• What is BRAF?
  • BRAF in Melanoma
  • BRAF c.1799T>G (V600G)
  • Clinical Trials

BRAF

BRAF belongs to a family of serine-threonine protein kinases that includes ARAF, BRAF, and CRAF (RAF1). RAF kinases are central mediators in the MAP kinase signaling cascade and exert their effect predominantly through phosphorylation and activation of MEK. This occurs following the dimerization (hetero- or homo-) of the RAF molecules. As part of the MAP kinase pathway, RAF is involved in many cellular processes, including cell proliferation, differentiation, and transcriptional regulation.

Mutant BRAF has been implicated in the pathogenesis of several cancers, including melanoma, non-small cell lung cancer, colorectal cancer, papillary thyroid cancer, and ovarian cancer (Davies et al. 2002). Mutant BRAF has been observed in these cancers as well as glioma and gastrointestinal stromal tumor (GIST).

mapk-pk13.png

Figure 1. Schematic of the MAPK and PI3K pathways. Growth factor binding to receptor tyrosine kinase results in activation of the MAPK signaling pathway (RAS-RAF-MEK-ERK) and the PI3K pathway (PI3K-AKT-mTOR). The letter "K" within the schema denotes the tyrosine kinase domain.

Related Pathways

Contributors: Christine M. Lovly, M.D., Ph.D., Leora Horn, M.D., M.Sc., William Pao, M.D., Ph.D. (through April 2014)

Suggested Citation: Lovly, C., L. Horn, W. Pao. 2015. BRAF. My Cancer Genome https://www.padiracinnovation.org/content/disease/melanoma/braf/?tab=0 (Updated December 7).

Last Updated: December 7, 2015

BRAF in Melanoma

Somatic mutations in BRAF have been found in 37-50% of all malignant melanomas (COSMIC; Davies et al. 2002; Hodis et al. 2012; Krauthammer et al. 2012; Maldonado et al. 2003). BRAF mutations are found in all melanoma subtypes but are the most common in melanomas derived from skin without chronic sun-induced damage (Curtin et al. 2005; Maldonado et al. 2003). In this category of melanoma, BRAF mutations are found in ~59% of samples (Curtin et al. 2005).

The most prevalent BRAF mutations detected in melanoma are missense mutations that introduce an amino acid substitution at valine 600. Approximately 80–90% of V600 BRAF mutations are V600E (valine to glutamic acid; COSMIC; Lovly et al. 2012; Rubinstein et al. 2010) while 5-12% are V600K (valine to lysine; COSMIC; Lovly et al. 2012; Rubinstein et al. 2010), and 5% or less are V600R (valine to arginine) or V600D (valine to aspartic acid; COSMIC;Lovly et al. 2012; Rubinstein et al. 2010). The result of these mutations is enhanced BRAF kinase activity and increased phosphorylation of downstream targets, particularly MEK (Wan et al. 2004). In the vast majority of cases, BRAF mutations are non-overlapping with other oncogenic mutations found in melanoma (e.g., NRAS mutations, KIT mutations, etc.).

While BRAF inhibitor therapy is associated with clinical benefit in the majority of patients with BRAF V600E- mutated melanoma, resistance to treatment and tumor progression occurs in nearly all patients, usually in the first year (Chapman et al. 2011; Sosman et al. 2012). A variety of mechanisms have been implicated in primary and acquired resistance to BRAF inhibitors, primarily through reactivation of the MAP kinase pathway and other cell signaling pathways. Secondary BRAF mutations have not been described. Mechanisms of resistance are described below (Table 1); the frequencies of each of these mechanisms of resistance are not yet known. Possible second-line and greater treatment options supported by preclinical rationale are listed, although clinical data are mostly lacking. First-line combination therapy with BRAF and MEK inhibitor therapy may delay or prevent some of the mechanisms below (Flaherty et al. 2012). Additionally, BRAF inhibitors have been investigated in combination with MEK inhibitors in subsets of patients with BRAF V600E-mutated melnaoma previously resistant to BRAF inhibitors (Johnson et al. 2014; Ribas et al. 2014).

Table 1. Mechanisms of Resistance to BRAF Inhibition.

