Physiopathology of Phosphate Disorders

Intracellular phosphate is critical for cellular processes such as signaling, nucleic acid synthesis, and membrane function. Extracellular phosphate (Pi) is an important component of the skeleton. Normal levels of serum phosphate are maintained by the coordinated actions of 1,25-dihydroxyvitamin D3, parathyroid hormone and fibroblast growth factor-23, which intersect in the proximal tubule to control the reabsorption of phosphate via the sodium-phosphate cotransporters Npt2a and Npt2c. Furthermore, 1,25-dihydroxyvitamin D3 participates in the regulation of dietary phosphate absorption in the small intestine. Clinical manifestations associated with abnormal serum phosphate levels are common and occur as a result of genetic or acquired conditions affecting phosphate homeostasis. For example, chronic hypophosphatemia leads to osteomalacia in adults and rickets in children. Acute severe hypophosphatemia can affect multiple organs leading to rhabdomyolysis, respiratory dysfunction, and hemolysis. Patients with impaired kidney function, such as those with advanced CKD, have high prevalence of hyperphosphatemia, with approximately two-thirds of patients on chronic hemodialysis in the United States having serum phosphate levels above the recommended goal of 5.5 mg/dL, a cutoff associated with excess risk of cardiovascular complications. Furthermore, patients with advanced kidney disease and hyperphosphatemia (>6.5 mg/dL) have almost one-third excess risk of death than those with phosphate levels between 2.4 and 6.5 mg/dL. Given the complex mechanisms that regulate phosphate levels, the interventions to treat the various diseases associated with hypophosphatemia or hyperphosphatemia rely on the understanding of the underlying pathobiological mechanisms governing each patient condition.

Approach to Hypophosphatemic Rickets

Hypophosphatemic rickets typically presents in infancy or early childhood with skeletal deformities and growth plate abnormalities. The most common causes are genetic (such as X-linked hypophosphatemia), and these typically will result in lifelong hypophosphatemia and osteomalacia. Knowledge of phosphate metabolism, including the effects of fibroblast growth factor 23 (FGF23) (an osteocyte produced hormone that downregulates renal phosphate reabsorption and 1,25-dihydroxyvitamin-D (1,25(OH)2D) production), is critical to determining the underlying genetic or acquired causes of hypophosphatemia and to facilitate appropriate treatment. Serum phosphorus should be measured in any child or adult with musculoskeletal complaints suggesting rickets or osteomalacia. Clinical evaluation incudes thorough history, physical examination, laboratory investigations, genetic analysis (especially in the absence of a guiding family history), and imaging to establish etiology and to monitor severity and treatment course. The treatment depends on the underlying cause, but often includes active forms of vitamin D combined with phosphate salts, or anti-FGF23 antibody treatment (burosumab) for X-linked hypophosphatemia. The purpose of this article is to explore the approach to evaluating hypophosphatemic rickets and its treatment options.

A Novel Synonymous Variant of PHEX in a Patient with X-Linked Hypophosphatemia

X-linked dominant hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets/osteomalacia, is caused by loss-of-function phosphate-regulating endopeptidase homolog X-linked gene (PHEX) variants. However, synonymous PHEX variants are rare in XLH. We report a 7-year-old boy with hypophosphatemia, short stature, and lower limb deformity. Whole-exome sequencing, reverse transcription-polymerase chain reaction, and Sanger sequencing were performed to identify the pathogenicity of the variant. A novel synonymous PHEX variant (NM_000444.4:c.1530 C>T, p.Arg510Arg) was detected in the proband. Further analysis revealed a 58-bp deletion at the 5' site of exon 14 during splicing. This study extends the genetic spectrum of XLH and confirms the rarity and significance of synonymous PHEX variants.

X-Linked Hypophosphatemia, Not Only a Skeletal Disease But Also a Chronic Inflammatory State

CONTEXT

X-linked hypophosphatemia (XLH) is a rare genetic disease caused by a primary excess of fibroblast growth factor 23 (FGF23). FGF23 has been associated with inflammation and impaired osteoclastogenesis, but these pathways have not been investigated in XLH.

OBJECTIVE

This work aimed to evaluate whether XLH patients display peculiar inflammatory profile and increased osteoclastic activity.

METHODS

We performed a prospective, multicenter, cross-sectional study analyzing transcript expression of 8 inflammatory markers (Il6, Il8, Il1β, CXCL1, CCL2, CXCR3, Il1R, Il6R) by real-time quantitative polymerase chain reaction on peripheral blood mononuclear cells (PBMCs) purified from total blood samples extracted from patients and healthy control individuals. The effect of native/active vitamin D on osteoclast formation was also assessed in vitro from XLH patients' PBMCs.

