The use of immunohistochemistry as an accurate tool in the assessment of BRAF V600E and TP53 mutations in primary and metastatic melanoma

Introduction: Metastatic melanoma is a fatal disease with poor prognosis. Since targeted therapy against oncogenic BRAF was approved, molecular profiling has become critical for the management of these patients. However, molecular tests are not available in all pathology laboratories but immunohistochemistry (IHC) emerges as a reliable screening option. The main objective of the present study was to evaluate if BRAF and p53 IHC are reliable surrogates for mutation detection. Material and methods: Formalin-fixed paraffin-embedded samples of melanomas for which molecular data was previously obtained by targeted Next Generation Sequencing (NGS) between 2014 and 2019 were immunostained with BRAF V600E and p53 antibodies. Two pathologists conducted a blinded evaluation of the IHC slides in order to evaluate inter-observer concordance (discordant cases reviewed by a third observer). The associations between IHC and molecular results were evaluated. Results: The study included a series of 55 cases of whom 21 cases (39.6%) harbored BRAF mutation and nine (17%) TP53 mutation. IHC had an overall diagnostic accuracy of 95.92% for BRAF V600E and 74.47% for TP53 compared to NGS. Statistically significant associations between the two diagnostic methods were obtained (p=0.01301 for TP53 and p=0.000001 for BRAF V600E). The kappa coefficient regarding p53 IHC assessment is 0.667 and for BRAF V600E 0.851. Conclusions: Our data evidenced that IHC staining is a reliable surrogate for NGS in identifying the BRAF V600E mutation, becoming an efficient screening tool. The p53 IHC aberrant expression is sometimes associated with TP53 mutations, but a direct link could not be established.