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Conjugated linolenic acids (CLnAs) are emerging as promising agents to trigger ferroptosis, a cell death driven by excessive lipid peroxidation, in cancer cells. Given the aggressive nature and treatment resistance of malignant melanoma, exploring CLnAs as therapeutic agents may offer a novel strategy to overcome these challenges. Here, we investigated the toxicity of four CLnA isomers on human (A375, WM266.4) and zebrafish (ZMEL1) melanoma cell lines. We observed a dose-dependent reduction in cell viability across all three tested cell lines. While human melanoma cells were more sensitive to CLnAs than ZMEL1 cells, treatment with ferroptosis inhibitors mitigated cell death in all models, confirming ferroptosis as the consistent primary mechanism of cell death. In addition, chemical inhibitors of ACSL4 and GPX4 modulated CLnA toxicity, further substantiating the ferroptotic mechanism by highlighting the role of these key regulators. Furthermore, fatty acid analysis revealed that CLnAs were effectively incorporated into phospholipids, generating substrates for lethal lipid peroxidation. At the transcriptional level, CLnA treatment significantly upregulated the pro-ferroptotic gene acsl4a in ZMEL1 cells. Overall, our study identifies specific CLnAs as potent ferroptosis inducers in both human and zebrafish melanoma cells and underscores the translational relevance of the zebrafish model based on a shared ferroptotic mechanism.

Cancer incidence increases exponentially with age when human telomeres are shorter. Similarly, telomerase reverse transcriptase (tert) mutant zebrafish have premature short telomeres and anticipate cancer incidence to younger ages. However, because short telomeres constitute a road block to cell proliferation, telomere shortening is currently viewed as a tumor suppressor mechanism and should protect from cancer. This conundrum is not fully understood. In our current study, we report that telomere shortening promotes cancer in a noncell autonomous manner. Using zebrafish chimeras, we show increased incidence of invasive melanoma when wild-type (WT) tumors are generated in tert mutant zebrafish. Tissues adjacent to melanoma lesions (skin) and distant organs (intestine) in tert mutants exhibited higher levels of senescence and inflammation. In addition, we transferred second generation (G2) tert blastula cells into WT to produce embryo chimeras. Cells with very short telomeres induced increased tumor necrosis factor1-α (TNF1-α) expression and senescence in larval tissues in a noncell autonomous manner, creating an inflammatory environment. Considering that inflammation is protumorigenic, we transplanted melanoma-derived cells into G2 tert zebrafish embryos and observed that tissue environment with short telomeres leads to increased tumor development. To test if inflammation was necessary for this effect, we treated melanoma transplants with nonsteroid anti-inflammatory drugs and show that higher melanoma dissemination can be averted. Thus, apart from the cell autonomous role of short telomeres in contributing to genome instability, we propose that telomere shortening with age causes systemic chronic inflammation leading to increased tumor incidence.

UV-induced cell pigmentation represents an important mechanism against skin cancers. Sun-exposed skin secretes α-MSH, which induces the lineage-specific transcriptional factor MITF and activates melanogenesis in melanocytes. Here, we show that the autophagic tumor suppressor UVRAG plays an integral role in melanogenesis by interaction with the biogenesis of lysosome-related organelles complex 1 (BLOC-1). This interaction is required for BLOC-1 stability and for BLOC-1–mediated cargo sorting and delivery to melanosomes. Absence of UVRAG dispersed BLOC-1 distribution and activity, resulting in impaired melanogenesis in vitro and defective melanocyte development in zebrafish in vivo. Furthermore, our results establish UVRAG as an important effector for melanocytes’ response to α-MSH signaling as a direct target of MITF and reveal the molecular basis underlying the association between oncogenic BRAF and compromised UV protection in melanoma.

