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Characterisation of the melanosphere CD20 sub-population

ABSTRACT: Background: Mutant BRAF targeted therapies remain a standard of care for the treatment of metastatic malignant melanoma (MM); however high initial response rates are tempered by the persistence of residual MM cells that eventually lead to disease recurrence and mortality. Since MM recurrence during targeted therapy can present with the simultaneous occurrence of multiple tumour nodules at the original body sites, we hypothesised the presence of an intrinsically resistant MM cell sub-population. Objectives: Identify an MM cell subpopulation that is intrinsically resistant to targeted therapy and so may be responsible for MM recurrence. Methods: Using melanoma cell lines, we define culture conditions for reproducible 3D growth of melanospheres to investigate putative cancer stem cell populations. We undertook RNA sequencing and bioinformatic analysis to characterise cell populations between adherent and non-adherent culture, and cells expressing or not expressing CD20. Furthermore, we define an in vitro assay to evaluate killing of melanoma cancer stem cells as a therapeutic test using combination therapies targeting driver mutation and CD20 marker. Results: We have described culture conditions that promote MM cells to form melanospheres with a reproducible colony forming efficiency of 0.3% to 1.3%. RNA sequencing of melanosphere versus conventional MM cell cultures (n=6), irrespective of the BRAF mutation status, showed that melanosphere formation was associated with growth and differentiation transcriptional signatures resembling MM tumours. Importantly, melanosphere formation also led to the emergence of a CD20+ MM cell subpopulation, similar to that observed in primary human MM tumours. CD20+ MM cells were resistant to BRAF inhibitor therapy, and consistent with this finding demonstrated a Forkhead box protein M1 (FOXM1) transcriptomic profile (n=6). Combining BRAF inhibitor and anti-CD20 antibody treatment led to the additional killing of previously resistant CD20+ BRAF mutant MM cells. Conclusions: In MM patients that harbour a CD20+ sub-population, combined therapy with BRAF inhibitor and anti-CD20 antibody could potentially kill residual MM cells and so prevent disease recurrence.

ORGANISM(S): Homo sapiens

PROVIDER: GSE248906 | GEO | 2024/02/22

REPOSITORIES: GEO

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Project description:Background: Mutant BRAF targeted therapies remain a standard of care for the treatment of metastatic malignant melanoma (MM); however high initial response rates are tempered by the persistence of residual MM cells that eventually lead to disease recurrence and mortality. Since MM recurrence during targeted therapy can present with the simultaneous occurrence of multiple tumour nodules at the original body sites, we hypothesised the presence of an intrinsically resistant MM cell sub-population. Objectives: Identify an MM cell subpopulation that is intrinsically resistant to targeted therapy and so may be responsible for MM recurrence. Methods: Using melanoma cell lines, we define culture conditions for reproducible 3D growth of melanospheres to investigate putative cancer stem cell populations. We undertook RNA sequencing and bioinformatic analysis to characterise cell populations between adherent and non-adherent culture, and cells expressing or not expressing CD20. Furthermore, we define an in vitro assay to evaluate killing of melanoma cancer stem cells as a therapeutic test using combination therapies targeting driver mutation and CD20 marker. Results: We have described culture conditions that promote MM cells to form melanospheres with a reproducible colony forming efficiency of 0.3% to 1.3%. RNA sequencing of melanosphere versus conventional MM cell cultures (n=6), irrespective of the BRAF mutation status, showed that melanosphere formation was associated with growth and differentiation transcriptional signatures resembling MM tumours. Importantly, melanosphere formation also led to the emergence of a CD20+ MM cell subpopulation, similar to that observed in primary human MM tumours. CD20+ MM cells were resistant to BRAF inhibitor therapy, and consistent with this finding demonstrated a Forkhead box protein M1 (FOXM1) transcriptomic profile (n=6). Combining BRAF inhibitor and anti-CD20 antibody treatment led to the additional killing of previously resistant CD20+ BRAF mutant MM cells. Conclusions: In MM patients that harbour a CD20+ sub-population, combined therapy with BRAF inhibitor and anti-CD20 antibody could potentially kill residual MM cells and so prevent disease recurrence.

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.

Project description:Nearly 50% of cutaneous melanomas carry activating mutations on the BRAF oncogene, and the combination of BRAF- and MEK-inhibitors (BRAF/MEKi) is frequently used for their clinical management. One major drawback of BRAF/MEKi targeted therapy is the rapid development of therapeutic resistance, which can occur via multiple mechanisms, including the metabolic rewiring of cancer cells that often involves the upregulation of mitochondrial bionergetics and NAD+ biosynthetic pathways. mTORC2 is a signaling complex that requires the presence of its essential RICTOR subunit to play its regulatory functions in cell growth and metabolism. mTORC2 is believed to play mostly pro-oncogenic roles in several tumor types, including melanoma. However, bioinformatics analysis of TCGA melanoma patients’ database revealed that low RICTOR levels in tumors correlate with an overall worse clinical outcome. GSEA analysis of low-RICTOR tumors evidenced also a gene expression signature suggestive of activation of mitochondrial energy producing pathways. On these bases, we have hypothesized that inhibition of mTORC2 activity may render BRAFV600E melanoma cells resistant to BRAFi/MEKi. We show here that RICTOR/mTORC2-deficient cells are intrinsically tolerant to BRAFi/MEKi, and anticipate the onset of resistance to BRAFi after sustained drug exposure both in vitro and in vivo, indicating that mTORC2 activity normally opposes the acquisition of targeted therapy resistance in BRAFV600E melanomas. Mechanistically, RICTOR-deficient cells show an enhanced mitochondrial respiratory potential and increased expression of nicotinamide phosphoribosyltransferase (NAMPT) protein, the rate-limiting enzyme of NAD+ salvage pathway, and pharmacological inhibition of these processes in RICTOR-deficient cells is sufficient to restore sensitivity to BRAFi. Thus, our work identifies a novel role for mTORC2 in favoring the responses of BRAF-mutated melanoma cells to targeted therapy, and suggest that the evaluation of the intratumor level of RICTOR may help to predict the responses of melanoma patients to these treatments.

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Characterisation of the melanosphere CD20 sub-population

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