ABSTRACT
Runx2 is a transcription factor that is not only critical in embryonic skeletal development but also important in regulating osteoblast function in the adult. Heterozygosity of RUNX2 (RUNX2(+/-)) leads to haploinsufficiency and manifests as a condition with distinctive skeletal features in humans and mice. Aged but not young RUNX2(+/-) adult mice may also display reduced intramembranous bone formation. To clarify the role of Runx2 in intramembranous bone formation in adult mice a histomorphometric study was performed to compare the osteogenic response to high-dose estrogen in RUNX2(+/-) and wild-type mice. Young (10 weeks) and aged (26 weeks) RUNX2(+/-) and wild-type littermate mice were treated with vehicle or high-dose estrogen (0.5 mg/animal/week) by subcutaneous injection for 4 weeks. Mice were divided into 8 groups according to age, genotype and treatment with 6 animals per group. Following sacrifice, longitudinal tibial sections were prepared and examined by static and dynamic histomorphometry. Estrogen treatment induced formation of new cancellous bone in both wild-type and RUNX2(+/-) mice. This occurred to the same extent in young mice of both genotypes. However, in the aged RUNX2(+/-) mice this response as assessed by bone volume (BV/TV%) was decreased by over 70% (p<0.001) when compared to aged wild-type mice. Furthermore, significant reductions in cancellous double-labelled surfaces (dls/TV, 1.7+/-0.2 vs 1.0+/-0.4 mm(2)/mm(3), p<0.05) and mineral apposition rate (1.8+/-0.1 vs 1.4+/-0.1 microm/day, p<0.01) were observed in aged RUNX2(+/-) mice compared to wild-types. Aged RUNX2(+/-) mice display an abrogated osteogenic response to high-dose estrogen. This may have occurred through combined reductions in recruitment of osteoprogenitor cells, osteoblast activity and mineralization. Since the characteristic histological changes in the marrow cavity which precede the formation of cancellous bone following estrogen treatment was seen in the aged RUNX2(+/-) mice we suggest that they may eventually be capable of a full osteogenic response but haploinsufficiency leads to delayed bone formation.
