Effects of tumor-induced osteomalacia on the bone mineralization process

Approaching virtual osteoid volume estimation and in-depth tissue characterization in patients with tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) poses a significant diagnostic challenge, leading to increased disease duration and patient burden also by missing clinical suspicion. Today, diagnosis of osteomalacia relies on invasive iliac crest biopsy, if needed. Therefore, a noninvasive method would be beneficial for patients with severe osteomalacia, such as TIO, to inform their clinical management and address specific needs, like estimating the regeneration capacity at high osteoid volumes (OVs) or the potential of a hungry bone syndrome after tumor removal. Furthermore, given the lack of comprehensive histological characterization of TIO, there is a need for additional tissue characterization. Therefore, our assessment encompassed iliac crest biopsies that were examined using quantitative electron backscattered microscopy, Raman spectroscopy, micro-computed tomography, and histology to analyze the biopsy tissue. Our clinical assessment encompassed DXA and high-resolution peripheral quantitative computed tomography (HR-pQCT) alongside with biochemical analyses and clinical evaluations. Combining imaging and clinical data, we established a model to predict the OV. We compared 9 TIO patients with 10 osteoporosis (OPO) patients and 10 healthy controls. Histological analyses confirmed a pronounced OV in TIO patients (OPO: 1.20% ± 1.23% vs TIO: 23.55% ± 12.23%, P < .0005), and spectroscopy revealed lower phosphate levels in TIO biopsies. By combining HR-pQCT and laboratory diagnostics, we developed a linear regression model to noninvasively predict the OV revealing significantly higher modeled OV/BVmodel values of 24.46% ± 14.22% for TIO compared to the control group (5.952% ± 3.44%, P ≤ .001). By combining laboratory diagnostics, namely, ALP and Tt.BMDRadius measured by HR-pQCT, we achieved the calculation of the virtual osteoid volume to bone volume ratio (OV/BVmodel) with a significant correlation to histology as well as reliable identification of TIO patients compared to OPO and control. This novel approach is potentially helpful for predicting OV by noninvasive techniques in diagnostic procedures and improving the clinical management of TIO.

Bone Material Properties in Bone Diseases Affecting Children

PURPOSE OF REVIEW

Metabolic and genetic bone disorders affect not only bone mass but often also the bone material, including degree of mineralization, matrix organization, and lacunar porosity. The quality of juvenile bone is moreover highly influenced by skeletal growth. This review aims to provide a compact summary of the present knowledge on the complex interplay between bone modeling and remodeling during skeletal growth and to alert the reader to the complexity of bone tissue characteristics in children with bone disorders.

RECENT FINDINGS

We describe cellular events together with the characteristics of the different tissues and organic matrix organization (cartilage, woven and lamellar bone) occurring during linear growth. Subsequently, we present typical alterations thereof in disorders leading to over-mineralized bone matrix compared to those associated with low or normal mineral content based on bone biopsy studies. Growth spurts or growth retardation might amplify or mask disease-related alterations in bone material, which makes the interpretation of bone tissue findings in children complex and challenging.

Shift in Calcium From Peripheral Bone to Axial Bone After Tumor Resection in Patients With Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. After successful tumor resection, patients can recover from hypophosphatemia quicky. However, data on the changes in bone mineral density (BMD) and microstructure in the short term after surgery remained unclear.

OBJECTIVE

This work aimed to investigate the postoperative changes in BMD and microstructure both in peripheral and axial bone in TIO patients.

METHODS

We evaluated BMD and microarchitecture in 22 TIO patients using high-resolution peripheral quantitative computed tomography (HR-pQCT) and dual-energy x-ray absorptiometry (DXA) before and 3 months after surgery in this retrospective study.

RESULTS

In this study, a total of 22 TIO patients who had recovered serum phosphate levels postoperatively were enrolled. After surgery, areal BMD (aBMD) increased by 21.6% in the femoral neck, by 18.9% in the total hip, and by 29.5% in the lumbar spine. Moreover, TBS increased by 14.1% (all P < .001). In contrast, trabecular or cortical volumetric BMD (vBMD), and microstructure of trabecular bone (trabecular number, separation and bone volume ratio) and cortical bone (cortical thickness and porosity) at the distal radius or tibia were further deteriorated. Correlation analyses found that changes in femoral neck and total hip aBMD were both conversely associated with changes in trabecular vBMD and bone volume ratio, while positively correlated with change in trabecular separation at the distal radius.

CONCLUSION

Although aBMD and microstructure in the axial bone were improved, vBMD and microstructure in the peripheral bone were further impaired shortly after surgery. Correlation of improvement of aBMD in the total hip and femoral neck with deterioration of vBMD and microstructure at the distal radius indicated a shift in calcium from the peripheral bone to the axial bone in the short term after tumor resection in TIO patients.

A Case of Tumor-Induced Osteomalacia Detected by Venous Sampling

Tumor-induced osteomalacia (TIO) can cause osteomalacia due to excessive production of fibroblast growth factor 23 (FGF23) by the tumor. Since TIO is a very rare disease, it is often misdiagnosed as intervertebral disc herniation, spondyloarthritis, or osteoporosis. We report a 65-year-old man who developed generalized arthralgia and difficulty walking two years ago and was diagnosed with multiple fractures throughout his body. He was initially diagnosed with osteoporosis and was treated with calcitriol. However, he was referred to our hospital since his symptoms did not improve. We diagnosed tumor-induced osteomalacia based on low serum phosphorus, high bone-type alkaline phosphatase, high FGF23 levels, and the presence of two tumors. The responsible tumor was identified using FGF23 levels in venous sampling. As the location of the tumor made surgical resection difficult, we selected treatment with burosumab, a human monoclonal antibody against FGF23, leading to improvement in the hypophosphatemia and pain, such that he was able to walk with a cane. In cases of osteoporosis with hypophosphatemia, general physicians should keep TIO in mind, and attempt to identify the responsible tumor lesion.

