Distinct Clinical and Pathological Features of Melorheostosis Associated With Somatic MAP2K1 Mutations

Melorheostosis is a rare hyperostotic disease of the long bones classically characterized by a "dripping candle-wax" radiographic appearance. We recently described somatic activating mutations in MAP2K1 as a cause of melorheostosis. Here, we report distinguishing characteristics of patients with MAP2K1-positive melorheostosis. Fifteen unrelated patients with radiographic appearance of melorheostosis underwent paired biopsies of affected and unaffected bone for whole-exome sequencing, histology, and cell culture. Eight patients with mutations in MAP2K1 in affected bone were compared to the seven MAP2K1-negative patients to identify distinguishing characteristics. Patients with MAP2K1-positive melorheostosis had a distinct phenotype with classic "dripping candle-wax" appearance on radiographs (p = 0.01), characteristic vascular lesions on skin overlying affected bone (p = 0.01), and higher prevalence of extraosseous mineralization and joint involvement (p = 0.04 for both). Melorheostotic bone from both MAP2K1-positive and MAP2K1-negative patients showed two zones of distinct morphology-an outer segment of parallel layers of primary lamellar bone and a deeper zone of intensely remodeled highly porous osteonal-like bone. Affected bone from MAP2K1-positive patients showed excessive osteoid (p = 0.0012), increased number of osteoblasts (p = 0.012) and osteoclasts (p = 0.04), and increased vascularity on histology in comparison to paired unaffected bone which was not seen in affected bone in most MAP2K1-negative patients. The identification of a distinct phenotype of patients with MAP2K1-positive melorheostosis demonstrates clinical and genetic heterogeneity among patients with the disease. Further studies are needed to better understand the underlying pathophysiology and associated skin findings. © 2018 American Society for Bone and Mineral Research.

Cortical bone properties in the Brtl/+ mouse model of Osteogenesis imperfecta as evidenced by acoustic transmission microscopy

Higher skeletal fragility has been established for the Brtl/+ mouse model of osteogenesis imperfecta at the whole bone level, but previous investigations of mechanical properties at the bone material level were inconclusive. Bone material was analyzed separately at endosteal (ER) and periosteal regions (PR) on transverse femoral midshaft sections for 2-month old mice (wild-type n = 6; Brtl/+ n = 6). Quantitative backscattered electron imaging revealed that the mass density computed from mineral density maps was higher in PR than in ER for both wild-type (+2.1%, p < 0.05) and Brtl/+ mice (+1.8%, p < 0.05). Electron induced X-ray fluorescence analysis indicated significantly lower atomic Ca/P ratios and higher Na/Ca, Mg/Ca and K/Ca ratios in PR bone compared to ER independently of genotype. Second harmonic generation microscopy indicated that the occurrence of periodically alternating collagen orientation in ER of Brtl/+ mice was strongly reduced compared to wild-type mice. Scanning acoustic microscopy in time of flight mode revealed that the sound velocity and Young's modulus (estimated based on sound velocity and mass density maps) were significantly greater in PR (respectively +6% and +15%) compared to ER in wild-type mice but not in Brtl/+ mice. ER sound velocity and Young's modulus were significantly increased in Brtl/+ mice (+9.4% and +22%, respectively) compared to wild-type mice. These data demonstrate that the Col1a1 G349C mutation in Brtl/+ mice affects the mechanical behavior of bone material predominantly in the endosteal region by altering the collagen orientation.

Longitudinal growth curves for children with classical osteogenesis imperfecta (types III and IV) caused by structural pathogenic variants in type I collagen

PURPOSE

Growth deficiency is a cardinal feature of osteogenesis imperfecta (OI) types III and IV, caused by pathogenic variants in type I collagen. OI-specific longitudinal growth charts are needed for patient care.

METHODS

We compiled longitudinal length, weight, head circumference, and body mass index (BMI) data from 100 children with types III and IV OI and known type I collagen pathogenic variants. Effects of gender, OI type, and pathogenic variant were examined using multilevel modeling. OI-specific centile curves were constructed using generalized additive model for location, scale, and shape (GAMLSS).

RESULTS

OI type and gender, but not the specific mutated collagen gene, significantly affect stature, but only OI type affects weight. Head circumference was not significantly different by gender, type, or mutated gene. In both genders, length curves for types III and IV OI overlap and the type IV 95th centile curve overlaps the lower US Centers for Disease Control and Prevention (CDC) curves for the general population. A pubertal growth spurt is generally absent or blunted in types III/IV OI. The body mass index 50th and 95th centile curves are distinctly shifted above respective US CDC curves in both genders.

