Somatic and germline mosaicism for a mutation of the PHEX gene can lead to genetic transmission of X-linked hypophosphatemic rickets that mimics an autosomal dominant trait

A De Novo Mosaic PHEX Variant Causing Sporadic X-Linked Hypophosphatemic Rickets in a 2-Year-Old Girl

Pathogenic variants in the PHEX gene are causative of X-linked hypophosphatemic rickets (XLH). We present a case of a 2-year-old girl with hypophosphatemic rickets with genu varum and short stature without any family history of XLH. Next generation sequencing of the PHEX gene identified a splice donor variant, NM_000444.6:c.1173 + 5G > A in intron 10. This variant had a mosaic pattern with only 22% of the sequence reads showing the variant allele and was not present in the girl's parents, both of whom had a normal phenotype. This is a sporadic case of a de novo mosaic splice-site variant in the PHEX gene.

A New de novo Mosaic Mutation of PHEX Gene: A Case Report of a Boy with Hypophosphatemic Rickets

BACKGROUND

X-linked hypophosphatemia is the most prevalent form of heritable rickets, characterized by an X-linked dominant inheritance pattern. The genetic basis of X-linked hypophosphatemia is a loss-of-function mutation in the PHEX gene (Phosphate regulating gene with Homology to Endopeptidases on the X chromosome), which leads to an enhanced production of phosphaturic hormone FGF23. X-linked hypophosphatemia causes rickets in children and osteomalacia in adults. Clinical manifestations are numerous and variable, including slowdown in growth, swing-through gait and progressive tibial bowing, related to skeletal and extraskeletal actions of FGF23. PHEX gene spans over 220 kb and consists of 22 exons. To date, hereditary and sporadic mutations are known (missense, nonsense, deletions and splice site mutations).

CASE PRESENTATION

Herein, we describe a male patient carrying a novel de novo mosaic nonsense mutation c.2176G>T (p.Glu726Ter) located in exon 22 of PHEX gene.

CONCLUSION

We highlight this new mutation among possible causative of X-linked hypophosphatemia and suggest that mosaicism of PHEX mutations is not so uncommon and should be excluded in diagnostic workflow of heritable rickets both in male and female patients.

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.

Interdisciplinary management of FGF23-related phosphate wasting syndromes: a Consensus Statement on the evaluation, diagnosis and care of patients with X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most frequent cause of hypophosphataemia-associated rickets of genetic origin and is associated with high levels of the phosphaturic hormone fibroblast growth factor 23 (FGF23). In addition to rickets and osteomalacia, patients with XLH have a heavy disease burden with enthesopathies, osteoarthritis, pseudofractures and dental complications, all of which contribute to reduced quality of life. This Consensus Statement presents the outcomes of a working group of the European Society for Clinical and Economic Aspects of Osteoporosis, Osteoarthritis and Musculoskeletal Diseases, and provides robust clinical evidence on management in XLH, with an emphasis on patients' experiences and needs. During growth, conventional treatment with phosphate supplements and active vitamin D metabolites (such as calcitriol) improves growth, ameliorates leg deformities and dental manifestations, and reduces pain. The continuation of conventional treatment in symptom-free adults is still debated. A novel therapeutic approach is the monoclonal anti-FGF23 antibody burosumab. Although promising, further studies are required to clarify its long-term efficacy, particularly in adults. Given the diversity of symptoms and complications, an interdisciplinary approach to management is of paramount importance. The focus of treatment should be not only on the physical manifestations and challenges associated with XLH and other FGF23-mediated hypophosphataemia syndromes, but also on the major psychological and social impact of the disease.