High prevalence of vertebral deformity in tumor-induced osteomalacia associated with impaired bone microstructure

PURPOSE

Patients with tumor-induced osteomalacia (TIO) often suffer from irreversible height loss due to vertebral deformity. However, the prevalence of vertebral deformity in TIO patients varies among limited studies. In addition, the distribution and type of vertebral deformity, as well as its risk factors, remain unknown. This study aimed to identify the prevalence, distribution, type and risk factors for vertebral deformity in a large cohort of TIO patients.

METHODS

A total of 164 TIO patients were enrolled in this retrospective study. Deformity in vertebrae T4-L4 by lateral thoracolumbar spine radiographs was evaluated according to the semiquantitative method of Genant. Bone microstructure was evaluated by trabecular bone score (TBS) and high-resolution peripheral QCT (HR-pQCT).

RESULTS

Ninety-nine (99/164, 60.4%) patients had 517 deformed vertebrae with a bimodal pattern of distribution (T7-9 and T11-L1), and biconcave deformity was the most common type (267/517, 51.6%). Compared with patients without vertebral deformity, those with vertebral deformity had a higher male/female ratio, longer disease duration, more height loss, lower serum phosphate, higher bone turnover markers, lower TBS, lower areal bone mineral density (aBMD), lower peripheral volumetric BMD (vBMD) and worse microstructure. Lower trabecular vBMD and worse trabecular microstructure in the peripheral bone and lower spine TBS were associated with an increased risk of vertebral deformity independently of aBMD. After adjusting for the number of deformed vertebrae, we found little difference in clinical indexes among the patients with different types of vertebral deformity. However, we found significant correlations of clinical indexes with the number of deformed vertebrae and the spinal deformity index.

CONCLUSION

We reported a high prevalence of vertebral deformity in the largest cohort of TIO patients and described the vertebral deformity in detail for the first time. Risk factors for vertebral deformity included male sex, long disease duration, height loss, abnormal biochemical indexes and bone impairment. Clinical manifestation, biochemical indexes and bone impairment were correlated with the number of deformed vertebrae and degree of deformity, but not the type of deformity.

Hyperparathyroidism in a Large Cohort of Chinese Patients with Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. Hyperparathyroidism (HPT) including secondary HPT (SHPT) and tertiary HPT (THPT) in TIO patients, which is considered to associate with phosphate supplementation, has not been well documented.

OBJECTIVES

To clarify the prevalence, clinical characteristics, and risk factors for HPT in a large cohort of Chinese TIO patients in our hospital.

DESIGN, SETTING AND PARTICIPANTS

This retrospective study enrolled 202 TIO patients.

MAIN OUTCOME MEASUREMENTS

Occurrence of HPT in TIO patients.

RESULTS

HPT was observed in 91 patients (91/202, 45.1%), as 84 patients (41.6%) were diagnosed as SHPT and seven patients (3.5%) were diagnosed as THPT. All patients with THPT underwent parathyroidectomy and only one patient experienced recurrence. Compared with patients without HPT, patients with SHPT had longer disease duration, higher rate of phosphate and calcitriol supplementation, lower serum calcium, lower urine calcium excretion, and higher urine phosphate excretion. Compared with patients with SHPT, patients with THPT had even longer disease duration, higher rate of phosphate and calcitriol supplementation. PTH levels showed positive correlation with intact FGF23 and 1,25(OH)2D levels, but not 25OHD level in TIO patients. Multivariate logistic regression analysis showed that long disease duration and phosphate supplementation were independently associated with occurrence of HPT in TIO patients. Further logistic regression analysis and restricted cubic spline model revealed dose-response relationship between cumulative dose of phosphate supplementation and PTH levels.

CONCLUSIONS

HPT is common in our TIO patients. To avoid the occurrence of HPT in TIO patients, timely diagnosis and tumor resection is necessary, and excessive dose of phosphate supplementation is not suggested before surgery.

Tumor-Induced Osteomalacia: A Systematic Clinical Review of 895 Cases

Tumor-induced osteomalacia (TIO) is a rare and largely underdiagnosed paraneoplastic condition. Previous reviews often reported incomplete data on clinical aspects, diagnosis or prognosis. The aim of this study was to present a systematic clinical review of all published cases of TIO. A search was conducted in Pubmed, Embase, Web of Science from inception until April 23rd, 2020. We selected case reports and case series of patients diagnosed with TIO, with information on tumor localization and serum phosphate concentration. Two reviewers independently extracted data on biochemical and clinical characteristics including bone involvement, tumor localization and treatment. 468 articles with 895 unique TIO cases were included. Median age was 46 years (range 9 months-90 years) and 58.3% were males. Hypophosphatemia and inappropriately low or normal 1,25-dihydroxyvitamin D levels, characteristic for TIO, were present in 98% of cases. Median tumor size was 2.7 cm (range 0.5 to 25.0 cm). Serum fibroblast growth factor 23 was related to tumor size (r = 0.344, P < 0.001). In 32% of the cases the tumor was detected by physical examination. Data on bone phenotype confirmed skeletal involvement: 62% of cases with BMD data had a T-score of the lumbar spine ≤ - 2.5 (n = 61/99) and a fracture was reported in at least 39% of all cases (n = 346/895). Diagnostic delay was longer than 2 years in more than 80% of cases. 10% were reported to be malignant at histology. In conclusion, TIO is a debilitating disease characterized by a long diagnostic delay leading to metabolic disturbances and skeletal impairment. Increasing awareness of TIO should decrease its diagnostic delay and the clinical consequences.

