The EGFR signalling pathway converts extracellular EGF binding into intracellular proliferative and survival signals via the KRAS–RAF–MEK–ERK cascade. In cancer, activating mutations in EGFR (exon 19 deletions, L858R) or constitutively activating KRAS mutations (G12C, G12D, G12V) decouple this cascade from upstream regulation, driving uncontrolled tumour cell proliferation. Understanding the EGFR–KRAS pathway mechanism is essential for interpreting why EGFR inhibitors fail in KRAS-mutant cancers, how resistance emerges, and how targeted therapies are selected across NSCLC, colorectal, and pancreatic cancers.
The EGFR signalling pathway begins when EGF or related ligands (TGF-α, amphiregulin, epiregulin) bind the extracellular domain of EGFR (ErbB1/HER1), inducing receptor dimerisation and trans-autophosphorylation of intracellular tyrosine residues. Phospho-Tyr1068 recruits the GRB2–SOS1 adaptor complex to the membrane, where SOS1 catalyses GDP→GTP exchange on membrane-anchored KRAS (and HRAS, NRAS). GTP-bound KRAS adopts an active conformation that recruits BRAF and CRAF kinases, initiating the RAF→MEK→ERK phosphorylation cascade. Nuclear ERK activates transcription factors ELK1, MYC, and c-FOS, which drive CCND1 (cyclin D1) expression and G1/S cell cycle entry. PI3K/AKT/mTOR survival signalling is simultaneously activated through p85 SH2 recruitment to phospho-Tyr992 and via direct GTP-KRAS binding to the p110α catalytic subunit of PI3K, reinforcing tumour cell survival independent of the MAPK cascade.
The EGFR signalling pathway begins when EGF or related ligands (TGF-α, amphiregulin, epiregulin) bind the extracellular domain of EGFR (ErbB1/HER1), inducing receptor dimerisation and trans-autophosphorylation of intracellular tyrosine residues. Phospho-Tyr1068 recruits the GRB2–SOS1 adaptor complex to the membrane, where SOS1 catalyses GDP→GTP exchange on membrane-anchored KRAS (and HRAS, NRAS). GTP-bound KRAS adopts an active conformation that recruits BRAF and CRAF kinases, initiating the RAF→MEK→ERK phosphorylation cascade. Nuclear ERK activates transcription factors ELK1, MYC, and c-FOS, which drive CCND1 (cyclin D1) expression and G1/S cell cycle entry. PI3K/AKT/mTOR survival signalling is simultaneously activated through p85 SH2 recruitment to phospho-Tyr992 and via direct GTP-KRAS binding to the p110α catalytic subunit of PI3K, reinforcing tumour cell survival independent of the MAPK cascade.
EGFR Signalling — NSCLC Driver Mutations & Therapeutic Targets
EGF binds bivalently across domains I and III of the EGFR extracellular region, stabilising a transition from the autoinhibited 'tethered' conformation (in which domains II and IV form an intramolecular tether) to the 'extended' conformation that exposes the domain II dimerisation arm. This structural shift increases EGFR's affinity for homo- and heterodimerisation with ErbB2 (HER2), ErbB3 (HER3), and ErbB4 (HER4) by ~100-fold. In EGFR-overexpressing cancers, elevated receptor density increases the probability of spontaneous ligand-independent dimerisation, bypassing the requirement for EGF.
EGFR dimerisation positions the intracellular kinase domains in an asymmetric head-to-tail configuration. The C-lobe of one kinase (the 'activator') contacts the N-lobe of the partner kinase (the 'receiver'), allosterically repositioning the activation loop of the receiver into the active DFG-in conformation without direct phosphoryl transfer between activation loops. This asymmetric mechanism explains why the gatekeeper mutation T790M sterically prevents first- and second-generation TKI binding (by enlarging the gatekeeper residue) while leaving the natural asymmetric activation mechanism fully intact.
The activated receiver kinase trans-phosphorylates the C-terminal tail of the activator kinase on six key tyrosines: Tyr992, Tyr1045, Tyr1068, Tyr1086, Tyr1148, and Tyr1173. Each phosphotyrosine recruits distinct SH2-domain effectors: Tyr1068 recruits GRB2 (→RAS/MAPK activation), Tyr992 and Tyr1173 recruit PLCγ1 (→PKC/calcium signalling), Tyr1045 recruits c-CBL (→receptor ubiquitination and lysosomal degradation), and Tyr1068/1086 recruit SHC adaptors (→amplified GRB2–SOS recruitment). EGFR exon 19 deletions and L858R mutations increase autophosphorylation kinetics ~10-fold by destabilising the inactive kinase conformation, explaining their ligand-independence.
