BRAF Gene Function
B-Raf Proto-Oncogene, Serine/Threonine Kinase
Overview
BRAF V600E is present in ~50% of melanomas, ~60% of papillary thyroid cancers, ~10% of colorectal cancers, and ~100% of hairy cell leukaemias. V600E mimics activation-loop phosphorylation, enabling monomer-active constitutive MEK/ERK signalling without RAS input — a key feature distinguishing it from Class 2 and 3 BRAF mutations. BRAF+MEK inhibitor combinations (dabrafenib+trametinib; encorafenib+binimetinib) achieve >60% response rates in melanoma. In colorectal cancer, EGFR feedback reactivation requires triple therapy (BRAF+MEK+EGFR inhibitor, BEACON trial). Resistance develops through NRAS mutations, BRAF splice variants, and BRAF amplification — all converging on ERK reactivation.
Molecular Mechanism
Mechanism Summary
BRAF V600E substitutes glutamate for valine at the kinase activation loop (position 600), mimicking phosphorylation and enabling constitutive MEK/ERK activation without upstream RAS engagement or kinase dimerisation. As a monomer-active kinase, V600E avoids the CRAF dimerisation-dependent ERK reactivation that occurs paradoxically when BRAF inhibitors are applied to RAS-wild-type cells — the mechanistic basis for mandatory MEK inhibitor co-treatment. Acquired resistance in melanoma predominantly restores ERK pathway output via NRAS mutations (~20%), BRAF splice variants lacking the RAS-binding domain (~15%), BRAF V600E amplification (~10%), or receptor tyrosine kinase upregulation — all converging on MEK/ERK reactivation.
Step-by-Step Mechanism
Wild-type BRAF is activated by GTP-RAS binding its RAS-binding domain (RBD), inducing dimerisation with CRAF or another BRAF monomer and promoting 14-3-3 displacement from inhibitory Ser365.
V600E substitution introduces a glutamate (mimicking phosphorylation) into the activation loop at position 600, stabilising the active DFG-in conformation and enabling catalytic activity without dimerisation.
Active BRAF (wild-type or V600E) phosphorylates MEK1 at Ser218 and MEK2 at Ser222. Activated MEK then dual-phosphorylates ERK1/2 at Thr185/Tyr187.
Nuclear ERK1/2 phosphorylates transcription factors ELK1, MYC, and RSK, driving expression of cyclin D1, FOS, JUN, and anti-apoptotic proteins. Cytoplasmic ERK phosphorylates RSK and MNK for translational control.
ERK-mediated negative feedback phosphorylates SOS1 and RAF kinases, normally attenuating signal. V600E-mutant BRAF is feedback-resistant, sustaining maximal ERK output.
First-generation BRAF inhibitors (vemurafenib) paradoxically activate ERK in RAS-mutant cells by promoting BRAF-CRAF heterodimerisation, transactivating CRAF and bypassing drug-bound BRAF — mechanistic basis for mandatory MEK inhibitor co-treatment.
Upstream Regulators
GTP-RAS binds BRAF RBD to promote dimerisation and activation; V600E bypasses this requirement
Bind phospho-Ser365/Ser729 to maintain BRAF in autoinhibited state; disrupted upon activation
Chaperones mutant BRAF V600E; Hsp90 inhibitors destabilise the oncoprotein
Downstream Targets
ERK1/2 activation → proliferation, survival, differentiation
Scaffold protein for ERK pathway assembly at membrane
Key Post-Translational Modifications
Activates kinase domain; mimicked by V600E mutation
Autoinhibitory when unphosphorylated; released by RAS binding
Proteasomal degradation of misfolded/excess BRAF
Disease Mechanism
BRAF V600E is present in ~50% of melanomas, ~60% of papillary thyroid cancers, ~10% of colorectal cancers, and virtually all hairy cell leukaemias. The mutation provides near-maximal ERK output from a single kinase domain, uncoupled from upstream growth factor control. In colorectal cancer, BRAF V600E confers a particularly poor prognosis and is associated with the CpG island methylator phenotype (CIMP) and microsatellite instability. Why BRAF-targeted therapy has tissue-specific limitations: (1) In colorectal cancer, EGFR feedback loop reactivation occurs within 2–4 hours of BRAF inhibition — EGFR upregulation re-engages KRAS/CRAF/MEK/ERK, rendering single-agent or dual BRAF+MEK inhibition largely ineffective; triple therapy (encorafenib + binimetinib + cetuximab, BEACON trial) achieved 26% response rates versus ~2% for monotherapy. (2) In melanoma, acquired resistance predominantly reactivates ERK: NRAS mutations (~20%) drive CRAF dimerisation that bypasses inhibited BRAF V600E; BRAF splice variants lacking the RAS-binding domain (~15%) dimerize constitutively without RAS interaction; BRAF amplification (~10%) overwhelms inhibitor stoichiometry. (3) Class 2 BRAF mutations (K601E, G469A) and Class 3 mutations (D594G, G596R) are not covered by V600E-selective inhibitors — they signal as active dimers or RAS-sensitised impaired kinases and require MEK inhibitors or pan-RAF strategies. (4) Re-challenge with an alternative BRAF/MEK inhibitor combination after first-line resistance shows activity in a subset of melanoma patients, suggesting incomplete cross-resistance.
