Abstract Dysregulated fibroblast growth factor-23 (FGF23) production is a demonstrated cause of hypophosphatemia and osteomalacia. Diseases associated with these conditions include phosphaturic mesenchymal tumor (PMT) causing tumor induced osteomalacia (TIO), various forms of rickets, and fibrous dysplasia (FD). Coexistence of two conditions that can increase FGF23 concentrations is rare. We report a case of a 79-year-old man who presented with rib and right flank pain. Imaging revealed bone lesions in the right iliac wing, left supra-acetabular area, and L4 vertebral body. Biopsies showed a right iliac PMT and left supra-acetabular FD. Cryoablation of both lesions resolved the phosphaturia with normalization of phosphorus level. Coexistence of PMT and FD in this patient with hypophosphatemia raised questions about the source of the FGF23, meaning of co-existence of PMT and FD in the same patient and, about the nature of the third lesion in the L4 vertebral body. Using FGF23 mRNA chromogenic in situ hybridization, we identified the PMT, rather than the FD, as the source of FGF23. Lack of GNAS mutation in the PMT suggested it being independent of FD. Assessment by the intact FGF23: total FGF23 ratio as well as gallium-DOTATATE scan suggested that the vertebral body lesion could represent FD. Other than understanding difference in underlying molecular processing of FGF23 in PMT and FD, testing for mutations, imaging studies as well as in situ hybridization helped solve the questions arising from this unique case of coexistence of PMT and FD.

Abstract A lifetime of successive bone remodeling events leads to trabeculae, which are composed of a patchwork of bone structural units (BSU) called hemi-osteons or trabecular packets. Traditionally, only intact surface BSU have been studied, which are those that have been created most recently. Accordingly, the complex changes in the size and distribution of BSU throughout the trabeculae have been overlooked. In this study, the BSU within the trabeculae of the second lumbar vertebrae were manually traced using ImageJ software, in osteopontin immunostained sections of eight young women (aged 19–38 years) and eight older women (aged 69–96 years). A series of BSU profile properties including area, width, length, and perimeter were quantified, along with properties of each trabecular profile such as the number of BSU and cement line length. The relationships between these properties and age, as well as selected trabecular microstructural properties assessed with microcomputed tomography, and bone strength assessed on the neighboring third lumbar vertebrae, were investigated. The median BSU profile length and perimeter decreased with age, while median BSU profile area and width was unchanged. Moreover, age was associated with an increase in the number of BSU profiles and cement line length per trabecular profile area. However, changes in BSU profile geometry, the number of BSU profiles, and the cement line length per trabecular profile were strongly correlated with trabecular bone volume fraction, structure model index, and bone strength. Further research is needed to understand how these changes in BSU properties affect the mechanical and failure properties of trabecular bone.

Abstract The R47H variant of the Triggering Receptor Expressed on Myeloid Cells 2 (TREM2) is a risk factor for Alzheimer’s disease in humans and leads to lower bone mass accrual in female but not male 12-month-old mice. To determine whether, as with aging, gonadectomy results in sex-specific musculoskeletal effects, gonad removal or SHAM surgery were performed in 4-month-old TREM2R47H/+ mice and wild type (WT) male and female littermates (n = 10-12/group), with sexes analyzed separately. Body weight was lower in males, but higher in females after gonadectomy, independently of their genotype. Gonadectomy also lead to decreased BMD in males at all sites and in whole body (total) and spine in female mice for both genotypes. Total and femur BMD was lower in gonadectomized male mice 6-weeks post-surgery, independently of the genotype. On the other hand, BMD was only lower in ovariectomized WT but not TREM2R47H/+ mice in all sites measured at this time point. Bone formation and resorption markers levels were not affected by orchiectomy, whereas CTX was higher 3 weeks after surgery and P1NP showed a tendency towards lower values at the 6 weeks timepoint only in ovariectomized WT mice. μCT analyses showed no differences resulting from gonadectomy in structural parameters in femoral cortical bone for either sex, but lower tissue mineral density in males of either genotype 6 weeks post-surgery. Nevertheless, biomechanical properties were overall lower in gonadectomized males of either genotype, and only for WT ovariectomized mice. Distal femur cancellous bone structure was also affected by gonadectomy in a genotype- and sex-dependent manner, with genotype-independent changes in males, and only in WT female mice. Thus, expression of the TREM2 R47H variant minimally alters the impact of gonadectomy in the musculoskeletal system in males, whereas it partially ameliorates the consequences of ovariectomy in female mice.

