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Summary X-linked hypophosphataemia (XLH) is a lifelong condition. Despite the mounting clinical evidence highlighting the long-term multi-organ sequelae of chronic phosphate wasting and consequent hypophosphatemia over the lifetime and the morbidities associated with adult age, XLH is still perceived as a paediatric disease. Introduction Children who have XLH need to transition from paediatric to adult healthcare as young adults. While there is general agreement that all affected children should be treated (if the administration and tolerability of therapy can be adequately monitored), there is a lack of consensus regarding therapy in adults. Methods To provide guidance in both diagnosis and treatment of adult XLH patients and promote better provision of care for this potentially underserved group of patients, we review the available clinical evidence and discuss the current challenges underlying the transition from childhood to adulthood care to develop appropriate management and follow-up patterns in adult XLH patients. Results and Conclusions Such a multi-systemic lifelong disease would demand that the multidisciplinary approach, successfully experienced in children, could be transitioned to adulthood care with an integration of specialized sub-disciplines to efficiently control musculoskeletal symptoms while optimizing patients’ QoL. Overall, it would be desirable that transition to adulthood care could be a responsibility shared by the paediatric and adult XLH teams. Pharmacological management should require an adequate balance between the benefits derived from the treatment itself with complicated and long-term monitoring and the potential risks, as they may differ across age strata.

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

Health-related quality of life (HRQoL) of patients with X-linked hypophosphatemia (XLH) is lower than that of both the general population and the patients with other chronic diseases, mainly due to diagnostic delay, treatment difficulties, poor psychosocial support, and problems with social integration. Early diagnosis and optimal treatment are paramount to control the disease in patients with XLH, avoid complications, and maintain or improve their HRQoL. We, therefore, analyzed the HRQoL of pediatric and adult patients with XLH treated with conventional therapy in Spain.We used several versions of the EuroQol-5 dimensions (EQ-5D) instrument according to the age of patients with XLH. Then we compared the HRQoL of patients to that of the general Spanish population. Children with XLH (n = 21) had moderate problems in walking about (61.9%), washing or dressing themselves (9.52%), and performing their usual activities (33.33%). They also felt moderate pain or discomfort (61.9%) and were moderately anxious or depressed (23.81%). Adults with XLH (n = 29) had lower HRQoL, with problems in walking (93%, with 3.45% unable to walk independently), some level of pain (86%, with 3.45% experiencing extreme pain), problems with their usual activities (80%) and self-care ( 50%), and reported symptoms of anxiety and/or depression (65%). There were important differences with the general Spanish population.XLH impacts negatively on physical functioning and HRQoL of patients. In Spanish patients with XLH, the HRQoL was reduced despite conventional treatment, clearly indicating the need to improve the therapeutic approach to this disorder.X-linked hypophosphatemia (XLH) is a severe inherited disease. It is caused by loss of phosphorus by kidneys. As a result, blood level of phosphorus is low, affectingX-linked hypophosphatemia (XLH) is a severe inherited disease. It is caused by loss of phosphorus by kidneys. As a result, blood level of phosphorus is low, affecting bones and muscles. Patients can have growth retardation, short stature, rickets, limb deformities, pain and other health problems despite traditional treatment. Consequently, their quality of life can be very bad. However, a recently available new treatment (burosumab) can improve this quality of life. We studied the quality of life of children and adults with XLH treated with traditional treatment in Spain. Results showed that children had moderate problems, but adults reported moderate-to-severe problems in walking and performing their usual activities and self-care. Pain and anxiety and/or depression were very frequent. There were important differences with the general Spanish population. Moreover, we also found that XLH is associated to high healthcare cost and even higher socioeconomic cost. Our results highlight the need of improving the treatment of XLH.bones and muscles. Patients can have growth retardation, short stature, rickets, limb deformities, pain and other health problems despite traditional treatment. Consequently, their quality of life can be very bad. However, a recently available new treatment (burosumab) can improve this quality of life. We studied the quality of life of children and adults with XLH treated with traditional treatment in Spain. Results showed that children had moderate problems, but adults reported moderate-to-severe problems in walking and performing their usual activities and self-care. Pain and anxiety and/or depression were very frequent. There were important differences with the general Spanish population. Moreover, we also found that XLH is associated to high healthcare cost and even higher socioeconomic cost. Our results highlight the need of improving the treatment of XLH.