Two De Novo Mosaic Variants Within the Same Site of PHEX Gene in a Girl with X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemic rickets (XLH) is the most common form of hypophosphatemic rickets, which is caused by the deficiencies of PHEX gene with an X-linked dominant inheritance pattern. As at least several thousands of XLH patients have been diagnosed, only several males and fewer females with mosaicism of PHEX gene were found. Here we describe an XLH girl with two de novo mosaic variants within the same site of PHEX gene. To rapidly screen all of the causative genes of hypophosphatemic rickets and rule out other diseases, DNA samples were initially analyzed using whole exome sequencing (WES). Interestingly, two different pathogenic mosaic variants, a known c.1809G > A(p.W603*) variant and a novel c.1809G > T(p.W603C) variant within the same site of PHEX gene, were identified in the proband by WES. Subsequent Sanger sequencing confirmed the presence and de novo pattern of these two mosaic variants in the proband, which were absent in her healthy parents. This is the first case to report two different mosaic variants of PHEX gene in an XLH individual. This XLH girl has a de novo mosaic genotype of c.1809 = /G > T/G > A in PHEX gene. Our report adds an unusual mocaicism case for XLH and expands the mutational event and spectrum of PHEX gene. Our report also alerts clinicians and geneticists to be cautious about mocaicism and detection methods.

Detection of Mosaic Variants in Mothers of MPS II Patients by Next Generation Sequencing

Mucopolysaccharidosis type II is an X-linked lysosomal storage disorder caused by mutations in the IDS gene that encodes the iduronate-2-sulfatase enzyme. The IDS gene is located on the long arm of the X-chromosome, comprising 9 exons, spanning approximately 24 kb. The analysis of carriers, in addition to detecting mutations in patients, is essential for genetic counseling, since the risk of recurrence for male children is 50%. Mosaicism is a well-known phenomenon described in many genetic disorders caused by a variety of mechanisms that occur when a mutation arises in the early development of an embryo. Sanger sequencing is limited in detecting somatic mosaicism and sequence change levels of less than 20% may be missed. The Next Generation Sequencing (NGS) has been increasingly used in diagnosis. It is a sensitive and fast method for the detection of somatic mosaicism. Compared to Sanger sequencing, which represents a cumulative signal, NGS technology analyzes the sequence of each DNA read in a sample. NGS might therefore facilitate the detection of mosaicism in mothers of MPS II patients. The aim of this study was to reanalyze, by NGS, all MPS II mothers that showed to be non-carriers by Sanger analysis. Twelve non-carriers were selected for the reanalysis on the Ion PGM and Ion Torrent S5 platform, using a custom panel that includes the IDS gene. Results were visualized in the Integrative Genomics Viewer (IGV). We were able to detected the presence of the variant previously found in the index case in three of the mothers, with frequencies ranging between 13 and 49% of the reads. These results suggest the possibility of mosaicism in the mothers. The use of a more sensitive technology for detecting low-level mosaic mutations is essential for accurate recurrence-risk estimates. In our study, the NGS analysis showed to be an effective methodology to detect the mosaic event.

A mosaic mutation of phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) in X-linked hypophosphatemic rickets with mild bone phenotypes

X-linked hypophosphatemic rickets (XLH) is primarily characterized by renal phosphate wasting with hypophosphatemia, short stature, and bone deformity of the leg. Here we present a male case of XLH with relatively mild bone deformity caused by a mosaic mutation of the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX). Polymerase chain reaction (PCR) direct sequencing revealed a novel in-frame deletion, NM-000444.6:c.671-685del p.Gln224-Ser228del, at exon 6 in PHEX as a mosaic pattern. This mutation was not found in any database and may result in a significant change in higher-order protein structure and function. TA cloning of the PCR product and clone sequencing estimated the mutation allele frequency at 21%. Literature review of the previously reported three cases with novel mosaic mutations in PHEX, together with the present case, suggests that the rates of the mutation allele correlate with phenotype severity to some extent. We initially treated him with nutritional vitamin D supplements and phosphate salts. However, to avoid the development of secondary/tertiary hyperparathyroidism, we had switched nutritional to active vitamin D supplementation with reduced phosphorus salts. The present report contributes to understanding the relationship between the mosaic rate, in addition to the mutation locus, of the PHEX gene, and clinical features of XLH.

