Multiscale, converging defects of macro-porosity, microstructure and matrix mineralization impact long bone fragility in NF1.
Bone fragility due to osteopenia, osteoporosis or debilitating focal skeletal dysplasias is a frequent observation in the Mendelian disease Neurofibromatosis type 1 (NF1). To determine the mechanisms underlying bone fragility in NF1 we analyzed two conditional mouse models, Nf1Prx1 (limb knock-out) and Nf1Col1 (osteoblast specific knock-out), as well as cortical bone samples from individuals with NF1. We examined mouse bone tissue with micro-computed tomography, qualitative and quantitative histology, mechanical tensile analysis, small-angle X-ray scattering (SAXS), energy dispersive X-ray spectroscopy (EDX), and scanning acoustic microscopy (SAM). In cortical bone of Nf1Prx1 mice we detected ectopic blood vessels that were associated with diaphyseal mineralization defects. Defective mineral binding in the proximity of blood vessels was most likely due to impaired bone collagen formation, as these areas were completely devoid of acidic matrix proteins and contained thin collagen fibers. Additionally, we found significantly reduced mechanical strength of the bone material, which was partially caused by increased osteocyte volume. Consistent with these observations, bone samples from individuals with NF1 and tibial dysplasia showed increased osteocyte lacuna volume. Reduced mechanical properties were associated with diminished matrix stiffness, as determined by SAM. In line with these observations, bone tissue from individuals with NF1 and tibial dysplasia showed heterogeneous mineralization and reduced collagen fiber thickness and packaging. Collectively, the data indicate that bone fragility in NF1 tibial dysplasia is partly due to an increased osteocyte-related micro-porosity, hypomineralization, a generalized defect of organic matrix formation, exacerbated in the regions of tensional and bending force integration, and finally persistence of ectopic blood vessels associated with localized macro-porotic bone lesions.
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany ; FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Jirko
J
Kühnisch
Department of Biomaterials, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany ; Department of Chemistry, Universität Konstanz, Konstanz, Germany.
Jong
J
Seto
Department of Biomaterials, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany ; Institut für Physiologische Chemie, MTZ, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Claudia
C
Lange
Julius Wolff Institute & Brandenburg School of Regenerative Therapies, Charité - Universitätsmedizin Berlin, Berlin, Germany.
Susanne
S
Schrof
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany.
Sabine
S
Stumpp
FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Karolina
K
Kobus
FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Julia
J
Grohmann
FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Nadine
N
Kossler
Julius Wolff Institute & Brandenburg School of Regenerative Therapies, Charité - Universitätsmedizin Berlin, Berlin, Germany.
Peter
P
Varga
FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Monika
M
Osswald
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany ; FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Denise
D
Emmerich
Division für Humangenetik, Medizinische Universität Innsbruck, Innsbruck, Austria ; Institut für Klinische Genetik, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Sigrid
S
Tinschert
Klinik für Orthopädie, Medizinische Fakultät Carl Gustav Carus, Technische Universität Dresden, Dresden, Germany.
Falk
F
Thielemann
Julius Wolff Institute & Brandenburg School of Regenerative Therapies, Charité - Universitätsmedizin Berlin, Berlin, Germany ; Berlin-Brandenburg Center for Regenerative Therapies, Berlin, Germany.
Georg
G
Duda
Institute for Vegetative Anatomy, Charité - Universitätsmedizin Berlin, Berlin, Germany.
Wenke
W
Seifert
Laboratory of Experimental Trauma Surgery Giessen, Justus-Liebig University Giessen, Giessen, Germany.
Thaqif
T
El Khassawna
University of Utah, Department of Pediatrics, Division of Medical Genetics, Salt Lake City, Utah, United States of America.
David A
DA
Stevenson
Department of Medicine, Pharmacology and Cancer Biology, Center for Bone Biology, Vanderbilt University - Medical Center, Nashville, Tennessee, United States of America.
Florent
F
Elefteriou
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany ; FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Uwe
U
Kornak
Julius Wolff Institute & Brandenburg School of Regenerative Therapies, Charité - Universitätsmedizin Berlin, Berlin, Germany.
Kay
K
Raum
Department of Biomaterials, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany ; Berlin-Brandenburg Center for Regenerative Therapies, Berlin, Germany.
Peter
P
Fratzl
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany ; FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany ; Berlin-Brandenburg Center for Regenerative Therapies, Berlin, Germany.
Stefan
S
Mundlos
Institute for Medical Genetics and Human Genetics, Charité - Universitätsmedizin Berlin, Berlin, Germany ; FG Development & Disease, Max Planck Institute for Molecular Genetics, Berlin, Germany.
Mateusz
M
Kolanczyk
D000818:Animals; D001696:Biomechanical Phenomena; D001808:Blood Vessels / Q000473:pathology; D015519:Bone Density; D001857:Bone Matrix / Q000473:pathology* / Q000503:physiopathology*; D001842:Bone and Bones / Q000098:blood supply / Q000473:pathology* / Q000503:physiopathology*; D002113:Calcification, Physiologic; D003094:Collagen / Q000378:metabolism; D018483:Diaphyses / Q000098:blood supply / Q000378:metabolism / Q000473:pathology; D018398:Homeodomain Proteins / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D009456:Neurofibromatosis 1 / Q000473:pathology* / Q000503:physiopathology*; D025542:Neurofibromin 1 / Q000172:deficiency / Q000378:metabolism; D010011:Osteocytes / Q000378:metabolism / Q000473:pathology; D016062:Porosity; D013977:Tibia / Q000473:pathology / Q000503:physiopathology
D018398:Homeodomain Proteins; D025542:Neurofibromin 1; C489936:Prrx1 protein, mouse; D003094:Collagen