Glioma-specific Domain IV EGFR cysteine mutations promote ligand-induced covalent receptor dimerization and display enhanced sensitivity to dacomitinib in vivo

A feature of many gliomas is the amplification of the epidermal growth factor receptor (EGFR), resulting in its overexpression. Missense mutations or deletions within the extracellular domain are associated with this amplification and can lead to constitutive activation of the receptor, with the Domain I/II deletion, EGFRvIII, being the most common. These changes have also been associated with increased sensitivity to EGFR inhibition using small molecule inhibitors. We have expressed, in human glioma cells, EGFR containing four glioma-specific EGFR missense mutations within Domain IV (C620Y, C624F, C628Y and C636Y) to analyze their biological properties and sensitivity to EGFR inhibition. One of these mutants, C620Y, exhibited an enhanced basal phosphorylation, which was partially dependent on an EGFR-ligand autocrine loop. All Domain IV mutants responded equally as well as wildtype EGFR (wtEGFR) to ligand stimulation. Biochemical analysis revealed that a pre-formed, disulfide-bonded dimer associated with these mutations was underglycosylated, inactive and cytoplasmically retained. Ligand stimulation resulted in the formation of a tyrosine-phosphorylated, disulfide-bonded dimer for all Domain IV mutants but not for wtEGFR. Following treatment with the next-generation, irreversible pan-ErbB inhibitor dacomitinib, the C620Y, C624F and EGFRvIII mutants were inactivated, covalently dimerized and were retained in the cytoplasm, resulting in cell-surface receptor loss and, for C620Y and C624F, decreased binding of EGF. Dacomitinib treatment significantly reduced the in vivo growth of human glioma xenografts bearing C620Y, but not wtEGFR. Collectively, these data indicate that the unique biochemical traits of Domain IV EGFR cysteine mutants can be exploited for enhanced sensitivity to EGFR small molecule inhibitors, with potential clinical applications.