Bone Matrix Mineralization and Response to Burosumab in Adult Patients With X-Linked Hypophosphatemia: Results From the Phase 3, Single-Arm International Trial

X-linked hypophosphatemia (XLH) is characterized by excess fibroblast growth factor 23 (FGF23) secretion, renal phosphate wasting, and low 1,25(OH)₂ D₃ . Adult patients present with osteomalacia, hypomineralized periosteocytic lesions, bone fragility, and pain. Burosumab is a fully human monoclonal FGF23 antibody approved for XLH treatment. UX023-CL304 was an open-label, phase 3 study investigating the effects of burosumab on osteomalacia in adults with XLH, who remained untreated at least 2 years prior enrollment. Here, we present the effect of burosumab on bone material properties. We analyzed transiliac bone biopsy samples from 11 individuals before and after 48 weeks of subcutaneous burosumab treatment (1.0 mg/kg administered every 4 weeks). We used quantitative backscattered electron imaging (qBEI) and Fourier transform infrared imaging (FTIRI) to assess bone mineralization density distribution (BMDD), mineralized bone volume, properties of the organic matrix, and size of periosteocytic lesions. The outcomes were compared with reference values from healthy adults and with four XLH patients either untreated or treated by conventional therapy. Prior to burosumab, the average mineralization in cancellous bone was lower than in healthy reference. CaLow, the fraction of lowly mineralized matrix, and CaHigh, the fraction of highly mineralized matrix, were both elevated resulting in a broad heterogeneity in mineralization (CaWidth). Burosumab resulted in a decrease of CaHigh toward normal range, whereas CaLow and CaWidth remained elevated. The mineralized bone volume was notably increased (+35.9%). The size of the periosteocytic lesions was variable but lower than in untreated XLH patients. FTIRI indicated decreased enzymatic collagen crosslink ratio heterogeneity. In summary, matrix mineralization in XLH is very heterogeneous. Highly mineralized regions represent old bone packets, probably protected from osteoclastic resorption by osteoid seams. The concomitant decrease of highly mineralized matrix, persistence of lowly mineralized matrix, and increase in mineralized bone volume after burosumab suggest a boost in mineralization of preexisting unmineralized or very lowly mineralized matrix, providing a potential explanation for previously observed improved osteomalacia. © 2022 American Society for Bone and Mineral Research (ASBMR).

Bone Impairment in a Large Cohort of Chinese Patients With Tumor-Induced Osteomalacia Assessed by HR-pQCT and TBS

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. Previous studies have revealed generalized mineralization defects and low areal bone mineral density (aBMD) in TIO. However, data on the bone microarchitecture in TIO are limited. In this study, we evaluated the microarchitecture in the peripheral (distal radius and tibia) and axial (lumbar spine) skeleton using high-resolution peripheral quantitative computed tomography (HR-pQCT) and trabecular bone score (TBS) and investigated related factors in a large cohort of Chinese patients with TIO. A total of 186 patients with TIO who had undergone dual-energy X-ray absorptiometry (DXA) or HR-pQCT scans were enrolled. Compared with age-, sex-, and body mass index (BMI)-matched healthy controls, TIO patients (n = 113) had lower volumetric BMD, damaged microstructure, and reduced bone strength in the peripheral skeleton, especially at the tibia. The average TBS obtained from 173 patients was 1.15 ± 0.16. The proportion of patients with abnormal TBS (<1.35) was higher than that with low L₁ to L₄ aBMD Z-score (Z ≤ -2) (43.9% versus 89.6%, p < 0.001). Higher intact fibroblast growth factor 23 (iFGF23), intact parathyroid hormone (iPTH), alkaline phosphatase, and β-isomerized C-terminal telopeptide of type I collagen (β-CTx) levels, more severe mobility impairment, and a history of fracture were associated with poorer HR-pQCT parameters but not with lower TBS. However, greater height loss and longer disease duration were correlated with worse HR-pQCT parameters and TBS. Moreover, TBS was correlated with both trabecular and cortical HR-pQCT parameters in TIO. In conclusion, we revealed impaired bone microarchitecture in the axial and peripheral skeleton in a large cohort of Chinese TIO patients. HR-pQCT parameters and TBS showed promising advantages over aBMD for assessing bone impairment in patients with TIO. A longer follow-up period is needed to observe changes in bone microarchitecture after tumor resection. © 2021 American Society for Bone and Mineral Research (ASBMR).

