Article Text

Download PDFPDF
Gene of the month: BRAF
  1. Dhirendra Govender1,
  2. Runjan Chetty2
  1. 1Division of Anatomical Pathology, University of Cape Town and National Health Laboratory Service, Groote Schuur Hospital, Cape Town, South Africa
  2. 2Department of Cellular Pathology and Nuffield Department of Clinical Laboratory Sciences, Oxford Biomedical Research Centre, Oxford University Hospitals Trust and University of Oxford University, Oxford, UK
  1. Correspondence to Professor Dhirendra Govender, Division of Anatomical Pathology, Faculty of Health Sciences, University of Cape Town, Anzio Road, Observatory, Cape Town 7925, South Africa; Dhiren.Govender{at}uct.ac.za

Statistics from Altmetric.com

Request Permissions

If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.

Second RAF paralogue (BRAF) is a proto-oncogene that encodes a serine/threonine kinase that transduces regulatory signals through the Rat Sarcoma (RAS)/RAF/ mitogen-activated protein kinase extracellular signal-regulated kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway. This pathway is hyperactivated in approximately 30% of human cancers.1 Activating mutations in the RAS oncogenes occur in 15%–30% of cancers.2 Kirsten murine sarcoma virus oncogene (KRAS) is an important and frequently mutated member of this group of oncogenes. Gain-of-function BRAF mutations results in aberrant activation of ERK signalling that is involved in several cancers such as thyroid papillary carcinoma, melanoma and colon carcinoma mainly but also as demonstrated in animal models in ovarian, skin, lung cancers and glioblastoma multiforme.3 ,4

Structure

There are three related RAF genes in mammals: first RAF paralogue (ARAF), BRAF and human homologue of v-RAF (CRAF) (RAF-1). The BRAF gene, located at 7q34, contains 18 exons and encodes a serine–threonine kinase in the RAS/RAF/MAPK signalling pathway.5 All the main RAF proteins share three highly conserved regions: CR1, CR2 and CR3; however, BRAF has a number of structural differences from the other RAF proteins (ARAF and CRAF). CR1 contains the RAS binding domain and the cysteine-rich domain.6–10 CR2 is rich in serine and threonine and contains regulatory phosphorylation sites. CR3 contains the P-loop and the important kinase domain with the activation segment.11 ,12

Function

The RAF proteins are pivotal in the RAS/RAF/MEK/ERK pathway by relaying signals from activated RAS proteins via the MEK/ERK kinases, which are the key effectors of this pathway13 ,14 (figure 1). The RAF proteins phosphorylate and activate MEK1–MEK2 which phosphorylates and activates ERK1–ERK2.15 ,16 MEK1–MEK2 are the only accepted downstream substrates for RAF proteins.

Figure 1

The RAS/RAF/MEK/ERK pathway: cellular interactions. Activate, line with arrow head; block, line with dot; ERK, extracellular signal-regulated kinase; GF, growth factor; Grb2, growth factor receptor bound-2; MEK, mitogen-activated protein kinase extracellular signal-regulated kinase; RTK, receptor tyrosine kinase; SOS, Son-of-sevenless.

The interaction of RAS with the RAS binding domain and the cysteine-rich domain in the N-terminal domain of the BRAF protein leads to the translocation of BRAF to the plasma membrane and subsequent activation of BRAF.17–19 Activated BRAF phosphorylates and activates MEK and ERK resulting in transcription of proliferation and survival factors.

Activated ERKs have many cellular substrates which influence cell fate (figure 1). Active ERKs as a result of upstream mutations or activations have been shown to activate mdm2, a negative regulator of p53.20–22 The consequence of this is downregulation of p53 and loss of cell cycle checkpoint control leading to limitless replicative potential.23 ERKs are also able to induce cell survival and evade apoptosis. This is mediated by interaction with different downstream substrates, including downregulation of p53, and inactivation of bcl2 associated agonist of cell death (BAD) and BIM.23 ,24

Unlike its related RAF proteins, normal unmutated BRAF has a highly restricted expression pattern.25 ,26 It is expressed in cells of neuronal origin, spleen and testis.

