A clinical and molecular genetic study of hypophosphatemic rickets in children

Characterization of Novel PHEX Variants in X-linked Hypophosphatemic Rickets and Genotype-PHEX Activity Correlation

BACKGROUND

X-linked hypophosphatemia (XLHR) is the most common genetic form of hypophosphatemic rickets (HR), which is caused by phosphate regulating endopeptidase homolog X-linked (PHEX) gene mutation. At present, the genotype-phenotype relationship of XLHR and the pathogenic role of PHEX are not fully understood.

METHODS

In this study, we summarized clinical features in a new cohort of 49 HR patients and detected 16 novel PHEX and 5 novel non-PHEX variants. Subsequently, we studied the pathogenesis of new variants by protein expression, glycosylation analysis, subcellular localization, and endopeptidase activity.

RESULTS

The results showed that missense variants (Q189H and X750R) slightly reduced protein expression without obviously altering protein length and localization, whereas truncating variants significantly impaired the synthesis of PHEX and produced a shorter immature protein in cells. Interestingly, no evident correlation was observed between mutation types and clinical phenotypes. However, when we analyzed the relationship between PHEX activity and serum phosphorus level, we found that patients with low PHEX activity tended to have severe hypophosphatemia and high rickets severity score. Following this observation, we established 2 new knock-in XLHR mouse models with 2 novel Phex variants (c.T1349C and c.C426G, respectively) using CRISPR/Cas9 technology. Both mouse models demonstrated clinical manifestations of XLHR seen in patients, and PhexC426G mice showed more severe phenotype than PhexT1349C mice, which further confirmed the rationality of genotype-PHEX enzymatic activity correlation analysis.

CONCLUSION

Therefore, our findings demonstrated that novel PHEX variants could disrupt protein function via affecting protein synthesis, post-translational modification, cellular trafficking, and catalytic activity. Our study facilitates a better understanding of XLHR pathogenic mechanism and PHEX activity-phenotype correlation, which is of crucial importance for future diagnosis and treatment of XLHR.

The contribution of a novel PHEX gene mutation to X-linked hypophosphatemic rickets: a case report and an analysis of the gene mutation dosage effect in a rat model

A Chinese family was identified to have two patients with rickets, an adult female and a male child (proband), both exhibiting signs related to X-linked hypophosphatemic rickets (XLH). Gene sequencing analysis revealed a deletion of adenine at position 1985 (c.1985delA) in the PHEX-encoding gene. To investigate the relationship between this mutation and the pathogenicity of XLH, as well as analyze the effects of different dosages of PHEX gene mutations on clinical phenotypes, we developed a rat model carrying the PHEX deletion mutation. The CRISPR/Cas9 gene editing technology was employed to construct the rat model with the PHEX gene mutation (c.1985delA). Through reproductive procedures, five genotypes of rats were obtained: female wild type (X/X), female heterozygous (-/X), female homozygous wild type (-/-), male wild type (X/Y), and male hemizygous (-/Y). The rats with different genotypes underwent analysis of growth, serum biochemical parameters, and bone microstructure. The results demonstrated the successful generation of a stable rat model inheriting the PHEX gene mutation. Compared to the wild-type rats, the mutant rats displayed delayed growth, shorter femurs, and significantly reduced bone mass. Among the female rats, the homozygous individuals exhibited the smallest body size, decreased bone mass, shortest femur length, and severe deformities. Moreover, the mutant rats showed significantly lower blood phosphorus concentration, elevated levels of FGF23 and alkaline phosphatase, and increased expression of phosphorus regulators. In conclusion, the XLH rat model with the PHEX gene mutation dosage demonstrated its impact on growth and development, serum biochemical parameters, and femoral morphology.

X-Linked Familial Hypophosphatemia: A Case Report of 27-Year Old Male and Review of Literature

X-linked hypophosphatemia (XLH) associated with short stature during childhood are mostly referred to the hospital and diagnosed as vitamin D deficiency rickets and received vitamin D before adulthood. A case is presented with clinical features of hypophosphatemia from childhood who did not seek medical care for diagnosis and treatment, nor did his mother or two brothers, who have short statures, bone pain, and fractures. The patient was assessed for sociodemographic, hematological, and biochemical parameters together with a genetic assessment. A DEXA scan and X-ray were done to determine the abnormalities and deformities of joints and bones despite clinical examination by an expert physician. All imaging, laboratory parameters, and the genetic study confirmed the diagnosis of XLH. A detailed follow-up of his condition was performed after the use of phosphate tablets and other treatments. X-linked hypophosphatemia needs a good assessment, care, and follow up through a complementary medical team including several specialties. Phosphate tablets in adulthood significantly affects clinical and physical improvement and prevention of further skeletal abnormality and burden on daily activity. The patients should be maintained with an adequate dose of phosphate for better patient compliance. More awareness is needed in society and for health professionals when conducting medical checkups during the presence of stress fractures, frequent dental and gum problems, rickets, short stature, or abnormality in the skeleton or walking to think of secondary causes such as hypophosphatemia. Further investigations including a visit to a specialist is imperative to check for the primary cause of these disturbances.

A genetic study of a Brazilian cohort of patients with X-linked hypophosphatemia reveals no correlation between genotype and phenotype

Aim

X-linked hypophosphatemia (XLH) is the most common inherited form of rickets, and it is caused by pathogenic inactivating variants of the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene. The main purpose of this study is to identify the presence of a genotype-phenotype correlation in a cohort of XLH patients.

Methods

This is a retrospective study including patients diagnosed with hypophosphatemic rickets, confirmed by clinical, radiological, and laboratory findings. Medical records were reviewed for phenotypic analyses. Genomic DNA was extracted from the peripheral blood lymphocytes, and PHEX sequencing was performed by exomic NGS sequencing. The Wilcoxon rank-sum test and the two-tailed Fisher's exact test were employed for the statistical analyses of this study.

Results

A total of 41 patients were included in this study, and 63.41% (26/41) of the patients were female. The mutation analyses identified 29.27% missense variants and 29.72% nonsense variants, most of them were considered deleterious (66.41%). Six novel deleterious variants in the PHEX gene were detected in seven patients. The median concentrations of pretreatment serum calcium, phosphorus, and parathyroid hormone (PTH) were not significantly different among patients with different genotypes. An orthopedic surgery due to bone deformity was required in 57.69%.

Conclusions

Our analysis did not identify any specific genotype as a predictor. No significant genotype-phenotype correlation was found, suggesting that the recognition of subjacent pathogenic mutation in the PHEX gene may have limited prognostic value. Despite this finding, genetic testing may be useful for identifying affected individuals early and providing appropriate treatment.

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.

Pathogenic Variants of the PHEX Gene

Twenty-five years ago, a pathogenic variant of the phosphate-regulating endopeptidase homolog X-linked (PHEX) gene was identified as the cause of X-linked hypophosphatemic rickets (XLH). Subsequently, the overproduction of fibroblast growth factor 23 (FGF23) due to PHEX defects has been found to be associated with XLH pathophysiology. However, the mechanism by which PHEX deficiency contributes to the upregulation of FGF23 and the function of PHEX itself remain unclear. To date, over 700 pathogenic variants have been identified in patients with XLH, and functional assays and genotype–phenotype correlation analyses based on pathogenic variant data derived from XLH patients have been reported. Genetic testing for XLH is useful for the diagnosis. Not only have single-nucleotide variants causing missense, nonsense, and splicing variants and small deletion/insertion variants causing frameshift/non-frameshift alterations been observed, but also gross deletion/duplication variants causing copy number variants have been reported as pathogenic variants in PHEX. With the development of new technologies including next generation sequencing, it is expected that an increasing number of pathogenic variants will be identified. This chapter aimed to summarize the genotype of PHEX and related analyses and discusses the pathophysiology of PHEX defects to seek clues on unsolved questions.

