Evaluation of diagnostic potential of CD38 in rickets

BACKGROUND

Rickets occurs in infants and children (aged 2 months to 3 years), compromising their skeletal development and damaging nervous, hematopoietic, immune, and other system functions. This study aimed to explore the significance of CD38 in rickets.

METHODS

The microarray dataset GSE22523 was analyzed to obtain differentially expressed genes in rickets patients. A total of 36 rickets patients and healthy controls were recruited for the study, and their blood samples were collected, followed by detecting mRNA levels of CD38 using quantitative real-time polymerase chain reaction (qRT-PCR). Moreover, the significance of CD38 in rickets patients was analyzed by receiver operating characteristic (ROC) analysis, while the correlation between CD38 and 25-hydroxy-vitamin D (25OHD)/parathyroid hormone (PTH) was analyzed with Pearson's correlation.

RESULTS

Results showed that CD38 mRNA levels and PTH contents were significantly increased in the rickets patients while 25OHD contents were decreased. Correlation analysis indicated that CD38 was positively correlated with PTH and negatively correlated with 25OHD in both serum and plasma samples of rickets patients. Moreover, ROC analysis showed that serum CD38 was 0.9005 (95 % CI: 0.8313-0.9696), and the AUCs of plasma CD38 was 0.7215 (95 % CI: 0.6031-0.8398) in differentiating rickets patients from healthy persons, advocating serum CD38 had better diagnostic value.

CONCLUSION

CD38 mRNA levels were upregulated in rickets patients and closely correlated with PTH and 25OHD contents, indicating CD38 might be a diagnostic marker of rickets patients. Further research on the diagnostic utility of CD38 is necessary for the diagnosis and treatment of ricketsin rickets in the future.

Characterization of Rickets Type II Model Rats to Reveal Functions of Vitamin D and Vitamin D Receptor

Vitamin D has been known to exert a wide range of physiological effects, including calcemic, osteogenic, anticancer, and immune responses. We previously generated genetically modified (GM) rats and performed a comparative analysis of their physiological properties to elucidate the roles of vitamin D and vitamin D receptor (VDR). In this study, our primary goal was to investigate the manifestations of type II rickets in rats with the VDR(H301Q) mutation, analogous to the human VDR(H305Q). Additionally, we created a double-mutant rat with the VDR(R270L/H301Q) mutation, resulting in almost no affinity for 1,25-dihydroxy-vitamin D3 (1,25D3) or 25-hydroxy-vitamin D3 (25D3). Notably, the plasma calcium concentration in Vdr(R270L/H301Q) rats was significantly lower than in wild-type (WT) rats. Meanwhile, Vdr(H301Q) rats had calcium concentrations falling between those of Vdr(R270L/H301Q) and WT rats. GM rats exhibited markedly elevated plasma parathyroid hormone and 1,25D3 levels compared to those of WT rats. An analysis of bone mineral density in the cortical bone of the femur in both GM rats revealed significantly lower values than in WT rats. Conversely, the bone mineral density in the trabecular bone was notably higher, indicating abnormal bone formation. This abnormal bone formation was more pronounced in Vdr(R270L/H301Q) rats than in Vdr(H301Q) rats, highlighting the critical role of the VDR-dependent function of 1,25D3 in bone formation. In contrast, neither Vdr(H301Q) nor Vdr(R270L/H301Q) rats exhibited symptoms of alopecia or cyst formation in the skin, which were observed in the Vdr-KO rats. These findings strongly suggest that unliganded VDR is crucial for maintaining the hair cycle and normal skin. Our GM rats hold significant promise for comprehensive analyses of vitamin D and VDR functions in future research.

Mutations in the CYP27B1 gene cause vitamin D dependent rickets in pugs

Rickets is a disorder of bone development and can be the result of either dietary or genetic causes. Here, related pugs from 2 litters were included. Three pugs had clinical signs including, lameness, bone deformities, and dyspnea. One other pug was found dead. Radiographs of 2 affected pugs, 5 and 6 months old, showed generalized widening, and irregular margination of the physes of both the appendicular and the axial skeleton with generalized decrease in bone opacity and bulbous swelling of the costochondral junctions. Two pugs had low serum calcium and 1,25 (OH)2 D3 concentrations. Test results further indicated secondary hyperparathyroidism with adequate concentrations of 25-hydroxyvitamin D. Necropsy revealed tongue-like projections of cartilage extending into the metaphysis consistent with rickets, loss of metaphyseal mineralization and lung pathology. Vitamin D-dependent rickets was diagnosed. A truncating mutation in the 1α-hydroxylase gene (CYP27B1) was identified by genome sequence analysis of the pugs with VDDR type 1A. Vitamin D-dependent rickets type 1A can occur in young pugs, and if left untreated is a life-threatening condition. Early medical intervention can reverse clinical signs and should be instituted as soon as possible.

Feline precision medicine using whole-exome sequencing identifies a novel frameshift mutation for vitamin D-dependent rickets type 2

OBJECTIVES

A 14-week-old female domestic longhair kitten presented with shifting lameness and disproportionately smaller size compared with a co-housed littermate.

METHODS

Hematology and serum biochemical testing were conducted to investigate causes for delayed growth, and radiographs of the appendicular skeleton were obtained.

RESULTS

The afflicted kitten had marked hypocalcemia, mild hypophosphatemia and substantial elevations in alkaline phosphatase activity, as well as pathognomonic radiographic findings consistent with rickets. Skeletal changes and hypocalcemia prompted testing of concentrations of parathyroid hormone (PTH) and vitamin D metabolites. Endocrine testing demonstrated significant increases in serum concentrations of PTH and 1,25-dihydroxycholecalciferol (calcitriol), supporting a diagnosis of vitamin D-dependent rickets type 2. Provision of analgesia, supraphysiologic doses of calcitriol and calcium carbonate supplementation achieved normalization of the serum calcium concentration and restoration of normal growth, although some skeletal abnormalities persisted. Once skeletally mature, ongoing calcitriol supplementation was not required. Whole-exome sequencing (WES) was conducted to identify the underlying DNA variant. A cytosine deletion at cat chromosome position B4:76777621 in VDR (ENSFCAT00000029466:c.106delC) was identified and predicted to cause a stop codon in exon 2 (p.Arg36Glufs*18), disrupting >90% of the receptor. The variant was unique and homozygous in this patient and absent in the sibling and approximately 400 other cats for which whole-genome and whole-exome data were available.

CONCLUSIONS AND RELEVANCE

A unique, heritable form of rickets was diagnosed in a domestic longhair cat. WES identified a novel frameshift mutation affecting the gene coding for the vitamin D3 receptor, determining the likely causal genetic variant. Precision medicine techniques, including whole-exome and whole-genome sequencing, can be a standard of care in cats to identify disease etiologies, and to target therapeutics and personalize treatment.

