Pigment Cell & Melanoma Research

Acral melanoma, which is not ultraviolet (UV)-associated, is the most common type of melanoma in several low- and middle-income countries including Mexico. Latin American samples are significantly underrepresented in global cancer genomics studies, which directly affects patients in these regions as it is known that cancer risk and incidence may be influenced by ancestry and environmental exposures. To address this, we characterise the genome and transcriptome of 123 acral melanoma tumours from 92 Mexican patients, a population notable because of its genetic admixture. Compared with other studies of melanoma, we found fewer frequent mutations in classical driver genes such as BRAF, NRAS or NF1. While most patients had predominantly Amerindian genetic ancestry, those with higher European ancestry had increased frequency of BRAF mutations and a lower median number of structural variants. The tumours with activating BRAF mutations have a transcriptional profile more similar to cutaneous non-volar melanocytes, suggesting that acral melanomas in these patients may arise from a distinct cell of origin compared to other tumours arising in these locations. KIT mutations were found in a subset of these tumours, and quadruple wild-type samples (non BRAF/NRAS/NF1/KIT) differed from mutated samples in their structural genomic profile and overall and recurrence-free survival patterns. Transcriptional profiling defined three expression clusters; these characteristics were associated with recurrence-free and overall survival. We highlight potential novel low-frequency drivers, such as PTPRJ, NF2 and RDH5. Our study enhances knowledge of this understudied disease and underscores the importance of including samples from diverse ancestries in cancer genomics studies.

A greater understanding of the biology of nevi will provide insights into the etiology of melanoma. Our large-scale meta-analysis of 14 nevus genome-wide association study (GWAS) included 85,967 individuals of European ancestry. We identified 29 nevus-associated loci (p < 5x10-8), of which 24 were not previously reported in a GWAS conducted for nevus count alone. We further identified 255 candidate genes for nevus loci, including SIKE1 which is involved in immune response regulation. This is of interest because immune response regulation influences the formation of nevi and melanoma susceptibility. Gene-set enrichment analyses prioritised immune response-related pathways and cancers that do not have a pigmentation component (e.g. breast, prostate, and glioma). This suggests that the biology underlying nevus count captures risk pathways beyond pigmentation that are relevant to melanoma. A nevus polygenic risk score explains 5% of the variance in nevus count, indicating its potential to enhance melanoma risk prediction.

The molecular diagnosis of albinism is hampered by a significant number of genetic variants of unknown significance (VUS) including a majority of missense and in-frame insertion deletion variants. This contributes to the high rate of unresolved genetic diagnosis for this disease. We designed a straightforward test of missense VUS in albinism genes based on functional rescue. As a proof of concept, the assay was set up for testing variants in the TYR gene associated with oculocutaneous albinism type 1. The TYR gene was knocked-out in the human melanogenic MNT1 cell line and the resulting unpigmented clones used as host cells for rescue experiments. Selected VUS and control sequences were run through the assay. Expression of tyrosinase was quantified by Western-blot, melanin synthesis was evaluated by direct observation as well as absorbance monitoring. One VUS, p.Ser270Phe (S270F) can be classified as pathogenic as it fails to restore pigmentation, whereas rescue was achieved with D305E and A391T. The two most frequent missense VUS of TYR, S192Y and R402Q, were also tested independently or in combination confirming the pathogenic effect of their association in cis. All in all, this new assay is straightforward enough to be transposed in diagnosis laboratories and can be considered for testing variants in other albinism genes such as TYRP1 and SLC45A2.

Loose Anagen Hair Syndrome is a form of non-scarring alopecia marked by easily and painlessly pluckable terminal hair during its active growth - anagen - phase. This condition is believed to result from poor hair shaft anchoring within the follicle due to premature keratinization. Our research identified the likely pathogenic c.296C>T (p.T99I) variant in KRT32, which was found to co-segregate with the disorder in a large family with autosomal dominant loose anagen hair syndrome. This study aimed to explore the role of KRT32, previously unassociated with loose anagen hair, in hair anchorage and assess the functional impact of the p.T99I variant. We hypothesized that the p.T99I variant reduces KRT32s binding affinity to KRT82, disrupting the intermediate filament structure in the hair shaft cuticle and leading to weak anagen hair anchorage. To test this hypothesis, we conducted a protein-protein interaction assay using far western blotting and performed in silico intermediate filament network segmentation analysis on high-resolution fluorescent microscopy images. Our results revealed a decreased binding affinity of KRT32T99I for KRT82 compared to KRT32WT, along with significant differences in segment count and filament brightness (thickness) between the two groups.