Effects of Burosumab Treatment on Two Siblings with X-Linked Hypophosphatemia. Case Report and Literature Review

X-linked hypophosphatemia (XLH) or vitamin D-resistant rickets (MIM#307800), is a monogenic disorder with X-linked inheritance. It is caused by mutations present in the Phosphate Regulating Endopeptidase Homolog X-Linked (PHEX) gene responsible for the degradation of the bone-derived hormone fibroblast growth factor 23 (FGF23) into inactive fragments, but the entire mechanism is currently unclear. The inactivation of the gene prevents the degradation of FGF23, causing increased levels of FGF23, which leads to decreased tubular reabsorbtion of phosphorus. Clinical aspects are growth delay, limb deformities, bone pain, osteomalacia, dental anomalies, and enthesopathy. Laboratory evaluation shows hypophosphatemia, elevated alkaline phosphatase (ALP), and normal serum calcium levels, whereas parathormone (PTH) may be normal or increased and FGF23 greatly increased. Conventional treatment consists of administration of oral phosphate and calcitriol. Treatment with Burosumab, a monoclonal antibody that binds to FGF23, reducing its activity, was approved in 2018. Methods. We describe a case of two siblings, a girl and a boy, diagnosed with XLH, monitored by the Genetic Department of the County Emergency Clinical Hospital since 2019. The clinical picture is suggestive for XLH, both siblings exhibiting short stature, lower limb curvature, bone pain, marked walking weakness, and fatigue. Radiological aspects showed marked deformity of the lower limbs: genu varum in the girl, genu varum and valgum in the boy. Laboratory investigations showed hypophosphathemia, hyperphosphaturia, elevated ALP, normal PTH, and highly increased FGF23 in both. DNA analysis performed on the two siblings revealed a nonsense mutation in exone 5 of the PHEX gene: NM_000444.6(PHEX):c.565C > T (p.Gln189Ter). Results. At the age of 13½ on 7 June 2021, the two children started treatment with Burosumab in therapeutic doses and were monitored clinically and biochemically at regular intervals according to the protocol established by the Endocrinology Commission of the Romanian Health Ministry. Conclusions. The first results of the Burosumab treatment in the two siblings are extremely encouraging and suggest a favorable long-term evolution under this treatment.

Hypophosphatemic Rickets in Colombia: A Prevalence-Estimation Model in Rare Diseases. 2018

Background

Hypophosphatemic rickets is a rare, genetic syndrome with multisystem involvement. It causes skeletal abnormalities, painful enthesopathies, increased risk of fracture, and short stature; leading to a substantial burden of disease, disability, and worsening of quality of life. To improve health conditions of people living with this disease, it is essential to know its prevalence which is currently unknown in Colombia. This study aimed to estimate the prevalence of hypophosphatemic rickets in Colombia by using a mathematical model and national statistic records.

Methods

We executed a model to estimate probabilities of transitions between health, disease, and death states (Markov chains). The model was fed with international prevalences taken from original studies (systematic review) and administrative records' data from SISPRO (a national health information system) using the International Classification of Diseases (ICD-10) E833 code, vital statistics, and census data. World Health Organization's (WHO) DISMOD II software was used to develop the model.

Findings

The estimated overall prevalence of hypophosphatemic rickets in Colombia in 2018 was 2·03 cases per 100 000 people (981 affected people), with a sensitive range of 1·97 to 2·09. The estimated prevalence by sex was 2·61 (645 people) and 1·43 (336 people) cases per 100 000 women and men, respectively.

Interpretation

Our overall estimated prevalence shows consistency with original international data. This is the first prevalence estimation of hypophosphatemic rickets in Colombia and will be relevant to support public health decisions for rare diseases and to provide a pre-test probability framework in clinical practice. DISMOD II and the model are useful tools to estimate the prevalence of rare and orphan diseases, when probabilistic studies cannot be carried out. There are limited bibliographic resources worldwide reporting prevalence values supported by original studies. Our study can be used as a cost-effective methodology reference in this regard, especially for Latin America.

Funding

Ultragenyx Pharmaceutical, as a donation.

Novel PHEX gene locus-specific database: Comprehensive characterization of vast number of variants associated with X-linked hypophosphatemia (XLH)

X‐linked hypophosphatemia (XLH), the most common form of hereditary hypophosphatemia, is caused by disrupting variants in the PHEX gene, located on the X chromosome. XLH is inherited in an X‐linked pattern with complete penetrance observed for both males and females. Patients experience lifelong symptoms resulting from chronic hypophosphatemia, including impaired bone mineralization, skeletal deformities, growth retardation, and diminished quality of life. This chronic condition requires life‐long management with disease‐specific therapies, which can improve patient outcomes especially when initiated early in life. To centralize and disseminate PHEX variant information, we have established a new PHEX gene locus‐specific database, PHEX LSDB. As of April 30, 2021, 870 unique PHEX variants, compiled from an older database of PHEX variants, a comprehensive literature search, a sponsored genetic testing program, and XLH clinical trials, are represented in the PHEX LSDB. This resource is publicly available on an interactive, searchable website (https://www.rarediseasegenes.com/), which includes a table of variants and associated data, graphical/tabular outputs of genotype‐phenotype analyses, and an online submission form for reporting new PHEX variants. The database will be updated regularly with new variants submitted on the website, identified in the published literature, or shared from genetic testing programs.

Clinical and Genetic Characteristics of 153 Chinese Patients With X-Linked Hypophosphatemia

X-linked hypophosphatemia (XLH) is caused by inactivating mutations in the phosphate-regulating endopeptidase homolog, X-linked (PHEX) gene, resulting in an excess of circulating intact fibroblast growth factor-23 (iFGF-23) and a waste of renal phosphate. In the present study, we retrospectively reviewed the clinical and molecular features of 153 Chinese patients, representing 87 familial and 66 sporadic cases with XLH. A total of 153 patients with XLH presented with signs or symptoms at a median age of 18.0 months (range, 9.0 months–26.0 years). Lower-limb deformity was the most frequent clinical manifestation, accounting for 79.1% (121/153). Biochemical screening showed increased serum levels of iFGF23 in patients with XLH, with a wide variation ranging from 14.39 to 730.70 pg/ml. Median values of serum iFGF23 in pediatric and adult patients were 94.87 pg/ml (interquartile range: 74.27–151.86 pg/ml) and 72.82 pg/ml (interquartile range: 39.42–136.00 pg/ml), respectively. Although no difference in circulating iFGF23 levels between these two groups was observed (P = 0.062), the proportion of patients with high levels of circulating iFGF23 (>42.2 pg/ml) was greater in the pediatric group than in the adult group (P = 0.026). Eighty-eight different mutations in 153 patients were identified, with 27 (30.7%) being novel. iFGF23 levels and severity of the disease did not correlate significantly with truncating and non-truncating mutations or N-terminal and C-terminal PHEX mutations. This study provides a comprehensive description of the clinical profiles, circulating levels of iFGF23 and gene mutation features of patients with XLH, further enriching the genotypic spectrum of the diseases. The findings show no evident correlation of circulating iFGF23 levels with the age or disease severity in patients with XLH.

A pathogenic PHEX variant (c.1483-1G>C) in a Korean patient with X-linked hypophosphatemic rickets

X-linked hypophosphatemic rickets is an X-linked dominantly inherited disorder characterized by defects in renal phosphate transport leading to phosphate wasting and hypophosphatemia. In this report, we describe a case of X-linked hypophosphatemic rickets in a patient with a rare pathogenic PHEX variant. The 25-year-old female patient came to our clinic for genetic counseling regarding presumed genetic disease and pregnancy. When she was 9 years old, she had been diagnosed with vitamin D-resistant rickets based on laboratory results and symptoms. She had undergone orthopedic surgery due to bowing leg deformities. Since then, she was intermittently self-prescribing oral phosphate and calcium supplements. At 25 years old, she was diagnosed with X-linked hypophosphatemic rickets with a rare pathogenic PHEX variant (c.1483-1G>C) by next-generation sequencing. This is the second report of the c.1483-1G>C variant to date, and her pathogenicity was confirmed based on the most recent guideline. Traditionally, the disease had been diagnosed mostly based on clinical findings. However, with advancements in genetic testing, genetic confirmation has become an imperative part of diagnostic workup. Herein, we report a 25-year-old female Korean patient diagnosed with X-linked hypophosphatemic rickets harboring a rare pathogenic PHEX variant.

The genetic polymorphisms of XPR1 and SCL34A3 are associated with Fanconi syndrome in Chinese patients of tumor-induced osteomalacia

PURPOSE

Tumor-induced osteomalacia (TIO) is an acquired form of hypophosphatemia caused by tumors with excess production of fibroblast growth factor 23 (FGF23). Some reports showed that TIO patients had renal Fanconi syndrome (FS) with unidentified mechanism. In this study, we investigated the association between genetic polymorphisms of phosphate transporters in renal proximal tubules and TIO with FS.

METHODS

We recruited 30 TIO patients with FS (TIO-FS) as well as 30 TIO patients (TIO-nonFS) without any urine abnormalities matched by age and gender. We collected clinical manifestations and conducted targeted sequencing of SLC34A1, SLC34A3 and XPR1 genes and the association analysis between variants in TIO with FS and phenotypes.

RESULTS

TIO-FS group had lower levels of serum phosphate (0.44 ± 0.12 vs. 0.51 ± 0.07 mmol/L, p < 0.05) than TIO-nonFS group. Among the 16 SNPs in SLC34A1, SLC34A3 and XPR1 genes, GG/GC genotypes of rs148196667 in XPR1 and AA/TA genotypes of rs35535797 in SLC34A3 were associated with a reduced susceptibility to have FS. The G allele of rs148196667 in XPR1 decreased the risk of FS. The GGAA haplotype in SLC34A3 and GCT haplotype in XPR1 were associated with a decreased risk for FS.

