EGFR Gene Function
Epidermal Growth Factor Receptor
Overview
EGFR is a receptor tyrosine kinase and major oncogenic driver mutated in 15–50% of non-small-cell lung adenocarcinomas. Activating mutations — exon 19 deletions and L858R — stabilise the active kinase without ligand, driving constitutive RAS/MAPK and PI3K/AKT/mTOR signalling. EGFR-mutant NSCLC responds dramatically to TKIs (osimertinib, ~19 months median PFS), but resistance emerges through T790M gatekeeper mutation, MET amplification, KRAS bypass mutations, and small cell transformation. Understanding these resistance mechanisms defines modern precision oncology strategy for lung cancer.
Molecular Mechanism
Mechanism Summary
Activating EGFR mutations — exon 19 deletions and L858R — stabilise the active kinase conformation without ligand, driving constitutive RAS/MAPK and PI3K/AKT/mTOR signalling in NSCLC. T790M increases ATP affinity by more than an order of magnitude, making reversible ATP-competitive inhibitors less effective. Third-generation osimertinib forms an irreversible covalent bond with Cys797, overcoming T790M — though C797S and bypass mechanisms such as MET amplification can drive later resistance.
Step-by-Step Mechanism
EGF, TGF-α, amphiregulin, or epiregulin binds between extracellular domains I and III of EGFR. Ligand binding stabilises an extended receptor conformation, releasing the intramolecular domain II–IV tether and exposing the domain II dimerisation arm for receptor–receptor contact.
EGFR forms homo- or heterodimers — most potently with HER2 (ERBB2), which acts as a preferred co-receptor with a constitutively open dimerisation arm. The asymmetric kinase dimer positions one subunit as the 'activator' and the other as the 'receiver', triggering trans-autophosphorylation at Tyr992, Tyr1045, Tyr1068, Tyr1086, Tyr1148, and Tyr1173.
Phospho-Tyr1068 recruits GRB2 via its SH2 domain. GRB2's SH3 domains constitutively bind the RAS GEF SOS1/SOS2, bringing SOS to the membrane where it catalyses GDP→GTP exchange on KRAS. GTP-KRAS binds and activates BRAF/CRAF → MEK1/2 (Ser217/221) → ERK1/2 (Thr185/Tyr187), driving nuclear transcription of cyclin D1, MYC, and FOS for G1/S progression.
Phospho-Tyr1173 and Tyr1068 recruit SHC and GAB1 adapter proteins, which recruit PI3K (p85/p110α). PI3Kα catalyses PIP2→PIP3 at the inner plasma membrane leaflet. PIP3 recruits AKT via its PH domain; PDK1 phosphorylates AKT Thr308 (~20% activity) and mTORC2 phosphorylates AKT Ser473 for full activation. Active AKT phosphorylates BAD (Ser136, blocking apoptosis), FOXO3a (nuclear exclusion), TSC2 (activating mTORC1→S6K1/4EBP1 for protein synthesis), and MDM2 (Ser166, promoting p53 degradation).
Signal termination occurs through c-CBL E3 ubiquitin ligase phosphorylation at Tyr1045, polyubiquitinating EGFR and directing it to lysosomes for degradation. Alternative routing via RAB11 recycling endosomes sustains signalling by returning EGFR to the cell surface. PTEN opposes PI3K signalling by dephosphorylating PIP3→PIP2, and GTPase-activating proteins (NF1, RASA1) accelerate KRAS GTP hydrolysis to terminate RAS signalling.
Exon 19 deletions (most commonly del746-750) remove the C-helix loop ELREA motif that normally contributes to autoinhibitory contacts, locking the kinase in the active DFG-in conformation. L858R replaces the small leucine with arginine in the activation loop, disrupting the molecular brake formed by the DFGφ motif. Both mutations make the kinase constitutively active without ligand and hypersensitive to ATP-competitive TKI binding, predicting 60–80% objective response rates to first-generation inhibitors.
T790M resistance: replacing the gatekeeper threonine with methionine increases ATP affinity by more than an order of magnitude. Reversible inhibitors such as erlotinib and gefitinib must therefore compete less effectively with cellular ATP. Biochemical and structural work indicates that this altered competition, rather than a simple steric blockade of inhibitor binding, is the primary resistance mechanism.
