New lens on congenital mild bone fragility: a novel Col1a1 knockout mouse model for osteogenesis imperfecta type 1

Osteogenesis imperfecta (OI) is a genetic disorder characterized by bone fragility. It is one of the most prevalent rare skeletal dysplasias. The mildest form, OI type 1, predominantly results from collagen type I haploinsufficiency due to pathogenic variants in the COL1A1 gene, leading to reduced collagen type I. Despite OI type 1 representing approximately half of the OI population, the lack of an effective mouse model has hindered research and therapy development. To address this gap, we developed a genetically engineered mouse model harboring a heterozygous deletion of the Col1a1 allele using the CRISPR/Cas system. The bone phenotype was characterized in 8- and 24-wk-old mice, assessing transcriptomics and serum markers for bone formation (procollagen type I N-terminal propeptide) and resorption (tartrate-resistant acid phosphatase 5b). Bone volume, microarchitecture, and strength were evaluated by micro-CT, histomorphometry, and three-point bending test. We showed that the decreased Col1a1 to Col1a2 mRNA ratio determines reduced collagen type I production in OI mice bones as the underlying mechanism of haploinsufficient OI. This was supported by COL1A1 to COL1A2 mRNA ratio findings in human OI cell models, including fibroblasts and induced mesenchymal stem cells, as well as in induced pluripotent and mesenchymal stem cell models that were edited to carry a heterozygous COL1A1 allele. Our findings indicate for the first time that reduced bone volume and altered bone microarchitecture in haploinsufficient OI depends on the Col1a1 to Col1a2 mRNA ratio regulation. This novel mouse model faithfully recapitulates OI type 1 and provides a vital tool for investigating the disease mechanism and developing targeted therapeutic strategies for this large neglected OI patient population.