Hereditary Hypophosphatemic Rickets Research Articles

Newly identified intronic and known pathogenic point mutations in SLC34A3/NPT2c cause hereditary hypophosphatemic rickets with hypercalciuria

Newly identified intronic and known pathogenic point mutations in SLC34A3/NPT2c cause hereditary hypophosphatemic rickets with hypercalciuria

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH), a complex disorder in need of precision medicine

Hereditary hypophosphatemic rickets with hypercalciuria is an autosomal recessive phosphate-wasting disorder, associated with kidney and skeletal pathologies, which is caused by pathogenic variants of SLC34A3. In this issue, Zhu etal. describe a pooled analysis of 304 individuals carrying SLC34A3 variants. Their study underscores the complexity of hereditary hypophosphatemic rickets with hypercalciuria, as kidney and bone phenotypes generally do not coexist, heterozygous carriers of SLC34A3 variants also can be affected, and the response to oral phosphate supplementation is dependent on the genetic status.

Lessons learned from the real-world diagnosis and management of hereditary hypophosphatemic rickets

Hypophosphatemic rickets, which is often hereditary, is still under- or misdiagnosed in both children and adults, denying these individuals access to optimal management and genetic counseling. There have been recent calls to compile real-world data and share best practice on these rare conditions to guide clinical decision-making. Here we present eight clinical vignettes of patients with hypophosphatemic rickets encountered in our tertiary pediatric endocrinology practice. We describe the clinical features, genetics, and management of four cases of X-linked hypophosphatemia (PHEX mutations), one each of autosomal recessive hypophosphatemic rickets (DMP1 mutation) and autosomal recessive vitamin D-dependent rickets type 1A (CYP27B1 mutation), and two cases of distal renal tubular acidosis with FOXI1 mutation-associated hypophosphatemic rickets. Our cases prompt consideration of the (i) frequent misdiagnosis of hypophosphatemic rickets in clinical practice and the importance of comprehensive genetic testing; (ii) variable expressivity of the causative mutations; and (iii) a lack of responsiveness and/or compliance to conventional therapy and the value of burosumab in modern management, provided access is equitable. These cases highlight common real-world themes and challenges to managing patients presenting with these diverse conditions, especially the burden of disease hidden by misdiagnosis. In sharing these cases, we hope to raise awareness of these conditions, promote best practice in genetic diagnosis and management, and further advocate for reimbursement equity for the best available therapies.

An update on clinical presentation and responses to therapy of patients with hereditary hypophosphatemic rickets with hypercalciuria (HHRH)

Pathogenic variants in solute carrier family 34, member 3 (SLC34A3), the gene encoding the sodium-dependent phosphate cotransporter 2c (NPT2c), cause hereditary hypophosphatemic rickets with hypercalciuria (HHRH). Here, we report a pooled analysis of clinical and laboratory records of 304 individuals from 145 kindreds, including 20 previously unreported HHRH kindreds, in which two novel SLC34A3 pathogenic variants were identified. Compound heterozygous/homozygous carriers show above 90% penetrance for kidney and bone phenotypes. The biochemical phenotype for heterozygous carriers is intermediate with decreased serum phosphate, tubular reabsorption of phosphate (TRP (%)), fibroblast growth factor 23, and intact parathyroid hormone, but increased serum 1,25-dihydroxy vitamin D, and urine calcium excretion causing idiopathic hypercalciuria in 38%, with bone phenotypes still observed in 23% of patients. Oral phosphate supplementation is the current standard of care, which typically normalizes serum phosphate. However, although in more than half of individuals this therapy achieves correction of hypophosphatemia it fails to resolve the other outcomes. The American College of Medical Genetics score correlated with functional analysis of frequent SLC34A3 pathogenic variants in vitro and baseline disease severity. The number of mutant alleles and baseline TRP (%) were identified as predictors for kidney and bone phenotypes, baseline TRP (%) furthermore predicted response to therapy. Certain SLC34A3/NPT2c pathogenic variants can be identified with partial responses to therapy, whereas with some overlap, others present only with kidney phenotypes and a third group present only with bone phenotypes. Thus, our report highlights important novel clinical aspects of HHRH and heterozygous carriers, raises awareness to this rare group of disorders and can be a foundation for future studies urgently needed to guide therapy of HHRH.