Vitamin D-Dependent Rickets Type 3: A Case Report and Systematic Review

Although vitamin D deficiency resulting from insufficient sunlight exposure or inadequate dietary vitamin D intake is the most common cause of rickets, mutations in genes involved in vitamin D metabolism can cause genetic forms of rickets termed Vitamin D-Dependent Rickets (VDDR). In 2018, Roizen et al. described a new type of VDDR, named VDDR3, caused by a recurrent missense mutation in the CYP3A4 gene that leads to accelerated inactivation of vitamin D metabolites. Here, we describe the third case of VDDR3 due to the same CYP3A4 mutation in a 2-year-old boy with bone deformities associated with poor growth. As in the previously reported cases, this patient had no family history of rickets. Serial measurements of vitamin D metabolites after a single 150,000 IU dose of cholecalciferol demonstrated an accelerated inactivation of 25(OH)D and 1,25(OH)2D. Significant improvement in growth velocity and healing of bone deformities were achieved after a short period of treatment with 10.000 IU of cholecalciferol daily, showing the importance of early recognition and prompt precision therapy of this condition.

Genotype-phenotype Description of Vitamin D-dependent Rickets 1A: CYP27B1 p.(Ala129Thr) Variant Induces a Milder Disease

INTRODUCTION

Vitamin D-dependent rickets type 1A (VDDR1A) is a rare genetic disease associated with loss-of-function variations in the gene encoding the vitamin D-activating enzyme 1α-hydroxylase (CYP27B1). Phenotype-genotype correlation is unclear. Long-term outcome data are lacking. The objective of this study was to describe characteristics and outcomes to search for a phenotype-genotype correlation.

METHODS

We retrospectively collected clinical data, genetic features, and outcomes from 24 genetically confirmed cases from 10 French centers; results are presented as median (min-max).

RESULTS

Clinical symptoms at diagnosis (age, 1.5 [0.5-8.7] years) were mainly bone and neurological abnormalities, and laboratory data showed hypocalcemia (1.97 [1.40-2.40] mmol/L), hypophosphatemia (-3.4 [-13.4 to (-)0.2] SD score for age), low 25OHD and low 1,25(OH)2D3, secondary hyperparathyroidism with PTH at 6.6 (1.3-13.7) times the upper limit for normal (ULN; PTH expressed as ULN to homogenize data presentation), and increased alkaline phosphatase (1968 [521-7000] IU/L). Bone radiographs were abnormal in 83% of patients. We identified 17 variations (11 missense, 3 frameshift, 2 truncating, and 1 acceptor splice site variations) in 19 families (homozygous state in 58% [11/19]). The partial loss-of-function variation p.(Ala129Thr) was associated with a milder phenotype: older age at diagnosis, higher serum calcium (2.26 vs 1.85 mmol/L), lower PTH (4.7 vs 7.5 ULN), and lower alkaline phosphatase (759 vs 2082 IU/L). Patients were treated with alfacalcidol. Clinical (skeletal, neurological), biochemical, and radiological outcomes were satisfactory, and complications occurred if there was bad adherence.

CONCLUSION

Overall, our findings highlight good outcomes under substitutive treatment and the need of a closer follow-up of eyes, teeth, kidneys, and blood pressure in VDDR1A.

The genetic and epigenetic contributions to the development of nutritional rickets

Nutritional rickets is an important disease in global health. Although nutritional rickets commonly manifests as bony deformities, there is an increased risk of life-threatening seizures secondary to hypocalcaemia. Dietary vitamin D deficiency is associated with the development of nutritional rickets among children and infants. This is especially true in populations of darker skinned individuals in high-latitude environments due to decreased ultraviolet light exposure, and in populations in tropical and subtropical climates due to cultural practices. A growing body of evidence has demonstrated that genetic factors might influence the likelihood of developing nutritional rickets by influencing an individual's susceptibility to develop deficiencies in vitamin D and/or calcium. This evidence has been drawn from a variety of different techniques ranging from traditional twin studies to next generation sequencing techniques. Additionally, the role of the epigenome in the development of rickets, although poorly understood, may be related to the effects of DNA methylation and non-coding RNAs on genes involved in bone metabolism. This review aims to provide an overview of the current evidence that investigates the genetic and epigenetic determinants of nutritional rickets.

Development of In Vitro and In Vivo Evaluation Systems for Vitamin D Derivatives and Their Application to Drug Discovery

We have developed an in vitro system to easily examine the affinity for vitamin D receptor (VDR) and CYP24A1-mediated metabolism as two methods of assessing vitamin D derivatives. Vitamin D derivatives with high VDR affinity and resistance to CYP24A1-mediated metabolism could be good therapeutic agents. This system can effectively select vitamin D derivatives with these useful properties. We have also developed an in vivo system including a Cyp27b1-gene-deficient rat (a type I rickets model), a Vdr-gene-deficient rat (a type II rickets model), and a rat with a mutant Vdr (R270L) (another type II rickets model) using a genome editing method. For Cyp27b1-gene-deficient and Vdr mutant (R270L) rats, amelioration of rickets symptoms can be used as an index of the efficacy of vitamin D derivatives. Vdr-gene-deficient rats can be used to assess the activities of vitamin D derivatives specialized for actions not mediated by VDR. One of our original vitamin D derivatives, which displays high affinity VDR binding and resistance to CYP24A1-dependent metabolism, has shown good therapeutic effects in Vdr (R270L) rats, although further analysis is needed.

Two novel CYP2R1 mutations in a family with vitamin D-dependent rickets type 1b

PURPOSE

Vitamin D-dependent rickets type 1b (VDDR1b) is a very rare autosomal recessive disorder caused by mutations in CYP2R1 that produces 25-hydroxylase. To date only five mutations in CYP2R1 have been identified. This study has reported the genetic results and the clinical characteristics of a family with VDDR1b and compared this family to the other families with VDDR1b in literature.

METHODS

After two probands were diagnosed with VDDR1b, all other family members were evaluated. Serum calcium, phosphorus, alkaline phosphatase, parathyroid hormone, 25-hydroxy vitamin D, and 1.25-dihydroxy vitamin D levels were measured in all family members. All individuals were evaluated radiographically, and a genetic analysis was done in all family members. The other families with VDDR1b in literature were reviewed.

RESULTS

Two novel mutations [c.367 + 1G > C and p.E339Q (c.1015G > C)] were identified. The clinic and laboratory findings were strikingly different among the members of this family regardless of the mutation and the number of alleles affected. The families having different mutations in literature had also extensive variation in both the clinical and the laboratory findings.

CONCLUSION

The current study further expands CYP2R1 mutation spectrum. The findings of both the current and the previous studies suggest that VDDR1b is a more complex disorder than the known autosomal recessive inheritance model and the phenotype may show an extensive variation regardless of the mutation type and the gene dosage.

