BRAF

The BRAF (B-type Raf kinase) gene encodes B-Raf, a RAF family serine/threonine protein kinase that regulates the MAPK/ERK signaling pathway located at 7q34. The pathway is controlling cell proliferation and differentiation. The most frequent (about 85-90%) pathogenic mutation is BRAF p.V600E (c.1799T>A), which constitutively activates MAPK signaling and promotes uncontrolled cell growth. The associated V600E protein product can be detected by immunohistochemistry (IHC) using mutation-specific primary antibodies giving a cytoplasmic staining reaction in tumor cells with the mutation variant. IHC can hereby serve as a surrogate for molecular testing.

The BRAF V600E mutation is an oncogenic mutation, that is among the most common oncogenic alterations in melanoma and have been identified in various other cancers, including non-Hodgkin lymphoma, colorectal cancer, thyroid papillary carcinoma, non-small cell lung carcinoma and hairy cell leukemia.

• Melanoma:  The BRAF V600E mutation is present in approximately 50% of melanomas. BRAF inhibitors have significantly improved survival in melanoma, which is why testing for BRAF V600E is now standard in advanced or metastatic disease. BRAF V600E IHC status serves can be used as an alternative to molecular testing. However, interpretation may be challenging in heavily pigmented lesions, as melanin can mimic the staining reaction.
• Colorectal carcinoma: The BRAF V600E mutation occurs in about 10% of colorectal carcinomas and is linked to aggressive disease and poor prognosis. In tumors with high microsatellite instability (MSI-H) or deficient mismatch repair (dMMR), BRAF V600E is absent in hereditary cases due to Lynch syndrome but common in sporadic dMMR tumors. Its presence therefore supports sporadic origin and helps distinguish from Lynch syndrome.
• Thyroid Cancer: In papillary thyroid carcinoma, the BRAF V600E mutation is common (approximately 30–80%), with higher rates in Asia than in Western populations. It is associated with a more aggressive phenotype. 
• Hairy Cell Leukemia (HCL): This rare B-cell neoplasm almost universally harbors the BRAF V600E mutation. This high frequency gives the mutation a significant diagnostic role, helping to distinguish HCL from other B-cell lymphomas with similar morphological appearance.
• Erdheim–Chester disease (ECD): This rare form of non-Langerhans cell histiocytosis is characterized by the presence of BRAF V600E mutations in more than 50% of cases. This finding has important diagnostic significance as it occurs far more frequently in ECD than in other types of non-Langerhans histiocytosis.

BRAF V600E serves as a predictive biomarker across different tumor types, guiding therapeutic decisions involving BRAF inhibitors, often in combination with MEK (mitogen-activated protein kinase) inhibitors.

Tumors such as melanoma or colorectal adenocarcinoma with a confirmed BRAF V600E mutation are recommended as positive tissue controls. The majority of neoplastic cells should exhibit at least weak but definite cytoplasmic staining reaction. Appendix or colon tissue is recommended as a negative tissue control, where no or minimal staining reaction should be observed in epithelial cells and smooth muscle cells. Currently, no data are available on consistently low-level expressing normal tissues or cells. Therefore, it is essential to ensure a distinct and as strong as possible staining reaction in BRAF-mutated tumors while maintaining no or minimal staining reaction in negative tissue controls. In some cases, aberrant nuclear staining in epithelial cells or weak granular cytoplasmic staining in smooth muscle cells may occur. These patterns should not be interpreted as positive.