[Tumor localization and treatment of tumor-induced osteomalacia]

Tumor-induced osteomalacia (TIO) or oncogenic osteomalacia (OOM) is a rare paraneoplastic renal phosphate wasting syndrome. The disease is mostly triggered by small, benign mesenchymal tumors that express somatostatin receptors (SSTR) and produce excessive levels of fibroblast growth factor 23 (FGF 23) or other phosphatonins. These reduce the phosphate back resorption in the proximal tubules of the kidneys, thereby causing hypophosphatemia and lead to an absolute or relatively low calcitriol serum concentration. The main symptoms include muscle weakness, bone pain and recurrent insufficiency fractures secondary to sometimes pronounced osteomalacia. The suspected diagnosis can only be confirmed by determination of the phosphate level. It can often take years before the tumor is successfully localized. The necessary tumor localization is often the most difficult step in the treatment before the OOM can be curatively treated by open surgical resection of the tumor. In recent years new approaches for faster tumor localization and treatment of the tumor have been developed. Positron emission tomography (PET) in co-registration with computed tomography (⁶⁸Ga-DOTA-TATE PET/CT) is currently the most sensitive imaging methodology for tumor detection. The application of the monoclonal FGF 23 antibody burosumab represents a promising new option in the treatment of inoperable adult OOM.

Microarchitectural parameters and bone mineral density in patients with tumour-induced osteomalacia by HR-pQCT and DXA

INTRODUCTION

Tumour-induced osteomalacia (TIO) is a rare paraneoplastic condition characterised by decreased tubular phosphate reabsorption. The purpose of this study is to evaluate bone mineral density (BMD) and microarchitecture in six TIO patients, compared with 18 healthy controls.

METHODS

Volumetric BMD and microarchitecture were evaluated by high-resolution peripheral quantitative computed tomography (HR-pQCT), and areal BMD by dual-energy X-ray absorptiometry (DXA). Differences between groups were significant for p < .05.

RESULTS

All TIO subjects were healthy until the development of diffuse bone pain and multiple skeletal fractures and deformities. At baseline, sPi and TmPi/GFR were low and patients were on vitamin D and phosphate replacement at the study. Compared with controls, TIO patients had lower aBMD at lumbar spine and hip, and lower vBMD at trabecular, cortical and entire bone, at distal radius (R) and distal tibia (T): trabecular vBMD (R = 118.3 × 177.1; T = 72.3 × 161.3 gHA/cm³ ); cortical vBMD (R = 782.3 × 866.5; T = 789.1 × 900.9 gHA/cm³ ); total region vBMD (R = 234.5 × 317; T = 167.1 × 295.8 gHA/cm³ ). Bone microarchitecture was very heterogeneous among patients and significantly different from controls: lower cortical thickness (R = 0.59 × 0.80; T = 0.90 × 1.31 mm), bone volume-to-total volume ratio (R = 0.09 × 0.14; T = 0.06 × 0.13) and Tb.N (R = 1.46 × 2.10; T = 0.93 × 1.96 mm^-1 ) and also higher Tb.Sp (R = 0.70 × 0.41; T = 1.28 × 0.45 mm) and Tb.1/N.SD (R = 0.42 × 0.18; T = 0.87 × 0.20 mm).

CONCLUSION

In this original study of TIO patients, DXA and HR-pQCT evaluation identified lower areal and volumetric BMD and severely impaired microarchitecture at cortical and trabecular bones, which probably contribute to bone fragility and fractures.

Impaired bone microarchitecture and strength in patients with tumor-induced osteomalacia

Some studies based on bone biopsy have demonstrated that in patients with tumor-induced osteomalacia (TIO) the mineralization process of the bone matrix is profoundly disturbed. However, the interrelationship between clinical and biochemical features and bone microarchitecture in this disease needs further analysis. With this purpose in mind, we set out three objectives: (i) to determine bone microarchitecture and estimated bone strength in a group of patients with tumor-induced osteomalacia using high-resolution peripheral quantitative computed tomography (HR-pQCT) and finite element analysis (FEA), (ii) to investigate correlations between duration of disease, biochemical features, bone density, HR-pQCT and FEA parameters, and (iii) to compare HR-pQCT and FEA parameters with a healthy control group. Ten patients with TIO were included. All patients had non-resolved disease. At the distal radius, all bone microarchitecture parameters were significantly affected in patients with TIO in comparison with healthy controls. At the distal tibia, all parameters were significantly impaired, except for trabecular thickness. All the parameters were more affected in the distal tibia than in the distal radius. Women with TIO (n = 7) had significantly lower bone strength parameters than healthy controls. In men (n = 3), bone strength parameters were significantly lower than in the control group at the distal tibia. Alkaline phosphatase levels exhibited a negative correlation with microarchitecture parameters, failure load, and stiffness. Higher levels of parathyroid hormone correlated with poorer microarchitecture parameters. We believe that in TIO, hormonal disturbances and the lack of mechanical stimulus specially converge to generate an extremely harmful combination for bone health. © 2021 American Society for Bone and Mineral Research (ASBMR).

3D Interrelationship between Osteocyte Network and Forming Mineral during Human Bone Remodeling

During bone remodeling, osteoblasts are known to deposit unmineralized collagenous tissue (osteoid), which mineralizes after some time lag. Some of the osteoblasts differentiate into osteocytes, forming a cell network within the lacunocanalicular network (LCN) of bone. To get more insight into the potential role of osteocytes in the mineralization process of osteoid, sites of bone formation are three-dimensionally imaged in nine forming human osteons using focused ion beam-scanning electron microscopy (FIB-SEM). In agreement with previous observations, the mineral concentration is found to gradually increase from the central Haversian canal toward pre-existing mineralized bone. Most interestingly, a similar feature is discovered on a length scale more than 100-times smaller, whereby mineral concentration increases from the LCN, leaving around the canaliculi a zone virtually free of mineral, the size of which decreases with progressing mineralization. This suggests that the LCN controls mineral formation but not just by diffusion of mineralization precursors, which would lead to a continuous decrease of mineral concentration from the LCN. The observation is, however, compatible with the codiffusion and reaction of precursors and inhibitors from the LCN into the bone matrix.

