A WNT/p21 Circuit Directed by the C-Clamp, a Sequence-Specific DNA Binding Domain in TCFs

The lymphoid enhancer factor 1/T cell factor (LEF/TCF) family of transcription factors are downstream effectors of theWNT signaling pathway, which drives colon tumorigenesis. LEF/TCFs have a DNA sequence-specific high-mobility group (HMG) box that bindsWnt response elements (WREs). The “E tail ” isoforms of TCFs are alternatively spliced to include a second DNA bind-ing domain called the C-clamp.We show that induction of a dominant negative C-clamp version of TCF1 (dnTCF1E) induces p21 expression and a stall in the growth of DLD1 colon cancer cells. Induction of a C-clampmutant did not efficiently induce p21, nor did it stall cell growth. Microarray analysis revealed that induction of p21 by wild-type dnTCF1E (dnTCF1EWT) corre-lated with a decrease in expression of multiple p21 suppressors that act at multiple levels from transcription (SP5, YAP1, and RUNX1), RNA stability (MSI2), and protein stability (CUL4A). We show that the C-clamp is a sequence-specific DNA binding domain that canmake contacts with 5=-RCCG-3 = elements upstream or downstream ofWREs. The C-clamp–RCCG interaction was critical for TCF1E-mediated transcriptional control of p21-connected target gene promoters. Our results indicate that a rap-id-responseWNT/p21 circuit is driven by C-clamp target gene selection. Overactive Wnt signaling is causally linked to carcinogenesis inmany different tumor types, with colon tumors having an especially strong link to this pathway (18, 21). Over 80 % of colon tumors derive from aberrantly activated Wnt signals, which are most often created by familial or sporadic mutation of the tumo