Audiometric evidence for two forms of X-linked hypophosphatemia in humans, apparent counterparts of Hyp and Gy mutations in mouse

Characterization of hearing-impairment in Generalized Arterial Calcification of Infancy (GACI)

BACKGROUND AND IMPORTANCE

Hearing loss (HL) has been sporadically described, but not well characterized, in Generalized Arterial Calcification of Infancy (GACI), a rare disease in which pathological calcification typically presents in infancy.

OBJECTIVES

This study aims to describe the clinical audiologic and otologic features and potential etiology of hearing impairment in GACI and gain pathophysiological insight from a murine model of GACI.

DESIGN

Cross-sectional cohort study of individuals with GACI. Murine ossicle micromorphology of the ENPP1^asj/asj mutant compared to wild-type.

SETTING

Clinical research hospital; basic science laboratory.

PARTICIPANTS

Nineteen individuals with GACI who met clinical, biochemical, and genetic criteria for diagnosis.

MAIN OUTCOMES AND MEASURES

Clinical, biochemical, and radiologic features associated with hearing status.

RESULTS

Pure-tone thresholds could be established in 15 (n = 30 ears) of the 19 patients who underwent audiological assessments. The prevalence of HL was 50% (15/30) of ears, with conductive HL in 80% and sensorineural HL in 20%. In terms of patients with HL (n = 8), seven patients had bilateral HL and one patient had unilateral HL. Degree of HL was mild to moderate for 87% of the 15 ears with hearing loss. Of those patients with sufficient pure-tone and middle ear function data, 80% (8/10) had audiometric configurations suggestive of ossicular chain dysfunction (OCD). Recurrent episodes of otitis media (ROM) requiring pressure-equalizing tube placement were common. In patients who underwent cranial CT, 54.5% (6/11) had auricular calcification. Quantitative backscattered electron imaging (qBEI) of murine ossicles supports an OCD component of auditory dysfunction in GACI, suggesting loss of ossicular osteocytes without initiation of bone remodeling.

CONCLUSIONS AND RELEVANCE

Hearing loss is common in GACI; it is most often conductive, and mild to moderate in severity. The etiology of HL is likely multifactorial, involving dysfunction of the ossicular chain and/or recurrent otitis media. Clinically, this study highlights the importance of early audiologic and otologic evaluation in persons with GACI. Novel findings of high rates of OCD and ROM may inform management, and in cases of unclear HL etiology, dedicated temporal bone imaging should be considered.

Incidence of Complications in 25 Adult Patients With X-linked Hypophosphatemia

CONTEXT

Adults with X-linked hypophosphatemia (XLH) present complications other than osteomalacia.

OBJECTIVE

To describe the incidence and severity of comorbidities in adults with XLH.

METHODS

This observational retrospective study included a total of 25 adults with XLH with thorough investigations, including spinal computed tomography scans, x-rays of hip/knee joints and Achilles tendons, abdominal ultrasounds, and audiograms. The index of ossification of the anterior/posterior longitudinal ligament and yellow ligament (OA/OP/OY index) and the sum of OA/OP/OY index (OS index) were utilized to evaluate the severity of spinal ligament ossification. The Kellgren-Lawrence (KL) classification was adopted to evaluate the severity of the hip/knee osteophytes.

RESULTS

The participants consisted of 13 male patients and 12 female patients from 21 families, with a median age of 43 (range, 18-72) years. In all, 20 patients (80%) showed spinal ligament ossification. The median OA/OP/OY/OS indices were 2 (0-22), 0 (0-15), 6 (0-13), and 12 (0-41), respectively. Hip/knee osteophytes were reported in 24 (96%) and 17 cases (68%). The median KL grade was 3 in the hip joint and 2 in the knee joint, and 18 cases (72%) developed enthesopathy in the Achilles tendon. Nephrocalcinosis and hearing impairment were observed in 18 (72%) and 8 (32%) cases.

CONCLUSION

This study revealed a high prevalence and severity of ectopic ossification and disclosed the incidence of nephrocalcinosis and hearing impairment in adults with XLH. In cases with severe spinal ligament ossification or noticeable osteophytes around the hip/knee joints, undiagnosed XLH should be considered as a possible underlying condition.