Database References
Key Pathways
- ·MAPK/ERK signaling
- ·RAS signaling
- ·RAF/MEK/ERK cascade
- ·BRAF inhibitor resistance
Disease Associations
- ·Melanoma
- ·Colorectal cancer (BRAF V600E poor prognosis)
- ·Papillary thyroid cancer
- ·Hairy cell leukemia
Research Activity
BRAF is an actively studied target: about 175+ 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 BRAF
What is the BRAF V600E mutation?
BRAF V600E substitutes glutamate for valine at position 600 in the kinase activation loop, mimicking phosphorylation and locking BRAF in a constitutively active conformation. The mutant kinase continuously signals through MEK and ERK to drive cell proliferation, completely bypassing the normal requirement for upstream RAS activation.
Is BRAF an oncogene or tumor suppressor?
BRAF is a proto-oncogene — its wild-type form promotes normal cell growth in response to RAS signalling, but activating mutations like V600E make it a driver oncogene. A single BRAF V600E mutation is sufficient to transform cells, consistent with the dominant gain-of-function behaviour of classical oncogenes.
How do BRAF inhibitors work and why are they combined with MEK inhibitors?
BRAF inhibitors (vemurafenib, dabrafenib) occupy the ATP-binding pocket of BRAF V600E, blocking ERK pathway activation. Monotherapy causes paradoxical ERK activation in RAS-wild-type cells through CRAF dimerisation; adding a MEK inhibitor (trametinib) downstream prevents this rebound and delays resistance.
What cancers carry BRAF V600E?
BRAF V600E occurs in approximately 50% of cutaneous melanomas, 60% of papillary thyroid cancers, 10% of colorectal cancers, and hairy cell leukaemia (~100%). Combined BRAF/MEK inhibition has transformed treatment outcomes in melanoma, with response rates exceeding 60%.
Why do BRAF inhibitors fail in colorectal cancer but work in melanoma?
The key difference is EGFR feedback reactivation. In colorectal cancer, BRAF inhibition causes rapid (within 2–4 hours) upregulation of EGFR ligand signalling that reactivates RAS→CRAF→MEK→ERK, completely bypassing the drug-inhibited BRAF V600E. Colorectal cells express high levels of EGFR and show robust feedback induction, whereas melanocytes express low EGFR and rely more exclusively on BRAF V600E for ERK output. This is why colorectal BRAF V600E requires triple therapy (encorafenib + binimetinib + cetuximab, BEACON trial) to block EGFR simultaneously, achieving response rates of ~26% versus ~2% for single-agent BRAF inhibition.
What are Class 1, 2, and 3 BRAF mutations and why does the classification matter?
BRAF mutations are classified by their signalling mechanism: Class 1 (V600E, V600K) are monomer-active, RAS-independent, and highly sensitive to V600E-selective inhibitors like vemurafenib and encorafenib. Class 2 (K601E, G469A, fusions) signal as active RAS-independent dimers — V600E-selective inhibitors paradoxically activate ERK in Class 2 tumours through CRAF transactivation; these require MEK inhibitors or next-generation pan-RAF inhibitors. Class 3 (D594G, G596R) are catalytically impaired and signal through RAS-driven dimerisation with CRAF, making them effectively RAS-dependent; standard BRAF inhibitors do not help and the approach is similar to treating upstream RAS-activating mutations. The classification determines which therapeutic strategy is appropriate.
What resistance mechanisms develop after BRAF/MEK inhibitor treatment in melanoma?
Acquired resistance to BRAF + MEK inhibitors in melanoma predominantly reactivates the MAPK/ERK pathway: NRAS activating mutations (~20% of resistance cases) provide RAS-GTP-driven CRAF dimerisation that bypasses inhibited BRAF V600E; BRAF V600E splice variants that lose the N-terminal RAS-binding domain (~15%) form constitutive dimers resistant to V600E-selective inhibitors; BRAF V600E amplification overwhelms inhibitor binding capacity; MEK1/2 mutations (MEK1 P124L/S, MEK2 C125S) reduce inhibitor binding or constitute activating alterations themselves. Non-MAPK bypass through AKT/PI3K pathway hyperactivation (PTEN loss, PIK3CA mutation) represents ~15% of resistance and requires combination PI3K or AKT inhibitor strategies.