TBX3 acts as tissue-specific component of the Wnt/β-catenin enhanceosome

Abstract

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 WNT/β‐catenin dependent transcription: A tissue‐specific business

“…The participation of TBX3 is strictly dependent on the presence of BCL9 and occurs on classical WREs. The BCL9‐dependent TBX3 genomic occupancy of WREs confirms this model (Zimmerli et al, 2020 ). It is tantalizing to speculate that the previously mentioned interplay with TBX5 (Figure 3 ; Rosenbluh et al, 2012 ) and that with TBX3 are simply two cases of a more general networking between Wnt/β‐catenin and the TBX TFs.…”

“…In an analogous example, the pituitary homeodomain factor PROP1 appears to benefit of this functional pliability of β‐catenin, which recruits HDAC/TLE complexes to repress certain PROP1 target genes and CBP to activate others in a binary manner within the same cell (Figure 3 ; Olson et al, 2006 ). A third example comes from our recent work unraveling the T‐box containing TF TBX3 as part of the Wnt‐dependent transcriptional complex (Zimmerli et al, 2020 ). The participation of TBX3 is strictly dependent on the presence of BCL9 and occurs on classical WREs.…”

“…In this context, BCL9/9L synergize with PYGO1/2, as the gene expression changes caused by their compound genetic deletion is broader than that provoked by recombination of Bcl9/9l or of Pygo1/2 (Mieszczanek, van Tienen, Ibrahim, Winton, & Bienz, 2019 ). Intriguingly however, deletion of Bcl9/9l prolonged lifespan of mice carrying the Apc 1322T mutation (which encodes a truncated but partially‐functional APC commonly found in CRC) significantly more than Pygo1/2 genetic recombination (Mieszczanek et al, 2019 ), and PYGO1/2 also seem to be not particularly relevant in chemically induced colorectal carcinogenesis (Zimmerli et al, 2020 ). It is possible that other BCL9/9L interacting partners are more relevant in CRC, such as TBX3 (Zimmerli et al, 2020 ).…”

“…Intriguingly however, deletion of Bcl9/9l prolonged lifespan of mice carrying the Apc 1322T mutation (which encodes a truncated but partially‐functional APC commonly found in CRC) significantly more than Pygo1/2 genetic recombination (Mieszczanek et al, 2019 ), and PYGO1/2 also seem to be not particularly relevant in chemically induced colorectal carcinogenesis (Zimmerli et al, 2020 ). It is possible that other BCL9/9L interacting partners are more relevant in CRC, such as TBX3 (Zimmerli et al, 2020 ). The exact mechanisms of Wnt target gene activation in CRC by BCL9/9L are still elusive but are certainly worth investigating.…”

A New CUT&RUN Low Volume-Urea (LoV-U) protocol uncovers Wnt/β-catenin tissue-specific genomic targets

“…When using chromatin immunoprecipitation followed by sequencing (ChIP-seq; Furey, 2012), the addition of doublecrosslinking steps aimed at preserving not only the DNA-protein - but also protein-protein interactions - yielded clear β-catenin profiles both in vitro (Schuijers et al, 2014) and in vivo (Cantù et al, 2018). These protocols, however, present several limitations, including the fact that they are at risk of generating numerous artifacts and, most importantly, require enormous amounts of cells, typically in the range of tens of millions (Doumpas et al, 2019; Zimmerli et al, 2020).…”