In situ 3D-patterning of electrospun fibers using two-layer composite materials

Polymeric electrospun nanofibers have extensive applications in filtration, sensing, drug delivery, and tissue engineering that often require the fibers to be patterned or integrated with a larger device. Here, we describe a highly versatile in situ strategy for three-dimensional electrospun fiber patterning using collectors with an insulative surface layer and conductive recessed patterns. We show that two-layer collectors with pattern dimensions down to 100-micrometers are easily fabricated using available laboratory equipment. We use finite element method simulation and experimental validation to demonstrate that the fiber patterning strategy is effective for a variety of pattern dimensions and fiber materials. Finally, the potential for this strategy to enable new applications of electrospun fibers is demonstrated by incorporating electrospun fibers into dissolving microneedles for the first time. These studies provide a framework for the adaptation of this fiber patterning strategy to many different applications of electrospun fibers.

FGF23 and its role in X-linked hypophosphatemia-related morbidity

Background

X-linked hypophosphatemia (XLH) is an inherited disease of phosphate metabolism in which inactivating mutations of the Phosphate Regulating Endopeptidase Homolog, X-Linked (PHEX) gene lead to local and systemic effects including impaired growth, rickets, osteomalacia, bone abnormalities, bone pain, spontaneous dental abscesses, hearing difficulties, enthesopathy, osteoarthritis, and muscular dysfunction. Patients with XLH present with elevated levels of fibroblast growth factor 23 (FGF23), which is thought to mediate many of the aforementioned manifestations of the disease. Elevated FGF23 has also been observed in many other diseases of hypophosphatemia, and a range of animal models have been developed to study these diseases, yet the role of FGF23 in the pathophysiology of XLH is incompletely understood.

Methods

The role of FGF23 in the pathophysiology of XLH is here reviewed by describing what is known about phenotypes associated with various PHEX mutations, animal models of XLH, and non-nutritional diseases of hypophosphatemia, and by presenting molecular pathways that have been proposed to contribute to manifestations of XLH.

Results

The pathophysiology of XLH is complex, involving a range of molecular pathways that variously contribute to different manifestations of the disease. Hypophosphatemia due to elevated FGF23 is the most obvious contributor, however localised fluctuations in tissue non-specific alkaline phosphatase (TNAP), pyrophosphate, calcitriol and direct effects of FGF23 have been observed to be associated with certain manifestations.

Conclusions

By describing what is known about these pathways, this review highlights key areas for future research that would contribute to the understanding and clinical treatment of non-nutritional diseases of hypophosphatemia, particularly XLH.

A novel auditory ossicles membrane and the development of conductive hearing loss in Dmp1-null mice

Genetic mouse models are widely used for understanding human diseases but we know much less about the anatomical structure of the auditory ossicles in the mouse than we do about human ossicles. Furthermore, current studies have mainly focused on disease conditions such as osteomalacia and rickets in patients with hypophosphatemia rickets, although the reason that these patients develop late-onset hearing loss is unknown. In this study, we first analyzed Dmp1 lac Z knock-in auditory ossicles (in which the blue reporter is used to trace DMP1 expression in osteocytes) using X-gal staining and discovered a novel bony membrane surrounding the mouse malleus. This finding was further confirmed by 3-D micro-CT, X-ray, and alizarin red stained images. We speculate that this unique structure amplifies and facilitates sound wave transmissions in two ways: increasing the contact surface between the eardrum and malleus and accelerating the sound transmission due to its mineral content. Next, we documented a progressive deterioration in the Dmp1-null auditory ossicle structures using multiple imaging techniques. The auditory brainstem response test demonstrated a conductive hearing loss in the adult Dmp1-null mice. This finding may help to explain in part why patients with DMP1 mutations develop late-onset hearing loss, and supports the critical role of DMP1 in maintaining the integrity of the auditory ossicles and its bony membrane.

Treatment of ear and bone disease in the Phex mouse mutant with dietary supplementation

HYPOTHESIS

Phosphorus and vitamin D (calcitriol) supplementation in the Phex mouse, a murine model for endolymphatic hydrops (ELH), will improve otic capsule mineralization and secondarily ameliorate the postnatal development of ELH and sensorineural hearing loss (SNHL).

BACKGROUND

Male Phex mice have X-linked hypophosphatemic rickets (XLH), which includes osteomalacia of the otic capsule. The treatment for XLH is supplementation with phosphorus and calcitriol. The effect of this treatment has never been studied on otic capsule bone and it is unclear if improving the otic capsule bone could impact the mice's postnatal development of ELH and SNHL.