Validation of a next-generation sequencing (NGS) panel to improve the diagnosis of X-linked hypophosphataemia (XLH) and other genetic disorders of renal phosphate wasting

BACKGROUND

Hypophosphataemic rickets (HR) comprise a clinically and genetically heterogeneous group of conditions, defined by renal-tubular phosphate wasting and consecutive loss of bone mineralisation. X-linked hypophosphataemia (XLH) is the most common form, caused by inactivating dominant mutations in PHEX, a gene encompassing 22 exons located at Xp22.1. XLH is treatable by anti-Fibroblast Growth Factor 23 antibody, while for other forms of HR such as therapy may not be indicated. Therefore, a genetic differentiation of HR is recommended.

OBJECTIVE

To develop and validate a next-generation sequencing panel for HR with special focus on PHEX.

DESIGN AND METHODS

We designed an AmpliSeq gene panel for the IonTorrent PGM next-generation platform for PHEX and ten other HR-related genes. For validation of PHEX sequencing 50 DNA-samples from XLH-patients, in whom 42 different mutations in PHEX and 1 structural variation have been proven before, were blinded, anonymised and investigated with the NGS panel. In addition, we analyzed one known homozygous DMP1 mutation and two samples of HR-patients, where no pathogenic PHEX mutation had been detected by conventional sequencing.

RESULTS

The panel detected all 42 pathogenic missense/nonsense/splice-site/indel PHEX-mutations and in one the known homozygous DMP1 mutation. In the remaining two patients, we revealed a somatic mosaicism of a PHEX mutation in one; as well as two variations in DMP1 and a very rare compound heterozygous variation in ENPP1 in the second patient.

CONCLUSIONS

This developed NGS panel is a reliable tool with high sensitivity and specificity for the diagnosis of XLH and related forms of HR.

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.

Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia

X-linked hypophosphataemia (XLH) is the most common cause of inherited phosphate wasting and is associated with severe complications such as rickets, lower limb deformities, pain, poor mineralization of the teeth and disproportionate short stature in children as well as hyperparathyroidism, osteomalacia, enthesopathies, osteoarthritis and pseudofractures in adults. The characteristics and severity of XLH vary between patients. Because of its rarity, the diagnosis and specific treatment of XLH are frequently delayed, which has a detrimental effect on patient outcomes. In this Evidence-Based Guideline, we recommend that the diagnosis of XLH is based on signs of rickets and/or osteomalacia in association with hypophosphataemia and renal phosphate wasting in the absence of vitamin D or calcium deficiency. Whenever possible, the diagnosis should be confirmed by molecular genetic analysis or measurement of levels of fibroblast growth factor 23 (FGF23) before treatment. Owing to the multisystemic nature of the disease, patients should be seen regularly by multidisciplinary teams organized by a metabolic bone disease expert. In this article, we summarize the current evidence and provide recommendations on features of the disease, including new treatment modalities, to improve knowledge and provide guidance for diagnosis and multidisciplinary care.

In this Evidence-Based Guideline on X-linked hypophosphataemia, the authors identify the criteria for diagnosis of this disease, provide guidance for medical and surgical treatment and explain the challenges of follow-up.

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.

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.

Targeted resequencing of phosphorus metabolism‑related genes in 86 patients with hypophosphatemic rickets/osteomalacia