Does Genotype-Phenotype Correlation Exist in Vitamin D-Dependent Rickets Type IA: Report of 13 New Cases and Review of the Literature

Vitamin D-dependent rickets type IA (VDDR-IA) is caused by biallelic mutations in CYP27B1. Data regarding genotype-phenotype correlation in VDDR-IA are scarce. Here, we aimed to investigate clinical/genotypic features and long-term follow-up of 13 new cases with VDDR-IA and genotype-phenotype correlation of reported cases in the literature. Thirteen patients with VDDR-IA were evaluated. Eight patients had reached their final height at the time of the study and, for whom, long-term outcome data were analyzed. Further, all VDDR-IA patients in the literature (n:183) were analyzed and clinical-genetic features were recorded. The median age of diagnosis was 2.55 ± 1.13 (1.0-12) years. Initial diagnoses before referral to our clinic were nutritional rickets (n:7), hypophosphatemic rickets (n:2), and pseudohypoparathyroidism (n:1). All had biochemical evidence suggestive of VDDR-IA; except one with elevated 1,25(OH)₂D3 and another with hyperphosphatemia, in whom pseudohypoparathyroidism was excluded with molecular tests. Combined analyses of our cohort and other series in the literature demonstrated that three most common CYP27B1 mutations are p.F443Pfs*24, c.195 + 2T > G, and p.V88Wfs*71. In Turkish population, p.K192E mutation along with the former two is the most common mutations. Comparison of clinical features demonstrated that c.195 + 2T > G mutation causes the most severe and p.K192E mutation causes the least severe phenotype with respect to age and height at presentation and calcitriol requirement. We found a clear genotype-phenotype correlation in VDDR-IA, notably CYP27B1 intronic c.195 + 2T > G mutation causes a more severe phenotype with lower height SDS at presentation and, higher calcitriol requirement, while less severe phenotype occurs in p.K192E mutation.

Combination of furosemide and fludrocortisone as a loading test for diagnosis of distal renal tubular acidosis in a pediatric case

Renal tubular acidosis (RTA) is a rare disease caused by a defect of urinary acidification. The ammonium chloride loading test is the gold standard method for determining the type of RTA. However, because this test has some side effects (e.g., nausea, vomiting, and stomach discomfort), applying this test for pediatric cases is difficult. Recently, a loading test with the combination of furosemide and fludrocortisone was reported to be an alternative to the ammonium chloride loading test, with 100% sensitivity and specificity in adult's cases. We report the first pediatric case of distal RTA in a patient who was successfully diagnosed by a drug loading test with the combination of furosemide and fludrocortisone without any side effects. We also performed genetic analysis and detected a known pathogenic variant in the SLC4A1 gene. The combination loading test of furosemide and fludrocortisone is a useful and safe diagnostic tool for pediatric cases of RTA.

Earlier Onset in Autosomal Dominant Hypophosphatemic Rickets of R179 than R176 Mutations in Fibroblast Growth Factor 23: Report of 20 Chinese Cases and Review of the Literature

Autosomal dominant hypophosphatemic rickets (ADHR) is a rare hereditary disorder characterized by variant onset ages and diverse phenotypes. Our aim is to explore the genotype-phenotype correlations between ADHR patients with R176 and R179 mutations in FGF23 gene. Clinical manifestations, laboratory examinations, and genetic analyses were collected from 20 patients in six Chinese ADHR kindreds in our hospital. Previously published ADHR literatures were reviewed. Among 20 Chinese ADHR mutation carriers, 11 patients revealed overt symptoms. 10/11 (90.9%) of which were females. Patients with R179 mutations presented with earlier onset than those with R176 mutation [1.3 (1.0, 37.0) years vs. 28.5 (19.0, 44.0) years]. More patients with R179 mutations had a history of rickets with lower extremity deformity [3/4 (75%) vs. 1/7 (14.3%), p < 0.05]. The serum phosphate, i-FGF23 and c-FGF23 levels of patients with R179 and R176 mutations were 0.47 ± 0.14 mmol/L versus 0.57 ± 0.17 mmol/L, 79.6 ± 87.0 pg/mL versus 79.9 ± 107.4 pg/mL, and 33.4 ± 3.0 RU/mL versus 121.3 ± 177.6 RU/mL, respectively. 7/11 of patients had iron deficiency at onset of disease. When combined with previously reported seven ADHR families, difference was observed in the age of onset among symptomatic patients with R179 and R176 mutations [1.0 (0.9, 37.0) years vs. 24.5 (1.2, 57.0) years, p < 0.05]. Patients with R179 mutation were more likely to have rickets than R176 mutation (11/13, 84.6% vs. 5/20, 25.0%, p < 0.01) and lower extremity deformity (10/13, 76.9% vs. 6/19, 31.6%, p < 0.01). ADHR patients with R179 mutations had earlier onset age and more rickets compared to those with mutations in R176, which partially explained the clinical heterogeneity of ADHR.

A novel heterozygous mutation c.680A>G (p. N227S) in SLC34A1 gene leading to autosomal dominant hypophosphatemia: A case report

RATIONALE

Currently, the relationship between heterozygous mutations in SLC34A1 and hypophosphatemia is controversial. Here we report an autosomal dominant hypophosphatemia pedigree carrying a novel heterozygous mutation in SLC34A1.

PATIENT CONCERNS

The proband is a 32-year old young man, presented with progressive pain and weakness in his lower extremities for more than 5 years. The proband showed persistent hypophosphatemia and low TmPO4/GFR values, indicating renal phosphate leak. His grandfather, father, and one of his uncles showed the similar symptoms.

DIAGNOSES

Autosomal dominant hypophosphatemia.

INTERVENTIONS AND OUTCOMES

Phosphorus supplement was prescribed to the proband and his affected uncle. Both their serum phosphorus levels recovered to normal and their symptoms such as back pain and lower extremity weakness were completely relieved. Whole exome sequencing was performed to identify disease-causing mutations in proband.

LESSONS

A novel heterozygous missense mutation c.680A>G (p. N227S) in exon 7 of SLC34A1 was found in proband by whole exome sequencing, which was also found in other 4 family members of this pedigree. Our report of an autosomal dominant hypophosphatemia pedigree with 5 mutant carriers enriches the clinical phenotype caused by the SLC34A1 mutations and further affirms the heterozygous mutations are causative for hypophosphatemia.

The relationship between maternal and child bone density in Nigerian children with and without nutritional rickets

We found a positive relationship between bone density in Nigerian children with and without rickets and that of their mothers. After treatment, children with rickets had greater bone density than children without rickets, indicating that children genetically programmed to have greater bone density may have a higher risk of rickets.

INTRODUCTION

To determine the relationship between bone density in children with and without rickets and that of their mothers METHODS: Using an unmatched case-control design, forearm areal bone mineral density (aBMD) was measured in 52 and 135 Nigerian children with and without rickets and their mothers, respectively. We performed multivariate linear regression analyses to assess the relationship between maternal and child aBMD Z-scores.

RESULTS

Forearm aBMD Z-scores in children were associated with maternal aBMD Z-scores at metaphyseal (effect estimate 0.23; 95% CI 0.08 to 0.37) and diaphyseal (effect estimate 0.16; 0.01 to 0.30) sites, after adjustment for rickets in the child, child's age and sex, height-for-age Z-score, and weight-for-age Z-score. In the adjusted model, rickets was inversely associated with child's aBMD Z-score at the diaphyseal site only (- 0.45, - 0.65 to - 0.24). The positive relationship between maternal and child aBMD Z-scores was marginally greater in children with rickets (slope 0.56, r = 0.47) than without rickets (slope 0.19, r = 0.20) at the diaphyseal site only (P = 0.06 for interaction) but not at the metaphyseal site (slopes 0.35 and 0.30, respectively, P = 0.48). After treatment with calcium for 6 months, metaphyseal aBMD Z-scores were greater in children with treated rickets (effect estimate 0.26; 95% CI 0.02 to 0.49) than in those without rickets.