We report the most comprehensive clinical and molecular characterization of XLH patients performed in Chile. We show high prevalence of musculoskeletal burden and pain, associated with significantly impaired physical capacity and quality of life, with many relevant complications presenting more frequently than previously reported in cohorts from developed countries. Our current understanding of the clinical presentation and natural history of X-linked hypophosphatemia (XLH) comes mainly from cohorts from developed countries, with limited data on the clinical and genetic abnormalities of XLH patients in South America. To describe the clinical, biochemical, and molecular presentation of patients with XLH in Chile. Patients with XLH referred by endocrinologist throughout Chile were included. Demographic data and clinical presentation were obtained from a clinical interview. Surveys were applied for quality of life (QoL), pain, and functionality. FGF23 was measured by ELISA, and genetic testing was performed. Imaging studies were conducted to assess skeletal and renal involvement. We included 26 patients, aged 2–64 years, from 17 unrelated Chilean families. All pediatric patients but only 40% of adults were receiving conventional therapy, while 65% of all patients had elevated alkaline phosphatase. All patients had mutations in PHEX, including 5 novel variants. Radiographic skeletal events (RSE) and enthesopathies in adults were frequent (34% and 85%, respectively). The duration of treatment was associated with fewer RSE (p < 0.05). Most adults reported pain and impaired QoL, and 50% had impaired physical capacity. The number of enthesopathies was associated with worse pain and stiffness scores (p < 0.05). Chilean patients with XLH have a high prevalence of musculoskeletal burden associated with pain and impaired physical capacity and QoL, especially in adults who were generally undertreated. These data identify a significant unmet need, inform our understanding of the current status of patients, and can guide care for XLH patients in similarly socioeconomically defined countries.

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. 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. 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. 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.

Aspects of bone remodeling have only been scarcely studied in X-linked hypophosphatemia (XLH). In this cross-sectional controlled study, we assessed biochemical indices of bone remodeling and sclerostin in 27 adult patients (median age 47 [range 24-79] years, 19 women, 8 men) with XLH matched with 81 healthy control subjects (1:3) with respect to age-, sex-, and menopausal status. Markers of bone resorption (carboxyterminal cross-linked telopeptide of type 1 collagen, CTX) and formation (N-terminal propeptide of type 1 procollagen, P1NP) were higher in XLH patients compared to controls (median [IQR] 810 [500-1340] vs 485 [265-715] ng/l and 90 [57-136] vs 49 [39-65] ug/l, respectively, both p

Background Osteogenesis imperfecta, fibrous dysplasia/McCune-Albright syndrome and X-linked hypophosphatemia are three rare musculoskeletal diseases characterised by bone deformities, frequent fractures and pain. Little high-quality research exists on appropriate treatment and long-term management of these conditions in adults. This is further worsened by limited research funding in rare diseases and a general mismatch between the existing research priorities and those of the patients. This partnership adopted the James Lind Alliance approach to identify the top 10 research priorities for rare musculoskeletal diseases in adults through joint patient, carer and healthcare professional collaboration. Results The initial survey for question collection recruited 198 respondents, submitting a total of 988 questions. 77% of the respondents were patients with a rare musculoskeletal disease. Following out-of-scope question exclusion, repeating query grouping and scientific literature check for answers, 39 questions on treatment and long-term management remained. In the second public survey, 220 respondents, of whom 85% were patients with a rare musculoskeletal disease, their carers, relatives or friends, prioritised these uncertainties, which allowed selection of the top 25. In the last stage, patients, carers and healthcare professionals gathered for a priority setting workshop to reach a consensus on the final top 10 research priorities. These focus on the uncertainties surrounding appropriate treatment and holistic long-term disease management, highlighting several aspects indirect to abnormal bone metabolism, such as extra-skeletal symptoms, psychological care of both patients and their families and disease course through ageing. Conclusions This James Lind Alliance priority setting partnership is the first to investigate rare bone diseases. The priorities identified here were developed jointly by patients, carers and healthcare professionals. We encourage researchers, funding bodies and other stakeholders to use these priorities in guiding future research for those affected by rare musculoskeletal disorders.

