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BCL9 and PYGO are β-catenin cofactors that enhance the transcription of Wnt target genes. They have been proposed as therapeutic targets to diminish Wnt signaling output in intestinal malignancies. Here we find that, in colorectal cancer cells and in developing mouse forelimbs, BCL9 proteins sustain the action of β-catenin in a largely PYGO-independent manner. Our genetic analyses implied that BCL9 necessitates other interaction partners in mediating its transcriptional output. We identified the transcription factor TBX3 as a candidate tissue-specific member of the β-catenin transcriptional complex. In developing forelimbs, both TBX3 and BCL9 occupy a large number of Wnt-responsive regulatory elements, genome-wide. Moreover, mutations in Bcl9 affect the expression of TBX3 targets in vivo, and modulation of TBX3 abundance impacts on Wnt target genes transcription in a β-catenin- and TCF/LEF-dependent manner. Finally, TBX3 overexpression exacerbates the metastatic potential of Wnt-dependent human colorectal cancer cells. Our work implicates TBX3 as context-dependent component of the Wnt/β-catenin-dependent transcriptional complex.

β‐catenin‐mediated Wnt signaling is an ancient cell‐communication pathway in which β‐catenin drives the expression of certain genes as a consequence of the trigger given by extracellular WNT molecules. The events occurring from signal to transcription are evolutionarily conserved, and their final output orchestrates countless processes during embryonic development and tissue homeostasis. Importantly, a dysfunctional Wnt/β‐catenin pathway causes developmental malformations, and its aberrant activation is the root of several types of cancer. A rich literature describes the multitude of nuclear players that cooperate with β‐catenin to generate a transcriptional program. However, a unified theory of how β‐catenin drives target gene expression is still missing. We will discuss two types of β‐catenin interactors: transcription factors that allow β‐catenin to localize at target regions on the DNA, and transcriptional co‐factors that ultimately activate gene expression. In contrast to the presumed universality of β‐catenin's action, the ensemble of available evidence suggests a view in which β‐catenin drives a complex system of responses in different cells and tissues. A malleable armamentarium of players might interact with β‐catenin in order to activate the right “canonical” targets in each tissue, developmental stage, or disease context. Discovering the mechanism by which each tissue‐specific β‐catenin response is executed will be crucial to comprehend how a seemingly universal pathway fosters a wide spectrum of processes during development and homeostasis. Perhaps more importantly, this could ultimately inform us about which are the tumor‐specific components that need to be targeted to dampen the activity of oncogenic β‐catenin. This article is categorized under: Cancer > Molecular and Cellular Physiology Cancer > Genetics/Genomics/Epigenetics Cancer > Stem Cells and Development

BCL9 and PYGO are b-catenin cofactors that enhance the transcription of Wnt target genes. They have been proposed as therapeutic targets to diminish Wnt signalling output in intestinal malignancies. Here we find that, in colorectal cancer cells and in developing mouse forelimbs, BCL9 proteins sustain the action of b-catenin in a largely PYGO-independent manner. Our genetic analyses implied that BCL9 necessitates other interaction partners in mediating its transcriptional output. We identified the transcription factor TBX3 as a candidate tissue-specific member of the b-catenin transcriptional complex. In developing forelimbs, TBX3 and BCL9 co-occupy a large number of Wnt-responsive regulatory elements, genome-wide. Moreover, mutations in Bcl9 affect the expression of TBX3 targets in vivo, and modulation of TBX3 abundance impacts on Wnt target genes transcription in a b-catenin-and TCF/LEF-dependent manner. Finally, TBX3 overexpression exacerbates the metastatic potential of Wntdependent human colorectal cancer cells. Our work implicates TBX3 as a new, context-dependent component of the Wnt/b-catenin-dependent enhanceosome.

The forkhead box transcription factor FOXQ1 contributes to the pathogenesis of carcinomas. In colorectal cancers, FOXQ1 promotes tumour metastasis by inducing epithelial-to-mesenchymal transition (EMT) of cancer cells. FOXQ1 may exacerbate cancer by activating the oncogenic Wnt/-catenin signalling pathway. However, the role of FOXQ1 in the Wnt pathway remains to be resolved. Here, we report that FOXQ1 is an activator of Wnt-induced transcription and regulator of b-catenin target gene expression. Upon Wnt pathway activation, FOXQ1 synergises with the b-catenin nuclear complex to boost the expression of major Wnt targets. In parallel, we find that FOXQ1 controls the differential expression of various Wnt target genes in a b-catenin-independent manner. Using RNA sequencing of colorectal cancer cell lines, we show that Wnt signalling and FOXQ1 converge on a transcriptional program linked to EMT and cell migration. Additionally, we demonstrate that FOXQ1 occupies Wnt-responsive elements in b-catenin target gene promoters and recruits a similar set of co-factors as the b-catenin-associated transcription factor Tcf7l1. Taken together, our results indicate a multifaceted role of FOXQ1 in Wnt/b-catenin signalling, which may drive the metastasis of colorectal cancers.