Hypophosphatemic rickets/osteomalacia is characterized by defective renal phosphate reabsorption and abnormal bone mineralization. Hypophosphatemic rickets/osteomalacia consists of inherited and acquired forms, many of which have unknown aetiology. In the present study, next‑generation sequencing‑based resequencing was used on samples from Chinese subjects with hypophosphatemic rickets/osteomalacia, aiming to detect the spectrum of pathogenic genes in these patients. A total of 86 hypophosphatemic rickets/osteomalacia patients (ranging from 3 to 70 years old) were recruited. Patients with tumour‑induced osteomalacia (TIO), renal tubular acidosis, renal osteodystrophy, and adefovir‑induced Fanconi syndrome were excluded. Targeted massively parallel resequencing of 196 candidate genes for hypophosphatemic rickets/osteomalacia was performed in the 86 affected unrelated individuals (cases) and in 100 unrelated healthy controls to identify new genes and mutations in known genes that cause hypophosphatemic rickets/osteomalacia. The results identified seven phosphate‑regulating gene with homologies to endopeptidases on the X chromosome (PHEX) mutations (of which two were novel) and one novel dentin matrix protein 1 (DMP1) mutation in eight patients. Following targeted exome sequencing data analysis, 14 candidate disease‑related gene loci were selected, two of which were of most concern regarding disease severity. Further validation of the present results is warranted, with additional sequencing projects and functional tests. To our knowledge, the present study is the largest cohort of cases with hypophosphatemic rickets/osteomalacia to undergo targeted resequencing. The diagnosis and understanding of the molecular aetiologies of these disorders will be improved by this fast and efficient approach.

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.

De novo mutation of PHEX in a type 1 diabetes patient

A new missense mutation on the X chromosome (PHEX) at exon 4(c.442C>T) in a 4-generation Chinese Han pedigree is reported. The proband and four family members were clinically identified as the X-linked hypophosphatemic rickets (XLH) which is a dominant inherited disorder characterized by renal phosphate wasting, aberrant vitamin D metabolism, and abnormal bone mineralization. The proband is identified as hemizygous with the four female family members to be heterozygous genotypes. The discovery was made through the complete sequencing of the exons and the intron-exon boundaries of the PHEX gene of this family. The mutation caused the S141 residue to change to Phe from Ser which is perfectly conserved among humans, mice, rats, cows and chickens. PolyPhen-2 software analysis of the mutation indicated it was probably damaging. The proband was also diagnosed with type 1 diabetes (T1D) and the relationship between XLH and diabetes phenotypes was discussed in the paper.

A de novo mosaic mutation of PHEX in a boy with hypophosphatemic rickets

X-linked dominant hypophosphatemic rickets (XLHR), is characterized mainly by renal phosphate wasting with hypophosphatemia, short stature and abnormal bone mineralization. PHEX, located at Xp22.1-p22.2, is the gene causing XLHR. We aim to characterize the pathogenesis of a Chinese boy who is apparently 'heterozygous' in PHEX gene. Direct sequencing showed two peaks: one was a wild-type 'G' and the other was one base substitution to 'A', though the patient was a male. TA clone assay clearly showed each sequences and the ratios. The mutation effect was predicted via bioinformatics and validated by exon-trapping assay. Real-time PCR was applied to determine the copy number of PHEX. TA clone assay showed the frequency of normal (G) to mutant allele (A) as 19:13. Normal karyotype and real-time PCR results indicate the normal copy number of PHEX. This splice site mutation leads to 4 bp of exon 18 skipping out causing frame shift p.Gly590Glufs*28 that ends up with a loss of active site and Zn(2+)-binding site of PHEX, which probably interfere with renal phosphate reabsorption and bone mineralization. In conclusion, mutation at conserved splice acceptor site resulted in aberrant splicing, ending up with a damaged protein product. This novel mutation is de novo in mosaic pattern that may be induced during early postzygotic period. Taking mosaic somatic mutation of PHEX into consideration is strongly suggested in genetic counseling and etiology research for XLHR.

Genetic mosaics and the germ line lineage

Genetic mosaics provide information about cellular lineages that is otherwise difficult to obtain, especially in humans. De novo mutations act as cell markers, allowing the tracing of developmental trajectories of all descendants of the cell in which the new mutation arises. De novo mutations may arise at any time during development but are relatively rare. They have usually been observed through medical ascertainment, when the mutation causes unusual clinical signs or symptoms. Mutational events can include aneuploidies, large chromosomal rearrangements, copy number variants, or point mutations. In this review we focus primarily on the analysis of point mutations and their utility in addressing questions of germ line versus somatic lineages. Genetic mosaics demonstrate that the germ line and soma diverge early in development, since there are many examples of combined somatic and germ line mosaicism for de novo mutations. The occurrence of simultaneous mosaicism in both the germ line and soma also shows that the germ line is not strictly clonal but arises from at least two, and possibly multiple, cells in the embryo with different ancestries. Whole genome or exome DNA sequencing technologies promise to expand the range of studies of genetic mosaics, as de novo mutations can now be identified through sequencing alone in the absence of a medical ascertainment. These technologies have been used to study mutation patterns in nuclear families and in monozygotic twins, and in animal model developmental studies, but not yet for extensive cell lineage studies in humans.