To find out if cardiovascular alterations are present in pediatric patients with X-linked hypophosphatemia (XLH). Multicentre prospective clinical study on pediatric patients included in the RenalTube database ( www.renaltube.com ) with genetically confirmed diagnosis of XLH by mutations in the PHEX gene. The study’s protocol consisted of biochemical work-up, 24-h ambulatory blood pressure monitoring (ABPM), carotid ultrasonography, and echocardiogram. All patients were on chronic treatment with phosphate supplements and 1-hydroxy vitamin D metabolites. Twenty-four patients (17 females, from 1 to 17 years of age) were studied. Serum concentrations (X ± SD) of phosphate and intact parathyroid hormone were 2.66 ± 0.60 mg/dl and 58.3 ± 26.8 pg/ml, respectively. Serum fibroblast growth factor 23 (FGF23) concentration was 278.18 ± 294.45 pg/ml (normal

Burosumab (Crysvita®), a fully human IgG1 monoclonal antibody directed at fibroblast growth factor 23 (FGF23), is indicated for the treatment of X-linked hypophosphatemia (XLH), a condition associated with excessive FGF23 production. It directly addresses the excessive FGF23 activity in patients with XLH by binding to FGF23, and inhibiting its signaling. This leads to increased gastrointestinal phosphate absorption and renal phosphate reabsorption, thereby improving serum phosphate levels, and, ultimately, bone mineralization and the risk of bone disease. In clinical trials, subcutaneous burosumab increased serum phosphorus levels in pediatric and adult patients with XLH, as well as significantly improving the severity of rickets in children, and improving pain, stiffness, physical functioning, and fracture/pseudofracture healing in adults. Burosumab is well tolerated by children and adults with XLH, with most treatment-emergent adverse events being of mild to moderate severity.

Bone mineral apparent density (BMAD) in children with X-linked hypophosphatemia (XLH) was evaluated, as they are unlikely to have extra-skeletal ossifications contributing to the elevated bone mineral density of the spine in adult patients. Children with XLH also had significantly higher BMAD of the spine compared to femoral neck. Introduction: BMAD obtained by dual-energy X-ray absorptiometry scans in children with XLH was evaluated, as they are unlikely to have the extra-skeletal ossifications contributing to the elevated bone mineral density of the spine in adult patients. Methods: A total of 15 children with biochemically and genetically verified XLH were recruited. Anthropometric measurements were performed, and to correct for the short stature (small bones), the BMAD of the spine and the femoral neck was evaluated. Results: Z-scores of BMAD of the spine (mean (95 % CI); 2.0 (1.3-2.7); p < 0.001) were significantly elevated compared to reference children. Z-scores of the femoral neck (1.0 (-0.0 to 2.1); p = 0.059) tended to be elevated. Spine Z-scores were significantly higher than the Z-scores of the femoral neck, (paired t test, p = 0.02). BMAD of the spine was evaluated according to the Molgaard's approach; XLH children had normal bone size of the spine for age due to a normal sitting height Z-score of -0.4 (-1.0 to 0.1); p = 0.1. Z-scores of bone mineral content (BMC) of the spine for bone area were elevated (1.4 (0.8-2.1); p < 0.001). No reference data were available to allow evaluation of the BMAD of the femoral neck by the Molgaard's approach. Conclusions: Children with XLH have an increased BMAD and a high BMC for bone area at the lumbar spine, and this was due to causes other than extra-skeletal ossifications and corrected for bone size. The BMAD of the spine was significantly higher compared to the femoral neck.

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.