Interim Analysis of a Phase 2 Open-Label Trial Assessing Burosumab Efficacy and Safety in Patients With Tumor-Induced Osteomalacia

Patients with tumor-induced osteomalacia (TIO), an acquired paraneoplastic condition characterized by osteomalacia due to hypophosphatemia, exhibit a similar clinical picture to those with X-linked hypophosphatemic rickets/osteomalacia (XLH). The human monoclonal anti-fibroblast growth factor 23 (FGF23) antibody burosumab (KRN23) increases serum phosphate and improves bone turnover, fracture healing, pain, and physical function in XLH patients by inhibiting circulating FGF23; thus, burosumab is expected to be an effective treatment for TIO. We report here an interim analysis of a multicenter, open-label, intraindividual dose-adjustment study of burosumab (0.3 to 2.0 mg/kg every 4 weeks) in Japanese and Korean TIO patients. The primary endpoint was the fasting serum phosphate level at each visit. Key secondary endpoints were changes over time in bone biomarkers, pharmacodynamic markers, bone histomorphometric parameters, motor function, and patient-reported outcomes. Safety was assessed based on treatment-emergent adverse events (TEAEs). Thirteen patients received burosumab treatment, of whom 4 underwent bone biopsy. The mean dose after week 112 was approximately 1.0 mg/kg. After the first burosumab administration, mean serum phosphate levels increased and remained above the lower limit of normal and in the normal range from weeks 14 to 112. Bone biomarkers initially increased, reaching maximum values at week 16 or 24, and then gradually decreased. After burosumab treatment, patients were able to walk further (evaluated by the 6-minute walk test), reported decreased pain levels, and showed a tendency toward healing of baseline fractures and pseudofractures. Two patients discontinued, one each due to disease progression and consent withdrawal. Burosumab was generally well tolerated, with no treatment-related TEAEs of grade ≥3 and no treatment-related serious AEs. In conclusion, the interim results of this first study of burosumab to treat TIO patients indicate that this drug has the potential to provide clinical benefit for patients with unresectable tumors. The full study results are eagerly anticipated. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Effects of burosumab on osteocalcin and bone mineral density in patient with 15-year history of nonremission tumor-induced osteomalacia initially treated with conventional therapy: Case report

Excess fibroblast growth factor 23 (FGF23) causes hypophosphatemic osteomalacia, which is associated with impaired bone matrix mineralization. Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by over-secretion of FGF23 from a tumor. Burosumab, a fully human monoclonal antibody with activities against FGF23, was initially approved in Japan before the rest of the world for treatment of FGF23-associated hypophosphatemic osteomalacia by TIO. We report here a patient with a 15-year history of non-remission TIO initially treated with conventional therapy who was then switched to burosumab treatment. Persistent hypophosphatemia and a relative low level of osteocalcin (bone Gla protein, BGP) compared with bone alkaline phosphatase (BAP) level, indicating poor matrix mineralization, developed during long-term conventional therapy. Repeated surgical and stereotactic body radiation treatments did not result in complete resection of the causable tumor, and bone mineral density (BMD) gradually decreased. Ultimately, burosumab treatment was administered and the serum Pi concentration immediately normalized, while both BGP and BMD also showed a good response. This is first known case report of the detailed efficacy of burosumab for nonremission TIO as an alternative to conventional therapy.

Mineral density differences between femoral cortical bone and trabecular bone are not explained by turnover rate alone

Bone mineral density distributions (BMDDs) are a measurable property of bone tissues that depends strongly on bone remodelling and mineralisation processes. These processes can vary significantly in health and disease and across skeletal sites, so there is high interest in analysing these processes from experimental BMDDs. Here, we propose a rigorous hypothesis-testing approach based on a mathematical model of mineral heterogeneity in bone due to remodelling and mineralisation, to help explain differences observed between the BMDD of human femoral cortical bone and the BMDD of human trabecular bone. Recent BMDD measurements show that femoral cortical bone possesses a higher bone mineral density, but a similar mineral heterogeneity around the mean compared to trabecular bone. By combining this data with the mathematical model, we are able to test whether this difference in BMDD can be explained by (i) differences in turnover rate; (ii) differences in osteoclast resorption behaviour; and (iii) differences in mineralisation kinetics between the two bone types. We find that accounting only for differences in turnover rate is inconsistent with the fact that both BMDDs have a similar spread around the mean, and that accounting for differences in osteoclast resorption behaviour leads to biologically inconsistent bone remodelling patterns. We conclude that the kinetics of mineral accumulation in bone matrix must therefore be different in femoral cortical bone and trabecular bone. Although both cortical and trabecular bone are made up of lamellar bone, the different mineralisation kinetics in the two types of bone point towards more profound structural differences than usually assumed.

