Desmosome mutations impact the tumor microenvironment to promote melanoma proliferation [scRNA-seq]

Desmosomes are transmembrane protein complexes that contribute to cell-cell adhesion in epithelia and other tissues. Here, we report the discovery of frequent genetic alterations in the desmosome in human cancers, with the strongest signal seen in cutaneous melanoma where desmosomes are mutated in >70% of cases. In primary but not metastatic melanoma biopsies, the burden of coding mutations in desmosome genes associates with a strong reduction in desmosome gene expression. Analysis by spatial transcriptomics and protein immunofluorescence suggests that these expression decreases occur in keratinocytes in the microenvironment rather than in primary melanoma cells. In further support of a microenvironmental origin, we find that desmosome gene knockdown in keratinocytes yields markedly increased proliferation of adjacent melanoma cells in keratinocyte/melanoma co-cultures. Similar increases in melanoma proliferation are observed in media preconditioned by desmosome-deficient keratinocytes. Thus, gradual accumulation of desmosome mutations in neighboring cells may prime melanoma cells for neoplastic transformation. Overall design: Melanoma tumors were collected post-operatively from two patients who consented and signed IRB. To obtain single-cell suspensions, samples were washed in PBS and cut into small pieces (4-5 mm3) followed by dissociation using the Miltenyi human tumor dissociation kit according to manufacturer instructions. Red blood cells were depleted using ACK lysis buffer for 3 minutes. Dead cells were removed, if needed, using the Milyenty dead cell separator. Single cell encapsulation and library preparation were performed using the inDrop platform followed by sequencing on an Illumina NextSeq.