The BRAF gene is located on chromosome arm 7q34. It encodes B-raf, a serine-threonine kinase that is part of the Ras-Raf-Mek-Erk-MAPK (mitogen-activated protein kinase) signaling cascade (see the illustration below). Activation of this pathway has been implicated in promoting growth, proliferation, and differentiation of cells.
Three functional RAF proteins exist in humans, ARAF, BRAF, and CRAF. Among them, BRAF has the highest basal kinase activity, and is the most potent activator of the MAPK pathway.
The BRAF gene is composed of 18 exons, and the most common activating mutation is found in exon 15 at nucleotide position 1799, involving a transversion of thymine to adenine. This results in a substitution of glutamic acid for valine at position 600, which is designated as V600E. This mutation occurs within the kinase domain of the B-raf protein and is reported to have 10 times more kinase activity than its normal counterpart. It therefore acts as an oncogene, promoting cell growth, differentiation, and survival.
More than 30 different BRAF mutations have been identified. However, V600E accounts for about 90% of all the BRAF mutations, and is the mutation commonly tested for in clinical laboratories.
The BRAF gene mutation is found in many different cancers, with various frequencies reported. The highest incidence is found in malignant melanoma (27–70%), papillary thyroid cancer (36–53%), colorectal cancer (5–22%) and serous ovarian cancer (30%). However, it can also occur at a low frequency (1-3%) in other cancers, including lung cancer, glioma, ependymoma, non-Hodgkin's lymphoma, acute lymphoblastic leukemia, liver cancer, stomach cancer, and esophageal cancer.
BFAR Gene Mutation Test
BRAF gene mutation testing has emerged as an important tool for diagnosis, prognosis, treatment, and predicting patient outcome in response to targeted therapy.
Evidence indicates BRAF gene mutation testing has a role in the following:
Differentiating papillary thyroid cancer from other types of thyroid cancer and benign thyroid nodules
Differentiating sporadic colorectal cancer with microsatellite instability-high (MSI-H) from hereditary nonpolyposis colorectal carcinoma (HNPCC) or Lynch syndrome
Differentiating hairy cell leukemia from other B-cell lymphomas with similar clinical and morphologic features
Determining prognosis, as cancers with BRAF mutation are found to be more aggressive than their counterparts without the mutation, particularly in papillary thyroid carcinoma and colorectal carcinoma
In treatment decisions, because in patients with advanced colorectal cancer,BRAF mutation testing has been suggested since its presence confers resistance to epidermal growth factor inhibitors (cetuximab and panitumumab), when k-ras is not mutated
In therapeutic targeting, because the presence of mutated BRAF has been a valuable therapeutic target, particularly in metastatic melanoma
What does the test result mean?
BRAF and melanoma
Approximately 40-60% of cutaneous melanomas carry mutations in the BRAF gene. About 90% of these mutations are found to be V600E.
Vemurafenib (PLX4032) is a highly selective and potent inhibitor of BRAF V600E. It has marked antitumor effects against melanoma cell lines with the BRAF V600E mutation but not against cells with wild-type (non-mutated) BRAF. Evidence exists that vemurafenib improves overall and progression-free survival in patients with advanced melanoma with the V600E mutation. Thus, establishing whether BRAF mutations exist in melanoma is now of critical therapeutic importance.
BRAF and colorectal cancer
BRAF mutation testing will likely be increasingly used in the management of colorectal cancer, as more evidence emerges regarding its usefulness. Some indication exists that BRAF mutation testing may be used to evaluate the likelihood of having hereditary nonpolyposis colorectal carcinoma (HNPCC) or Lynch syndrome versus sporadic colorectal cancer. Distinguishing HNPCC from sporadic colon cancer with MSI-H is important because patients with HNPCC and their family members can be offered genetic counseling and provided with opportunities to prevent such second cancers through close surveillance and prophylactic surgery.
The presence of microsatellite instability (MSI) is one of the major mechanisms of disease in HNPCC; however, MSI-high (MSI-H) is also found in approximately 12% of sporadic cases of colorectal cancer. Full gene sequencing, albeit expensive, can detect all of the defective mismatch repair genes in HNPCC. Pembrolizumab, a PD-1 inhibitor, gained accelerated approval from the FDA in May 2017 for unresectable or metastatic colon cancer that has tested positive for microsatellite instability-high (MSI-H) or deficient mismatch repair (dMMR), and has progressed following treatment with a fluoropyrimidine, oxaliplatin, and irinotecan. It is also approved for any solid tumor that has tested positive for MSI-H or MMR-deficiency in patients who have had prior treatment and have no satisfactory alternative treatment options.
Preliminary tests, such as BRAF mutation testing, may be useful in determining which patients are likely to benefit from full gene sequencing. Approximately 91% of sporadic colorectal cancers harbor BRAF mutation, whereas BRAF is almost never mutated in colorectal cancers that arise as a consequence of Lynch syndrome. Thus, persons with colorectal cancers with the BRAF mutation may be unlikely to have HNPCC, and further testing with full gene sequencing is probably unwarranted.
Some evidence indicates that the presence of BRAF mutation may render colorectal cancer resistant to epidermal growth factor (EGFR)-inhibitors, such as cetuximab and panitumumab. K-ras, which lies upstream of BRAF, is mutated in 30-50% of colorectal cancersand is predictive of unresponsiveness to EGFR-inhibitor targeted therapy. Although not yet standard of practice, there is some suggestion that BRAF mutation testing may be a useful adjunct to k-ras testing, since the presence of mutated BRAF likewise confers resistance to anti-EGFR treatment
In addition, the presence of BRAF mutation may possibly serve as a useful prognostic marker, as one study reported its significant association with decreased progression-free and overall survival in colorectal cancer.
BRAF and thyroid cancer
The V600E BRAF mutation is the most common genetic alteration involved in the most prevalent type of thyroid cancer, papillary thyroid cancer (PTC), with an overall prevalence of 45% .
Some studies indicate that the presence of the BRAF gene mutation may potentially be a putative prognostic marker of papillary thyroid cancer. A systematic review and meta-analysis reported that BRAF mutation was significantly associated with recurrence, lymph node metastases, extrathyroidal extension, and advanced stage in papillary thyroid cancer.
BRAF gene mutation testing has also been reported as a specific marker for papillary thyroid carcinoma when used in conjunction with fine-needle aspiration biopsy (FNAB). Mutated BRAF has been demonstrated to occur exclusively in papillary thyroid cancer and PTC-derived anaplastic thyroid cancer and is virtually absent in follicular, Hurthle cell, medullary carcinomas, and in benign thyroid tumors.
BRAF and other cancers
The BRAF mutation may be a potential diagnostic tool to distinguish Hairy-cell leukemia (HCL) from other B-cell lymphomas with similar clinical and morphologic features, such as the HCL-variant and splenic marginal zone lymphoma. A study by Tiacci et al demonstrated that BRAF mutation was present in all patients with HCL but not in other peripheral B-cell lymphomas or leukemia, which implicates the potential usefulness of BRAF mutation testing in diagnosis of HCL.
As mentioned, BRAF mutations are found to occur in many other cancer types, such as lung cancer, glioma, Ependymoma, non-Hodgkin lymphoma, acute lymphoblastic leukemia, liver cancer, stomach cancer, and esophageal cancer, although at a low frequency (1-3%). However, the clinical implication and utility of BRAF gene mutation testing in these types of cancers have yet to be determined.
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