CONCLUSIONS

The polymorphisms of XPR1 and SCL34A3 are associated with TIO patients with Fanconi syndrome. It provides novel insight to the relationship of phosphate transportation and general functions of renal proximal tubules.

Outcome of primary tubular tubulopathies diagnosed in pediatric age

BACKGROUND AND OBJECTIVE

Primary tubulopathies are rare and usually present at pediatric age. Recent advances in genetic diagnosis and treatment have changed its natural history. This study provides the clinical spectrum of a series of primary tubulopathies diagnosed in a Pediatric Nephrology Unit and to offer long-term follow-up data regarding growth, estimated glomerular filtration (eGFR) and intercurrent complications.

PATIENTS AND METHODS

Observational study in 53 patients with primary tubulopathies and identified genetic defect: Gitelman syndrome (36%), distal renal tubular acidosis (15%), cystinuria (11%), X-linked hypophosphatemic rickets (7%), Dent-syndrome Lowe (7%), cystinosis (6%), and 1-2 cases of other tubulopathies. Demographic, analytical and clinical data were collected at diagnosis, during evolution and at the time of the study.

RESULTS

The age (median and interquartile range) at diagnosis was 5.08 years (1.33-8.50). The most frequent presentation manifestations were metabolic decompensations associated with intercurrent processes (40%) and short stature (38%). Height (mean ± SD) was -1.39 ± 1.49 at diagnosis and 1.07 ± 1.54 after a follow-up of 18.92 (6.25-24.33) years. Sixteen (32%) developed an eGFR <90 ml/min/1.73 m². Three patients required replacement renal replacement. Eleven patients had metabolic decompensations that required hospitalization, 9 renal colic and/or kidney stones and 10 mental problems. Six of 8 patients with distal renal tubular acidosis developed sensorineural deafness.

CONCLUSIONS

Primary tubulopathies are a heterogeneous group of diseases that cause growth impairment, largely reversible with treatment, risk of eGFR reduction and significant extrarenal complications derived or associated.

Phenotypic characterization of X-linked hypophosphatemia in pediatric Spanish population

Background

X-linked hypophosphatemia (XLH) is a hereditary rare disease caused by loss-of-function mutations in PHEX gene leading tohypophosphatemia and high renal loss of phosphate. Rickets and growth retardation are the major manifestations of XLH in children, but there is a broad phenotypic variability. Few publications have reported large series of patients. Current data on the clinical spectrum of the disease, the correlation with the underlying gene mutations, and the long-term outcome of patients on conventional treatment are needed, particularly because of the recent availability of new specific medications to treat XLH.

Results

The RenalTube database was used to retrospectively analyze 48 Spanish patients (15 men) from 39 different families, ranging from 3 months to 8 years and 2 months of age at the time of diagnosis (median age of 2.0 years), and with XLH confirmed by genetic analysis. Bone deformities, radiological signs of active rickets and growth retardation were the most common findings at diagnosis. Mean (± SEM) height was − 1.89 ± 0.19 SDS and 55% (22/40) of patients had height SDS below—2. All cases had hypophosphatemia, serum phosphate being − 2.81 ± 0.11 SDS. Clinical manifestations and severity of the disease were similar in both genders. No genotype—phenotype correlation was found. Conventional treatment did not attenuate growth retardation after a median follow up of 7.42 years (IQR = 11.26; n = 26 patients) and failed to normalize serum concentrations of phosphate. Eleven patients had mild hyperparathyroidism and 8 patients nephrocalcinosis.

Conclusions

This study shows that growth retardation and rickets were the most prevalent clinical manifestations at diagnosis in a large series of Spanish pediatric patients with XLH confirmed by mutations in the PHEX gene. Traditional treatment with phosphate and vitamin D supplements did not improve height or corrected hypophosphatemia and was associated with a risk of hyperparathyroidism and nephrocalcinosis. The severity of the disease was similar in males and females.

Genotype and Phenotype Analysis in X-Linked Hypophosphatemia

Background: X-linked hypophosphatemia (XLH) is the most frequent form of hypophosphatemic rickets and is caused by mutations in the PHEX gene. We analyzed genotype-phenotype correlations in XLH patients with proven PHEX mutations. Methods: PHEX mutations were detected in 55 out of 81 patients who clinically presented with hypophosphatemic rickets. The patients were grouped into nontruncating (n = 9) and truncating (n = 46) mutation groups; their initial presentation as well as long-term clinical findings were evaluated according to these groups. Results: Initial findings, including presenting symptoms, onset age, height standard deviation scores (SDS), and laboratory tests, including serum phosphate level and tubular resorption of phosphate, were not significantly different between the two groups (onset age: nontruncating mutation group, 2.0 years, truncating mutation group, 2.2 years; height SDS: nontruncating mutation group, -1.9, truncating mutation group, -1.7; serum phosphate: nontruncating mutation group, 2.5 mg/dL, truncating mutation group, 2.6 mg/dL). However, at their last follow-up, the serum phosphate level was significantly lower in patients with truncating mutations (nontruncating mutation group: 3.2 mg/dl, truncating mutation group: 2.3 mg/dl; P = 0.006). Additionally, 62.5% of patients with truncating mutations developed nephrocalcinosis at their last follow-up, while none of the patients with nontruncating mutations developed nephrocalcinosis (P = 0.015). Orthopedic surgery due to bony deformations was performed significantly more often in patients with truncating mutations (52.3 vs. 10.0%, P = 0.019). Conclusion: Although considerable inconsistency exists regarding the correlation of truncating mutations and their disease phenotype in several other studies, we cautiously suggest that there would be genotype-phenotype correlation in some aspects of disease manifestation after long-term follow-up. This information can be used when consulting patients with confirmed XLH regarding their disease prognosis.

Novel variants and uncommon cases among southern Chinese children with X-linked hypophosphatemia

PURPOSE

X-linked hypophosphatemia (XLH) is the most common inherited renal phosphate wasting disorder and is often misdiagnosed as vitamin D deficiency. This study aims to provide clinical and mutational characteristics of 65 XLH pediatric patients in southern China.

METHODS

In this work, a combination of DNA sequencing and qPCR analysis was used to study the PHEX gene in 80 pediatric patients diagnosed with hypophosphatemia. The clinical and laboratory data of confirmed 65 XLH patients were assessed and analyzed retrospectively.

RESULTS

In 65 XLH patients from 61 families, 51 different variants in the PHEX gene were identified, including 23 previously reported variants and 28 novel variants. In this cohort of XLH patients, the c.1601C>T(p.Pro534Leu) variant appears more frequently. Fourteen uncommon XLH cases were described, including four boys with de novo mosaic variants, eight patients with large deletions and a pair of monozygotic twins. The clinical manifestations in this cohort are very similar to those previously reported.

CONCLUSION

This study extends the mutational spectrum of the PHEX gene, which will contribute to accurate diagnosis. This study also suggests a supplementary qPCR or MLPA assay may be performed along with classical sequencing to confirm the gross insertion/deletion.

Functional Characterization of PHEX Gene Variants in Children With X-Linked Hypophosphatemic Rickets Shows No Evidence of Genotype-Phenotype Correlation

X-linked hypophosphatemia (XLHR) is caused by loss-of-function mutations in the phosphate regulating endopeptidase homolog X-linked (PHEX) gene. Considerable controversy exists regarding genotype-phenotype correlations in XLHR. The present study describes the clinical features and molecular genetic bases of 53 pediatric patients with XLHR. Overall, 47 different mutations were identified, of which 27 were not previously described in the literature or entered in the Human Gene Mutation Database (HGMD). A high prevalence (72.34%) of truncating variants was observed in XLHR patients. The clinical presentation and severity of XLHR did not show an evident correlation between the truncating and non-truncating mutation types in our cohort. To further delineate the characteristics of PHEX variants underlying this nonsignificant trend, we assessed the effects of 10 PHEX variants on protein expression, cellular trafficking, and endopeptidase activity. Our results showed that the nonsense mutations p.Arg567*, p.Gln714*, and p.Arg747* caused a reduction of protein molecular weight and a trafficking defect. Among seven non-truncating mutations, the p.Cys77Tyr, p.Cys85Ser, p.Ile281Lys, p.Ile333del, p.Ala514Pro, and p.Gly572Ser mutants were not secreted into the medium and remained trapped inside cells in an immature form, whereas the p.Gly553Glu mutant was terminally glycosylated and secreted into the medium. We further assessed the endopeptidase activity of the p.Gly553Glu mutant using a quenched fluorogenic peptide substrate and revealed that the activity of p.Gly553Glu significantly reduced to 13% compared with the wild type, which indicated disruption of catalytic function. These data not only support the clinical results showing no correlation between disease severity and the type of PHEX mutation but also provide helpful molecular insights into the pathogenesis of XLHR. © 2020 American Society for Bone and Mineral Research.

