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Key References for
Epidermal Growth Factor Receptor (EGFR)
The EGFR gene is located on the short arm of chromosome 7 (7p11.2) and encodes a 170-kDa type I transmembrane growth factor receptor with Tyrosine kinase activity. EGFR belongs to the ErbB family of receptor tyrosine kinases (RTKs), which includes EGFR, ErbB2(neu, HER2), ErbB3 (HER3), and ErbB4(HER4).
The activation of EGFR is believed to be induced through sequential receptor dimerization occurring at two distinct sites of homo- or heterodimers with other ErbB family members upon ligand stimulation; one between the extracellular domains of EGFR and one between the intracellular domains of the receptor. Ligand-induced dimerization of the receptor’s extracellular domain leads to approximation of the intracellular domains, followed by asymmetric dimerization of the two kinase domains, where the C-lobe of one kinase domain (the “activator” monomer) binds to the N-lobe of the second kinase domain (the “receiver” monomer) and allosterically activates it.
Following these receptor dimerization events, EGFR becomes fully activated and undergoes phosphorylation at multiple tyrosine residues within its C-terminal tail. Signaling molecules containing Src homology 2 (SH2) domains and phosphotyrosine binding (PTB) domains are then recruited to specific phosphotyrosines of the receptor, subsequently priming the induction of downstream signaling cascades, which leading to increased proliferation, angiogenesis, metastasis, and decreased apoptosis.
Key EGFR mutations in various Cancer
Several different classes of genomic mutations within EGFR have been found in various cancer types, including lung adenocarcinoma, glioblastoma (GBM), and colorectal adenocarcinoma. In addition, numerous structural and functional studies have shown that a subset of patient-derived EGFR mutations are directly associated with ligand-independent receptor activation and dysregulation of the EGFR signaling cascade, consequently resulting in cellular transformation. In lung adenocarcinoma, the most dominant mutations are within the kinase domain of EGFR, including L858R in exon 21 and various small in-frame deletions in exon 19 (Ex19Del), with these genomic alterations making up ∼90% of EGFR mutations identified from lung adenocarcinoma. In particular, these two prevalent types of somatic mutations have clinical significance because lung tumors harboring such mutations are highly sensitive to EGFR kinase targeted inhibitors such as gefitinib and erlotinib. Thus, the EGFR mutation status in lung cancer patients serves as a crucial genomic determinant and biomarker for clinical outcomes with EGFR-directed therapy.
In GBM, EGFR alterations were frequently identified as either intragenic deletions between exons 2-7 (denoted as EGFR vIII), exons 14-15 (denoted as EGFRvII) or somatic alterations within the extracellular domain of the receptor, but lung cancer prevalent kinase domain mutations are relatively rare. Furthermore, recent genomic studies identified several additional intragenic deletions of exons encoding the EGFR C-terminal domain in GBM, also found in lung adenocarcinoma. Subsequent studies have showed that these resulting C-terminal truncation mutants retain the ability to induce oncogenic transformation and tumorigenesis. Notably, in vitro and in vivo experiments clearly demonstrated that therapeutic anti-EGFR monoclonal antibodies such as cetuximab are highly effective against GBM-derived C-terminal deletion EGFR variants. However, the clinical efficacy of cetuximab-directed therapy for GBM patients harboring such mutations has not been proven.
In colorectal cancer, according to the public genomic databases (e.g. cBioportal), EGFR mutations have been known to be very rare (∼3%). However, very recent systematic functional and biochemical studies with recurrent EGFR mutations selected from public genomic datasets of colorectal cancer revealed that a subset of colon cancer-derived EGFR mutants function as a strong oncogenic driver in a ligand-independent manner. This result suggests that although EGFR mutations are not as common as other cancer types, these somatic alterations may contribute to the pathogenesis of colorectal cancer. Significantly, the oncogenic potential of these EGFR mutants is efficiently inhibited by cetuximab or panitumumab in vivo and in vitro. Thus, further exploration is needed as to whether these EGFR mutations can serve as clinically beneficial genomic biomarkers for anti‐EGFR antibodies in colon cancer patients with such mutations as shown in lung adenocarcinoma.
