Growth Plate Function Research Articles

Combined ADAMTS10 and ADAMTS17 inactivation exacerbates bone shortening and compromises extracellular matrix formation.

Weill-Marchesani syndrome (WMS) is characterized by severe short stature, short hands and feet (brachydactyly), joint contractures, tight skin, and heart valve, eye, and skin anomalies. Whereas recessive WMS is caused by mutations in ADAMTS10 , ADAMTS17 , or LTBP2 , dominant WMS is caused by mutations in FBN1 (encoding fibrillin-1). Since bone growth is driven by chondrocyte proliferation and hypertrophy in the growth plates, the genetics of WMS suggests that the affected ECM proteins act within the same pathway to regulate chondrocyte and growth plate function. Here, we investigated the role of the secreted ADAMTS proteases ADAMTS10 and ADAMTS17 in growth plate function and ECM formation. We generated Adamts10 ; Adamts17 double knockout (DKO) mice, which showed significant postnatal lethality compared to single Adamts10 or Adamts17 KO mice. Importantly, we observed severe bone shortening DKO mice, which correlated with a narrower hypertrophic zone in their growth plates. ADAMTS17 substrates identified by N-terminomics and yeast two-hybrid screening identified the ECM proteins fibronectin and collagen VI (COL6). However, validation experiments did not reveal direct proteolysis of either fibronectin or COL6 by ADAMTS17. We then investigated ECM formation in primary ADAMTS10- and ADAMTS17-deficient skin fibroblasts and observed compromised fibronectin deposition concomitant with aberrant intracellular accumulation of fibrillin-1. These findings support a role for ADAMTS17 in ECM protein secretion and assembly. Collectively, our data suggest that ADAMTS10 and ADAMTS17 regulate bone growth by regulating chondrocyte hypertrophy or hypertrophic chondrocyte turnover. Mechanistically, ADAMTS17 appears to be a critical regulator of ECM protein secretion or pericellular matrix assembly, whereas ADAMTS10 likely modulates ECM formation at later stages, possibly regulating the spatio-temporal deposition of fibrillin isoforms.

Up IGF-I via High-toughness Adaptive Hydrogels for Remodeling Growth Plate of Children

Abstract The growth plate is crucial for skeletal growth in children, but research on repairing growth plate damage and restoring growth is limited. Here, a high-toughness adaptive dual-crosslinked hydrogel designed to mimic the growth plate's structure, supporting regeneration and bone growth. Composed of aldehyde-modified bacterial cellulose (DBNC), methacrylated gelatin (GelMA), and sodium alginate (Alg), the hydrogel is engineered through ionic bonding and Schiff base reactions, creating a macroporous structure. This structure can transform into a denser form by binding with calcium ions. In vitro, the loose macroporous structure of the hydrogels can promote chondrogenic differentiation, and when it forms a dense structure by binding with calcium ions, it also can activate relevant chondrogenic signaling pathways under the influence of IGF-1, further inhibiting osteogenesis. In vivo experiments in a rat model of growth plate injury demonstrated that the hydrogel promoted growth plate cartilage regeneration and minimized bone bridge formation by creating a hypoxic microenvironment that activates IGF-1-related pathways. This environment encourages chondrogenic differentiation while preventing the undesired formation of bone tissue within the growth plate area. Overall, the dual-crosslinked hydrogel not only mimics the growth plate's structure but also facilitates localized IGF-1 expression, effectively reshaping the growth plate's function. This approach represents a promising therapeutic strategy for treating growth plate injuries, potentially addressing challenges associated with skeletal growth restoration in pediatric patients.

Retinoid-impregnated nanoparticles enable control of bone growth by site-specific modulation of endochondral ossification in mice.