Mucosal melanoma (MM) is a deadly cancer derived from mucosal melanocytes. To test the consequences of MM genetics, we develop a zebrafish model in which all melanocytes experience CCND1 expression and loss of PTEN and TP53. Surprisingly, melanoma only develops from melanocytes lining internal organs, analogous to the location of patient MM. We find that zebrafish MMs have a unique chromatin landscape from cutaneous melanomas. Internal melanocytes are labeled using a MM-specific transcriptional enhancer. Normal zebrafish internal melanocytes share a gene expression signature with MMs. Patient and zebrafish MMs show increased migratory neural crest and decreased antigen presentation gene expression, consistent with the increased metastatic behavior and decreased immunotherapy sensitivity of MM. Our work suggests that the cell state of the originating melanocyte influences the behavior of derived melanomas. Our animal model phenotypically and transcriptionally mimics patient tumors, allowing this model to be used for MM therapeutic discovery. As this is a non-MAPK driven genetically engineered model of melanoma, our work also has implications for the 15% of cutaneous melanoma patients who lack MAPK-driving mutations. Mucosal melanoma (MM) is a cancer with poor prognosis derived from mucosal melanocytes. Here, the authors implement a combination of genetic changes that occur in MM patients in a zebrafish model, revealing the potential MM cell of origin and showing that patient and zebrafish MMs share a gene signature that is more metastatic and immune-evasive.

During tumor progression, cancer cells come into contact with various non-tumor cell types, but it is unclear how tumors adapt to these new environments. Here, we integrate spatially resolved transcriptomics, single-cell RNA-seq, and single-nucleus RNA-seq to characterize tumor-microenvironment interactions at the tumor boundary. Using a zebrafish model of melanoma, we identify a distinct “interface” cell state where the tumor contacts neighboring tissues. This interface is composed of specialized tumor and microenvironment cells that upregulate a common set of cilia genes, and cilia proteins are enriched only where the tumor contacts the microenvironment. Cilia gene expression is regulated by ETS-family transcription factors, which normally act to suppress cilia genes outside of the interface. A cilia-enriched interface is conserved in human patient samples, suggesting it is a conserved feature of human melanoma. Our results demonstrate the power of spatially resolved transcriptomics in uncovering mechanisms that allow tumors to adapt to new environments. During tumor progression, cancer cells contact different neighboring cell types, but it is unclear how these interactions affect cancer cell behavior. Here, the authors use spatially resolved transcriptomics and single-cell RNA-seq to study the role of cilia at the tumormicroenvironment interface.

Genome-wide association studies (GWAS) have identified ~20 melanoma susceptibility loci, most of which are not functionally characterized. Here we report an approach integrating massively-parallel reporter assays (MPRA) with cell-type-specific epigenome and expression quantitative trait loci (eQTL) to identify susceptibility genes/variants from multiple GWAS loci. From 832 high-LD variants, we identify 39 candidate functional variants from 14 loci displaying allelic transcriptional activity, a subset of which corroborates four colocalizing melanocyte cis -eQTL genes. Among these, we further characterize the locus encompassing the HIV-1 restriction gene, MX2 (Chr21q22.3), and validate a functional intronic variant, rs398206. rs398206 mediates the binding of the transcription factor, YY1, to increase MX2 levels, consistent with the cis -eQTL of MX2 in primary human melanocytes. Melanocyte-specific expression of human MX2 in a zebrafish model demonstrates accelerated melanoma formation in a BRAF V600E background. Our integrative approach streamlines GWAS follow-up studies and highlights a pleiotropic function of MX2 in melanoma susceptibility. There are more than 20 known melanoma susceptibility genes. Here, using a massively parallel reporter assay, the authors identify risk-associated variants that alter gene transcription, and demonstrate that expression of one such gene, MX2 , leads to the promotion of melanoma in a zebrafish model.

Long non-coding RNAs (lncRNAs) play crucial roles in regulating gene expression. Some are essential for organismal development and physiology, and they can contribute to diseases including cancer. Whilst most lncRNAs exhibit little sequence similarity, conservation of lncRNA transcription relative to neighbouring protein-coding genes suggests potential functional significance. Most positionally equivalent lncRNAs are uncharacterized and it remains unclear whether they exert similar roles in distant species. Here, we identified melanoma-associated lncRNAs predicted to be components of the MITF gene regulatory network in human melanoma that have positionally equivalent transcripts in zebrafish. We prioritized the cancer-associated lncRNA Differentiation Antagonizing Non-Protein Coding RNA (DANCR) as an exemplar for functional investigation. DANCR is a multi-exonic, cytoplasmically-enriched lncRNA and small RNA host gene transcribed from syntenic regions in the human and zebrafish genomes. MITF and c-MYC, key melanoma transcription factors, regulate human DANCR expression and melanoma patients with high DANCR display significantly decreased survival. DANCR is a melanoma oncogene that controls cancer-associated gene expression networks to promote human melanoma cell proliferation and migration. Zebrafish dancr is essential for embryonic development. It is dynamically expressed across multiple different cell types in the developing embryo, transcriptionally activated by mitfa during early zebrafish development and it regulates genes involved in cell death. Our work suggests that cancer-critical lncRNAs such as DANCR, expressed from similar regions in vertebrate genomes, may control related genes and processes involved in both embryonic development and tumorigenesis across species.

