A multi-stem cell basis for craniosynostosis and calvarial mineralization.

Craniosynostosis is a group of disorders of premature calvarial suture fusion. The identity of the calvarial stem cells (CSCs) that produce fusion-driving osteoblasts in craniosynostosis remains poorly understood. Here we show that both physiologic calvarial mineralization and pathologic calvarial fusion in craniosynostosis reflect the interaction of two separate stem cell lineages; a previously identified cathepsin K (CTSK) lineage CSC (CTSK CSC) and a separate discoidin domain-containing receptor 2 (DDR2) lineage stem cell (DDR2 CSC) that we identified in this study. Deletion of Twist1, a gene associated with craniosynostosis in humans, solely in CTSK CSCs is sufficient to drive craniosynostosis in mice, but the sites that are destined to fuse exhibit an unexpected depletion of CTSK CSCs and a corresponding expansion of DDR2 CSCs, with DDR2 CSC expansion being a direct maladaptive response to CTSK CSC depletion. DDR2 CSCs display full stemness features, and our results establish the presence of two distinct stem cell lineages in the sutures, with both populations contributing to physiologic calvarial mineralization. DDR2 CSCs mediate a distinct form of endochondral ossification without the typical haematopoietic marrow formation. Implantation of DDR2 CSCs into suture sites is sufficient to induce fusion, and this phenotype was prevented by co-transplantation of CTSK CSCs. Finally, the human counterparts of DDR2 CSCs and CTSK CSCs display conserved functional properties in xenograft assays. The interaction between these two stem cell populations provides a new biologic interface for the modulation of calvarial mineralization and suture patency.