A subset of major EGFR mutations derived from cancer-patients are presented according to the location of each exon.
(BMB Reports 2020; 53(3): 133-141)
EGFR targeted therapy and drug resistance
The success of genome-directed small molecule inhibitors targeted against aberrantly activated tyrosine kinases have changed the clinical paradigm of cancer treatments and ushered in the age of precision medicine. In particular, epidermal growth factor receptor (EGFR)-targeted therapy with tyrosine kinase inhibitors (TKIs) such as erlotinib, gefitinib and afatinib have been effective in a subset of patients with non-small cell lung cancer (NSCLC) harboring EGFR activating mutations. Two common EGFR somatic alterations, the L858R mutation in exon 21 and exon 19 in-frame deletions encompassing amino acids 747 to 749, represent about 90% of EGFR mutations in lung adenocarcinoma, and predict clinical responses to EGFR-TKIs.
Dramatic radiologic responses are observed with the EGFR-TKIs, however, almost all patients become resistant less than 1 year after initial treatment. The most prevalent mechanism of acquired resistance, accounting for ~50% of resistant cases, is the acquisition of a secondary EGFR mutation, a substitution of threonine at the “gatekeeper” amino acid 790 to methionine (T790M) in exon 20, resulting in increased binding affinity of EGFR to ATP over inhibitors. In addition to the EGFR gatekeeper mutation, altered expression profiles, somatic single nucleotide variants and copy number alterations have also been found as mechanisms driving acquired resistance. These include gene amplification of MET, ERBB2 or CRKL, somatic mutations in PI3KCA or BRAF, NF1 loss, and increased levels of IGF1R or AXL. Furthermore, epithelial-to-mesenchymal transition (EMT) or histological transformation to small-cell lung cancer has been reported to be responsible for EGFR-TKIs resistance. Nevertheless, the mechanism of acquired resistance is still unknown for about 30% of remaining cases.
Several EGFR-directed therapeutic monoclonal antibodies (mAb) such as cetuximab and panitumumab are effective in the treatment of a subset of tumors such as colon cancer. Although the mechanisms are unclear, previous structural studies suggested that one proposed pharmacological mode of action of these EGFR-targeted Abs is to interfere with EGFR dimerization via specifically binding to the ligand interacting region of the receptor so that the drugs block the enzymatic activation of EGFR. Given that a subset of oncogenic EGFR mutants such as exon 19 deletion and exon 20 insertion mutants are oncogenically active irrespective of receptor dimerization, it is postulated that antibodies directed at the “upstream” extracellular domain may be ineffective against these dimerization-independent oncogenic mutant EGFR. Several recent in vivo and in vitro studies provided compelling evidence to support this hypothesis, showing that there is a close relationship between dimerization dependency and EGFR-directed mAb responses among mutant EGFR. For example, mouse lung tumors formed by dimerization-dependent L858R and G719S EGFR mutants are dramatically reduced by cetuximab treatment, whereas tumors induced by dimerization-independent mutant EGFR such as the exon 20 insertion mutant are resistant to cetuximab. Similarly, the oncogenic activity of GBM-derived C-terminal deletion mutants, caused by their constitutive dimerization, is efficiently inhibited by cetuximab in vivo and in vitro. Furthermore, recent reports demonstrated that all dimerization-dependent colorectal cancer-derived EGFR have a dramatic response to cetuximab and panitumumab. Taken together, these data suggest that ligand-independent constitutive receptor dimerization caused by somatic mutations within EGFR is a key molecular mechanism leading to oncogenic activation of EGFR and disruption of dimerization may be among the pharmacological mechanisms of EGFR-targeted mAbs. Thus, it was proposed that the requirement of dimerization for oncogenic activation among mutant EGFR may be a crucial predictive factor of clinical response to cetuximab, as a close correlation exists between dimerization dependency and its pharmacological effects. This hypothesis needs to be further explored in future studies in clinical settings.
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