GRB2 constitutively binds SOS1 via its two flanking SH3 domains binding proline-rich regions of SOS1; this GRB2–SOS1 complex is pre-assembled in the cytoplasm. Phospho-Tyr1068-EGFR recruits the complex en bloc via GRB2's SH2 domain, concentrating SOS1 at the inner leaflet of the plasma membrane where prenylated KRAS resides. Membrane localisation is essential: KRAS is constitutively membrane-anchored via C-terminal farnesylation and polybasic region electrostatics, so SOS1's GDP-exchange activity on KRAS requires co-localisation at the same membrane compartment. The SHC→GRB2→SOS1 route provides a parallel, amplified input at lower EGFR activation thresholds.
Membrane-localised SOS1 inserts its CDC25 catalytic domain into the nucleotide-binding cleft of KRAS, displacing the switch I loop and opening the nucleotide pocket to solvent. GDP dissociates rapidly (intrinsic rate ≪1 min⁻¹; SOS1-catalysed rate ~100-fold faster), and GTP — present at ~10-fold higher cytoplasmic concentration than GDP — rebinds and resets switch I and II loops into the active closed conformation. GTP-KRAS presents a dramatically remodelled effector-binding surface with high affinity for BRAF and CRAF RBDs (Kd ~20 nM), the p110α RBD of PI3K, and RALGDS. Oncogenic KRAS mutations (G12C, G12D, G12V, G13D) impair GTP hydrolysis by sterically obstructing the arginine finger of GAP proteins, locking KRAS in this active state permanently.
GTP-KRAS recruits BRAF and CRAF (RAF1) to the plasma membrane via their RAS-binding domains (RBDs) and cysteine-rich domains (CRDs). Membrane localisation relieves autoinhibitory 14-3-3 contacts at BRAF Ser365/Ser729, enabling BRAF–BRAF homodimerisation or the more potent BRAF–CRAF heterodimerisation. Dimerisation releases N-terminal regulatory inhibition and positions the DFG motif into the active 'DFG-in' kinase conformation. BRAF V600E bypasses this mechanism entirely: the V600E substitution disrupts an intramolecular hydrophobic interaction that stabilises the inactive monomer, enabling constitutive kinase activity independent of RAS-mediated dimerisation — explaining why BRAF V600E tumours retain RAF/MEK/ERK signalling even when upstream RAS is wild-type.
Active RAF phosphorylates MEK1 (MAP2K1) at Ser218 and Ser222, and MEK2 (MAP2K2) at Ser222 and Ser226, within their activation segment. MEK is a structurally constrained dual-specificity kinase with exceptionally narrow substrate specificity — its only known physiological substrates are ERK1 and ERK2. MEK achieves dual phosphorylation of ERK in a single processive event, phosphorylating first Tyr and then Thr without releasing the substrate. This narrow specificity is the mechanistic basis for the high selectivity of allosteric MEK inhibitors (trametinib, cobimetinib, binimetinib), which stabilise an inactive MEK conformation adjacent to the ATP-binding site without competing with ATP.
Doubly-phosphorylated ERK1 (pThr202/pTyr204) and ERK2 (pThr185/pTyr187) undergo a conformational change that enables homodimerisation and nuclear import via importin-independent mechanisms involving nuclear pore components. Nuclear ERK phosphorylates ELK1 (Ser383/Ser389) to drive c-FOS transcription, phosphorylates and stabilises MYC (Ser62), and activates RSK2 (which phosphorylates CREB at Ser133). The resulting transcriptional programme upregulates CCND1 (cyclin D1), CDK4, anti-apoptotic BCL-XL, and ribosome biogenesis genes — directly coupling EGFR→KRAS→ERK signal amplitude to G1/S cell cycle commitment. Cytoplasmic ERK simultaneously phosphorylates RSK1/2, MNK1/2 (→eIF4E-mediated cap-dependent translation), and cytoskeletal proteins, integrating proliferative, translational, and migratory outputs of the EGFR–KRAS signal.