RESULTS

In total, 28 XLH patients (17 children, among them 6 undergoing standard of care [SOC] and 11 burosumab therapy) and 19 controls were enrolled. Expression of most inflammatory markers was significantly increased in PBMCs from XLH patients compared to controls. No differences were observed between the burosumab and SOC subgroups. Osteoclast formation was significantly impaired in XLH patients. XLH mature osteoclasts displayed higher levels of inflammatory markers, being however lower in cells derived from the burosumab subgroup (as opposed to SOC).

CONCLUSION

We describe for the first time a peculiar inflammatory profile in XLH. Since XLH patients have a propensity to develop arterial hypertension, obesity, and enthesopathies, and because inflammation can worsen these clinical outcomes, we hypothesize that inflammation may play a critical role in these extraskeletal complications of XLH.

Hypoplastic amelogenesis imperfecta, bilateral nephrolithiasis and FGF-23-mediated hypophosphataemia: a triad of FAM20A-related enamel renal syndrome

Enamel renal syndrome (ERS) due to loss of function (LOF) mutation of FAM20A gene typically consists of hypoplastic amelogenesis imperfecta (AI) and bilateral nephrolithiasis/nephrocalcinosis. Recent evidence suggests that FAM20A interacts with FAM20C and increases its activity; thus LOF mutation of FAM20A leads to impaired FAM20C action. FAM20C, a golgi casein kinase, phosphorylates fibroblast growth factor (FGF)-23, prevents its glycosylation and makes it more susceptible to degradation by furine proteases. Consequently, inactivating mutations of FAM20C lead to increased concentration of bioactive and intact FGF-23 in circulation and resultant hypophosphataemia. LOF mutation of FAM20A, thus, might also be associated with FGF-23-mediated hypophosphataemia; however, such an association has never been reported in the literature. We describe, for the first time, a triad of AI, bilateral nephrolithiasis and FGF-23-mediated hypophosphataemia in LOF mutation of FAM20A. We suggest that serum phosphate should be measured in all patients with ERS to avoid metabolic and skeletal complications of undiagnosed, hence untreated hypophosphataemia.

Analysis of Electrolyte Abnormalities in Adolescents and Adults and Subsequent Diagnosis of an Eating Disorder

IMPORTANCE

Eating disorders lead to increased mortality and reduced quality of life. While the acute presentations of eating disorders frequently involve electrolyte abnormalities, it remains unknown whether electrolyte abnormalities may precede the future diagnosis of an eating disorder.

OBJECTIVE

To determine whether outpatient electrolyte abnormalities are associated with the future diagnosis of an eating disorder.

DESIGN, SETTING, AND PARTICIPANTS

This population-level case-control study used provincial administrative health data for residents of Ontario, Canada aged 13 years or older from 2008 to 2020. Individuals without an eating disorder (controls) were matched 4:1 to individuals diagnosed with an incident eating disorder (cases) based on age and sex. Both groups had outpatient electrolyte measurements between 3 years and 30 days prior to index. Index was defined as the date of an eating disorder diagnosis in any inpatient or outpatient clinical setting for cases. Controls were assigned a pseudo-index date according to the distribution of index dates in the case population. Individuals with any prior eating disorder diagnosis were excluded. The data analyzed was from January 1, 2008, through June 30, 2020.

EXPOSURES

Any electrolyte abnormality, defined as abnormal test results for a composite of hypokalemia, hyperkalemia, hyponatremia, hypernatremia, hypomagnesemia, hypophosphatemia, metabolic acidosis, or metabolic alkalosis.

OUTCOMES AND MEASURES

Eating disorder diagnosis including anorexia nervosa, bulimia nervosa, and eating disorder not otherwise specified.

