Runx2 and Axin2 regulate skeletal development. We recently determined that Axin2 
and Runx2 molecularly interact in differentiating osteoblasts to regulate 
intramembranous bone formation, but the relationship between these factors in 
endochondral bone formation was unresolved. To address this, we examined the 
effects of Axin2 deficiency on the cleidocranial dysplasia (CCD) phenotype of 
Runx2(+/-) mice, focusing on skeletal defects attributed to improper 
endochondral bone formation. Axin2 deficiency unexpectedly exacerbated calvarial 
components of the CCD phenotype in the Runx2(+/-) mice; the endocranial layer of 
the frontal suture, which develops by endochondral bone formation, failed to 
mineralize in Axin2(-/-):Runx2(+/-) mice, resulting in a cartilaginous, fibrotic 
and larger fontanel than observed in Runx2(+/-) mice. Transcripts associated 
with cartilage development (e.g., Acan, miR140) were expressed at higher levels, 
whereas blood vessel morphogenesis transcripts (e.g., Slit2) were suppressed in 
Axin2(-/-):Runx2(+/-) calvaria. Cartilage maturation was impaired, as primary 
chondrocytes from double mutant mice demonstrated delayed differentiation and 
produced less calcified matrix in vitro. The genetic dominance of Runx2 was also 
reflected during endochondral fracture repair, as both Runx2(+/-) and double 
mutant Axin2(-/-):Runx2(+/-) mice had enlarged fracture calluses at early stages 
of healing. However, by the end stages of fracture healing, double mutant 
animals diverged from the Runx2(+/-) mice, showing smaller calluses and 
increased torsional strength indicative of more rapid end stage bone formation 
as seen in the Axin2(-/-) mice. Taken together, our data demonstrate a dominant 
role for Runx2 in chondrocyte maturation, but implicate Axin2 as an important 
modulator of the terminal stages of endochondral bone formation.