Molecular analysis of enthesopathy in a mouse model of hypophosphatemic rickets

Understanding the structural biology of osteomalacia through multiscale 3D X-ray and electron tomographic imaging: a review of X-linked hypophosphatemia, the Hyp mouse model, and imaging methods

Biomineralization in bones and teeth is a highly regulated extracellular event. In the skeleton, mineralization at the tissue level is controlled within the collagenous extracellular matrix by both circulating and local factors. While systemic regulation of mineral ion homeostasis has been well-studied over many decades, much less is known about the regulation of mineralization at the local level directly within the extracellular matrix. Some local regulators have been identified, such as tissue-nonspecific alkaline phosphatase (TNAP), phosphate-regulating endopeptidase homolog X-linked (PHEX), pyrophosphate, and osteopontin, and others are currently under investigation. Dysregulation of the actions of enzyme-inhibitor substrate pairs engaged in mineralization (as we describe by the Stenciling Principle for extracellular matrix mineralization) leads to osteomalacic "soft bone" diseases, such as hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH). This review addresses how advances in 3D imaging tools and software now allow contextual and correlative viewing and interpretation of mineralized tissue structure across most length scales. Contextualized and integrated 3D multiscale data obtained from these imaging modalities have afforded an unprecedented structural biology view of bone from the macroscale to the nanoscale. Such correlated volume imaging data is highly quantitative, providing not only an integrated view of the skeleton in health, but also a means to observe alterations that occur in disease. In the context of the many hierarchical levels of skeletal organization, here we summarize structural features of bone over multiple length scales, with a focus on nano- and microscale features as viewed by X-ray and electron tomography imaging methods (submicron μCT and FIB-SEM). We additionally summarize structural changes observed after dysregulation of the mineralization pathway, focusing here on the Hyp mouse model for XLH. More specifically, we summarize how mineral patterns/packs at the microscale (3D crossfibrillar mineral tessellation), and how this is defective in Hyp mouse bone and Hyp enthesis fibrocartilage.

Clinical presentation and burden of ENPP1 deficiency in adults

While the clinical consequences of severe ENPP1 deficiency leading to the rare disorders generalized arterial calcification of infancy (GACI) and autosomal recessive hypophosphatemic rickets type 2 (ARHR2) are well defined and understood, much less is known about how this evolves into adulthood and how moderate ENPP1 deficiency can first manifest in adulthood. Moreover, growing evidence substantiates an association of genetic variants in the ENPP1 gene with a wide range of further clinical manifestations including early-onset osteoporosis, osteoarthritis, and different forms of spinal ligament calcifications, i.e., diffuse idiopathic skeletal hyperostosis (DISH) and ossification of the posterior/anterior longitudinal ligament (OPLL/OALL). Furthermore, conditions with primarily extraskeletal signs and symptoms such as Cole disease, coagulopathies, and metabolic syndrome can seemingly result from ENPP1 variants. The causality and the pathophysiology behind these different clinical presentations appear complex and require further research, especially since the coincidence of these different phenotypes is rarely described and available evidence suggests that part of the aforementioned manifestations may result from ENPP1 effects beyond the catalytic activity of processing ATP to AMP and inorganic pyrophosphate (PPi). Growing awareness of the additional ENPP1-related manifestations across the lifespan will advance our understanding of this complex condition and help to standardize diagnostic approaches and develop individually tailored treatment concepts.

The role of GDF5 in regulating enthesopathy development in the Hyp mouse model of XLH

X-linked hypophosphatemia (XLH) is caused by mutations in PHEX, leading to rickets and osteomalacia. Adults affected with XLH develop a mineralization of the bone-tendon attachment site (enthesis), called enthesopathy, which causes significant pain and impaired movement. Entheses in mice with XLH (Hyp) have enhanced bone morphogenetic protein (BMP) and Indian hedgehog (IHH) signaling. Treatment of Hyp mice with the BMP signaling blocker palovarotene attenuated BMP/IHH signaling in Hyp entheses, thus indicating that BMP signaling plays a pathogenic role in enthesopathy development and that IHH signaling is activated by BMP signaling in entheses. It was previously shown that mRNA expression of growth/differentiation factor 5 (Gdf5) is enhanced in Hyp entheses at P14. Thus, to determine a role for GDF5 in enthesopathy development, Gdf5 was deleted globally in Hyp mice and conditionally in Scx + cells of Hyp mice. In both murine models, BMP/IHH signaling was similarly decreased in Hyp entheses, leading to decreased enthesopathy. BMP/IHH signaling remained unaffected in WT entheses with decreased Gdf5 expression. Moreover, deletion of Gdf5 in Hyp entheses starting at P30, after enthesopathy has developed, partially reversed enthesopathy. Taken together, these results demonstrate that while GDF5 is not essential for modulating BMP/IHH signaling in WT entheses, inappropriate GDF5 activity in Scx + cells contributes to XLH enthesopathy development. As such, inhibition of GDF5 signaling may be beneficial for the treatment of XLH enthesopathy.

