Immunohistochemical Detection of Mutated BRAF V600E Supports the Clonal Origin of BRAF-Induced Thyroid Cancers Along the Spectrum of Disease Progression

Background: The mutated BRAF V600E protein has been specifically detected in papillary thyroid carcinomas (PTCs) using immunohistochemical (IHC) analysis. The clonal origin of PTCs harboring BRAF mutations has recently been called into question. Objectives: The purpose of this study was 2-fold: (1) to compare BRAF V600E IHC expression in PTCs, poorly differentiated thyroid carcinomas (PDTCs), and anaplastic thyroid carcinomas (ATCs) with DNA mutation analysis; and (2) to study the distribution of BRAF V600E IHC staining within thyroid cancer tissues. Methods: Whole sections and tissue microarrays from 31 PTCs, 38 PDTCs, and 22 ATCs were subjected to both mass spectrometry genotyping for the BRAF(T1799A) mutation as well as IHC staining for BRAF V600E protein. Results: Of the 31 PTCs, 16 (52%) showed strong (3+) IHC staining and harbored BRAF(T1799A), whereas the remaining 15 (48%) showed absent/faint (0/1+) staining, and were wild type for BRAF (BRAF-wt). Only 5 of 38 (13%) PDTCs harbored mutant BRAF, and these were the only ones with moderate (2+) or 3+ IHC staining. All 14 ATCs with a staining intensity of 3+ harbored BRAF(T1799A), whereas the 2 ATCs with 0/1+ staining were BRAF-wt. Six ATCs showed staining of 2+, 5 of which had high background staining. Of those 6 cases, BRAF(T1799A) was present only in the tumor without background. Homogeneous staining was found in 13 of 14 (93%) PTCs, 3 of 3 (100%) PDTCs, and 12 of 14 ATCs (86%). Conclusions: First, absent/faint staining for BRAF V600E correlates perfectly with the lack of the BRAF(T1799A) mutation, whereas strong staining is highly specific for the BRAF(T1799A) mutation in PTCs, PDTCs, and ATCs. Moderate staining intensity cannot be relied on and should lead to genotypic analysis. Second, homogeneous staining occurs in the vast majority of cases, demonstrating that the BRAF(T1799A) mutation is a clonal event in thyroid cancer.