Growth-plates (physes), which are cartilage tissues near the ends of bones, support normal bone growth in children. Growth plate injures in limbs and other sites can impair growth plate function, leading to inhibited or imbalanced bone growth, skeletal deformities, decreased motion, discomfort or pain. At present, surgical interventions are the only means available to correct these conditions. Here, we aimed to develop a pharmacologic treatment for bone growth imbalance. Nanoparticles loaded with a selective agonist for the retinoic acid nuclear receptor gamma were prepared and implanted near the tibial growth plate in juvenile mice. The growth plate underwent involution and closure, and overall tibia length was shortened compared to the contralateral element implanted with drug-free control nanoparticles. Importantly, when the same drug nanoparticles were implanted in only one side of the tibia, the tibia was tilted toward the injection site. Our findings provide novel evidence that retinoic acid receptor gamma agonist-loaded nanoparticles can control activity, function and directionality of a targeted growth plate, offering a potential and clinically-relevant alternative or supplementation to surgical correction of limb length imbalances and deformities.

The antioxidant protective effect of resveratrol on long-term exposure to acrylamide-induced skeletal toxicity in female mice.

Acrylamide (AA) is a toxic substance formed when cooking starch-based foods at high temperatures. Studies have shown that AA can cause neurotoxicity, reproductive toxicity and so on. However, there remains limited understanding of the potential skeletal toxicity of AA. The aim of this study was to investigate the potential skeletal toxicity of AA, as well as the potential bone protective effects of Resveratrol (RVT). Based on the daily intake of adult women, adult female mice was treated with AA at 0, 0.01, 0.1, 1 mg/kg/d or AA/RVT (1 mg/kg/d AA +10 mg/kg/d RVT) for 8 weeks, and skeletal toxicity were evaluated by RT-qPCR and histopathological techniques. The results found that exposure to AA (0.1 or 1 mg/kg/d) after 8 weeks, osteogenesis exhibited pathological damage characteristics such as inhibition of growth plate function, and reduction of fibrous tissue, and cartilage exhibited pathological damage characteristics such as irregular cell morphology and arrangement, and damage to the tidal line. The results of cellular functional gene testing showed a decrease in the expression of functional genes in osteoblasts and chondrocytes. Meanwhile, after further co-treatment with AA (1 mg/kg/d) and resveratrol (RVT) (10 mg/kg/d), we found that RVT restored AA-induced damage to osteogenesis and cartilage, and reduced the high apoptosis and oxidative stress levels in osteogenesis/cartilage after AA exposure. In summary, this study confirmed the skeletal toxicity of AA on female adult mice, and further clarified the antioxidant protective effect of RVT on this toxicity.

Efficacy of cartilage-targeted IGF-1 in a mouse model of growth hormone insensitivity.

Recombinant human IGF-1 is used to treat severe primary IGF-1 deficiency, but this treatment requires twice-daily injection, often does not fully correct the growth deficit, and has important off-target effects. We therefore sought to target IGF-1 to growth plate cartilage by generating fusion proteins combining IGF-1 with single-chain human antibody fragments that target matrilin-3, a cartilage matrix protein. We previously showed that this cartilage-targeting IGF-1 fusion protein (CV1574-1) promoted growth plate function in a GH-deficient (lit) mouse model. Here, we studied CV1574-1 in a second mouse model, C57BL/6 wild-type mice treated with pegvisomant to induce GH resistance. In this model, once-daily injections of CV1574-1 for 5 days partially restored the pegvisomant-induced decrease in growth plate height without increasing kidney cell proliferation. Furthermore, we found that subcutaneous CV1574-1 showed significantly reduced hypoglycemic effect compared to injection of IGF-1 itself. Lastly, to gain mechanistic insights into the role of matrilin-3 targeting, we assessed the ability of CV1574-1 to activate AKT signaling in vitro and found that CV1574-1 caused a prolonged increase in AKT signaling compared to IGF-1 and that this effect was dependent on matrilin-3. Taken together, our findings provide further evidence that cartilage-targeted therapy could provide new pharmacological approaches for the treatment of childhood growth disorders, such as GH-insensitivity syndrome.

