Oncogenic osteomalacia caused by a phosphaturic mesenchymal tumor of the oral cavity: a case report

Phosphaturic Mesenchymal Tumor in the Maxillofacial Region: A Diagnostic Dilemma

Oncogenic osteomalacia is a rare paraneoplastic syndrome and is associated with the presence of phosphaturic mesenchymal tumor (PMT) which results in renal phosphate wasting with hypophosphatemia. In total, 95% of cases reported in upper and lower extremities and in head and neck are a rare site for these tumors. Besides osteomalacia, the clinical presentation includes bone pain and multiple bone fractures. Only fewer cases of PMT are reported in the oral cavity. The presentation of this rare case in a young patient was palatal swelling mimicking like an abscess which was clinically and by advanced imaging evaluated and histopathological findings confirmed the rare presentation. Following the surgical excision, the serum level of FGF23 rapidly decreased, hypophosphatemia improved, and the clinical symptoms greatly improved. The result suggests that the overexpressed FGF23 primary tumor in the palate was the cause of osteomalacia which is a rare entity.

Ameloblastic Fibroodontoma of Mandible Causing Tumor Induced Osteomalacia: A Case Report with Review of 88 Phosphaturic Oral Neoplasms

Tumor induced osteomalacia (TIO) is a rare paraneoplastic syndrome associated with tumors secreting fibroblast growth factor 23, which induces osteomalacia. Microscopically, these tumors most commonly show benign phosphaturic mesenchymal tumors. We report the first case of phosphaturic ameloblastic fibro-odontoma (AFO) manifesting as osteomalacia. Our index patient was a 33-year-old male who was diagnosed with TIO and AFO in the mandible was identified as the cause. Our case is unique as AFO is considered as a hamartoma. To the best of our knowledge, there is no hamartoma reported till date causing phosphaturic osteomalacia. As AFO demonstrates mixed epithelial and mesenchymal origin, we propose a new histopathological subtype of TIO-"phosphaturic tumor of mixed epithelial and mesenchymal origin". A review of literature focused on TIO caused by oral lesions revealed 88 oral neoplasms which matched our search criteria. Due to the rarity and unpredictable behavior of TIOs, a high index of suspicion, a broad diagnostic approach, detailed history and multidisciplinary investigations are crucial for establishing the definitive diagnosis and proper treatment recommendations.

Clinical, histopathologic, subtype, and immunohistochemical analysis of jaw phosphaturic mesenchymal tumors

Jaw phosphaturic mesenchymal tumors (PMTs) are a rare neoplasm with uncertain histogenesis. This study aimed to clarify the clinical and pathological features of jaw PMTs.We reviewed the clinical records of 39 patients diagnosed with PMTs in the jaws, and investigated clinical and morphologic characteristics, histologic subtypes, and immunophenotypes of all cases.Microscopic analyses revealed 2 major histologic tumor subtypes: "phosphaturic mesenchymal tumors of mixed epithelial and connective tissue" (PMTMECT), and "phosphaturic mesenchymal tumors of mixed connective tissue" (PMTMCT). PMTMECTs and PMTMCTs accounted for 29 and 10 cases of PMTs, respectively. Most PMTMECT diagnoses were made predominantly in males aged <45 years, and the incidence was similar in both the mandible and maxilla. In contrast, patients with PMTMCTs are predominantly females aged ≥45 years, and all tumors were in the mandible. Histologically, PMTMECT had lower cellularity and a more elongated and spindled mesenchymal component with less elaborate intrinsic microvasculature than PMTMCT. Immunohistochemically, the epithelia of all PMTMECTs was immunoreactive for AE1/AE3. Other immunohistochemical staining of PMTMECTs revealed positive expression of vimentin, SATB2, ERG, CD99, Bcl-2, CD56, S-100, D2-40, CD68, SMA, and CD34 in either one or both components. Immunohistochemical staining of PMTMCTs was diffusely positive for vimentin and a varied ratio of positivity for SATB2, ERG, CD99, Bcl-2, CD56, S-100, D2-40, CD68, SMA, and CD34, but negative for AE1/AE3. Most patients were cured by complete resection, except 2 patients who had repeated recurrences, one of which also had multiple metastasis.Jaw PMT can be divided into 2 major histological subtypes. PMTMECTs are more common than are PMTMCTs, and can transform into malignant PMTMCTs during the progression. PMTMECTs were more commonly observed in males and the incidence was similar in both the maxilla and mandible. PMTMCTs were almost always observed in the mandible of females. Compared with PMTMCTs, PMTMECTs have an admixture of epithelial components with less prominent vasculature and lower cellularity. There were no statistically significant differences in the expression of immunohistochemical markers except AE1/AE3 between PMTMECTs and PMTMCTs. However, immunohistochemical markers have great significance for differentiating other mesenchymal tumors.

