DPC4 TRANSCRIPTIONAL ACTIVATION AND DYSFUNCTION IN CANCER-CELLS

Dpc4 (Smad4) is implicated in mediation of signals from transforming g rowth factor (TGF) beta and related ligands, and wild-type Dpc4 mediat es TGF-beta-stimulated gene transcription at specific DNA sequences bo und by Dpc4 [Smad binding element (SBE)]. We characterized panels of D PC4 tumor mutations and cancer cell lines. Amino acid substitutions wi thin the NH2-terminal third of Dpc4 weakened or ablated SBE-mediated g ene regulation by a disruption of DNA binding. An interaction of the C OOH-terminal end with the DNA-binding domain of Dpc4 was evident but w as not required to explain the functional impairment produced by NH2-t erminal DPC4 mutations, Both substitution and truncation mutations of the COOH-terminal half of DPC4 lacked the ability to regulate transcri ption while retaining the sequence-specific DNA-binding function, but through differing mechanisms. A modular domain to redistribute Dpc4 to the nuclear compartment allowed SBE-mediated transcriptional activati on in a cell line having a TGF-beta receptor defect and was sufficient to restore SBE-mediated transactivation ability to COOH-terminal DPC4 missense mutants. Cells harboring DPC4 alterations had a universal im pairment of the TGF-beta-stimulated SBE transcriptional response. Thes e studies identify the loss of SBE-mediated gene regulation as a unifo rm outcome of the selection for DPC4 alterations during tumorigenesis. They raise the possibility of restoration of some Dpc4-associated tra nscriptional events in cancer cells through the targeted redistributio n of wild-type and some missense mutant forms of Dpc4 to the nucleus.