Molecular mechanisms of long bone growth and chondrocyte regulation: A narrative review

Long bone growth is a fundamental determinant of final height. Growth, metabolism, and differentiation of chondrocytes, which are the key cellular players in this process, are regulated by systemic hormones, local factors, and cellular signaling pathways. This review provides an overview of the structural aspects of the growth plate, factors influencing chondrocyte function, and their impact on longitudinal bone growth. Systemic factors, including growth hormone, sex hormones, thyroid hormone, glucocorticoids, leptin, and insulin significantly affect chondrocyte proliferation and hypertrophy. Local factors, including transcription factors such as SRY-box 9 protein (SOX9), Runt-related transcription factor 2 (RUNX2), and bone morphogenetic proteins (BMPs), along with signaling pathways such as the Wnt pathway, play critical roles in chondrocyte proliferation and differentiation. These factors regulate gene expression, cell differentiation, and extracellular matrix synthesis. Additionally, Indian hedgehog (Ihh) and C-type natriuretic peptide (CNP) are involved in the complex signaling network governing chondrocyte function. Understanding molecular mechanisms underlying normal growth plate function has provided valuable insights into the genetic defects that impact growth and foundation for the development of effective therapeutic strategies for individuals with growth disorders.

Impact of telescopic intramedullary rodding on the growth of tibia: Comparative experimental study in dogs

IntroductionThe most commonly accepted method of long bone deformity correction in children with osteogenesis imperfecta is surgical realignment with transphyseal telescopic intramedullary rodding. This approach ensures reinforcement of the bone throughout the growth period. Although longitudinal growth does occur with these implants there has been very little work carried out to calculate the effect of such factors as rod position or implant material on growth. We carried out a prospective comparative study on 12 puppies using titanium alloy telescopic tibial rods with and without hydroxyl-apatite coating. The aim of this non-randomized controlled experimental study was to assess the impact of telescopic intramedullary rodding on spontaneous growth of the tibia. Material and methodsTwelve mongrel puppies aged of 5 months underwent intramedullary transphyseal rodding of the right tibia. In group I (6 dogs) a titanium telescopic rod was used, in group II (6 dogs) a titanium rod with hydroxyapatite (HA)-coated threaded end was used. The following radiological criteria were assessed before surgery and every month until age of 12 months (natural fusion of physes in dogs): length of tibia, amount of superposition of inner (male) rod into external (female) rod; alteration of anatomy in terms of joint angles (mMPTA, mLDTA, mPPTA, mADTA); positioning of threaded ends in proximal and distal epiphyses and evidence of premature growth arrest. Parameters were compared with left tibia serving as control segment. The null hypothesis was that neither rod position nor implant material altered growth. ResultsThe transphyseal rods did not lead to irreversible epiphysiodesis in either group. In group II (HA-coated) some loss of residual length was found in all six dogs, over 7mm (5.9%) in comparison to left intact tibia. In contrast to that, in group I (titanium nail) only one animal (16.7%) demonstrated a tibia length discrepancy of 8mm (4.8%). Eccentric ( posterior) positioning of the rod in the distal epiphysis resulted in a procurvatum deformity (increased anterior distal tibial angle) in both groups. We found no failure of telescoping and no loss of fixation of threaded parts in either epiphyses. DiscussionThe presence of telescopic rods with HA-coated threads parts clearly contributes to inhibition of spontaneous longitudinal growth. We hypothesize that HA stimulates maturation of chondrocytes of growth plate. Our findings regarding the potential adverse effect of thread position in the distal physis demonstrate the importance of attempting to place the rod as central as possible. ConclusionTitanium alloy telescopic rods did not reveal significant effect on physeal growth in puppies in comparison to HA-coated implants. Transphyseal HA-coated implants did however inhibit growth plate function with mean loss of length of 5.2% compared to the other side. Eccentric positioning of rods relative to center of physis resulted in angular deformity due to irregular growth. There were no cases of mechanical failure or loss of telescopic function with either group of titanium implant. Level of evidenceII; prospective comparative experimental study.