Identification of a novel variant in the PHEX gene using targeted gene panel sequencing in a 24-month-old boy with hypophosphatemic rickets

Familial hypophosphatemic rickets (FHR) is a disorder characterized by phosphate wasting and hypophosphatemia due to defects in renal phosphate transport regulation. There are 4 known inherited forms of FHR that differ in their molecular causes. Very few studies have been conducted that focused on the molecular analysis of FHR in Koreans. Eighteen mutations of the PHEX gene have been identified to this date in Korea. Herein, we report the clinical case of a 24-month-old boy presenting with bowed legs and short stature. The biochemical profile showed hypophosphatemia with decreased tubular reabsorption of phosphate. Several family members were identified with short stature and genu varum. Therefore, he was diagnosed with FHR. To identify the molecular causes of FHR, we performed targeted gene panel sequencing and found a novel hemizygous missense variant, c.1949T&gt;C (p.Leu650Pro), in the PHEX gene. This variant was also detected in the boy's mother who exhibited genu varum and short stature.

PHEX Gene Mutation in a Patient with X-Linked Hypophosphatemic Rickets in a Developing Country

Introduction

X-linked hypophosphatemic rickets is part of a larger group of hereditary diseases characterized by renal phosphate loss, which causes growth disorders, rickets, and osteomalacia. These conditions are characterized by disorders in phosphate equilibrium, which is essential for bone formation.

Case Report

A female patient presented with bone deformities of the inferior extremities, prominent joints, and loss of teeth. She received initial management with oral calcium and orthotics in inferior extremities, with poor clinical outcome. PHEX gene sequencing revealed a pathogenic variant c.1601C>T (p.Pro534Leu).

Discussion

XLHR is caused by mutations in the PHEX gene; to date, more than 460 mutations have been associated with the disease. Clinically, it is characterized by bowing of the lower extremities, decreased growth, musculoskeletal complaints, dental abscesses, and other clinical signs and symptoms of rickets.

Diagnosis, treatment-monitoring and follow-up of children and adolescents with X-linked hypophosphatemia (XLH)

Early diagnosis, optimal therapeutic management and regular follow up of children with X-linked hypophosphatemia (XLH) determine their long term outcomes and future quality of life. Biochemical screening of potentially affected newborns in familial cases and improving physician's knowledge on clinical signs, symptoms and biochemical characteristics of XLH for de novo cases should lead to earlier diagnosis and treatment initiation. The follow-up of children with XLH includes clinical, biochemical and radiological monitoring of treatment (efficacy and complications) and screening for XLH-related dental, neurosurgical, rheumatological, cardiovascular, renal and ENT complications. In 2018, the European Union approved the use of burosumab, a humanized monoclonal anti-FGF23 antibody, as an alternative therapy to conventional therapy (active vitamin D analogues and phosphate supplements) in growing children with XLH and insufficiently controlled disease. Diagnostic criteria of XLH and the principles of disease management with conventional treatment or with burosumab are reviewed in this paper.

Clinical and genetic analysis in a large Chinese cohort of patients with X-linked hypophosphatemia

X-linked Hypophosphatemia (XLH) is caused by loss of function mutations in the PHEX gene. Given the recent availability of a new therapy for XLH, a retrospective analysis of the most recent 261 Chinese patients with XLH evaluated at Peking Union Medical College Hospital was conducted. Clinical, biochemical, radiographic studies, as well as genetic analyses, including Sanger sequencing for point mutations and Multiplex Ligation-dependent Probe Amplification (MLPA) to detect large deletions/duplications were employed. Based on the structure of Neprilysin (NEP), a member of M13 family that includes PHEX, a three-dimensional (3D) model of PHEX was constructed, missense and nonsense mutations were positioned on the predicted structure to visualize relative positions of these two types of variants. Sex differences and genotype-phenotype correlations were also undertaken. Genetic analyses identified 166 PHEX mutations in 261 XLH patients. One hundred and eleven of the 166 mutations were unreported. Four mutational 'hot-spots' were identified in this cohort (P534L, G579R, R747X, c.1645+1 G>A). Missense mutations, but not nonsense mutations, clustered in the two putative lobes of the PHEX protein, suggesting these are functionally important regions of the molecule. Circulating levels of intact FGF23 were significantly elevated (median level 101.9 pg/mL; reference range 16.1-42.2 pg/mL). No significant sex differences, as well as no phenotypic differences were identified between patients with putative truncating and non-truncating PHEX mutations. However, patients with N-terminal PHEX mutations had an earlier age of onset of disease (P = 0.015) and higher iFGF23 levels (P = 0.045) as compared to those with C-terminal mutations. These data provide a comprehensive characterization of the largest cohort of patients with XLH reported to date from China, which will help in evaluating the applicability of emerging therapies for this disease in this ethnic group.

Nephrocalcinosis and Nephrolithiasis in X-Linked Hypophosphatemic Rickets: Diagnostic Imaging and Risk Factors

Context

Nephrocalcinosis (NC) and nephrolithiasis (NL) are described in hypophosphatemic rickets, but data regarding their prevalence rates and the presence of metabolic risk factors in X-linked hypophosphatemic rickets (XLH) are scarce.

Objective

To determine the prevalence rates of NC and NL and their risk factors in patients with XLH with confirmed PHEX mutations.

Methods

Renal ultrasonography (US) and CT were performed in 16 children and 23 adults. The images were evaluated by two blinded radiologists specializing in US and two specializing in CT. Confirmation of NC was determined with a positive result on both US and CT, whereas the diagnosis of NL was confirmed by CT alone. The presence of hypercalciuria, hypocitraturia, and hyperoxaluria was determined from 24-hour urinary samples from each patient. The glomerular filtration rate was estimated.

Results

NC was identified in 15 patients (38.4%), and stratification by age group showed a higher prevalence of NC in children than in adults (56.2% vs 26.1%). CT identified NL in four adults (10.2%). Patients in the pediatric group required intensive use of phosphate, started treatment earlier, and presented greater phosphaturia than those in the adult group (P < 0.01). In addition to hyperphosphaturia, which was present in all patients with XLH, hypocitraturia was the most common metabolic factor (28.2%), whereas hypercalciuria occurred in two patients (5.1%). None had hyperoxaluria. Most patients had normal renal function.

Conclusions

NC was more prevalent than NL. The main metabolic factor was hyperphosphaturia, and intensive phosphate treatment appears to be a worsening factor for kidney calcification.

Regulation of Fibroblast Growth Factor 23 by Iron, EPO, and HIF

Purpose of review

Fibroblast growth factor-23 (FGF23) is the key hormone produced in bone critical for phosphate homeostasis. Elevated serum phosphorus and 1,25dihydroxyvitaminD stimulates FGF23 production to promote renal phosphate excretion and decrease 1,25dihydroxyvitaminD synthesis. Thus completing the feedback loop and suppressing FGF23. Unexpectedly, studies of common and rare heritable disorders of phosphate handling identified links between iron and FGF23 demonstrating novel regulation outside the phosphate pathway.

Recent Findings

Iron deficiency combined with an FGF23 cleavage mutation was found to induce the autosomal dominant hypophosphatemic rickets phenotype. Physiological responses to iron deficiency, such as erythropoietin production as well as hypoxia inducible factor activation, have been indicated in regulating FGF23. Additionally, specific iron formulations, used to treat iron deficiency, alter post-translational processing thereby shifting FGF23 protein secretion.

Summary

Molecular and clinical studies revealed that iron deficiency, through several mechanisms, alters FGF23 at the transcriptional and post-translational level. This review will focus upon the novel discoveries elucidated between iron, its regulators, and their influence on FGF23 bioactivity.

A novel de novo mosaic mutation in PHEX in a Korean patient with hypophosphatemic rickets

X-linked hypophosphatemic rickets is caused by loss-of-function mutations in PHEX, which encodes a phosphate-regulating endopeptidase homolog. We report a 26-year-old man with X-linked hypophosphatemic rickets who showed decreased serum phosphate accompanied by bilateral genu valgum and short stature. He had received medical treatment with vitamin D (alfacalcidol) and phosphate from the age of 3 to 20 years. He underwent surgery due to valgus deformity at the age of 14 and 15. Targeted gene panel sequencing for Mendelian genes identified a nonsense mutation in PHEX (c.589C>T; p.Gln197Ter) and a mosaic pattern where only 38% of sequence reads showed the variant allele. This mutation was not found in his mother, who had a normal phenotype. This is a case of a sporadic nonsense mutation in PHEX and up to date, this is the first case of a mosaic mutation in PHEX in Korea.