Family analysis and literature study of hereditary hypophosphatemic rickets with hypercalciuria

BackgroundHereditary hypophosphatemia rickets with hypercalciuria (HHRH) is a rare autosomal recessive disorder characterised by reduced renal phosphate reabsorption leading to hypophosphataemia, rickets and bone pain. Here, we present a case of HHRH in a Chinese boy.Case presentationWe report a 11-year-old female proband, who was admitted to our hospital with bilateral genuvarum deformity and short stature. Computed Tomography (CT) showed kidney stones, blood tests showed hypophosphatemia, For a clear diagnosis, we employed high-throughput sequencing technology to screen for variants. Our gene sequencing approach encompassed whole exome sequencing, detection of exon and intron junction regions, and examination of a 20 bp region of adjacent introns. Flanking sequences are defined as ±50 bp upstream and downstream of the 5′ and 3′ ends of the coding region.The raw sequence data were compared to the known gene sequence data in publicly available sequence data bases using Burrows-Wheeler Aligner software (BWA, 0.7.12-r1039), and the pathogenic variant sites were annotated using Annovar. Subsequently, the suspected pathogenic variants were classified according to ACMG’s gene variation classification system. Simultaneously, unreported or clinically ambiguous pathogenic variants were predicted and annotated based on population databases. Any suspected pathogenic variants identified through this analysis were then validated using Sanger sequencing technology. At last, the proband and her affected sister carried pathogenic homozygous variant in the geneSLC34A3(exon 13, c.1402C > T; p.R468W). Their parents were both heterozygous carriers of the variant. Genetic testing revealed that the patient has anLRP5(exon 18, c.3917C > T; p.A1306V) variant of Uncertain significance, which is a rare homozygous variant.ConclusionThis case report aims to raise awareness of the presenting characteristics of HHRH. The paper describes a unique case involving variants in both theSLC34A3andLRP5genes, which are inherited in an autosomal recessive manner. This combination of gene variants has not been previously reported in the literature. It is uncertain whether the presence of these two mutated genes in the same individual will result in more severe clinical symptoms. This report shows that an accurate diagnosis is critical, and with early diagnosis and correct treatment, patients will have a better prognosis.

Impaired Growth Plate Maturation in XLH Is due to Both Excess FGF23 and Decreased 1,25-Dihydroxyvitamin D Signaling.

X-linked hypophosphatemia (XLH) is the most common form of hereditary hypophosphatemic rickets. The genetic basis for XLH is loss of function mutations in the phosphate-regulating endopeptidase X-linked (PHEX), which leads to increased circulating fibroblast growth factor 23 (FGF23). This increase in FGF23 impairs activation of vitamin D and attenuates renal phosphate reabsorption, leading to rickets. Previous studies have demonstrated that ablating FGF23 in the Hyp mouse model of XLH leads to hyperphosphatemia, high levels of 1,25-dihydroxyvitamin D, and is not associated with the development of rickets. Studies were undertaken to define a role for the increase in 1,25-dihydroxyvitamin D levels in the prevention of rickets in Hyp mice lacking FGF23. These mice were mated to mice lacking Cyp27b1, the enzyme responsible for activating vitamin D metabolites, to generate Hyp mice lacking both FGF23 and 1,25-dihydroxyvitamin D (FCH mice). Mice were fed a special diet to maintain normal mineral ion homeostasis. Despite normal mineral ions, Hyp mice lacking both FGF23 and Cyp27b1 developed rickets, characterized by an interrupted, expanded hypertrophic chondrocyte layer and impaired hypertrophic chondrocyte apoptosis. This phenotype was prevented when mice were treated with 1,25-dihydroxyvitamin D from day 2 until sacrifice on day 30. Interestingly, mice lacking FGF23 and Cyp27b1 without the PHEX mutation did not exhibit rickets. These findings define an essential PHEX-dependent, FGF23-independent role for 1,25-dihydroxyvitamin D in XLH and have important therapeutic implications for the treatment of this genetic disorder.

Novel Variant of SLC34A3 in a Compound Heterozygous Brazilian Girl with Hereditary Hypophosphatemic Rickets with Hypercalciuria.

