Spectrum and Frequency of BRAF Mutations in Skin Melanomas in the Dalmatian Region of Croatia.

Mutation of the BRAF oncogene is one of the most common mutations detected in human neoplasia, occurring in 40-60% of all cutaneous melanoma. BRAF is a serine/threonine protein kinase which is an essential part of the mitogen-activated protein kinase (MAPK) pathway. It is physiologically activated by RAS, but in mutated form, due to molecular conformational change, BRAF becomes constitutively active with subsequent persistent activation of downstream cytoplasmic and nuclear proteins (MEK, ERK, ETS), which finally leads to gene expression that promotes cell growth and survival. Inhibition of the altered MAPK pathway by BRAF inhibitors and combined BRAF/MEK inhibitors in BRAF mutated melanoma has become a standard therapeutic approach (1,2). We recently reported the frequency and clinicopathological features of BRAF V600E mutated melanomas in the Dalmatian region of Croatia. This report included 80 cutaneous melanomas with BRAF analyses performed at our institution until the second half of 2017, using a kit which detected only BRAF V600E mutation (3). From the second half of 2017, we started using a kit which detects several types of BRAF mutations along with NRAS mutation. The aim of this report was to determine the spectrum and frequency of different BRAF mutations in a group of skin melanomas in the Dalmatian region of Croatia and to comment on the relationship between type of BRAF mutation and therapeutic response to MAPK pathway inhibition. The analysis included 179 patients with stage 3 and stage 4 cutaneous melanoma with known BRAF/NRAS mutational status. The paraffin blocks were forwarded from four Dalmatian hospitals (Split: 139 cases, Zadar: 17 cases, Šibenik: 13 cases, Dubrovnik: 10 cases). BRAF/NRAS mutation analysis was performed at the Institute of Pathology, Clinical Hospital Center Split, Croatia, in the period from the second half of 2017 to the end of 2022. For DNA extraction analysis, hematoxylin and eosin stained slides from each submitted sample were reviewed by a pathologist, and tumor tissue was identified for analysis. For all tissue specimens, DNA was extracted from sections (10 mm thick) using the cobas® DNA Sample Preparation Kit (Roche Molecular Diagnostics), following the manufacturer's protocol. The amount of genomic DNA was quantified using the Qubit® 2.0 Fluorometer (Life Technologies) and adjusted to a fixed concentration to be added to the amplification/detection mixture. For mutation analysis, the target DNA was amplified and detected on the cobas z 480 analyzer using the amplification and detection reagents provided in the Roche BRAF/NRAS mutation test (LSR) kit, according to the manufacturer's protocol. The test results were reported as follows: BRAF exon 11 mutation detected, BRAF V600E/E2/D mutation detected, BRAF V600K mutation detected, BRAF V600R mutation detected, BRAF K601E mutation detected, NRAS (G12X, G13X, A18T, Q61X, other NRAS Ex3/4) mutation detected, mutation not detected, or invalid result (no result was obtained on the cobas test). BRAF mutation was observed in 87 patients (48.6%), NRAS mutation was found in 27 patients (15.1%), while 65 patients (36.3%) were without BRAF/NRAS mutation (Table 1). In the group of BRAF mutated melanomas, 61 cases (70.1%) had V600E/E2/D mutation, 20 cases (23%) had V600K mutation, 3 cases (3.4%) had exon 11 mutation, 2 cases (2.3%) had V600R mutation, and 1 case (1.2%) had K601E mutation (Table 2). The observed frequency of BRAF mutated melanomas in this study was similar to the frequency reported in our previous study (48.6% and 47.5%, respectively) (3). The vast majority were BRAF V600 mutations, while BRAF non-V600 mutations were rare (95.4% and 4.6%, respectively). In the group of BRAF V600 mutations, V600E/E2/D mutations predominated, followed by V600K mutations, while V600R mutations were rare. Greaves et al. reported similar frequency of BRAF V600 mutations in a group of 499 BRAF-mutated cutaneous melanomas, with V600E/E2/D mutations observed in 77.2% cases, followed by V600K mutations observed in 17.2% cases, and V600R mutations observed in 2.6% cases (4). BRAF non-V600 mutations (exon 11 and K601E mutations) were rarely observed in this study, confirming the findings of other authors (4,5). A three-class system of BRAF mutations was recently proposed that takes into account the differences in their kinase activity, with class I containing mutants with high kinase activity and high response rate to BRAF and BRAF/MEK inhibitors. Class II BRAF mutations have lower kinase activity than class I mutants, but higher than wild-type BRAF, showing resistance to BRAF inhibitor monotherapy and sensitivity to MEK and BRAF/MEK inhibitors. Finally, class III BRAF mutations are characterized by low kinase activity and low response rate to targeted therapy (6). BRAF V600 mutations belong to class I mutations, which means that the large majority of BRAF-positive melanomas in this study (95.4%) were sensitive to targeted therapy. However, the sensitivity to targeted therapy is different among different class I BRAF mutations. While large randomized controlled trials on combined BRAF/MEK inhibition showed good overall response (63-68%) and improvement of progression-free survival (PFS) and overall survival (OS) for the melanomas with most prevalent V600E and V600K mutations, Menzer et al. showed lower response rate to MAPK pathway inhibition (45%) in the group of metastatic melanomas with BRAF V600 mutations other than V600E/K. The overall response rate to MAPK pathway inhibition in the same group of melanomas with BRAF non-V600 mutations (class II and III mutations) was only 18% (7). In our group of BRAF mutated skin metastatic melanomas, we found only 6 cases (6.9%) with expected lower response rate to MAPK pathway inhibition: 2 cases with V600R mutation (class I non-V600E/K mutation), 1 case with K601E mutation (class II mutation), and 3 cases with exon 11 mutation (class II and III mutations).