Newly formed and remodeled human bone exhibits differences in the mineralization process

During human skeletal growth, bone is formed via different processes. Two of them are: new bone formation by depositing bone at the periosteal (outer) surface and bone remodeling corresponding to a local renewal of tissue. Since in remodeling formation is preceded by resorption, we hypothesize that modeling and remodeling could require radically different transport paths for ionic precursors of mineralization. While remodeling may recycle locally resorbed mineral, modeling implies the transport over large distances to the site of bone apposition. Therefore, we searched for potential differences of size, arrangement and chemical composition of mineral particles just below surfaces of modeling and remodeling sites in femur midshaft cross-sections from healthy children. These bone sites were mapped using scanning synchrotron X-ray scattering, Raman microspectroscopy, energy dispersive X-ray analysis and quantitative backscattered electron microscopy. The results show clear differences in mineral particle size and composition between the sites, which cannot be explained by a change in the rate of mineral apposition or accumulation. At periosteal modeling sites, mineral crystals are distinctly larger, display higher crystallinity and exhibit a lower calcium to phosphorus ratio and elevated Na and Mg content. The latter may originate from Mg used for phase stabilization of mineral precursors and therefore indicate different time periods for mineral transport. We conclude that the mineralization process is distinctively different between modeling and remodeling sites due to varying requirements for the transport distance and, therefore, the stability of non-crystalline ionic precursors, resulting in distinct compositions of the deposited mineral phase. STATEMENT OF SIGNIFICANCE: In growing children new bone is formed either due to apposition of bone tissue e.g. at the outer ridge of long bones to allow growth in thickness (bone modeling), or in cavities inside the mineralized matrix when replacing tissue (bone remodeling). We demonstrate that mineral crystal shape and composition are not the same between these two sites, which is indicative of differences in mineralization precursors. We suggest that this may be due to a longer mineral transport distance to sites of new bone formation as compared to remodeling where mineral can be locally recycled.

Tumor-induced osteomalacia with the culprit lesion located in the palm: a case report

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome. We herein report a rare case of TIO in a 58-year-old Chinese man who presented with a large lump in the right palm. Clinical, biochemical, and radiological assessments were performed. Laboratory examination showed severe hypophosphatemia, phosphaturia, an elevated serum alkaline phosphatase level, and an elevated serum fibroblast growth factor 23 (FGF-23) level. Dual-energy X-ray absorptiometry showed low bone mineral density. Magnetic resonance imaging revealed an irregular mass located in the right palm and abnormal findings in several metacarpal bones. During the operation, the surgeons found that the tumor had penetrated the surrounding muscles. The tumor had unique characteristics of local tissue invasion. The patient’s symptoms fully resolved and his serum phosphorus level normalized, although his serum FGF-23 level remained slightly high in the postoperative phase. Our findings suggest that in some patients with TIO, the serum phosphorus level might return to the normal range despite a relatively high postoperative serum FGF-23 level. These patients should be kept under close observation and regularly surveyed for any evidence of a residual tumor.

Hypophosphatemia, Severe Bone Pain, Gait Disturbance, and Fatigue Fractures After Iron Substitution in Inflammatory Bowel Disease: A Case Report

Intravenous infusions of different iron formulations are recognized as a cause of hypophosphatemia. Chronic hypophosphatemia can alter bone metabolism and bone material structure. As a consequence, osteomalacia may develop and lead to bone fragility. Herein, we report a patient with Crohn's disease presenting with persistent hypophosphatemia and insufficiency fractures while receiving regular iron infusions due to chronic gastrointestinal bleeding. Previously, the patient regularly received vitamin D and also zoledronic acid. The patient underwent bone biopsy of the iliac crest that showed typical signs of osteomalacia with dramatically increased osteoid volume and decreased bone formation. Analysis of the bone mineralization density distribution (BMDD) revealed a more complex picture: On the one hand, there was a shift to higher matrix mineralization, presumably owing to low bone turnover; on the other hand, a broadening of the BMDD indicating more heterogeneous mineralization due to osteomalacia was also evident. This is the first report on changes of bone histomorphometry and bone matrix mineralization in iron-induced osteomalacia. © 2017 American Society for Bone and Mineral Research.

PAI-1 is a critical regulator of FGF23 homeostasis

Elevated levels of fibroblast growth factor 23 (FGF23), a bone-derived phosphaturic hormone, are associated with a number of pathologic conditions including chronic kidney disease, cardiac hypertrophy, and congestive heart failure. Currently, there are no specific treatments available to lower plasma FGF23 levels. We have recently reported that genetic plasminogen activator inhibitor-1 (PAI-1) deficiency provided a significant reduction in circulating FGF23 levels while simultaneously prolonging the life span of Klotho-deficient mice. We extend our investigations into the effect of PAI-1 on FGF23 homeostasis. Transgenic overexpression of PAI-1 resulted in threefold increase in FGF23 levels compared to wild-type littermates. Moreover, pharmacological modulation of PAI-1 activity with the small-molecule PAI-1 antagonist TM5441 significantly reduced FGF23 levels in PAI-1 transgenic and Klotho-deficient mice. In addition, TM5441 treatment or PAI-1 deficiency significantly accelerated the clearance of endogenous FGF23 and recombinant human FGF23 from circulation in mice with acute kidney injury. On the basis of these observations, we studied the effects of plasminogen activators (PAs), tissue-type PA (tPA) and urokinase-type PA (uPA), on FGF23. We demonstrate that both PAs directly cleave FGF23; however, it is not known whether the PA-generated FGF23 peptides retain or acquire functions that affect binding and/or signaling properties of intact FGF23. PAI-1 inhibits the PA-dependent cleavage of FGF23, and TM5441 inhibition of PAI-1 restores the proteolysis of FGF23. Furthermore, top-down proteomic analysis indicates that tPA cleaves FGF23 at multiple arginines including the proconvertase sensitive site R176. In summary, our results indicate that PAI-1 prevents the PA-driven proteolysis of FGF23 and PAI-1 inhibition provides a novel therapeutic approach to prevent the pathologic consequences of increased FGF23.

