Addiction to multiple oncogenes can be exploited to prevent the emergence of therapeutic resistance

Many cancers exhibit sensitivity to the inhibition of a single genetic lesion, a property that has been successfully exploited with oncogene-targeted therapeutics. However, inhibition of single oncogenes often fails to result in sustained tumor regression due to the emergence of therapy-resistant cells. Here, we report that MYC-driven lymphomas frequently acquire activating mutations in β-catenin, including a previously unreported mutation in a splice acceptor site. Tumors with these genetic lesions are highly dependent on β-catenin for their survival and the suppression of β-catenin resulted in marked apoptosis causally related to a decrease in Bcl-xL expression. Using a novel inducible inhibitor of β-catenin, we illustrate that, although MYC withdrawal or β-catenin inhibition alone results in initial tumor regression, most tumors ultimately recurred, mimicking the clinical response to single-agent targeted therapy. Importantly, the simultaneous combined inhibition of both MYC and β-catenin promoted more rapid tumor regression and successfully prevented tumor recurrence. Hence, we demonstrated that MYC-induced tumors are addicted to mutant β-catenin, and the combined inactivation of MYC and β-catenin induces sustained tumor regression. Our results provide a proof of principle that targeting multiple oncogene addicted pathways can prevent therapeutic resistance.Cancer cells are highly sensitive to the targeted inhibition of single driver mutations, eliciting a phenomenon known as “oncogene addiction” (1). The identification of genetic dependencies in multiple tumor types has resulted in the development of several molecularly targeted therapeutics, including the BCR-ABL kinase inhibitor imatinib for the treatment of chronic myelogenous leukemia (CML), the EGFR kinase inhibitor gefitinib for the treatment of non–small cell lung cancer (NSCLC), and the BRAF kinase inhibitor vemurafenib for the treatment of advanced melanoma (2–4). Although these oncogene-targeted agents have provided promising clinical responses, many patients ultimately experience a recurrence of their disease due to the development of drug resistance (4–6). Thus, it has become evident that monotherapy with targeted drugs is insufficient for achieving sustained tumor regression.Resistance to targeted therapy can arise through multiple mechanisms, depending on the tumor type and the targeted oncogenic pathway (7). Cells frequently acquire resistance through mutations in the targeted oncogene itself that disrupt drug binding, as in the case of BCR-ABL and EGFR (8, 5, 6). In addition, resistance to EGFR inhibition in NSCLC and BRAF inhibition in melanoma has been found to occur through a variety of mechanisms that activate downstream signaling proteins or alternative pathways, which can functionally substitute for loss in activity of the targeted oncogene (9–11). Although significant progress has been made in the identification and inhibition of resistance pathways, it may prove challenging to anticipate and suppress all of the potential mechanisms of resistance for each oncogene-addicted cancer and targeted therapeutic agent.Combination therapy has been successfully applied to prevent resistance in the treatment of infectious diseases such as HIV (12, 13) and tuberculosis (14). In the context of oncogene-targeted therapy for cancer, it has been proposed that a similar strategy, using combinations directed against multiple dependencies, is the most likely to prevent resistance (7). Indeed, mathematical modeling indicates that targeting at least two independently required pathways may be sufficient to prevent tumor recurrence (15). However, there exists little experimental evidence directly testing such an approach and it remains unclear which combinations of targets would be most effective at inducing long-term remissions.MYC is one of the most frequently amplified oncogenes in human cancer (16). In the Eμ-tTA/tetO-MYC conditional mouse model, overexpression of MYC results in the development of aggressive T-cell lymphoma, and MYC inactivation in established tumors is sufficient to induce tumor regression through processes such as proliferative arrest, cellular senescence, apoptosis, and the shutdown of angiogenesis (17–19). The extent of regression is dependent on both cell-intrinsic and host-dependent contexts, and in particular, tumors frequently recur following MYC inactivation in the absence of an intact adaptive immune system (20). Recurring tumors restore expression of the MYC transgene or up-regulate expression of endogenous Myc, demonstrating that resistance occurs primarily through reactivation of the MYC pathway (21). Thus, MYC oncogene addiction and tumor recurrence in the Eμ-tTA/tetO-MYC lymphoma model resembles the clinical course of human cancers treated with single agent targeted therapy.Here, we demonstrate that the combined inactivation of two oncogene addiction pathways can result in sustained tumor regression. Moreover, we describe a previously unidentified splice acceptor site mutation in β-catenin that is associated with MYC-induced lymphomagenesis. Tumors with mutations in β-catenin are also highly addicted to this mutant gene product for their survival. We demonstrate that in MYC-induced lymphomas, combined addiction to both MYC and β-catenin can be exploited in a rational manner to prevent the emergence of therapeutic resistance.