SUMMARY Accumulating evidence implicates heterogeneity within cancer cell populations in the response to stressful exposures, including drug treatments. While modeling the acute response to various anti-cancer agents in drug-sensitive human tumor cell lines, we consistently detected a small subpopulation of reversibly “drug-tolerant” cells. These cells demonstrate >100-fold reduced drug sensitivity, and maintain viability via engagement of IGF-1 receptor signaling and an altered chromatin state that requires the histone demethylase RBP2/KDM5A/Jarid1A. This drug-tolerant phenotype is transiently acquired and relinquished at low frequency by individual cells within the population, implicating the dynamic regulation of phenotypic heterogeneity in drug tolerance. The drug-tolerant subpopulation can be selectively ablated by treatment with IGF-1 receptor inhibitors or chromatin-modifying agents, potentially yielding a therapeutic opportunity. Together, these findings suggest that cancer cell populations employ a dynamic survival strategy in which individual cells transiently assume a reversibly drug-tolerant state to protect the population from eradication by potentially lethal exposures.

Lung cancers caused by activating mutations in the epidermal growth factor receptor (EGFR) are initially responsive to small molecule tyrosine kinase inhibitors (TKIs), but the efficacy of these agents is often limited because of the emergence of drug resistance conferred by a second mutation, T790M. Threonine 790 is the ''gatekeeper'' residue, an important determinant of inhibitor specificity in the ATP binding pocket. The T790M mutation has been thought to cause resistance by sterically blocking binding of TKIs such as gefitinib and erlotinib, but this explanation is difficult to reconcile with the fact that it remains sensitive to structurally similar irreversible inhibitors. Here, we show by using a direct binding assay that T790M mutants retain low-nanomolar affinity for gefitinib. Furthermore, we show that the T790M mutation activates WT EGFR and that introduction of the T790M mutation increases the ATP affinity of the oncogenic L858R mutant by more than an order of magnitude. The increased ATP affinity is the primary mechanism by which the T790M mutation confers drug resistance. Crystallographic analysis of the T790M mutant shows how it can adapt to accommodate tight binding of diverse inhibitors, including the irreversible inhibitor HKI-272, and also suggests a structural mechanism for catalytic activation. We conclude that the T790M mutation is a ''generic'' resistance mutation that will reduce the potency of any ATP-competitive kinase inhibitor and that irreversible inhibitors overcome this resistance simply through covalent binding, not as a result of an alternative binding mode. lung cancer ͉ tyrosine kinase ͉ x-ray crystallography

Non-small-cell lung cancers (NSCLCs), the most common lung cancers, are known to have diverse pathological features. During the past decade, in-depth analyses of lung cancer genomes and signalling pathways have further defined NSCLCs as a group of distinct diseases with genetic and cellular heterogeneity. Consequently, an impressive list of potential therapeutic targets was unveiled, drastically altering the clinical evaluation and treatment of patients. Many targeted therapies have been developed with compelling clinical proofs of concept; however, treatment responses are typically short-lived. Further studies of the tumour microenvironment have uncovered new possible avenues to control this deadly disease, including immunotherapy.