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia as a result of an inactivating mutation of the PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) gene. PHEX encodes an endopeptidase that, when inactivated, results in elevated circulating levels of FGF-23, a novel phosphate-regulating hormone (a phosphatonin), thereby resulting in increased phosphate excretion and impaired bone mineralization. A generalized and severe mineralizing enthesopathy in patients with XLH was first reported in 1985; we likewise report a survey in which we found evidence of enthesopathy in fibrocartilaginous insertion sites, as well as osteophyte formation, in the majority of patients. Nonetheless, there has been very little focus on the progression and pathogenesis underlying the paradoxical heterotopic calcification of tendon and ligament insertion sites. Such studies have been hampered by lack of a model of mineralizing enthesopathy. We therefore characterized the involvement of the most frequently targeted fibrocartilaginous tendon insertion sites in Hyp mice, a murine model of the XLH mutation that phenocopies the human syndrome in every detail including hypophosphatemia and elevated FGF-23. Histological examination of the affected entheses revealed that mineralizing insertion sites, while thought to involve bone spur formation, were not due to bone-forming osteoblasts but instead to a significant expansion of mineralizing fibrocartilage. Our finding that enthesis fibrocartilage cells specifically express fibroblast growth factor receptor 3 (FGFR3)/Klotho suggests that the high circulating levels of FGF-23, characteristic of XLH and Hyp mice, may be part of the biochemical milieu that underlies the expansion of mineralizing enthesis fibrocartilage.

Background To determine the genetic lesions and to modify the clinical diagnosis for a Chinese family with significant intrafamilial phenotypic diversities and unusual presentations. Methods Three affected patients and the asymptomatic father were included and received comprehensive systemic examinations. Whole exome sequencing (WES) was performed for mutation detection. Structural modeling test was applied to analyze the potential structural changes caused by the missense substitution. Results The proband showed a wide spectrum of systemic anomalies, including bilateral ectopia lentis, atrial septal defect, ventricular septal defect, widening of tibial metaphysis with medial bowing, and dolichostenomelia in digits, while her mother and elder brother only demonstrated similar skeletal changes. A recurrent mutation, PHEX p.R291*, was found in all patients, while a de novo mutation, FBN1 p.C792F, was only detected in the proband. The FBN1 substitution was also predicted to cause significant conformational change in fibrillin-1 protein, thus changing its physical and biological properties. Conclusions Taken together, we finalized the diagnosis for this family as X-linked hypophosphatemia (XLH), and diagnosed this girl as Marfan syndrome combined with XLH, and congenital heart disease. Our study also emphasizes the importance of WES in assisting the clinical diagnosis for complicated cases when the original diagnoses are challenged. Electronic supplementary material The online version of this article (doi:10.1186/s12967-015-0534-9) contains supplementary material, which is available to authorized users.

This work was supported by the National Plan I + D + I 2008-2011, Instituto de Salud Carlos III (PI12/00987) and also by the National Plan I + D + I 2013-2016 Instituto de Salud Carlos III (PI14/00702 and PI15/02122), European Regional Development Funds 2013-2016 (ERDF, Grupín 14-020), the Foundation of the University of Oviedo (FUO), and by the Severo Ocha Grant 2013-2017 in the framework of “Programa Estatal de Promoción del Talento y su Empleabilidad en I+D+I, Subprograma Estatal de Formación”.

Background. X-linked hypophosphatemia (XLH) is the most common form of familial hypophosphatemic rickets, which results from impaired renal phosphate (Pi) reabsorption, renal vitamin D metabolism, and skeletal mineralization. PHEX the gene responsible for XLH, is homologous to members of the neutral endopeptidase family. The role that the PHEX gene plays in Pi transport is not yet known. Methods. In the present study, we investigated the molecular mechanisms of the disorder of renal Pi transport in XLH in seven individuals from Japanese XLH families. Also, to characterize the effect of XLH serum from these patients on the expression of a renal phosphate transporter gene, we performed a functional analysis of the human type II Na/Pi cotransporter (NPT2) gene promoter in a kidney cell line, OK cells (named OK-B2400), which had stable expression of the luciferase reporter vector p3P2400 with 2462 base pairs (nucleotides −2409 to +53) of the NPT2 gene. Results. XLH serum significantly decreased the transcriptional activity of the NPT2 promoter. The mutations identified in our patients included three large deletions, frame shift mutations, and a stop mutation in the PHEX gene, which suggest that the gene is nonfunctional in affected individuals. Conclusion. The present study demonstrated that the serum of XLH patients with PHEX mutations contains inhibitory factor(s) for the transcriptional activity of the NPT2 gene. In addition, the transcription assay of the NPT2 promoter may provide a useful tool for the characterization of the putative PHEX protein substrate.