RESULTS

A total 6970 eligible Ontario residents with an eating disorder (mean [SD] age, 28 (19) years; 6075 [87.2%] female, 895 [12.8%] male) were matched with 27 878 age- and sex-matched residents without an eating disorder diagnosis (mean [SD] age, 28 [19] years; 24 300 [87.2%] female, 3578 [12.8%] male). Overall, 18.4% of individuals with an eating disorder had a preceding electrolyte abnormality vs 7.5% of individuals without an eating disorder (adjusted odds ratio [aOR], 2.12; [95% CI, 1.86-2.41]). The median (IQR) time from the earliest electrolyte abnormality to eating disorder diagnosis was 386 (157-716) days. Specific electrolyte abnormalities associated with a higher risk of an eating disorder were: hypokalemia (aOR, 1.98; 95% CI, 1.70-2.32), hyperkalemia (aOR, 1.97; 95% CI, 1.48-2.62), hyponatremia (aOR, 5.26; 95% CI, 3.32-8.31), hypernatremia (aOR, 3.09; 95% CI, 1.01-9.51), hypophosphatemia (aOR, 2.83; 95% CI, 1.82-4.40), and metabolic alkalosis (aOR, 2.60; 95% CI, 1.63-4.15).

CONCLUSIONS AND RELEVANCE

In this case-control study, individuals with an eating disorder were associated with a preceding outpatient electrolyte abnormality compared with matched controls. Otherwise unexplained electrolyte abnormalities may serve to identify individuals who may benefit from screening for an underlying eating disorder.

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.

A Unique Mechanism of a Novel Synonymous PHEX Variant Causing X-Linked Hypophosphatemia

CONTEXT

Synonymous mutations are usually nonpathogenic.

OBJECTIVE

We report here a family with X-linked hypophosphatemia (XLH) due to a novel synonymous PHEX variant with a unique mechanism.

METHODS

We studied a 4-member family (a mother, a son, and 2 daughters), all affected with XLH. Genomic DNA was extracted from peripheral leucocytes. Whole exome sequencing (WES) was used to identify the underlying genetic variant in the proband (the son). Sanger sequencing was used to confirm this variant in the proband and his family members. RT-PCR and sequencing of the cDNA revealed the effect of this variant on the PHEX structure and function.

RESULTS

A synonymous variant in the PHEX gene (c.1701A>C) was identified in all affected members. This variant changes the first nucleotide of exon 17 from adenine to cytosine. Using RT-PCR, this variant was shown to interfere with splicing of exons 16 with 17 resulting in a single shorter PHEX transcript in the proband compared to normal control. Sanger sequencing of the cDNA revealed a complete skipping of exon 17 and direct splicing of exons 16 and 18. This led to a frameshift and an introduction of a new stop codon in the next codon (codon 568), which ultimately led to truncation and loss of the final 183 amino acids of PHEX.

CONCLUSION

This novel variant shows how a synonymous exonic mutation may induce a complex series of changes in the transcription and translation of the gene and causes a disease, a mechanism that is not commonly recognized.

Clinical practice recommendations for the diagnosis and treatment of X-linked hypophosphatemia: A consensus based on the ADAPTE method

BACKGROUND AND OBJECTIVE

The objective of this project was to adapt to our setting following a systematic process based on the ADAPTE method the first clinical practice guidelines on X-linked hypophosphatemia (XLH) that were published in 2019.

MATERIALS AND METHODS

The adaptation of the guidelines to our application and implementation setting was carried out in three phases -start-up, adaptation, and finalization- by a group of experts involved in the management of patients with XLH.

RESULTS

Following the original guide, the recommendations agreed by the group that elaborated the guidelines for diagnosis, frequency and scope of visits and specific follow-up in children and adults are presented. On the other hand, recommendations are established for both age groups with conventional treatment, as well as with burosumab in children or adults and those related to the controversial use of growth hormone in children. Suggestions are also proposed regarding the monitoring and management of musculoskeletal disorders and orthopedic treatment in children, dental health and hearing, and neurosurgical complications. Finally, a series of questions and areas are raised in order to deepen the possible future investigation.

CONCLUSIONS

These recommendations constitute the systematic adaptation to our setting of the first evidence-based clinical practice guide for the diagnosis and management of XLH and we hope that they can contribute to the adequate management of the disease.

Effect of Burosumab Compared With Conventional Therapy on Younger vs Older Children With X-linked Hypophosphatemia

CONTEXT

Younger age at treatment onset with conventional therapy (phosphate salts and active vitamin D; Pi/D) is associated with improved growth and skeletal outcomes in children with X-linked hypophosphatemia (XLH). The effect of age on burosumab efficacy and safety in XLH is unknown.

OBJECTIVE

This work aimed to explore the efficacy and safety of burosumab vs Pi/D in younger (< 5 years) and older (5-12 years) children with XLH.

