β-catenin–sensitive isoforms of lymphoid enhancer factor-1 are selectively expressed in colon cancer

Constitutive activation of the Wnt signaling pathway is a root cause of many colon cancers1,2,3. Activation of this pathway is caused by genetic mutations that stabilize the β-catenin protein, allowing it to accumulate in the nucleus and form complexes with any member of the lymphoid enhancer factor (LEF1) and T-cell factor (TCF1, TCF3, TCF4) family of transcription factors (referred to collectively as LEF/TCFs) to activate transcription of target genes3,4. Target genes such as MYC, CCND1, MMP7 and TCF7 (refs. 5–9) are normally expressed in colon tissue, so it has been proposed that abnormal expression levels or patterns imposed by β-catenin/TCF complexes have a role in tumor progression. We report here that LEF1 is a new type of target gene ectopically activated in colon cancer. The pattern of this ectopic expression is unusual because it derives from selective activation of a promoter for a full-length LEF1 isoform that binds β-catenin, but not a second, intronic promoter that drives expression of a dominant-negative isoform. β-catenin/TCF complexes can activate the promoter for full-length LEF1, indicating that in cancer high levels of these complexes misregulate transcription to favor a positive feedback loop for Wnt signaling by inducing selective expression of full-length, β-catenin–sensitive forms of LEF/TCFs.