Induced pluripotent stem cells from homozygous Runx2 -deficient mice show poor response to vitamin D during osteoblastic differentiation

Cleidocranial dysplasia (CCD) is a hereditary disorder associated with skeletal dysplasia and dental abnormalities. CCD arises from heterozygous loss of function mutations in the Runt-related transcription factor 2 (RUNX2) gene. Osteoporosis is often observed in CCD patients and conventional vitamin D supplementation is recommended. However, sufficient evidences have not been presented yet. This study investigated the role of RUNX2 in osteoblastic differentiation and sought to identify potential target genes for the treatment of osteoporosis associated with CCD, using induced pluripotent stem cell (iPSC) technology. We successfully established Runx2 −/− , Runx2 +/− and wild-type miPSCs from litter-matched mice and found poor Vdr expression in Runx2 −/− cells. Significant down-regulation of osteoblastic differentiation in Runx2 −/− miPSCs was observed. Gene expression array revealed unexpected results such as remarkable increase of Rankl expression and decrease of Vdr in Runx2 −/− cells. Insufficient response to vitamin D in Runx2 −/− cells was also observed. Our results suggest that RUNX2 functions as a regulator of Rankl and Vdr and thereby controls bone density. These findings also suggest that conventional vitamin D supplementation may not be as effective as previously expected, in the treatment of osteoporosis associated with CCD, and that inhibiting RANKL function might be worth considering as an alternative treatment strategy.