METHODS

This post hoc analysis of a 64-week, open-label, randomized controlled study took place at 16 academic centers. Sixty-one children aged 1 to 12 years with XLH (younger, n = 26; older, n = 35) participated. Children received burosumab starting at 0.8 mg/kg every 2 weeks (younger, n = 14; older, n = 15) or continued Pi/D individually titrated per recommended guidelines (younger, n = 12; older, n = 20). The main outcome measure included the least squares means difference (LSMD) in Radiographic Global Impression of Change (RGI-C) rickets total score from baseline to week 64.

RESULTS

The LSMD in outcomes through 64 weeks on burosumab vs conventional therapy by age group were as follows: RGI-C rickets total score (younger, +0.90; older, +1.07), total Rickets Severity Score (younger, -0.86; older, -1.44), RGI-C lower limb deformity score (younger, +1.02; older, +0.91), recumbent length or standing height Z-score (younger, +0.20; older, +0.09), and serum alkaline phosphatase (ALP) (younger, -31.15% of upper normal limit [ULN]; older, -52.11% of ULN). On burosumab, dental abscesses were not reported in younger children but were in 53% of older children.

CONCLUSION

Burosumab appears to improve outcomes both in younger and older children with XLH, including rickets, lower limb deformities, growth, and ALP, compared with Pi/D.

Accelerated-strategy renal replacement therapy for critically ill patients: A systematic review and meta-analysis

BACKGROUND

The aim of this study was to investigate the clinical effect and safety of accelerated-strategy initiation of renal replacement therapy (RRT) in critically ill patients.

METHODS

PubMed, Embase, OVID, EBSCO, and the Cochrane Library databases were searched for relevant articles from inception to December 30, 2020. Only RCTs that compared the clinical efficacy and safety between accelerated-strategy RRT and standard-strategy RRT among critically ill adult patients with acute kidney injury (AKI) were included. The primary outcome was 28-day mortality.

RESULTS

A total of 5279 patients in 12 RCTs were included in this meta-analysis. The 28-day mortality rates of patients treated with accelerated and standard RRT were 37.3% (969/2596) and 37.9% (976/2573), respectively. No significant difference was observed between the groups (OR, 0.92; 95% CI, 0.70-1.12; I2 = 60%). The recovery rates of renal function were 54.5% and 52.5% in the accelerated- and standard-RRT groups, respectively, with no significant difference (OR, 1.03; 95% CI, 0.89-1.19; I2 = 56%). The rate of RRT dependency was similar in the accelerated- and standard-RRT strategies (6.7% vs 5.0%; OR, 1.11; 95% CI, 0.71-1.72; I2 = 20%). The accelerated-RRT group displayed higher risks of hypotension, catheter-related infection, and hypophosphatemia than the standard-RRT group (hypotension: OR, 1.26; 95% CI, 1.10-1.45; I2 = 36%; catheter-related infection: OR, 1.90; 95% CI, 1.17-3.09; I2 = 0%; hypophosphatemia: OR, 2.11; 95% CI, 1.43-3.15; I2 = 67%).

CONCLUSIONS

Accelerated RRT does not reduce the risk of death and does not improve the recovery of kidney function among critically ill patients with AKI. In contrast, an increased risk of adverse events was observed in patients receiving accelerated RRT. However, these findings were based on low quality of evidence. Further large-scale RCTs is warranted.

Physical function and physical activity in adults with X-linked hypophosphatemia

We described physical function and activity in UK adults with X-linked hypophosphatemia (XLH). Our data indicate that low physical activity and impaired mobility are common in adults with XLH. Deficits in lower limbs muscle power and functional capacity contribute to the loss of physical function in adults with XLH.

INTRODUCTION

There is a dearth of literature on physical function and physical activity in adults with X-linked hypophosphatemia (XLH). We described muscle strength and power, functional capacity, mobility and physical activity level and explored the relationships among these variables in adults with XLH.

METHODS

Participants were recruited as part of a UK-based prospective cohort study, the RUDY Study. They underwent a clinical visit and physical examination, including assessment of handgrip strength, jump power (mechanography), six-minute walk test (6MWT) and short physical performance battery (SPPB), and completed the International Physical Activity Questionnaire (IPAQ). Performance data were analysed using parametric and non-parametric tests, whereas correlations were assessed by univariate analysis.

RESULTS

Twenty-six adults with XLH (50% males) with a mean age of 44 ± 16.1 years were recruited. Jump power and 6MWT distances (p < 0.0001) were 54.4% and 38.6% lower respectively in individuals with XLH compared with normative values. These deficits were not associated with age or sex. Handgrip strength values were similar to expected values. Deficits in muscle power were more pronounced than those reported at 6MWT (p < 0.0001). Univariate analysis revealed only a correlation between total physical activity and muscle power (r = 0.545, p = 0.019).