CONCLUSIONS

OI type is a stronger contributing factor than gender for OI growth, while curves do not differ for COL1A1 versus COL1A2 pathogenic variants. Types III and IV OI-specific growth curves are presented.

Low Dose of Bisphosphonate Enhances Sclerostin Antibody-Induced Trabecular Bone Mass Gains in Brtl/+ Osteogenesis Imperfecta Mouse Model

Osteogenesis imperfecta (OI) is a genetic disorder characterized by altered bone quality and imbalanced bone remodeling, leading to skeletal fractures that are most prominent during childhood. Treatments for OI have focused on restoring pediatric bone density and architecture to recover functional strength and consequently reduce fragility. Though antiresorptive agents like bisphosphonates (BPs) are currently the most common intervention for the treatment of OI, a number of studies have shown efficacy of sclerostin antibody (SclAb) in inducing gains in bone mass and reducing fragility in OI mouse models. In this study, the effects of the concurrent use of BP and SclAb were evaluated during bone growth in a mouse harboring an OI-causing Gly→Cys mutation on col1a1. A single dose of antiresorptive BP facilitated the anabolic action of SclAb by increasing availability of surfaces for new bone formation via retention of primary trabeculae that would otherwise be remodeled. Chronic effects of concurrent administration of BP and SclAb revealed that accumulating cycles conferred synergistic gains in trabecular mass and vertebral stiffness, suggesting a distinct advantage of both therapies combined. Cortical gains in mass and strength occurred through SclAb alone, independent of presence of BP. In conclusion, these preclinical results support the scientific hypothesis that minimal antiresorptive treatment can amplify the effects of SclAb during early stages of skeletal growth to further improve bone structure and rigidity, a beneficial outcome for children with OI. © 2018 American Society for Bone and Mineral Research.

Somatic activating mutations in MAP2K1 cause melorheostosis

Melorheostosis is a sporadic disease of uncertain etiology characterized by asymmetric bone overgrowth and functional impairment. Using whole exome sequencing, we identify somatic mosaic MAP2K1 mutations in affected, but not unaffected, bone of eight unrelated patients with melorheostosis. The activating mutations (Q56P, K57E and K57N) cluster tightly in the MEK1 negative regulatory domain. Affected bone displays a mosaic pattern of increased p-ERK1/2 in osteoblast immunohistochemistry. Osteoblasts cultured from affected bone comprise two populations with distinct p-ERK1/2 levels by flow cytometry, enhanced ERK1/2 activation, and increased cell proliferation. However, these MAP2K1 mutations inhibit BMP2-mediated osteoblast mineralization and differentiation in vitro, underlying the markedly increased osteoid detected in affected bone histology. Mosaicism is also detected in the skin overlying bone lesions in four of five patients tested. Our data show that the MAP2K1 oncogene is important in human bone formation and implicate MEK1 inhibition as a potential treatment avenue for melorheostosis.

4-PBA ameliorates cellular homeostasis in fibroblasts from osteogenesis imperfecta patients by enhancing autophagy and stimulating protein secretion

The clinical phenotype in osteogenesis imperfecta (OI) is attributed to the dominant negative function of mutant type I collagen molecules in the extracellular matrix, by altering its structure and function. Intracellular retention of mutant collagen has also been reported, but its effect on cellular homeostasis is less characterized. Using OI patient fibroblasts carrying mutations in the α1(I) and α2(I) chains we demonstrate that retained collagen molecules are responsible for endoplasmic reticulum (ER) enlargement and activation of the unfolded protein response (UPR) mainly through the eukaryotic translation initiation factor 2 alpha kinase 3 (PERK) branch. Cells carrying α1(I) mutations upregulate autophagy, while cells with α2(I) mutations only occasionally activate the autodegradative response. Despite the autophagy activation to face stress conditions, apoptosis occurs in all mutant fibroblasts. To reduce cellular stress, mutant fibroblasts were treated with the FDA-approved chemical chaperone 4-phenylbutyric acid. The drug rescues cell death by modulating UPR activation thanks to both its chaperone and histone deacetylase inhibitor abilities. As chaperone it increases general cellular protein secretion in all patients' cells as well as collagen secretion in cells with the most C-terminal mutation. As histone deacetylase inhibitor it enhances the expression of the autophagic gene Atg5 with a consequent stimulation of autophagy. These results demonstrate that the cellular response to ER stress can be a relevant target to ameliorate OI cell homeostasis.

