beta-Catenin signaling dosage dictates tissue-specific tumor predisposition in Apc-driven cancer

Apc-driven tumor formation in patients and Apc-mutant mouse models is generally attributed to increased levels of beta-catenin signaling. We and others have proposed that a specific level of b-catenin signaling is required to successfully initiate tumor formation, and that each tissue prefers different dosages of signaling. This is illustrated by APC genotype - tumor phenotype correlations in cancer patients, and by the different tumor phenotypes displayed by different Apc-mutant mouse models. Apc1638N mice, associated with intermediate b-catenin signaling, characteristically develop intestinal tumors (< 10) and extra-intestinal tumors, including cysts and desmoids. Apc1572T mice associated with lower levels of b-catenin signaling are free of intestinal tumors, but instead develop mammary tumors. Although the concept of b-catenin signaling dosage and its impact on tumor growth among tissues is gaining acceptance, it has not been formally proven. Additionally, alternative explanations for Apc-driven tumor formation have been proposed. To obtain direct evidence for the dominant role of b-catenin dosage in tumor formation and tissue-specific tumor predisposition, we crossed Apc1638N mice with heterozygous b-catenin knockout mice, thereby reducing b-catenin levels. Whereas all the Apc1638N; Ctnnb1(+/+) mice developed gastrointestinal tumors, none were present in the Apc1638N; Ctnnb1(-/-) mice. Incidence of other Apc1638N-associated lesions, including desmoids and cysts, was strongly reduced as well. Interestingly, Apc1638N; Ctnnb1(-/-) females showed an increased incidence of mammary tumors, which are normally rarely observed in Apc1638N mice, and the histological composition of the tumors resembled that of Apc1572T-related tumors. Hereby, we provide in vivo genetic evidence confirming the dominant role of b-catenin dosage in tumor formation and in dictating tumor predisposition among tissues in Apc-driven cancer.