Faculty Bibliography

Mitogen-activated protein kinase (MAPK) pathway inhibitors show promise in treating melanoma, but are unsuccessful in achieving long-term remission. Concordant with clinical data, BRAF^V600E melanoma cells eliminate glycolysis upon inhibition of BRAF^V600E or MEK with the targeted therapies Vemurafenib or Trametinib, respectively. Consequently, exposure to these therapies reprograms cellular metabolism to increase mitochondrial respiration and restrain cell death commitment. As the inner mitochondrial membrane (IMM) is sub-organellar site of oxidative phosphorylation (OXPHOS), and the outer mitochondrial membrane (OMM) is the major site of anti-apoptotic BCL-2 protein function, we hypothesized that suppressing these critical mitochondrial membrane functions would be a rational approach to maximize the pro-apoptotic effect of MAPK inhibition. Here, we demonstrate that disruption of OXPHOS with the mitochondria-specific protonophore BAM15 promotes the mitochondrial pathway of apoptosis only when oncogenic MAPK signaling is inhibited. Based on RNA-sequencing analyses of nevi and primary melanoma samples, increased pro-apoptotic BCL-2 family expression positively correlates with high-risk disease suggesting a highly active anti-apoptotic BCL-2 protein repertoire likely contributes to worse outcome. Indeed, combined inhibition of the anti-apoptotic BCL-2 repertoire with BH3-mimetics, OXPHOS, and oncogenic MAPK signaling induces fulminant apoptosis and eliminates clonogenic survival. Altogether, these data suggest that dual suppression of IMM and OMM functions may unleash the normally inadequate pro-apoptotic effects of oncogenic MAPK inhibition to eradicate cancer cells, thus preventing the development of resistant disease, and ultimately, supporting long-term remission.

BACKGROUND:Melanoma is a heterogeneous malignancy. We set out to identify the molecular underpinnings of high-risk melanomas, those that are likely to progress rapidly, metastasize, and result in poor outcomes. METHODS:We examined transcriptome changes from benign states to early-, intermediate-, and late-stage tumors using a set of 78 treatment-naive melanocytic tumors consisting of primary melanomas of the skin and benign melanocytic lesions. We utilized a next-generation sequencing platform that enabled a comprehensive analysis of protein-coding and -noncoding RNA transcripts. RESULTS:Gene expression changes unequivocally discriminated between benign and malignant states, and a dual epigenetic and immune signature emerged defining this transition. To our knowledge, we discovered previously unrecognized melanoma subtypes. A high-risk primary melanoma subset was distinguished by a 122-epigenetic gene signature ("epigenetic" cluster) and TP53 family gene deregulation (TP53, TP63, and TP73). This subtype associated with poor overall survival and showed enrichment of cell cycle genes. Noncoding repetitive element transcripts (LINEs, SINEs, and ERVs) that can result in immunostimulatory signals recapitulating a state of "viral mimicry" were significantly repressed. The high-risk subtype and its poor predictive characteristics were validated in several independent cohorts. Additionally, primary melanomas distinguished by specific immune signatures ("immune" clusters) were identified. CONCLUSION/CONCLUSIONS:The TP53 family of genes and genes regulating the epigenetic machinery demonstrate strong prognostic and biological relevance during progression of early disease. Gene expression profiling of protein-coding and -noncoding RNA transcripts may be a better predictor for disease course in melanoma. This study outlines the transcriptional interplay of the cancer cell's epigenome with the immune milieu with potential for future therapeutic targeting. FUNDING/BACKGROUND:National Institutes of Health (CA154683, CA158557, CA177940, CA087497-13), Tisch Cancer Institute, Melanoma Research Foundation, the Dow Family Charitable Foundation, and the Icahn School of Medicine at Mount Sinai.

A well-defined risk factor and precursor for cutaneous melanoma is the dysplastic nevus. These benign tumors represent clonal hyperproliferation of melanocytes that are in a senescent-like state, but with occasional malignant transformation events. To portray the mutational repertoire of dysplastic nevi in patients with the dysplastic nevus syndrome and to determine the discriminatory profiles of melanocytic nevi (including dysplastic nevi) from melanoma, we sequenced exomes of melanocytic nevi including dysplastic nevi (n = 19), followed by a targeted gene panel (785 genes) characterization of melanocytic nevi (n = 46) and primary melanomas (n = 42). Exome sequencing revealed that dysplastic nevi harbored a substantially lower mutational load than melanomas (21 protein-changing mutations versus >100). Known "driver" mutations in genes for melanoma, including CDKN2A, TP53, NF1, RAC1, and PTEN, were not found among any melanocytic nevi sequenced. Additionally, melanocytic nevi including dysplastic nevi showed a significantly lower frequency and a different UV-associated mutational signature. These results show that although melanocytic nevi and dysplastic nevi harbor stable genomes with relatively few alterations, progression into melanomas requires additional mutational processes affecting key tumor suppressors. This study identifies molecular parameters that could be useful for diagnostic platforms.