Sclerostin modulates mineralization degree and stiffness profile in the fibrocartilaginous enthesis for mechanical tissue integrity

Fibrocartilaginous entheses consist of tendons, unmineralized and mineralized fibrocartilage, and subchondral bone, each exhibiting varying stiffness. Here we examined the functional role of sclerostin, expressed in mature mineralized fibrochondrocytes. Following rapid mineralization of unmineralized fibrocartilage and concurrent replacement of epiphyseal hyaline cartilage by bone, unmineralized fibrocartilage reexpanded after a decline in alkaline phosphatase activity at the mineralization front. Sclerostin was co-expressed with osteocalcin at the base of mineralized fibrocartilage adjacent to subchondral bone. In Scx-deficient mice with less mechanical loading due to defects of the Achilles tendon, sclerostin+ fibrochondrocyte count significantly decreased in the defective enthesis where chondrocyte maturation was markedly impaired in both fibrocartilage and hyaline cartilage. Loss of the Sost gene, encoding sclerostin, elevated mineral density in mineralized zones of fibrocartilaginous entheses. Atomic force microscopy analysis revealed increased fibrocartilage stiffness. These lines of evidence suggest that sclerostin in mature mineralized fibrochondrocytes acts as a modulator for mechanical tissue integrity of fibrocartilaginous entheses.

Engineered Decellularized Tendon Matrix Putty Preserves Native Tendon Bioactivity to Promote Cell Proliferation and Enthesis Repair

Rotator cuff tears are a common soft tissue injury that can significantly decrease function of the shoulder and cause severe pain. Despite progress in surgical technique, rotator cuff repairs (RCRs) do not always heal efficiently. Many failures occur at the bone-tendon interface as a result of poor healing capacity of the tendon and failure to regenerate the native histological anatomy of the enthesis. While allografts are commercially available, clinical use is limited as they do not stimulate tissue regeneration and are associated with a structural failure of up to 40% in re-tear cases. Novel tissue engineering strategies are being developed with promise, but most involve addition of cells and/or growth factors which extends the timeline for clinical translation. Thus, there exists a significant unmet clinical need for easily translatable surgical augmentation approaches that can improve healing in RCR. Here we describe the development of a decellularized tendon matrix (DTM) putty that preserves native tendon bioactivity using a novel processing technique. In vitro, DTM promoted proliferation of tenocytes and adipose-derived stem cells with an increase in expression-specific transcription factors seen during enthesis development, Scleraxis and Sox9. When placed in a rabbit model of a chronic rotator cuff tear, DTM improved histological tissue repair by promoting calcification at the bone-tendon interface more similar to the normal fibrocartilaginous enthesis. Taken together, these data indicate that the engineered DTM putty retains a pro-regenerative bioactivity that presents a promising translational strategy for improving healing at the enthesis.

Impaired 1,25-dihydroxyvitamin D3 action underlies enthesopathy development in the Hyp mouse model of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by high serum fibroblast growth factor 23 (FGF23) levels, resulting in impaired 1,25-dihydroxyvitamin D3 (1,25D) production. Adults with XLH develop a painful mineralization of the tendon-bone attachment site (enthesis), called enthesopathy. Treatment of mice with XLH (Hyp) with 1,25D or an anti-FGF23 Ab, both of which increase 1,25D signaling, prevents enthesopathy. Therefore, we undertook studies to determine a role for impaired 1,25D action in enthesopathy development. Entheses from mice lacking vitamin D 1α-hydroxylase (Cyp27b1) (C-/-) had a similar enthesopathy to Hyp mice, whereas deletion of Fgf23 in Hyp mice prevented enthesopathy, and deletion of both Cyp27b1 and Fgf23 in mice resulted in enthesopathy, demonstrating that the impaired 1,25D action due to high FGF23 levels underlies XLH enthesopathy development. Like Hyp mice, enthesopathy in C-/- mice was observed by P14 and was prevented, but not reversed, with 1,25D therapy. Deletion of the vitamin D receptor in scleraxis-expressing cells resulted in enthesopathy, indicating that 1,25D acted directly on enthesis cells to regulate enthesopathy development. These results show that 1,25D signaling was necessary for normal postnatal enthesis maturation and played a role in XLH enthesopathy development. Optimizing 1,25D replacement in pediatric patients with XLH is necessary to prevent enthesopathy.