EGFR and KRAS mutations represent two mechanistically distinct — and clinically mutually exclusive — mechanisms of constitutive EGFR pathway activation. EGFR activating mutations (exon 19 in-frame deletions removing LREA residues 746–750; L858R point mutation in exon 21) destabilise the inactive kinase conformation, increasing basal autophosphorylation and reducing ATP Km, producing ligand-independent receptor activation. These mutations occur in 10–15% of NSCLC overall (30–40% in East Asian populations) and are strongly associated with adenocarcinoma histology and non-smoking history. The exon 19 deletion and L858R mutations together account for ~90% of all EGFR-activating mutations in NSCLC. KRAS oncogenic mutations — G12C (27% of KRAS-mutant NSCLC), G12D (35% of pancreatic KRAS mutations), G12V, G12R, and G13D — impair intrinsic GTPase activity by disrupting catalytic Gln61 coordination of the water nucleophile, or by sterically blocking the arginine finger of RasGAP proteins (Arg789 of NF1-GAP), locking KRAS permanently in the GTP-bound active state. Because mutant KRAS propagates RAF, MEK, ERK, and PI3K signals constitutively and independently of upstream receptor status, no EGFR-targeting agent can suppress downstream signalling in KRAS-mutant tumours. KRAS mutations occur in ~90% of pancreatic ductal adenocarcinomas, ~40% of colorectal cancers, and ~25% of lung adenocarcinomas, making KRAS the most commonly mutated oncogene across human solid tumours. EGFR and KRAS mutations are mutually exclusive because they activate the same downstream pathway at different nodes — co-occurrence confers no additional proliferative advantage and is essentially absent in clinical datasets.
EGFR-targeted therapies exploit kinase domain dependency in EGFR-mutant cancers. First-generation reversible TKIs (erlotinib, gefitinib) competitively displace ATP from the kinase active site; second-generation irreversible TKIs (afatinib, dacomitinib) covalently modify Cys797; osimertinib (third-generation) was designed to covalently target Cys797 while accommodating the T790M gatekeeper mutation that bulked out the ATP pocket, overcoming primary resistance. Osimertinib achieves 18–25 month median PFS in EGFR-mutant NSCLC, the longest of any EGFR TKI. KRAS mutations are the primary reason anti-EGFR therapies fail in large patient subsets. Constitutively GTP-bound KRAS (G12C, G12D, G12V) drives RAF→MEK→ERK and PI3K→AKT signal continuously regardless of EGFR inhibitor exposure — the downstream signal simply bypasses the drug's target. This is why KRAS/NRAS mutation testing is FDA-mandated before cetuximab or panitumumab in colorectal cancer (since 2012), and why NSCLC patients are stratified by EGFR, KRAS, ALK, ROS1, and MET status before treatment selection. Beyond primary resistance, acquired KRAS mutations emerge in ~15–20% of osimertinib-resistant NSCLC tumours, alongside MET amplification, HER2 amplification, BRAF V600E, and PIK3CA mutation. KRAS G12C-specific inhibitors (sotorasib, adagrasib) exploit a cryptic switch-II pocket exposed only in the inactive GDP-bound state, covalently trapping KRAS G12C and achieving ~35–40% objective response rates in KRAS G12C-mutant NSCLC. Key genes in the EGFR–KRAS pathway: EGFR (receptor tyrosine kinase; driver through mutation in NSCLC and amplification in GBM), KRAS (central RAS GTPase; most frequently mutated oncogene in human cancer), BRAF (RAF serine/threonine kinase; V600E is the defining driver in 50% of melanomas and 60% of papillary thyroid cancers), PIK3CA (PI3Kα catalytic subunit; hotspot mutations E542K/E545K/H1047R activate survival signalling parallel to MAPK), and AKT1 (Ser/Thr kinase; E17K activating mutation in breast and colorectal cancer). Together these genes constitute the core EGFR cancer signalling network.
What does EGFR do in cancer?
EGFR (epidermal growth factor receptor) is a receptor tyrosine kinase that, when activated by EGF ligand, dimerises and autophosphorylates intracellular tyrosine residues to initiate the RAS/MAPK and PI3K/AKT proliferative and survival cascades. In cancer, EGFR is dysregulated through activating kinase domain mutations (exon 19 deletions, L858R), gene amplification (up to 40-fold in glioblastoma), or autocrine ligand loops — causing ligand-independent, constitutive pathway activation that drives uncontrolled proliferation. EGFR mutations are the primary oncogenic driver in 10–15% of NSCLC cases and define a patient population with high sensitivity to EGFR TKIs.
