In This Issue

See article, p. 1355.

Negative immune checkpoint receptors such as programmed cell death 1 (PD-1) and cytotoxic T-lymphocyte–associated protein 4 (CTLA-4) suppress antitumor T-cell activity and facilitate tumor immune escape. Blocking antibodies against PD-1 and its ligand PD-L1 have shown clinical activity in various tumor types, including a subset of patients with non–small cell lung cancer (NSCLC). However, it is unclear whether oncogenic activation of the EGF receptor (EGFR) in NSCLC contributes to immunosuppression and modulates the response to PD-1 inhibition. Akbay and colleagues found that expression of mutant EGFR in mouse models of NSCLC generated an immunosuppressive microenvironment characterized by upregulation of T-cell inhibitory molecules including PD-1 and PD-L1, reduction of CD8⁺ cytotoxic T cells, and induction of tumor-promoting cytokines. Treatment with a PD-1 blocking antibody triggered tumor cell apoptosis, resulting in impaired EGFR-driven NSCLC growth and prolonged survival of tumor-bearing mice. This antitumor effect was mediated by increased T-cell infiltration into tumors and enhanced IFNγ-producing CD8⁺ cytotoxic T-cell function, as well as decreased expression of immunosuppressive, protumorigenic cytokines. Furthermore, induction of PD-L1 in human NSCLC cell lines was dependent on activation of EGFR signaling, as treatment with EGFR kinase inhibitors decreased PD-L1 levels. These results identify a non–cell-autonomous role of oncogenic EGFR in remodeling the immune microenvironment to promote immune evasion in lung cancer and suggest that combined inhibition of EGFR and PD-1 may be therapeutically beneficial in EGFR-mutant NSCLC.

See article, p. 1364.

Few therapies are available for small cell lung cancer (SCLC), a deadly cancer characterized by the rapid growth and dissemination of cells with neuroendocrine features. To identify quickly implementable therapeutic strategies for SCLC, Jahchan and colleagues computationally screened publicly available gene expression signatures of FDA-approved compounds for their predicted ability to revert SCLC gene expression signatures to healthy lung signatures. This drug repositioning approach identified the tricyclic antidepressant imipramine and the antihistamine promethazine, which each significantly reduced SCLC tumor growth and metastasis in vivo in association with induction of cell stress pathways and cell death. Given that these drugs broadly target neurotransmitter-activated G protein-coupled receptors (GPCR), the authors evaluated GPCR expression and ligand production and found that SCLC cells express GPCRs for which imipramine and promethazine have affinity and produce the ligands that activate these GPCRs, suggesting that the drugs work by inhibiting autocrine GPCR-dependent survival signaling. Indeed, SCLC cells were sensitive to other tricyclic antidepressants and selective GPCR inhibitors that inhibited protein kinase A signaling downstream of the Gα_s subunit. Interestingly, other neuroendocrine tumor subtypes express GPCR targets of imipramine and promethazine, and cell lines derived from these tumor types were sensitive to these drugs in vitro. Moreover, imipramine treatment significantly increased survival in a mouse model of pancreatic neuroendocrine tumor. Autocrine GPCR signaling may therefore be a shared survival mechanism of neuroendocrine tumor types that may be targetable by clinically available drugs.

See article, p. 1378.

Progression of melanomas to aggressive, metastatic tumors involves a dynamic switch from a highly proliferative to a more invasive phenotype. This switch is regulated by hypoxia within the tumor microenvironment and increased noncanonical WNT signaling via WNT5A and its receptor ROR2 (receptor tyrosine kinase-like orphan receptor 2). However, the contribution of phenotype switching to therapy resistance and the role of the related receptor ROR1 in melanoma remain unclear. O'Connell and colleagues found that, in contrast to ROR2, ROR1 was selectively expressed in poorly invasive, proliferative melanoma cells and was downregulated in metastatic cells by WNT5A-driven protein kinase C signaling. ROR1 depletion reduced tumor growth but augmented melanoma cell invasion and metastasis via increased ROR2 expression, suggesting that reciprocal regulation of these receptors promotes melanoma progression. Consistent with this idea, hypoxic exposure induced WNT5A and triggered a shift from ROR1 to ROR2 expression; this switch was dependent on stabilization of hypoxia-inducible factor 1α (HIF1α), which was regulated via a positive feedback loop by WNT5A-mediated activation of the E3 ubiquitin ligase SIAH2 under hypoxia. Furthermore, elevated WNT5A expression was correlated with resistance to BRAF inhibitor treatment and was predictive of diminished clinical response to vemurafenib. ROR1 expression was diminished in resistant cells, whereas increased ROR2 expression decreased sensitivity to BRAF inhibition. These findings implicate WNT5A-induced phenotype switching in therapy resistance and suggest that ROR2 blockade may be therapeutically beneficial in melanoma.

