Wnt pathway mutations selected by optimal beta-catenin signaling for tumorigenesis

Mutations in components of the Wnt/beta-catenin pathway are observed to be the earliest initiating event for most colorectal tumors. The majority of the mutations occur in the tumor suppressor adenomatous polyposis coli (APC), even though there are other genes that are capable of modulating the pathway activity. Moreover, the specific APC mutations associated in colon cancer indicate the possibility that the tumor selects for certain truncated forms of APC that partially retain its function, namely, inhibition of beta-catenin. We estimated the effects of various mutations in APC and other known mutations using a recent mathematical model of the Wnt pathway that was constructed to represent the conserved core molecular events. We provide evidence that APC mutations are selected not based on the maximal level of beta-catenin but rather based on distinct state of activity that appears to be optimal for the tissue-specific tumorigenesis. This optimal level is determined by balancing beta-catenin signaling and the induction of Axin2 that acts as a potent negative feedback. The predominant pattern of APC mutations may provide synergistic oncogenic effects that promote colorectal tumorigenesis: the optimal signaling for cell survival and renewal, disrupted cell adhesion, chromosomal instability, and altered asymmetric division of stem cells.