Lgr6 is a stem cell marker in mouse skin squamous cell carcinoma

The G-protein-coupled receptors LGR4, LGR5 and LGR6 are Wnt signaling mediators, but their functions in squamous cell carcinoma (SCC) are unclear. Using lineage tracing in Lgr5-EGFP-CreERT2/Rosa26-Tomato and Lgr6-EGFP-CreERT2/Rosa26Tomato reporter mice, we demonstrate that Lgr6, but not Lgr5, acts as an epithelial stem cell marker in SCCs in vivo. We identify, by single-molecule in situ hybridization and cell sorting, rare cells positive for Lgr6 expression in immortalized keratinocytes and show that their frequency increases in advanced SCCs. Lgr6 expression is enriched in cells with stem cell characteristics, and Lgr6 downregulation in vivo causes increased epidermal proliferation with expanded lineage tracing from epidermal stem cells positive for Lgr6 expression. Surprisingly, mice with germline knockout of Lgr6 are predisposed to SCC development, through a mechanism that includes compensatory upregulation of Lgr5. These data provide a model for human patients with germline lossof- function mutations in Wnt pathway genes, including RSPO1 or LGR4, who show increased susceptibility to squamous tumor development.