Insights into the Molecular and Hormonal Regulation of Complications of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by mutations in the PHEX gene, leading to elevated serum levels of FGF23, decreased production of 1,25 dihydroxyvitamin D3 (1,25D), and hypophosphatemia. Those affected with XLH manifest impaired growth and skeletal and dentoalveolar mineralization as well as increased mineralization of the tendon–bone attachment site (enthesopathy), all of which lead to decreased quality of life. Many molecular and murine studies have detailed the role of mineral ions and hormones in regulating complications of XLH, including how they modulate growth and growth plate maturation, bone mineralization and structure, osteocyte-mediated mineral matrix resorption and canalicular organization, and enthesopathy development. While these studies have provided insight into the molecular underpinnings of these skeletal processes, current therapies available for XLH do not fully prevent or treat these complications. Therefore, further investigations are needed to determine the molecular pathophysiology underlying the complications of XLH.

Eccentric contraction-dominant exercise leads to molecular biological changes in enthesis and enthesopathy-like morphological changes

Entheses, which are tendon-to-bone attachment sites in the musculoskeletal system, play important roles in optimizing the mechanical stress and force transmitted from the muscle to the bone. Sports-related enthesopathy shows pathological features, including hyperplasia of the fibrocartilage (FC) region in the enthesis. The amount of exercise and type of muscle contraction during movement is involved in the pathogenesis of sports-related enthesopathy; however, the details of this condition are unclear. Here we examined the molecular pathways involved in the morphological changes of the muscle-tendon-enthesis complex and enthesis FC region in the supraspinatus muscle enthesis of mice under different exercise conditions. Following intervention, morphological changes in the muscle-tendon-enthesis complex were initiated in the eccentric contraction-dominant exercise group at 2 weeks, with activation of the transforming growth factor-β (TGFβ) superfamily pathway predicted by proteome and ingenuity pathway analyses. Histological and molecular biological analyses confirmed the activation of the TGFβ/bone morphogenetic protein (BMP)-Smad pathway. The concentric contraction-dominant exercise group showed no change in the morphology of the muscle-tendon-enthesis complex or activation of the TGFβ/BMP-Smad pathway, despite overuse exercise. Statement of Clinical Significance: These results suggest that eccentric contraction-dominant exercise induces sports-related enthesopathy-like morphological changes in the early stages as well as molecular biological changes, mainly in the transforming growth factor-β superfamily pathway in enthesis. Statement of Clinical Significance: These results suggest that eccentric contraction-dominant exercise induces sports-related enthesopathy-like morphological changes in the early stages as well as molecular biological changes, mainly in the transforming growth factor-β superfamily pathway in enthesis.

The Enthesopathy of XLH Is a Mechanical Adaptation to Osteomalacia: Biomechanical Evidence from Hyp Mice

A major comorbidity of X-linked hypophosphatemia (XLH) is fibrocartilaginous tendinous insertion site mineralization resulting in painful enthesophytes that contribute to the adult clinical picture and significantly impact physical function. Enthesophytes in Hyp mice, a murine model of XLH are the result of a hyperplastic expansion of resident alkaline phosphatase, Sox9-positive mineralizing fibrochondrocytes. Here, we hypothesized hyperplasia as a compensatory physical adaptation to aberrant mechanical stresses at the level of the entheses interface inserting into pathologically soft bone. To test this hypothesis, we examined the Achilles insertion of the triceps surae developed under normal and impaired loading conditions in Hyp and WT mice. Tensile stiffness, ultimate strength, and maximum strain were measured and compared. Biomechanical testing revealed that under normal loading conditions, despite inserting into a soft bone matrix, both the enthesophyte development (9 weeks) and progression (6-8 months) of Hyp mice were equivalent to the mechanical properties of WT mice. Unloading the insertion during development significantly reduced alkaline phosphatase, Sox9-positive fibrochondrocytes. In WT mice, this correlated with a decrease in stiffness and ultimate strength relative to the control limb, confirming the critical role of mechanical loading in the development of the enthesis. Most significantly, in response to unloading, maximum strain was increased in tensile tests only in the setting of subchondral osteomalacia of Hyp mice. These data suggest that mineralizing fibrochondrocyte expansion in XLH occurs as a compensatory adaptation to the soft bone matrix.