CONCLUSIONS

Adults with XLH have a marked deficit in lower limb muscle power and a reduced functional capacity, with a high incidence of impaired mobility and inactivity. In addition to metabolic effects of XLH, low physical activity may contribute to deficits in lower limb power. Further studies are required to develop novel treatment approaches to improve physical function and mobility.

Challenging case of tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is an uncommon paraneoplastic syndrome which poses a diagnostic challenge. The hallmark feature is severe acquired hypophosphataemic osteomalacia due to renal phosphate wasting because of increased secretion of fibroblast growth factor 23 (FGF-23). A man in his 30s, presented with a 4-year history of severe muscle aches, bone pain and proximal muscle weakness, was referred for evaluation. His laboratory examination revealed severe hypophosphataemia as a result of urinary phosphate wasting, low 1,25-dihydroxyvitamin D, high alkaline phosphatase and elevated FGF-23. We could localise the tumour to his right femur and the biopsy showed a mesenchymal origin. The treatment with pharmacotherapy and radiofrequency ablation helped in the normalisation of blood chemistry and resulted in significant clinical improvement. Hypophosphataemia, phosphaturia, elevated FGF-23 and low 1,25-dihydroxyvitamin D level with severe musculoskeletal pain and muscle weakness necessitate careful evaluation of TIO.

Phosphate-Sensing

The blood level of phosphate is tightly regulated in a narrow range. Hyperphosphatemia and hypophosphatemia both lead to the development of diseases, such as hyperphosphatemic tumoral calcinosis and rickets/osteomalacia, respectively. Although several humoral factors have been known to affect blood phosphate levels, fibroblast growth factor 23 (FGF23) is the principal hormone involved in the regulation of blood phosphate. This hormone is produced by bone, particularly by osteocytes and osteoblasts, and has the effect of lowering the blood level of phosphate in the renal proximal tubules. Therefore, some phosphate-sensing mechanism should exist, at least in the bone. However, the mechanisms through which bone senses changes in the blood level of phosphate, and through which the bone regulates FGF23 production remain to be fully elucidated. Our recent findings demonstrate that high extracellular phosphate phosphorylates FGF receptor 1c (FGFR1c). Its downstream extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK signaling pathway regulates the expression of several transcription factors and the GALNT3 gene, which encodes GalNAc-T3, which plays a role in the regulation of posttranslational modification of FGF23 protein, which in turn enhances FGF23 production. The FGFR1c-GALNT3 gene axis is considered to be the most important mechanism for regulating the production of FGF23 in bone in the response to a high phosphate diet. Thus-in the regulation of FGF23 production and blood phosphate levels-FGFR1c may be considered to function as a phosphate-sensing molecule. A feedback mechanism, in which FGFR1c and FGF23 are involved, is present in blood phosphate regulation. In addition, other reports indicate that PiT1 and PiT2 (type III sodium-phosphate cotransporters), and calcium-sensing receptor are also involved in the phosphate-sensing mechanism. In the present chapter, we summarize new insights on phosphate-sensing mechanisms.

Potential influences on optimizing long-term musculoskeletal health in children and adolescents with X-linked hypophosphatemia (XLH)

In recent years, much progress has been made in understanding the mechanisms of bone growth and development over a lifespan, including the crosstalk between muscle and bone, to achieve optimal structure and function. While there have been significant advances in understanding how to help improve and maintain bone health in normal individuals, there is limited knowledge on whether these mechanisms apply or are compromised in pathological states. X-linked hypophosphatemia (XLH) (ORPHA:89936) is a rare, heritable, renal phosphate-wasting disorder. The resultant chronic hypophosphatemia leads to progressive deterioration in musculoskeletal function, including impaired growth, rickets, and limb deformities in children, as well as lifelong osteomalacia with reduced bone quality and impaired muscle structure and function. The clinical manifestations of the disease vary both in presentation and severity in affected individuals, and many of the consequences of childhood defects persist into adulthood, causing significant morbidity that impacts physical function and quality of life. Intervention to restore phosphate levels early in life during the critical stages of skeletal development in children with XLH could optimize growth and may prevent or reduce bone deformities in childhood. A healthier bone structure, together with improved muscle function, can lead to physical activity enhancing musculoskeletal health throughout life. In adults, continued management may help to maintain the positive effects acquired from childhood treatment, thereby slowing or halting disease progression. In this review, we summarize the opinions from members of a working group with expertise in pediatrics, epidemiology, and bone, joint and muscle biology, on potential outcomes for people with XLH, who have been optimally treated from an early age and continue treatment throughout life.