METHODS

Four cohorts were studied: 1) wild-type control, 2) Phex control, 3) Phex prevention, and 4) Phex rescue. The control groups were not given any dietary supplementation. The Phex prevention group was supplemented with phosphorus added to its drinking water and intraperitoneal calcitriol from postnatal day (P) 7-P40. The Phex rescue group was also supplemented with phosphorus and calcium but only from P20 to P40. At P40, all mice underwent auditory brainstem response (ABR) testing, serum analysis, and temporal bone histologic analysis. Primary outcome was otic capsule mineralization. Secondary outcomes were degree of SNHL and presence ELH.

RESULTS

Both treatment groups had markedly improved otic capsule mineralization with less osteoid deposition. The improved otic capsule mineralized did not prevent the development of ELH or SNHL.

CONCLUSION

Supplementation with phosphorus and calcitriol improves otic capsule bone morphology in the Phex male mouse but does not alter development of ELH or SNHL.

A Genetic Murine Model of Endolymphatic Hydrops: The Phex Mouse

Animal models of endolymphatic hydrops (ELH) provide critical insight into the pathophysiology of Meniere's disease (MD). A new genetic murine model, called the Phex mouse, circumvents prior need for a time and cost-intensive surgical procedure to create ELH. The Phex mouse model of ELH, which also has X-linked hypophosphatemic rickets, creates a postnatal, spontaneous, and progressive ELH whose phenotype has a predictable decline of vestibular and hearing function reminiscent of human MD. The Phex mouse enables real-time histopathologic analysis to assess diagnostic and therapeutic interventions as well as further our understanding of ELH's adverse effects. Already the model has validated electrocochleography and cervical vestibular evoked myogenic potential as useful diagnostic tools. New data on caspase activity in apoptosis of the spiral ganglion neurons may help target future therapeutic interventions. This paper highlights the development of the Phex mouse model and highlights its role in characterizing ELH.

Therapeutic management of hypophosphatemic rickets from infancy to adulthood

In children, hypophosphatemic rickets (HR) is revealed by delayed walking, waddling gait, leg bowing, enlarged cartilages, bone pain, craniostenosis, spontaneous dental abscesses, and growth failure. If undiagnosed during childhood, patients with hypophosphatemia present with bone and/or joint pain, fractures, mineralization defects such as osteomalacia, entesopathy, severe dental anomalies, hearing loss, and fatigue. Healing rickets is the initial endpoint of treatment in children. Therapy aims at counteracting consequences of FGF23 excess, i.e. oral phosphorus supplementation with multiple daily intakes to compensate for renal phosphate wasting and active vitamin D analogs (alfacalcidol or calcitriol) to counter the 1,25-diOH-vitamin D deficiency. Corrective surgeries for residual leg bowing at the end of growth are occasionally performed. In absence of consensus regarding indications of the treatment in adults, it is generally accepted that medical treatment should be reinitiated (or maintained) in symptomatic patients to reduce pain, which may be due to bone microfractures and/or osteomalacia. In addition to the conventional treatment, optimal care of symptomatic patients requires pharmacological and non-pharmacological management of pain and joint stiffness, through appropriated rehabilitation. Much attention should be given to the dental and periodontal manifestations of HR. Besides vitamin D analogs and phosphate supplements that improve tooth mineralization, rigorous oral hygiene, active endodontic treatment of root abscesses and preventive protection of teeth surfaces are recommended. Current outcomes of this therapy are still not optimal, and therapies targeting the pathophysiology of the disease, i.e. FGF23 excess, are desirable. In this review, medical, dental, surgical, and contributions of various expertises to the treatment of HR are described, with an effort to highlight the importance of coordinated care.

Hearing loss and fluctuating hearing levels in X-linked hypophosphataemic osteomalacia

Background and objective:

X-linked hypophosphataemic osteomalacia is the most common of the genetically determined forms of osteomalacia. The occurrence of hearing loss in X-linked hypophosphataemic osteomalacia has been known since 1984. However, observations on the progression of such hearing loss, and suggestions regarding possible therapy, have not previously been published.

Methods:

Case report of a patient with X-linked hypophosphataemic osteomalacia and hearing loss, with three years' audiological follow up, description of empirical therapy and literature review.

Results:

The patient presented with fluctuating hearing. An audiogram showed mild to severe sensorineural hearing loss mainly in the low and high frequencies. A temporary improvement of 20–40 dB after steroid therapy was observed. Four weeks later, hearing had deteriorated again, mainly in the low frequencies. After one year of fluctuating hearing levels, stabilisation occurred.