Diffusion-tensor magnetic resonance imaging of the growth plates: substantiation of the method and evaluation of its reproducibility

INTRODUCTION: In recent years there is an increasing interest in assessing the state of the physis among pediatric orthopedic traumatologists, due to the availability of surgical correction of conditions associated with impaired growth plate function. Application of these methods requires dynamic assessment of the state of the physis. The existing methods for assessing the functional state of the physis are few and imperfect. Magnetic resonance imaging (MRI), in particular diffusion tensor imaging (DTI), is promising for assessing the growth plates.OBJECTIVE: To assess the possibility of applying DTI technique to assess the state of the physis (growth plate).MATERIALS AND METHODS: The study was approved by the local ethical committee of the Institute of the Human Brain named after N.P. Bekhtereva of Russian Academy of Sciences, protocol No. 32, dated 21.12.2020. Informed consent was obtained from each legal representative. At the Turner National Medical Research Center for Сhildren’s Orthopedics and Trauma Surgery of the Ministry of Health of the Russian Federation a standard magnetic resonance examination of the knee joints was performed, additionally diffusion tensor tomography was performed in 75 patients aged 4.1 to 14.3 years (41 males, 34 females), both deemed healthy and with a known physeal pathology. The studies were performed using a Philips Ingenia ElitionX machine with a magnetic field strength of 3 Tesla, DTI was performed in the coronal plane, TE=71.2 ms, TR=4000 ms, b-factor value 600 s/mm2, 15 diffusion directions were used, slice thickness was 3 mm without gaps, the matrix size was 1.25×1.25 mm. The resulting images were processed using the DSI Studio software, including tract construction, estimation of the total volume and average length of the tracts. The reproducibility of the technique was assessed.RESULTS: Higher volumes and mean tract lengths were found in healthy males at puberty compared to other groups. When assessing reproducibility, the standard deviations were about 10% of the final value.DISCUSSION: Images of growth plate tracts were obtained. In individuals of pubertal age that were deemed healthy, the tract volume values and the average length of the tracts were higher than in individuals of prepubertal age and those with pathological conditions affecting the state of the growth plates. The average tract length turned out to be more reproducible than the total tract volume parameter.CONCLUSION: With the help of DTI, it is possible to evaluate the arrangement of the structures in the growth plates.

Reactivation of Vertebral Growth Plate Function in Vertebral Body Tethering in an Animal Model.

Flexible spine tethering is a relatively novel fusionless surgical technique that aims to correct scoliosis based on growth modulation due to the pressure exerted on the vertebral body epiphyseal growth plate. The correction occurs in two phases: immediate intraoperative and postoperative with growth. The aim of this study was to evaluate the reactivation of vertebral growth plate function after applying corrective forces. The rat tail model was used. Asymmetric compression and distraction of caudal growth plates were performed using a modified external fixation apparatus. Radiological and histopathological data were analysed. After three weeks of correction, the activity of the structures increased across the entire growth plate width, and the plate was thickened. The height of the hypertrophic layer and chondrocytes on the concave side doubled in height. The height of chondrocytes and the cartilage thickness on the concave and central sides after the correction did not differ statistically significantly from the control group. Initiation of the correction of scoliosis in the growing spine, with relief of the pressure on the growth plate, allows the return of the physiological activity of the growth cartilage and restoration of the deformed vertebral body.

Vitamin D deficiency and anatomical region alters porcine growth plate properties

Ossification of growth plate cartilage mediates longitudinal extension of long bones. Biomechanical and biochemical disruptions of growth plate function may lead to abnormal bone growth. In humans and animals, severe dietary vitamin D deficiency can lead to rickets which features growth plate widening, resulting in abnormalities in growth. However, effects of marginal vitamin D deficiencies on growth plates are not well understood. The purpose of this study was to examine the effects of a vitamin D deficient diet in the 26-day nursery phase on mechanical properties (ultimate normal stress, ultimate shear stress, ultimate strain, and tangent modulus) of porcine growth plate. Standard uniaxial tensile tests were applied on bone-growth plate-bone sections and the total stress was decomposed into normal stress and shear stress. Ultimate shear stress and ultimate strain traits were lower in the vitamin D deficient group than in the control. Regional differences were observed in all four variables. Ultimate normal stress was higher in the anterior region, which was consistent with a previous study. Sex differences were detected in ultimate normal stress, which was higher in females than in males. Interestingly, the classical finding of growth plate widening seen in severe vitamin D deficiency was not observed in the pigs with marginal vitamin D deficiency utilized in this study.