CONCLUSION

In Nigerian children with and without rickets, forearm aBMD Z-scores were positively associated with maternal aBMD Z-scores. Active rickets in the child marginally modified the relationship at the diaphyseal site only. After treatment, children with rickets had greater metaphyseal aBMD Z-scores than children without rickets.

Soluble Klotho causes hypomineralization in Klotho-deficient mice

The type I transmembrane protein αKlotho (Klotho) serves as a coreceptor for the phosphaturic hormone fibroblast growth factor 23 (FGF23) in kidney, while a truncated form of Klotho (soluble Klotho, sKL) is thought to exhibit multiple activities, including acting as a hormone, but whose mode(s) of action in different organ systems remains to be fully elucidated. FGF23 is expressed primarily in osteoblasts/osteocytes and aberrantly high levels in the circulation acting via signaling through an FGF receptor (FGFR)-Klotho coreceptor complex cause renal phosphate wasting and osteomalacia. We assessed the effects of exogenously added sKL on osteoblasts and bone using Klotho-deficient (kl/kl) mice and cell and organ cultures. sKL induced FGF23 signaling in bone and exacerbated the hypomineralization without exacerbating the hyperphosphatemia, hypercalcemia and hypervitaminosis D in kl/kl mice. The same effects were seen in rodent bone models in vitro, in which we also detected formation of a sKL complex with FGF23-FGFR and decreased Phex (gene responsible for X-linked hypophosphatemic rickets (XLH)/osteomalacia) expression. Further, sKL-FGF23-dependent hypomineralization in vitro was rescued by soluble PHEX. These data suggest that exogenously added sKL directly participates in FGF23 signaling in bone and that PHEX is a downstream effector of the sKL-FGF23-FGFR axis in bone.

CYP3A4 mutation causes vitamin D-dependent rickets type 3

Genetic forms of vitamin D-dependent rickets (VDDRs) are due to mutations impairing activation of vitamin D or decreasing vitamin D receptor responsiveness. Here we describe two unrelated patients with early-onset rickets, reduced serum levels of the vitamin D metabolites 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, and deficient responsiveness to parent and activated forms of vitamin D. Neither patient had a mutation in any genes known to cause VDDR; however, using whole exome sequencing analysis, we identified a recurrent de novo missense mutation, c.902T>C (p.I301T), in CYP3A4 in both subjects that alters the conformation of substrate recognition site 4 (SRS-4). In vitro, the mutant CYP3A4 oxidized 1,25-dihydroxyvitamin D with 10-fold greater activity than WT CYP3A4 and 2-fold greater activity than CYP24A1, the principal inactivator of vitamin D metabolites. As CYP3A4 mutations have not previously been linked to rickets, these findings provide insight into vitamin D metabolism and demonstrate that accelerated inactivation of vitamin D metabolites represents a mechanism for vitamin D deficiency.

Genetic Diseases of Vitamin D Metabolizing Enzymes

Vitamin D metabolism involves 3 highly specific cytochrome P450 (CYP) enzymes (25-hydroxylase, 1α-hydroxylase, and 24-hydroxylase) involved in the activation of vitamin D₃ to the hormonal form, 1,25-(OH)₂D₃, and the inactivation of 1,25-(OH)₂D₃ to biliary excretory products. Mutations of the activating enzymes CYP2R1 and CYP27B1 cause lack of normal 1,25-(OH)₂D₃ synthesis and result in rickets whereas mutations of the inactivating enzyme CYP24A1 cause build-up of excess 1,25-(OH)₂D₃ and result in hypercalcemia, nephrolithiasis, and nephrocalcinosis. This article reviews the literature for 3 clinical conditions. Symptoms, diagnosis, treatment, and management of vitamin D-dependent rickets and idiopathic infantile hypercalcemia are discussed.

Transgenic Expression of the Vitamin D Receptor Restricted to the Ileum, Cecum, and Colon of Vitamin D Receptor Knockout Mice Rescues Vitamin D Receptor-Dependent Rickets

Although the intestine plays the major role in 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] action on calcium homeostasis, the mechanisms involved remain incompletely understood. The established model of 1,25(OH)2D3-regulated intestinal calcium absorption postulates a critical role for the duodenum. However, the distal intestine is where 70% to 80% of ingested calcium is absorbed. To test directly the role of 1,25(OH)2D3 and the vitamin D receptor (VDR) in the distal intestine, three independent knockout (KO)/transgenic (TG) lines expressing VDR exclusively in the ileum, cecum, and colon were generated by breeding VDR KO mice with TG mice expressing human VDR (hVDR) under the control of the 9.5-kb caudal type homeobox 2 promoter. Mice from one TG line (KO/TG3) showed low VDR expression in the distal intestine (<50% of the levels observed in KO/TG1, KO/TG2, and wild-type mice). In the KO/TG mice, hVDR was not expressed in the duodenum, jejunum, kidney, or other tissues. Growth arrest, elevated parathyroid hormone level, and hypocalcemia of the VDR KO mice were prevented in mice from KO/TG lines 1 and 2. Microcomputed tomography analysis revealed that the expression of hVDR in the distal intestine of KO/TG1 and KO/TG2 mice rescued the bone defects associated with systemic VDR deficiency, including growth plate abnormalities and altered trabecular and cortical parameters. KO/TG3 mice showed rickets, but less severely than VDR KO mice. These findings show that expression of VDR exclusively in the distal intestine can prevent abnormalities in calcium homeostasis and bone mineralization associated with systemic VDR deficiency.

Genetic disorders of Vitamin D biosynthesis and degradation

Vitamin D, an inactive secosteroid pro-hormone, is produced by the action of ultraviolet light on 7-dehydrocholesterol in the skin. The active hormone, 1,25(OH)₂D is produced by sequential 25-hydroxylation in the liver, principally by CYP2R1, and 1α-hydroxylation in the kidney by CYP27B1. Mutations in CYP27B1 cause 1α-hydroxylase deficiency, also known as vitamin D dependent rickets type I or hereditary pseudo-vitamin D deficient rickets; very rare mutations in CYP2R1 can cause 25-hydroxylase deficiency. Both deficiencies cause hypocalcemia, secondary hyperparathyroidism, severe rickets in infancy, and low serum concentrations of 1,25(OH)₂D; both disorders respond to hormonal replacement therapy with calcitriol. The inactivation of vitamin D is principally initiated by its 23- and 24-hydroxylation by CYP24A1. Mutations in CYP24A1 can cause both severe neonatal hypercalcemia and a less severe adult hypercalcemic syndrome. Other pathways of vitamin D metabolism are under investigation, notably its 20-hydroxylation by the cholesterol side-chain cleavage enzyme, CYP11A1.