Abnormally High and Heterogeneous Bone Matrix Mineralization After Childhood Solid Organ Transplantation: A Complex Pathology of Low Bone Turnover and Local Defects in Mineralization

Chronic renal, liver, and heart failure in children associates with multiple skeletal complications. Increased fracture incidence often persists after transplantation and could be related to alterations in bone material properties. In the present cohort study we evaluated bone mineralization density distribution (BMDD) by quantitative backscattered electron imaging (qBEI) in 23 pediatric solid organ allograft recipients with suspected osteoporosis. We measured BMDD in the entire cross-sectional area of transiliac bone biopsies obtained from kidney (n = 9), liver (n = 9), and heart (n = 5) transplant recipients (aged 7.6 to 19.7 years; 6.0 ± 5.6 years posttransplantation, patients with a history of clinical fractures: n = 14). The BMDD findings were compared with age-appropriate references and with a previously presented cohort of children with chronic kidney disease on dialysis (CKD5D, n = 18). Furthermore, we related the BMDD parameters with patients' clinical and bone histomorphometric outcomes. Compared to healthy children, qBEI results for cancellous and cortical bone in transplant recipients revealed an increase in the most frequently occurring calcium concentration (+2.9%, p = 0.001; +3.5%, p = 0.014), in the portion of fully mineralized bone (fivefold; 10-fold, both p < 0.0001) and in heterogeneity of mineralization (+26,5% and +27.8%, both p < 0.0001), respectively. Moreover, the BMDD parameters were nonsignificantly distinct from CKD5D cohort except that the heterogeneity in mineralization was higher posttransplantation. There was a strong inverse correlation between the average calcium content of the bone matrix and patients' biochemical ALP levels, histomorphometric indices of bone formation and resorption. The abnormally high bone matrix mineralization in transplant recipients, consistent with serum and histomorphometric outcomes, suggests a history of low bone turnover with accumulation of fully mineralized bone packets. Additionally, the increased heterogeneity of mineralization suggests local alterations in mineralization kinetics, which may be linked to dysfunctional osteocytes that were recently shown to accumulate within the bone matrix during organ failure and concomitant glucocorticoid and immunosuppressive medication. © 2017 American Society for Bone and Mineral Research.

A Digital Model to Simulate Effects of Bone Architecture Variations on Texture at Spatial Resolutions of CT, HR-pQCT, and μCT Scanners

The quantification of changes in the trabecular bone structure induced by musculoskeletal diseases like osteoarthritis, osteoporosis, rheumatoid arthritis, and others by means of a texture analysis is a valuable tool which is expected to improve the diagnosis and monitoring of a disease. The reaction of texture parameters on different alterations in the architecture of the fine trabecular network and inherent imaging factors such as spatial resolution or image noise has to be understood in detail to ensure an accurate and reliable determination of the current bone state. Therefore, a digital model for the quantitative analysis of cancellous bone structures was developed. Five parameters were used for texture analysis: entropy, global and local inhomogeneity, local anisotropy, and variogram slope. Various generic structural changes of cancellous bone were simulated for different spatial resolutions. Additionally, the dependence of the texture parameters on tissue mineralization and noise was investigated. The present work explains changes in texture parameter outcomes based on structural changes originating from structure modifications and reveals that a texture analysis could provide useful information for a trabecular bone analysis even at resolutions below the dimensions of single trabeculae.

Quantifying mineralization using bone mineral density distribution in the mandible

BACKGROUND

Micro-computed tomography is an efficient method for quantifying the density and mineralization of mandibular microarchitecture. Conventional radiomorphometrics such as bone and tissue mineral density are useful in determining the average overall mineral content of a scanned specimen; however, relying solely on these metrics has limitations. Using bone mineral density distribution (BMDD), the complex array of mineralization densities within a bone sample can be portrayed. This information is particularly useful as a computational feature reflective of the rate of bone turnover. We demonstrate the utility of BMDD analyses in the rat mandible and generate a platform for further exploration of mandibular pathology and treatment.

METHODS

Male Sprague-Dawley rats (n = 8) underwent micro-computed tomography, and histogram data were generated from a selected volume of interest. A standard curve was derived for each animal, and reference criteria were defined. An average histogram was produced for the group, and descriptive analyses including the means and SDs are reported for each of the normative metrics.

RESULTS

M(peak) (3444 Hounsfield units [SD, 138]) and M(width) (2221 Hounsfield units [SD, 628]) are 2 metrics demonstrating reproducible parameters of BMDD with minimal variance. A total of 8 valuable metrics quantifying biologically significant events concerning mineralization are reported.

CONCLUSIONS

We quantify the vast wealth of information depicted in the complete spectrum of mineralization established by the BMDD analysis. We demonstrate its potential in delivering mineralization data that encompass and enhance conventional reporting of radiomorphometrics. Moreover, we explore its role and translational potential in craniofacial experimentation.

