Time Course and Gene Expression Analysis of Juvenile Mouse Re-synostosis

Objectives: Craniosynostosis is the pathologic fusion of the cranial sutures and requires complex calvarial reconstruction. In up to 40% of cases, the bones re-fuse resulting in re-synostosis that requires surgical intervention associated with an extremely high incidence of life threatening complications. The objective of this study is to completely characterize the time course and changes in gene expression in a juvenile mouse model of re-synostosis.

Methods: Under approval of the GA Tech IACUC, a 1.5 by 2.5 mm defect was created in 21 day old C57Bl/6J male mice removing the posterior frontal suture. On post-operative days 1, 2 , 3, 4, 5 ,14, and 21 five mice were randomized for imaging with micro-CT and histology. The mean defect distance and bone volume in the defect were quantified using our novel snake algorithm previously validated with serial histology. Analysis of genes associated with suture fusion was by qPCR.  A P<0.05 was considered statistically significant by ANOVA and Bonferroni T-test.

Results: The defect distance decreased on post-op days 3-6, followed by an increase in bone volume on post-op days 14 - 21. Expression of mRNAs for markers of chondrogenesis (Sox9,ColII, ColX, and Comp) all increased on post-op days 3-5. This was followed by an increase in Bmp4 on day 5 then Bmp2 on day 7. The increase in bone volume on day 14 was associated with an increase in expression of osteocalcin, Vitamin D receptor, and TGFB2.

Conclusion: The results from this study show that the rapid re-synostosis of the defect occurs through a biphasic process with a thin bridge of bone forming 3 days post-op and then a large increase in bone volume by 14 days post-op. Following the bridging there was an increase in expression of markers associated with cartilage development, which suggests that this re-synostosis occurs through endochondral ossification.