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is a rare FGF23-independent disorder caused by biallelic variants in the SLC34A3 gene. The disease severity varies, and patients have an increased risk of developing renal complications. Phosphate supplementation is standard of care and active vitamin D analogs are not indicated as they could worsen the hypercalciuria. We report a Brazilian girl with HHRH who presented with knee pain and progressive genu valgum deformity that became apparent later in childhood (at age 8). Nephrocalcinosis was also identified at age 13. Next-generation sequencing (NGS) target panel directed to inherited forms of rickets detected compound heterozygous pathogenic variants in SLC34A3, including a novel missense variant c.1217G>T (p.Gly406Val). Compliance to oral phosphorus therapy was suboptimal and adjunctive chlorthalidone therapy improved hypercalciuria. Our case highlights the phenotypic variability of patients with HHRH and expands the growing list of SLC34A3 variants associated with this disorder. An accurate diagnosis is crucial for proper treatment, and a thiazide diuretic may be useful as adjunctive therapy for controlling hypercalciuria.

Regulation of Phosphate Transporters and Novel Regulator of Phosphate Metabolism

Phosphorus is essential for all living organisms. It plays an important role in maintaining biological functions, such as energy metabolism, cell membrane formation, and bone mineralization. Various factors in the intestine, kidneys, and bones regulate the homeostasis of the inorganic phosphate (Pi) concentration in the body. X-linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets, is characterized by an impaired mineralization of the bone matrix, hypertrophic chondrocytes with hypophosphatemia, and active vitamin D resistance in childhood. Phosphate-regulating gene with homologies to endopeptidases on the X chromosome was recognized as the responsible gene for XLH. XLH is classified as fibroblast growth factor 23 (FGF23)-related hypophosphatemic rickets. The enhanced FGF23 stimulates renal phosphate wasting by downregulating sodium-dependent Pi cotransporters, NaPi2a and NaPi2c proteins, in the proximal tubules. Recently, transmembrane protein (Tmem) 174 has been identified as a novel regulator of phosphate transporters. This review introduces the role of Tmem174 in the Pi homeostasis in the body.

#3320 GENETIC TESTING FOR MOLECULAR DIAGNOSIS OF NEPHROLITHIASIS AND NEPHROCALCINOSIS IN ADULT PATIENTS: A SINGLE-CENTER COHORT

Abstract Background and Aims Hereditary factors represent an important cause of nephrolithiasis which will generate stone disease in adult. Molecular analysis in patients with nephrolithiasis (NL) and/or nephrocalcinosis (NC) for a genetic mutation has become more accessible and the benefits are highlighted by an increasing number of publications. The aim of the paper was to study genetic screening for 36 NL and/or NC patients in order to identify the cases that can be confirmed by mutations in known kidney stone genes. Method Between 2020–2022 we included 35 adult patients with NL/NC with onset on pediatric age or in young adults, plus one of the following criteria: family history for NL/NC, indicative phenotype, recurrent NL. All the patients were diagnosed with NL and/or NC by ultrasound or computed tomography scan. Inform consent was signed and dated before blood samples and we performed genetic testing using nephrolithiasis panel (that include 45 genes) from 2 laboratories. In addition, we performed clinical assessment in a multidisciplinary team, underwent metabolic assessment, and caring out the genealogical tree. Results The study included 18 females and 17 males. The mean age of studied patients was 34.9 ± 10.3 years (range 18 – 54 years), although mean age of NL/NC diagnosis was 19.4 ± 12.0 years (range 0.5 – 34 years). 29 patients presented NL of any type, 12 patients presented both NL and NC, and 3 patients isolated NC. All the patients presented positive family history of NL/NC, 17 (48.5%) patients had pediatric age of onset, 17 (48.5%) patients presented indicative phenotype and 19 (54.2%) recurrent stone disease. Causative monogenic mutations were detected in 25 of 35 NL/NC. We identified 20 deleterious variants in 12 out of 45 analyzed genes. Genetic testing was positive with a definite diagnosis (had pathogenic variant) in 17 (48.6%) of cases, while 3 (8.6%) patients presented likely pathogenic variants, and 5 (14.3%) patients had variants of uncertain significance (VUS). In our cohort, the most common cause of kidney stone disease was cystine nephrolithiasis in 8 (22.8%) patients, followed by hereditary distal renal tubular acidosis in 4 patients, Dent disease in 3 patients, primary hyperoxaluria type 1 in 2 patients, familial hypomagnesaemia with hypercalciuria and nephrocalcinosis in 2 patients. Other causes of kidney stone disease included: renal hypouricemia type 1, hereditary hypophosphatemic rickets with hypercalciuria, primary mitochondrial disorders, autosomal dominant familial idiopathic hypercalciuria, Bartter syndrome type 3, and autosomal dominant tubulo-interstitial disease with hyperuricemia. Pathogenic mutations were detected in the following 5 dominant disease genes: SLC7A9 (4 patients), SLC4A1 (3 patients), ADCY10 (1 patient), HNF1B (2 patients), POLG (1 patient). Also, we identified pathogenic mutations in the following 7 recessive disease genes: CLCN5 (3 patients), AGXT (2 patients), CLDN16 (2 patients), SLC3A1 (3 patients), SLC34A3 (1 patient), WDR72 (1 patient), CLCNKB (1 patient). The mean eGFR for the study group was 71.2 ± 37.6 ml/min/1.72 m2. Seventeen patients presented eGFR < 60 ml/min/1.73 m2: seven patients CKD stage 3, four patients CKD stage 4, two patients CKD stage 5, and two patients were with renal replacement therapy. Conclusion Genetic kidney stone disease is an underdiagnosed condition. Although 48.5% of the patients had NL/NC onset on pediatric age, the molecular diagnosis was performed in adulthood, and, for some of them, when they suffer of advanced kidney disease. In our cohort, genetic testing had a high rate of positive molecular diagnosis of NL/NC due to selection criteria. Five of our patients presented VUS, but with disease-specific phenotype. We emphasize the importance of reporting these cases to generate additional evidence that could allow the reclassification of these variants. We conclude that the molecular diagnosis improves patient management, prevent or delay chronic kidney disease, offer possibility of genetic counseling and an extended screening to the family. Thus, our study showed the potential benefits of genetic testing, especially in high-risk groups for stone disease.

