The RAS/RAF/MEK/ERK (MAPK) cascade is one of the most ancient and conserved proliferative signalling pathways in eukaryotes, from yeast to humans. KRAS, BRAF, or NRAS mutations — each constitutively activating the pathway at different nodes — are collectively present in approximately 40% of all human cancers. The pathway drives transcription of proliferative genes (cyclin D1, MYC, FOS, JUN) and survival genes (BCL2-family members), making it a central oncogenic axis and high-priority therapeutic target.
RTK activation recruits GRB2–SOS to the membrane, where SOS catalyses GDP→GTP exchange on RAS. GTP-RAS binds BRAF (and CRAF) RBD domains, promoting RAF dimerisation and kinase activation. RAF phosphorylates MEK1/2, which dually phosphorylates ERK1/2. Nuclear ERK activates transcription factors (ELK1, FOS, JUN, MYC), while cytoplasmic ERK activates RSK and MNK kinases for translational control and ribosome biogenesis.
RTK activation recruits GRB2–SOS to the membrane, where SOS catalyses GDP→GTP exchange on RAS. GTP-RAS binds BRAF (and CRAF) RBD domains, promoting RAF dimerisation and kinase activation. RAF phosphorylates MEK1/2, which dually phosphorylates ERK1/2. Nuclear ERK activates transcription factors (ELK1, FOS, JUN, MYC), while cytoplasmic ERK activates RSK and MNK kinases for translational control and ribosome biogenesis.
RAS / MAPK Signalling Cascade
SOS catalyses GDP→GTP exchange on KRAS (and HRAS, NRAS). GTP-RAS undergoes conformational change in switch I and II loops, creating an effector-binding surface with high affinity for RAF RBD domains.
Active RAF phosphorylates MEK1 at Ser218/Ser222 and MEK2 at Ser222/Ser226. MEK is a dual-specificity kinase — it must phosphorylate both Thr and Tyr on ERK. MEK's narrow substrate specificity (only ERK) makes MEK inhibitors highly specific.
Phospho-MEK dually phosphorylates ERK1 at Thr202/Tyr204 and ERK2 at Thr185/Tyr187. Active ERK dimerises and translocates to the nucleus where it phosphorylates transcription factors ELK1, MYC, FOS, and RSK. Nuclear ERK output drives cyclin D1, MYC, and anti-apoptotic protein expression.
ERK phosphorylates SOS1 at multiple serines, promoting GRB2 dissociation and SOS inactivation. ERK also phosphorylates BRAF and CRAF at inhibitory sites, creating amplitude-limiting negative feedback. Oncogenic KRAS/BRAF mutations partially disrupt these feedback mechanisms, sustaining maximal ERK output.
KRAS is mutated in ~90% of pancreatic, ~40% of colorectal, and ~25% of lung adenocarcinomas — collectively making it the most commonly mutated oncogene across solid tumours. BRAF V600E drives ~50% of melanomas, ~60% of papillary thyroid cancers, and ~10% of colorectal cancers. NRAS mutations occur in ~20% of melanomas. The ubiquity of RAS/MAPK pathway activation means nearly half of all cancer patients have a tumour with a direct mutation in this cascade.
BRAF V600E-targeted therapy (dabrafenib + trametinib) achieves ~70% response rates in melanoma with median PFS of ~12–15 months. KRAS G12C inhibitors (sotorasib, adagrasib) achieve ~35–40% response rates in NSCLC. MEK inhibitors (trametinib, cobimetinib, binimetinib) are used in combination with BRAF inhibitors to prevent paradoxical ERK activation. RAS inhibitors targeting G12D and other mutations are in clinical trials, potentially expanding targetability to pancreatic cancer.
Why do BRAF inhibitors require MEK inhibitor combination therapy?
BRAF inhibitors (vemurafenib, dabrafenib) paradoxically activate ERK in RAS-wild-type cells by promoting BRAF–CRAF heterodimerisation. Drug-bound BRAF acts as an 'activator' in the asymmetric dimer, transactivating CRAF and bypassing the drug-bound monomer. Adding a MEK inhibitor (trametinib) downstream prevents this paradoxical ERK activation and delays resistance.
What is the difference between KRAS G12C and G12D targeting?
G12C inhibitors (sotorasib, adagrasib) exploit the unique cysteine at position 12, covalently forming a bond that locks KRAS in the inactive GDP-bound state. G12D (the most common KRAS mutation, dominant in pancreatic cancer) lacks this cysteine. G12D inhibitors using non-covalent or alternative covalent strategies are in early clinical development, with MRTX1133 showing promising preclinical activity.
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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.