Performance evaluation of the new chemiluminescent intact FGF23 assay relative to the existing assay system

INTRODUCTION

This study assessed the performance of a new fully automated immunoassay for fibroblast growth factor (FGF) 23 (Determinar CL FGF23 CL) among healthy individuals and those with chronic hypophosphatemia compared with the previous assay (Kainos FGF23 KI).

MATERIALS AND METHODS

A total of 380 serum samples from healthy participants were collected to determine the reference range of FGF23 levels with CL. A total of 200 serum samples from 22 hypophosphatemic patients were collected simultaneously to compare the difference in FGF23 levels between CL and KI. The Mann-Whitney U test and linear regression analysis were adopted to assess the differences and linearity between the two assays.

RESULTS

The median FGF23 levels among healthy individuals was 31.7 (interquartile: 26.4-37.5) pg/mL. When the reference range was calculated as the mean  ± 2 standard deviation (2SD), it was 16.1-49.3 pg/mL. A total of 363 individuals (96%) among normal cases fell in this range. Among 200 samples from patients with chronic hypophosphatemic disorder, the median FGF23 levels analyzed by CL and KI were 123.0 (90.2-237.7) and 172.5 (115.8-290.7) pg/mL. KI yielded significantly higher FGF23 values than CL (p < 0.001). A linear regression model revealed the correlation between KI (x) and CL (y), which had a slope of 0.76 with a y-intercept of -0.32 and high linearity (R² = 0.99).

CONCLUSION

The new measurement kit yielded lower FGF23 values when compared with the previous assay. Clinicians should consider this discrepancy when they assay intact FGF23 values with CL.

Clinical and genetic characteristics of 29 Chinese patients with X-linked hypophosphatemia

OBJECTIVE

The aim of this study was to fully describe the clinical and genetic characteristics, including clinical manifestations, intact fibroblast growth factor 23 (iFGF23) levels, and presence of PHEX gene mutations, of 22 and 7 patients with familial and sporadic X-linked dominant hypophosphatemia (XLH), respectively.

METHODS

Demographic data, clinical features, biochemical indicators, and imaging data of 29 patients were collected. All 22 exons and exon-intron boundaries of the PHEX gene were amplified by polymerase chain reaction (PCR) and directly sequenced. The serum level of iFGF23 was measured in 15 of the patients.

RESULTS

Twenty-nine patients (male/female: 13:16, juvenile/adult: 15:14) with XLH were included. The main symptoms were bowed lower extremities (89.7%), abnormal gait (89.7%), and short stature/growth retardation (78.6%). Hypophosphatemia with a high alkaline phosphatase level was the main biochemical feature and the median value of serum iFGF23 was 55.7 pg/ml (reference range: 16.1-42.2 pg/ml). Eight novel mutations in the PHEX gene were identified by Sanger sequencing, including two missense mutations (p. Gln682Leu and p. Phe312Ser), two deletions (c.350_356del and c.755_761del), one insertion (c.1985_1986insTGAC), and three splice mutations (c.1700+5G>C, c.1966-1G>T, and c.350-14_350-1del). Additionally, the recurrence rate after the first orthopedic surgery was 77.8% (7/9), and five of them had their first surgery before puberty.

CONCLUSION

Our study expanded the clinical phenotypes and gene mutation spectrum of XLH and provided a reference for the optimal timing of orthopedic surgeries.

Clinical performance of a novel chemiluminescent enzyme immunoassay for FGF23

INTRODUCTION

Measurement of fibroblast growth factor 23 (FGF23) has been reported to be clinically useful for the differential diagnosis of chronic hypophosphatemia. However, assays for research use only are available in Japan. Thus, the objective of this study was to examine the clinical utility of a novel and automated chemiluminescent enzyme immunoassay for the measurement of FGF23.

MATERIALS AND METHODS

Participants were recruited from July 2015 to January 2017 at six facilities in Japan. Thirty-eight patients with X-linked hypophosphatemic rickets (XLH 15 males, 23 females, age 0-66 years), five patients with tumour-induced osteomalacia (TIO 3 males, 2 females, age 60-73 years), and twenty-two patients with hypophosphatemia (11 males, 11 females, age 1-75 years) caused due to other factors participated in this study.

