Targeting the multifaceted BRAF in cancer: New directions

Toye et al., 2024. Oncotarget. DOI: 10.18632/oncotarget.28612. PMID: 39018217

• MAPK activation predominantly occurs through genomic alterations in RAS and BRAF.
• BRAF is an effector kinase that functions downstream of RAS and propagates this oncogenic activity through MEK and ERK.
• Across cancers, BRAF alterations include gain of function mutations, copy number alterations, and structural rearrangements.

[MAPK Signaling]

Functional members of this pathway are the cytoplasmic serine/threonine kinases RAS, RAF, MEK, and ERK.

Canonical activation of receptor tyrosine kinases leads to:

• RAS-GDP phosphorylation
• Activated RAS-GTP
• Phosphorylation of RAF and dimerization
• Phosphorylation of MEK1/2
• Activation of ERK1/2
• Regulation of cellular growth, proliferation, and cell death.

Three distinct isoforms of RAF proteins encoded by: ARAF, BRAF, CRAF (RAF1) genes.

Each of these isoforms contains the same conserved regions (CR):

• Ras-binding domain (CR1)
• Regulatory domain (CR2)
• Functional kinase domain (CR3)

All RAF kinases promote MAPK signaling, BRAF is the most consistently altered RAF family gene.

[Classes of BRAF alterations in Cancer]

1. Class 1 BRAF alterations (point mutation) / “Monomers”
2. Involve missense mutations at valine 600 position (V600).
3. Leads to RAS-independent BRAF monomeric activation with strongly elevated kinase activity.
4. Can be treated with BRAF inhibitors or combinations of BRAF/MEK inhibitors.
5. Dominant form of BRAF alterations in melanomas, thyroid, colorectal, and ovarian cancers.

• Class II BRAF alterations (fusions) / “Ras-independent, constitutively activated dimers”
• Non-V600 mutations and structural rearrangements that yield homodimers that are RAS-independent and have moderate to high kinase activity.
• Dominant form of BRAF alterations in prostate, bladder, and non-small cell lung cancers.

• Class III BRAF alterations (fusions) / “Ras-dependent, hypoactive dimers”
• Non-V600 mutations which heterodimerize with ARAF or WT BRAF that are RAS-dependent and have low kinase activity. 
• Dominant form of BRAF alterations in cervical cancer, hepatobiliary cancer, and non-small cell lung cancer.
• Since they are upstream RAS-dependent, Class III mutants may be susceptible to RAS-targeted therapies.

[Evolution of BRAF inhibition in the clinic]

Vemurafenib and Dabrafenib (for metastatic melanomas) were foundational for establishing the utility of Class I BRAF inhibitors. Metastatic melanomas resistant to BRAF inhibition -> combined use of MEK inhibitors.

• In 2022, FDA granted accelerated approval of BRAFi + MEKi in all Class I BRAF-mutant metastatic solid tumors. (One of the major successes in precision oncology to date).

i.e. Approval of dabrafenib/trametinib in patients with metastatic BRAF V600E-mutant solid tumors.

In Contrast,

Agents targeting Class II and III BRAF alterations have yet to achieve similar success.

Mechanistically, Class II and III BRAF alterations are now susceptible to Class I inhibitors.

• Class I inhibitors:

Attenuate BRAF activity by binding monomeric BRAF and inducing dimerization.

Paradoxically,

• These drugs enhance Class II and III altered BRAF signaling in which the dimerization promotes enhanced MAPK signaling and ultimately tumor growth.

Therefore, recently, a new focus has been on developing paradox breakers and type II pan-RAF inhibitors that ablate the upregulated MAPK activity seen in Class II and III BRAF-altered cancers treated with currently FDA-approved RAF inhibitors.

• Example of type-II pan-RAF inhibitor: tovorafenib (phase II FIREFLY-1 clinical trial) showed high tolerability and strong efficacy in non-Class 1 BRAF-mutant pediatric low-grade gliomas harboring BRAF structural rearrangements.
• FDA therefore recently approved tovorafenib for relapsed/refractory pLGG with BRAFV600E mutation or BRAF structural rearrangement.

[BRAF structural rearrangements – gene fusions]

• SLC45A3-BRAF and TMPRSS2-BRAF gene fusions only found in prostate cancers and in no other tumor types.

[Conclusion]

Class I BRAF inhibitors are one of the landmark achievements in precision oncology – recently evidenced by the FDA approval of dabrafenib/trametinib in patients with metastatic BRAF p.V600E mutant solid tumors.

• Although targeted therapies against Class II and Class III alterations and gene fusions are largely still in early development, the accelerated approval of tovorafenib for patients with relapsed/refractory BRAF-altered pediatric low-grade glioma underscores the therapeutic potential of this and other next-generation strategies to target aberrant MAPK signaling.