BRAF activation

Activating BRAF mutations lead to unchecked signalling. Most BRAF mutations occur in the CR3 domain, in the P-loop and the activating segment of the kinase domain (figure 2). The most common activating mutation which is a thymine to adenine transversion involves exon 15 at the nucleotide position 1799. This results in the substitution of valine by glutamate.3 This mutation often referred to as BRAFV600E encodes a protein that has 10 times more kinase activity than the wild type protein.3 Although many different BRAF mutations have been detected, BRAFV600E accounts for the majority of activating mutations.27 BRAF kinase domain mutations, the majority of which are V600E, occur in approximately 8% of human cancers.3 ,28

Figure 2

Schematic representation of BRAF structure showing domains and some of the common BRAF mutations.CRD, cystine-rich domain; RBD, RAS binding domain.

Mutant BRAF acts as an oncogene. V600E promotes tumour cell viability, proliferation and growth.

BRAF abnormalities and therapeutic implications

Thyroid papillary carcinoma

In general, the rates of BRAF mutations encountered in thyroid papillary carcinoma vary from 29% to 69%.3 ,29–32 In all, 55%–100% of cases of tall cell variant of papillary thyroid cancer are associated with BRAF mutations but only up to 15% of cases of follicular variant papillary carcinoma have BRAF mutations.33 BRAF mutations are absent in follicular neoplasms and only found in 13% of poorly differentiated thyroid carcinomas and 35% of undifferentiated cancers.3 ,29–32 In the latter two categories of thyroid cancer, the presence of BRAF mutations is thought to be related to the presence of pre-existing papillary thyroid cancer.

Point mutations of exon 15 involving BRAFV600E is the most common alteration in sporadic papillary carcinoma.32 The follicular variant of papillary carcinoma is characterised by the BRAFK601E mutation, while an inframe VK600–1E deletion (BRAFVK600–1E ) is seen in solid variant papillary carcinoma.32–34 These genotypic variations suggest that there is a distinct phenotype–genotype correlation between the type of BRAF mutation and the morphological variation of papillary carcinoma.

The presence of BRAF mutations in papillary thyroid carcinoma is said to correlate with distant metastasis and more advanced clinical stage.29 Furthermore, BRAF mutations occur at a higher frequency in older patients.33 In view of this, it has been suggested that papillary cancers harbouring BRAF mutations pursue a more aggressive course.30–33

Malignant melanoma

Over 60% of melanomas carry mutations in the mitogen-activated protein kinase pathway.35 BRAF mutations occur in 36%–59% of primary melanomas and 42%–66% of metastatic melanomas.36–38 The V600E mutation is the most common mutation found in BRAF mutant melanomas.38 Recently, a drug, PLX4032, was found to be a powerful inhibitor of V600E-mutated BRAF. In a phase I and II clinical trial conducted by Flaherty and colleagues, patients had partial (the majority) or complete response to PLX4032.35 Thus, establishing whether BRAF mutations exist in melanoma is now of critical therapeutic importance.

Colorectal cancer

It has been shown that cetuximab, a monoclonal antibody directed against epidermal growth factor receptor (EGFR), improves progression-free survival in colorectal cancer.39 KRAS lies upstream of BRAF and is mutated in 30%–50% of colorectal cancers. Mutations in KRAS make the colorectal carcinoma unresponsive to anti-EGFR therapies. BRAF mutations occur in approximately 8%–10% of primary and metastatic colorectal cancers and also lead to stimulation of the MAPK pathway.27 ,40 BRAF and KRAS mutations are mutually exclusive and patients with BRAF-mutated tumours had a significantly shorter median progression-free and median overall survival than patients with wild type BRAF tumours.40

References

Footnotes

  • Competing interests None.

  • Provenance and peer review Commissioned; externally peer reviewed.