A Novel PHEX Mutation in Japanese Patients with X-Linked Hypophosphatemic Rickets

X-linked hypophosphatemic rickets (XLH) is a dominant inherited disorder characterized by renal phosphate wasting, aberrant vitamin D metabolism, and abnormal bone mineralization. Inactivating mutations in the gene encoding phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX) have been found to be associated with XLH. Here, we report a 16-year-old female patient affected by hypophosphatemic rickets. We evaluated her serum fibroblast growth factor 23 (FGF23) levels and conducted sequence analysis of the disease-associated genes of FGF23-related hypophosphatemic rickets: PHEX, FGF23, dentin matrix protein 1, and ectonucleotide pyrophosphatase/phosphodiesterase 1. She was diagnosed with XLH based on her clinical features and family history. Additionally, we observed elevated FGF23 levels and a novel PHEX exon 9 mutation (c.947G>T; p.Gly316Val) inherited from her father. Although bioinformatics showed that the mutation was neutral, Gly316 is perfectly conserved among humans, mice, and rats, and there were no mutations in other FGF23-related rickets genes, suggesting that in silico analysis is limited in determining mutation pathogenicity. In summary, we present a female patient and her father with XLH harboring a novel PHEX mutation that appears to be causative of disease. Measurement of FGF23 for hypophosphatemic patients is therefore useful for the diagnosis of FGF23-dependent hypophosphatemia.

FGF23 neutralization improves bone quality and osseointegration of titanium implants in chronic kidney disease mice

Chronic kidney disease (CKD) is a worldwide health problem. Serum levels of FGF23, a phosphaturic hormone, increase at the earliest stages of CKD, and have been found to be independently associated with the mortality and morbidity of CKD patients. The purpose of this study was to evaluate whether FGF23 neutralization was able to improve bone quality and osseointegration of titanium implants. Uremia was induced by 5/6 nephrectomy in adult female mice. Postsurgery, the mice were injected with vehicle or FGF23 neutralizing antibody (5 mg/kg body weight) 3 times a week. Experimental titanium implants were inserted in the distal end of the femurs. FGF23 neutralization significantly increased serum phosphate, 1,25(OH)2D and BUN, and decreased serum PTH and FGF23, relative to vehicle-treated CKD mice. Histomorphometric analysis of the tibiae indicated that FGF23 neutralization normalized the osteoidosis observed in vehicle-treated CKD mice. Although bone-implant contact ratio remained unchanged by anti-FGF23 antibody treatment, the strength of osseointegration, as evidenced by a biomechanical push-in test, was significantly improved by FGF23 neutralization. Our findings revealed that FGF23 neutralization effectively improves bone quality and osseointegration of titanium implants in CKD mice, suggesting FGF23 as a key factor of CKD related bone diseases.

Identification of a novel PHEX mutation in a Chinese family with X-linked hypophosphatemic rickets using exome sequencing

Familial hypophosphatemic rickets (HR), the most common inherited form of rickets, is a group of inherited renal phosphate wasting disorders characterized by growth retardation, rickets with bone deformities, osteomalacia, poor dental development, and hypophosphatemia. The purpose of this study was to identify the genetic defect responsible for familial HR in a four-generation Chinese Han pedigree by exome sequencing and Sanger sequencing. Clinical features include skeletal deformities, teeth abnormalities, hearing impairments and variable serum phosphate level in patients of this family. A novel deletion mutation, c.1553delT (p.F518Sfs*4), was identified in the X-linked phosphate regulating endopeptidase homolog gene (PHEX). The mutation is predicted to result in prematurely truncated and loss-of-function PHEX protein. Our data suggest that exome sequencing is a powerful tool to discover mutation(s) in HR, a disorder with genetic and clinical heterogeneity. The findings may also provide new insights into the cause and diagnosis of HR, and have implications for genetic counseling and clinical management.

