Frequent Loss of PTEN and NM23H1 in Melanomas Harboring a BRAF V600E Mutation-Implications for Targeted Therapy
Anthony N Snow, Mohammed M Milhem, Aaron D Bossler, Deqin Ma. University of Iowa Hospitals and Clinics, Iowa City, IA
Background: BRAF is a member of the MAP kinase signaling pathway. BRAF V600E mutation is present in >50% of melanomas and associated with a more aggressive clinical course. Although BRAF V600E inhibitor, vemurafenib, has demonstrated improved progression-free and overall survival in patients with advanced disease, patients develop drug resistance and metastatic disease is rarely curable. Identification of other treatment targets is necessary. PTEN is a tumor suppressor in the phosphatidylinositol 3 Kinase (PI3K) pathway. Patients with PTEN loss in their tumors may benefit from combined therapy against PI3K pathway and vemurafenib or after vemurafenib failure. In this study, we evaluated the expression of PTEN and NM23H1, a metastatic suppressor, in BRAF wild type and V600E positive melanomas.
Design: Melanoma cases with (n=10) or without (n=12) BRAF V600E mutation were selected. BRAF mutation analysis was performed by Sanger sequencing. Immunohistochemical (IHC) stains for PTEN (Dako; clone 6H2.1, 1:100) and NM23H1 (Santa Cruz Biotechnology, clone: Nm301, 1:50) were performed on the same blocks used for molecular testing. The expression of PTEN and NM23H1 were scored using a 4-tier system. P: positive (≤90% of cells intensely positive); H: heterogeneous (regional positivity with >10% of cells negative); R: reduced (>10% of cells negative and decreased intensity of staining); and L: loss (≤1% of cells positive). Stromal cells served as positive controls.
Results: Decreased or absent expression of PTEN was observed in 41% of all cases examined. A higher frequency of reduced/loss of PTEN expression was seen in the BRAF V600E group (60%, n=10) versus BRAF wild-type tumors (25%, n=12). PTEN expression was also absent or reduced in 60% of cases with metastatic disease (n=10). 40% of BRAF V600E tumors showed absent or reduced NM23H staining. Four (all V600E positive) of 9 cases with PTEN loss/reduced expression also showed loss or reduced expression of NM23H1.