Loss of beta-catenin regulation by the APC tumor suppressor protein correlates with loss of structure due to common somatic mutations of the gene.

The mutation cluster region in the APC gene defines a region of approximately 660 bp, in which the vast majority of its somatic mutations are found. These mutations disrupt the polypeptide chain, typically eliminating five of the seven repeated sequences of 20 amino acids (aa) each in the central region of the APC protein. To examine the relationship between loss of this structure and loss of function, we constructed APC deletion mutants that progressively truncated the protein across the mutation cluster region. The mutants were tested for their association with beta-catenin and their ability to down-regulate it in SW480 cells. The binding of beta-catenin to APC fragments required the inclusion of only a single 20-aa repeat sequence, whereas down-regulation required the presence of at least three of these repeat sequences, and those including the second repeat exhibited the highest activity. The mutation of three conserved serine residues in the second repeat greatly reduced the activity of an otherwise highly active APC fragment. Thus, the repeated 20-aa sequence is directly implicated in beta-catenin turnover. The elimination of at least five of these seven repeats due to somatic mutations suggests that loss of beta-catenin regulation by APC is selected for during tumor progression.