Persistent expression of Twist1 in chondrocytes causes growth plate abnormalities and dwarfism in mice

Evidence from various in vitro gain and loss of function studies indicate that the bHLH transcription factor Twist1 negatively regulates chondrocyte differentiation; however limited information regarding Twist1 function in postnatal cartilage development and maintenance is available. Twist1 expression within the postnatal growth plate is restricted to immature, proliferating chondrocytes, and is significantly decreased or absent in hypertrophic chondrocytes. In order to examine the effect of maintaining the expression of Twist1 at later stages of chondocyte differentiation, we used type II collagen Cre (Col2-Cre) mice to activate a Cre-inducible Twist1 transgene specifically in chondrocytes (Col2-Twist1). At two weeks, postnatal growth was inhibited in Col2-Twist1 mice, as evidenced by limb shortening. Histological examination revealed abnormal growth plate structure, characterized by poor columnar organization of proliferating cartilaginous cells, decreased cellularity, and expansion of the hypertrophic zone. Moreover, structural defects within the growth plates of Col2-Twist1 transgenic mice included abnormal vascular invasion and focal regions of bony formation. Quantitative analysis of endochondral bone formation via micro-computed topography revealed impaired trabecular bone formation in the hindlimbs of Col2-Twist1 transgenic mice at various timepoints of postnatal development. Taken together, these findings indicate that regulated Twist1 expression contributes to growth plate organization and endochondral ossification to modulate postnatal longitudinal bone growth.