Objectives: Osteogenesis imperfecta (OI) is an extremely heterogeneous connective tissue disorder characterized by low bone mass, which together with altered bone matrix properties leads to skeletal fragility. Due to the wide range of symptoms, the pathophysiology of the OI is not fully understood. Null mutations in SERPINF1, encoding the potent antiangiogenic factor PEDF, cause type VI OI with excessive osteoid formation, abnormal osteoblast-osteocyte development and increased matrix mineralization. A very severe OI case has been reported recently with reduced PEDF secretion by osteoblasts, similar to OI type VI, but with normal PEDF serum levels. This atypical type VI OI was caused by a loss-of-function (p.S40L) in IFITM5the causative gene for type V OI. Six cases have been reported since then but it is unclear how the mutation impacts bone phenotype. To achieve further insights into the bone material properties, vascularization and thus pathophysiology of atypical type VI OI, we investigated a new knock-in (KI) mouse model carrying a leucine substitution for the BRIL p.Serine42 residue.
Methods: We analyzed longitudinal sections of distal femurs of 8 weeks-old heterozygous male mutants (KI, n=10) and wild-types (WT, n=9) using quantitative backscattered electron imaging (qBEI) performed with a scanning electron microscope (DSM962, Zeiss). Bone mineralization density distribution (BMDD) was measured in cancellous metaphyseal bone and midshaft cortical bone. The qBEI images were used to evaluate the osteocyte lacunae sections (OLS) in cortical bone and the structural histomorphometric parameters in cancellous bone. We used X-ray microcomputed tomography(micro-CT) to evaluate vascularization in the femoral third trochanter.
Results: qBEI revealed that bone matrix mineralization was significantly increased in KI compared to WT cancellous (CaPeak: +2.38%, P=0.0331) and cortical bone (CaPeak: +2.81%, P=0.0085; CaMean: +2.48%, P=0.0023; CaWidth: +11.24%, P<0.0001, CaHigh: +51%, P= 0.0027). We further observed in KI mice an increased OLS density (+23.11%, P<0.0001) and decreased OLS mean area and perimeter (-20.25%, P<0.0001; -13%, P<0.0001, respectively) versus WT. Histomorphometry revealed no changes of mineralized BV/TV, BS/TV, Tb.N and Tb.Th between the two genotypes. Micro-CT analyzes yielded increased pore volume/bone volume in KI (+14.28%, P=0.044) mirroring increased vascularity.
Conclusion: Our new mouse model for atypical type VI OI has elevated bone matrix mineralization as in other forms of OI. The increased bone vascular volume is consistent with defective PEDF secretion in bone as reported in affected patients. Further analysis of osteoblasts function and osteoid formation will provide additional insights in atypical OI type VI.
19 - 21 Sep 2019
European Society for Paediatric Endocrinology