Pathogenesis of FGF23-Related Hypophosphatemic Diseases Including X-linked Hypophosphatemia

Since phosphate is indispensable for skeletal mineralization, chronic hypophosphatemia causes rickets and osteomalacia. Fibroblast growth factor 23 (FGF23), which is mainly produced by osteocytes in bone, functions as the central regulator of phosphate metabolism by increasing the renal excretion of phosphate and suppressing the production of 1,25-dihydroxyvitamin D. The excessive action of FGF23 results in hypophosphatemic diseases, which include a number of genetic disorders such as X-linked hypophosphatemic rickets (XLH) and tumor-induced osteomalacia (TIO). Phosphate-regulating gene homologous to endopeptidase on the X chromosome (PHEX), dentin matrix protein 1 (DMP1), ectonucleotide pyrophosphatase phosphodiesterase-1, and family with sequence similarity 20c, the inactivating variants of which are responsible for FGF23-related hereditary rickets/osteomalacia, are highly expressed in osteocytes, similar to FGF23, suggesting that they are local negative regulators of FGF23. Autosomal dominant hypophosphatemic rickets (ADHR) is caused by cleavage-resistant variants of FGF23, and iron deficiency increases serum levels of FGF23 and the manifestation of symptoms in ADHR. Enhanced FGF receptor (FGFR) signaling in osteocytes is suggested to be involved in the overproduction of FGF23 in XLH and autosomal recessive hypophosphatemic rickets type 1, which are caused by the inactivation of PHEX and DMP1, respectively. TIO is caused by the overproduction of FGF23 by phosphaturic tumors, which are often positive for FGFR. FGF23-related hypophosphatemia may also be associated with McCune-Albright syndrome, linear sebaceous nevus syndrome, and the intravenous administration of iron. This review summarizes current knowledge on the pathogenesis of FGF23-related hypophosphatemic diseases.

Musculoskeletal Comorbidities and Quality of Life in ENPP1-Deficient Adults and the Response of Enthesopathy to Enzyme Replacement Therapy in Murine Models

Ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) deficiency leads to cardiovascular calcification in infancy, fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic rickets in childhood, and osteomalacia in adulthood. Excessive enthesis mineralization and cervical spine fusion have been previously reported in patients with biallelic ENPP1 deficiency, but their effect on quality of life is unknown. We describe additional musculoskeletal complications in patients with ENPP1 deficiency, namely osteoarthritis and interosseous membrane ossification, and for the first time evaluate health-related quality of life (HRQoL) in patients with this disease, both subjectively via narrative report, and objectively via the Brief Pain Inventory-Short Form, and a Patient Reported Outcome Measurement Information System Physical Function (PROMIS PF) short form. Residual pain, similar in magnitude to that identified in adult patients with X-linked hypophosphatemia, was experienced by the majority of patients despite use of analgesic medications. Impairment in physical function varied from mild to severe. To assess murine ENPP1 deficiency for the presence of enthesopathy, and for the potential response to enzyme replacement therapy, we maintained Enpp1asj/asj mice on regular chow for 23 weeks and treated cohorts with either vehicle or a long-acting form of recombinant ENPP1. Enpp1asj/asj mice treated with vehicle exhibited robust calcification throughout their Achilles tendons, whereas two-thirds of those treated with ENPP1 enzyme replacement exhibited complete or partial suppression of the Achilles tendon calcification. Our combined results document that musculoskeletal complications are a significant source of morbidity in biallelic ENPP1 deficiency, a phenotype which is closely recapitulated in Enpp1asj/asj mice. Finally, we show that a long-acting form of recombinant ENPP1 prevents the development of enthesis calcification at the relatively modest dose of 0.3 mg/kg per week, suggesting that suppression of enthesopathy may be attainable upon dose escalation. © 2021 American Society for Bone and Mineral Research (ASBMR). This article has been contributed to by US Government employees and their work is in the public domain in the USA.