Metastatic skin cutaneous melanomas that contain wild-type BRAF alleles typically possess an activating mutation in a RAS allele or a loss-of-function mutation in an NF1 allele ("BRAF-WT&RAS/NF1-mutant melanomas"). Nonetheless, these tumors remain a significant clinical challenge; they are resistant to MEK and BRAF inhibitors, their response to immune checkpoint inhibitors is less robust than the response of BRAF mutant melanomas to these agents, and additional validated targets for therapeutic intervention have yet to be identified. Previous work from our laboratory has demonstrated that ERBB4 is required for the proliferation of the IPC-298, MEL-JUSO, MeWo, and SK-MEL-2 BRAF-WT&RAS/NF1-mutant melanoma cell lines. Surprisingly, the synthetic constitutively dimerized and active Q646C ERBB4 mutant allele appears to strongly inhibits the proliferation of BRAF-WT&RAS/NF1- mutant melanoma cell lines. Given that we have also previously demonstrated that ERBB4-ERBB2 and ERBB4-EGFR heterodimers are more potent drivers of proliferation than are ERBB4 homodimers, here we begin to test the hypothesis that ERBB4 heterodimers drive the proliferation of BRAF-WT&RAS/NF1-mutant melanoma cell lines. Here we demonstrate that the kinase-deficient (dominant-negative) ERBB2 K753A mutant allele inhibits the clonogenic proliferation of the IPC-298, MEL-JUSO, and MeWo ERBB4-dependent, BRAF-WT&RAS/NF1-mutant melanoma cell lines. Moreover, the kinase-deficient (dominant-negative) EGFR K721A mutant allele inhibits the clonogenic proliferation of the MeWo cell line, but not the IPC-298 or MEL-JUSO cell lines. Finally, the clonogenic proliferation of the SK-MEL-2 ERBB4-dependent, BRAF-WT&RAS/NF1-mutant melanoma cell line is unaffected by the ERBB2 K753A or EGFR K721A dominant-negative mutant alleles. We discuss these findings in the context of our hypothesis that ERBB4 heterodimers drive the proliferation of BRAF-WT&RAS/NF1-mutant melanoma cell lines.

Acral melanoma (AM) is a distinct and understudied subtype of melanoma that is reported to represent the majority of melanoma diagnoses in various Latin American, African and Asian countries. Patients with ulcerated AM diagnoses face a worse prognosis and increased risk of recurrence. While tumour ulceration has been shown to influence therapy response in cutaneous melanoma patients, the clinical and molecular traits of AM tumours remain poorly understood. In this study, we performed transcriptomic profiling of 59 AM samples and proteomic analysis of 45 AM samples from patients with extensively annotated clinical information. Our analysis revealed immunological differences in ulcerated tumours, including a significant upregulation of processes related to humoral immunity and markers for macrophages/monocytes, alongside a downregulation of keratins, epidermis-associated processes and cell adhesion. Notably, ulcerated tumours exhibited disruption of tight junctions and desmosomes, potentially explaining their compromised tissue integrity. We identified a significant increase of plasma cells, M0 macrophages and eosinophils within the ulcerated tumour microenvironment, suggesting that inflammation and infection might accompany these lesions. Moreover, fibronectin, CD8, PD-1, CD14 and CD68 protein levels were useful in distinguishing between ulcerated and non-ulcerated samples using a random forest classifier. These findings indicate that persistent inflammation and dysregulated immune responses characterise ulcerated AM, potentially offering new avenues for targeted therapeutic interventions in this aggressive melanoma subtype.