Tumor induced osteomalacia in head and neck region: single center experience and systematic review

Tumor-induced osteomalacia in the head and neck region remains a challenging diagnosis to manage. Literature pertaining to management and outcome details remains sparse. We describe two cohorts: cohort 1 included seven patients from a single center in Western India with tumors located in paranasal sinuses (n = 3), intracranial (n = 2) and maxilla (n = 2). The unique features from our series is the management of persistent disease with radiation therapy (n = 2) and peptide receptor radionuclide therapy (PRRT) (n = 1). Cohort two has 163 patients identified from 109 publications for systematic review. Paranasal sinuses, mandible, intracranial disease, maxilla and oral cavity, in descending order, are reportedly common tumor sites. Within this cohort, mean age was 46 ± 14 years at presentation with 44.1% having local symptoms. Duration of symptoms varied from 1 to 240 months. Pre-surgery mean serum phosphorus was 1.4 ± 0.4 mg/dL and median FGF-23 levels were 3.6 (IQR:1.8-6.8) times of normal upper limit of normal. Majority (97.5%) were managed primarily with surgical excision; however, primary radiotherapy (n = 2) and surgery combined with radiotherapy (n = 2) were also reported. Twenty patients had persistent disease while nine patients had recurrence, more commonly noted with intracranial and oral cavity tumors. Surgery was the most common second mode of treatment employed succeeded by radiotherapy. Four patients had metastatic disease. The most common histopathological diagnosis reported is PMT mixed connective tissue, while the newer terminology 'PMT mixed epithelial and connective tissue type' has been described in 15 patients.

Tumor-induced hypophosphatemic osteomalacia caused by a mesenchymal tumor of the mandible managed by a segmental mandibulectomy and microvascular reconstruction with a free fibula flap

BACKGROUND

Tumor-induced osteomalacia is a rare paraneoplastic syndrome in which patients develop hypophosphatemia and osteomalacia.

METHODS AND RESULTS

Here, we report a unique case of a 42-year-old man who presented to our institution with a 1-year history of pain in his ribs, hips, lower back, and feet. Radiologic examination revealed a decrease in bone density and multiple insufficiency fractures. Laboratory evaluation revealed hypophosphatemia, low serum 1,25 dihydroxy vitamin D₃ , and elevated fibroblast growth factor 23 (FGF23). A positron emission tomography/CT scan showed increased uptake in the right mandibular third molar region. Panoramic radiography and CT scanning showed a lytic expansile bone lesion. A mandibular bone biopsy revealed a mixed connective tissue tumor. A right segmental mandibulectomy was performed, followed by microvascular reconstruction. The resection was confirmed by normalization of serum phosphate and FGF23.

CONCLUSION

Successful management of this condition was achieved, with complete surgical resection of the tumor and reconstructive surgery.