Conclusions:

In X-linked hypophosphataemic osteomalacia, hearing loss occurs predominantly in the low and high frequencies. The hearing loss type and progression pattern point to an endolymphatic hydrops as the pathogenetic mechanism. Steroid therapy may be of some benefit.

New intragenic deletions in the Phex gene clarify X-linked hypophosphatemia-related abnormalities in mice

X-linked hypophosphatemic rickets (XLH) in humans is caused by mutation in the PHEX gene. Previously, three mutations in the mouse Phex gene have been reported: Phex(Hyp), Gy, and Phex(Ska1). Here we report analysis of two new spontaneous mutation in the mouse Phex gene, Phex(Hyp-2J) and Phex(Hyp-Duk). Phex(Hyp-2J) and Phex(Hyp-Duk) involve intragenic deletions of at least 7.3 kb containing exon 15, and 30 kb containing exons 13 and 14, respectively. Both mutations cause similar phenotypes in males, including shortened hind legs and tail, a shortened square trunk, hypophosphatemia, hypocalcemia, and rachitic bone disease. In addition, mice carrying the Phex(Hyp-Duk) mutation exhibit background-dependent variable expression of deafness, circling behavior, and cranial dysmorphology, demonstrating the influence of modifying genes on Phex-related phenotypes. Cochlear cross-sections from Phex(Hyp-2J)/Y and Phex(Hyp-Duk)/Y males reveal a thickening of the temporal bones surrounding the cochlea with the presence of a precipitate in the scala tympani. Evidence of the degeneration of the organ of Corti and spiral ganglion also are present in the hearing-impaired Phex(Hyp-Duk)/Y mice, but not in the normal-hearing Phex(Hyp-2J)/Y mice. Analysis of the phenotypes noted in Phex(Hyp-Duk)/Y and Phex(Hyp-2J)/Y males, together with those noted in Phex(Ska1)/Y and Phex(Hyp)/Y males, now allow XLH-related phenotypes to be separated from non-XLH-related phenotypes, such as those noted in Gy/Y males. Also, identification of the genetic modifiers of hearing and craniofacial dysmorphology in Phex(Hyp-Duk)/Y mice could provide insight into the phenotypic variation of XLH in humans.

Partial deletion of both the spermine synthase gene and the Pex gene in the X-linked hypophosphatemic, gyro (Gy) mouse

Gy, along with Hyp, is a dominant mutation of the normal gene Pex causing X-linked hypophosphatemia in the mouse. Hemizygous Gy male mice, however, have greater defects in survival, bodily growth, skeletal mineralization, and neurological function than those found in heterozygous Gy females or in Hyp mice. Since the gene for spermine synthase is immediately upstream of the homologous human gene PEX, we compared the effects of the Gy and Hyp mutations on both the spermine synthase gene and the Pex gene. Barely detectable levels of spermine (< 5% of normal) with elevated levels of its precursor, spermidine, were found in organs of Gy male mice compared to normal male littermates. Neither Gy females nor Hyp male mice were significantly affected. Four missing introns of the spermine synthase gene were identified in Gy male mice, suggesting extensive gene disruption. A pseudogene for spermine synthase was also identified in the mouse genome. Pex mRNA was found in several but not all tissues studied in adult normal mice. Pex mRNA was altered in both Gy and Hyp mice. All male Hyp mice were lacking the 3' end of the Pex message, whereas all male Gy mice were deficient at the 5' end. In summary, the Gy mutation is associated with a recessively expressed mutation of the spermine synthase gene, leading to spermine deficiency, and a dominantly expressed mutation of the Pex gene, leading to hypophosphatemia. Alterations in two contiguous genes in Gy may explain the additional phenotypic abnormalities present in the Gy male mouse.

Pex gene deletions in Gy and Hyp mice provide mouse models for X-linked hypophosphatemia

X-linked hypophosphatemic rickets in humans is caused by mutations in the PEX gene which codes for a protein homologous to neutral endopeptidases. Hyp and Gy mice both have X-linked hypophosphatemic rickets, although genetic data and the different phenotypic spectra observed have previously suggested that two different genes are mutated. In addition to the metabolic disorder observed in Hyp mice, male Gy mice are sterile and show circling behavior and reduced viability. We now report the cloning of the mouse homolog of PEX which is highly conserved between man and mouse. The 3' end of this gene is deleted in Hyp mice. In Gy mice, the first three exons and the promotor region are deleted. Thus, Hyp and Gy are allelic mutations and both provide mouse models for X-linked hypophosphatemia.