Abstract Osteoporosis is a metabolic bone disorder characterized by low bone mass and bone mineral density. It is the most prevalent bone disease and a common cause of fracture in aging adults. Low bone mass, as seen in osteoporosis, results from an imbalance between osteoblast and osteoclast activity. Gli1+ cells are indispensable to the maintenance of bone tissue homeostasis. These cells give rise to osteoprogenitors and are present at the osteogenic fronts of long bones in adult mice. Runx2 is a key regulator of osteogenesis and plays a crucial role in osteoblastic differentiation and maturation during development. However, its function in maintaining adult bone tissue homeostasis remains unclear. In this study, we investigated the role of Runx2 in maintaining adult bone homeostasis in the context of ovariectomy-induced estrogen deficiency, a model for postmenopausal osteoporosis. Our results show that deletion of Runx2 in the Gli1+ osteogenic progenitor population prevents loss of both cortical and trabecular bone mass and mineralization after ovariectomy. At the cellular level, loss of Runx2 leads to a decrease in osteoclast activity. Our study indicates that Runx2 is essential for maintaining adult bone tissue homeostasis by regulating osteoclast differentiation.

Abstract Iron deficiency anemia is treated by iron supplementation. Increasing evidence has shown that the carbohydrate components in iron infusions can cause hypophosphatemia and subsequent osteomalacia due to excess iFGF23 (intact Fibroblast Growth Factor 23). We here undertook an in-depth characterization of 13 patients with iron infusion-induced osteomalacia (IIIO). Patients were characterized (monocentric institutional practice) by means of laboratory, bone density, HR-pQCT and virtual osteoid volume estimation. We additionally report a patient who was treated with burosumab because iron infusions had to be continued despite osteomalacia. All 13 patients received ferric carboxymaltose (FCM) infusions and presented with low phosphate levels in the laboratory. Stopping the FCM infusions and supportive treatment by substitution of phosphate, calcium, native and/or active vitamin D was the chosen therapeutic approach. Pain, mobility, biochemical data, such as serum phosphate levels, bone mineral density, bone microstructure and virtual osteoid volume were the main outcome measures. Our results indicate biochemical normalization (e. g. phosphate levels pre: 0.50 mmol/l ± 0.23 mmol/l, post: 0.93 mmol/l ± 0.32 mmol/l, P<.001) after stopping the FCM infusion and establishing supportive treatment. Additionally, pain levels on the visual analogue scale (VAS) decreased (VASpre 7.31 ± 1.22, VASpost 2.73 ± 1.27, P<.0001) and areal bone mineral density (expressed as T-score) improved significantly (T-scorepre: -1.85 ± 1.84, T-scorepost: -0.91 ± 2.13, P<.05). One patient requiring ongoing FCM infusions experienced significant additional benefits from burosumab treatment. In conclusion, our results highlight the importance of monitoring phosphate in patients treated with FCM infusions. Stopping FCM infusions is effective in addressing the excess of iFGF23 and thereby phosphate wasting. Supportive therapy enables quick recovery of the musculoskeletal system and improves pain levels in these patients.

Abstract Osteoclasts are large multinucleated cells that degrade bone mineral and extracellular matrix. Investigating the epigenetic mechanisms orchestrating osteoclast differentiation is key to our understanding of the pathogenesis of skeletal related diseases such as periodontitis and osteoporosis. Lysine specific demethylase 1 (LSD1/KDM1A) is a member of the histone demethylase family that mediates the removal of mono- and dimethyl groups from H3K4 and H3K9 to elicit dichotomous effects on gene expression. Prior to our study, little was known about the contributions of LSD1 to skeletal development and osteoclast differentiation. Here we show that conditional deletion of Lsd1 within the myeloid lineage or macrophage/osteoclast precursors results in enhanced bone mass of male and female mice accompanied by diminished osteoclast size in vivo. Furthermore, Lsd1 deletion decreased osteoclast differentiation and activity within in vitro assays. Our bulk RNA-SEQ data suggests Lsd1 ablation in male and female mice inhibits osteoclast differentiation due to enhanced expression of IFN-β target genes. Lastly, we demonstrate that LSD1 forms an immune complex with HDAC1 and HDAC2. These data suggest that the combination of methylation and acetylation of histone residues, facilitated by LSD1, mechanistically promotes osteoclast gene expression.

Abstract Tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR) is a key diagnostic test for renal phosphate wasting. However, there is a lack of international consensus regarding the reporting of age-related reference intervals. The TmP/GFR Harmonization Working Group, formed by The Australasian Association for Clinical Biochemistry and Laboratory Medicine and The Australian and New Zealand Bone and Mineral Society, aimed to evaluate analytical differences amongst commercial creatinine and phosphate assays, harmonize age-specific cut-offs and compare the ANZBMS TmP/GFR online calculator with existing products. A total of 11 360 results from The Royal College of Pathologists of Australasia Quality Assurance Programs were analyzed to assess creatinine and phosphate assay performance amongst five In Vitro Diagnostic (IVD) companies. Ortho-Clinical Diagnostics analyzers had a positive bias of up to 32% for serum phosphate and up to 29% for TmP/GFR. The other IVD companies produced comparable results and are suitable for harmonized reference intervals. To date, the AACB-ANZBMS TmP/GFR online calculator is the only validated Isotope Dilution Mass Spectrometry-creatinine aligned tool which caters for both pediatric and adult individuals, providing automatic interpretive comments to aid clinicians managing patients with hypophosphatemia.