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

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 interferon-inducible transmembrane (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 such as 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 toward a 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; +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, similar 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. © 2017 American Society for Bone and Mineral Research.

Cyclophilin B Deficiency Causes Abnormal Dentin Collagen Matrix

Cyclophilin B (CypB) is an endoplasmic reticulum-resident protein that regulates collagen folding, and also contributes to prolyl 3-hydroxylation (P3H) and lysine (Lys) hydroxylation of collagen. In this study, we characterized dentin type I collagen in CypB null (KO) mice, a model of recessive osteogenesis imperfecta type IX, and compared to those of wild-type (WT) and heterozygous (Het) mice. Mass spectrometric analysis demonstrated that the extent of P3H in KO collagen was significantly diminished compared to WT/Het. Lys hydroxylation in KO was significantly diminished at the helical cross-linking sites, α1/α2(I) Lys-87 and α1(I) Lys-930, leading to a significant increase in the under-hydroxylated cross-links and a decrease in fully hydroxylated cross-links. The extent of glycosylation of hydroxylysine residues was, except α1(I) Lys-87, generally higher in KO than WT/Het. Some of these molecular phenotypes were distinct from other KO tissues reported previously, indicating the dentin-specific control mechanism through CypB. Histological analysis revealed that the width of predentin was greater and irregular, and collagen fibrils were sparse and significantly smaller in KO than WT/Het. These results indicate a critical role of CypB in dentin matrix formation, suggesting a possible association between recessive osteogenesis imperfecta and dentin defects that have not been clinically detected.

P4HA1 mutations cause a unique congenital disorder of connective tissue involving tendon, bone, muscle and the eye

Collagen prolyl 4-hydroxylases (C-P4Hs) play a central role in the formation and stabilization of the triple helical domain of collagens. P4HA1 encodes the catalytic α(I) subunit of the main C-P4H isoenzyme (C-P4H-I). We now report human bi-allelic P4HA1 mutations in a family with a congenital-onset disorder of connective tissue, manifesting as early-onset joint hypermobility, joint contractures, muscle weakness and bone dysplasia as well as high myopia, with evidence of clinical improvement of motor function over time in the surviving patient. Similar to P4ha1 null mice, which die prenatally, the muscle tissue from P1 and P2 was found to have reduced collagen IV immunoreactivity at the muscle basement membrane. Patients were compound heterozygous for frameshift and splice site mutations leading to reduced, but not absent, P4HA1 protein level and C-P4H activity in dermal fibroblasts compared to age-matched control samples. Differential scanning calorimetry revealed reduced thermal stability of collagen in patient-derived dermal fibroblasts versus age-matched control samples. Mutations affecting the family of C-P4Hs, and in particular C-P4H-I, should be considered in patients presenting with congenital connective tissue/myopathy overlap disorders with joint hypermobility, contractures, mild skeletal dysplasia and high myopia.

Phenotypic Spectrum in Osteogenesis Imperfecta Due to Mutations in TMEM38B: Unraveling a Complex Cellular Defect

CONTEXT

Recessive mutations in TMEM38B cause type XIV osteogenesis imperfecta (OI) by dysregulating intracellular calcium flux.

OBJECTIVES

Clinical and bone material phenotype description and osteoblast differentiation studies.

DESIGN AND SETTING

Natural history study in pediatric research centers.

PATIENTS

Eight patients with type XIV OI.

MAIN OUTCOME MEASURES

Clinical examinations included bone mineral density, radiographs, echocardiography, and muscle biopsy. Bone biopsy samples (n = 3) were analyzed using histomorphometry, quantitative backscattered electron microscopy, and Raman microspectroscopy. Cellular differentiation studies were performed on proband and control osteoblasts and normal murine osteoclasts.