Sex-related differences in the skeletal phenotype of aged vitamin D receptor global knockout mice

The role of the vitamin D receptor (VDR) in maintaining skeletal health appears to be complex and dependent on the physiological context. Global Vdr deletion in a mouse model (Vdr^-/-) results in hypocalcemia, secondary hyperparathyroidism and bone features typical of vitamin D-dependent rickets type II. When weanling Vdr^-/- mice are fed a diet containing high levels of calcium, phosphorus and lactose, termed the rescue diet, normalisation of serum calcium, phosphate and parathyroid hormone levels results in prevention of rickets at 10 weeks of age. However, 17 week old male Vdr^-/- mice, fed the rescue diet, have been reported as osteopenic due to a decrease in bone formation when compared to wild type mice. We now report confirmation of this finding with further data on the effect of the rescue diet on appendicular and axial skeletal structures in male and female Vdr^-/- mice at 26 weeks of age compared to Vdr^+/- controls. All Vdr^-/- mice were normocalcemic with no evidence of any mineralization defect. However, male Vdr^-/- mice exhibited significantly reduced mineral in femoral and vertebral bones when compared to control littermate Vdr^+/- mice, consistent with the previously reported data. In contrast, 26-week-old female Vdr^-/- mice demonstrated significantly increased femoral trabecular bone volume although there was decreased vertebral trabecular bone volume, similar to males, and femoral cortical bone volume was unchanged. Thus, the Vdr^-/- mouse model displays sex- and site-specific differences in skeletal structures with long-term feeding of a rescue diet. Although the global Vdr^-/- ablation does not permit the determination of skeletal mechanisms producing these differences, these data confirm skeletal changes even when fed the rescue diet.

MUCOLIPIDOSIS II INFANTS PRESENTING WITH SKELETAL DEFORMITIES MIMICKING RICKETS AND A NEW MUTATION IN GNPTAB GENE

Mucolipidosis II or I-cell disease is a rare lysosomal enzyme hydrolase trafficking due to deficient activity of the multimeric enzyme UDP-Nacetylglucosamine-l-phosphotransferase. It is a severe inborn error of lysosomal storage that causes progressive multisystem deterioration and death within the first year of life. The diagnosis of ML II is often difficult in an infant due to clinical variety, phenotypic overlap and the enzyme analysis required. Mucolipidosis II and rickets may have similar physical, biochemical and radiographic findings in newborns. The diagnosis of Mucolipidosis II is often missed, as it may present with rickets-like picture. In this article, we describe two neonatal mucolipidosis II patients mimicking rickets, and we evaluated them by clinical, metabolic and imaging findings via literature and also emphasized the difficulties in diagnosis of this rare disease.

Novel Vitamin D Receptor Mutations in Hereditary Vitamin D Resistant Rickets in Chinese

Hereditary 1, 25-dihydroxyvitamin D-resistant rickets (HVDRR), a rare recessive disease, is caused by mutation in the VDR gene encoding the vitamin D receptor leading to the resistance to vitamin D. We described a female toddler with initial presentation of leg tenderness and clinical features of HVDRR including severe rickets, hypocalcemia and hypophosphatemia without alopecia. Genetic analysis revealed novel compound heterozygous mutations of p.M4I and p.H229Q in patient’s VDR gene. In cis p.M4I with FOKI-F eliminated both translation start sites of the VDR protein. The p.H229Q VDR exhibited significantly reduced VDR transactivation activity with intact dimerization with RXR. Our report expanded the mutation spectrum of HVDRR, and provided the first case of a benign variant p.M4I plus a common p.M1T polymorphism leading to a pathogenic allele.

CYP2R1 Mutations Impair Generation of 25-hydroxyvitamin D and Cause an Atypical Form of Vitamin D Deficiency

CONTEXT

Production of the active vitamin D hormone 1,25-dihydroxyvitamin D requires hepatic 25-hydroxylation of vitamin D. The CYP2R1 gene encodes the principal vitamin D 25-hydroxylase in humans.

OBJECTIVE

This study aimed to determine the prevalence of CYP2R1 mutations in Nigerian children with familial rickets and vitamin D deficiency and assess the functional effect on 25-hydroxylase activity.

DESIGN AND PARTICIPANTS

We sequenced the CYP2R1 gene in subjects with sporadic rickets and affected subjects from families in which more than one member had rickets.

MAIN OUTCOME MEASURES

Function of mutant CYP2R1 genes as assessed in vivo by serum 25-hydroxyvitamin D values after administration of vitamin D and in vitro by analysis of mutant forms of the CYP2R1.

RESULTS

CYP2R1 sequences were normal in 27 children with sporadic rickets, but missense mutations were identified in affected members of 2 of 12 families, a previously identified L99P, and a novel K242N. In silico analyses predicted that both substitutions would have deleterious effects on the variant proteins, and in vitro studies showed that K242N and L99P had markedly reduced or complete loss of 25-hydroxylase activity, respectively. Heterozygous subjects were less affected than homozygous subjects, and oral administration of vitamin D led to significantly lower increases in serum 25-hydroxyvitamin D in heterozygous than in control subjects, whereas homozygous subjects showed negligible increases.

CONCLUSION

These studies confirm that CYP2R1 is the principal 25-hydroxylase in humans and demonstrate that CYP2R1 alleles have dosage-dependent effects on vitamin D homeostasis. CYP2R1 mutations cause a novel form of genetic vitamin D deficiency with semidominant inheritance.

Normal bone mass and normocalcemia in adulthood despite homozygous vitamin D receptor mutations

Adding to the debate around vitamin D's effects on skeletal health, we report the long-term follow-up of two patients with severe vitamin D receptor mutations, who had normal bone mass acquisition and normalization of calcemia around puberty, suggesting that vitamin D might not be essential for skeletal health in adulthood.

INTRODUCTION

Vitamin D plays a pivotal role in calcium homeostasis, and the consequences of vitamin D insufficiency for skeletal health, as well as the importance of its supplementation, are a matter of great interest. Individuals bearing homozygous vitamin D receptor (VDR) defects present with severe hypocalcemic rickets in early infancy due to vitamin D resistance.

METHODS

Here, we report the follow-up of two patients with hereditary vitamin D-resistant rickets (HVDRR), focusing on bone mass acquisition and evolution of calcemia.

RESULTS

Patient 1 is a 30-year-old male bearing a homozygous p.Arg30* nonsense mutation in the VDR DNA-binding domain, who presented at 6 months. From 9 years of age, treatment requirement decreased progressively. Follow-up with DXA showed normal bone mass acquisition. In adulthood, he maintains normocalcemia without calcium supplementation and has no signs of bone fragility. Patient 2 is a 37-year-old female with milder HVDRR and alopecia due to a homozygous p.Gly319Val mutation in the VDR ligand-binding domain. Around puberty, hypercalciuria and kidney stones were detected, resulting in suspension of treatment. Follow-up with DXA revealed normal bone mass, and she maintained normocalcemia without supplementation during gestation and lactation.

CONCLUSIONS

The long-term follow-up of HVDRR provides insights into the role of vitamin D in human calcium homeostasis and bone health. The normalization of calcemia and normal bone mass acquisition despite a permanently dysfunctional VDR suggest that vitamin D might not be essential for skeletal health in adulthood. Extrapolation of these findings may have implications in broader clinical settings, especially considering widespread vitamin D supplementation.