Phosphaturic mesenchymal tumor with an admixture of epithelial and mesenchymal elements in the jaws: clinicopathological and immunohistochemical analysis of 22 cases with literature review

Information on the heterogeneity of phosphaturic mesenchymal tumor, a rare entity associated with tumor-induced osteomalacia, is limited. In this retrospective analysis of 222 phosphaturic mesenchymal tumors, 22 cases exhibited mixed mesenchymal and epithelial elements, which we propose to term "phosphaturic mesenchymal tumor, mixed epithelial, and connective tissue type." Phosphaturic mesenchymal tumor of the mixed epithelial and connective tissue type showed a distinctive and significant male predominance (male:female = 2.67:1), with most patients diagnosed at <40 years old. Moreover, all tumors were mainly located in the alveolar bone with focal invasion into surrounding soft tissue and oral mucosa, which could be detected preoperatively by oral examination. The mesenchymal component, composed of spindled cells resembling fibroblasts or myofibroblasts arranged in a storiform or fascicular pattern, exhibited a less prominent vasculature and lower cellularity than the typical phosphaturic mesenchymal tumor (mixed connective tissue type). The epithelial component was typically haphazardly and diffusely distributed throughout the tumor, forming small, irregular nests resembling odontogenic epithelial nests. All cases were immunoreactive for fibroblast growth factor-23, somatostatin receptor 2A, and NSE in both components. Mostly also demonstrated positive staining for CD99 (21/22, 96%), CD56 (16/22, 73%), Bcl-2 (21/22, 96%), and D2-40 (19/22, 86%) in one or both components. S100 was positive in both components in one of seven cases. Interestingly, immunoreactivity was typically stronger and more diffuse in the epithelial than in the paired mesenchymal components. The mesenchymal component was also diffusely positive for CD68 (17/17, 100%) and showed variable focal staining for SMA (15/22, 68%) and CD34 (9/19, 47 %). These results indicate that phosphaturic mesenchymal tumor of the mixed epithelial and connective tissue type has distinctive clinicopathological characteristics and a polyimmunophenotypic profile.

Phosphaturic Mesenchymal Tumor: 2 New Oral Cases and Review of 53 Cases in the Head and Neck

Phosphaturic mesenchymal tumor (PMT) is a rare neoplasm that secretes fibroblast growth factor-23 (FGF-23) and causes oncogenic osteomalacia. It occurs in adults with equal gender distribution and the most common location is the lower extremities, followed by the head and neck. Besides osteomalacia, the clinical presentation includes bone pain and multiple bone fractures. Microscopic features consist of spindle cells, multinucleated giant cells, and calcifications embedded in a chondromyxoid matrix. Laboratory findings indicate normal calcium and parathyroid levels, hypophosphatemia, and increased levels of FGF-23 that usually revert to normal after surgical removal. Due to its rarity, the purpose of the study was to report 2 new oral cases of PMT and to review the literature in the head and neck. The first case occurred in the gingiva and had been present for 6 years. The second case was a recurrence of a previously diagnosed PMT in the right mandible that metastasized to the lung and soft tissue. The literature review included 53 cases in the head and neck. There was a predilection for extra-oral sites (76%) compared to intra-oral sites (24%) with paranasal sinuses considered the most common location (38%) followed by the mandible (15%). There were 9 recurrences that included 3 malignant cases indicating a potentially aggressive tumor. Due to the indeterminate biological behavior of PMT and its rarity, a comprehensive evaluation of medical, laboratory, radiographic, and histological findings are crucial for a definitive diagnosis and treatment.

Sinonasal Phosphaturic Mesenchymal Tumor: A Rare and Misinterpreted Entity

Objectives Oncogenic osteomalacia is a paraneoplastic syndrome in which the tumor secretes a peptide-like hormone, fibroblast growth factor, resulting in urinary loss of phosphates. Methods We present the case of a 50-year-old woman with a benign phosphaturic mesenchymal tumor (PMT) involving the ethmoid sinus with obstruction of the ostiomeatal complex causing unilateral nasal airway obstruction. Results The tumor was initially thought to be an esthesioneuroblastoma based on primary pathology interpretation and on clinical and radiographic appearance. However, a benign PMT was later confirmed by further testing. Conclusion The tumor was removed entirely by the endoscopic transnasal approach, leading to a full resolution of symptoms.

