Mechanisms of regression and recurrence of Wnt1-initiated mouse mammary tumors following Wnt pathway down-regulation

The Wnt signaling pathway is a master regulator of embryonic tissue morphogenesis that is frequently co-opted to participate in malignant transformation. Preclinical models suggest that inhibition of the Wnt pathway may represent a potent anticancer strategy. Notably, abrogation of doxycycline-regulated Wnt1 expression in a transgenic mouse model of Wnt-initiated mammary tumorigenesis results in regression of established mammary adenocarcinomas. Studies using this conditional Wnt-initiated mammary tumor model provide important experimental support for the reversibility of established Wnt-initiated tumors in vivo, but also suggest potential obstacles to an anticancer strategy involving Wnt pathway inhibition. Specifically, despite sustained down-regulation of the initiating oncogenic signal via ongoing doxycycline withdrawal, spontaneous tumor relapses commonly occur during long-term monitoring of mice harboring regressed, clinically silent mammary tumors. Here, the mechanisms underlying tumor regression and relapse in this model were investigated to provide a framework for understanding how cancers might evade Wnt pathway inhibition. Mammary tumor cells responded to reversal of oncogenic Wnt signaling by undergoing both proliferation arrest and programmed cell death. A subset of regressed tumors reactivated the Wnt pathway en route to relapse, and these tumors were readily identifiable by their expression of Wnt pathway target genes. Interestingly, several tumors from this subset lacked Wnt1 transgene expression and instead activated the pathway through acquired spontaneous somatic mutations in the Wnt pathway effector beta-catenin. Thus, inhibition of oncogenic signaling by targeted down-regulation of an upstream effector molecule selected for mutational activation of a downstream effector molecule. In contrast, in the setting of p53-deficiency, relapse rarely was associated with over-expression of Wnt pathway target genes and uniformly involved an epithelial-to-mesenchymal transition (EMT) that was apparent on morphologic and molecular characterization. These findings are the first to link the p53 pathway to suppression of EMT. Finally, pharmacologic inhibitors of oncogenic signaling are typically tested in cancer patients who have previously undergone treatment with conventional cytotoxins. In principle, the mutagenic properties of clinically important cytotoxins might compromise the long-term efficacy of a subsequent targeted therapy. To explore this possibility in vivo, mice harboring Wnt-initiated mammary tumors were pre-treated with cytotoxins/mutagens prior to abrogating oncogenic Wnt signaling. Disease relapse was hastened in the setting of mutagen exposure, and the mechanisms underlying tumor recurrence provided a genetic link between mutagen exposure and relapse. These findings have implications for determining the optimal sequence for administering traditional cytotoxins in combination with newer targeted therapeutics.