RESULTS

Type XIV OI clinical phenotype ranges from asymptomatic to severe. Previously unreported features include vertebral fractures, periosteal cloaking, coxa vara, and extraskeletal features (muscular hypotonia, cardiac abnormalities). Proband lumbar spine bone density z score was reduced [median -3.3 (range -4.77 to +0.1; n = 7)] and increased by +1.7 (1.17 to 3.0; n = 3) following bisphosphonate therapy. TMEM38B mutant bone has reduced trabecular bone volume, osteoblast, and particularly osteoclast numbers, with >80% reduction in bone resorption. Bone matrix mineralization is normal and nanoporosity low. We demonstrate a complex osteoblast differentiation defect with decreased expression of early markers and increased expression of late and mineralization-related markers. Predominance of trimeric intracellular cation channel type B over type A expression in murine osteoclasts supports an intrinsic osteoclast defect underlying low bone turnover.

CONCLUSIONS

OI type XIV has a bone histology, matrix mineralization, and osteoblast differentiation pattern that is distinct from OI with collagen defects. Probands are responsive to bisphosphonates and some show muscular and cardiovascular features possibly related to intracellular calcium flux abnormalities.

Osteogenesis imperfecta: new genes reveal novel mechanisms in bone dysplasia

Osteogenesis imperfecta (OI) is a skeletal dysplasia characterized by fragile bones and short stature and known for its clinical and genetic heterogeneity which is now understood as a collagen-related disorder. During the last decade, research has made remarkable progress in identifying new OI-causing genes and beginning to understand the intertwined molecular and biochemical mechanisms of their gene products. Most cases of OI have dominant inheritance. Each new gene for recessive OI, and a recently identified gene for X-linked OI, has shed new light on its (often previously unsuspected) function in bone biology. Here, we summarize the literature that has contributed to our current understanding of the pathogenesis of OI.

Single dose of bisphosphonate preserves gains in bone mass following cessation of sclerostin antibody in Brtl/+ osteogenesis imperfecta model

Sclerostin antibody has demonstrated a bone-forming effect in pre-clinical models of osteogenesis imperfecta, where mutations in collagen or collagen-associated proteins often result in high bone fragility in pediatric patients. Cessation studies in osteoporotic patients have demonstrated that sclerostin antibody, like intermittent PTH treatment, requires sequential anti-resorptive therapy to preserve the anabolic effects in adult populations. However, the persistence of anabolic gains from either drug has not been explored clinically in OI, or in any animal model. To determine whether cessation of sclerostin antibody therapy in a growing OI skeleton requires sequential anti-resorptive treatment to preserve anabolic gains in bone mass, we treated 3week old Brtl/+ and wild type mice for 5weeks with SclAb, and then withdrew treatment for an additional 6weeks. Trabecular bone loss was evident following cessation, but was preserved in a dose-dependent manner with single administration of pamidronate at the time of cessation. In vivo longitudinal near-infrared optical imaging of cathepsin K activation in the proximal tibia suggests an anti-resorptive effect of both SclAb and pamidronate which is reversed after three weeks of cessation. Cortical bone was considerably less susceptible to cessation effects, and showed no structural or functional deficits in the absence of pamidronate during this cessation period. In conclusion, while SclAb induces a considerable anabolic gain in the rapidly growing Brtl/+ murine model of OI, a single sequential dose of antiresorptive drug is required to maintain bone mass at trabecular sites for 6weeks following cessation.

Non-Lethal Type VIII Osteogenesis Imperfecta Has Elevated Bone Matrix Mineralization

CONTEXT

Type VIII osteogenesis imperfecta (OI; OMIM 601915) is a recessive form of lethal or severe OI caused by null mutations in P3H1, which encodes prolyl 3-hydroxylase 1.

OBJECTIVES

Clinical and bone material description of non-lethal type VIII OI.

DESIGN

Natural history study of type VIII OI.

SETTING

Pediatric academic research centers.

PATIENTS

Five patients with non-lethal type VIII OI, and one patient with lethal type VIII OI.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Clinical examinations included bone mineral density, radiographs, and serum and urinary metabolites. Bone biopsy samples were analyzed for histomorphometry and bone mineral density distribution by quantitative backscattered electron imaging microscopy. Collagen biochemistry was examined by mass spectrometry, and collagen fibrils were examined by transmission electron microscopy.

RESULTS

Type VIII OI patients have extreme growth deficiency, an L1-L4 areal bone mineral density Z-score of -5 to -6, and normal bone formation markers. Collagen from bone and skin tissue and cultured osteoblasts and fibroblasts have nearly absent 3-hydroxylation (1-4%). Collagen fibrils showed abnormal diameters and irregular borders. Bone histomorphometry revealed decreased cortical width and very thin trabeculae with patches of increased osteoid, although the overall osteoid surface was normal. Quantitative backscattered electron imaging showed increased matrix mineralization of cortical and trabecular bone, typical of other OI types. However, the proportion of bone with low mineralization was increased in type VIII OI bone, compared to type VII OI.