RESULTS

With the clinical cut-off value of FGF23 at 30.0 pg/mL indicated in the Diagnostic Guideline of Rickets/Osteomalacia in Japan, the sensitivity and specificity of FGF23-related hypophosphatemic rickets/osteomalacia without vitamin D deficiency (disease group-1) were 100% and 81.8%, respectively, which distinguished it from non-FGF23-related hypophosphatemia (disease group-2). Furthermore, the diagnostic sensitivity of FGF23-related hypophosphatemia with vitamin D deficiency remained at 100%. Among the four patients with FGF23 levels ≥ 30.0 pg/mL in disease group-2, two patients with relatively higher FGF23 values were suspected to have genuine FGF23-related hypophosphatemia, due to the ectopic production of FGF23 in pulmonary and prostate small cell carcinomas.

CONCLUSION

The novel FGF23 assay tested in this study is useful for the differential diagnosis of hypophosphatemic rickets/osteomalacia in a clinical setting.

Tumor induced osteomalacia: A single center experience on 17 patients

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome due to a phosphaturic tumor, which overproduces Fibroblast Growth Factor 23 (FGF-23), causing hyperphosphaturia, hypophosphatemia, low 1,25(OH)₂D and osteomalacia. Tumor localization is critical, diagnostic delay ranges from 2.5 to 28 years and to date surgical removal is considered effective treatment. We retrospectively evaluated patients with definite diagnosis of TIO referred to a tertiary Rheumatology Center between September 2000 and May 2020, investigating clinical management and disease outcome. We included 17 patients: 10 (58.8%) were females, mean age at diagnosis was 55.3 ± 13.9 years (mean ± standard deviation), with a diagnostic delay from symptoms onset to tumor detection of 6.6 ± 6.25 years. Biochemical data were: serum phosphorus 1.3 ± 0.4 mg/dL (Reference Range: 2.5-4.6), serum 1,25(OH)₂D 31.8 ± 22.9 ng/mL (RR: 25-86), intact FGF-23, 358.9 ± 677 pg/mL (RR: 25-45); 24 h-Urine Phosphorus was increased in only 2 patients, while tubular reabsorption of phosphate (TRP) was decreased in all patients confirming a renal phosphate wasting. In 2013 ⁶⁸Ga- DOTA-based PET/CT was introduced in routinely practice and diagnostic delay was consistently reduced (from 8.6 ± 7.9 to 4.3 ± 2.4 years). Thirteen patients underwent surgery, one patient underwent radiofrequency ablation; 3 patients, not eligible for surgery, were treated only with supplements of phosphorus and calcitriol. One was started on Burosumab after several unsuccessful surgical attempts. After surgery or ablation, 8 patients had complete remission, 3 TIO persistence, and 3 had overtime relapse. Relapses were observed only in patients who previously underwent closed biopsy. To our knowledge, this is the widest European cohort of TIO patients in the last two decades. We confirm a usual diagnostic delay and recommend a stepwise diagnostic approach. Tumor biopsy is not recommended due to the potential cell spilling. Surgery is generally considered a definitive treatment, even though other approaches have been successful in curing TIO. Active surveillance on possible recurrence is always needed. Burosumab appears a promising therapy.

[Osteomalacia following intravenous iron suppletion]

BACKGROUND

Ferric carboxymaltose (FCM) is highly effective in supplementing iron-deficiency anemia and frequently used. However, it can severely interfere with the phosphate metabolism.

CASE DESCRIPTION

A 63 year old man with iron-deficiency anemia due to hereditary hemorrhagic telangiectasia was treated with intravenous FCM. After initiation of the FCM he developed generalized bone and muscle pain as well as insufficiency fractures. Treatment with colecalciferol was started. However, the bone pain increased and further investigation showed a hypophosphatemicosteomalacia with high urine phosphate loss suggesting renal phosphate wasting. Serum FGF23 level was increased confirming the diagnosis of FGF23 mediated hypophosphatemicosteomalacia induced by intravenous iron suppletion.

CONCLUSION

FCM injections may cause FGF23 mediated hypophosphatemia already 4 weeks after suppletion. Therefore it is recommended that serum phosphate levels should be checked frequently. In patients developing hypophosphatemia, a non-maltose form of iron suppletion must be started as well as active vitamin D.

Burosumab treatment for fibrous dysplasia

BACKGROUND

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare mosaic disorder of Gα_s activation. Fibroblast Growth Factor 23 (FGF23)-mediated hypophosphatemia is a feature of FD/MAS that has been associated with poor skeletal outcomes. Standard therapy includes oral phosphorus and vitamin D analogs; however, treatment is limited by potential adverse renal and gastrointestinal effects. Burosumab is a monoclonal antibody to FGF23 approved to treat patients with X-linked hypophosphatemia and tumor-induced osteomalacia. There is currently no safety or efficacy data to support burosumab use in patients with FD/MAS.

