Structural, Biochemical, and Clinical Characterization of Epidermal Growth Factor Receptor (EGFR) Exon 20 Insertion Mutations in Lung Cancer

Crystal structure and detailed analysis of different EGFR mutants explain why some mutations in exon 20 make lung cancers resistant to EGFR inhibitors and others make them more sensitive. A Crystal Clear Cause of Drug Resistance Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) are used to treat a variety of cancers, including non–small cell lung cancer. EGFR mutations have a wide range of effects on the success of TKI treatment in this cancer type, with some sensitizing the tumors to TKI inhibitors and others making them resistant to targeted therapy. For example, most of the mutations in exon 20, a relatively common mutation site, prevent cancer cells from responding to EGFR inhibitors. Here, Yasuda and co-workers determined the crystal structure of EGFR with an exon 20 mutation and used a combination of kinetic studies and structural analysis to elucidate the mechanism for these mutants’ differential sensitivity to TKIs. The findings of Yasuda et al. clarify the reasons for the drug resistance of most exon 20 mutations and show the mechanism for the rare mutation in the same exon that increases tumors’ sensitivity to treatment. In addition to explaining which of the mutants are resistant to targeted inhibition of EGFR and the reasons for this phenomenon, this work could help with the development of future therapeutics. By taking advantage of the crystal structure and detailed insights into the function of mutant EGFR, researchers may be able to design drugs that exploit the unique structural features of resistant mutants and specifically target them for treatment. Epidermal growth factor receptor (EGFR) gene mutations (G719X, exon 19 deletions/insertions, L858R, and L861Q) predict favorable responses to EGFR tyrosine kinase inhibitors (TKIs) in advanced non–small cell lung cancer (NSCLC). However, EGFR exon 20 insertion mutations (~10% of all EGFR mutations) are generally associated with insensitivity to available TKIs (gefitinib, erlotinib, and afatinib). The basis of this primary resistance is poorly understood. We studied a broad subset of exon 20 insertion mutations, comparing in vitro TKI sensitivity with responses to gefitinib and erlotinib in NSCLC patients, and found that most are resistant to EGFR TKIs. The crystal structure of a representative TKI-insensitive mutant (D770_N771insNPG) reveals an unaltered adenosine triphosphate–binding pocket, and the inserted residues form a wedge at the end of the C helix that promotes the active kinase conformation. Unlike EGFR-L858R, D770_N771insNPG activates EGFR without increasing its affinity for EGFR TKIs. Unexpectedly, we find that EGFR-A763_Y764insFQEA is highly sensitive to EGFR TKIs in vitro, and patients whose NSCLCs harbor this mutation respond to erlotinib. Analysis of the A763_Y764insFQEA mutant indicates that the inserted residues shift the register of the C helix in the N-terminal direction, altering the structure in the region that is also affected by the TKI-sensitive EGFR-L858R. Our studies reveal intricate differences between EGFR mutations, their biology, and their response to EGFR TKIs.