Renal Na(+)-phosphate cotransporter gene expression in X-linked Hyp and Gy mice

The X-linked Hyp and Gy mutations are murine homologues of X-linked hypophosphatemia (XLH), a dominant disorder of phosphate (Pi) homeostasis characterized by growth retardation, rickets, hypophosphatemia and decreased renal tubular maximum for Pi reabsorption relative to glomerular filtration rate (Tmp/GFR). In Hyp and Gy mice, the decrease in Tmp/GFR is associated with a reduction in renal brush-border membrane (BBM) Na(+)-Pi cotransport that can be ascribed to a decrease in renal-specific, Na(+)-Pi cotransporter (NPT2) mRNA and protein abundance. Although renal NPT2 gene expression is reduced in Hyp and Gy mice, the NPT2 gene does not map to the X chromosome. These findings exclude NPT2 as a candidate gene for murine and human X-linked hypophosphatemias and suggest that genes at the Hyp, Gy and XLH (HYP) loci are involved in regulation of NPT2 gene expression. Both Hyp and Gy mice respond to low Pi diet with an increase in BBM Na(+)-Pi cotransport, NPT2 mRNA and protein. The increase in NPT2 protein in Pi-depleted mice far exceeds the increase in NPT2 mRNA, suggesting that translational or post-translational mechanisms are involved in the adaptive process. NPT2 protein is localized to the apical surface of the proximal tubule, where immunostaining in both normal and Hyp mice is increased in response to low Pi diet. Pi-deprived Hyp and Gy mice fail to show an increase in Tmp/GFR, indicating that adaptation at the BBM is not sufficient for the overall increase in Tmp/GFR in response to low Pi diet.

Effects of altered diet on serum levels of 1,25-dihydroxyvitamin D and parathyroid hormone in X-linked hypophosphatemic (Hyp and Gy) mice

X-linked hypophosphatemia is a metabolic bone disease occurring in both humans and mice. In mice, two different mutations (Hyp and Gy), occurring at separate but closely linked loci, have been proposed as models for this disease. Varying reports of the Vitamin D status of these two mutants has led us to reexamine the influence of diet on circulating calcitrophic hormones and mineral metabolism in both mutants. Hyp and Gy mice were raised on the B6C3H background, and both normal females and heterozygous mutant females were studied at 10 weeks of age. Animals were fed one of three diets at random: high (1.5% Ca and 1.0% P); medium (0.6% Ca and 0.6% P); or low (0.0% Ca and 0.6% P). After 3 days, serum and urine samples were collected. In comparison to mutant mice fed the high diet, both Hyp and Gy mice fed the low diet had decreased serum calcium levels, and further elevations in both serum alkaline phosphatase and serum parathyroid hormone (PTH). Serum 1,25-dihydroxyvitamin D levels were elevated by both the medium and low diets in all groups of mice over values obtained with the high diet. Mutant mice were significantly higher in serum PTH on all diets compared to normal mice fed the same diet. Mutant mice were not elevated in serum 1,25-dihydroxyvitamin D over normal mice when fed the high diet. However, both Hyp and Gy mice fed the medium and low diets were elevated in serum 1,25-dihydroxyvitamin D over normal mice. Serum PTH levels were correlated to serum 1,25-dihydroxyvitamin D levels with Hyp and Gy mice lying on the same line (r = 0.86; p < 0.0001). In summary, when Hyp and Gy mice are studied on the same genetic background and fed the same diet, similar responses are seen in PTH levels and 1,25-dihydroxyvitamin D levels. Both mutants should be useful in elucidating the pathophysiology of this poorly understood human disease.

Femoral abnormalities and vitamin D metabolism in X-linked hypophosphatemic (Hyp and Gy) mice

X-linked hypophosphatemia is a genetic bone disease in humans and mice. Two closely linked mutations in mice, Hyp and Gy, cause low plasma phosphate and a rachitic and osteomalacic bone disease. Because of the controversy as to whether Gy is a good model for X-linked hypophosphatemia, the phenotypic severity of these two mutations was compared in both sexes and on two genetic backgrounds. The depression in plasma levels of phosphate was similar in all 10-week-old mutant mice. Male Hyp mice and heterozygous female Hyp mice were affected with similar severity in terms of reduced tail growth, shortened femora, reduced femoral mineral content, and abnormal mineral composition of the femoral matrix. In contrast, male Gy mice did not survive on the C57BL/6J background and were more severely affected than female Gy mice on the B6C3H background. The hybrid B6C3H background ameliorated the bone disease compared with the inbred C57BL/6J background for both mutant strains. There was no evidence of change in the plasma levels of 1,25-dihydroxyvitamin D, duodenal level of vitamin D-dependent calcium-binding protein, or urinary level of calcium in these adult mutant mice. In summary, Gy mice have a sexual dimorphism not present in Hyp mice. These two genes may indicate the presence of multiple gene loci in the human disease, with multiple proteins involved in the pathophysiology of the bone disease.