Abstract #1178966: A Rare Case of Idiopathic Osteoporosis with COL10A1 and COL27A1 Genetic Mutations in a Premenopausal Woman

The association between bone mineral density (BMD) and fracture differs in premenopausal women when compared to the postmenopausal women. Irrespective of the BMD, any fragility fracture in those aged 20-50 requires evaluation for secondary causes of osteoporosis. In the absence of secondary causes, idiopathic osteoporosis (IOP) is considered. We present a case of a premenopausal woman with IOP whose mutational analysis revealed variants in COL10A1 and COL27A1. These genetic mutations are found in Schmid-type metaphyseal chondrodysplasia (SMCD) and Steel syndrome, respectively. To our knowledge, this is the first case report establishing a linkage between these specific mutations and IOP. A 36-year-old Hispanic woman with a history of fragility fracture of the lumbar spine at age 23 was referred to Endocrinology for further evaluation. Dual-energy x-ray absorptiometry (DEXA) reported a Z-score of -3.0 in L1-L4, consistent with osteoporosis of the lumbar spine. A careful history, physical examination, and laboratory evaluation ruled out secondary causes of osteoporosis. In the interim, the patient presented a recurrent fragility fracture of the left proximal humerus requiring surgical intervention where bone marrow (BM) was obtained for cytology. BM cytology was found normocellular without increase in blasts or lymphocytes. Notably, the mutational analysis revealed heterozygous variants of uncertain significance (VUS) in COL10A1 [variant c682 c >T (p.Pro228Ser)] and COL27A1 [variant c.1814 c >T (p.Thr 6605 Met)]. Whole exome sequencing was not able to be performed due to insurance issues. The most common secondary causes of premenopausal bone loss include estrogen deficiency as seen in hypogonadism, drug exposure such as glucocorticoids, malabsorption disorders like celiac disease, hyperthyroidism, hypercalciuria, hyperparathyroidism, and disturbances in calcium and vitamin D metabolism. Osteoporosis with no known secondary causes is known as IOP. IOP is a disorder with multiple proposed mechanisms including impairment in growth hormone and insulin like growth factor axis, osteoblast dysfunction, and genetic mutations. Mutational analysis in selected IOP cases, have shown pathogenic variants and VUS in LRP5, LRP6, COL1A1, COL1A2, PLS3, WNT1, DKK1, FKBP10, HGD, SLC34A3. Our patient was found with heterozygous VUS in COL10A1 and COL27A1. These genetic mutations are found in (SMCD) and Steel syndrome, respectively. These rare osteochondrodysplasias, more commonly found in childhood, are associated with skeletal abnormalities including scoliosis, congenital hip dislocations, short stature, and characteristic facial features. More importantly, COL10A1 encodes the alpha 1 chain of collagen type X which is involved in growth plate function, and COL27A1 gene encodes a member of the fibrillar collagen family involved in the calcification of cartilage and the transition of cartilage to bone. To our knowledge, COL10A1 and COL27A1 mutations have not been yet associated as primary causes of IOP. We report this case aiming for further research to identify additional genetic etiologies of IOP.

Wnt signaling in chondroprogenitors during long bone development and growth.

Wnt signaling together with other signaling pathways governs cartilage development and the growth plate function during long bone formation and growth. β-catenin-dependent Wnt signaling is a specific lineage determinant of skeletal mesenchymal cells toward chondrogenic or osteogenic direction. Once cartilage forms and the growth plate organize, Wnt signaling continues to regulate proliferation and differentiation of the growth plate chondrocytes. Although chondrocytes in the growth plate have a high capacity to proliferate, new cells must be supplied to the growth plate from chondroprogenitor population. Advances in in vivo cell tracking techniques have demonstrated the importance of Wnt signaling in driving tissue renewal. The Wnt-responsive cells, genetically marked by the Wnt-reporter system, are found as stem cells in various tissues. Similarly, Wnt-responsive cells are found in the periphery of the growth plate and expanded to constitute entire column structure, indicating that Wnt signaling participates in the regulation of chondroprogenitors in the growth plate. This review will discuss advancements in research of progenitors in the growth plate, specifically focusing on Wnt/β-catenin signaling.