See article, p. 1394.

Gene fusions arising from chromosomal translocations act as oncogenic drivers in many human cancers. Yuan and colleagues hypothesized that oncogenic chromosomal rearrangements lead to constitutive expression of chimeric RNAs with physiologic functions that are normally expressed in untransformed cells. Both normal bone marrow–derived mesenchymal stem cells (MSC) induced to differentiate into skeletal muscle and human fetal muscle tissue were found to express a chimeric PAX3–FOXO1 RNA identical to the fusion transcript expressed in over half of patients with alveolar rhabdomyosarcoma (ARMS), yet lacked the corresponding t(2;13)(q35;q14) translocation. The chimeric PAX3–FOXO1 RNA was only transiently expressed in normal differentiating MSCs, and always preceded expression of myogenic early stage markers such as MYOD and MYOG. However, constant expression of PAX3–FOXO1 in differentiating MSCs led to sustained MYOD and MYOG expression, which blocked expression of mature muscle markers and suppressed terminal differentiation, consistent with the PAX3–FOXO1-induced myogenic differentiation block observed in ARMS cells. Additional chimeric RNAs were also expressed during various stages of muscle differentiation, raising the possibility that other fusion products have roles in development. Although the mechanism responsible for the generation of the chimeric PAX3–FOXO1 RNA remains unclear, the observation that PAX3–FOXO1 is expressed during normal muscle development suggests that ARMS arises from immature progenitor cells that transiently express this chimeric RNA, and has implications for the diagnosis and management of PAX3–FOXO1-positive ARMS.

See article, p. 1404.

The efficacy of EGF receptor (EGFR) inhibitors in patients with EGFR-mutant non–small cell lung cancer (NSCLC) is limited by development of acquired resistance, often due to secondary EGFR^T790M mutation, and on-target toxicity in the skin and intestine resulting from inhibition of wild-type EGFR. Second-generation EGFR inhibitors that target EGFR^T790M in vitro have failed to induce responses in clinical trials because of dose-limiting wild-type EGFR toxicity, emphasizing the need to develop improved mutant-selective EGFR inhibitors that overcome clinical T790M-mediated resistance. Walter and colleagues characterized CO-1686, a first-in-class irreversible EGFR inhibitor that covalently modifies Cys797 in the EGFR kinase domain, resulting in potent and selective inhibition of mutant EGFR proteins, including EGFR^T790M. CO-1686 inhibited proliferation and triggered apoptosis specifically in NSCLC cell lines expressing mutant but not wild-type EGFR. Moreover, in contrast to erlotinib and afatinib, CO-1686 was well tolerated and spared wild-type EGFR signaling in vivo. Treatment with single-agent CO-1686 induced tumor regression in erlotinib-resistant, EGFR^T790M NSCLC xenograft models, as well as in transgenic mouse models of EGFR-mutant and erlotinib-resistant NSCLC. In addition, NSCLC cell lines with acquired resistance to CO-1686 were enriched for genes associated with epithelial–mesenchymal transition (EMT) and exhibited decreased dependence on EGFR expression and upregulation of AKT signaling; treatment with AKT inhibitors was synergistic and partially restored sensitivity to CO-1686. These results identify a potential strategy to treat resistant T790M-positive tumors and support ongoing clinical trials of CO-1686 in EGFR-mutant NSCLC.

See article, p. 1416.

Acute myeloid leukemia (AML) can be categorized into distinct subtypes based on recurring genetic abnormalities, but this information has not effectively guided use of targeted therapies, and outcomes in response to conventional chemotherapy remain poor. By performing ex vivo drug sensitivity and resistance testing (DSRT) of 187 chemotherapeutic agents and targeted compounds on 28 patient-derived AML samples and normal bone marrow mononuclear cells, Pemovska and colleagues developed an individualized systems medicine (ISM) approach to predict which drugs would be leukemia-specific and most effective in each patient. Although each sample had a unique DSRT profile, the AML samples clustered into five subgroups based on their drug responses, with one group sensitive to BCL-2 inhibition, one group sensitive to immunosuppressive drugs, and three groups sensitive to different classes of tyrosine kinase inhibitors (TKI). Three of eight evaluable patients had clinical responses to drug combinations that were predicted to be effective based on DSRT of their AML cells. Subsequent DSRT and molecular profiling after these patients relapsed showed decreased sensitivity to the initial treatments consistent with the in vivo responses but also revealed the acquisition of new TKI sensitivities concomitant with the emergence of new subclones. Although these findings need to be verified in a randomized setting, they suggest that an ISM strategy is clinically implementable in hematologic cancers and can be performed continually to guide ongoing personalized treatment in response to clonal selection and mechanisms of acquired resistance.

Note: In This Issue is written by Cancer Discovery Science Writers. Readers are encouraged to consult the original articles for full details.