Vitamin D and vitamin D dependency

Administration of large doses of vitamin D2 brought about a marked increase of 25-hydroxyvitamin D in both the patients with vitamin D dependency and hypophosphatemic vitamin D-resistant rickets. During the administration of vitamin D2, increment of 1,25-dihydroxyvitamin D was marked in hypophosphatemic vitamin D-resistant rickets, but far smaller in vitamin D dependency. In the latter, however, the 1,25-dihydroxyvitamin D reached the level close to the normal adult values. 1 alpha-hydroxyvitamin D3 was found 50 approximately 100 times as effective as vitamin D2 in 2 patients with vitamin D dependency (optimum maintenance dose: 0.05 micrograms/kg/day). It was concluded that 1 alpha-hydroxylation in the renal tubules is markedly defective in the patients with vitamin D dependency, but that a large dose of vitamin D2 is able to cause a definite increase in serum concentration of 1,25-dihydroxyvitamin D resulting in improvement of the rickets.

First report of a novel missense CLDN19 mutations causing familial hypomagnesemia with hypercalciuria and nephrocalcinosis in a Chinese family

Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is an autosomal recessive disorder caused by mutations in the CLDN16 or CLDN19 genes, encoding claudin-16 and claudin-19 in the thick ascending limb of Henle's loop. In patients with claudin-19 mutations, severe ocular involvement (macular coloboma, pigmentary retinitis, nystagmus, or visual loss) has been described. In this report, we presented a 12-year-old girl with rickets, polyuria, and polydipsia. She was the daughter of consanguineous parents, and she had a history of recurred hypocalcemic and hypomagnesemic tetany. On physical examination, bilateral horizontal nystagmus and severe myopia were detected. Laboratory examination revealed hypomagnesemia, hypocalcemia, hypercalciuria, nephrocalcinosis, and renal stone. A clinical diagnosis of FHHNC caused possibly by claudin-19 mutation was decided with the ocular findings. DNA analysis revealed a novel homozygous missense mutation c.241C>T in the CLDN19 gene. In conclusion, in a patient with hypomagnesemia, hypercalciuria, nephrocalcinosis, and ocular findings, a diagnosis of FHHNC caused by claudin-19 mutation should be considered. This is the first study of FHHNC in Chinese population. Our findings of the novel mutation c.241C>T in exon 2 add to the list of more than 16 mutations of CLDN19 gene reported.

Genetic rescue of glycosylation-deficient Fgf23 in the Galnt3 knockout mouse

Fibroblast growth factor 23 (FGF23) is a hormone that inhibits renal phosphate reabsorption and 1,25-dihydroxyvitamin D biosynthesis. The FGF23 subtilisin-like proprotein convertase recognition sequence ((176)RHTR(179)↓) is protected by O-glycosylation through ppGalNAc-T3 (GALNT3) activity. Thus, inactivating GALNT3 mutations render FGF23 susceptible to proteolysis, thereby reducing circulating intact hormone levels and leading to hyperphosphatemic familial tumoral calcinosis. To further delineate the role of glycosylation in the Fgf23 function, we generated an inducible FGF23 transgenic mouse expressing human mutant FGF23 (R176Q and R179Q) found in patients with autosomal dominant hypophosphatemic rickets (ADHR) and bred this animal to Galnt3 knockout mice, a model of familial tumoral calcinosis. Due to the low intact Fgf23 level, Galnt3 knockout mice with wild-type Fgf23 alleles were hyperphosphatemic. In contrast, carriers of the mutant FGF23 transgene, regardless of Galnt3 mutation status, had significantly higher serum intact FGF23, resulting in severe hypophosphatemia. Importantly, serum phosphorus and FGF23 were comparable between transgenic mice with or without normal Galnt3 alleles. To determine whether the presence of the ADHR mutation could improve biochemical and skeletal abnormalities in Galnt3-null mice, these mice were also mated to Fgf23 knock-in mice, carrying heterozygous or homozygous R176Q ADHR Fgf23 mutations. The knock-in mice with functional Galnt3 had normal Fgf23 but were slightly hypophosphatemic. The stabilized Fgf23 ADHR allele reversed the Galnt3-null phenotype and normalized total Fgf23, serum phosphorus, and bone Fgf23 mRNA. However, the skeletal phenotype was unaffected. In summary, these data demonstrate that O-glycosylation by ppGaINAc-T3 is only necessary for proper secretion of intact Fgf23 and, once secreted, does not affect Fgf23 function. Furthermore, the more stable Fgf23 ADHR mutant protein could normalize serum phosphorus in Galnt3 knockout mice.

A homozygous CaSR mutation causing a FHH phenotype completely masked by vitamin D deficiency presenting as rickets

CONTEXT

Heterozygous inactivating calcium-sensing receptor (CaSR) mutations lead to familial hypocalciuric hypercalcemia (FHH), whereas homozygous mutations usually cause neonatal severe hyperparathyroidism.

OBJECTIVE

The objective of the study was to investigate the pathophysiological mechanisms of a homozygous inactivating CaSR mutation identified in a 16-year-old female.

DESIGN

Clinical, biochemical, and genetic analyses of the index patient and her family were performed. Functional capacity of CaSRQ459R and CaSR mutants causing FHH (Q27R, P39A, S417C) or neonatal severe hyperparathyroidism (W718X) was assessed. Activation of the cytosolic calcium pathway and inhibition of PTH-induced cAMP signaling were measured.

RESULTS

A 16-year-old girl presented with adolescent rickets, vitamin D deficiency, and secondary hyperparathyroidism. Vitamin D treatment unmasked features resembling FHH, and genetic testing revealed a homozygous CaSRQ459R mutation. Two apparently healthy siblings were homozygous for CaSRQ459R and had asymptomatic hypercalcemia and hypocalciuria. The CaSRQ459R mutation leads to mild functional inactivation in vitro, which explains the FHH-like phenotype in homozygous family members and the grossly exaggerated PTH response to vitamin D deficiency in the index case. The patient's parents and two other siblings were heterozygous, had normal serum calcium and PTH, but had marked hypocalciuria, which appeared to be associated with impaired in vitro activation of the calcium signaling pathway by CaSRQ459R. The Q459R mutation responded well to calcimimetic treatment in vitro.

CONCLUSION

CaSR mutations causing mild functional impairment can lead to FHH, even in homozygous patients. The skeletal deformities in the index case were mainly due to severe vitamin D deficiency, and the CaSR mutation did not appear to have played a major independent role in the skeletal phenotype.

A novel compound mutation of CYP27B1 in a Chinese family with vitamin D-dependent rickets type 1A

OBJECTIVES

Mutations in the CYP27B1 gene, which encodes vitamin D 1α-hydroxylase, are the genetic basis of vitamin D-dependent rickets type 1A (VDDR1A, MIM 264700). The aim of this study was to investigate a novel CYP27B1 mutation and its clinical manifestations.

METHODS

VDDR1A was diagnosed based on clinical presentation, a physical examination, bone characteristics on an X-ray, and laboratory results. A molecular model of the CYP27B1 protein was constructed using the SWISS-MODEL server and Swiss-PdbViewer.