Inorganic polyphosphates stimulate FGF23 expression through the FGFR pathway

Polyphosphate (polyP) is composed of linear polymers of orthophosphate residues linked by high-energy phosphoanhydride bonds. It has been reported to improve osteoblastic differentiation, stimulate periodontal tissue regeneration, and accelerate bone repair. The aim of this study was to evaluate the effect of polyP on the expression of FGF23, a hormone secreted mostly be mature osteoblasts and osteocytes. In this study, different types of polyP were synthesized and co-cultured with osteoblast-like UMR-106 cells. Real-time PCR and western blot were used to analyze the gene and protein expression of FGF23. We found that 1 mM polyP was able to increase FGF23 expression after 4 h, reaching a peak after 12-24 h, with expression decreasing by 48 h. We also found that polyP could activate the FGFR pathway, as evidenced by increased phosphorylation of FGFR, FRS2, and Erk1/2. When FGFR signaling was inhibited by the specific inhibitor SU5402, the effect of polyP on FGF23 expression was significantly reduced. Our results indicate that polyP is able to stimulate osteoblastic FGF23 expression and that this effect is associated with activation of the FGFR pathway. These findings provide support for the clinical use of polyP by indicating a mechanism for polyP in bone regeneration.

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.

X-linked adrenoleukodystrophy: ABCD1 de novo mutations and mosaicism

X-linked adrenoleukodystrophy (X-ALD) is a progressive peroxisomal disorder affecting adrenal glands, testes and myelin stability that is caused by mutations in the ABCD1 (NM_000033) gene. Males with X-ALD may be diagnosed by the demonstration of elevated very long chain fatty acid (VLCFA) levels in plasma. In contrast, only 80% of female carriers have elevated plasma VLCFA; therefore targeted mutation analysis is the most effective means for carrier detection. Amongst 489 X-ALD families tested at Kennedy Krieger Institute, we identified 20 cases in which the ABCD1 mutation was de novo in the index case, indicating that the mutation arose in the maternal germ line and supporting a new mutation rate of at least 4.1% for this group. In addition, we identified 10 cases in which a de novo mutation arose in the mother or the grandmother of the index case. In two of these cases studies indicated that the mothers were low level gonosomal mosaics. In a third case biochemical, molecular and pedigree analysis indicated the mother was a gonadal mosaic. To the best of our knowledge mosaicism has not been previously reported in X-ALD. In addition, we identified one pedigree in which the maternal grandfather was mosaic for the familial ABCD1 mutation. Less than 1% of our patient population had evidence of gonadal or gonosomal mosaicism, suggesting it is a rare occurrence for this gene and its associated disorders. However, the residual maternal risk for having additional ovum carrying the mutant allele identified in an index case that appears to have a de novo mutation is at least 13%.

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.