Abstract #1413209: An Interesting Case of Hereditary Adult Onset Hypophosphatemic Osteomalacia

Hereditary rickets/osteomalacia is uncommon disorder previously being referred as Vitamin D -resistant rickets. This group of rare disorder can be differentiated from other causes of rickets by inheritance pattern, hypophosphatemia with normocalcemia, FGF-23 dependent or independent nature and detection of genetic defects. Here we present an interesting case of autosomal dominant hypophosphatemic rickets/osteomalacia (ADHR). A 32-year-old lady presented with complains of bony pains, proximal myopathy and recurrent insufficiency fractures for last 15 years. She was evaluated elsewhere and was found to have FGF23 dependent hypophosphatemic osteomalacia. She underwent Ga68 DOTATATE scan twice in the span of last 5 years as she was thought to have tumor induced osteomalacia (TIO), however both the scans were not able to localize any definitive lesion. She was advised to take oral phosphorus supplements, calcitriol and to undergo serial functional imaging every 3-5 years to localize FGF-23 secreting tumor. She was re-evaluated in our center for these complaints and was found to have low serum phosphorus, low TMP/GFR, normal serum calcium & electrolytes and elevated FGF-23 levels. On reexamining her history, she told that her mother as well as her 5-year-old daughter have knock knees. She also told that both she and her daughter started walking late at the age of 20 months. With this additional history, we considered the possibility of hereditary hypophosphatemic rickets and genetic testing in the form of clinical exome by next generation sequencing with further confirmation by sanger sequencing was done. Both index patient and her daughter were found to have a pathogenic heterogenous missense mutation in exon 3 of the FGF23 gene (chr12:c.527G >A ;p.Arg176Gln) confirming the diagnosis of ADHR. ADHR is a rare disorder caused by mutations in FGF-23 gene. This gain of function mutation leads to resistance of FGF-23 to proteolytic cleavage and processing resulting in its impaired inactivation. ADHR exhibits incomplete penetrance with variable age of onset as well as presentation. Some patients present in childhood with lower extremity deformity and phosphate wasting similar to classical presentation of X-linked hypophosphatemic rickets whereas other patients present clinically in adolescence or adulthood with bony pains, insufficiency fractures without lower extremity involvement. Patients with adult onset ADHR have clinical and biochemical profile similar to TIO. Hence clinical history of similar milder illness in other family members is an important clue to differentiate ADHR from TIO as was with this index case.

