Oncogenic mutant forms of EGFR: lessons in signal transduction and targets for cancer therapy

Abstract

The EGF-receptor is frequently mutated in a large variety of tumors. Here we review the most frequent mutations and conclude that they commonly enhance the intrinsic tyrosine kinase activity, or they represent loss-of-function of suppressive regulatory domains. Interestingly, the constitutive activity of mutant receptors translates to downstream pathways, which are subtly different from those stimulated by the wild-type receptor. Cancer drugs intercepting EGFR signaling have already entered clinical application. Both kinase inhibitors specific to EGFR, and monoclonal antibodies to the receptor are described, along with experimental approaches targeting the HSP90 chaperone. Deeper understanding of signaling pathways downstream to mutant receptors will likely improve the outcome of current EGFR-targeted therapies, as well as help develop new drugs and combinations.

Figure 1. Activation mechanisms and signaling pathways engaged by EGFR

In the absence of ligand, EGFR predominantly exists in a monomeric form, in which the kinase domain is auto-inhibited by the carboxyl-terminal tail of the receptor. Ligand binding traps the extracellular domain of EGFR in a conformation poised to form receptor dimers, which are mediated by mutual interactions of the dimerization loops. The resulting conformational change is relayed into the intracellular receptor portion, resulting in the formation of an asymmetric dimer of kinase domains. In this asymmetric dimer, the C-lobe of the kinase domain of the “activator kinase” buttresses the kinase domain N-lobe of the “receiver kinase”, resulting in catalytic activation of the latter. The receiver kinase then phosphorylates intracellular tyrosine residues of the activator kinase, which cluster in the carboxyl-terminal tail. Phosphorylated tyrosine residues function as docking sites for signaling adaptors (e.g., Grb2), intracellular enzymes (e.g., PLCgamma) or transcription factors (e.g., STAT3). Thereby, activated EGFR couples to a myriad of intracellular signaling pathways mediating such diverse cellular responses as cell lineage determination, proliferation, survival, or migration. The abbreviations used are: PI3K, phosphoinositide-3-kinase; PIP₃, phosphatidylinositol-3,4,5-trisphosphate; Akt, v-akt murine thymoma viral oncogene homolog; mTOR, mechanistic target of rapamycin; STAT, signal transducer and activator of transcription; PLCgamma, phospholipase C gamma; IP₃, inositol 1,4,5-triphosphate; DAG, diacylglycerol; PKC, protein kinase C; CAMK, calcium/calmodulin-dependent protein kinase; Grb2, growth factor receptor-bound protein 2; Shc, Src homology 2 domain containing transforming protein; SOS, son of sevenless homolog; Ras, Ras viral oncogene homolog; Raf, Raf murine viral oncogene homolog; MEK, mitogen-activated protein kinase kinase; ERK, mitogen-activated protein kinase (extracellular signal-regulated kinase)

Figure 2. Receptor aberrations cluster in functionally relevant hotspots of EGFR

Structural constraints impose ligand binding for receptor activation. Brain tumors such as glioblastoma multiforme (GBM) circumvent this requirement by deletions or point mutations in the ligand-binding domain (e.g., EGFRvIII) of EGFR, which promote receptor dimerization and activation. Alternately, GBMs may enhance the EGFR catalytic activity through duplications of its kinase domain, as well as by deletions in the carboxyl-terminal tail, which may otherwise execute an auto-inhibitory role on the kinase activity. Conversely, non-small cell lung cancers (NSCLC) often exhibit prototypical point mutations or small insertions/deletions in kinase domain regions regulating the basal catalytic activity of EGFR.

Figure 3. Therapies targeting EGFR signaling

Oncogenic aberrations of the EGFR extracellular domain may result in exposure of novel epitopes not displayed by the wild-type receptor. In context of the EGFRvIII deletion, monoclonal antibodies targeting such epitopes (e.g., mAb 806) have already demonstrated clinical activity. Monoclonal antibodies targeting the wild type EGFR have demonstrated substantial activity in patients with colorectal cancer, head and neck cancer, and non-small cell lung cancer (NSCLC). Tyrosine kinase inhibitors (TKIs) represent another class of anti-EGFR agents. These small ATP-mimetics readily penetrate into the cell, with gefitinib and erlotinib binding to the kinase domain in its active conformation, and inhibiting transfer of phosphate moieties. Patients with NSCLC harboring mutations in the EGFR kinase domain are particularly sensitive to TKIs (kinase marked in red). Erlotinib is approved also for patients with pancreatic cancer. Another, non-direct strategy entails HSP90 inhibition; mutated and activated EGF receptors have been shown to serve as HSP90 clients, demonstrating significantly reduced activity upon its inhibition by benzoquinoid ansamycins, such as geldanamycin derivatives.