Sinonasal phosphaturic mesenchymal tumor: Case report and systematic review

We report a case of sinonasal phosphaturic mesenchymal tumor (PMT) and conduct a systematic review of the literature to highlight a unique paraneoplastic syndrome associated with PMT. We used English language publications from Medline and Cochrane databases (1970-2013) as data sources. A systematic review of the literature was conducted. All reported cases of head and neck PMTs were included. The presence or absence of the associated paraneoplastic syndrome was noted. We found 33 cases of PMT in the head and neck reported in the literature, 17 of which occurred in the sinonasal area. Approximately 5% of all PMTs are located in the head and neck. Just greater than half are concentrated in the sinonasal area, and the remaining involve various bony and soft tissue structures of the head and neck. PMT is sometimes associated with a paraneoplastic syndrome of tumor-induced (oncogenic) osteomalacia (TIO) causing bone pain, muscle weakness, and pathologic fractures. We present the 18th reported case of sinonasal PMT. A smooth mucosa-covered midline intraseptal mass filling the posterior nasal cavity with destruction and erosion of the skull base was found in an adult male. The patient underwent successful endoscopic resection with wide negative margins and is without recurrence at 24-month follow-up. PMT is a benign, locally aggressive tumor with rare malignant transformation. Knowledge of the bony invasion and destruction caused by this tumor is essential in planning surgical resection with wide negative margins. Familiarity with the associated TIO is essential to investigate for and manage any associated bony morbidity.

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.

Mesenchymal phosphaturic neoplasm in the maxillary sinus: a case report

The authors describe a case of oncogenic osteomalacia due to a mesenchymal phosphaturic tumour in the maxillary sinus. This is a paraneoplastic syndrome in which a tumour produces a peptide hormone-like substance (phosphatonin) that causes a urinary loss of phosphates resulting in a debilitating systemic condition. In this case, the patient experienced muscle stiffness, reduction of muscle tone, loss of weight and pathological fractures. Clinical and radiological examination revealed a tumour in the right maxillary sinus; all other results were negative. The diagnosis following pathology examination was mesenchymal phosphaturic tumour with a haemangiopericytoma-like vascular pattern. Different histological types of mesenchymal tumours can be associated with paraneoplastic syndrome, but their localization in the paranasal sinuses is rare. The correct diagnosis allows the appropriate therapeutic approach, which can lead to an almost immediate resolution of the clinical situation after surgical removal of the neoplasm as in the present case. Oncogenic osteomalacia is rare, particularly in the maxillofacial region, and only a few cases have been reported.

Phosphaturic mesenchymal tumor, mixed connective tissue variant, of the mandible: report of a case and review of the literature

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome that results in renal phosphate wasting with hypophosphatemia. In most cases, the underlying cause of TIO is a small mesenchymal neoplasm that is often difficult to detect, resulting in delayed diagnosis. One such neoplasm is the phosphaturic mesenchymal tumor, mixed connective tissue variant (PMTMCT), an unusual entity with unique morphologic and biochemical features. Most of these tumors are found at appendicular sites with only rare cases reported in the jaws. We describe a PMTMCT involving the mandible in a patient with a protracted history of osteomalacia. A review of the current literature is provided with emphasis on the clinical and histologic features, etiopathogenesis, and management of PMTMCT in the setting of TIO.