CASE DESCRIPTION

A 7-year-old boy with severe FD/MAS presented with persistent hypophosphatemia and skeletal complications despite conventional treatment with oral phosphate and calcitriol. He was started on burosumab and achieved sustained normalization of serum phosphorus and marked improvement in alkaline phosphatase levels. This was accompanied by an encouraging clinical response, including decreased bone pain, improved muscle strength, and improved ambulation. No adverse effects of burosumab therapy were observed.

CONCLUSIONS

This is the first reported case of burosumab treatment in a patient with FD/MAS. The encouraging biochemical and clinical response in this patient highlights the need for future studies to explore the safety and efficacy of burosumab in the FD/MAS pediatric population.

Epidemiology of Tumor-Induced Osteomalacia in Denmark

Tumor-induced osteomalacia (TIO) is a rare, acquired condition of phosphate wasting due to phosphaturic mesenchymal tumors. Because the incidence and prevalence of TIO is unknown, we conducted an observational cohort study using national Danish health registers for the period 2008 to 2018 to obtain such information. The study also aimed to describe the demographics of the TIO population and the prognosis. The operational definition was based on hypophosphatemia or adult osteomalacia diagnoses, combined with prescriptions used in the initial management and procedures consistent with advanced imaging used for locating tumors. The incidence of TIO in Denmark was found to be below 0.13 per 100,000 person years for the total population of the country and 0.10 per 100,000 in adult-onset disease. The prevalence of TIO was estimated to be no more than 0.70 per 100,000 persons for the total population and 0.43 per 100,000 in adults. In 2018, there were a maximum of nine new cases of TIO in Danish adults. Mortality was low but few patients fulfilled the protocol cure criterion during the observation period. TIO has no ICD-10 code and limitations to the study include lack of information on serum biochemistry and on the use of phosphate supplements. Strengths include the use of long-term longitudinal, national hospital and prescription data from a country with universal healthcare. Given the very small patient population with TIO and the known delay to diagnosis and cure, management of patients with suspected TIO should be centralized.

Characterization of Oral Health Status in Chilean Patients with X-Linked Hypophosphatemia

X-Linked Hypophosphatemia (XLH) is the most common cause of inherited hypophosphatemic rickets. Dental involvement, including spontaneous abscesses and/or fistulae, is an important part of the disease and has not been completely defined, especially in cohorts from developing countries. To describe oral health status in a cohort of Chilean patients with XLH and explore its correlation with biochemical presentation and treatment, we conducted a cross-sectional observational study of patients with PHEX mutation-confirmed XLH. All patients had an oral clinical exam, radiographic evaluation; clinical and biochemical data were obtained to determine their association with oral features. Twenty-six patients were included, 77% adults and 23% children. Most adults (89%) had past or current dental pulp pathology (abscesses and/or fistulae). Pulpal chamber enlargement and radiolucent apical lesions were common radiological features (94 and 74%, respectively). In children, abscess and/or fistulae were also common (33%). Caries index, which was determined by dmft/DMFT, was higher than the Chilean national average. Early and long-term therapy with phosphate and activated vitamin D was associated with lower carious index and attachment loss. XLH patients frequently present with high pulpal involvement and carious index. Conventional therapy was associated with lower carious index and attachment loss. These data highlight the importance of early and periodical dental care in order to prevent dental damage and assure a good quality of oral health for XLH patients.

Acquired Fanconi Syndrome in a Patient with Nontyphoidal Salmonella Bacteremia

Nontyphoidal Salmonella is a common cause of bacterial gastroenteritis, occasionally causing bacteremia. We herein report the case of an 80-year-old man who presented with bacteremia and pre-renal acute kidney injury (AKI) secondary to diarrhea caused by nontyphoidal Salmonella. Despite AKI improvement on fluid administration, some serological abnormalities, such as hypokalemia, hypophosphatemia, and hypouricemia, and abnormal urinary findings emerged, including renal glycosuria and aminoaciduria. Fractional excretion of phosphate and uric acid was increased, suggesting that the serological and urinary abnormalities may have arisen from Fanconi syndrome. Physicians should consider acquired Fanconi syndrome when patients with nontyphoidal Salmonella bacteremia present with electrolyte disorders.