I.Cutaneous skin melanomas with wild-type BRAF alleles ("BRAF-WT melanomas") remain relatively difficult to treat, even though they typically possess driver mutations in a RAS gene or NF1. For example, these tumors respond relatively poorly to combinations of MEK and BRAF inhibitors, and their response to ICIs is muted compared to the response of BRAF-mutant melanomas. ERBB2 and ERBB4, which encode receptor tyrosine kinase genes, are necessary and sufficient for the proliferation of multiple BRAF-WT melanoma cell lines. Consequently, we have postulated that ERBB4-ERBB2 heterodimerization drives BRAF-WT melanomas. This mechanism is consistent with the observation that elevated ERBB4 transcription or ERBB4 mutations are found in a significant fraction of BRAF-WT melanoma tumor samples. Moreover, a subset of ERBB4 mutations found in BRAF-WT melanoma samples increases proliferation in a BRAF-WT melanoma cell line. Because the elevated ERBB4 transcription observed in BRAF- WT melanomas is typically insufficient to cause ligand-independent ERBB4 signaling, we have postulated that ligands for ERBB family receptors drive the elevated ERBB4-ERBB2 heterodimerization responsible for the proliferation of BRAF-WT melanoma cell lines. We have explored this hypothesis by analyzing data found in the Broad Institutes Cancer Cell Line Encyclopedia. These data suggest that some EGF family hormones are required for the proliferation of BRAF-WT melanoma cell lines. Likewise, the G11/Gq pathway, which can stimulate cleavage and maturation of EGF family hormones, is also required for the proliferation of BRAF-WT melanoma cell lines. Thus, these data suggest additional therapeutic targets in BRAF-WT melanomas. Moreover, because many uveal (ocular) melanomas possess elevated G11/Gq signaling, these data suggest that ligand stimulation of ERBB receptor signaling may contribute to uveal melanomagenesis or progression.

Peripheral melanin and neuromelanin share a common biosynthetic initiation. Peripheral melanin (eumelanin and pheomelanin) is cyclically produced and degraded, while neuromelanin accumulates in dopaminergic neurons over time. Neurons containing excess neuromelanin (e.g., substantia nigra) exhibit increased degeneration in Parkinsons patients, suggesting a potential genetic interplay between pigmentation pathways and Parkinsons Disease (PD). We used linkage disequilibrium score regression (LDSC), polygenic risk score (PRS) analysis, Mendelian Randomization (MR), and multi-trait association analysis to examine shared genetic architecture between PD and nine pigmentation-related traits (basal cell carcinoma, brown hair, melanoma, nevi, red hair, skin colour, tanning response, vitiligo, vitamin D levels). PRS analyses identified limited shared genetic variation (max 0.15% for nevi), and MR analyses did not provide evidence of a causal relationship. Together, the ten-trait and pairwise multi-trait analyses identified 48 SNPs with suggestive pleiotropy, 31 of which were protein-coding and could be mapped to 22 different genes. Overall, while some genetic overlap exists, no definitive correlative or causal relationships were established. These results contribute to the broader understanding of the differing roles of melanin and neuromelanin, as well as potential implications in neurodegenerative diseases.

Melanoma being one of the most common and deadliest skin cancers, has been rising since the past decade. Patients at advanced stages of the disease have very poor prognoses, as opposed to at the earlier stages. Nowadays the standard-of-care of advanced melanoma is resection followed by immune checkpoint inhibition based immunotherapy. However, a substantial proportion of patients either do not respond or develop resistances. This underscores a need for novel approaches and therapeutic targets as well as a better understanding of the mechanisms of melanoma pathogenesis. Long non-coding RNAs (lncRNAs) comprise a poorly characterized class of functional players and promising targets in promoting malignancy. Certain lncRNAs have been identified to play integral roles in melanoma progression and drug resistances, however systematic screens to uncover novel functional lncRNAs are scarce. Here, we profile differentially expressed lncRNAs in patient derived short-term metastatic cultures and BRAF-MEK-inhibition resistant cells. We conduct a focused growth-related CRISPR-inhibition screen of overexpressed lncRNAs, validate and functionally characterize lncRNA hits with respect to cellular growth, invasive capacities and apoptosis in vitro as well as the transcriptomic impact of our lead candidate the novel lncRNA XLOC_030781. In sum, we extend the current knowledge of ncRNAs and their potential relevance on melanoma. SignificancePreviously considered as transcriptional noise, lncRNAs have emerged as novel players in regulating many cellular aspects in health and disease including melanoma. However, the number and as well as the extent of functional significance of most lncRNAs remains elusive. We provide a comprehensive strategy to identify functionally relevant lncRNAs in melanoma by combining expression profiling with CRISPR-inhibition growths screens lowering the experimental effort. We also provide a larger resource of differentially expressed lncRNAs with potential implications in melanoma growth and invasion. Our results broaden the characterized of lncRNAs as potential targets for future therapeutic applications.