Genetic analysis of three families with X-linked dominant hypophosphatemic rickets

Background Hypophosphatemic rickets, including familial hypophosphatemic vitamin D-resistant rickets, which commonly manifests in childhood, is generally hereditary. X-linked dominant hypophosphatemic rickets (XLH, MIM307800), caused by inactivating mutations in the PHEX gene, is the most common form. This study aimed to identify the gene mutations responsible for three cases of XLH and its clinical phenotype. Methods We conducted a genetic diagnosis and clinical phenotypic linkage analysis of three pedigrees with XLH. Three probands finally diagnosed as XLH were analyzed by next-generation sequencing (NGS). Sanger sequencing was used for mutation scanning in other family members. Results For the three patients with XLH, the age of onset ranged from 1.5 to 2 years and their heights were less than three standard deviations (SDs) below the median. The patients exhibited curved deformities in both lower limbs, hypophosphatemia, elevated serum FGF23 levels and elevated levels of blood alkaline phosphatase, with normal levels of blood parathyroid hormone (PTH) and calcium. X-ray analysis of the limbs and chest revealed characteristic rickets signs. Three candidate pathogenic mutations were identified in PHEX (NM_000444.5): c.433G>T (p.Glu145*, p.E145*) in exon 4, c.1735G>A (p.Gly579Arg, p.G579R) (rs875989883) in exon 17 and c.2245T>C (p.Trp749Arg, p.W749R) in exon 22. The nonsense mutation (p.E145*) in PHEX is novel and is predicted to cause a truncation of the encoded protein, resulting in loss of function. Conclusions The novel nonsense mutation (p.E145*) in PHEX is possibly involved in inherited XLH.

Two novel variants of the PHEX gene in patients with X‑linked dominant hypophosphatemic rickets and prenatal diagnosis for fetuses in these families

X-linked hypophosphatemic rickets (XLHR; OMIM 307800) is an X-linked dominant disorder caused by mutations in the phosphate-regulating neutral endopeptidase homolog X-linked (PHEX) gene, which is located at Xp22.11. In the present study, two novel variants of the PHEX gene were identified in two unrelated families with XLHR by directly sequencing all 22 exon regions and intron/exon boundaries of the PHEX gene. One missense variant, NM_000444.5: c.1721T>A, was identified in exon 17 of the PHEX gene in Family 1, which led to an amino acid change in the p.Ile574Lys protein. The other splicing variant identified was NM_000444.5: c.591A>G, in exon 5 in Family 2, resulting in a deletion of 77 bp in the 3′ site of exon 5 during splicing, which was verified by direct cDNA sequencing of the PHEX gene. According to the results of reverse transcription-quantitative polymerase chain reaction analysis, the affected male with the splicing variant c.591A>G showed normal gene expression of PHEX, whereas the affected female exhibited low gene expression, compared with normal females. Based on these findings, prenatal diagnoses were made for the fetuses with a family history of XLHR using the backup amniotic fluid samples. One fetus without the missense variant was confirmed to be a healthy girl in a follow-up visit 1 month following birth.

Hypertension is a characteristic complication of X-linked hypophosphatemia

X-linked hypophosphatemia (XLH) is a group of rare disorders caused by defective proximal tubular reabsorption of phosphate. Mutations in the PHEX gene are responsible for the majority of cases. There are very few reports of long-term complications of XLH other than skeletal and dental diseases. The aim of this study was to identify the phenotypic presentation of XLH during adulthood including complications other than skeletal and dental diseases. The clinical and biochemical phenotype of 22 adult patients with a PHEX gene mutation were examined retrospectively from their medical records. 6 patients had hypertension. The average age of hypertension onset was 29.0 years. Secondary hyperparathyroidism preceded the development of hypertension in 5 patients. 1 patient developed tertiary hyperparathyroidism. 15 patients had nephrocalcinosis. 2 patients had chronic renal dysfunction. Patients with hypertension had a significantly lower eGFR (p=0.010) compared to patients without hypertension. No significant difference was found in any other parameters. To examine the genotype-phenotype correlation, 10 adult males were chosen for analysis. No significant genotype-phenotype correlation analysis was revealed in any of the complications. However, there was a possibility that the age at nephrocalcinosis onset was younger in the non-missense mutation group than in the missense mutation group (p=0.063). This study corroborated the view that early-onset hypertension could be one of the characteristic complications seen in XLH patients. Considering the limited number of our patients, further study is necessary to address a potential cause of hypertension. XLH patients require careful lifelong treatment.

Delayed presentation of rickets in a child with labyrinthine aplasia, microtia and microdontia (LAMM) syndrome

BACKGROUND

Labyrinthine Aplasia, Microtia and Microdontia (LAMM) syndrome is characterized by the complete absence of inner ear structures (Michel aplasia), microtia and microdontia. Hypophosphatemic rickets results from defects in the renal tubular reabsorption of filtered phosphate.

CASE CHARACTERISTICS

13-year-old Indian girl presented with deafness since infancy and progressive wrist widening and genu valgum for last one year.

OBSERVATION

Homozygous novel missense mutation in fibroblast growth factor 3.

MESSAGE

LAMM syndrome and hypophosphatemic rickets may be associated.

Hereditary hypophosphatemia in Norway: a retrospective population-based study of genotypes, phenotypes, and treatment complications

OBJECTIVE

Hereditary hypophosphatemias (HH) are rare monogenic conditions characterized by decreased renal tubular phosphate reabsorption. The aim of this study was to explore the prevalence, genotypes, phenotypic spectrum, treatment response, and complications of treatment in the Norwegian population of children with HH.

DESIGN

Retrospective national cohort study.

METHODS

Sanger sequencing and multiplex ligand-dependent probe amplification analysis of PHEX and Sanger sequencing of FGF23, DMP1, ENPP1KL, and FAM20C were performed to assess genotype in patients with HH with or without rickets in all pediatric hospital departments across Norway. Patients with hypercalcuria were screened for SLC34A3 mutations. In one family, exome sequencing was performed. Information from the patients' medical records was collected for the evaluation of phenotype.

RESULTS

Twety-eight patients with HH (18 females and ten males) from 19 different families were identified. X-linked dominant hypophosphatemic rickets (XLHR) was confirmed in 21 children from 13 families. The total number of inhabitants in Norway aged 18 or below by 1st January 2010 was 1,109,156, giving an XLHR prevalence of ∼1 in 60,000 Norwegian children. FAM20C mutations were found in two brothers and SLC34A3 mutations in one patient. In XLHR, growth was compromised in spite of treatment with oral phosphate and active vitamin D compounds, with males tending to be more affected than females. Nephrocalcinosis tended to be slightly more common in patients starting treatment before 1 year of age, and was associated with higher average treatment doses of phosphate. However, none of these differences reached statistical significance.

CONCLUSIONS

We present the first national cohort of HH in children. The prevalence of XLHR seems to be lower in Norwegian children than reported earlier.

Clinical and molecular heterogeneity in a large series of patients with hypophosphatemic rickets

CONTEXT

Hypophosphatemic rickets (HR) is a rare disease that includes a group of hereditary and sporadic conditions characterized by renal phosphate loss associated with normal to low vitamin D serum concentration. The most common form is the X-linked hypophosphatemic rickets, with an incidence of 1:20,000. Several mutations have recently been identified in the PHEX, FGF23, DMP1 and ENPP1 genes in patients with HR. Moreover, in vitro and in vivo studies suggested an involvement of MEPE for defective mineralization in HR.

OBJECTIVE

The present case series describes the clinical features and the analysis of genes implicated in HR in a cohort of 26 Italian HR patients.

SETTING AND DESIGN

All patients were analyzed for the PHEX and FGF23 genes by direct sequencing. When no mutations were detected, Multiplex Ligation-dependent Probe Amplification (MLPA) analysis was performed. The negative patients were screened for the DMP1, MEPE and ENPP1 genes by direct sequencing.

RESULTS

Twenty-two patients (84%) harbored mutations in the PHEX gene. In particular, we detected 19 different mutations, 15 of which were novel. One patient presented a novel splice variation in the ENPP1 gene while no alterations were identified in the FGF23, DMP1 and MEPE genes. The genetic study of the families showed that 11 patients (55%) had de novo mutations. Clinical presentation and disease severity did not show an evident correlation between the mutation types.

CONCLUSIONS

This report represents the first large familial study performed on Italian patients. It confirms that mutations in PHEX are the most frequent cause of HR. Furthermore, the variety of clinical manifestations identified in our HR patients underlines the extreme clinical and genetic heterogeneity of this disease.