CONCLUSIONS

P3H1 is the unique enzyme responsible for collagen 3-hydroxylation in skin and bone. Bone from non-lethal type VIII OI children is similar to type VII, especially bone matrix hypermineralization, but it has distinctive features including extremely thin trabeculae, focal osteoid accumulation, and an increased proportion of low mineralized bone.

PrP-containing aggresomes are cytosolic components of an ER quality control mechanism

Limited detoxification capacity often directs aggregation-prone, potentially hazardous, misfolded proteins to be deposited in designated cytosolic compartments known as 'aggresomes'. The roles of aggresomes as cellular quality control centers, and the cellular origin of the deposits contained within these structures, remain to be characterized. Here, we utilized the observation that the prion protein (PrP, also known as PRNP) accumulates in aggresomes following the inhibition of folding chaperones, members of the cyclophilin family, to address these questions. We found that misfolded PrP molecules must pass through the endoplasmic reticulum (ER) in order to be deposited in aggresomes, that the Golgi plays no role in this process and that cytosolic PrP species are not deposited in pre-existing aggresomes. Prior to their deposition in the aggresome, PrP molecules lose the ER localization signal and have to acquire a GPI anchor. Our discoveries indicate that PrP aggresomes are cytosolic overflow deposition centers for the ER quality control mechanisms and highlight the importance of these structures for the maintenance of protein homeostasis within the ER.

MBTPS2 mutations cause defective regulated intramembrane proteolysis in X-linked osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a collagen-related bone dysplasia. We identified an X-linked recessive form of OI caused by defects in MBTPS2, which encodes site-2 metalloprotease (S2P). MBTPS2 missense mutations in two independent kindreds with moderate/severe OI cause substitutions at highly conserved S2P residues. Mutant S2P has normal stability, but impaired functioning in regulated intramembrane proteolysis (RIP) of OASIS, ATF6 and SREBP transcription factors, consistent with decreased proband secretion of type I collagen. Further, hydroxylation of the collagen lysine residue (K87) critical for crosslinking is reduced in proband bone tissue, consistent with decreased lysyl hydroxylase 1 in proband osteoblasts. Reduced collagen crosslinks presumptively undermine bone strength. Also, proband osteoblasts have broadly defective differentiation. These mutations provide evidence that RIP plays a fundamental role in normal bone development.

Osteogenesis imperfecta (OI) is genetically linked to autosomal dominant or autosomal recessive mutations. Here, Marini et al. describe two families with X-chromosome-linked OI with mutations in MBTPS2 that alter regulated intramembrane proteolysis and subsequent defects in collagen crosslinking and osteoblast function.

Non-surgical alternatives to invasive procedures in mice

We have developed and validated catheterization protocols in mice that allow for simultaneous infusion and sampling. A sampling catheter was inserted in the lateral vein of the tail, while the animals were infused either intravenously or intragastrically through a second catheter placed in the contralateral lateral vein or via an intragastric catheter, respectively. The applicability of these methods of infusion and blood sampling were validated by conducting urea kinetics utilizing stable isotopes. These non-surgical procedures are non-invasive, inexpensive, fast to perform and animals do not require a recovery period before their use.

Bone mineral properties in growing Col1a2(+/G610C) mice, an animal model of osteogenesis imperfecta