Phenotype presentation of hypophosphatemic rickets in adults

Hypophosphatemic rickets (HR) is a group of rare disorders caused by excessive renal phosphate wasting. The purpose of this cross-sectional study of 38 HR patients was to characterize the phenotype of adult HR patients. Moreover, skeletal and endodontic severity scores were defined to assess possible gender differences in disease severity in patients with genetically verified X-linked HR. Compared to normal reference data, i.e., z = 0, HR patients had significantly lower final height, with a mean difference in z-score of -1.9 (95% CI -2.4 to -1.4, P < 0.001). Compared to paired z-scores of final height, z-scores of leg length were significantly lower and those of sitting height were significantly higher (P < 0.001), resulting in disproportion as indicated by the significantly elevated sitting height ratio, mean difference in z-score of 2.6 (95% CI 2.1-3.1, P < 0.001). Z-scores of head circumference (median 1.4, range -0.4 to 5.5, P < 0.001) and z-scores of bone mineral density (BMD) of the lumbar spine (median 1.9, range -1.5 to 8.6, P < 0.001) were significantly elevated compared to normal reference data. The relative risk (RR) of fracture was reduced (RR = 0.34, 95% CI 0.20-0.57, P < 0.001). The skeletal severity score tended to be higher in males compared to females (P = 0.07), and no gender difference in endodontic severity was found. In conclusion, adult HR patients were characterized by short stature and were disproportioned. They had elevated BMD of the lumbar spine and a reduced risk of fractures. We found a tendency for males to be more severely affected than females.

Germline mosaicism in keratitis-ichthyosis-deafness syndrome: pre-natal diagnosis in a familial lethal form

Keratitis-ichthyosis-deafness (KID) syndrome is an autosomal dominant congenital ectodermal defect characterized by the association of skin lesions, hearing loss and keratitis. Most of the cases appear to be sporadic. KID syndrome is mostly related to mutations of GJB2 gene encoding connexin-26. Recently, a lethal form of the disease during the first year of life has been reported in two unrelated Caucasian patients. This rare lethal form is caused by the G45E mutation of GJB2 gene. We here report the first pre-natal molecular genetic diagnosis of the lethal form of KID syndrome relating to a G45E mutation. In the same family, the occurrence of this condition in three other siblings born to African non-consanguineous healthy parents lead to perform pre-natal diagnosis for this last pregnancy. Molecular analysis confirms the diagnosis of the lethal form of KID for the fetus. These results establish the role of germline mosaicism in KID syndrome and warrant careful genetic counseling. Furthermore, analysis of our cases and the literature allowed us to define a characteristic severe neonatal phenotype including facial dysmorphy, severe cornification with massive focal hyperkeratosis of the skin with erythroderma, dystrophic nails, complete atrichia and absence of foreskin.

Mutational analysis of the PHEX gene: novel point mutations and detection of large deletions by MLPA in patients with X-linked hypophosphatemic rickets

X-Linked hypophosphatemic rickets (HYP, XLH) is a disorder of phosphate homeostasis, characterized by renal phosphate wasting and hypophosphatemia, with normal to low 1,25-dihydroxy vitamin D3 serum levels. The purpose of our study was the detection of inactivating mutations in the PHEX gene, the key enzyme in the pathogenesis of XLH. The 16 patients, representing eight families, presented with suspected XLH from biochemical and clinical evidence. All 16 were referred for mutational analysis of the PHEX gene. We detected three novel disease-causing mutations, C59S, Q394X, and W602, for which a loss of function can be predicted. A G28S variation, found in two healthy probands, may be a rare polymorphism. Another mutation, A363 V, is localized on the same allele as the C59S mutation, thus its functional consequences cannot be proven. Furthermore, we detected a deletion of three nucleotides in exon 15 which resulted in the loss of amino acid threonine 535. Heterozygosity of this mutation in a male patient without any chromosomal aberrations suggests its presence as a mosaic. Novel large deletions were detected using multiplex ligation-dependent probe amplification (MLPA) analysis. Two of these deletions, loss of exon 22 alone or exons 21 and 22 together, may result in the translation of a C-terminal truncated protein. Two large deletions comprise exons 1-9 and exons 4-20, respectively, and presumably result in a nonfunctional protein. We conclude that molecular genetic analysis confirms the clinical diagnosis of XLH and should include sequence analysis as well as the search for large deletions, which is facilitated by MLPA.