Mechanism of resistance Implications for Targeted Therapeutics
BRAF V600 alternate splicing Unknown at this timea
BRAF V600 gene amplification Unknown at this timeb
COT overexpression Unknown at this timec
CRAF overexpression Unknown at this timed
HGF overexpression Unknown at this timee
IGF1R overexpression Unknown at this timef
Acquired MEK1 (MAP2K1) mutations Unknown at this timeg
NF1 loss of function (via mutation, deletion, etc.) Unknown at this timeh
Acquired NRAS mutations Unknown at this timei
PDGFRβ overexpression Unknown at this timej
PI3K/AKT1 mutations Unknown at this timek
PTEN loss Unknown at this timel

 

a See Poulikakos et al. 2011.

b See Shi H. et al. 2012.

c In preclinical studies, cell lines overexpressing COT (the product of the MAP3K8 gene) were sensitive to combinations of BRAF and MEK inhibitors but resistant to BRAF or MEK inhibitors alone (Johannesson et al. 2010).

d A BRAF inhibitor–resistant cell line demonstrating CRAF (the product of the RAF1 gene) overexpression was sensitive to the HSP inhibitor geldanamycin in preclinical studies (Montagut et al. 2008).

e In preclinical studies, cell lines overexpressing HFG were sensitive to combinations of BRAF and HGF inhibitors and combinations of BRAF and MET inhibitors (Straussman et al. 2012; Wilson et al. 2012).

f In preclinical studies, cell lines overexpressing IGF1R were sensitive to combinations of MEK and IGF1R inhibitors and combinations of MEK and PI3K inhibitors (Villanueva et al. 2010).

g See Emery et al. 2009 and Wagle et al. 2011.

h See Gibney and Smalley 2013; Maertens et al. 2013; Nissan et al. 2014; Whittaker et al. 2013.

i NRAS Q61K mutations resulted in increased levels of activated NRAS in two cell lines. These cell lines were sensitive to MEK inhibition with AZD6244 in preclinical studies (Nazarian et al. 2010).

j See Nazarian et al. 2010.

k See Shi et al. 2014 and Van Allen et al. 2014. l See Paraiso et al. 2011.

Contributors: Christine M. Lovly, M.D., Ph.D., Douglas Johnson, M.D., William Pao, M.D., Ph.D. (through April 2014), Jeff Sosman, M.D.

Suggested Citation: Lovly, C., D. Johnson, W. Pao, J. Sosman. 2015. BRAF in Melanoma. My Cancer Genome https://www.padiracinnovation.org/content/disease/melanoma/braf/ (Updated June 16).

Last Updated: June 16, 2015

BRAF c.1799T>G (V600G) Mutation in Melanoma

Properties
Location of mutation Kinase domain (exon 15)
Frequency of BRAF mutations in melanoma 37–50% (COSMIC; Davies et al. 2002; Hodis et al. 2012; Krauthammer et al. 2012; Maldonado et al. 2003)​
Frequency of V600G mutation among BRAF-mutated melanomas < 1% (COSMIC)
Implications for Targeted Therapeutics
Response to BRAF inhibitors Confers increased sensitivitya
Response to MEK inhibitors Unknown at this timeb
Response to dabrafenib-trametinib combination therapy Unknown at this timec
Response to vemurafenib-cobimetinib combination therapy Unknown at this timed

 

The V600G mutation results in an amino acid substitution at position 600 in BRAF, from a valine (V) to a glycine (G). This mutation occurs within the activation segment of the kinase domain (Figure 1). Mutations at V600 result in increased kinase activity and are transforming in vitro. BRAF mutations are usually found in tumors with no driver mutations detected in NRAS, KIT, and other genes.

a BRAF V600 mutations are associated with increased sensitivity to BRAF inhibitors (Chapman et al. 2011​; Falchook et al. 2012a; Flaherty et al. 2010; Flaherty et al. 2012a; Hauschild et al. 2012; Sosman et al. 2012​). BRAF V600G mutations were not specifically evaluated in these trials.

b Patients whose tumors harbored V600E and V600K mutations showed better responses to the MEK inhibitor, trametinib, than to chemotherapy (dacarbazine or paclitaxel; Flaherty et al. 2012b​); patients with V600E or V600K-mutated tumors also showed better responses to trametinib than patients with BRAF wild type tumors (Falchook et al. 2012b).

c BRAF V600E and V600K mutations are associated with response to the combination of dabrafenib and trametinib (FDA 2014; Long et al. 2014). The FDA approved use of this combination in melanoma patients with BRAF V600E or V600K mutations, based on interim results from a phase III clinical trial (NCT01584648; Long et al. 2014). This trial is only evaluating BRAF V600E and V600K mutations.

d Patients with V600-mutated tumors and no prior BRAF inhibitors treated with combination vemurafenib and cobimetinib experienced improved response rate and progression-free survival compared to patients with V600-mutated tumors and recent progression on vemurafenib (Ribas et al. 2014). This study included patients with V600E and V600K mutations (Ribas et al. 2014).

