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 CSC¹ (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^2,3, 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.