Abstract Radiofrequency echographic multi spectrometry (REMS) is an innovative, non-ionizing diagnostic technique that has shown high accuracy and precision, making it a promising alternative to dual-energy X-ray absorptiometry (DXA) for osteoporosis diagnosis in clinical settings. With economic considerations playing an increasingly crucial role in healthcare decisions, this study aims to evaluate the cost-effectiveness and economic impact of improved osteoporosis diagnosis using REMS followed by treatment in the United States (US). A microsimulation-based Markov model was constructed to estimate the cost per quality-adjusted life year (QALY) gained (in US$2022) for REMS followed by treatment versus no diagnosis and treatment in US women aged 50 years and older with osteoporosis. Women were categorized as high risk (receiving alendronate monotherapy for 5 years) or very high risk (receiving an 18-month course of anabolic treatment, abaloparatide, followed by 5 years of alendronate). The study evaluated two medication adherence scenarios: one assuming full adherence to treatment and the other reflecting real-world adherence. The results indicate that REMS followed by treatment is associated with improved health outcomes, including more QALYs and fewer fractures, and reduced fracture-related costs compared to no diagnosis and treatment. The incremental cost-effectiveness ratio of REMS was estimated at $33 891 and $49 198 per QALY gained, under the full adherence and real-world adherence scenarios, respectively. These values are below the US cost-effectiveness thresholds of $100 000 to $150 000 per QALY. Moreover, a 5% increase in the diagnosis and treatment of women over 50 years at high and very high risk of fractures using REMS is projected to save approximately 30 000 life years, 43 500 QALYs, and prevent 100 000 fractures over a lifetime under real-world medication adherence. In conclusion, this study suggests that REMS is a cost-effective strategy for the diagnosis and management of osteoporosis in US women, offering substantial potential economic benefits and improved health outcomes.

Thalassemic osteopathy includes low bone mass and impaired bone microarchitecture. We aimed to evaluate the prevalence and determinants of bone quantity (osteoporosis) and quality (microarchitecture) in a cohort of adult patients with transfusion-dependent thalassemia (TDT). Patients with TDT (n = 63) and age- and BMI-matched controls (n = 63) were recruited in the study. Areal bone mineral density (BMD) was measured using DXA Hologic scanner. P1NP and β-CTX were estimated by electrochemiluminescence assay. Bone geometry and volumetric BMD (vBMD) were estimated by second-generation high-resolution peripheral quantitative computed tomography. Bone turnover marker β-CTX was significantly lower in the TDT group, but there was no difference in P1NP levels. Low bone mass (Z ≤ -2) was present in greater proportion of patients both at lumbar spine (LS) (54 vs 0%; p = .001) and femoral neck (FN) (33 vs 8%; p = .001). Hypogonadism was associated with low BMD at FN (OR 10.0; 95% CI, 1.2-86; p = .01) and low hemoglobin with low BMD at LS (OR 1.58; 95% CI, 0.96-2.60; p = .07). The mean trabecular bone score was also significantly lower in patients compared with controls (1.261 ± 0.072 vs 1.389 ± 0.058). Total, cortical and trabecular vBMD were significantly lower in cases than controls. The trabecular number and cortical thickness were significantly lower and trabecular separation higher in cases than controls. Adults with TDT have significantly lower areal, cortical and trabecular vBMD. The bone microarchitecture is also significantly impaired in terms of lower number and wider spacing of trabeculae as well as lower cortical thickness and area at both radius and tibia.

Osteoporosis results in low-trauma fractures affecting millions globally, in particular elderly populations. Despite the inclusion of physical activity in fracture prevention strategies, the optimal bone-strengthening exercises remain uncertain, highlighting the need for a deeper understanding of lower limb joint loading dynamics across various exercise types and levels. This study examines lower limb joint loading during high-impact exercises across different intensities. A total of 40 healthy, active participants were recruited (mean ± SD: age of 40.3 ± 13.1yr; height 1.71 ± 0.08m; and mass 68.44 ± 11.67kg). Motion capture data and ground reaction forces of 6 different exercises: a self-selected level of walking, running, countermovement jump, squat jump, unilateral hopping, and bilateral hopping were collected for each participant. Joint reaction forces were estimated using lower body musculoskeletal models developed in OpenSim. Running and hopping increased joint forces compared to walking, notably at the hip (83% and 21%), knee (134% and 94%), and ankle (94% and 77%), while jump exercises reduced hip and ankle loading compared to walking (36% and 19%). Joint loading varied with exercise type and intensity, with running faster increasing forces on all joints, particularly at the hip. Sprinting increased forces at the hip but lowered knee and ankle forces. Higher jumps intensified forces on all joints, while faster hopping reduced forces. The wide variation of lower limb joint loading observed across the exercises tested in this study underscores the importance of implementing diverse exercise routines to optimize overall bone health and strengthen the musculoskeletal structure. Practitioners must therefore ensure that exercise programs include movements that are specifically suitable for their intended purpose.