Non-EGFR bypass resistance: MET gene amplification (~15% of acquired resistance) produces excess MET receptor that transphosphorylates ERBB3 at multiple YXXM motifs, directly activating PI3K/AKT independent of EGFR. KRAS mutations (~3%) provide constitutive RAS→RAF→MEK→ERK and PI3K→AKT signalling downstream of EGFR, rendering upstream EGFR inhibition irrelevant. Small cell transformation (~5%) represents epigenetic reprogramming in which EGFR-mutant adenocarcinoma cells acquire neuroendocrine identity, lose EGFR dependency, and require platinum/etoposide chemotherapy.
Osimertinib (AZD9291) forms an irreversible covalent thioether bond with Cys797 in the hinge region of the EGFR ATP-binding pocket, selectively targeting both activating mutations and T790M while sparing wild-type EGFR. First-line osimertinib achieves 18.9 months median PFS (FLAURA trial). Acquired C797S resistance disrupts covalent bond formation; combined with T790M, the triple-mutant EGFR (exon19del/L858R + T790M + C797S) is resistant to all approved EGFR inhibitors, driving development of allosteric fourth-generation inhibitors.
Upstream Regulators
Ligand-induced extracellular domain rearrangement, dimerisation arm exposure, and kinase activation
Sterically modulate EGFR ligand access at the cell surface; MUC4 promotes EGFR–HER2 dimerisation
Phosphorylates EGFR Tyr845 in the kinase domain activation loop, potentiating kinase activity and enabling ligand-independent signalling
In MET-amplified tumours, MET transphosphorylates HER3 to activate PI3K/AKT as a bypass signal independent of EGFR
Downstream Targets
Cyclin D1 and MYC transcription → G1/S progression; FOS/JUN activation → AP-1-driven pro-proliferative gene expression
BAD inactivation (apoptosis resistance); FOXO3a cytoplasmic sequestration (BIM/p27 suppression); mTORC1-driven S6K1/4EBP1 → protein synthesis
Direct EGFR kinase phosphorylation of STAT3 Tyr705 → STAT3 dimerisation → BCL-XL, MCL1, and VEGFA transcription
Key Post-Translational Modifications
Scaffold for RAS/MAPK and PI3K/AKT cascade recruitment; primary oncogenic signalling outputs
Receptor internalisation and lysosomal degradation; primary signal termination mechanism
Potentiates kinase domain activity; contributes to ligand-independent signalling in EGFR-overexpressing tumours
Disease Mechanism
EGFR mutations occur in approximately 15% of Western NSCLC patients and 30–50% of East Asian NSCLC patients, with strong enrichment in never-smokers and lung adenocarcinoma histology. Exon 19 deletions and L858R account for most sensitising mutations and predict response to EGFR TKIs in the appropriate disease setting. Acquired resistance is heterogeneous: T790M increases ATP affinity by more than an order of magnitude, reducing the effectiveness of reversible ATP-competitive inhibitors, while osimertinib can overcome T790M through covalent binding to Cys797. Other routes include bypass signalling such as MET amplification, downstream pathway alterations, histologic transformation and C797S-mediated loss of osimertinib covalent binding. Frequencies and treatment implications vary with line of therapy, assay, tumour context and the specific combination of alterations.
Database References
Key Pathways
- ·EGFR/RAS/MAPK signalling
- ·PI3K/AKT/mTOR signalling
- ·RAS/RAF/MEK/ERK cascade
- ·JAK-STAT3 signalling
- ·EGFR TKI resistance pathways
Disease Associations
- ·EGFR-mutant non-small-cell lung cancer (NSCLC)
- ·Glioblastoma (EGFRvIII deletion)
- ·Head and neck squamous cell carcinoma
- ·Colorectal cancer (EGFR overexpression)
Research Activity
EGFR is an actively studied target: about 600+ clinical trials that mention it are currently recruiting on ClinicalTrials.gov. Trial activity reflects research interest, not proven benefit — designs, endpoints and populations vary widely.
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Functional Partners
Common Questions About EGFR
What is the most common EGFR mutation in lung cancer?