Presentation and Diagnosis of Pediatric X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is a rare type of hereditary hypophosphatemic rickets. Patients with XLH have various symptoms that lower their QOL as defined by HAQ, RAPID3, SF36-PCS, and SF36-MCS in adult patients and SF-10 and PDCOI in pediatric patients. Early diagnosis and treatment are needed to reduce the burden, but the condition is often diagnosed late in childhood. The present review aims to summarize the symptoms, radiological and biological characteristics, and long-term prognosis of pediatric XLH. Typical symptoms of XLH are lower leg deformities (age six months or later), growth impairment (first year of life or later), and delayed gross motor development with progressive lower limb deformities (second year of life or later). Other symptoms include dental abscess, bone pain, hearing impairment, and Chiari type 1 malformation. Critical, radiological findings of rickets are metaphyseal widening, cupping, and fraying, which tend to occur in the load-bearing bones. The Rickets Severity Score, validated for XLH, is useful for assessing the severity of rickets. The biochemical features of XLH include elevated FGF23, hypophosphatemia, low 1,25(OH)2D, and elevated urine phosphate. Renal phosphate wasting can be assessed using the tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR), which yields low values in patients with XLH. XLH should be diagnosed early because the multisystem symptoms often worsen over time. The present review aims to help physicians diagnose XLH at an early stage.

Relationship between clinical phenotype and in vitro analysis of 13 NPT2c/SCL34A3 mutants

Biallelic pathogenic variants in the SLC34A3 gene, encoding for the NPT2c cotransporter, cause Hereditary Hypophosphatemic Rickets with Hypercalciuria (HHRH). However, the associated phenotype is highly variable. In addition, mice deleted for Slc34a3 exhibit a different phenotype compared to humans, without urinary phosphate leakage. The mechanisms by which SLC34A3 variants disrupt phosphate/calcium metabolism are un-completely understood. In this study we explored these mechanisms in vitro using SLC34A3 variants identified in patients with urinary phosphate leakage. We analyzed the consequences of these variants on NPT2c function and the link with the phenotype of the patients. We studied 20 patients with recurrent nephrolithiasis and low serum phosphate concentration harboring variants in the SLC34A3 gene. Half of the patients carried homozygous or composite heterozygous variants. Three patients had in addition variants in SLC34A1 and SLC9A3R1 genes. All these patients benefited from a precise analysis of their phenotype. We generated 13 of these mutants by site-directed mutagenesis. Then we carried out transient transfections of these mutants in HEK cells and measured their phosphate uptake capacity under different conditions. Among the 20 patients included, 3 had not only mutations in NPT2c but also in NPT2a or NHERF1 genes. Phosphate uptake was decreased in 8 NPT2c mutants studied and normal for 5. Four variants were initially categorized as variants of uncertain significance. Expression of the corresponding mutants showed that one did not modify phosphate transport, two reduced it moderately and one abolished it. Co-transfection of the NPT2c mutants with the wild-type plasmid of NPT2c or NPT2a did not reveal dominant negative effect of the mutants on NPT2c-mediated phosphate transport. A detailed analysis of patient phenotypes did not find a link between the severity of the disorder and the level of phosphate transport impairment. NPT2c mutations classified as ACMG3 identified in patients with renal phosphate leak should be characterized by in vitro study to check if they alter NPT2c-mediated phosphate transport since phosphate uptake capacity may not be affected. In addition, research for mutations in NHERF1 and NPT2a genes should always be associated to NPT2c sequencing.

Pharmacology of Mammalian Na+-Dependent Transporters of Inorganic Phosphate.

Inorganic phosphate (Pi) is an essential component of many biologically important molecules such as DNA, RNA, ATP, phospholipids, or apatite. It is required for intracellular phosphorylation signaling events and acts as pH buffer in intra- and extracellular compartments. Intestinal absorption, uptake into cells, and renal reabsorption depend on a set of different phosphate transporters from the SLC20 (PiT transporters) and SLC34 (NaPi transporters) gene families. The physiological relevance of these transporters is evident from rare monogenic disorders in humans affecting SLC20A2 (Fahr's disease, basal ganglia calcification), SLC34A1 (idiopathic infantile hypercalcemia), SLC34A2 (pulmonary alveolar microlithiasis), and SLC34A3 (hereditary hypophosphatemic rickets with hypercalciuria). SLC34 transporters are inhibited by millimolar concentrations of phosphonoformic acid or arsenate while SLC20 are relatively resistant to these compounds. More recently, a series of more specific and potent drugs have been developed to target SLC34A2 to reduce intestinal Pi absorption and to inhibit SLC34A1 and/or SLC34A3 to increase renal Pi excretion in patients with renal disease and incipient hyperphosphatemia. Also, SLC20 inhibitors have been developed with the same intention. Some of these substances are currently undergoing preclinical and clinical testing. Tenapanor, a non-absorbable Na+/H+-exchanger isoform 3 inhibitor, reduces intestinal Pi absorption likely by indirectly acting on the paracellular pathway for Pi and has been tested in several phase III trials for reducing Pi overload in patients with renal insufficiency and dialysis.

