No abstract

Tumors can develop a blood supply not only by promoting angiogenesis but also by forming vessel-like structures directly from tumor cells, known as vasculogenic mimicry (VM). Understanding mechanisms that regulate VM is important, as these might be exploitable to inhibit tumor progression. Here, we reveal the adhesion molecule desmoglein 2 (DSG2) as a novel mediator of VM in melanoma. Analysis of patient-derived melanoma cell lines and tumor tissues, and interrogation of The Cancer Genome Atlas (TCGA) data, revealed that DSG2 is frequently overexpressed in primary and metastatic melanomas compared to normal melanocytes. Notably, this overexpression was associated with poor clinical outcome. DSG2+ melanoma cells self-organized into tube-like structures on Matrigel, indicative of VM activity, which was inhibited by DSG2 knockdown or treatment with a DSG2-blocking peptide. Mechanistic studies revealed that DSG2 regulates adhesion and cell-cell interactions during tube formation, but does not control melanoma cell viability, proliferation or motility. Finally, analysis of patient tumors revealed a correlation between DSG2 expression, VM network density and expression of VM-associated genes. These studies identify DSG2 as a key regulator of VM activity in human melanoma and suggest this molecule might be therapeutically targeted to reduce tumor blood supply and metastatic spread.

Melanoma is usually driven by mutations in BRAF or NRAS, which trigger hyperactivation of MAPK signaling. However, MAPK-targeted therapies are not sustainably effective in most patients. Accordingly, characterizing mechanisms that co-operatively drive melanoma progression is key to improving patient outcomes. One possible mechanism is the Hippo signaling pathway, which regulates cancer progression via its central oncoproteins YAP and TAZ, although is thought to be only rarely affected by direct mutation. As YAP hyperactivation occurs in uveal melanoma, we investigated this oncogene in cutaneous melanoma. YAP protein expression was elevated in most benign nevi and primary cutaneous melanomas but present at only very low levels in normal melanocytes. In patient-derived xenografts and melanoma cell lines, we observed variable reliance of cell viability on Hippo pathway signaling that was independent of TAZ activity and also of classical melanoma driver mutations such as BRAF and NRAS. Finally, in genotyping studies of melanoma, we observed the first ever hyperactivating YAP mutations in a human cancer, manifest as seven distinct missense point mutations that caused serine to alanine transpositions. Strikingly, these mutate four serine residues known to be targeted by the Hippo pathway and we show that they lead to hyperactivation of YAP.Implications: Our studies highlight the YAP oncoprotein as a potential therapeutic target in select subgroups of melanoma patients, although successful treatment with anti-YAP therapies will depend on identification of biomarkers additional to YAP protein expression. Mutations and methylation changes in Hippo pathway genes in the TCGA melanoma cohort. A, Somatic coding mutations (top) and CN changes (bottom) affecting Hippo pathway genes identified in patients from the TCGA cohort (56), organized horizontally according to the BRAF-, NRAS-, NF1-, TP53-, RAC1-or PTEN-mutant melanoma subtype of each patient and vertically according the mutation frequency in each Hippo pathway gene. Mutations colored by type: amp, amplification; hemi, hemizygous; homo, homozygous; del, deletion. B, top: distributions of methylation values across 473 TCGA patients in 18 known/predicted oncogenes and 33 known/predicted tumor suppressor genes in the Hippo pathway. Bottom: Spearman coefficient values for correlations between methylation and mRNA expression for each gene. C, Methylation versus mRNA expression plots for YAP and TAZ. Each dot represents values for an individual patient. Blue dots, mutated gene; red dots, wild-type gene. r is the Spearman correlation coefficient; Ã , P < 0.001. YAP and Melanoma