Melanoma mortality is driven by the formation and growth of distant metastases. Here, we interrogated the role of tumor oxidative phosphorylation (OXPHOS) in the formation of distant metastases in melanoma. OXPHOS was the most upregulated metabolic pathway in primary tumors that formed distant metastases in the RCAS-TVA mouse model of spontaneous lung and brain metastases, and in melanoma patients that developed brain or other distant metastases. Knockout of PGC1 in melanocytes in the RCAS-TVA melanoma mouse model had no impact on primary tumor formation, but markedly reduced the incidence of lung and brain metastases. Genetic knockout of a component of electron transport chain complex I, NDUFS4, in B16-F10 and D4M-UV2 murine melanoma cell lines did not impact tumor incidence following subcutaneous, intravenous, or intracranial injection, but decreased tumor burden specifically in the lungs and brain. Together, these data demonstrate that OXPHOS is critical for the formation of metastases in melanoma. STRUCTURED ABSTRACTO_ST_ABSPurposeC_ST_ABSMelanoma mortality is driven by the formation and growth of distant metastases. However, the process and pathogenesis of melanoma metastasis remain poorly understood. Here, we interrogate the role of tumor oxidative phosphorylation (OXPHOS) in the formation of distant metastases in melanoma. Experimental DesignThis study includes (1) new RNA-seq analysis of primary melanomas from patients characterized for distant metastasis events; (2) RNA-seq analysis and functional testing of genetic OXPHOS inhibition (PGC1 KO) the RCAS-TVA model, which is the only existing immunocompetent murine model of autochthonous lung and brain metastasis formation from primary melanoma tumors; and (3) functional experiments of genetic OXPHOS inhibition (NDUFS4 KO) in the B16-F10 and D4M-UV2 murine melanoma cell lines, including evaluation of subcutaneous, lung, and brain metastatic site dependencies. ResultsOXPHOS was the most upregulated metabolic pathway in primary tumors that formed distant metastases in the RCAS-TVA mouse model of spontaneous lung and brain metastases, and in melanoma patients that developed brain or other distant metastases. Knockout of PGC1a in melanocytes in the RCAS-TVA melanoma mouse model had no impact on primary tumor formation, but markedly reduced the incidence of lung and brain metastases. Genetic knockout of a component of electron transport chain complex I, NDUFS4, in B16-F10 and D4M-UV2 murine melanoma cell lines did not impact tumor incidence following subcutaneous, intravenous, or intracranial injection, but decreased tumor burden specifically in the lungs and brain. ConclusionsTogether, these data demonstrate that OXPHOS is critical for the formation of metastases in melanoma. TRANSLATIONAL RELEVANCEMelanoma is the most aggressive form of skin cancer. One hallmark of this disease is a high risk of distant metastasis formation. The process and pathogenesis of metastasis in this disease remain poorly understood and there is controversy regarding the role of oxidative phosphorylation (OXPHOS) in melanoma metastasis. This study incorporates RNAseq analysis of primary melanoma tumors from patients characterized for distant metastasis events, RNAseq analysis of the only existing immunocompetent murine model of autochthonous lung and brain metastasis formation from primary melanoma tumors, and functional testing in multiple syngeneic models of melanoma at different tissue sites. This integrated analysis consistently demonstrates that melanoma OXPHOS promotes distant metastasis to the lungs and brain, two of the most common and clinically relevant sites of melanoma metastasis. This improved understanding of tumor OXPHOS may represent novel vulnerabilities for therapeutics development and surveillance/preventative strategies for melanoma metastasis.