Phosphaturic mesenchymal tumor diagnosed by fine-needle aspiration and core biopsy: a case report and review of literature

Oncogenic (tumor-induced) osteomalacia is a rare paraneoplastic syndrome of phosphate wasting that is frequently associated with phosphaturic mesenchymal tumor (PMT). As the cytologic features of this tumor apparently have not been reported, we describe the fine-needle aspiration (FNA) findings for PMT that arose from the gluteal soft tissue in a patient with hypophosphatemia and multiple fractures secondary to osteomalacia. Smears from the computerized tomography (CT)-guided FNA showed groups of spindle cells having elongated nuclei, fine to moderately coarsely granular chromatin, inconspicuous nucleoli, and delicate cytoplasm. Marked nuclear atypia, mitotic figures, and necrosis were absent. The differential diagnosis included a variety of benign and malignant spindle cell neoplasms such as monophasic synovial sarcoma, leiomyoma, peripheral nerve sheath tumor, fibrosarcoma, and, less likely, metastatic melanoma and sarcomatoid carcinoma. The bland-appearing cytologic features of a spindle cell tumor in a patient with osteomalacia should suggest the diagnosis of PMT.

Oncogenic osteomalacia in a case with a maxillary sinus mesenchymal tumor

We herein describe the rare case of a 41-year-old woman with oncogenic osteomalacia due to a tumor in the maxillary sinus who presented with chronic general pain that had been gradually deteriorating. The patient's laboratory findings revealed hypophosphatemia due to renal phosphate wasting, an inappropriately low serum 1 alpha,25-dihydroxyvitamin D3 level for hypophosphatemia and an unusually high serum level of fibroblast growth factor 23 (FGF23). The causative tumor was surgically removed, resulting in a rapid resolution of the patient's biochemical abnormalities. An improvement of the abnormal multiple deposits on (99)Technetium-methylene diphosphonate bone scintigraphy and an increase in the bone metabolism markers suggested the development of bone remodeling within 49 days after the operation. The pathologic diagnosis of the tumor was a "phosphaturic mesenchymal tumor, mixed with a connective tissue variant." The expression of FGF23 was demonstrated in the tumor by the immunohistochemical techniques and a Western analysis.

Phosphaturic mesenchymal tumor (mixed connective tissue variant): a case report with spectral analysis

We present a further case of a rare mesenchymal neoplasm termed phosphaturic mesenchymal tumor (mixed connective tissue variant). The patient was a 42-year-old man with a long history of osteomalacia of unknown etiology with pathological bone fracture, abnormality of parathyroid glands, kyphosis, scoliosis, and spondylosis. Laboratory investigation disclosed hypophosphatemia, elevated serum alkaline phosphatase activity, and normal serum calcium level. The patient had a soft tissue mass in the right inguinal area, measuring 11 x 6 x 5 cm, which was previously interpreted as a calcified hematoma on sonography. The tumor was surgically removed. Grossly, the tumor was well circumscribed, unencapsulated, and had soft to dense consistency. The cut surface had a variegated appearance due to the presence of large hemorrhagic areas admixed with foci of grey-yellow tissue. Histologically, the tumor was composed of primitive mesenchymal cells, osteoclast-like cells, and cells showing myofibroblastic features without cytologic atypia. There were a well developed vascular network, microcystic areas, and poorly formed cartilaginous foci. Unusual and hitherto unpublished prominent features were flower-like, slate-gray crystals, widespread hemosiderin deposits and large areas of hemorrhages, with the latter comprising approximately 60% of the tumor. A spectral analysis indicated that chemically, the crystals mainly consisted of calcium phosphate and sodium nitrate.

Transgenic mice overexpressing human fibroblast growth factor 23 (R176Q) delineate a putative role for parathyroid hormone in renal phosphate wasting disorders