Role of Mechanical Force on the Growth Plate

The shape and size of the limb is determined by genetic, biochemical, and mechanical factors resulting from weight bearing activity or muscle contraction. Changes in bone shape in response to mechanical forces was first reported by Wolf, 1892. Observations by Hueter‐Volkman indicated that supraphysiological forces lead to reduced skeletal growth. More recently, Frost (1990) proposed the concept of chondral modeling to describe physiological loading. In this case, both tension and compression lead to increased skeletal growth up to a peak then compression beyond that peak leads to inhibition of growth. Recent experiments using zebra fish, chick, and muscle‐less Spd mice indicated a specific connection between mechanical load and endochondral bone formation in embryonic skeleton. A role for mechanical forces on postnatal skeletal development is apparent in clinical manifestations of children with hemiplegia resulting in unequal loading of developing limbs. A large proportion of children with hemiplegia demonstrated significant limb length discrepancy with the paralyzed limb being shorter than the other limb. An age dependent effect of force on limb development in mice during spaceflight and tail suspended rats indicated a critical developmental window where loading regulates growth plate function. Longitudinal growth of limbs occurs through cartilaginous structures called the growth plate in a process called endochondral bone formation. One important feature of the growth plate is that the cells align into columns that represent a continuum of differentiation. It was suggested that the magnitude of bone growth is dependent on chondrocyte proliferation, matrix deposition, and hypertrophy; however, without the columnar structure, growth would not be longitudinal and the proper length and shape of skeletal elements would be disrupted. Recent experimental data indicate that mechanical load is required to regulate growth plate function including proliferation of chondrocytes and column formation. The molecular mechanism governing proliferation and column formation in response to load are not known but may involve chondrocytes sensing mechanical changes in the environment thought their actin cytoskeleton. Unloading mouse hind limb results in disorganization of the actin structure in chondrocytes, loss of columnar structure in the growth plate, and reduced limb length. Mice with conditional deletion of genes associated with regulation of the actin cytoskeleton demonstrate similar alterations in their growth plates. These observations highlight the importance of mechanical loading on growth plate function but there is still limited understanding of the molecular mechanisms of how limb length is regulated in response to mechanical loads.Support or Funding InformationNIH/NIAMS R21 AR070097 and the US‐Israel Bi‐national Science Foundation

New developments in the management of achondroplasia

SummaryAchondroplasia is the most common form of disproportionate short stature. A dominantly inherited FGFR3 mutation permanently activates the fibroblast growth factor receptor 3 (FGFR3) and its downstream mitogen-activated protein kinase (MAPK) signalling pathway. This inhibits chondrocyte differentiation and puts a break on growth plate function, in addition to causing serious medical complications such as foramen magnum and spinal stenosis and upper airway narrowing. A great deal has been learned about complications and consequences of FGFR3 activation and management guidance is evolving aimed to reduce the increased mortality and morbidity in this condition, particularly deaths from spinal cord compression and sleep apnoea in infants and small children. To date, no drugs are licensed for treatment of achondroplasia. Here, we report on the various substances in the drug development pipeline which target elements in molecular disease mechanism such as FGF (fibroblast growth factor) ligands, FGFR3, MAPK signalling as well as the C‑type natriuretic peptide receptor NPR‑B (natriuretic peptide receptor B).

The effect of a high-calorie diet on bone growth is mediated by the insulin receptor.