X-Linked hypophosphatemic rickets: a disease often unknown to affected patients

X-Linked hypophosphatemic rickets (XLH) is an X-linked dominant disorder that is secondary to renal phosphate wasting. Affected individuals frequently present the following characteristics: short stature, lower-extremity deformity, bone pain, dental abscesses, enthesopathy, rickets, and osteomalacia. Since the disorder is characterized by evident phenotypic abnormalities, we hypothesized that there would be a high degree of knowledge about the disease in affected kindreds. Thus, we constructed a six-page, self-administered questionnaire to determine whether family members are, in fact, aware of their disease and properly diagnosed and treated. We also designed the survey to determine rates of symptoms thought to be associated with rickets/osteomalacia in a population with a lower referral bias than is usually seen in tertiary care centers. We administered the questionnaire to 234 study subjects (57 affected) who were members of one of three large kindreds. Although 62% of affected individuals knew they had some problem with their bones, only 22.6% were told by a physician that they had rickets or osteomalacia. This apparent lack of awareness occurred in spite of 61.1% of affected subjects complaining of bone or joint problems to their personal physician. Indeed, of those patients who had persistent complaints, only 34.5% were told they had rickets or osteomalacia. Only one patient was taking phosphate and vitamin D.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic linkage studies of X-linked hypophosphataemic rickets in a Saudi Arabian family

OBJECTIVE, PATIENTS AND DESIGN: X-linked hypophosphataemic rickets (HYP) is the most common inherited form of rickets and the gene causing this disorder has been localized to Xp22.3-p21.3 by linkage studies of affected families of Northern European origin. In addition, the locus order Xpter-(DXS207-DXS43,DXS197)-HYP-DXS41-X cen has been established and the flanking markers are useful for the presymptomatic diagnosis of HYP. However, a recent study indicates locus heterogeneity and this may hinder the use of the flanking markers for presymptomatic diagnosis in additional families and in particular those from different populations. We have therefore investigated one Saudi-Arabian family (13 affected and six unaffected members) with hypophosphataemic rickets for linkage to these and other X-linked markers. A total of 17 cloned human X chromosome sequences identifying restriction fragment length polymorphisms were used to localize the mutant gene causing this disorder in the Saudi Arabian family.

RESULTS

Nine (four from Xp and five from Xq) of the 17 X-linked DNA probes proved informative and linkage was established between HYP and the DSX41 locus, peak LOD score = 4.22 (recombination fraction, theta = 0.00). A positive peak LOD score of 2.32 (theta = 0.05) was also obtained between HYP and the DXS207 locus. Thus, the HYP gene in this Saudi Arabian family is linked to two of the four flanking markers which demonstrated linkage in families of Northern European origin.

CONCLUSION

We conclude that the X-linked hypophosphataemic rickets gene in a Saudi Arabian family is located in the Xp22.3-p21.3, a region where this gene has previously been mapped by linkage studies of families of Northern European origin. Our studies have not demonstrated locus heterogeneity, so the flanking markers for HYP previously established in the families of Northern-European origin will be useful in the genetic counselling and presymptomatic diagnosis of this disorder in the Saudi Arabian family.

Mouse models of human single gene disorders. I: Nontransgenic mice

Mouse models of human genetic disorders provide a valuable resource for investigating the pathogenesis of genetic disease and for testing potential therapies. The high degree of resolution of linkage mapping in the mouse allows mutant phenotypes to be mapped precisely which, combined with the accurate definition of areas of homology between the mouse and human genomes, greatly facilitates the identification of mouse models. We describe here mouse models of human single gene disorders dividing them into three categories depending on the information available; phenotypic similarities, comparative mapping and identification of the underlying genetic lesion.

