A L225A substitution in the human tumour suppressor HIC1 abolishes its interaction with the corepressor CtBP.

HIC1 (hypermethylated in cancer) is a tumour suppressor gene located in 17p13.3, a region frequently hypermethylated or deleted in many types of prevalent human tumour. HIC1 is also a candidate for a contiguous-gene syndrome, the Miller-Dieker syndrome, a severe form of lissencephaly accompanied by developmental anomalies. HIC1 encodes a BTB/POZ-zinc finger transcriptional repressor. HIC1 represses transcription via two autonomous repression domains, an N-terminal BTB/POZ and a central region, by trichostatin A-insensitive and trichostatin A-sensitive mechanisms, respectively. The HIC1 central region recruits the corepressor CtBP (C-terminal binding protein) through a conserved GLDLSKK motif, a variant of the consensus C-terminal binding protein interaction domain PxDLSxK/R. Here, we show that HIC1 interacts with both CtBP1 and CtBP2 and that this interaction is stimulated by agents increasing NADH levels. Furthermore, point mutation of two CtBP2 residues forming part of the structure of the recognition cleft for a PxDLS motif also ablates the interaction with a GxDLS motif. Conversely, in perfect agreement with the structural data and the universal conservation of this residue in all C-terminal binding protein-interacting motifs, mutation of the central leucine residue (leucine 225 in HIC1) abolishes the interaction between HIC1 and CtBP1 or CtBP2. As expected from the corepressor activity of CtBP, this mutation also impairs the HIC1-mediated transcriptional repression. These results thus demonstrate a strong conservation in the binding of C-terminal binding protein-interacting domains despite great variability in their amino acid sequences. Finally, this L225A point mutation could also provide useful knock-in animal models to study the role of the HIC1-CtBP interaction in tumorigenesis and in development.