Microphthalmia-associated transcription factor (MITF) is a master regulator of melanocyte function, development and plays a significant role in melanoma pathogenesis. MITF genomic amplification promotes melanoma development, and it can facilitate resistance to multiple therapies. Here, we show that MITF regulates a global antioxidant program that increases survival of melanoma cell lines by protecting the cells from reactive oxygen species (ROS)-induced damage. In addition, this redox program is correlated with MITF expression in human melanoma cell lines and patient-derived melanoma samples. Using a zebrafish melanoma model, we show that MITF decreases ROS-mediated DNA damage in vivo. Some of the MITF target genes involved, such as IDH1 and NNT , are regulated through direct MITF binding to canonical enhancer box (E-BOX) sequences proximal to their promoters. Utilizing functional experiments, we demonstrate the role of MITF and its target genes in reducing cytosolic and mitochondrial ROS. Collectively, our data identify MITF as a significant driver of the cellular antioxidant state.

In cancer biology, a tumor begins from a single cell within a group of precancerous cells that share genetic mutations. Kaufman et al. used a zebrafish melanoma model to visualize cancer initiation (see the Perspective by Boumahdi and Blanpain). They used a fluorescent reporter that specifically lit up neural crest progenitors that are only present during embryogenesis or during adult melanoma tumor formation. The appearance of this tumor correlated with a set of gene regulatory elements, called super-enhancers, whose identification and manipulation may prove beneficial in detecting and preventing melanoma initiation. Science , this issue p. 10.1126/science.aad2197 ; see also p. 453 Melanocytes with oncogenic or tumor suppressor mutations revert to expressing the crestin gene early in melanoma formation. [Also see Perspective by Boumahdi and Blanpain ] The “cancerized field” concept posits that cancer-prone cells in a given tissue share an oncogenic mutation, but only discreet clones within the field initiate tumors. Most benign nevi carry oncogenic BRAF V600E mutations but rarely become melanoma. The zebrafish crestin gene is expressed embryonically in neural crest progenitors (NCPs) and specifically reexpressed in melanoma. Live imaging of transgenic zebrafish crestin reporters shows that within a cancerized field ( BRAF V600E - mutant; p53- deficient), a single melanocyte reactivates the NCP state, revealing a fate change at melanoma initiation in this model. NCP transcription factors, including sox10 , regulate crestin expression. Forced sox10 overexpression in melanocytes accelerated melanoma formation, which is consistent with activation of NCP genes and super-enhancers leading to melanoma. Our work highlights NCP state reemergence as a key event in melanoma initiation.

Cancers contain a wide diversity of cell types that are defined by differentiation states, genetic mutations and altered epigenetic programmes that impart functional diversity to individual cells. Elevated tumour cell heterogeneity is linked with progression, therapy resistance and relapse. Yet, imaging of tumour cell heterogeneity and the hallmarks of cancer has been a technical and biological challenge. Here we develop optically clear immune-compromised rag2 E450fs (casper) zebrafish for optimized cell transplantation and direct visualization of fluorescently labelled cancer cells at single-cell resolution. Tumour engraftment permits dynamic imaging of neovascularization, niche partitioning of tumour-propagating cells in embryonal rhabdomyosarcoma, emergence of clonal dominance in T-cell acute lymphoblastic leukaemia and tumour evolution resulting in elevated growth and metastasis in BRAF V600E -driven melanoma. Cell transplantation approaches using optically clear immune-compromised zebrafish provide unique opportunities to uncover biology underlying cancer and to dynamically visualize cancer processes at single-cell resolution in vivo. Direct visualisation of heterogeneous cell populations in live animals has been challenging. Here, the authors optimize cell transplantation into optically clear immune-deficient zebrafish, and use intravital imaging to track and to assess functional diversity of individual cancer cells in vivo .