PHEX analysis in 118 pedigrees reveals new genetic clues in hypophosphatemic rickets

Familial hypophosphatemic rickets is a rare disease, which is mostly transmitted as an X-linked dominant trait, and mutations on the phosphate regulating gene with homologies to endopeptidases on the X-chromosome (PHEX) gene are responsible for the disease in most familial cases. In this study we analyzed PHEX in a large cohort of 118 pedigrees representing 56 familial cases and 62 sporadic cases. The high-resolution melting curves technique was tested as a screening method, along with classical sequencing. PHEX mutations have been found in 87% of familial cases but also in 72% of sporadic cases. Missense mutations were found in 16 probands, two of which being associated with other PHEX mutations resulting into truncated proteins. By plotting missense mutations described so far on a 3D model of PHEX we observed that these mutations focus on two regions located in the inner part of the PHEX protein. Family members of 13 sporadic cases were analyzed and a PHEX mutation was detected in one of the apparently healthy mother. These results highlight the major role of PHEX in X-linked dominant hypophosphatemic rickets, and give new clues regarding the genetic analysis of the disease. A screening of the different family members should be mandatory when a PHEX mutation is assessed in a sporadic case and the search for another PHEX mutation should be systematically proceed when facing a missense mutation.

Two silent substitutions in the PDHA1 gene cause exon 5 skipping by disruption of a putative exonic splicing enhancer

BACKGROUND

Synonymous mutations within exons may cause aberrant splicing by disrupting exonic splicing enhancer (ESE) motifs in the vicinity of non consensus splice sites. Mutational analysis of PDHA1 revealed only one silent single nucleotide substitution in exon 5 in two unrelated boys and a girl (c.483C>T and c.498C>T variants, respectively). For both patients, pyruvate dehydrogenase complex activity was low and the immunoreactive E1alpha protein was defective in cultured fibroblasts.

METHODS AND RESULTS

One of the boys was a somatic mosaic for the c.483C>T variant, as shown by the variable ratio of mutant to normal alleles in fibroblast, lymphocyte and single hair root DNA. Transcript analysis in fibroblasts from the three patients revealed the presence of both normal and truncated cDNAs, with the splicing out of exon 5 predicted to result in a frame shift and premature termination (p.Arg141AlafsX11). The treatment of fibroblasts with emetine before harvesting to prevent nonsense mRNA-mediated decay increased the amount of mutant mRNA. In silico analysis revealed that each variant disrupted a putative SRp55 binding site and that the intron 5 donor splice site (5'ss) contained a weak splicing signal. Transient transfection of COS-7 or Hela cells with hybrid minigene constructs containing wild-type or mutant PDHA1 exon 5, followed by RT-PCR demonstrated that each variant resulted in the incomplete inclusion of PDHA1 exon 5, and that this defect was corrected following the restoration of a perfect consensus sequence for the 5' splice site by site-directed mutagenesis.

CONCLUSION

These two synonymous mutations expand the spectrum of rare PDHA1 splicing mutations, all of which are located in non canonical splice sites.

Regulation of phosphate homeostasis in infants, children, and adolescents, and the role of phosphatonins in this process

PURPOSE OF REVIEW

Unlike calcium metabolism, the control of phosphate homeostasis has long been poorly understood. The identification of 'phosphatonins' in the serum of hypophosphatemic patients, the unveiling of the genetic causes of hypo and hyperphosphatemic diseases in patients, and the creation of finely adapted animal models have revolutionized our understanding of phosphate homeostasis.

RECENT FINDINGS

Original reports published in 2006/2007 bring valuable pieces of information that enable better understanding of the physiological regulation of phosphate homeostasis by more precisely defining the interplay between PHEX, vitamin D, and phosphatonins; identification of new genes causing hypophosphatemic rickets, aside from PHEX and fgf23, namely the genes encoding for a renal sodium-phosphate cotransporter, NaPiIIc, and for a bone matrix protein, DmpI; and improved diagnosis of tumor-induced osteomalacia with more precise imaging techniques for tumor localization and more precise fibroblast growth factor 23 assays.

SUMMARY

From a clinical point of view, these findings offer new tools for the diagnosis of hypophosphatemic rickets (biologic, genetic, imaging techniques) and open the way to new treatment strategies.