X-linked hypophosphataemia: a homologous phenotype in humans and mice with unusual organ-specific gene dosage

XLH (X-linked hypophosphataemia, gene symbol HYP, McKusick 307800, 307810) and its murine counterparts (Hyp and Gy) map to a conserved segment on the X-chromosome (Xp 22.31-p.21.3, human; distal X, mouse). Gene dosage has received relatively little attention in the long history of research on this disease, which began over 50 years ago. Bone and teeth are sites of the principal disease manifestations in XLH (rickets, osteomalacia, interglobular dentin). Newer measures of quantitative XLH phenotypes reveal gene dose effects in bone and teeth with heterozygous values distributed between those in mutant hemizygotes and normal homozygotes. On the other hand, serum phosphate concentrations (which are low in the mutant phenotype and thereby contribute to bone and tooth phenotypes) do not show gene dosage. In Hyp mice serum values in mutant hemizygotes, mutant homozygotes and heterozygotes are similar. Phosphate homeostasis reflects its renal conservation. Renal absorption of phosphate on a high-affinity, Na+ ion-gradient coupled system in renal brush border membrane is impaired and gene dosage is absent at this level; the mutant phenotype is fully dominant. Synthesis and degradation of 1,25(OH)2D are also abnormal in XLH (and Hyp), but gene dosage in these parameters has not yet been measured. An (unidentified) inhibitory trans-acting product of the X-linked locus, affecting phosphate transport and vitamin D metabolism, acting perhaps through cytosolic protein kinase C, could explain the renal phenotype. But why would it have a normal gene dose effect in bone and teeth? Since the locus may have duplicated (to form Hyp and Gy), and shows evidence of variable expression in different organs (inner ear, bone/teeth, kidney), it may have been recruited during evolution to multiple functions.

Craniofacial abnormalities in mice with X-linked hypophosphatemic genes (Hyp or Gy)

X-Linked hypophosphatemia is the most common cause of metabolic rickets in humans and is characterized by a reduced renal TmP/GFR and hypophosphatemia. Clinically, these changes are associated with growth retardation including attenuated craniofacial growth, femoral and tibial bowing, and radiologic and histomorphometric evidence of rickets and osteomalacia. Similar mutations occur in mice at the Hyp and Gy gene loci. Direct craniometric measurements were made on mouse skulls to investigate the pattern of craniofacial growth differences in the Hyp/+, Hyp/Hyp and Gy/+ genotypes and to compare these to littermate normals in the C57BL/6J mouse strain. There was generalized attenuation in craniofacial growth in all mutants. The heterozygous Hyp and Gy mutants showed similar patterns of craniofacial growth with diminished neurocranial length, viscerocranial length, and mandibular height. The Gy/+ was significantly smaller than the Hyp/+ in neurocranial width. The homozygous Hyp mouse was not affected more severely than the heterozygous Hyp except in overall cranial length, nasal bone length, and mandibular length from mandibular foramen to third molar. In summary, the heterozygous Hyp and Gy mutant mice showed similar patterns of craniofacial growth. The homozygous Hyp mouse was not affected more severely than the heterozygous Hyp except in three of the 15 measured variables. Thus, these data demonstrate the almost complete dominance of the Hyp gene. In contrast, the Gy gene is incompletely dominant. The heterozygous Gy females survive, but the hemizygous Gy males do not, on a C57BL/6J background. This suggests that there is a family of closely linked genes on the X chromosome which, while similar in their effects on phosphate homeostasis, have differing mechanisms of action.

Conserved loci on the X chromosome confer phosphate homeostasis in mice and humans

Several genes expressed in kidney and other tissues determine phosphate homeostasis in extracellular fluid. The major form of inherited hypophosphatemia in humans involves an X-linked locus (HPDR, Xp22.31-p21.3). It has two murine homologues (Hyp and Gy) which map to closely-linked but separate loci (crossover value 0.4%-0.8%). Both murine mutations impair Na(+)-phosphate cotransport in renal brush border membrane; an associated renal disorder of 1,25-dihydroxyvitamin D3 (1,25(OH)2D) metabolism has been characterized in Hyp mice. Whereas experiments with cultured Hyp renal epithelium indicate that the gene is expressed in kidney, studies showing the development of the mutant renal phenotype in normal mice parabiosed to Hyp mice implicate a circulating factor; these findings can be reconciled if the humoral factor is of renal origin. The gene dose effect of HPDR, Hyp and Gy on serum phosphorus values is consistently deviant and heterozygotes resemble affected hemizygotes. The deviant effect is also seen on renal phosphate transport; all mutant females (Hyp/Hyp and Hyp/+) have similar phenotypes. On the other hand, there is a normal gene dose effect of HPDR in mineralized tissue; tooth PRATIO (pulp area/tooth area) values for heterozygotes are distributed between those for affected males and normals. The tooth data imply that the X chromosome locus is expressed in both renal and non-renal cells. The polypeptide product of the X chromosome gene(s) is still unknown.

