hematopoietic stem/progenitor cell (HSPC) biology and heavily implicated in acute myeloid leukaemia (AML). The central mediator β-catenin is an attractive therapeutic target in AML however its targeting has been hampered by poor characterisation of its molecular interactions in haematopoietic cells. Our previous β-catenin interactome study identified the significant enrichment of RNA-binding proteins (RBP) implying post-transcriptional roles for β-catenin in myeloid cells. To identify β-catenin-associated mRNAs we performed β-catenin RNA-immunoprecipitation coupled to RNA-sequencing (RIP-seq) and identified significantly enriched Wnt signalling pathway transcripts. Using β-catenin cross-linking immunoprecipitation (CLIP) we demonstrated a limited capacity for β-catenin to bind RNA directly implying dependence on other RBPs. β-Catenin was found to interact with MSI2 in both myeloid cell lines and AML patient samples, where expression was significantly correlated. MSI2 knockdown reduced Wnt signalling output (TCF/LEF activity), through suppression of LEF-1 expression and nuclear localization. Through both RIP and CLIP we demonstrate MSI2 binds LEF1 mRNA in a partly β-catenin dependent fashion, and may impact the post-transcriptional control of LEF-1 expression. Finally, we show that MSI2-mediated expansion of human HSPCs could be partly driven through LEF1 regulation. This is the first study to show functional crosstalk between MSI2 and Wnt signalling in human cells, and indicates potential novel post-transcriptional roles for β-catenin in a haematological context.