Cranial Bone Ossification Trajectories in a Mouse Model of Osteogenesis Imperfecta

Purpose: Osteogenesis imperfecta (OI) is a genetic disorder that affects the production of type I collagen. Altered collagen production results in delayed or impaired skeletal formation and biomineralization. It also results in the defining characteristics of OI: brittle bones and high rates of fractures. Investigations of skeletal growth in OI have primarily focused on the postcranial skeleton, where interrupted, atypical, and disorganized ossification is seen at long bone growth plates. However, few studies have investigated changes in craniofacial growth in OI and there are currently few early interventions to improve growth trajectories in this region. The current medication prescribed for children with OI to improve skeletal growth, such as bisphosphonates, have major side effects and are not suitable for long-term use. A better understanding of craniofacial development in OI can help with targeting specific developmental stages when new treatments can be administered to provide the best results. The aim of this study is to examine cranial ossification from birth to weaning to determine where and when differences in growth occur in OI. We hypothesize that starting at birth mice with OI will have delayed craniofacial growth due to the poor collagen formation.

Methods: To test our hypothesis, we collected cranial bone volumes from micro-CT scans of the homozygous recessive OI murine model (OIM or B6C3Fe a/a-Col1a2oim/oim) and compared them to their wild type (WT) littermates. The OIM model has a COL1A2 mutation that has been found to express a similar skeletal phenotype to the severe form (type III) of OI in humans. Bone volumes were collected from birth (P0) and weaning (P21) from the nasal, frontal, parietal, interparietal, and occipital bones (n=2/genotype/timepoint).

Results: At birth, OIM and WT bone volumes were similar. By weaning, bone volume was lower in OIM mice compared to WT mice. Our results demonstrate that OIM mice have reduced rates of bone ossification between birth and weaning, and these differences are most profound in the facial and occipital regions. Additionally, OIM skulls are characterized by low bone volume and potential delays in the closure of cranial sutures and fontanelles.

Conclusions: This study suggests that the divergence in cranial ossification rates related to COL1A2 mutations occurs postnatally. Interventions to recover craniofacial bone growth in this experimental model should focus on the critical growth period between birth and weaning. Results from this research have the potential to assist in developing treatments and highlight the importance of early life development of the craniofacial bones in human patients with OI.