BRAF(V600E) immunopositive Melanomas Show Low Frequency of Heterogeneity and Association With Epithelioid Tumor Cells: A STROBE-Compliant article

Treatment of BRAF(V600E)-mutant melanoma by small molecule inhibitors that target BRAF or MEK kinases is increasingly used in clinical practice and significantly improve patient outcome. However, patients eventually become resistant and therapeutic improvement is required. Molecular diversity within individual tumors (intratumor heterogeneity) and between tumors within a single patient (intrapatient heterogeneity) poses a significant challenge to precision medicine. Using immunohistochemistry, we determined the extent of BRAF(V600E) intratumor and intrapatient heterogeneity and the influence of morphological heterogeneity in a large series of 171 melanomas of 81 patients. The BRAF(V600E) mutation rate found in our melanoma series is 44%, with none of 22 (0%) melanoma in situ, 23 of 56 (41%) primary tumors, 28 of 59 (48%) regional metastases, and 24 of 34 (71%) distant metastases harboring the mutation. In general, a diffuse homogeneous immunostaining was seen, even in tumors consisting of more than one cell type, that is, epithelioid, spindle, and/or small cell types. Nevertheless, BRAF(V600E)-mutant melanomas more often had a purely epithelioid cell population (P = 0.063), that is more evident among distant metastases (P = 0.014). Only two of 75 (3%) mutated specimens (one primary and one metastasis) displayed heterogeneous BRAF(V600E) expression. The primary tumor was also morphologically heterogeneous and exclusively displayed BRAF(V600E) in the epithelioid component, confirming an association between BRAF(V600E) and epithelioid cells. Twenty-eight of 30 patients (93%) had concordant BRAF mutation status between their tumors. Taken together, BRAF(V600E) intratumor and intrapatient heterogeneity in melanoma is diminutive, nevertheless, the identified exceptions will have important implications for the clinical management of this disease.