Fibroblast growth factor 23 (FGF23) is a recently characterized protein likely involved in the regulation of serum phosphate homeostasis. Increased circulating levels of FGF23 have been reported in patients with renal phosphate-wasting disorders, but it is unclear whether FGF23 is the direct mediator responsible for the decreased phosphate transport at the proximal renal tubules and the altered vitamin D metabolism associated with these states. To examine this question, we generated transgenic mice expressing and secreting from the liver human FGF23 (R176Q), a mutant form that fails to be degraded by furin proteases. At 1 and 2 months of age, mice carrying the transgene recapitulated the biochemical (decreased urinary phosphate reabsorption, hypophosphatemia, low serum 1,25-dihydroxyvitamin D(3)) and skeletal (rickets and osteomalacia) alterations associated with these disorders. Unexpectantly, marked changes in parameters of calcium homeostasis were also observed, consistent with secondary hyperparathyroidism. Moreover, in the kidney the anticipated alterations in the expression of hydroxylases associated with vitamin D metabolism were not observed despite the profound hypophosphatemia and increased circulating levels of PTH, both major physiological stimuli for 1,25-dihydroxyvitamin D(3) production. Our findings strongly support the novel concept that high circulating levels of FGF23 are associated with profound disturbances in the regulation of phosphate and vitamin D metabolism as well as calcium homeostasis and that elevated PTH levels likely also contribute to the renal phosphate wasting associated with these disorders.

Most osteomalacia-associated mesenchymal tumors are a single histopathologic entity: an analysis of 32 cases and a comprehensive review of the literature

Oncogenic osteomalacia (OO) is a rare paraneoplastic syndrome of osteomalacia due to phosphate wasting. The phosphaturic mesenchymal tumor (mixed connective tissue variant) (PMTMCT) is an extremely rare, distinctive tumor that is frequently associated with OO. Despite its association with OO, many PMTMCTs go unrecognized because they are erroneously diagnosed as other mesenchymal tumors. Expression of fibroblast growth factor-23 (FGF-23), a recently described protein putatively implicated in renal tubular phosphate loss, has been shown in a small number of mesenchymal tumors with known OO. The clinicopathological features of 32 mesenchymal tumors either with known OO (29) or with features suggestive of PMTMCT (3) were studied. Immunohistochemistry for cytokeratin, S-100, actin, desmin, CD34, and FGF-23 was performed. The patients (13 male, 19 female) ranged from 9 to 80 years in age (median 53 years). A long history of OO was common. The cases had been originally diagnosed as PMTMCT (15), hemangiopericytoma (HPC) (3), osteosarcoma (3), giant cell tumor (2), and other (9). The tumors occurred in a variety of soft tissue (21) and bone sites (11) and ranged from 1.7 to 14 cm. Twenty-four cases were classic PMTMCT with low cellularity, myxoid change, bland spindled cells, distinctive "grungy" calcified matrix, fat, HPC-like vessels, microcysts, hemorrhage, osteoclasts, and an incomplete rim of membranous ossification. Four of these benign-appearing PMTMCTs contained osteoid-like matrix. Three other PMTMCTs were hypercellular and cytologically atypical and were considered malignant. The 3 cases without known OO were histologically identical to the typical PMTMCT. Four cases did not resemble PMTMCT: 2 sinonasal HPC, 1 conventional HPC, and 1 sclerosing osteosarcoma. Three cases expressed actin; all other markers were negative. Expression of FGF-23 was seen in 17 of 21 cases by immunohistochemistry and in 2 of 2 cases by RT-PCR. Follow-up (25 cases, 6-348 months) indicated the following: 21 alive with no evidence of disease and with normal serum chemistry, 4 alive with disease (1 malignant PMTMCT with lung metastases). We conclude that most cases of mesenchymal tumor-associated OO, both in the present series and in the reported literature, are due to PMTMCT. Improved recognition of their histologic spectrum, including the presence of bone or osteoid-like matrix in otherwise typical cases and the existence of malignant forms, should allow distinction from other mesenchymal tumors. Recognition of PMTMCT is critical, as complete resection cures intractable OO. Immunohistochemistry and RT-PCR for FGF-23 confirm the role of this protein in PMTMCT-associated OO.

