BRAFV600E Expression Is Homogenous and Associated with Nonrecurrent Disease and Better Survival in Primary Melanoma

Abstract

Background: Superficial spreading melanomas (SSMs) are the most common type of melanoma and cause the majority of skin cancer deaths. More than 50% of cases harbor a mutation in the BRAF gene that activates the mitogen-activated protein kinase (MAPK) cancer signaling pathway. BRAFV600E is the most common BRAF mutation, and it represents an important biomarker that guides treatment selection. However, the relationship between the BRAFV600E gene expression and intratumoral protein distribution, on one side, and clinicopathological factors and patient outcomes, on the other, is not fully described. Additionally, whether MAPK cancer signaling activation in melanoma is due to increased biochemical activity of BRAFV600E, increased mRNA levels, or both requires further investigation. Here, we addressed these questions by examining expression patterns of BRAFV600E in primary treatment-naive melanomas and correlating them to clinicopathological factors and patient outcomes.

Methods: In 166 SSM cases, we performed immunohistochemical staining to investigate the protein expression of BRAFV600E, and we measured BRAF mRNA levels using NanoString nCounter system.

Results: Ninety-seven (49%) melanomas stained positive for BRAFV600E, with nearly 100% intratumoral homogeneity observed. Positive BRAFV600E expression was significantly associated with nonrecurrent disease and was found to be an independent predictor of better prognosis in univariate and multivariable analyses. Furthermore, presence of tumor-infiltrating lymphocytes, sentinel lymph node biopsy negativity, and low Breslow thickness were all independent predictors of better prognosis. We observed no difference in the BRAF mRNA levels in BRAFV600E-negative and BRAFV600E-positive melanomas, respectively. Validation in a larger publicly available cohort confirmed that there is only a weak correlation (Spearman 0.4) between BRAFV600E mRNA and protein levels and no differences in mRNA between BRAFV600E mutated and non-mutated patients.

Conclusion: Our findings indicated that BRAFV600E is homogeneously present throughout the whole tumor and is associated with nonrecurrent disease and better survival in primary melanoma. We also showed that BRAFV600E mutation does not result in higher transcriptional levels, suggesting that activation of the MAPK signaling pathway in BRAFV600E mutated patients can be attributed to the increased biochemical activity caused by the mutation.

Keywords: BRAFV600E; Immunohistochemical staining; Intratumor heterogeneity; Melanoma; Melanoma outcome; Skin cancer.