The Col1a2(+/G610C) knock-in mouse, models osteogenesis imperfecta in a large old order Amish family (OOA) with type IV OI, caused by a G-to-T transversion at nucleotide 2098, which alters the gly-610 codon in the triple-helical domain of the α2(I) chain of type I collagen. Mineral and matrix properties of the long bones and vertebrae of male Col1a2(+/G610C) and their wild-type controls (Col1a2(+/+)), were characterized to gain insight into the role of α2-chain collagen mutations in mineralization. Additionally, we examined the rescuability of the composition by sclerostin inhibition initiated by crossing Col1a2(+/G610C) with an LRP(+/A214V) high bone mass allele. At age 10-days, vertebrae and tibia showed few alterations by micro-CT or Fourier transform infrared imaging (FTIRI). At 2-months-of-age, Col1a2(+/G610C) tibias had 13% fewer secondary trabeculae than Col1a2(+/+), these were thinner (11%) and more widely spaced (20%) than those of Col1a2(+/+) mice. Vertebrae of Col1a2(+/G610C) mice at 2-months also had lower bone volume fraction (38%), trabecular number (13%), thickness (13%) and connectivity density (32%) compared to Col1(a2+/+). The cortical bone of Col1a2(+/G610C) tibias at 2-months had 3% higher tissue mineral density compared to Col1a2(+/+); Col1a2(+/G610C) vertebrae had lower cortical thickness (29%), bone area (37%) and polar moment of inertia (38%) relative to Col1a2(+/+). FTIRI analysis, which provides information on bone chemical composition at ~7μm-spatial resolution, showed tibias at 10-days did not differ between genotypes. Comparing identical bone types in Col1a2(+/G610C) to Col1a2(+/+) at 2-months-of-age, tibias showed higher mineral-to-matrix ratio in trabeculae (17%) and cortices (31%). and in vertebral cortices (28%). Collagen maturity was 42% higher at 10-days-of-age in Col1a2(+/G610C) vertebral trabeculae and in 2-month tibial cortices (12%), vertebral trabeculae (42%) and vertebral cortices (12%). Higher acid-phosphate substitution was noted in 10-day-old trabecular bone in vertebrae (31%) and in 2-month old trabecular bone in both tibia (31%) and vertebrae (4%). There was also a 16% lower carbonate-to-phosphate ratio in vertebral trabeculae and a correspondingly higher (22%) carbonate-to-phosphate ratio in 2month-old vertebral cortices. At age 3-months-of-age, male femurs with both a Col1a2(+/G610C) allele and a Lrp5 high bone mass allele (Lrp5+/A214V) showed an improvement in bone composition, presenting higher trabecular carbonate-to-phosphate ratio (18%) and lower trabecular and cortical acid-phosphate substitutions (8% and 18%, respectively). Together, these results indicate that mutant collagen α2(I) chain affects both bone quantity and composition, and the usefulness of this model for studies of potential OI therapies such as anti-sclerostin treatments.

Osteogenesis imperfecta

Osteogenesis imperfecta is a phenotypically and molecularly heterogeneous group of inherited connective tissue disorders that share similar skeletal abnormalities causing bone fragility and deformity. Previously, the disorder was thought to be an autosomal dominant bone dysplasia caused by defects in type I collagen, but in the past 10 years discoveries of novel (mainly recessive) causative genes have lent support to a predominantly collagen-related pathophysiology and have contributed to an improved understanding of normal bone development. Defects in proteins with very different functions, ranging from structural to enzymatic and from intracellular transport to chaperones, have been described in patients with osteogenesis imperfecta. Knowledge of the specific molecular basis of each form of the disorder will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches. In this Seminar, together with diagnosis, management, and treatment, we describe the defects causing osteogenesis imperfecta and their mechanism and interrelations, and classify them into five groups on the basis of the metabolic pathway compromised, specifically those related to collagen synthesis, structure, and processing; post-translational modification; folding and cross-linking; mineralisation; and osteoblast differentiation.

Cyclophilin-B Modulates Collagen Cross-linking by Differentially Affecting Lysine Hydroxylation in the Helical and Telopeptidyl Domains of Tendon Type I Collagen

Covalent intermolecular cross-linking provides collagen fibrils with stability. The cross-linking chemistry is tissue-specific and determined primarily by the state of lysine hydroxylation at specific sites. A recent study on cyclophilin B (CypB) null mice, a model of recessive osteogenesis imperfecta, demonstrated that lysine hydroxylation at the helical cross-linking site of bone type I collagen was diminished in these animals (Cabral, W. A., Perdivara, I., Weis, M., Terajima, M., Blissett, A. R., Chang, W., Perosky, J. E., Makareeva, E. N., Mertz, E. L., Leikin, S., Tomer, K. B., Kozloff, K. M., Eyre, D. R., Yamauchi, M., and Marini, J. C. (2014) PLoS Genet 10, e1004465). However, the extent of decrease appears to be tissue- and molecular site-specific, the mechanism of which is unknown. Here we report that although CypB deficiency resulted in lower lysine hydroxylation in the helical cross-linking sites, it was increased in the telopeptide cross-linking sites in tendon type I collagen. This resulted in a decrease in the lysine aldehyde-derived cross-links but generation of hydroxylysine aldehyde-derived cross-links. The latter were absent from the wild type and heterozygous mice. Glycosylation of hydroxylysine residues was moderately increased in the CypB null tendon. We found that CypB interacted with all lysyl hydroxylase isoforms (isoforms 1-3) and a putative lysyl hydroxylase-2 chaperone, 65-kDa FK506-binding protein. Tendon collagen in CypB null mice showed severe size and organizational abnormalities. The data indicate that CypB modulates collagen cross-linking by differentially affecting lysine hydroxylation in a site-specific manner, possibly via its interaction with lysyl hydroxylases and associated molecules. This study underscores the critical importance of collagen post-translational modifications in connective tissue formation.

