Guest post by Jennifer Zieba
Osteogenesis Imperfecta (OI) is the most common genetic bone dysplasia that is phenotypically and genetically complex. It is characterized by bone deformities and fractures caused by low bone mass and impaired bone quality. Roughly 85-90% of cases are dominantly inherited and result from mutations in genes encoding type I collagen (COL1A1 and COL1A2), the major protein of the bone matrix. 10-15% of OI cases are recessively inherited and the majority of those result from mutations in members of the prolyl-3-hydroxylation complex including Cartilage Associated Protein (CRTAP) involved in collagen posttranslational modification.
OI patients are at an increased risk of fracture throughout their lifetimes and anecdotal evidence suggests successful fracture recovery. However, non-union has been reported in 24% of fractures and 52% of osteotomies and many stabilization techniques result in additional surgery due to re-fracture. Re-fractures typically go unreported making the frequency of re-fractures in OI patients unknown. Thus, there is an unmet need to better understand the mechanisms by which OI affects fracture healing. Assessing fracture healing in human patients is a difficult task as neither X-Ray nor CT analysis provide accurate information concerning fracture callus composition, remodeling rate, or the final bone composition. Mouse models for OI have been proven to accurately reflect OI pathogenesis and phenotype. Furthermore, using mice as models for fracture healing allow us to observe in greater detail the lengthy process of fracture healing in a smaller time frame with more informative in vivo techniques such as histochemistry, uCT analysis and biomechanical testing of the fracture tissue at several timepoints. It is our hypothesis that OI fractures undergo suboptimal healing and that this process results in ultimately weaker bone leading to the increased possibility of re-fracture and we are using two murine models to assess this hypothesis.
Using an open tibial fracture model, we show a decrease in callus size in both Col1a1G610c/+ and Crtap–/– OI mouse models post-fracture indicating delayed healing and decreased cartilage content indicating decreased callus cell proliferation. Additionally, fracture calluses in both models exhibited a significant decrease in polar moment of inertia (pMOI) indicating a decrease in resistance to torsional stress supporting a potential functional deficit in newly healed bone. This data provides valuable insight into the effect of the ECM on fracture healing, a poorly understood mechanism. Most importantly, we performed biomechanical testing via three-point-bending of fully healed Crtap–/– tibia to determine the mechanical strength of the fracture site. In wild type bone, the healed fracture site resulted in stronger bone when compared to the unfractured tibia. However, Crtap–/– healed fractured tibia are mechanically weaker than the contralateral unfractured bone. This implies the possibility that OI fractures do not heal properly and may be a prime location for re-fracture. These data may support aggressive prevention of primary fractures as well as a need for therapies during fracture healing to decrease incidence of refracture and deformity in OI patients.