Single-cell analysis of human fibrous dysplasia bone reveals a fibrotic transcriptome and GNAS variants in endothelial, perivascular, and stromal cells.

Genetic mosaicism is a leading cause of human disease across the lifespan. Improving the tools to detect somatic mosaicism and applying them to understand the cellular and molecular mechanisms that contribute to disease is of critical importance for improving human health. Fibrous dysplasia (FD) is a prototypical disease of Gs-GPCR activation caused by somatic, mosaic GNAS variants (c.602G>A [p.Arg201His] or c.601C>T [p.Arg201Cys]) that result in fibrotic bone. Utilizing single-cell RNA sequencing and a GNAS genotyping strategy, we analyzed non-hematopoietic cells from FD and non-FD human bone. FD bone showed an altered fibroblast composition with an FD-specific osteoblastic cluster. Surprisingly, in addition to the skeletal stromal lineages, endothelial and perivascular cells also expressed GNAS c.602G>A and c.601C>T variants, which was confirmed using BaseScope, suggesting that these variants are present in multiple non-osteogenic cell lineages. We also identified a common fibrotic transcriptomic signature across FD cell lineages. Our results highlight the effects of GNAS mosaicism on the cellular and transcriptomic landscapes of FD, identify previously unrecognized cell types that may be relevant to FD pathogenesis, and reframe our understanding of GNAS c.602G>A (p.Arg201His) and c.601C>T (p.Arg201Cys) function in bone.