Abstract B87: Genomic complexity and BRAF/MEK‐dependence in V600E BRAF mutant melanoma

Constitutive ERK activation, often the result of BRAF mutation, is a common finding in human cancer. BRAF mutations occur in approximately 8% of human tumors, with the highest frequency observed in melanoma (40–70%). Supporting its classification as an oncogene, V600E BRAF stimulates ERK signaling, induces proliferation and is capable in model systems of promoting transformation. BRAF mutations are, however, common in nevi suggesting that BRAF mutation alone is insufficient for tumorigenesis. These observations suggest that additional alterations cooperate with BRAF in promoting melanoma formation and that inhibition of oncogenic BRAF may not be sufficient in many tumors to induce apoptosis in some BRAF mutant melanomas. Prior studies have demonstrated that loss of PTEN is a common event in melanoma and PTEN has been shown to cooperate with BRAF mutation in accelerating invasive melanoma formation in genetic murine models. With the goal of identifying model systems in which to study the the role played by PTEN in promoting tumorigenesis, we performed MALDI‐TOF mass spectrometry genotyping on a panel of 116 melanoma cell lines. Seventy‐two (62%) harbored BRAF mutations (66 V600E, 3 V600K, 3 in exon 11). Proteomic profiling of the class of V600E BRAF mutant cell lines demonstrated significant variability in the expression of PTEN and pAKT. Nine cells lines harboring V600E BRAF mutations were found to express no PTEN protein and high pAKT by immunoblotting. The majority of these cell lines had small (1–2) base‐pair insertions and deletions leading to a frame shift and early truncation. Array CGH confirmed focal homozygous deletion of the PTEN gene in three of the models. These deletions were small and in one case detected by the Agilent 1M but not the 244K platform. We have previously reported that mutations of BRAF are associated with enhanced and selective sensitivity to MEK inhibition when compared to either receptor tyrosine kinase‐driven cells or cells harboring a RAS mutation. To determine the MAPK pathway dependence of V600E BRAF mutant cell lines as a function of PTEN expression, we used PD0325901, a selective allosteric inhibitor of MEK1/2. A subset of the V600E BRAF mutant, PTEN null or mutant cells were dependent upon MEK for proliferation with IC50 values ranging from 1.1 to >500nM. These data suggest that PTEN loss is insufficient to confer absolute resistance to MAPK‐pathway inhibition. MEK inhibition was, however, cytostatic in all V600E BRAF, PTEN null cell lines whereas MEK inhibition induced significant apoptosis in a subset of V600E BRAF, PTEN wild‐type models. PTEN null, BRAF mutant cell lines were sensitive to two selective PI3 kinase inhibitors and the combination of MEK and PI3 kinase inhibition was more effective than either alone in a subset of models. In summary, our data suggest that PTEN is one of several concurrent genetic/epigenetic alterations that condition MEK‐dependence in melanoma cell lines harboring V600E BRAF mutations. Given the promising preliminary data with BRAF and MEK selective inhibitors in early stage clinical trials, these data may aid in the identification of those patients most likely to benefit from selective inhibitors of the MAPK pathway. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B87.