Tumor escape in a Wnt1-dependent mouse breast cancer model is enabled by p19Arf/p53 pathway lesions but not p16Ink4a loss

Abstract

Breast cancers frequently progress or relapse during targeted therapy, but the molecular mechanisms that enable escape remain poorly understood. We elucidated genetic determinants underlying tumor escape in a transgenic mouse model of Wnt pathway-driven breast cancer, wherein targeted therapy is simulated by abrogating doxycycline-dependent Wnt1 transgene expression within established tumors. In mice with intact tumor suppressor pathways, tumors typically circumvented doxycycline withdrawal by reactivating Wnt signaling, either via aberrant (doxycycline-independent) Wnt1 transgene expression or via acquired somatic mutations in the gene encoding β-catenin. Germline introduction of mutant tumor suppressor alleles into the model altered the timing and mode of tumor escape. Relapses occurring in the context of null Ink4a/Arf alleles (disrupting both the p16 Ink4a and p19 Arf tumor suppressors) arose quickly and rarely reactivated the Wnt pathway. In addition, Ink4a/Arf-deficient relapses resembled p53-deficient relapses in that both displayed morphologic and molecular hallmarks of an epithelial-to-mesenchymal transition (EMT). Notably, Ink4a/Arf deficiency promoted relapse in the absence of gross genomic instability. Moreover, Ink4a/Arf-encoded proteins differed in their capacity to suppress oncogene independence. Isolated p19 Arf deficiency mirrored p53 deficiency in that both promoted rapid, EMT-associated mammary tumor escape, whereas isolated p16 Ink4a deficiency failed to accelerate relapse. Thus, p19 Arf /p53 pathway lesions may promote mammary cancer relapse even when inhibition of a targeted oncogenic signaling pathway remains in force. IntroductionBreast cancer research offers a clinically important venue for exploring resistance to targeted therapy. Antagonists of estrogen receptor-dependent (ER-dependent) and human epidermal growth factor receptor 2 (HER2-dependent) signaling are mainstays of modern breast cancer treatment that enhance cure rates when applied against early-stage disease and contribute to disease remissions when applied against late-stage disease (1, 2). Even so, potent targeted agents impose strong selective pressure that ultimately favors tumor escape, wherein treatment-resistant cancer cells survive and proliferate (3). Indeed, resistance to targeted agents, when not encountered de novo, routinely emerges during treatment (4, 5). As a result, targeted agents supplement traditional breast cancer treatment strategies but do not yet obviate the need for surgery, radiation, and cytotoxic chemotherapy. Moreover, incorporating targeted agents into routine clinical practice does not yet permit cure of advanced disease. Thus, tumor escape sets profound limits on the clinical usefulness of targeted therapy in breast cancer patients.In principle, tumors can escape growth constraints imposed by targeted therapy either by reactivating the targeted signaling pathway or by perturbing untargeted compensatory pathways. Both mechanisms appear capable of promoting tumor escape in breast cancer patients....

Reversibility and recurrence of IGF-IR-induced mammary tumors

“…Mammary tumor escape from the initiating oncogene, accompanied by EMT, has been reported in similar doxycycline inducible breast cancer models. In these models, recurrent tumors displaying features of EMT were found to contain increased mRNA levels of the E-cadherin transcriptional repressors Snail (Moody et al, 2005;Debies et al, 2008) and to a lesser degree Slug (Moody et al, 2005). In addition, Snail itself was shown to play a causal role in promoting recurrence of fully regressed erbB2-induced tumors (Moody et al, 2005).…”

Regulation of transgenes in three-dimensional cultures of primary mouse mammary cells demonstrates oncogene dependence and identifies cells that survive deinduction

“…An important characteristic of such doxycyclinedependent cancer models is the frequent recurrence of tumors, with varied latencies, but usually with reactivation of one or both of the initiating oncogenes (Chin et al 1999;Felsher and Bishop 1999;Podsypanina et al 2008) or with an activating mutation in the same signaling pathway (Debies et al 2008). Tumor recurrence, without a second round of induction of regulated oncogenes, implies that some of the original tumor cells may survive deinduction of the oncogenes and remain in a latent state until additional events occur to allow tumor regrowth.…”

Oncogene cooperation in tumor maintenance and tumor recurrence in mouse mammary tumors induced by Myc and mutant Kras

“…3A). It is expected that tumor cells giving rise to recurrent tumors become independent of the transgenic oncogenes by acquiring genetic lesions or epigenetic changes that substitute for the requirement of MYC and/or mutant Kras, as was demonstrated in other models of doxycyclineindependent relapses (7,11,25,27,34). The high prevalence of doxycycline-bypassing mutations in the MMTV-rtTA transgene that confer doxycycline-independence in the recurrent tumors from TOM;TOR;MTB mice reflects the essential role of oncogene cooperation in tumor relapse, because none of the recurrent tumors displayed reinduction of one oncogene alone.…”