Background and methods: Pigment epithelium-derived factor (PEDF) is a potent antiangiogenic factor, ubiquitously expressed and secreted in human tissues. Hypertrophic cartilage and osteoblasts express PEDF that binds to type I collagen and glycosaminoglycans in the extracellular matrix. Two rare forms of osteogenesis imperfecta (OI) with intact collagen synthesis are associated with PEDF deficiency. Histological observations revealed excessive osteoid formation and prolonged mineralization lag time, suggesting a distinctive mineralization defect. The goal of this study was to characterize the mineralization pattern in bone with deficient PEDF secretion.
We used quantitative backscattered electron imaging (qBEI) to assess bone mineralization density distribution in transiliac biopsies from nine children with SERPINF1 mutations causing PEDF loss-of-function (OI type VI)* and in two biopsies (obtained at the ages of 7 and 25 years) from a patient with a novel mutation in IFITM5 (p.S40L substitution, atypical type VI OI) encoding BRIL, a transmembrane protein enriched in osteoblasts during mineralization. This patient has normal PEDF serum level but severely decreased PEDF secretion in osteoblasts. We further characterized bone tissue by high-resolution backscattered electron imaging (hrBEI) and measured thickness, shape and arrangement of mineral particles by synchrotron small-angle X-ray scattering (SAXS) in a subset of patients with SERPINF1 mutations.
Results: qBEI revealed hypermineralized bone matrix in all cases, with a similarly increased typical calcium content, coexisting with areas having unusually lower mineral content than controls and OI patients with collagen-gene mutations. In bone with the IFITM5 mutation we found in the childhood biopsy accumulation of highly mineralized non-remodeled bone. In the young adult biopsy, most bone appears remodeled although average mineral content was still highly increased.
HrBEI showed high density of oddly shaped lacunae from young osteocytes with abnormal collagen fibril organization in the perilacunar regions. SAXS revealed smaller mineral particles with a less ordered arrangement than age-matched controls and OI with collagen-gene mutations.
In conclusion, our data suggest that local deficiency of PEDF within the bone matrix impairs the early steps of mineralization at the onset of osteoblastosteocyte differentiation, resulting in the previously undescribed occurrence of abnormally highly mineralized bone matrix coexisting with extended regions of poorly mineralized bone.
*BONE, in press
Disclosure: The authors declared no competing interests.
27 - 30 Jun 2015