Question 1

What is the BRAF V600E mutation?

Accepted Answer

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.

Question 2

Is BRAF an oncogene or tumor suppressor?

Accepted Answer

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.

Question 3

How do BRAF inhibitors work and why are they combined with MEK inhibitors?

Accepted Answer

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.

Question 4

What cancers carry BRAF V600E?

Accepted Answer

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%.

Question 5

Why do BRAF inhibitors fail in colorectal cancer but work in melanoma?

Accepted Answer

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.

Question 6

What are Class 1, 2, and 3 BRAF mutations and why does the classification matter?

Accepted Answer

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.

Question 7

What resistance mechanisms develop after BRAF/MEK inhibitor treatment in melanoma?

Accepted Answer

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.

BRAF Gene Function

Overview

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