Sost deficiency restores calvarial bone thickness without recovery of sutural growth in Runx2 missense mutant mice.

Runt-related transcription factor 2 (Runx2) is a key transcription factor essential for osteogenesis, and RUNX2 haploinsufficiency causes cleidocranial dysplasia, whereas sclerostin encoded by Sost negatively regulates bone mass by antagonizing Wnt/β-catenin signaling. Although the individual roles of Runx2 and sclerostin in skeletal biology have been extensively studied, their combined effects on calvarial development have not been directly examined in vivo. Here, we investigated whether Sost deficiency affects the cranial phenotype associated with Runx2 haploinsufficiency. Using skeletal preparations, micro-computed tomography, and three-dimensional morphometric analysis of the interparietal bone, we analyzed Sost-deficient, Runx2 missense mutant, and Runx2-Sost compound mutant mice at embryonic day 18.5 (E18.5) and postnatal day 90 (P90). At E18.5, both Runx2R232Q/R232Q;Sost+/+ and Runx2R232Q/R232Q;SostΔ26/Δ26 mice lacked ossification throughout the body. At P90, Runx2R232Q/+;Sost+/+ mice exhibited cranial hypoplasia with delayed suture closure. In Runx2R232Q/+;SostΔ26/Δ26 mice, interparietal bone thickness was restored to wild-type levels, whereas the bone volume showed only partial recovery. Bone volume/tissue volume and bone mineral density increased to levels higher than those of wild-type, comparable to those of Runx2+/+;SostΔ26/Δ26. However, the bone surface area exhibited no significant improvement, and delayed suture closure around the interparietal bone and anterior and posterior fontanelles persisted. Thus, Sost-deficiency augments postnatal calvarial mass primarily by thickening without expanding the surface area and does not normalize suture/fontanelle morphogenesis under Runx2 haploinsufficiency. Our findings reveal a differential genetic effect of Sost deficiency on calvarial thickness and sutural growth in mice with Runx2 haploinsufficiency.