Effect of anti-sclerostin therapy and osteogenesis imperfecta on tissue-level properties in growing and adult mice while controlling for tissue age

Bone composition and biomechanics at the tissue-level are important contributors to whole bone strength. Sclerostin antibody (Scl-Ab) is a candidate anabolic therapy for the treatment of osteoporosis that increases bone formation, bone mass, and bone strength in animal studies, but its effect on bone quality at the tissue-level has received little attention. Pre-clinical studies of Scl-Ab have recently expanded to include diseases with altered collagen and material properties such as osteogenesis imperfecta (OI). The purpose of this study was to investigate the role of Scl-Ab on bone quality by determining bone material composition and tissue-level mechanical properties in normal wild type (WT) tissue, as well as mice with a typical OI Gly➔Cys mutation (Brtl/+) in type I collagen. Rapidly growing (3-week-old) and adult (6-month-old) WT and Brtl/+ mice were treated for 5weeks with Scl-Ab. Fluorescent guided tissue-level bone composition analysis (Raman spectroscopy) and biomechanical testing (nanoindentation) were performed at multiple tissue ages. Scl-Ab increased mineral to matrix in adult WT and Brtl/+ at tissue ages of 2-4wks. However, no treatment related changes were observed in mineral to matrix levels at mid-cortex, and elastic modulus was not altered by Scl-Ab at any tissue age. Increased mineral-to-matrix was phenotypically observed in adult Brtl/+ OI mice (at tissue ages>3wks) and rapidly growing Brtl/+ (at tissue ages>4wks) mice compared to WT. At identical tissue ages defined by fluorescent labels, adult mice had generally lower mineral to matrix ratios and a greater elastic modulus than rapidly growing mice, demonstrating that bone matrix quality can be influenced by animal age and tissue age alike. In summary, these data suggest that Scl-Ab alters the matrix chemistry of newly formed bone while not affecting the elastic modulus, induces similar changes between Brtl/+ and WT mice, and provides new insight into the interaction between tissue age and animal age on bone quality.

Alzheimer's disease-causing proline substitutions lead to presenilin 1 aggregation and malfunction

Do different neurodegenerative maladies emanate from the failure of a mutual protein folding mechanism? We have addressed this question by comparing mutational patterns that are linked to the manifestation of distinct neurodegenerative disorders and identified similar neurodegeneration-linked proline substitutions in the prion protein and in presenilin 1 that underlie the development of a prion disorder and of familial Alzheimer's disease (fAD), respectively. These substitutions were found to prevent the endoplasmic reticulum (ER)-resident chaperone, cyclophilin B, from assisting presenilin 1 to fold properly, leading to its aggregation, deposition in the ER, reduction of γ-secretase activity, and impaired mitochondrial distribution and function. Similarly, reduced quantities of the processed, active presenilin 1 were observed in brains of cyclophilin B knockout mice. These discoveries imply that reduced cyclophilin activity contributes to the development of distinct neurodegenerative disorders, propose a novel mechanism for the development of certain fAD cases, and support the emerging theme that this disorder can stem from aberrant presenilin 1 function. This study also points at ER chaperones as targets for the development of counter-neurodegeneration therapies.

