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Identification of distinct molecular signatures in BRAF V600E and BRAF wt cutaneous melanomas

ABSTRACT: The purpose of this study was to investigate the presence of a gene expression signature in BRAF V600E melanomas compared to wild type ones, all derived from sun-exposed sites. Microdissected tissues from excisional biopsies of 18 cutaneous melanomas were analyzed to detect the presence of BRAF and NRAS mutations and to profile whole genome expression by means of oligonucleotide microarrays. Class comparison methods were used to select differentially expressed genes between wild type and mutated lesions. Real Time RT-PCR and immunohistochemistry were applied to validate differences at the mRNA and protein levels on an independent cohort of samples. BRAF mutations were evidenced in 67% of melanomas. All of them consisted of the oncogenic change V600E (and the mutation event was independent of Clark's level). Data indicate that in V600E melanomas there is an over-expression of cancer stem cell markers and an upregulation of important oncogenes, like KRAS. This, together with the downregulation of genes involved in oxidative UV stress response and immuno system defense, confers an advantage to V600E melanomas compared to wt lesions. Moreover, the downregulation of topoisomerase I and CDKN2A results in an increased replicative potential associated with a decrease in senescence markers and a diminished DNA damage response. As far as the wild-type lesions, we interestingly pointed out the overexpression of PML, PIK3CA and the downregulation of two tumour suppressor genes (BRCA1 and TP73) relevant to DNA repair.

ORGANISM(S): Homo sapiens

PROVIDER: GSE22838 | GEO | 2024/12/31

REPOSITORIES: GEO

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Project description:Background Main drawback of BRAF/MEK inhibitors (BRAF/MEKi)-based targeted therapy in the management of BRAF- mutated cutaneous metastatic melanoma (MM) is the development of therapeutic resistance. We aimed to define in this context the specific role of mTORC2, a signaling complex defined by the presence of the essential RICTOR subunit and regarded as an oncogenic driver in several tumor types, including MM. Methods After analyzing the TCGA MM patients’ database to explore both overall survival and molecular signatures as a function of intra-tumor RICTOR levels, we investigated the effects of RICTOR downregulation in BRAF V600E MM cell lines on their response to BRAF/MEKi in vitro. We performed a proteomic screening to identify proteins modulated by changes in RICTOR expression and Seahorse analysis to evaluate the effects of RICTOR depletion on mitochondrial respiration. The combination of BRAFi with drugs targeting proteins and processes emerged in the proteomic screening was carried out on RICTOR-deficient cells in vitro and in a xenograft setting in vivo. Results We found that low RICTOR levels in MM correlate with an overall worse clinical outcome. GSEA of low- RICTOR tumors revealed a gene expression signature suggestive of activation of the mitochondrial Electron Transport Chain (ETC) energy producing pathway. RICTOR-deficient BRAF V600E cells are intrinsically tolerant to BRAFi/MEKi and anticipate the onset of resistance to BRAFi upon prolonged drug exposure. In RICTOR- depleted cells, both mitochondrial respiration and expression of nicotinamide phosphoribosyltransferase (NAMPT) are enhanced, while their pharmacological inhibition restores sensitivity to BRAFi. Conclusions Our work unveils a novel tumor suppressing role for mTORC2 in the responses of BRAF V600E melanoma cells to targeted therapy and identifies the NAMPT-ETC axis as a potential therapeutic vulnerability of low- RICTOR tumors. Importantly, our findings support the concept that the evaluation of intra-tumor RICTOR levels in MM has a prognostic value, and may help predicting the response of patients to targeted therapy.

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Identification of distinct molecular signatures in BRAF V600E and BRAF wt cutaneous melanomas

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