Proper serum phosphate concentrations are maintained by a complex and poorly understood process. Identification of genes responsible for inherited disorders involving disturbances in phosphate homeostasis may provide insight into the pathways that regulate phosphate balance. Several hereditary disorders of isolated phosphate wasting have been described, including X-linked hypophosphataemic rickets1 (XLH), hypophosphataemic bone disease2 (HBD), hereditary hypophosphataemic rickets with hypercalciuria3 (HHRH) and autosomal dominant hypophosphataemic rickets4,5 (ADHR). Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH (refs 6,7). ADHR (MIM 193100) is characterized by low serum phosphorus concentrations, rickets, osteomalacia, lower extremity deformities, short stature, bone pain and dental abscesses4,5. Here we describe a positional cloning approach used to identify the ADHR gene which included the annotation of 37 genes within 4 Mb of genomic sequence. We identified missense mutations in a gene encoding a new member of the fibroblast growth factor (FGF) family, FGF23. These mutations in patients with ADHR represent the first mutations found in a human FGF gene.

Objective. To evaluate the bone mineral status of children being treated for X-linked hypophosphatemia, including potential differences between cortical bone in the radial diaphysis and combined cortical and trabecular bone in the lumbar spine. Design and patients. Forty-four bone mineral evaluations were performed in 11 children and adolescents with X-linked hypophosphatemia. Bone mineral density (BMD) of the lumbar spine and the radial diaphysis were measured by dual X-ray absorptiometry (DXA), second metacarpal cortical thickness was measured on hand radiographs, and these results were expressed as Z-scores (standard deviations from the mean). Results. For the 11 initial examinations, Z-scores (mean±SD) were: radial BMD, –2.73±1.15, lumbar BMD, +1.28±1.53; and cortical thickness, –2.21±0.95. Lumbar BMD Z-scores were significantly greater than those for radial BMD and cortical thickness. On follow-up examinations there was a mild increase in radial BMD and decrease in lumbar BMD. Although these changes were statistically significant, they were quite small and the discordance between radial and lumbar BMD was not corrected. Conclusions. Children and adolescents who are being treated for X-linked hypophosphatemia manifest a bone mineral disorder characterized by decreased BMD in the appendicular skeleton and increased BMD in the lumbar spine. Although current therapy is successful in its anti-rachitic effects, it does not correct this bone mineral disorder and additional therapeutic trials should be considered.

Mutations in the PHEX gene (phosphate-regulating gene with homology to endopeptidases on the X-chromosome) are responsible for X-linked hypophosphatemia (HYP). We previously reported the full-length coding sequence of murine Phex cDNA and provided evidence of Phex expression in bone and tooth. Here, we report the cloning of the entire 3.5-kb 3'UTR of the Phex gene, yielding a total of 6248 bp for the Phex transcript. Southern blot and RT-PCR analyses revealed that the 3' end of the coding sequence and the 3'UTR of the Phex gene, spanning exons 16 to 22, are deleted in Hyp, the mouse model for HYP. Northern blot analysis of bone revealed lack of expression of stable Phex mRNA from the mutant allele and expression of Phex transcripts from the wild-type allele in Hyp heterozygous females. Expression of the Phex protein in heterozygotes was confirmed by Western analysis with antibodies raised against a COOH-terminal peptide of the mouse Phex protein. Taken together, these results indicate that the dominant pattern of Hyp inheritance in mice is due to Phex haploinsufficiency.

Recent investigations of X-linked hypophosphatemia support the concept that the kidney is intrinsically normal in this disorder, and that the characteristic phosphaturia is caused by a humoral factor. The mechanisms involved in the pathophysiology of X-linked hypophosphatemia, Hyp, and oncogenic hypophosphatemic osteomalacia are complex and are the results of mutations in a putative zinc metalloprotease. Inactivation inPHEX gene function initiates a series of events that result in severe perturbations in renal Pi transport and metabolism of vitamin D. There are a number of possible working models that could explain the experimental observations. However, our studies clearly show that a humoral factor (phosphatonin) inhibits the transcription of the type II Na+/Pi cotransporter gene (Fig. 4). Phosphatonin may be a key modulator of phosphate homeostasis.