Familial renal hypophosphatemia, minor facial anomalies, intracerebral calcifications, and non-rachitic bone changes: apparently new syndrome?

We report on two brothers with renal hypophosphatemia, intracerebral calcifications, minor facial anomalies, and short distal phalanges. The children presented with recurrent dental abscesses; one had premature closure of the anterior fontanelle. Biochemical findings included hypophosphatemia and elevated serum alkaline phosphatase with normocalcemia. Blood levels of parathyroid hormone, 1,25(OH)2 and 25(OH) vitamin D levels were normal; TRP (the fractional tubular reabsorption of PO4) and TmP/GFR (the tubular maximum rate of PO4 reabsorption in relation to GFR) were low. Both parents had a normal serum phosphate and brain CT scan without evidence of calcifications. This apparently new syndrome of renal hypophosphatemia associated with intracerebral calcifications appears to be inherited as either an autosomal recessive or an X-linked trait.

Normal regulation of calcitriol production in Gy mice. Evidence for biochemical heterogeneity in the X-linked hypophosphatemic diseases

Phenotypic heterogeneity in X-linked hypophosphatemic rickets (XLH) is ascribed to variable penetrance of the genetic abnormality. However, studies of hypophosphatemic (Hyp) and gyrorotary (Gy) mice indicate that mutations at different loci along the X chromosome may underlie the genetically transmitted hypophosphatemic disorders. Thus, genetic heterogeneity may be a determinant of the phenotypic variability in XLH. To determine if such variance includes biochemical diversity, we examined whether Gy mice, similar to Hyp mice, exhibit abnormal regulation of renal 25-hydroxyvitamin D (25[OH]D)-1 alpha-hydroxylase. Serum phosphorus in Gy (4.7 +/- 0.3 mg/dl) and phosphate (P)-depleted mice (4.9 +/- 0.4) was significantly less than normal (8.4 +/- 0.5). Consistent with P depletion, the Gy mice exhibited enhanced renal 25(OH)D-1 alpha-hydroxylase activity (9.3 +/- 0.6 fmol/mg kidney per min), similar to that of P-depleted normals (9.1 +/- 1.5), but significantly greater than that of controls (3.1 +/- 0.3). Such normal enzyme responsiveness was confirmed upon PTH stimulation (1 IU/h s.c.), which revealed that Gy mice increased renal 1-hydroxylase (59 +/- 7.7) similarly to normals (65 +/- 7.7) and P-depleted animals (58.4 +/- 7.8). Calcitonin administration also enhanced enzyme function comparably in the animal models. Evidence confirming normally responsive calcitriol production in untreated Gy mice included increased serum 1,25-dihydroxyvitamin D levels, gastrointestinal calcium absorption, and urinary calcium. The normally regulated vitamin D metabolism in Gy mice indicates that biochemically diverse disease may result from mutations in the gene family regulating renal P transport and underlying X-linked hypophosphatemia. We suspect such heterogeneity is due to altered P transport at variable segments of the proximal convoluted tubule.

Initiation and characterization of primary mouse kidney epithelial cultures

Primary cultures of murine renal epithelial cells were established from a preparation of proximal tubule fragments. Confluent cultures exhibited multiple dome formation, indicating the presence of tight junctions and an intact transcellular transport process. Ultrastructural analysis revealed a monolayer of polarized cells, with a sparse but clearly defined microvillar surface facing the growth medium and a basolateral surface attached to the substratum. Cultures grown on collagen gels did not show domes. The epithelial monolayer exhibited several differentiated functions of the proximal tubule: a) parathyroid hormone (PTH)-stimulated cAMP synthesis; b) production of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; c) high alkaline phosphatase activity; and d) Na+-dependent transport of phosphate (Pi) and alpha-methylglucoside (alpha-MG). The sugar uptake was selectively inhibited by phlorizin, a competitive inhibitor of glucose uptake at the luminal membrane. Kinetic analysis revealed independent transport systems for Pi and alpha-MG, with Km values corresponding to the high affinity systems identified in brush border membrane vesicles derived from the proximal tubule. Pi uptake by the epithelial monolayers was regulated by the concentration of Pi in the growth medium. Phorbol esters and PTH did not exert an effect on Pi and alpha-MG transport in mouse primary cultures. The present study demonstrates that primary cultures provide a useful in vitro preparation to investigate renal proximal tubular function.