Oncogenic Adenomatous Polyposis Coli Mutants Impair the Mitotic Checkpoint through Direct Interaction with Mad2

The majority of colorectal tumors are aneuploid because of the underlying chromosome instability (CIN) phenotype, in which a defective mitotic checkpoint is implicated. Adenomatous polyposis coli (APC), a tumor suppressor gene that is commonly mutated in colon cancers, has been suggested in causing CIN; however, the molecular mechanism remains unresolved. In this study, we report an interaction of tumor-associated N-terminal APC fragments (N-APC) with Mad2, an essential mitotic checkpoint protein, providing a direct molecular support for linking APC mutations to the generation of CIN. N-APC interacts with Mad2 in Xenopus egg extracts, colon cancer cells, and in vitro with purified components. The interaction between N-APC and Mad2 decreases the soluble pool of Mad2, which is essential for Mad2 cycling and releasing from unattached kinetochores to produce a diffusible wait anaphase signal. Addition of such an N-APC mutant of egg extracts inactivates the mitotic checkpoint. Expressing a tumor-associated N-APC mutant in mammalian cells with an intact mitotic checkpoint produces premature anaphase onset with missegregated chromosomes.