X–linked hypophosphatemic rickets (HYP) is a dominant disorder characterised by impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules. By positional cloning, we have isolated a candidate gene from the HYP region in Xp22.1. This gene exhibits homology to a family of endopeptidase genes, members of which are involved in the degradation or activation of a variety of peptide hormones. This gene (which we have called PEX) is composed of multiple exons which span at least five cosmids. Intragenic non–overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients, which suggest that the PEX gene is involved in the HYP disorder.

Hypophosphatemic vitamin D-resistant rickets, an X-linked dominant disorder, is the most common form of vitamin D–resistant rickets in humans (McKusick number 307800). Biochemically, these patients exhibit hypophosphatemia due to a defect in the renal tubular reabsorption of phosphate. The human cDNA encoding for the renal phosphate transporter has been recently cloned using the expression system in the Xenopus laevis oocytes. Because hypophosphatemic vitamin D–resistant rickets has an X-linked mode of transmission, we hypothesized that the gene encoding the renal phosphate transporter might map to the X chromosome. In this report, we determined the chromosomal localization of the human renal phosphate transporter using three independent methods. First, DNA from somatic cell hybrid panels was examined by Southern blotting for the phosphate transporter. Second, the polymerase chain reaction was used to amplify DNA from somatic cell hybrids. Third, fluorescent in situ hybridization was used to sublocalize the renal phosphate transporter. All three methods localized the renal phosphate transporter to chromosome 5q13. Our results indicate that either derangement of a gene other than the phosphate transporter gene that is encoded on chromosome 5 is responsible for X-linked hypophosphatemic rickets or, alternatively, a gene encoded on the X chromosome has an epistatic effect on the expression of the renal phosphate transporter on chromosome 5.

Wrist and knee radiographs from children with X-linked hypophosphatemic rickets were analyzed and compared with those from normal children and children with established rickets to assess whether radiographically apparent rickets is a consistent abnormality in X-linked hypophosphatemia. The absence or presence of rickets was correctly identified in 94.8% of wrist and knee films from normal and positive controls. In contrast, patients with X-linked hypophosphatemia exhibited rachitic abnormalities in only 5 of 11 wrist and 13 of 15 knee radiographs. As a result, 4 patients within this study group had rickets at the knee and not at the wrist, whereas 5 displayed classic defects at both sites. Perhaps more important, 2 patients, aged 3.8 and 5.2 years, displayed no evidence of rickets in either wrist or knee films, although relatives exhibited demonstrable rachitic abnormalities. Our data indicate that radiographically detectable rickets is a variable abnormality of X-linked hypophosphatemia and does not provide an unambiguous index for the diagnosis of this disease.

Three decades of work on X-linked hypophosphatemia in human and mouse reached fruition with the recent discovery of PEX, the ‘‘Phosphate regulating gene with homologies to Endopeptidases on the X chromosome’’ (HYP Consortium 1995; Strom et al. 1997). The mouse story began in 1966, when six males with shortened trunk and hindlimbs were discovered during a linkage experiment at The Jackson Laboratory (Eicher et al. 1976). Biochemical detective work identified hypophosphatemia as a critical defect, and administration of phosphate to the drinking water corrected many of the skeletal abnormalities and the dwarfing. Abnormal renal phosphate reabsorption accounts for the low level of phosphate in blood and the elevated level in urine. The mutation was inherited as an X-linked dominant and designated hypophosphatemia (Hyp). In 1986, a second dominant X-linked hypophosphatemia mutation, gyro (Gy), was described (Lyon et al. 1986). In addition to hypophosphatemia and skeletal defects that closely resemble Hyp, gyro mice exhibit circling behavior due to degeneration of hair cells in the vestibular apparatus and cochlea. The neuroepithelial abnormalities of the inner ear also result in a hearing deficiency (Steel 1995). Because the phenotypes of Hyp and Gy are dominant, it was not possible to draw conclusions about possible allelism from the phenotype of the compound heterozygote. Evidence for very close linkage was obtained from a cross between Gy +/+ Hyp females and normal males (Lyon et al. 1986). Among the 239 male offspring of this cross, one had normal blood phosphate levels and generated normal male offspring, indicating that he carried a recombinant + + chromosome. This recombination was interpreted as evidence that Hyp and Gy were mutations in distinct but closely linked and functionally related genes. However, recent molecular analysis found that Hyp and Gy are caused by independent mutations in the Pex gene that are separated by approximately 100 kb (Strom et al. 1997). The original recombination between Gy and Hyp was thus a rare example of intragenic recombination and reminds us that a single recombination does not provide a reliable estimate of genetic distance.