RESULTS

We sequenced the CYP27B1 gene in a 5-year-old male child who presented with growth retardation and a history of frequent hand, leg, and perioral twitching since the age of 12 months. We identified a compound heterozygous mutation consisting of two missense mutations: one in exon 7 (R389C [c.1165C>T]) and one in exon 8 (R459C [c.1375C>T]). We used the wild-type CYP27B1 as a receptor and calcidiol as a ligand to predict the interaction between the R459 site and calcidiol. According to the predicted structure, the wild-type R459 residue localizes to the pocket where CYP27B1 binds to its ligand.

CONCLUSIONS

According to the Human Gene Mutation Database, the compound heterozygous mutation identified in our patient is novel and has not yet been reported in the literature. This mutation provides a new basis for further research on VDDR1A and for the development of clinical diagnostics.

[Updates on rickets and osteomalacia: vitamin D dependency]

Vitamin D dependency was first termed for patients resembling vitamin D-deficiency but require high doses of vitamin D administration. Now this disease is known to be caused by defective conversion of 25OHD to 1,25 (OH) 2D, which is termed vitamin D-dependent rickets type 1 or 1α-hydroxylase deficiency, or by end-organ unresponsiveness to 1,25 (OH) 2D, which is called vitamin D-dependent rickets type 2 or hereditary vitamin D-resistant rickets. Recent advance in the molecular analysis of these diseases revealed variety in the presentation and in the inheritance patterns. Molecular diagnosis would be preferable for some atypical cases for adequate therapy.

[Updates on rickets and osteomalacia: various roles of Klotho and FGF23 in vivo ]

Recent understandings of phosphate regulation have, at least in part, depended upon the findings of fibroblast growth factor (FGF) 23, the hormone specific for phosphate regulation. Moreover, FGF23 would be the most important marker for prognosis in chronic kidney disease (CKD) . On the other hand, Klotho was firstly developed as a responsible gene for senescence. Although the role of Klotho has been so far established as a co-receptor for FGF23 in mineral metabolism, Klotho would play rather various roles than FGF23 signaling. Thus Klotho is not necessarily equivalent to FGF23. Further studies for both Klotho and FGF23 will elucidate to understand mineral homeostasis.

[Updates on rickets and osteomalacia: biological function of osteocyte and FGF23/klotho system]

Osteocytes are known to synthesize FGF23 which would bind to FGFR1c/klotho complex in proximal renal tubules in kidney, thereby, reducing serum concentration of Pi and the activity of 1α-hydroxylase. Meanwhile, recent studies suggest the possibility that osteocytes might induce osteolysis of lacuna walls. Compact, cortical bone develops well-organized distribution of osteocyte-lacunar canalicular system (OLCS) , which appears to be efficient for osteocytic function. There seems some relation between the geometrical regularity of OLCS and osteocytic regulation of systemic and local mineral balance.

[Updates on rickets and osteomalacia: the role of NaPi-2c/SLC34A3 and hypophosphataemic rickets]

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) , an autosomal recessive disorder first identified in a large Bedouin tribe, is characterized by hypophosphatemia secondary to renal inorganic phosphate (Pi) wasting, resulting in increased serum1,25-dihydroxyvitamin D3 concentrations with associated intestinal calcium hyperabsorption, hypercalciuria, rickets, and osteomalacia. Recent studies identified several mutations in the NaPi-2c/NPT2c transporter gene (SLC34A3) as the cause of HHRH. The fact that HHRH is caused by NaPi-2c loss-of-function mutations is compatible with the HHRH phenotype and the prevailing view of renal Pi regulation. The NaPi-2c mutants in HHRH show defective processing and stability.

Osteocyte regulation of phosphate homeostasis and bone mineralization underlies the pathophysiology of the heritable disorders of rickets and osteomalacia

Although recent studies have established that osteocytes function as secretory cells that regulate phosphate metabolism, the biomolecular mechanism(s) underlying these effects remain incompletely defined. However, investigations focusing on the pathogenesis of X-linked hypophosphatemia (XLH), autosomal dominant hypophosphatemic rickets (ADHR), and autosomal recessive hypophosphatemic rickets (ARHR), heritable disorders characterized by abnormal renal phosphate wasting and bone mineralization, have clearly implicated FGF23 as a central factor in osteocytes underlying renal phosphate wasting, documented new molecular pathways regulating FGF23 production, and revealed complementary abnormalities in osteocytes that regulate bone mineralization. The seminal observations leading to these discoveries were the following: 1) mutations in FGF23 cause ADHR by limiting cleavage of the bioactive intact molecule, at a subtilisin-like protein convertase (SPC) site, resulting in increased circulating FGF23 levels and hypophosphatemia; 2) mutations in DMP1 cause ARHR, not only by increasing serum FGF23, albeit by enhanced production and not limited cleavage, but also by limiting production of the active DMP1 component, the C-terminal fragment, resulting in dysregulated production of DKK1 and β-catenin, which contributes to impaired bone mineralization; and 3) mutations in PHEX cause XLH both by altering FGF23 proteolysis and production and causing dysregulated production of DKK1 and β-catenin, similar to abnormalities in ADHR and ARHR, but secondary to different central pathophysiological events. These discoveries indicate that ADHR, XLH, and ARHR represent three related heritable hypophosphatemic diseases that arise from mutations in, or dysregulation of, a single common gene product, FGF23 and, in ARHR and XLH, complimentary DMP1 and PHEX directed events that contribute to abnormal bone mineralization.

[Mutation analysis of FAH gene in patients with tyrosinemia type 1]

OBJECTIVE

To investigate the clinical features and mutations of the FAH gene.

METHOD

Clinical records of two cases were collected, and diagnosis was made according to the diagnostic criteria of the International Organization for Rare Disorders (NORD). Genomic DNA was extracted from peripheral blood leukocytes with QIAamp DNA Mini Kit. The DNA extracts were subjected to direct sequencing for 14 exons together with adjacent fragments of FAH gene using ABI Prism 3730 Genetic Analyzer (Applied Biosystems, Foster City, CA) after PCR based on genomic DNA. The mutation source was verified by analyzing parents' exons corresponding to patients' mutation exons. The homology between human FAH enzyme and that of other species was surveyed using software Clustal X(European Bioinformatics Institute, Hinxton, Saffron Walde, UK). Polyphen (Polymorphism Phenotyping), available online, were used to predict possible impact of an amino acid substitution on structure and function of FAH enzyme. Polyphen calculates position-specific independent counts (PISC) scores for two amino acid variants in polymorphic position. A PISC scores that differ by > 2 were regarded as indicating the probability of damaging variants.