Phosphate homeostasis and genetic mutations of familial hypophosphatemic rickets

Hypophosphatemic rickets (HR) is a syndrome of hypophosphatemia and rickets that resembles vitamin D deficiency, which is caused by malfunction of renal tubules in phosphate reabsorption. Phosphate is an essential mineral, which is important for bone and tooth structure. It is regulated by parathyroid hormone, 1,25-dihydroxyvitamin D and fibroblast-growth-factor 23 (FGF23). X-linked hypophosphatemia (XLH), autosomal dominant HR (ADHR), and autosomal recessive HR (ARHR) are examples of hereditary forms of HR, which are mainly caused by mutations in the phosphate regulating endopeptidase homolog, X-linked (PHEX), FGF23, and, dentin matrix protein-1 (DMP1) and ecto-nucleotide pyro phosphatase/phosphodiesterase 1 (ENPP1) genes, respectively. Mutations in these genes are believed to cause elevation of circulating FGF23 protein. Increase in FGF23 disrupts phosphate homeostasis, leading to HR. This review aims to summarize phosphate homeostasis and focuses on the genes and mutations related to XLH, ADHR, and ARHR. A compilation of XLH mutation hotspots based on the PHEX gene database and mutations found in the FGF23, DMP1, and ENPP1 genes are also made available in this review.

Results of deformity correction in children with X-linked hereditary hypophosphatemic rickets by external fixation or combined technique

BACKGROUND

The operative procedures to correct multiplanar bone deformities may be indicated for prevention of secondary orthopaedic complications in children with X-linked hereditary hypophosphatemic rickets (XHPR). Different problems related to surgical correction were reported: increased rate of non-union, delayed union, recurrent deformity, deep intramedullary infection, refracture, nerve palsy, and pin tract infection. The aim of this retrospective study was comparison of results of correction in children with XHPR who underwent the treatment with either the Ilizarov device alone or a combined technique: the Ilizarov fixator with flexible intramedullary nailing (FIN) with hydroxyapatite bioactive coating and FIN.

MATERIAL AND METHODS

We retrospectively analysed 47 cases (children of age under 14 years) affected by XHPR. Simultaneous deformity correction in femur and tibia was performed with the Ilizarov device (group I) or the combined method (group II). This article is based on the results of a historical comparative retrospective study from the same institution.

RESULTS

The duration of external fixation is noted to be shorter applying the combined technique: 124.7 days (group I) vs 87.4 days (group II). In both groups deformity correction was achieved with a proper alignment. Nevertheless, while a child continues to grow during long-term follow-up, deviations of the mechanic axis from the centre of the knee joint have been developing again and values of mLDFA, mMPTA have become pathologic in the most of the cases. In group I location of a newly developed deformity resembled a pre-operative one, whereby both diaphyseal and metaphyseal parts were deformed. In group II in all the cases an apex of deformity was located in distal metadiaphyseal zone of the femur and proximal metadiaphyseal zone of the tibia. It is important to note that all of those in group II were out of the zone of the intramedullary nail.

CONCLUSION

Simultaneous correction of femoral and tibial deformities by means of circular external fixators is preferable. Application of a combined osteosynthesis allows to considerably reduce the duration of external fixation and decrease the number of complications. There were no recurrent deformities in parts of bone reinforced by intramedullary nails.

Whole Exome Sequencing Reveals Novel PHEX Splice Site Mutations in Patients with Hypophosphatemic Rickets

Objective

Hypophosphatemic rickets (HR) is a heterogeneous genetic phosphate wasting disorder. The disease is most commonly caused by mutations in the PHEX gene located on the X-chromosome or by mutations in CLCN5, DMP1, ENPP1, FGF23, and SLC34A3. The aims of this study were to perform molecular diagnostics for four patients with HR of Indian origin (two independent families) and to describe their clinical features.

Methods

We performed whole exome sequencing (WES) for the affected mother of two boys who also displayed the typical features of HR, including bone malformations and phosphate wasting. B-lymphoblast cell lines were established by EBV transformation and subsequent RT-PCR to investigate an uncommon splice site variant found by WES. An in silico analysis was done to obtain accurate nucleotide frequency occurrences of consensus splice positions other than the canonical sites of all human exons. Additionally, we applied direct Sanger sequencing for all exons and exon/intron boundaries of the PHEX gene for an affected girl from an independent second Indian family.

Results

WES revealed a novel PHEX splice acceptor mutation in intron 9 (c.1080-3C>A) in a family with 3 affected individuals with HR. The effect on splicing of this mutation was further investigated by RT-PCR using RNA obtained from a patient’s EBV-transformed lymphoblast cell line. RT-PCR revealed an aberrant splice transcript skipping exons 10-14 which was not observed in control samples, confirming the diagnosis of X-linked dominant hypophosphatemia (XLH). The in silico analysis of all human splice sites adjacent to all 327,293 exons across 81,814 transcripts among 20,345 human genes revealed that cytosine is, with 64.3%, the most frequent nucleobase at the minus 3 splice acceptor position, followed by thymidine with 28.7%, adenine with 6.3%, and guanine with 0.8%. We generated frequency tables and pictograms for the extended donor and acceptor splice consensus regions by analyzing all human exons. Direct Sanger sequencing of all PHEX exons in a sporadic case with HR from the Indian subcontinent revealed an additional novel PHEX mutation (c.1211_1215delACAAAinsTTTACAT, p.Asp404Valfs*5, de novo) located in exon 11.

Conclusions

Mutation analyses revealed two novel mutations and helped to confirm the clinical diagnoses of XLH in two families from India. WES helped to analyze all genes implicated in the underlying disease complex. Mutations at splice positions other than the canonical key sites need further functional investigation to support the assertion of pathogenicity.

Iron and fibroblast growth factor 23 in X-linked hypophosphatemia

BACKGROUND

Excess fibroblast growth factor 23 (FGF23) causes hypophosphatemia in autosomal dominant hypophosphatemic rickets (ADHR) and X-linked hypophosphatemia (XLH). Iron status influences C-terminal FGF23 (incorporating fragments plus intact FGF23) in ADHR and healthy subjects, and intact FGF23 in ADHR. We hypothesized that in XLH serum iron would inversely correlate to C-terminal FGF23, but not to intact FGF23, mirroring the relationships in normal controls.

METHODS

Subjects included 25 untreated outpatients with XLH at a tertiary medical center and 158 healthy adult controls. Serum iron and plasma intact FGF23 and C-terminal FGF23 were measured in stored samples.

RESULTS

Intact FGF23 was greater than the control mean in 100% of XLH patients, and >2SD above the control mean in 88%, compared to 71% and 21% respectively for C-terminal FGF23. In XLH, iron correlated negatively to log-C-terminal FGF23 (r=-0.523, p<0.01), with a steeper slope than in controls (p<0.001). Iron was not related to log-intact FGF23 in either group. The log-ratio of intact FGF23 to C-terminal FGF23 was higher in XLH (0.00±0.44) than controls (-0.28±0.21, p<0.01), and correlated positively to serum iron (controls r=0.276, p<0.001; XLH r=0.428, p<0.05), with a steeper slope in XLH (p<0.01).

CONCLUSION

Like controls, serum iron in XLH is inversely related to C-terminal FGF23 but not intact FGF23. XLH patients are more likely to have elevated intact FGF23 than C-terminal FGF23. The relationships of iron to FGF23 in XLH suggest that altered regulation of FGF23 cleaving may contribute to maintaining hypophosphatemia around an abnormal set-point.

A novel de novo mutation within PHEX gene in a young girl with hypophosphatemic rickets and review of literature

X-linked hypophosphatemia (XLH) is the most common form of familial hypophosphatemic rickets and it is caused by loss-of-function mutations in the PHEX gene. Recently, a wide variety of PHEX gene defects in XLH have been revealed; these include missense mutations, nonsense mutations, splice site mutations, insertions, and deletions. Recently, we encountered a 2-year-9-month-old female with sporadic hypophosphatemic rickets. She underwent osteotomy, dental abscess was evident, and there was severe bowing of the legs. A low serum phosphorus level in combination with elevated serum alkaline phosphatase activity and normal serum calcium is suggestive of hypophosphatemic rickets. PHEX gene analysis revealed a splice acceptor site mutation, c.934-1G>T (IVS8(-1)G>T), at the intron8 and exon9 junction. To the best of our knowledge, this mutation is novel and has not been reported. The results of this study expand and improve our understanding of the clinical and molecular characteristics and the global pool of patients with sporadic hypophosphatemic rickets.

