An inducible mouse model of OI type V reveals aberrant osteogenesis caused by Ifitm5 c.-14C > T mutation.

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Osteogenesis imperfecta (OI) Type V is typically characterized by radial head dislocation, calcification of interosseous membrane and hyperplastic callus. It is caused by the c.-14C > T mutation in the 5' UTR of IFITM5 gene, adding five amino acids (MALEP) to the N-terminal of IFITM5 protein. Previous studies have suggested a neomorphic function of the MALEP-IFITM5 protein. However, the underlying mechanisms remain unclear due to embryonic lethality in previous mouse models. Therefore, we developed an inducible mouse model (Ifitm5flox c.-14C > T) that could be induced by Cre expressed at different developmental stages to explore the pathogenic effects of the neomorphic MALEP-IFITM5. The mutant Ifitm5 allele could be regulated by the endogenous regulatory elements after Cre recombination, maintaining its spatiotemporal expression pattern and physiological level. Specifically, Prx1-Cre; Ifitm5flox c.-14C > T mutant mice were born with fractures in all limbs, showing impaired ossification and enhanced chondrogenesis associated with increased SOX9 abundance. Analyses of single-cell RNA sequencing data revealed arrested osteogenesis in Prx1-Cre; Ifitm5flox c.-14C > T mouse. A major population of cells expressing both osteogenic and chondrogenic signature genes was identified in the mutant mouse. Reduced expression of SP7 and SOST in the cortical regions of mutant mice confirmed delayed osteocyte maturation and compromised osteogenesis. Elevated bone marrow adipocytes were found in the adult mutant mice. Ectopic chondrogenesis and SOX9 expression were also observed in the perichondrium regions of Col1a1-Cre; Ifitm5flox c.-14C > T and Ocn-Cre; Ifitm5flox c.-14C > T mutant mice. The inducible Ifitm5flox c.-14C > T mouse model and integrated single-cell transcriptomic analyses elucidated that ectopic expression of SOX9 and disrupted homeostatic balance among osteogenesis, chondrogenesis and adipogenesis may contribute to the pathogenesis caused by MALEP-IFITM5, helping to gain deeper insights into the molecular mechanisms of type V OI.