Desmogleins (DSG) are a family of cadherin adhesion proteins that were first identified in desmosomes and provide cardiomyocytes and epithelial cells with the junctional stability to tolerate mechanical stress. However, one member of this family, DSG2, is emerging as a protein with additional biological functions on a broader range of cells. Here we reveal that DSG2 is expressed by non-desmosome-forming human endothelial progenitor cells as well as their mature counterparts [endothelial cells (ECs)] in human tissue from healthy individuals and cancer patients. Analysis of normal blood and bone marrow showed that DSG2 is also expressed by CD34+CD45dim hematopoietic progenitor cells. An inability to detect other desmosomal components, i.e., DSG1, DSG3 and desmocollin (DSC)2/3, on these cells supports a solitary role for DSG2 outside of desmosomes. Functionally, we show that CD34+CD45dimDSG2+ progenitor cells are multi-potent and pro-angiogenic in vitro. Using a ‘knockout-first’ approach, we generated a Dsg2 loss-of-function strain of mice (Dsg2lo/lo) and observed that, in response to reduced levels of Dsg2: (i) CD31+ ECs in the pancreas are hypertrophic and exhibit altered morphology, (ii) bone marrow-derived endothelial colony formation is impaired, (iii) ex vivo vascular sprouting from aortic rings is reduced, and (iv) vessel formation in vitro and in vivo is attenuated. Finally, knockdown of DSG2 in a human bone marrow EC line reveals a reduction in an in vitro angiogenesis assay as well as relocalisation of actin and VE-cadherin away from the cell junctions, reduced cell–cell adhesion and increased invasive properties by these cells. In summary, we have identified DSG2 expression in distinct progenitor cell subpopulations and show that, independent from its classical function as a component of desmosomes, this cadherin also plays a critical role in the vasculature.Electronic supplementary materialThe online version of this article (doi:10.1007/s10456-016-9520-y) contains supplementary material, which is available to authorized users.

Although melanoma is initiated by acquisition of point mutations and limited focal copy number alterations in melanocytes-of-origin, the nature of genetic changes that characterise lethal metastatic disease is poorly understood. Here, we analyze the evolution of human melanoma progressing from early to late disease in 13 patients by sampling their tumours at multiple sites and times. Whole exome and genome sequencing data from 88 tumour samples reveals only limited gain of point mutations generally, with net mutational loss in some metastases. In contrast, melanoma evolution is dominated by whole genome doubling and large-scale aneuploidy, in which widespread loss of heterozygosity sculpts the burden of point mutations, neoantigens and structural variants even in treatment-naïve and primary cutaneous melanomas in some patients. These results imply that dysregulation of genomic integrity is a key driver of selective clonal advantage during melanoma progression.

The stability of markers that identify cancer cells that propagate disease is important to the outcomes of targeted therapy strategies. In human melanoma, conflicting data exist as to whether hierarchical expression of CD271/p75/NGFR (nerve growth factor receptor) marks cells with enriched tumorigenicity, which would compel their specific targeting in therapy. To test whether these discrepancies relate to differences among groups in assay approaches, we undertook side-by-side testing of published methods of patient-derived melanoma xenografting (

Melanoma is a deadly form of skin cancer that accounts for a disproportionally large proportion of cancer-related deaths in younger people. Compared to most other skin cancers, a feature of melanoma is its high metastatic capacity, although molecular mechanisms that confer this are not well understood. The Hippo pathway is a key regulator of organ growth and cell fate that is deregulated in many cancers. To analyse the Hippo pathway in cutaneous melanoma, we generated a transcriptional signature of pathway activity in melanoma cells. Hippo-mediated transcriptional activity varied in melanoma cell lines but failed to cluster with known genetic drivers of melanomagenesis such as BRAF and NRAS mutation status. Instead, it correlated strongly with published gene expression profiles linked to melanoma cell invasiveness. Consistent with this, the central Hippo oncogene, YAP, was both necessary and sufficient for melanoma cell invasion in vitro. In in vivo murine studies, YAP promoted spontaneous melanoma metastasis, whilst the growth of YAP-expressing primary tumours was impeded. Finally, we identified the YAP target genes AXL, THBS1 and CYR61 as key mediators of YAP-induced melanoma cell invasion. These data suggest that the Hippo pathway is a critical regulator of melanoma metastasis.

Targeting the right cancer-specific peptides presented by Human Leukocyte antigen (HLA) class I and II molecules on the tumor cell surface is a crucial step in cancer immunotherapy. Numerous approaches have been proposed to predict the presentation of potential neoepitopes that may be targeted through immune-based therapies. Often founded on patient specific somatic mutations, the routine validation of their actual appearance on the tumor cell surface is a significant barrier to realising personalized cancer immunotherapy. This can be attributed to the lack of robust and adaptable assays for antigen presentation that offer the required sensitivity to deal with the limited amounts of patient tumor tissue available. Rather than personalize individual assays we propose the use mass spectrometry to identify tumor neoepitopes from HLA-bound peptides directly isolated form the surface of tumor biopsies. We have developed a microscale HLA-peptide complex immunoprecipitation protocol combined with tandem mass tagging (TMT) to directly sequence HLA-bound peptides using mass spectrometry. Using this strategy, we identified HLA-bound peptides from as few as ~1000 cultured cells and from a small piece (~1 mg) of whole melanoma tumour tissue, encompassing epitopes derived from Melanoma-associated antigens and potential neoantigens.