Tumor-induced osteomalacia: an important cause of adult-onset hypophosphatemic osteomalacia in China: Report of 39 cases and review of the literature

Tumor-induced osteomalacia (TIO) is an acquired form of hypophosphatemia. Tumor resection leads to cure. We investigated the clinical characteristics of TIO, diagnostic methods, and course after tumor resection in Beijing, China, and compared them with 269 previous published reports of TIO. A total of 94 patients with adult-onset hypophosphatemic osteomalacia were seen over a 6-year period (January, 2004 to May, 2010) in Peking Union Medical College Hospital. After physical examination (PE), all patients underwent technetium-99m octreotide scintigraphy ((99) Tc(m) -OCT). Tumors were removed after localization. The results demonstrated that 46 of 94 hypophosphatemic osteomalacia patients had high uptake in (99) Tc(m) -OCT imaging. Forty of them underwent tumor resection with the TIO diagnosis established in 37 patients. In 2 patients, the tumor was discovered on PE but not by (99) Tc(m) -OCT. The gender distribution was equal (M/F = 19/20). Average age was 42 ± 14 years. In 35 patients (90%), the serum phosphorus concentration returned to normal in 5.5 ± 3.0 days after tumor resection. Most of the tumors (85%) were classified as phosphaturic mesenchymal tumor (PMT) or mixed connective tissue variant (PMTMCT). Recurrence of disease was suggested in 3 patients (9%). When combined with the 269 cases reported in the literature, the mean age and sex distribution were similar. The tumors were of bone (40%) and soft tissue (55%) origins, with 42% of the tumors being found in the lower extremities. In summary, TIO is an important cause of adult-onset hypophosphatemia in China. (99) Tc(m) -OCT imaging successfully localized the tumor in the overwhelming majority of patients. Successful removal of tumors leads to cure in most cases, but recurrence should be sought by long-term follow-up.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare and fascinating paraneoplastic syndrome in which patients present with bone pain, fractures, and muscle weakness. The cause is high blood levels of the recently identified phosphate and vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23). In TIO, FGF23 is secreted by mesenchymal tumors that are usually benign, but are typically very small and difficult to locate. FGF23 acts primarily at the renal tubule and impairs phosphate reabsorption and 1α-hydroxylation of 25-hydroxyvitamin D, leading to hypophosphatemia and low levels of 1,25-dihydroxy vitamin D. A step-wise approach utilizing functional imaging (F-18 fluorodeoxyglucose positron emission tomography and octreotide scintigraphy) followed by anatomical imaging (computed tomography and/or magnetic resonance imaging), and, if needed, selective venous sampling with measurement of FGF23 is usually successful in locating the tumors. For tumors that cannot be located, medical treatment with phosphate supplements and active vitamin D (calcitriol or alphacalcidiol) is usually successful; however, the medical regimen can be cumbersome and associated with complications. This review summarizes the current understanding of the pathophysiology of the disease and provides guidance in evaluating and treating these patients. Novel imaging modalities and medical treatments, which hold promise for the future, are also reviewed.

Evidence of reduced bone turnover and disturbed mineralization process in a boy with Stickler syndrome

We describe a tall-statured 14-year-old boy who illustrated the full phenotypic and radiographic features of Stickler syndrome type I. A bone biopsy showed evidence of reduced bone mass and bone turnover, such as reduced BV/TV (-43%), TbTh (-29%), and OS/BS (-48%), Ob.S/BS (-27%), and Oc/BS (-47%) compared to "age-matched" controls. Moreover, there was evidence that the mineralization process was severely disturbed. Quantitative backscattered electron imaging revealed that the bone mineralization density distribution (BMDD) of cancellous (Cn) as well as cortical (Ct) bone was shifted toward lower mineralization compared to a young control reference cohort. BMDD parameters of mean degree of mineralization, Cn Ca (-9.8%) and Ct Ca (-18.0%), were dramatically decreased. To the best of our knowledge this is the first clinical report describing bone biopsy findings in a boy with Stickler syndrome. Such a severe undermineralization of bone matrix might essentially contribute to the compromised mechanical competence of the skeleton found in this patient.

Bone turnover and the osteoprotegerin-RANKL pathway in tumor-induced osteomalacia: a longitudinal study of five cases

To evaluate serum levels of osteoprotegerin (OPG), soluble receptor activator of the nuclear factor-kappaB (RANKL), and their relationship with FGF-23, lumbar bone mineral density (BMD), and bone turnover markers, five patients with tumor-induced osteomalacia (TIO) and 40 healthy controls were studied. TIO patients were followed for 360 days after surgical removal of underlying tumor (n = 2) or beginning of therapy with phosphate and calcitriol when surgical treatment was impossible (n = 3). At diagnosis, TIO patients had higher levels of FGF-23 and bone-specific alkaline phosphatase (bALP) and lower levels of cathepsin K (CathK), RANKL, and RANKL/OPG ratio compared to controls. During the follow-up, FGF-23 decreased significantly only in patients who underwent a surgical excision, while phosphate and BMD increased in all patients. The increases in BMD, phosphate, and renal phosphate reabsorption rate were directly related. In the first 60 days of follow-up, we observed a prolonged inhibition of RANKL, CathK, and bone resorption markers associated with a persistence of TIO symptoms and an increase in bALP. From day 60, levels of bone turnover markers returned progressively within the normal range and a clinical remission was observed. The inhibition of the RANKL/OPG pathway and the uncoupling of bone formation and resorption observed in patients with active TIO may be a compensatory mechanism, attempting to reduce worsening of osteomalacia. The BMD increase during TIO treatment is related to the improvement of phosphate rather than FGF-23 levels. A "hungry bone"-like syndrome was observed after surgical or pharmacological treatment.