Monogenic causation of pediatric nephrolithiasis

Nephrolithiasis is a condition in which crystals precipitate out of the urine forming kidney stones in the renal calyces and pelvis. Approximately 80% of stones are composed of calcium oxalate and calcium phosphate. In recent years, there has been a significant increase in the prevalence of nephrolithiasis across populations, specifically in that of the pediatric population. The etiology of stone disease is multifactorial, and includes environmental, dietary, hormonal, and genetic factors. Evidence for monogenic causation (also known as Mendelian or single-gene disorders) in nephrolithiasis includes the finding that 30% of children with stone disease report a positive family history, with monogenic nephrolithiasis accounting for approximately 30% of cases. Monogenic nephrolithiasis can occur in isolation or may be the result of an underlying genetic disorder including autosomal dominant hypocalcemia (ADH), primary hyperoxalurias, and hereditary hypophosphatemic rickets with hypercalciuria (HHRH), to name a few. Currently, there are 41 known genes that represent monogenic causes of human nephrolithiasis. Since early detection of these mutations can in some cases prevent the progression to end stage kidney disease in pediatric patients, establishing the genetic basis for nephrolithiasis is increasingly important. Here we provide an overview of kidney stone disease in children with a focus on monogenic causation in the pediatric population.

PSAT228 Hereditary Hypophosphatemic Rickets: Management in Adults

Abstract Introduction Hereditary Hypophosphatemic Rickets (HHR) is a group of rare inherited disorders consisting of X-linked hypophosphatemic rickets (XLHR, prevalence 1 in 20,000 births), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR). XLHR is caused by loss of function mutation of the PHEX gene, ARHR is caused by loss of function mutation of the DMP1/ENPP1 gene and ADHR is caused by activating point mutations in FGF23. These mutations result in renal phosphate wasting, hypophosphatemia, and impaired bone mineralization. It presents in childhood as skeletal deformities, abnormal gait, skeletal pain, dental abnormalities, and stunted growth. Adults have osteomalacia-related pain, fractures, and arthritis. The diagnosis is made by findings consistent with rickets and normal serum calcium, normal 25-hydroxy vitamin D, low-normal 1,25-dihydroxy vitamin D, elevated alkaline phosphatase activity, and low phosphate levels. Urine studies show low urine calcium and decreased tubular maximum reabsorption of phosphate, confirming renal wasting of phosphate. The goal of treatment in adults is to reduce pain, prevent fractures, enhance mobility, and decrease osteomalacia. Phosphate and vitamin D supplementations are used with close monitoring as it is associated with severe adverse events. It can cause hypercalcemia leading to nephrocalcinosis, nephrolithiasis, chronic kidney disease, and hyperparathyroidism. There is no evidence that the treatment affects long-term skeletal complications. Burosumab, a monoclonal antibody that inhibits FGF23, is approved for the treatment of XLHR. No studies are available about the use of Burosumab in ADHR or ARHR. Case Presentation A 18-year-old male with past medical history of hypophosphatemic rickets with multiple fractures, on oral phosphate tablets and calcitriol, presented to the Endocrinology clinic to establish care. He had been following Pediatric Endocrinology in the past. His blood work showed normal calcium, low phosphate of 1.6 mg/dl (normal: 2.5-4.5 mg/dl), vitamin D 25 hydroxy level of 26 ng/ml (normal: 20-100 ng/ml), vitamin D 1,25-dihydroxy level of 25 pg/ml (normal: 30-80 pg/ml), intact parathyroid hormone level of 549 pg/ml (normal: 15-65 pg/ml), and alkaline phosphatase of 438 U/l (normal: 40-150 U/l). Urine calcium was low and urine phosphorus was elevated with phosphate/creatinine ratio more than 2000 (normal: 60-962), consistent with renal phosphate wasting. Genetic testing in the past was negative for PHEX gene mutation, so he was not a candidate for Burosumab. Conclusion There is limited knowledge about Hereditary Hypophosphatemic Rickets, especially ADHR and ARHR due to limited cases (less than 100 cases reported so far). The current standard of treatment of HHR is sub-optimal and associated with adverse effects. Burosumab has not been tested in ADHR or ARHR, so it is unclear if it will be beneficial in this subgroup. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m., Sunday, June 12, 2022 12:30 p.m. - 2:30 p.m.

