Hypermineralization and High Osteocyte Lacunar Density in Osteogenesis Imperfecta Type V Bone Indicate Exuberant Primary Bone Formation

Abstract

In contrast to “classical” forms of Osteogenesis imperfecta (OI) types I to IV, caused by a mutation in COL1A1/A2, OI type V is due to a gain-of-function mutation in the IFITM5 gene, encoding the interferon-induced transmembrane protein 5, or bone-restricted ifitm-like protein (BRIL). Its phenotype distinctly differs from OI types I to IV by absence of blue sclerae and dentinogenesis imperfecta, by the occurrence of ossification disorders like hyperplastic callus and forearm interosseous membrane ossification. Little is known about the impact of the mutation on bone tissue/material level in untreated and bisphosphonate treated patients. Therefore investigations of transiliac bone biopsy samples from a cohort of OI type V children (n=15, 8.7±4 years old) untreated at baseline and a subset (n=8) after pamidronate treatment (2.6 years in average) were performed. Quantitative backscattered electron imaging (qBEI) was used to determine bone mineralization density distribution (BMDD) as well as osteocyte lacunar density. The BMDD of type V OI bone was distinctly shifted towards higher degree of mineralization. The most frequently occurring calcium concentration (CaPeak) in cortical (Ct) and cancellous (Cn) bone was markedly increased (+11.5%, +10.4%, respectively, P<0.0001) compared to healthy reference values. Treatment with pamidronate resulted in only a slight enhancement of mineralization. The osteocyte lacunar density derived from sectioned bone area was elevated in OI type V Ct and Cn bone (+171%, P<0.0001 and +183.3%, P<0.01, respectively) versus controls. The high osteocyte density was associated with an overall immature primary bone structure (”mesh-like”) as visualized by polarized light microscopy. In summary the bone material from OI type V patients is hypermineralized, similarly to other forms of OI. The elevated osteocyte lacunar density in connection with lack of regular bone lamellation points to an exuberant primary bone formation and an alteration of the bone remodeling process in OI type V.

Bone biology: insights from osteogenesis imperfecta and related rare fragility syndromes

“…Primary osteoblasts from OI type V bone biopsies show high expression of osteoblastic markers and increased mineralization in the presence of decreased type I collagen expression, secretion, and matrix incorporation [36]. These data are consistent with the hypermineralized bone matrix and high osteocyte lacunar density in immature bone recently described in iliac crest bone samples from affected children [37]. These observations, together with low trabecular bone volume and bone formation rate [28], point to a peculiar characteristic of this OI form, in which exuberant primary bone formation is associated with an osteoporotic phenotype [4].…”

Melorheostotic Bone Lesions Caused by Somatic Mutations in MAP2K1 Have Deteriorated Microarchitecture and Periosteal Reaction

“…In fact, periosteal cells can lay down either highly organized and lowly mineralized lamellar bone as observed in the melorheostotic lesions or highly disorganized and highly mineralized woven bone. The latter is formed when a bony scaffold has to be quickly formed during fetal development, postnatal growth spurt, in callus during fracture healing, or after osteotomy . In contrast, periosteal lamellar bone deposition occurs much more slowly, in agreement with a progression of melorheostotic lesions over years .…”

“…Osteocyte lacunae were evaluated in all samples using qBEI analysis and thus investigated parameters refer to 2D sections through the 3D lacunae. The OLS size and shape were analyzed with a custom‐made macro in ImageJ as previously described . Six to 12 qBEI scans with 130× nominal magnification (0.88 μm/pixel) were performed throughout the section to characterize the OLS by five parameters: OLS‐density, the number of OLS per mineralized bone matrix area; OLS‐porosity, OLS total area/(mineralized bone matrix area + OLS total area); OLS‐area, mean value of the OLS areas per sample; OLS‐perimeter, mean value of the OLS perimeters per sample; and OLS‐AR, mean value of the OLS aspect ratio (AR) per sample.…”

Biomechanical and Bone Material Properties of Schnurri‐3 Null Mice

“…Additionally, the calibrated qBEI images were also analyzed for 2D‐information about the osteocyte lacunae sections (OLS) characteristics in the cortical bone area . For this purpose, qBEI images were transformed to binary images using a threshold based on a fixed gray level (corresponding to 5.2 wt% Ca).…”

“…For this purpose, qBEI images were transformed to binary images using a threshold based on a fixed gray level (corresponding to 5.2 wt% Ca). The OLS were extracted using a minimum and maximum size threshold of 5 μm 2 and 80 μm 2 , and analyzed based on a custom‐made macro in ImageJ software (version 1.50f; NIH, Bethesda, MD, USA) as described previously . The calculated parameters included …”