CARDIOVASCULAR (CV) ABNORMALITIES IN PATIENTS (PTS) WITH X-LINKED HYPOPHOSPHATEMIA (XLH). † 554

We studied the effect of 1,25(OH)2D3 on phosphate (Pi) homeostasis in the Hyp mouse, a homologue of X-linked hypophosphatemia in man. 1,25(OH)2D3 (83 pg/g·day, infused subcut. × 10 days) produced significant hypercalcemia in normal mice (11.34±0.07 vs 10.14±0.15 mg/dl, p

Stability of genes on the X chromosome during evolution and homologous mutant phenotypes in human X-linked hypophosphatemia (XLH), and Hyp mouse convinced us to study phosphate (Pi) transport in the latter to delineate the mutant gene product. Net reabsorption of Pi in the Hyp/Y (hemizygous) mouse is impaired in vivo. The defect in transepithelial transport of anion is not dependent on PTH. Hyp/Y cortex slices which expose the basolateral membrane take up and efflux Pi and incorporate anion into cellular pools normally. Accordingly neither “transport run out” nor “metabolic run out” of Pi, from the cytosol pool in equilibrium with luminal Pi across the brush border membrane, are affected in the mutant phenotype. Brush-border membrane vesicles (BBMV) purified ten fold from normal renal cortex transport Pi against an electrochemical gradient by an Na+-dependent, arsenate-inhibited process; an Na+-independent diffusional process is also present. Hyp/Y BBMVs have a partial deletion of the Na+-dependent Pi transport process (Hyp/Y, 227 pmoles/mg protein at 60s; +/Y littermate control, 408 pmoles/mg; p

438 SERUM OSTEOCALCIN: RESPONSE TO EXOGENOUS PARATHYROID HORMONE (PTH) IN X-LINKED HYPOPHOSPHATEMIA (XLH)

The mineralization defect in vitamin D deficiency (animal model) results in decreased turnover of osteocalcin, a unique vitamin K-dependent calcium-binding protein of bone matrix. GLA is the modified glutamate residue that is excreted quantitatively after osteocalcin degradation. To determine whether a similar effect occurs in mendelian rachitic disorders of man, we assayed urinary content of GLA. We studied 11 XLH patients (5M, 6F) treated with calcitriol (1,25-(OH)2D3) and phosphate. Urinary GLA excretion (50±15(SD) μmol/g creat) was not different from normal (44±11 μmol/g creat, n=63). We compared GLA excretion in 5 ARVDD patients in a vitamin D-depleted state with 7 ARVDD patients receiving a therapeutic dose of 1,25-(OH)2D3 (1.0-1.5 μg/day). GLA excretion in the D-depleted ARVDD group was significantly lower than normal (31±6 μmol/g creat, p=.006). GLA excretion was higher in the 1,25-(OH)2D3 treated group (44±4 μmol/g creat, p < .001) and not different from controls. These observations suggest that osteocalcin turnover is decreased in untreated individuals with impaired mineral metabolism. GLA excretion appears to be a useful marker for turnover of bone matrix in disorders of mineralization.

Serum Pi in dwarfed, rachitic C57BL/6J Hyp/Y mice is low (4.6±0.9mg/dl) vs normal +/Y male littermates (7.6±1.0mg/ dl,p