Osteocytes and the pathogenesis of hypophosphatemic rickets.

Since phosphorus is a component of hydroxyapatite, its prolonged deprivation affects bone mineralization. Fibroblast growth factor 23 (FGF23) is essential for maintaining phosphate homeostasis and is mainly produced by osteocytes. FGF23 increases the excretion of inorganic phosphate (Pi) and decreases the production of 1,25-dihydroxyvitamin D in the kidneys. Osteocytes are cells of osteoblastic lineage that have undergone terminal differentiation and become embedded in mineralized bone matrix. Osteocytes express FGF23 and other multiple genes responsible for hereditary hypophosphatemic rickets, which include phosphate-regulating gene homologous to endopeptidase on X chromosome (PHEX), dentin matrix protein 1 (DMP1), and family with sequence similarity 20, member C (FAM20C). Since inactivating mutations in PHEX, DMP1, and FAM20C boost the production of FGF23, these molecules might be considered as local negative regulators of FGF23. Mouse studies have suggested that enhanced FGF receptor (FGFR) signaling is involved in the overproduction of FGF23 in PHEX-deficient X-linked hypophosphatemic rickets (XLH) and DMP1-deficient autosomal recessive hypophosphatemic rickets type 1. Since FGFR is involved in the transduction of signals evoked by extracellular Pi, Pi sensing in osteocytes may be abnormal in these diseases. Serum levels of sclerostin, an inhibitor Wnt/β-catenin signaling secreted by osteocytes, are increased in XLH patients, and mouse studies have suggested the potential of inhibiting sclerostin as a new therapeutic option for the disease. The elucidation of complex abnormalities in the osteocytes of FGF23-related hypophosphatemic diseases will provide a more detailed understanding of their pathogenesis and more effective treatments.

Corrective osteotomy for complex tibial deformity in a patient with hereditary vitamin D-resistant hypophosphatemic rickets (HVDRR) using CT-based navigation system and 3D printed osteotomy model

Planning a three-dimensional (3D) osteotomy using computed tomography (CT) data is useful especially in cases with complex deformities. Furthermore, CT-based navigation system allows the preoperative virtual planning to be replicated in actual surgery. However, one disadvantage when using navigation systems is that when osteotomies are performed on tracker-attached bone, the bone fragments on the side that were cut away cannot be tracked. This is especially problematic when performing multiple osteotomies on bones with complex deformities. We solved this problem by creating a 3D printed bone model that can be referenced intraoperatively and used in combination with the navigation system. We applied these techniques to perform segmental corrective osteotomy for a complex tibial deformity with intramedullary nail (IMN) fixation case of hereditary vitamin D-resistant hypophosphatemic rickets (HVDRR) in an adult man. Due to the patient’s history of multiple surgeries, the affected tibia had a narrow and partially closed medullary canal. Therefore, we planned to use an IMN for correction and fixation of tibial deformity to protect the thin and stretched skin around the deformed tibia. With the assistance of CT-based navigation, we could perform an accurate three-dimensional tibial osteotomy. Moreover, we could perform accurate preparation of closed medullary canal for the IMN placement by referring to the 3D printed bone models. Six months after the operation, the bone union at the osteotomy sites was confirmed and the patient was able to return to his normal life and work.

Novel Small Molecule Fibroblast Growth Factor 23 Inhibitors Increase Serum Phosphate and Improve Skeletal Abnormalities in Hyp Mice.