Heat-shock factor 1 (HSF1) orchestrates the heat-shock response in eukaryotes. Although this pathway has evolved to help cells adapt in the presence of challenging conditions, it is co-opted in cancer to support malignancy. However, the mechanisms that regulate HSF1 and thus cellular stress response are poorly understood. Here we show that the ubiquitin ligase FBXW7alpha interacts with HSF1 through a conserved motif phosphorylated by GSK3beta and ERK1. FBXW7alpha ubiquitylates HSF1 and loss of FBXW7alpha results in impaired degradation of nuclear HSF1 and defective heat-shock response attenuation. FBXW7alpha is either mutated or transcriptionally downregulated in melanoma and HSF1 nuclear stabilization correlates with increased metastatic potential and disease progression. FBXW7alpha deficiency and subsequent HSF1 accumulation activates an invasion-supportive transcriptional program and enhances the metastatic potential of human melanoma cells. These findings identify a post-translational mechanism of regulation of the HSF1 transcriptional program both in the presence of exogenous stress and in cancer.

BACKGROUND: Melanoma is a heterogeneous tumor with subgroups requiring distinct therapeutic strategies. Genetic dissection of melanoma subgroups and identification of therapeutic agents are of great interest in the field. These efforts will ultimately lead to treatment strategies, likely combinatorial, based on genetic information. METHODS: To identify "driver" genes that can be targeted therapeutically, we screened metastatic melanomas for somatic mutations by exome sequencing followed by selecting those with available targeted therapies directed to the gene product or its functional partner. The FBXW7 gene and its substrate NOTCH1 were identified and further examined. Mutation profiling of FBXW7, biological relevance of these mutations and its inactivation, and pharmacological inhibition of NOTCH1 were examined using in vitro and in vivo assays. RESULTS: We found FBXW7 to be mutated in eight (8.1%) melanoma patients in our cohort (n = 103). Protein expression analysis in human tissue samples (n = 96) and melanoma cell lines (n = 20) showed FBXW7 inactivation as a common event in melanoma (40.0% of cell lines). As a result of FBXW7 loss, we observed an accumulation of its substrates, such as NOTCH1. Ectopic expression of mutant forms of FBXW7 (by 2.4-fold), as well as silencing of FBXW7 in immortalized melanocytes, accelerated tumor formation in vivo (by 3.9-fold). Its inactivation led to NOTCH1 activation, upregulation of NOTCH1 target genes (by 2.6-fold), and promotion of tumor angiogenesis and resulted in tumor shrinkage upon NOTCH1 inhibition (by fivefold). CONCLUSIONS: Our data provides evidence on FBXW7 as a critical tumor suppressor mutated and inactivated in melanoma that results in sustained NOTCH1 activation and renders NOTCH signaling inhibition as a promising therapeutic strategy in this setting.

GAB2 is a scaffold protein with diverse upstream and downstream effectors. MAPK and PI3K signaling pathways are known effectors of GAB2. It is amplified and overexpressed in a variety of human tumors including melanoma. Here we show a previously undescribed role for GAB2 in NRAS-driven melanoma. Specifically, we found that GAB2 is co-expressed with mutant NRAS in melanoma cell lines and tumor samples and its expression correlated with metastatic potential. Co-expression of GAB2(WT) and NRAS(G12D) in melanocytes and in melanoma cells increased anchorage-independent growth by providing GAB2-expressing cells a survival advantage through upregulation of BCL-2 family of anti-apoptotic factors. Of note, collaboration of GAB2 with mutant NRAS enhanced tumorigenesis in vivo and led to an increased vessel density with strong CD34 and VEGFR2 activity. We found that GAB2 facilitiated an angiogenic switch by upregulating HIF-1alpha and VEGF levels. This angiogenic response was significantly suppressed with the MEK inhibitor PD325901. These data suggest that GAB2-mediated signaling cascades collaborate with NRAS-driven downstream activation for conferring an aggressive phenotype in melanoma. Second, we show that GAB2/NRAS signaling axis is non-linear and non-redundant in melanocytes and melanoma, and thus are acting independent of each other. Finally, we establish a link between GAB2 and angiogenesis in melanoma for the first time. In conclusion, our findings provide evidence that GAB2 is a novel regulator of tumor angiogenesis in NRAS-driven melanoma through regulation of HIF-1alpha and VEGF expressions mediated by RAS-RAF-MEK-ERK signaling.

Adaptor or scaffolding proteins mediate protein-protein interactions that drive the formation of protein complexes. Grb2-associated binding protein 2 (GAB2) scaffolding protein is an intermediary molecule that links plasma membrane receptor signaling including receptor tyrosine kinases with the downstream effectors, such as protein tyrosine phosphatase, nonreceptor type 11 (SHP2), p85 subunit of phosphoinositide-3 kinase (PI3-K), phospholipase C-gamma 1 (PLC-γ), v-crk sarcoma virus CT10 (CRK), Src homology 2 domain containing transforming protein 1 (SHC), and SH2 containing inositol phosphatase (SHIP). Although, well described in signal transduction, its role in cancer has recently been emerging especially in leukemia, breast and ovarian cancer, and melanoma. GAB2 is essential for two major signal transduction pathways in cancer, the PI3-K-AKT and extracellular signal-regulated kinase (ERK) signaling pathways, and thus regulates a number of key cellular processes. This review focuses on structure and function of GAB2, its regulatory proteins, emerging role in cancer, and potential as a therapeutic target.