How does KRAS affect EGFR signalling?
KRAS is the primary downstream effector of EGFR in the proliferative cascade. Normally, EGFR activates GRB2–SOS1 to transiently exchange GDP for GTP on KRAS. When KRAS itself is mutated (G12C, G12D, G12V, G13D), its GTPase activity is abolished and it remains constitutively GTP-bound regardless of upstream EGFR signalling. This renders all EGFR-targeted therapies — both TKIs and monoclonal antibodies — ineffective, because the downstream proliferative signal propagates through mutant KRAS independently of EGFR activity. KRAS mutation is therefore the dominant mechanism of primary resistance to anti-EGFR therapy.
What is the EGFR–KRAS signalling pathway step-by-step?
The EGFR–KRAS signalling pathway: (1) EGF binds EGFR extracellular domain → (2) receptor dimerisation and asymmetric kinase activation → (3) tyrosine autophosphorylation (Tyr1068, Tyr992, etc.) → (4) GRB2–SOS1 recruited to phospho-Tyr1068 → (5) SOS1 catalyses GDP→GTP exchange on membrane-anchored KRAS → (6) GTP-KRAS recruits and activates BRAF/CRAF via RBD → (7) RAF phosphorylates MEK1/2 (Ser218/Ser222) → (8) MEK dually phosphorylates ERK1/2 (Thr202/Tyr204 on ERK1; Thr185/Tyr187 on ERK2) → (9) Nuclear ERK activates ELK1, MYC, and c-FOS, driving cyclin D1 expression and G1/S entry. Simultaneously, GTP-KRAS and phospho-Tyr992-EGFR activate PI3K→AKT→mTOR survival signalling in parallel.
Why are EGFR inhibitors ineffective in some cancers?
EGFR inhibitors fail primarily in tumours with downstream activating mutations that bypass EGFR. KRAS mutations (G12C/D/V/R) are the most common mechanism — constitutively GTP-bound KRAS activates RAF→MEK→ERK independently of EGFR status. Secondary resistance to EGFR TKIs involves EGFR T790M (addressed by osimertinib), C797S (prevents covalent osimertinib binding), MET amplification, HER2 amplification, BRAF V600E, and PIK3CA/PTEN alterations that sustain survival through AKT/mTOR even when MAPK is suppressed. These co-occurring alterations mean single-agent EGFR blockade is insufficient in molecularly heterogeneous tumours.
What is the difference between EGFR and KRAS mutations in lung cancer?
EGFR mutations (exon 19 deletions, L858R) and KRAS mutations are mutually exclusive in NSCLC because both activate the same downstream pathway — simultaneous mutation provides no additional oncogenic benefit. EGFR-mutant NSCLC responds to EGFR TKIs with ~70% response rates and 18–25 month PFS on osimertinib. KRAS-mutant NSCLC was undruggable for decades; sotorasib and adagrasib now target KRAS G12C (27% of KRAS-mutant NSCLC) with ~35–40% response rates, but G12D and G12V mutations — dominant in pancreatic cancer — lack approved targeted therapies and require distinct inhibitor strategies.
What role does the EGFR–KRAS pathway play in pancreatic cancer?
In pancreatic ductal adenocarcinoma (PDAC), KRAS mutations — predominantly G12D (~35%), G12V (~18%), G12R (~16%) — are the initiating oncogenic event present in >90% of cases. EGFR is overexpressed in ~30–40% of PDAC and contributes upstream activation, but the dominant signal runs through constitutively active mutant KRAS, rendering cetuximab-based anti-EGFR strategies ineffective. Sotorasib and adagrasib cover only KRAS G12C (rare in PDAC: <2%), while non-covalent G12D inhibitors (MRTX1133) and pan-KRAS inhibitors (RMC-6236) targeting multiple KRAS mutations simultaneously are in clinical trials for PDAC.
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Content is based on peer-reviewed scientific literature including data from NCBI, UniProt, PubMed, and TCGA. Gene links reference curated molecular biology databases. For educational purposes only; does not constitute clinical advice.