Altered cytoskeletal organization characterized lethal but not surviving Brtl+/- mice: insight on phenotypic variability in osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable bone disease with dominant and recessive transmission. It is characterized by a wide spectrum of clinical outcomes ranging from very mild to lethal in the perinatal period. The intra- and inter-familiar OI phenotypic variability in the presence of an identical molecular defect is still puzzling to the research field. We used the OI murine model Brtl(+/-) to investigate the molecular basis of OI phenotypic variability. Brtl(+/-) resembles classical dominant OI and shows either a moderately severe or a lethal outcome associated with the same Gly349Cys substitution in the α1 chain of type I collagen. A systems biology approach was used. We took advantage of proteomic pathway analysis to functionally link proteins differentially expressed in bone and skin of Brtl(+/-) mice with different outcomes to define possible phenotype modulators. The skin/bone and bone/skin hybrid networks highlighted three focal proteins: vimentin, stathmin and cofilin-1, belonging to or involved in cytoskeletal organization. Abnormal cytoskeleton was indeed demonstrated by immunohistochemistry to occur only in tissues from Brtl(+/-) lethal mice. The aberrant cytoskeleton affected osteoblast proliferation, collagen deposition, integrin and TGF-β signaling with impairment of bone structural properties. Finally, aberrant cytoskeletal assembly was detected in fibroblasts obtained from lethal, but not from non-lethal, OI patients carrying an identical glycine substitution. Our data demonstrated that compromised cytoskeletal assembly impaired both cell signaling and cellular trafficking in mutant lethal mice, altering bone properties. These results point to the cytoskeleton as a phenotypic modulator and potential novel target for OI treatment.

Rapidly growing Brtl/+ mouse model of osteogenesis imperfecta improves bone mass and strength with sclerostin antibody treatment

Osteogenesis imperfecta (OI) is a heritable collagen-related bone dysplasia, characterized by brittle bones with increased fracture risk that presents most severely in children. Anti-resorptive bisphosphonates are frequently used to treat pediatric OI and controlled clinical trials have shown that bisphosphonate therapy improves vertebral outcomes but has little benefit on long bone fracture rate. New treatments which increase bone mass throughout the pediatric OI skeleton would be beneficial. Sclerostin antibody (Scl-Ab) is a potential candidate anabolic therapy for pediatric OI and functions by stimulating osteoblastic bone formation via the canonical Wnt signaling pathway. To explore the effect of Scl-Ab on the rapidly growing OI skeleton, we treated rapidly growing 3week old Brtl/+ mice, harboring a typical heterozygous OI-causing Gly→Cys substitution on col1a1, for 5weeks with Scl-Ab. Scl-Ab had anabolic effects in Brtl/+ and led to new cortical bone formation and increased cortical bone mass. This anabolic action resulted in improved mechanical strength to WT Veh levels without altering the underlying brittle nature of the material. While Scl-Ab was anabolic in trabecular bone of the distal femur in both genotypes, the effect was less strong in these rapidly growing Brtl/+ mice compared to WT. In conclusion, Scl-Ab was able to stimulate bone formation in a rapidly growing Brtl/+ murine model of OI, and represents a potential new therapy to improve bone mass and reduce fracture risk in pediatric OI.

Type V OI primary osteoblasts display increased mineralization despite decreased COL1A1 expression

CONTEXT

Patients with type V osteogenesis imperfecta (OI) are heterozygous for a dominant IFITM5 c.-14C>T mutation, which adds five residues to the N terminus of bone-restricted interferon-induced transmembrane-like protein (BRIL), a transmembrane protein expressed in osteoblasts. Type V OI skeletal findings include hyperplastic callus formation, ossification of the forearm interosseous membrane, and dense metaphyseal bands.

OBJECTIVE

The objective of this study was to examine the role of osteoblasts in the active mineralization traits of type V OI and the effect of the IFITM5 mutation on type I collagen.

METHODS

We identified eight patients with the IFITM5 c.-14C>T mutation. Cultured osteoblasts from type V OI patients were used to study osteoblast differentiation and mineralization.

RESULTS

We verified the expression and stability of mutant IFITM5 transcripts. In differentiated type V OI primary osteoblasts in culture, the IFITM5 expression and BRIL level is comparable with control. Both early and late markers of osteoblast differentiation are increased in type V OI osteoblasts. Mineralization, assayed by alizarin red staining, was increased in type V OI osteoblasts compared with control. However, type V OI osteoblasts have significantly decreased COL1A1 transcripts in mid- to late differentiation. Type I collagen protein is concomitantly decreased, with decreased cross-linked collagen in matrix and altered appearance of fibrils deposited in culture.

CONCLUSIONS

This study demonstrates that type V OI mineralization has a gain-of-function mechanism at the osteoblast level, which likely underlies the overactive tissue mineralization seen in patients. Decreased type I collagen expression, secretion, and matrix incorporation establish type V OI as a collagen-related defect.