Obese children grow faster than their normal-weight peers. Insulin resistance and hyperinsulinemia have been associated with obesity-related growth acceleration. To determine whether obesity-associated hyperinsulinemia promotes bone growth by activating the insulin receptor in the growth plate, we generated TamCartIRflox/flox mice. The injection of 4 doses of tamoxifen in these mice (beginning at postnatal day 5th with 2 days interval between injections) resulted in the Insulin Receptor (IR) gene excision exclusively in the cartilage. TamCartIRflox/flox tamoxifen-treated mice (KO mice) and their IRflox/flox control littermates (C mice) at 3 weeks of age were exposed to a standard or hypercaloric (high-fat) diet for 4 weeks. At the end of study, C and KO mice fed with a high-fat diet exhibited greater weight gain than the respective strains fed with a standard diet. Body and tibial growth and growth plate height of C mice fed with high-fat diet were greater than those of standard-diet-fed C mice; however, no difference was observed between KO mice fed with standard or high-fat diet with respect to body and tibial growth and growth plate height. Circulating levels of insulin, IGF-1 and leptin were significantly higher in C and KO mice exposed to high-fat diet compared to those in the same strain exposed to standard diet. Increased phosphorylation of Akt (one of the intracellular mediators of insulin action in bone) in the growth plate of C mice on high-fat diet (compared to those on standard diet) suggests that high-fat-mediated increased circulating insulin levels may directly affect growth plate function and bone growth. High-fat diet was not associated with any change of Akt phosphorylation in KO mice. In addition, in vitro studies in cultured primary chondrocytes revealed that Akt mediates the stimulatory effects of insulin on chondrocyte proliferation and differentiation. In conclusion, the activation of the insulin receptor in the growth plate of mice fed with a hypercaloric diet stimulates skeletal growth and growth plate chondrogenesis.

Advances in the pathogenesis and possible treatments for multiple hereditary exostoses from the 2016 international MHE conference

ABSTRACTMultiple hereditary exostoses (MHE) is an autosomal dominant disorder that affects about 1 in 50,000 children worldwide. MHE, also known as hereditary multiple exostoses (HME) or multiple osteochondromas (MO), is characterized by cartilage-capped outgrowths called osteochondromas that develop adjacent to the growth plates of skeletal elements in young patients. These benign tumors can affect growth plate function, leading to skeletal growth retardation, or deformations, and can encroach on nerves, tendons, muscles, and other surrounding tissues and cause motion impairment, chronic pain, and early onset osteoarthritis. In about 2–5% of patients, the osteochondromas can become malignant and life threatening. Current treatments consist of surgical removal of the most symptomatic tumors and correction of the major skeletal defects, but physical difficulties and chronic pain usually continue and patients may undergo multiple surgeries throughout life. Thus, there is an urgent need to find new treatments to prevent or reverse osteochondroma formation. The 2016 International MHE Research Conference was convened to provide a forum for the presentation of the most up-to-date and advanced clinical and basic science data and insights in MHE and related fields; to stimulate the forging of new perspectives, collaborations, and venues of research; and to publicize key scientific findings within the biomedical research community and share insights and relevant information with MHE patients and their families. This report provides a description, review, and assessment of all the exciting and promising studies presented at the Conference and delineates a general roadmap for future MHE research targets and goals.

The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development.

The primary cilium is a solitary, antenna-like sensory organelle with many important roles in cartilage and bone development, maintenance, and function. The primary cilium's potential role as a signaling nexus in the growth plate makes it an attractive therapeutic target for diseases and disorders associated with bone development and maintenance. Many signaling pathways that are mediated by the cilium-such as Hh, Wnt, Ihh/PTHrP, TGFβ, BMP, FGF, and Notch-are also known to influence endochondral ossification, primarily by directing growth plate formation and chondrocyte behavior. Although a few studies have demonstrated that these signaling pathways can be directly tied to the primary cilium, many pathways have yet to be evaluated in context of the cilium. This review serves to bridge this knowledge gap in the literature, as well as discuss the cilium's importance in the growth plate's ability to sense and respond to chemical and mechanical stimuli. Furthermore, we explore the importance of using the appropriate mechanism to study the cilium in vivo and suggest IFT88 deletion is the best available technique. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:533-545, 2018.