Excess fibroblast growth factor (FGF) 23 causes hereditary hypophosphatemic rickets, such as X-linked hypophosphatemia (XLH) and tumor-induced osteomalacia (TIO). A small molecule that specifically binds to FGF23 to prevent activation of the fibroblast growth factor receptor/α-Klotho complex has potential advantages over the currently approved systemically administered FGF23 blocking antibody. Using structure-based drug design, we previously identified ZINC13407541 (N-[[2-(2-phenylethenyl)cyclopenten-1-yl]methylidene]hydroxylamine) as a small molecule antagonist for FGF23. Additional structure-activity studies developed a series of ZINC13407541 analogs with enhanced drug-like properties. In this study, we tested in a preclinical Hyp mouse homolog of XLH a direct connect analog [(E)-2-(4-(tert-butyl)phenyl)cyclopent-1-ene-1-carbaldehyde oxime] (8n), which exhibited the greatest stability in microsomal assays, and [(E)-2-((E)-4-methylstyryl)benzaldehyde oxime] (13a), which exhibited increased in vitro potency. Using cryo-electron microscopy structure and computational docking, we identified a key binding residue (Q156) of the FGF23 antagonists, ZINC13407541, and its analogs (8n and 13a) in the N-terminal domain of FGF23 protein. Site-directed mutagenesis and bimolecular fluorescence complementation-fluorescence resonance energy transfer assay confirmed the binding site of these three antagonists. We found that pharmacological inhibition of FGF23 with either of these compounds blocked FGF23 signaling and increased serum phosphate and 1,25-dihydroxyvitamin D [1,25(OH)2D] concentrations in Hyp mice. Long-term parenteral treatment with 8n or 13a also enhanced linear bone growth, increased mineralization of bone, and narrowed the growth plate in Hyp mice. The more potent 13a compound had greater therapeutic effects in Hyp mice. Further optimization of these FGF23 inhibitors may lead to versatile drugs to treat excess FGF23-mediated disorders. SIGNIFICANCE STATEMENT: This study used structure-based drug design and medicinal chemistry approaches to identify and optimize small molecules with different stability and potency, which antagonize excessive actions of fibroblast growth factor 23 (FGF23) in hereditary hypophosphatemic rickets. The findings confirmed that these antagonists bind to the N-terminus of FGF23 to inhibit its binding to and activation of the fibroblast growth factor receptors/α-Klotho signaling complex. Administration of these lead compounds improved phosphate homeostasis and abnormal skeletal phenotypes in a preclinical Hyp mouse model.

Growth in height and body proportion from birth to adulthood in hereditary hypophosphatemic rickets: a retrospective cohort study.

Patients with hereditary hypophosphatemic rickets are short and disproportionate and very little information is available on segmental growth, but the body disproportion at adulthood leads us to think that the growth velocity of legs is slower. A total of 96 children were included and molecular testing was carried out in 42. Children who reached adult heightwere classified into two groups according to their compliance to conventional treatment (phosphate supplement and calcitriol). Individual growth records of height and sitting height/height were plotted using Argentine reference data in 96 children and growth curves were estimated by fitting Preece-Baines Model 1 in 19 of the children. Molecular testing revealed sequence deleterious alterations or large deletions in 36/42 patients. During childhood, 76% of children grew below -1.88 standard deviation score (SDS) and 97% had body disproportion. During adolescence, the mean peak height velocity for the good and poor compliance to treatment groups was 7.8 (0.6) and 5.4 (0.4) cm/year in boys and 7.0 (0.7) and 5.2 (0.8) cm/year in girls, respectively. At adulthood, the median sitting height/height ratio was 2.32 and 6.21 SDS for the good and poor compliance to treatment groups, respectively. The mean pubertal growth spurt of the trunk was -0.8 (1.4) SDS, with a short pubertal growth spurt of -1.8 (0.4) SDS for limbs in the good compliance group. Median adult height in 13/29 males and 30/67 females was -4.56 and -3.16 SDS, respectively. For all patients the growth spurt was slower, secondary to a short growth spurt of limbs, reaching a short adult height with body disproportion that was more prominent in the poor compliance group.

Cellular and Molecular Alterations Underlying Abnormal Bone Growth in X-Linked Hypophosphatemia.

X-linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemic rickets, is caused by inactivating mutations of the phosphate-regulating endopeptidase gene (PHEX). XLH is mainly characterized by short stature, bone deformities and rickets, while in hypophosphatemia, normal or low vitamin D levels and low renal phosphate reabsorption are the principal biochemical aspects. The cause of growth impairment in patients with XLH is not completely understood yet, thus making the study of the growth plate (GP) alterations necessary. New treatment strategies targeting FGF23 have shown promising results in normalizing the growth velocity and improving the skeletal effects of XLH patients. However, further studies are necessary to evaluate how this treatment affects the GP as well as its long-term effects and the impact on adult height.