A number of genomic regions have been associated with melanoma risk through genome-wide association studies, however, the causal variants underlying the majority of these associations remain unknown. Here, we sequenced either the full locus or the functional regions including exons of 19 melanoma-associated loci in 1,977 British melanoma cases and 754 controls. Variant filtering followed by Fisher's exact test analyses identified 88 variants associated with melanoma risk. Sequential conditional logistic regressions identified the distinct haplotypes on which variants lie, and massively parallel reporter assays (MPRA) provided biological insights into how these variants influence gene function. We performed further analyses to link variants to melanoma risk phenotypes and assess their association with melanoma-specific survival. Our analyses replicate previously known associations in the MC1R and TYR loci, while identifying novel potentially causal variants at the MTAP/CDKN2A and CASP8 loci. These results increase our understanding of the architecture of melanoma risk and outcome.

Cutaneous melanoma is a very aggressive type of skin cancer with remarkable phenotypic plasticity that contributes to adaptation and resistance to targeted therapies against the MAPK pathway. Previous research described that non-muscle myosin II (NMII) of the actomyosin cytoskeleton, which is essential for cell migration and metastasis, is overactivated in BRAF inhibitor-resistant melanomas. Since the combination of BRAF and MEK inhibitors (BMi) is the current standard of care, we investigated if and how NMII activity is regulated during adaptation to BMi. Here, we find that most dedifferentiating BMi-resistant melanomas overactivate NMII compared to their parental counterparts. NMII activity generally increases during the first 2 weeks of BMi treatment, and it is followed by elevated total NMII levels due partly to transcriptional modulation. Although ERK activity rebounds with similar kinetics, NMII overactivation is not prevented by ERK inhibition but by blockade of ROCK. In melanomas that hyperdifferentiate during adaptation to BMi, NMII activity is not increased upon BMi treatment due, in part, to MITF. We also find that co-targeting NMII along BMi in some melanomas reduces survival of drug-tolerant persister cells, which would delay the development of resistance. Therefore, our study identifies elevated NMII activity as a potential marker of adaptation to MAPK in some melanoma subpopulations, and also provide an approach to delay the emergence of resistance to MAPK-targeted therapy.

Uveal melanoma is the most prevalent primary intraocular cancer, with a significant metastatic risk. This risk is dependent on the genetic drivers. Secondary mutations in EIF1AX, SF3B1 and BAP1 correlate with clinical outcomes and are recognized for their distinct transcriptomic and epigenetic profiles. Previously, we identified 480 genes involved in the development of ocular melanocytes. Top ranking genes RBFOX2 and FOXD1 were significantly associated with BAP1 UM and were independently correlated to poor progression. However, it is uncertain whether either RBFOX2 or FOXD1 have biological contribution to disease progression or are solely indicative. This study investigates if and how these high-risk associated transcription regulators FOXD1 and RBFOX2 could influence tumor progression through knock-out and overexpression models. Our results indicate that loss of RBFOX2 affects cell morphology, attachment and proliferation, particularly in BAP1neg cells. Additionally, both RBFOX2 and FOXD1 contribute to tumor growth and dissemination in zebrafish xenografts. Loss of either RBFOX2 or FOXD1 reduced tumor volume and cell dissemination, with the greatest effects seen in BAP1neg cells. Overexpression models demonstrated different morphological and invasive behavior depending on the genetic background, suggesting complex roles in a context dependent fashion. Overexpression of RBFOX2 did not alter BAP1pos cells yet made BAP1neg cells more aggressive in vitro and in vivo. This study underscores the influence of RBFOX2 and FOXD1 as important factors for UM progression.