The two most common sensitising EGFR mutations in NSCLC are exon 19 deletions (del746-750 and similar variants, ~45% of all EGFR mutations) and the L858R point mutation in exon 21 (~40%). Together they account for approximately 85% of all clinically relevant EGFR alterations. Both stabilise the active kinase conformation and confer strong sensitivity to EGFR TKIs, with exon 19 deletions generally associated with slightly superior outcomes.
How does T790M cause resistance to EGFR inhibitors?
T790M substitutes threonine at the gatekeeper position 790 with methionine and increases EGFR's affinity for ATP by more than an order of magnitude. Reversible inhibitors such as erlotinib and gefitinib therefore compete less effectively with cellular ATP; biochemical and structural evidence indicates that altered ATP competition, rather than a simple steric block, is the primary resistance mechanism. Osimertinib can overcome T790M by forming an irreversible covalent bond with Cys797, although later resistance mechanisms can emerge.
Why do KRAS mutations cause EGFR inhibitor resistance?
KRAS lies immediately downstream of EGFR in the signalling cascade. Oncogenic KRAS mutations (G12C, G12D, G12V) lock KRAS in the GTP-bound active state, providing constitutive downstream RAF/MEK/ERK and PI3K/AKT signalling that is completely independent of upstream EGFR activity. When EGFR is inhibited in a KRAS-mutant tumour, the downstream pathway remains fully active — exactly as if EGFR were never engaged. This is why KRAS mutations mandate RAS testing before anti-EGFR antibody therapy in colorectal cancer and explain the ~3% of NSCLC EGFR inhibitor resistance cases attributed to KRAS.
What drugs target EGFR mutations and how do they differ?
First-generation TKIs (erlotinib, gefitinib) are reversible ATP-competitive inhibitors that block the EGFR kinase domain competitively. Second-generation agents (afatinib, dacomitinib) form irreversible covalent bonds with Cys797 and additionally inhibit HER2 and HER4. Third-generation osimertinib forms an irreversible Cys797 bond with selectivity for mutant over wild-type EGFR, covering both activating mutations and T790M. Osimertinib is now the preferred first-line agent, achieving 18.9 months median PFS versus 10.2 months for first-generation TKIs in the FLAURA trial.
What is EGFR signalling and why does it matter in cancer?
EGFR is a receptor tyrosine kinase that, upon ligand binding, activates two major oncogenic cascades: EGFR→GRB2→SOS→KRAS→BRAF→MEK→ERK (driving cyclin D1 and MYC for proliferation) and EGFR→GAB1→PI3K→PIP3→AKT→mTOR (driving BAD inactivation and protein synthesis for survival). In EGFR-mutant NSCLC, both cascades run constitutively without ligand — providing an unbroken signal for unrestrained tumour growth. The pathway's clinical importance stems from the high response rates to matched TKI therapy in the 15–50% of NSCLC patients carrying activating mutations.
What happens after osimertinib resistance develops?
Resistance to osimertinib arises through diverse mechanisms detectable by liquid biopsy: C797S tertiary EGFR mutation (disrupts the covalent bond, ~20% of resistance), MET amplification (~15%), KRAS/BRAF mutations, RET fusions, and EGFR amplification. No single subsequent therapy is approved post-osimertinib failure. Strategies include platinum-based chemotherapy, EGFR-MET bispecific antibodies (amivantamab), and fourth-generation allosteric EGFR inhibitors (BLU-945, BBT-176) designed to overcome triple-mutant EGFR (del19/L858R + T790M + C797S).
What is the difference between EGFR exon 19 deletion and L858R?
Both are sensitising mutations that constitutively activate EGFR kinase, but through different structural mechanisms. Exon 19 deletions remove the ELREA motif in the C-helix loop that normally contributes to autoinhibitory contacts, releasing the kinase from its resting state. L858R inserts arginine into the DFGφ activation loop motif, disrupting the 'molecular brake' that normally holds the loop in the inactive position. Both mutations are hypersensitive to EGFR TKIs, but exon 19 deletions are associated with slightly superior progression-free survival on osimertinib and may have different sensitivity to specific TKIs in head-to-head comparisons.