Melanoma to rhabdomyosarcoma plasticity in the setting of immunotherapy.

BACKGROUND: Acquired resistance to immune checkpoint inhibitors (ICIs) remains a significant challenge in the treatment of metastatic melanoma. Phenotypic plasticity, such as dedifferentiation and transdifferentiation, is an increasingly recognized mechanism of treatment resistance. METHODS: We present a case of a 70-year-old man with metastatic melanoma who experienced progression through sequential treatments including pembrolizumab in combination with the HDAC inhibitor entinostat and ipilimumab. During treatment, a histologically distinct pleomorphic rhabdomyosarcoma (RMS) emerged at metastatic sites. Longitudinally acquired tumor samples representing both phenotypes were analyzed using whole-exome sequencing (WES), RNA sequencing (RNA-seq), and high-plex tissue imaging (spatial proteomics). FINDINGS: WES revealed driver mutations (e.g., NRAS and NF1) and loss of heterozygosity (LOH) shared between phenotypes indicating a common ancestral clone. Phylogenetic analysis demonstrated an early divergence of the phenotypes, with each later acquiring unique mutations. RNA-seq showed mutually exclusive expression of lineage-specific markers as well as epithelial-mesenchymal transition and myogenic gene set enrichment in the RMS samples. High-plex imaging identified distinct tumor microenvironments, with RMS lesions enriched in CD163+ macrophages. CONCLUSIONS: This case provides molecular evidence of phenotypic plasticity occurring under the selective pressure of immune checkpoint inhibitor therapy. FUNDING: This study was supported by the National Institutes of Health (K12CA087723 and K08CA234458), the Doris Duke Charitable Foundation, and Adelson Medical Research Foundation.