FGF23 signalling and physiology

Fibroblast growth factor 23 (FGF23) is a phosphotropic hormone that belongs to a subfamily of endocrine FGFs with evolutionarily conserved functions in worms and fruit flies. FAM20C phosphorylates FGF23 post-translationally, targeting it to proteolysis through subtilisin-like proprotein convertase FURIN, resulting in secretion of FGF23 fragments. O-glycosylation of FGF23 through GALNT3 appears to prevent proteolysis, resulting in secretion of biologically active intact FGF23. In the circulation, FGF23 may undergo further processing by plasminogen activators. Crystal structures show that the ectodomain of the cognate FGF23 receptor FGFR1c binds with the ectodomain of the co-receptor alpha-KLOTHO. The KLOTHO-FGFR1c double heterodimer creates a high-affinity binding site for the FGF23 C-terminus. The topology of FGF23 deviates from that of paracrine FGFs, resulting in poor affinity for heparan sulphate, which may explain why FGF23 diffuses freely in the bone matrix to enter the bloodstream following its secretion by cells of osteoblastic lineage. Intact FGF23 signalling by this canonical pathway activates FRS2/RAS/RAF/MEK/ERK1/2. It reduces serum phosphate by inhibiting 1,25-dihydroxyvitamin D synthesis, suppressing intestinal phosphate absorption, and by downregulating the transporters NPT2a and NPT2c, suppressing phosphate reabsorption in the proximal tubules. The physiological role of FGF23 fragments, which may be inhibitory, remains unclear. Pharmacological and genetic activation of canonical FGF23 signalling causes hypophosphatemic disorders, while its inhibition results in hyperphosphatemic disorders. Non-canonical FGF23 signalling through binding and activation of FGFR3/FGFR4/calcineurin/NFAT in an alpha-KLOTHO-independent fashion mainly occurs at extremely elevated circulating FGF23 levels and may contribute to mortality due to cardiovascular disease and left ventricular hypertrophy in chronic kidney disease.

Bi-fated tendon-to-bone attachment cells are regulated by shared enhancers and KLF transcription factors

The mechanical challenge of attaching elastic tendons to stiff bones is solved by the formation of a unique transitional tissue. Here, we show that murine tendon-to-bone attachment cells are bi-fated, activating a mixture of chondrocyte and tenocyte transcriptomes, under regulation of shared regulatory elements and Krüppel-like factors (KLFs) transcription factors. High-throughput bulk and single-cell RNA sequencing of humeral attachment cells revealed expression of hundreds of chondrogenic and tenogenic genes, which was validated by in situ hybridization and single-molecule ISH. ATAC sequencing showed that attachment cells share accessible intergenic chromatin areas with either tenocytes or chondrocytes. Epigenomic analysis revealed enhancer signatures for most of these regions. Transgenic mouse enhancer reporter assays verified the shared activity of some of these enhancers. Finally, integrative chromatin and motif analyses and transcriptomic data implicated KLFs as regulators of attachment cells. Indeed, blocking expression of both Klf2 and Klf4 in developing limb mesenchyme impaired their differentiation.

X-Linked Hypophosphatemia: A New Era in Management

X-linked hypophosphatemia (XLH) is a rare, hereditary, progressive musculoskeletal disease that often causes pain and short stature, as well as decreased physical function, mobility, and quality of life. Hypophosphatemia in XLH is caused by loss of function mutations in the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene, resulting in excess levels of the phosphate-regulating hormone fibroblast growth factor 23 (FGF23), which leads to renal phosphate wasting and decreased serum 1,25-dihydroxyvitamin D production. Historically, treatment options were limited to oral phosphate and active vitamin D analogues (conventional management) dosed several times daily in an attempt to improve skeletal mineralization by increasing serum phosphorus. The recent approval of burosumab, a fully human monoclonal antibody to FGF23, has provided a new, targeted treatment option for patients with XLH. This review summarizes our current understanding of XLH, the safety and efficacy of conventional management and burosumab, existing recommendations for managing patients, and unanswered questions in the field.