DEVELOPMENTAL SCENARIOS OF THE EPIPHYSIS AND GROWTH PLATE UPON MECHANICAL LOADING: A COMPUTATIONAL MODEL

Long bone growth relies on the continuous bone formation from cartilaginous tissue (endochondral ossification). This process starts in the central region (diaphysis) of the forming bone and short before birth, ossification starts in bone extremes (epiphysis). A cartilaginous region known as the growth plate is maintained until adolescence between epiphysis and diaphysis to further contribute to longitudinal growth. Even though there are several biochemical factors controlling this process, there is evidence revealing an important regulatory role of mechanical stimuli. Up to now approaches to understand mechanical effects on ossification have been limited to epiphysis. In this work, based on Carter's mathematical model for epiphyseal ossification, we explored human growth plate response to mechanical loads. We analyzed growth plate stress distribution using finite element method for a generic bone considering different stages of bone development in order to shed light on mechanical contribution to growth plate function. Results obtained revealed that mechanical environment within the growth plate change as epiphyseal ossification progresses. Furthermore, results were compared with physiological behavior, as reported in literature, to analyze the role of mechanical stimulus over development. Our results suggest that mechanical stimuli may play different regulation roles on growth plate behavior through normal long bone development. However, as this approach only took into account mechanical aspects, failed to accurately predict biological behavior in some stages. In order to derive biologically relevant information from computational models it is necessary to consider biological contribution and possible mechanical–biochemical interactions affecting human growth plate physiology. Along these lines, we propose the dilatatorial parameter k used by Carter et al. should assume different values corresponding to the developmental stage in question. Thus, reflecting biochemical contribution changes over time.

Palovarotene Inhibits Heterotopic Ossification and Maintains Limb Mobility and Growth in Mice With the Human ACVR1(R206H) Fibrodysplasia Ossificans Progressiva (FOP) Mutation.

Fibrodysplasia ossificans progressiva (FOP), a rare and as yet untreatable genetic disorder of progressive extraskeletal ossification, is the most disabling form of heterotopic ossification (HO) in humans and causes skeletal deformities, movement impairment, and premature death. Most FOP patients carry an activating mutation in a bone morphogenetic protein (BMP) type I receptor gene, ACVR1(R206H) , that promotes ectopic chondrogenesis and osteogenesis and, in turn, HO. We showed previously that the retinoic acid receptor γ (RARγ) agonist palovarotene effectively inhibited HO in injury-induced and genetic mouse models of the disease. Here we report that the drug additionally prevents spontaneous HO, using a novel conditional-on knock-in mouse line carrying the human ACVR1(R206H) mutation for classic FOP. In addition, palovarotene restored long bone growth, maintained growth plate function, and protected growing mutant neonates when given to lactating mothers. Importantly, palovarotene maintained joint, limb, and body motion, providing clear evidence for its encompassing therapeutic potential as a treatment for FOP. © 2016 American Society for Bone and Mineral Research.

Human monoclonal antibody fragments targeting matrilin-3 in growth plate cartilage.

Many genetic disorders, including chondrodysplasias, and acquired disorders impair growth plate function, resulting in short and sometimes malformed bones. There are multiple endocrine and paracrine factors that promote chondrogenesis at the growth plate, which could potentially be used to treat these disorders. Targeting these growth factors specifically to the growth plate might augment the therapeutic skeletal effect while diminishing undesirable effects on non-target tissues. Using yeast display technology, we selected single-chain variable antibody fragments that bound to human and mouse matrilin-3, an extracellular matrix protein specifically expressed in cartilage tissue. The ability of the selected antibody fragments to bind matrilin-3 and to bind cartilage tissue in vitro and in vivo was assessed by ELISA and immunohistochemistry. We identified antibody fragments that bound matrilin-3 with high affinity and also bound with high tissue specificity to cartilage homogenates and to cartilage structures in mouse embryo sections. When injected intravenously in mice, the antibody fragments specifically homed to cartilage. Yeast display successfully selected antibody fragments that are able to target cartilage tissue in vivo. Coupling these antibodies to chondrogenic endocrine and paracrine signaling molecules has the potential to open up new pharmacological approaches to treat childhood skeletal growth disorders.