The cyclic AMP pathway promotes melanocyte differentiation in part by triggering gene expression changes mediated by CREB and its coactivators (CRTC1-3). Differentiation is dysregulated in melanomas, although the contributions of different cAMP effectors in this setting is unclear. We report a selective differentiation impairment in CRTC3 KO melanocytes and melanoma cells, due to downregulation of OCA2 and block of melanosome maturation. CRTC3 stimulated OCA2 expression via binding to CREB on a conserved enhancer, a regulatory site for pigmentation and melanoma risk in humans. Response to cellular signaling differed between CRTC3 and its family members; CRTC3 was uniquely activated by ERK1/2-mediated phosphorylation at Ser391 and by low levels of cAMP. Phosphorylation at Ser391 was constitutively elevated in human melanoma cells with hyperactivated ERK1/2 signaling; knockout of CRTC3 in this setting impaired anchorage-independent growth, migration and invasiveness while CRTC3 overexpression supported cell survival in response to MAPK inhibition by vemurafenib. Human melanomas expressing gain of function mutations in CRTC3 were associated with poorer clinical outcome. Our results suggest that CRTC3 inhibition may provide benefit in the treatment of hyperpigmentation and melanoma, and potentially other disorders with deregulated cAMP/MAPK crosstalk.

Melanoma is a highly aggressive and frequently metastatic cancer with its incidence reported to be on the rise. Although most oncogenic drivers in melanoma converge on activation of the RAS>RAF>MEK>ERK MAPK signaling pathway, not all MAPK-activating mutations are recurrently observed in this disease, suggesting a unique functional role for BRAFV600E, which is present in [~]50% of all melanoma cases. However, the prevalence of BRAFV600E alterations over other known MAPK-promoting oncoproteins raises questions regarding whether BRAFV600E possesses additional functions outside of MAPK pathway activation. Thus, we performed TurboID to differentiate the interactome between wild-type BRAF and BRAFV600E. We identified novel interacting partners of normal vs. BRAFV600E, most strikingly being the tumor suppressor TP53. While TP53 is commonly altered or lost across many malignancies, it is notable that TP53 alterations are rare in melanoma. Our studies suggest that BRAFV600E can interact with and inactivate TP53, thus providing potential mechanistic explanation as to why TP53 inactivation or loss is infrequent in BRAFV600E-driven melanoma.

Monosomy 3 is a negative indicator for uveal melanoma (UM). A key tumor suppressor on chromosome 3 is the deubiquitinase BAP1, which usually has a second hit in cases with monosomy 3. Here, we investigated the role of Bap1 loss in the GNAQQ209L mouse UM model. We found that heterozygous Bap1 mutations increased the proportion of lung lesions reaching an unusually large size and permitted the growth of liver lesions. Comparison of RNAseq data from mouse and human UM identified a set of 270 genes differentially expressed in the same direction when BAP1 is mutant. The most significant pathway in this gene set was Epithelial to Mesenchymal Transition (EMT). The expression of five apical junction complex genes known to be down-regulated in association with EMT was very significantly correlated with survival in human UM patients. Activation of EMT through Bap1 deficiency could increase melanoma plasticity and adaptation to new microenvironments.

Melanoma (MEL) is an aggressive form of skin cancer, causing over 60,000 deaths every year and it is considered one of the fastest-growing cancer forms. Genome-wide association studies have identified numerous genetic variants (SNPs) independently associated with MEL. The effects of such SNPs can be combined into a single polygenic risk score (PRS). Stratification of individuals according to PRS could be introduced to the primary prevention of melanoma. Our aim was to combine PRS with health behavior recommendations to develop a personalized recommendation for primary prevention of melanoma. Previously published PRS models for predicting the risk of melanoma were collected from the literature. Models were validated on the UK Biobank dataset consisting of a total of 487,410 quality-controlled genotypes with 3791 prevalent and 2345 incident cases. The best performing sex-specific models were selected based on the AUC in prevalent data and independently validated on an independent UKBB incident dataset for females and males separately. The best performing model included 28 SNPs. The C-index of the best performing model in the dataset was 0.59 (0.009) and hazard ratio (HR) per unit of PRS was 1.38 (standard error of log (HR) = 0.03) for both males and females. We performed absolute risk simulations on the Estonian population and developed individual risk-based clinical follow-up recommendations. Both models were able to identify individuals with more than a 2-fold risk increase. The observed 10-year risks of developing melanoma for individuals in the 99th percentile exceeded the risk of individuals in the 1st percentile more than 4.5-fold. We have developed a PRS-based recommendations pipeline for individual health behavior suggestions to support melanoma prevention.