Loss and gain of bone in spondyloarthritis: what drives these opposing clinical features?

The breadth of bone lesion types seen in spondyloarthritis is unprecedented in medicine and includes increased bone turnover, bone loss and fragility, osteitis, osteolysis and erosion, osteosclerosis, osteoproliferation of soft tissues adjacent to bone and spinal skeletal structure weakness. Remarkably, these effects can be present simultaneously in the same patient. The search for a potential unifying cause of effects on the skeleton necessarily focuses on inflammation arising from the dysregulation of immune response to microorganisms, particularly dysregulation of T_H17 lymphocytes, and the dysbiosis of established gut and other microbiota. The compelling notion that a common antecedent pathological mechanism affects existing bone and tissues with bone-forming potential (entheses), simultaneously with variable effect in the former but bone-forming in the latter, drives basic research forward and focuses our awareness on the effects on these bone mechanisms of the increasing portfolio of targeted immunotherapies used in the clinic.

Immunofluorescent Staining of Adult Murine Paraffin-Embedded Skeletal Tissue

Immunohistochemistry, or immunolabeling, is a key method for the identification of protein expression and localization. Successful detection relies on a low signal-to-noise ratio, which is affected greatly by antibody specificity as well as the staining protocol. Immunohistochemistry in the mouse is challenging, particularly in adult skeletal tissue, due to the need for long decalcification, high autofluorescence and high levels of endogenous peroxidase. Here, we describe a highly sensitive protocol for protein detection in decalcified paraffin-embedded sections from adult mouse skeletal tissue. By using four levels of amplification, this method allows for the identification of even low-abundance proteins.

Mineralized tissues in hypophosphatemic rickets

Hypophosphatemic rickets is caused by renal phosphate wasting that is most commonly due to X-linked dominant mutations in PHEX. PHEX mutations cause hypophosphatemia indirectly, through the increased expression of fibroblast growth factor 23 (FGF23) by osteocytes. FGF23 decreases renal phosphate reabsorption and thereby increases phosphate excretion. The lack of phosphate leads to a mineralization defect at the level of growth plates (rickets), bone tissue (osteomalacia), and teeth, where the defect facilitates the formation of abscesses. The bone tissue immediately adjacent to osteocytes often remains unmineralized ("periosteocytic lesions"), highlighting the osteocyte defect in this disorder. Common clinical features of XLH include deformities of the lower extremities, short stature, enthesopathies, dental abscesses, as well as skull abnormalities such as craniosynostosis and Chiari I malformation. For the past four decades, XLH has been treated by oral phosphate supplementation and calcitriol, which improves rickets and osteomalacia and the dental manifestations, but often does not resolve all aspects of the mineralization defects. A newer treatment approach using inactivating FGF23 antibodies leads to more stable control of serum inorganic phosphorus levels and seems to heal rickets more reliably. However, the long-term benefits of FGF23 antibody treatment remain to be elucidated.

Development of Enthesopathies and Joint Structural Damage in a Murine Model of X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia, caused by inactivating mutations in the Phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. With aging, adult patients develop paradoxical heterotopic calcifications of tendons and ligaments at their insertion sites (enthesophytes), and joint alterations. Understanding the progression of this structural damage that severely affects patients’ quality of life will help to improve the management of XLH. Here, we characterized the occurrence of enthesophytes and joint alterations through a 12 month in vivo micro-CT follow-up in the Hyp mouse, a murine model of XLH (n = 5 mice per group). Similar to adult patients with XLH, Hyp mice developed calcaneal enthesophytes, hip joint alterations, erosions of the sacroiliac joints and periarticular calcifications. These lesions were already present at month 3 and gradually worsened over time. In sharp contrast, no abnormalities were observed in control mice at early time points. Histological analyses confirmed the presence of bone erosions, calcifications and expansion of mineralizing enthesis fibrocartilage in Hyp mice and their absence in controls and suggested that new bone formation is driven by altered mechanical strain. Interestingly, despite a strong deformation of the curvature, none of the Hyp mice displayed enthesophyte at the spine. Peripheral enthesophytes and joint alterations develop at the early stages of the disease and gradually worsen overtime. Overall, our findings highlight the relevance of this preclinical model to test new therapies aiming to prevent bone and joint complications in XLH.