INVESTIGATING THE CONTRIBUTION OF OSTEOBLAST DIFFERENTIATION IN THE PATHOGENESIS OF OSTEOGENESIS IMPERFECTA TYPE V - PROJECT SUMMARY Osteogenesis imperfecta (OI) is the most common osteodysplasia affecting between 1 in 15,000 and 1 in 20,000 live births. While variants in COL1A1 and COL1A2 are the most common causes of OI, up to 15% of OI cases are attributed to variants in other genes. Osteogenesis imperfecta type V (OI-V) is caused by a recurrent, heterozygous pathogenic variant (c.-14C>T) in the 5’ UTR of IFITM5, which encodes an osteoblast transmembrane protein. This point mutation creates a new in-frame start codon upstream of the endogenous start site which results in the addition of five amino acids to the intracellular N-terminus of IFITM5 that is hypothesized to prevent normal osteoblast differentiation in a neomorphic manner. Patients with OI-V may present with classic OI phenotypes like short stature, low bone mass, and recurrent fractures, in addition to OI- V specific phenotypes such as interosseous membrane calcification, hyperplastic callus formation, and radial head dislocation. The current standard of care for patients with OI-V is off-label use of bisphosphonates, which is ineffective at treating the complete phenotypic spectrum of OI-V. As such, the overall goal of this proposal is to develop targeted therapies that better address the underlying mechanism of OI-V. However, the temporal consequence of IFITM5(c.-14C>T) expression is not understood and further investigation has been limited by perinatal lethality of global Ifitm5(c.-14C>T) mouse models. Our lab has generated an OI-V conditional mouse model by inserting mutant Ifitm5(c.-14C>T) cDNA into the Rosa26 locus and induced expression of Ifitm5(c.-14C>T) in mesenchymal progenitor cells and mature osteoblasts using Prx1-Cre, and Ocn-Cre, respectively. Prx1- Cre;Ifitm5(c.-14C>T) mice demonstrate a severe skeletal phenotype including low bone mass, long bone deformities, and growth plate abnormalities that persist into skeletal maturity while the vertebrae and femora of Ocn- Cre;Ifitm5(c.-14C>T) mice do not show gross skeletal abnormalities, suggesting that early expression of Ifitm5(c.- 14C>T) disrupts normal osteoblast development. TGF-β is a known regulator of osteoblast differentiation, is a well- described pathogenic driver of other subtypes of OI, and anti-TGF-β therapies are in clinical trials to treat OI. Preliminary data from our lab suggests that downstream targets of TGF-β are upregulated in our OI-V animal models. We hypothesize that Ifitm5(c.-14C>T) results in increased TGF-β signaling which is a primary driver of the OI-V osteoblast differentiation defect and therapies that target either TGF-β signaling or the pathogenic point mutation early in development will improve OI-V phenotypes. This central hypothesis will be tested by the following two specific aims: 1) determine the role of TGF-β signaling in Ifitm5(c.-14C>T) mediated abnormal osteoblast differentiation and 2) determine the effect of IFITM5(c.-14C>T) targeting ASO therapy on osteoblast differentiation. By assessing these aims, we will gain important insight into the pathogenesis of OI-V that has the potential to directly impact preclinical development and testing of novel OI-V therapies.