Hypophosphatemic rickets due to perturbations in renal tubular function

The common denominator for all types of rickets is hypophosphatemia, leading to inadequate supply of the mineral to the growing bone. Hypophosphatemia can result from insufficient uptake of the mineral from the gut or its disproportionate losses in the kidney, the latter being caused by either tubular abnormalities per se or the effect on the tubule of circulating factors like fibroblast growth factor-23 and parathyroid hormone (PTH). High serum levels of the latter result in most cases from abnormalities in vitamin D metabolism which lead to decreased calcium absorption in the gut and hypocalcemia, triggering PTH secretion. Rickets is a disorder of the growth plate and hence pediatric by definition. However, it is important to recognize that the effect of hypophosphatemia on other parts of the skeleton results in osteomalacia in both children and adults. This review addresses the etiology, pathophysiologic mechanisms, clinical manifestations and treatment of entities associated with hypophosphatemic rickets due to perturbations in renal tubular function.

Clinical and etiological profile of refractory rickets from western India

OBJECTIVE

To present clinical and etiological profile of refractory rickets from Mumbai.

METHODS

Case records of 36 patients presenting over 2½ y with refractory rickets were evaluated with respect to clinical presentation, biochemical, radiological features and where needed, ophthalmological examination, ultrasonography and special tests on blood and urine.

RESULTS

Twenty three (63 %) patients had renal tubular acidosis (RTA)-distal RTA in 20 and proximal RTA in 3 patients; 5 (14 %) had vitamin D dependent rickets (VDDR I in 2 and VDDR II in 3 patients), 4 (11 %) had chronic renal failure (CRF) and 2 each (6 %) had hypophosphatemic rickets and chronic liver disease as cause of refractory rickets. A significant proportion of patients with RTA and VDDR showed skeletal changes of rickets in the first 2 y of life, while those with hypophosphatemic rickets presented later. Patients with hypophosphatemic rickets had predominant involvement of lower limbs, normal blood calcium and PTH levels and phosphorus leak in urine. All patients with RTA presented with failure to thrive, polyuria and marked rickets; blood alkaline phosphatase levels being normal in almost 50 % patients. Three (75 %) patients with rickets due to CRF had GFR < 30 ml/min/1.73 m(2) and hyperphosphatemia. Patients with cirrhosis due to biliary atresia had rickets inspite of taking high dose of vitamin D orally.

CONCLUSIONS

Refractory rickets is a disorder of multiple etiologies; a good history and clinical examination supplemented with appropriate investigations helps to determine its cause.

PHEX gene mutation in a Chinese family with six cases of X-linked hypophosphatemic rickets

OBJECTIVE

X-linked hypophosphatemic (XLH) rickets is caused by inactivating mutations in the PHEX gene, which encodes a metalloprotease that cleaves small peptide hormone. So far there are only a few reports on XLH patients from China. In the present study, we report on six XLH patients from one family. A PHEX missense mutation was found in exon 22, and a literature review on the mutations of Chinese patients was undertaken.

CASE DESCRIPTION

The family included six XLH patients with five females and one male (the proband). All the patients showed a low serum phosphorus, increased blood alkaline phosphatase and normal calcium levels. Mutation analysis revealed a PHEX mutation in exon 22 (c.2237G>A). In total, 15 PHEX mutations have been reported in Chinese populations at this time.

CONCLUSION

These data extend the spectrum of mutations in the PHEX gene in Chinese populations.

Novel and de novo PHEX mutations in patients with hypophosphatemic rickets

X-linked hypophosphatemic rickets (XLH) is the most common inherited rickets. XLH is caused by inactivating mutations in the PHEX gene and is transmitted as an X-linked dominant disorder. We investigated PHEX mutation in 10 patients from 6 unrelated Turkish families by PCR-sequence analysis. Six different PHEX mutations were detected in the patients. Four of them were novel: c.1217G>A (p.C406Y) in exon 11, c.2078G>T (p.C693F) in exon 21, a splice donor site mutation in intron 13 (IVS13+1G>T), and a splice acceptor site mutation in intron 13 (IVS13-2A>G). De novo PHEX mutations were found exclusively in female patients from 4 families and inherited mutations were detected from remaining two families. The patients' phenotype was consistent with the loss of PHEX function. Literature review of 78 sporadic cases shows that de novo mutations are present in 83% female patients and female/male ratio is 5 to 1. One patient had biallilic PHEX mutations at c.1735G>A (p.G579R) whereas her mother and two siblings carried a monoallelic mutation. The clinical and laboratory findings of the patient with biallilic PHEX mutation were similar to those with monoallelic mutation. The study shows that PHEX mutation is a common cause of either familial or sporadic hypophosphatemic rickets in Turkish population. Gene dosage effect is not observed. The frequent de novo mutations found in the female patients are likely resulting from mutagenesis of X chromosome in paternal germ cells.

Genetic disorders of phosphate regulation

Regulation of phosphate homeostasis is critical for many biological processes, and both hypophosphatemia and hyperphosphatemia can have adverse clinical consequences. Only a very small percentage (1%) of total body phosphate is present in the extracellular fluid, which is measured by routine laboratory assays and does not reflect total body phosphate stores. Phosphate is absorbed from the gastrointestinal tract via the transcellular route [sodium phosphate cotransporter 2b (NaPi2b)] and across the paracellular pathway. Approximately 85% of the filtered phosphate is reabsorbed from the kidney, predominantly in the proximal tubule, by NaPi2a and NaPi2c, which are present on the brush border membrane. Renal phosphate transport is tightly regulated. Dietary phosphate intake, parathyroid hormone (PTH), 1,25 (OH)2 vitamin D3, and fibroblast growth factor 23 (FGF23) are the principal regulators of phosphate reabsorption from the kidney. Recent advances in genetic techniques and animal models have identified many genetic disorders of phosphate homeostasis. Mutations in NaPi2a and NaPi2c; and hormonal dysregulation of PTH, FGF23, and Klotho, are primarily responsible for most genetic disorders of phosphate transport. The main focus of this educational review article is to discuss the genetic and clinical features of phosphate regulation disorders and provide understanding and treatment options.

Three novel PHEX gene mutations in four Chinese families with X-linked dominant hypophosphatemic rickets

BACKGROUND

X-linked hypophosphatemia (XLH), the most common form of inherited rickets, is a dominant disorder that is characterized by renal phosphate wasting with hypophosphatemia, abnormal bone mineralization, short stature, and rachitic manifestations. The related gene with inactivating mutations associated with XLH has been identified as PHEX, which is a phosphate-regulating gene with homologies to endopeptidases on the X chromosome. In this study, a variety of PHEX mutations were identified in four Chinese families with XLH.

METHODS

We investigated four unrelated Chinese families who exhibited typical features of XLH by using PCR to analyze mutations that were then sequenced. The laboratory and radiological investigations were conducted simultaneously.

RESULTS

Three novel mutations were found in these four families: one frameshift mutation, c.2033dupT in exon 20, resulting in p.T679H; one nonsense mutation, c.1294A>T in exon 11, resulting in p.K432X; and one missense mutation, c.2192T>C in exon 22, resulting in p.F731S.

CONCLUSIONS

We found that the PHEX gene mutations were responsible for XLH in these Chinese families. Our findings are useful for understanding the genetic basis of Chinese patients with XLH.

Mutational analysis of PHEX, FGF23, DMP1, SLC34A3 and CLCN5 in patients with hypophosphatemic rickets

This study aimed to identify the underlying genetic mutation in patients with hypophosphatemic rickets (HR). Genomic DNA was analysed for mutations in PHEX, FGF23 and CLCN5 by polymerase chain reaction (PCR) followed by denaturing high-performance liquid chromatography (dHPLC). Bi-directional sequencing was performed in samples with deviating chromatographic profiles. DMP1 and SLC34A3 were sequenced, only. In addition, a multiplex ligation-dependent probe amplification (MLPA) analysis was performed to detect larger deletions/duplications in PHEX or FGF23. Familial cases accounted for 12 probands while 12 cases were sporadic. In 20 probands, mutations were detected in PHEX of which 12 were novel, and one novel frameshift mutation was found in DMP1. Three PHEX mutations were identified by the MLPA analysis only; that is, two large deletions and one duplication. No mutations were identified in FGF23, SLC34A3 or CLCN5. By the methods used, a disease causing mutation was identified in 83% of the familial and 92% of the sporadic cases, thereby in 88% of the tested probands. Genetic analysis performed in HR patients by PCR, dHPLC, sequencing and in addition by MLPA analysis revealed a high identification rate of gene mutations causing HR, including 12 novel PHEX and one novel DMP1 mutation.