Reference Study Type / Phase Line of Treatment Treatment Agent Mutation Status # Patients in Study Response Rate PFS OS
Chapman et al. 2011; McArthur et al. 2014 Phase 3 1st vemurafenib (BRAF inhibitor) BRAF mutant (intended to include only BRAF V600E) 337 48% 6.9 13.6
V600E 328   6.9 13.3
V600K 33   5.9 14.5
dacarbazine (crossover to the vemurafenib arm permitted at progression) BRAF mutant (intended to include only BRAF V600E) 338 5% 1.6 9.7
V600E 329   1.6 10.0
V600K 24   1.7 7.6
Sosman et al. 2012​ Phase 2 2nd line or greater vemurafenib (BRAF inhibitor) BRAF V600 mutations 132 (122 with V600E, 10 with V600K) 53% 6.8 15.9
V600K 10 40%    
V600E 122 54%    
Hauschild et al. 2012​ Phase 3 1st line or greater dabrafenib (BRAF inhibitor) V600E 187 50% 5.1  
dacarbazine (crossover to dabrafenib arm permitted at progression) V600E 63 6% 2.7  
Flaherty et al. 2012b​ Phase 3 1st line or greater trametinib (MEK inhibitor) V600E or V600K 214   4.8 81% at 6 months
chemotherapy (crossover to trametinib arm permitted at progression) V600E or V600K 108   1.5 67% at 6 months
Long et al. 2014 Phase 3 1st or 2nd line dabrafenib + trametinib V600E or V600K 211 67% 9.3 93% at 6 months
dabrafenib V600E or V600K 211 51% 8.8 85% at 6 months
Robert et al. 2014 Phase 3 1st line dabrafenib + trametinib V600E or V600K 352 64% 11.4  
V600E 284 64%    
V600K 34 65%    
vemurafenib V600E or V600K 352 51% 7.3  
V600E 284 52%    
V600K 34 44%    
Flaherty et al. 2012a Phase 1 and 2 1st line or greater 150 mg dabrafenib twice daily + 1 mg trametinib per day V600E or V600K 54 (45 with V600E, 9 with V600K) 50% 9.2  
150 mg dabrafenib twice daily + 2 mg trametinib per day V600E or V600K 54 (47 with V600E, 7 with V600K) 76% 9.4  
150 mg dabrafenib 2x daily V600E or V600K 54 (45 with V600E, 9 with V600K) 54% 5.8  
Larkin et al. 2014 Phase 3 1st line or greater vemurafenib + cobimetinib V600E or V600K 56 68% 9.9 81% at 9 months
vemurafenib + placebo V600E or V600K 56 45% 6.2 73% at 9 months
Ribas et al. 2014 Phase 1b 1st line or greater vemurafenib + cobimetinib V600 mutant, no prior BRAF inhibitor 63 87% 13.7  
V600 mutant, recent progression on vemurafenib 66 15% 2.8  
Long et al. 2012 Phase 2 3rd or greater dabrafenib (no prior treatment for brain mets) V600E 74 37.8% 16.1 33.1
V600K 65 30.8% 16.6 31.4
dabrafenib (disease progression after surgery) V600E 15 0% 8.1 16.3
V600K 18 27.8% 15.9 21.9

​NOTE: PFS = progression-free survival; OS = overall survival; TTD = time to death; TTP = time to progression.

 

braf-v600g.png

Figure 1.
Schematic of BRAF V600G mutation. Functional domains of BRAF are depicted. CR1: conserved regions 1. CR2: conserved region 2.

Contributors: Christine M. Lovly, M.D., Ph.D., William Pao, M.D., Ph.D. (through April 2014), Jeff Sosman, M.D.

Suggested Citation: Lovly, C., W. Pao, J. Sosman. 2015. BRAF c.1799T>G (V600G) Mutation in Melanoma. My Cancer Genome https://www.padiracinnovation.org/content/disease/melanoma/braf/115/ (Updated June 16).

Last Updated: June 16, 2015

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