Fibroblast growth factor 23 in oncogenic osteomalacia and X-linked hypophosphatemia

BACKGROUND

Mutations in fibroblast growth factor 23 (FGF-23) cause autosomal dominant hypophosphatemic rickets. Clinical and laboratory findings in this disorder are similar to those in oncogenic osteomalacia, in which tumors abundantly express FGF-23 messenger RNA, and to those in X-linked hypophosphatemia, which is caused by inactivating mutations in a phosphate-regulating endopeptidase called PHEX. Recombinant FGF-23 induces phosphaturia and hypophosphatemia in vivo, suggesting that it has a role in phosphate regulation. To determine whether FGF-23 circulates in healthy persons and whether it is elevated in those with oncogenic osteomalacia or X-linked hypophosphatemia, an immunometric assay was developed to measure it.

METHODS

Using affinity-purified, polyclonal antibodies against [Tyr223]FGF-23(206-222)amide and [Tyr224]FGF-23(225-244)amide, we developed a two-site enzyme-linked immunosorbent assay that detects equivalently recombinant human FGF-23, the mutant form in which glutamine is substituted for arginine at position 179 (R179Q), and synthetic human FGF-23(207-244)amide. Plasma or serum samples from 147 healthy adults (mean [+/-SD] age, 48.4+/-19.6 years) and 26 healthy children (mean age, 10.9+/-5.5 years) and from 17 patients with oncogenic osteomalacia (mean age, 43.0+/-13.3 years) and 21 patients with X-linked hypophosphatemia (mean age, 34.9+/-17.2 years) were studied.

RESULTS

Mean FGF-23 concentrations in the healthy adults and children were 55+/-50 and 69+/-36 reference units (RU) per milliliter, respectively. Four patients with oncogenic osteomalacia had concentrations ranging from 426 to 7970 RU per milliliter, which normalized after tumor resection. FGF-23 concentrations were 481+/-528 RU per milliliter in those with suspected oncogenic osteomalacia and 353+/-510 RU per milliliter (range, 31 to 2335) in those with X-linked hypophosphatemia.

CONCLUSIONS

FGF-23 is readily detectable in the plasma or serum of healthy persons and can be markedly elevated in those with oncogenic osteomalacia or X-linked hypophosphatemia, suggesting that this growth factor has a role in phosphate homeostasis. FGF-23 measurements might improve the management of phosphate-wasting disorders.

The autosomal dominant hypophosphatemic rickets R176Q mutation in fibroblast growth factor 23 resists proteolytic cleavage and enhances in vivo biological potency

Missense mutations in fibroblast growth factor 23 (FGF23) are the cause of autosomal dominant hypophosphatemic rickets (ADHR). The mutations (R176Q, R179W, and R179Q) replace Arg residues within a subtilisin-like proprotein convertase (SPC) cleavage site (RXXR motif), leading to protease resistance of FGF23. The goals of this study were to examine in vivo the biological potency of the R176Q mutant FGF23 form and to characterize alterations in homeostatic mechanisms that give rise to the phenotypic presentation of this disorder. For this, wild type and R176Q mutant FGF23 were overexpressed in the intact animals using a tumor-bearing nude mouse system. At comparable circulating levels, the mutant form was more potent in inducing hypophosphatemia, in decreasing circulating concentrations of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and in causing rickets and osteomalacia in these animals compared with wild type FGF23. Parameters of calcium homeostasis were also altered, leading to secondary hyperparathyroidism and parathyroid gland hyperplasia. However, the raised circulating levels of parathyroid hormone were ineffective in normalizing the reduced 1,25(OH)(2)D(3) levels by increasing renal expression of 25(OH)D(3)-1alpha-hydroxylase (Cyp40) to promote its synthesis and by decreasing that of 25(OH)D(3)-24-hydroxylase (Cyp24) to prevent its catabolism. The findings provide direct in vivo evidence that missense mutations from ADHR kindreds are gain-of-function mutations that retain and increase the protein's biological potency. Moreover, for the first time, they define a potential role for FGF23 in dissociating parathyroid hormone actions on mineral fluxes and on vitamin D metabolism at the level of the kidney.