Growth in PHEX-associated X-linked hypophosphatemic rickets: the importance of early treatment

Inactivating mutations in phosphate-regulating endopeptidase (PHEX) cause X-linked hypophosphatemic rickets (XLHR) characterized by phosphaturia, hypophosphatemia, bony deformities, and growth retardation. We assessed the efficacy of combined calcitriol and orally administered phosphate (Pi) therapy on longitudinal growth in relation to age at treatment onset in a retrospective, single-center review of children with XLHR and documented PHEX mutations. Growth was compared in those who started treatment before (G1; N = 10; six boys) and after (G2; N = 13; five boys) 1 year old. Median height standard deviation score (HSDS) at treatment onset was normal in G1: 0.1 [interquartile range (IR) -1.3 to 0.4) and significantly (p = 0.004) lower in G2 (IR -2.1 (-2.8 to -1.4). Treatment duration was similar [G1 8.5 (4.0-15.2) vs G2 11.9 (6.2-14.3) years; p = 0.56], as were prescribed phosphate and calcitriol doses. Recent HSDS was significantly (p = 0.009) better in G1 [-0.7 (-1.5 to 0.3)] vs G2 [-2.0 (-2.3 to -1.0)]. No effects of gender or genotype on growth could be identified. Children with PHEX-associated XLHR benefit from early treatment and can achieve normal growth. Minimal catchup growth was seen in those who started treatment later. Our findings emphasize the importance of early diagnosis to allow treatment before growth has been compromised.

Evaluation of hypophosphatemia: lessons from patients with genetic disorders

Phosphate is a key component of several physiologic pathways, such as skeletal development, bone mineralization, membrane composition, nucleotide structure, maintenance of plasma pH, and cellular signaling. The kidneys have a key role in phosphate homeostasis, with 3 hormones having important roles in renal phosphate handling: parathyroid hormone, fibroblast growth factor 23 (FGF-23), and 1,25-dihydroxyvitamin D. Independent of the genetic diseases affecting the FGF-23 pathway (such as hypophosphatemic rickets), hypophosphatemia is a frequent condition encountered in daily practice, and untreated critical hypophosphatemia can induce hemolysis, rhabdomyolysis, respiratory failure, cardiac dysfunction, and neurologic impairment. Rapid correction thus is necessary to avoid severe complications. The aims of this teaching case are to summarize the causes and biological evaluation of hypophosphatemia and provide an overview of our current understanding of phosphate metabolism.

Genetic diagnosis of X-linked dominant Hypophosphatemic Rickets in a cohort study: tubular reabsorption of phosphate and 1,25(OH)2D serum levels are associated with PHEX mutation type

Background

Genetic Hypophosphatemic Rickets (HR) is a group of diseases characterized by renal phosphate wasting with inappropriately low or normal 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D) serum levels. The most common form of HR is X-linked dominant HR (XLHR) which is caused by inactivating mutations in the PHEX gene. The purpose of this study was to perform genetic diagnosis in a cohort of patients with clinical diagnosis of HR, to perform genotype-phenotype correlations of those patients and to compare our data with other HR cohort studies.

Methods

Forty three affected individuals from 36 non related families were analyzed. For the genetic analysis, the PHEX gene was sequenced in all of the patients and in 13 cases the study was complemented by mRNA sequencing and Multiple Ligation Probe Assay. For the genotype-phenotype correlation study, the clinical and biochemical phenotype of the patients was compared with the type of mutation, which was grouped into clearly deleterious or likely causative, using the Mann-Whitney and Fisher's exact test.

Results

Mutations in the PHEX gene were identified in all the patients thus confirming an XLHR. Thirty four different mutations were found distributed throughout the gene with higher density at the 3' end. The majority of the mutations were novel (69.4%), most of them resulted in a truncated PHEX protein (83.3%) and were family specific (88.9%). Tubular reabsorption of phosphate (TRP) and 1,25(OH)₂D serum levels were significantly lower in patients carrying clearly deleterious mutations than in patients carrying likely causative ones (61.39 ± 19.76 vs. 80.14 ± 8.80%, p = 0.028 and 40.93 ± 30.73 vs. 78.46 ± 36.27 pg/ml, p = 0.013).

Conclusions

PHEX gene mutations were found in all the HR cases analyzed, which was in contrast with other cohort studies. Patients with clearly deleterious PHEX mutations had lower TRP and 1,25(OH)₂D levels suggesting that the PHEX type of mutation might predict the XLHR phenotype severity.

Three novel mutations in the PHEX gene in Chinese subjects with hypophosphatemic rickets extends genotypic variability

Mutations in the phosphate-regulating endopeptidase homolog, X-linked, gene (PHEX), which encodes a zinc-dependent endopeptidase that is involved in bone mineralization and renal phosphate reabsorption, cause the most common form of hypophosphatemic rickets, X-linked hypophosphatemic rickets (XLH). The distribution of PHEX mutations is extensive, but few mutations have been identified in Chinese with XLH. We extracted genomic DNA and total RNA from leukocytes obtained from nine unrelated Chinese subjects (three males and six females, age range 11-36 years) who were living in Taiwan. The PHEX gene was amplified from DNA by PCR, and the amplicons were directly sequenced. Expression studies were performed by reverse-transcription PCR of leukocyte RNA. Serum levels of FGF23 were significantly greater in the patients than in normal subjects (mean 69.4 ± 18.8 vs. 27.2 ± 8.4 pg/mL, P < 0.005), and eight of the nine patients had elevated levels of FGF23. Germline mutations in the PHEX gene were identified in five of 9 patients, including novel c.1843 delA, donor splice site mutations c.663+2delT and c.1899+2T>A, and two previously reported missense mutations, p.C733Y and p.G579R. These data extend the spectrum of mutations in the PHEX gene in Han Chinese and confirm variability for XLH in Taiwan.

Age-related stature and linear body segments in children with X-linked hypophosphatemic rickets

Children with X-linked hypophosphatemic rickets (XLH) are prone to severe stunting. A multicenter mixed-longitudinal study was conducted to assess age-related stature, sitting height, arm and leg length in XLH patients on continuous treatment with phosphate and calcitriol. Mean standard deviation scores (SDS) for all body dimensions were markedly reduced and differed significantly among each other at the initial and subsequent evaluations (baseline: stature -2.48 SDS; sitting height -0.99 SDS; arm length -1.81 SDS; leg length -2.90 SDS; each p<0.001). A strong association between stature and leg length (r (2)=0.87, p<0.001) was noted. Leg length SDS decreased progressively during childhood (2-9 years) and adolescence (12-15 years; each p<0.001). Sitting height SDS increased significantly during late childhood, indicating uncoupled growth of the legs and trunk and resulting in an ever increasing sitting height index (i.e. ratio of sitting height to stature; age 2 years 2.0 SDS; age 10 years 3.3 SDS; p<0.001) that was associated with the degree of stunting (r (2)=0.314, p<0.001). Mean serum phosphate levels were positively associated with stature and leg length, but negatively with sitting height index. Based on these results, we can conclude that growth of the legs and trunk is uncoupled in XLH and related to serum phosphate levels.

Familial hypophosphatemia: an unusual presentation with low back ache, heel pain, and a limp in a young man, and literature review

A case of young man with low back ache and heel pains who was examined in a rheumatology outpatient and diagnosed as familial hypophosphatemia (FH), probably X-linked (XL), is presented. FH is most commonly transmitted as XL. The role of PHEX gene and fibroblast growth factor 23 is also described.

Prenatal diagnosis for a novel splice mutation of PHEX gene in a large Han Chinese family affected with X-linked hypophosphatemic rickets

BACKGROUND

X-linked hypophosphatemia (XLH) is the most common form of heritable rickets characterized by X-linked dominant inheritance, renal phosphate wasting, hypophosphatemia, and defective bone mineralization. Inactivating mutations of the PHEX gene located at Xp22.1 have been linked with this disease. Ethnic distribution of such mutations seems widespread but only a few mutations in the Chinese population have been reported to date.

MATERIALS AND METHODS

We report on a large Han Chinese family affected with XLH rickets, which included 13 patients from four generations. Polymerase chain reaction and direct sequencing were performed for all exons and intron-exon boundaries of the PHEX gene. The effect of nucleotide changes was analyzed using bioinformatic software. Prenatal diagnosis was performed on umbilical cord blood at the 20th gestational week.

RESULTS

A novel G-->A splice mutation in intron 7 (c.849+1G>A) was identified in all patients from the family. As confirmed by reverse-transcription (RT)-polymerase chain reaction (PCR), the mutation has rendered loss of a normal splice donor site (c.849+1G) while activating a cryptic one at c.849+519G, which resulted in addition of 518 nucleotides to the mature RNA. Prenatal diagnosis had excluded the fetus for carrying the same mutation. A healthy boy was born later.

CONCLUSIONS

A novel splice mutation c.849+1G>A in the PHEX gene is responsible for XLH in the studied family. Further studies may enhance our understanding of the role of this mutation in the pathogenesis of XLH.