The Microphthalmia associated transcription factor (Mitf) is a critical regulator of the melanocyte lineage and also plays an important role in eye development. Mitf activity in different cell types is controlled in part by ten alternative promoters and their resulting isoforms. A useful tool for melanocyte-based research, the Mitf-cre transgene was designed to express Cre recombinase from the Mitf-M promoter, which is melanocyte specific. However, Mitf-cre mice are also microphthalmic, perhaps because of insertional mutagenesis or disrupted gene expression. Here, we investigated these possibilities. We determined that the eye phenotype arises early, with Mitf-cre embryos at E13.5 exhibiting variable ocular sizes and abnormalities, but all with coloboma. Targeted locus amplification and next generation sequencing indicated that multiple copies of the transgene integrated into an intergenic region on chromosome 2, in between Spred1 and Meis2. The BAC transgene used to make Mitf-cre was larger than expected, carrying three upstream alternative promoters, Mitf-H, Mitf-D, and Mitf-B, which could express their isoforms intact off the transgene. RT-qPCR using eye tissue demonstrated a 5-fold increase in Mitf transcripts containing exon 1B1b, which is shared by Mitf-H, Mitf-D, and Mitf-B, while Spred1 and Meis2 did not differ in their expression. These findings clarify and support the usage of Mitf-cre in conditional mutagenesis in melanocytes. The specific over-expression of the Mitf-H and Mitf-D isoforms, which are preferentially expressed in the RPE, presents a unique resource for those interested in eye development and coloboma.

Keratinocytes, the dominant cell type in the melanoma microenvironment during tumor initiation, exhibit diverse effects on melanoma progression. Using a zebrafish model of melanoma and human cell co-cultures, we observed that keratinocytes undergo an Epithelial-Mesenchymal Transition (EMT)-like transformation in the presence of melanoma, reminiscent of their behavior during wound healing. Surprisingly, overexpression of the EMT transcription factor Twist in keratinocytes led to improved overall survival in zebrafish melanoma models, despite no change in tumor initiation rates. This survival benefit was attributed to reduced melanoma invasion, as confirmed by human cell co-culture assays. Single-cell RNA-sequencing revealed a unique melanoma cell cluster in the Twist-overexpressing condition, exhibiting a more differentiated, less invasive phenotype. Further analysis nominated homotypic jam3b-jam3b and pgrn-sort1a interactions between Twist-overexpressing keratinocytes and melanoma cells as potential mediators of the invasive restraint. Our findings suggest that EMT in the tumor microenvironment (TME) may limit melanoma invasion through altered cell-cell interactions.

Distinct NRAS mutants are enriched in various tumor types. Here, we generated a suite of fully congenic, conditional, Nras knock-in mouse models (LSL-Nras Q61R, -K, -L, -H, -P, -Q; G12D and G13D, -R) to test the hypothesis that melanocyte transformation requires functions specific to the NRAS mutants enriched in human melanoma (Q61R and Q61K). Consistent with the rarity of NRAS codon 12 and 13 mutants in human melanoma, spontaneous melanomas were rare or absent in mice expressing NRAS G12D, G13D or G13R. Mice expressing less common codon 61 alleles (Q61H, Q61P) also developed few or no tumors. NRAS Q61R, Q61K, or Q61L expression, by contrast, induced rapid melanoma onset with high penetrance. Cohorts of heterozygous mice containing one LSL-Nras Q61R and one LSL-Nras Q61K, -L, -H, -P, or -Q allele were generated to assess potential interactions between NRAS mutants. The ability of each Nras variant to substitute for an Nras Q61R allele was consistent with its own ability to drive spontaneous melanoma formation. However, LSL-Nras Q61Q/Q61R mice rarely developed tumors. In vitro experiments in mouse embryonic fibroblasts (MEFs) highlighted activation of the MAPK pathway as a defining difference between tumorigenic and non-tumorigenic NRAS mutants. Enhanced MAPK activation was associated with the promotion of BRAF-BRAF and BRAF-CRAF dimers. These results support the development of cancer preventative strategies specific to the properties of the commonly observed RAS mutants in each tumor type.