RESULT

Patient 1 was a 5 months and 21 days-old boy who suffered from persistent diarrhea, hepatomegaly, ascites; Alpha-fetoprotein > 1210 µg/L, levels of tyrosine in blood and succinylacetone in urine were 110.8 µmol/L and 83.7 µmol/L. His sister suffered from tyrosinemia type 1. Direct sequencing showed a G to A transition in CDS position 455 and 1027. He was compound heterozygous for the mutation c.455G > A/c.1027G > A, which predicts a change from tryptophan to a stop codon (TGG > TAG) at position 152 (W152X) and a change from glycine to arginine (GGG > AGG) at position 343 respectively. Patient 2 was a 6 year and 1 month-old girl with late-onset rickets who had signs of hepatosplenomegaly, rachitic rosary, windswept knees. Hypophosphatemia and alkaline phosphatase 1620 IU/L were detected. Alpha-fetoprotein 412.8 µg/L, levels of tyrosine in blood and succinylacetone in urine were 835.8 µmol/L and 27.48 µmol/L. Rickets did not improve after administration of calcium and vitamine D3. She is homozygous for the mutation c.1027G > A/c.1027G > A, which predicts G343R. The parents were mutation carriers. Analysis by Clustal X on the alignment of amino acids residual reservation among different species showed that the locative amino acid was highly conserved. Polyphen software predicted G343R was probably damaging (PISC score 3.235).

CONCLUSION

Children with tyrosinemia type 1 can have manifestations of persistent diarrhea or late-onset rickets. Physical examination can reveal hepatosplenomegaly, laboratory tests indicate markedly elevated serum concentration of alpha-fetoprotein and alkaline phosphatase in plasma and succinylacetone in urine, other members in family may have tyrosinemias or parents are consanguineous. Mutations c.455G > A and c.1027G > A can be detected in FAH gene of Chinese children.

Vitamin D receptor in osteoblasts is a negative regulator of bone mass control

The physiological and beneficial actions of vitamin D in bone health have been experimentally and clinically proven in mammals. The active form of vitamin D [1α,25(OH)(2)D(3)] binds and activates its specific nuclear receptor, the vitamin D receptor (VDR). Activated VDR prevents the release of calcium from its storage in bone to serum by stimulating intestinal calcium absorption and renal reabsorption. However, the direct action of VDR in bone tissue is poorly understood because serum Ca(2+) homeostasis is maintained through tightly regulated ion transport by the kidney, intestine, and bone. In addition, conventional genetic approaches using VDR knockout (VDR-KO, VDR(-/-)) mice could not identify VDR action in bone because of the animals' systemic defects in calcium metabolism. In this study, we report that systemic VDR heterozygous KO (VDR(+/L-)) mice generated with the Cre/loxP system as well as conventional VDR heterozygotes (VDR(+/-)) showed increased bone mass in radiological assessments. Because mineral metabolism parameters were unaltered in both types of mice, these bone phenotypes imply that skeletal VDR plays a role in bone mass regulation. To confirm this assumption, osteoblast-specific VDR-KO (VDR(ΔOb/ΔOb)) mice were generated with 2.3 kb α1(I)-collagen promoter-Cre transgenic mice. They showed a bone mass increase without any dysregulation of mineral metabolism. Although bone formation parameters were not affected in bone histomorphometry, bone resorption was obviously reduced in VDR(ΔOb/ΔOb) mice because of decreased expression of receptor activator of nuclear factor kappa-B ligand (an essential molecule in osteoclastogenesis) in VDR(ΔOb/ΔOb) osteoblasts. These findings establish that VDR in osteoblasts is a negative regulator of bone mass control.

Fanconi-Bickel syndrome and autosomal recessive proximal tubulopathy with hypercalciuria (ARPTH) are allelic variants caused by GLUT2 mutations

CONTEXT

Many inherited disorders of calcium and phosphate homeostasis are unexplained at the molecular level.

OBJECTIVE

The objective of the study was to identify the molecular basis of phosphate and calcium abnormalities in two unrelated, consanguineous families.

PATIENTS

The affected members in family 1 presented with rickets due to profound urinary phosphate-wasting and hypophosphatemic rickets. In the previously reported family 2, patients presented with proximal renal tubulopathy and hypercalciuria yet normal or only mildly increased urinary phosphate excretion.

METHODS

Genome-wide linkage scans and direct nucleotide sequence analyses of candidate genes were performed. Transport of glucose and phosphate by glucose transporter 2 (GLUT2) was assessed using Xenopus oocytes. Renal sodium-phosphate cotransporter 2a and 2c (Npt2a and Npt2c) expressions were evaluated in transgenically rescued Glut2-null mice (tgGlut2-/-).

RESULTS

In both families, genetic mapping and sequence analysis of candidate genes led to the identification of two novel homozygous mutations (IVS4-2A>G and R124S, respectively) in GLUT2, the gene mutated in Fanconi-Bickel syndrome, a rare disease usually characterized by renal tubulopathy, impaired glucose homeostasis, and hepatomegaly. Xenopus oocytes expressing the [R124S]GLUT2 mutant showed a significant reduction in glucose transport, but neither wild-type nor mutant GLUT2 facilitated phosphate import or export; tgGlut2-/- mice demonstrated a profound reduction of Npt2c expression in the proximal renal tubules.

CONCLUSIONS

Homozygous mutations in the facilitative glucose transporter GLUT2, which cause Fanconi-Bickel syndrome, can lead to very different clinical and biochemical findings that are not limited to mild proximal renal tubulopathy but can include significant hypercalciuria and highly variable degrees of urinary phosphate-wasting and hypophosphatemia, possibly because of the impaired proximal tubular expression of Npt2c.

Novel mutations of CYP27B1 gene lead to reduced activity of 1α-hydroxylase in Chinese patients

Pseudovitamin D-deficiency rickets (PDDR) is an autosomal recessive disorder resulting from a defect in renal 25-hydroxyvitamin D 1α-hydroxylase, the key enzyme in the pathway of vitamin D metabolism. We identified ten different mutations in the 1α-hydroxylase gene (CYP27B1) in eight Chinese families with PDDR by DNA-sequence analysis. Six of them are novel missense mutations: G57V, G73W, L333F, R432C, R459C, and R492W; three are novel deletion mutations: c48-60del, c1310delG, and c1446delA; and an insertion mutation c1325-1332insCCCACCC reported previously. Functional assay revealed that the missense mutants identified in this study retain 5.5-12.1% 1α-hydroxylase activity of the wild type. The study describes nine novel mutations in addition to 37 known mutations of CYP27B1 gene and shows the correlation between these mutations and the clinical findings of 1α-hydroxylase deficiency.

Clinical and genetic analysis of patients with vitamin D-dependent rickets type 1A

CONTEXT

Vitamin D-dependent rickets type 1A (VDDR-IA, OMIM 264700) is a rare autosomal recessive disorder and is caused by mutations in the CYP27B1 gene.

OBJECTIVES

We aim to investigate CYP27B1 mutation in seven patients from four separate families and characterize the genotype-phenotype correlation.

METHODS

The entire coding region of the CYP27B1 gene was sequenced, and genotype-phenotype correlation among patients was assessed.

RESULTS

Sequencing analysis identified biallelic CYP27B1 mutations in all patients and monoallelic mutations in their parents. One patient from the first family was compound heterozygous for c.1166G>A (p.Arg389His) and a novel nonsense mutation c.1079 C>A (p.Ser360*). Two patients from the second family were homozygous for a novel splice donor site mutation in intron 1 (c.195 + 2 T>G), causing partial retention of the intron and a shift in the reading frame. Both novel mutations lead to the complete loss of vitamin D1α-hydroxylase activity. Four patients from families 3 and 4 were homozygous for a previously reported duplication mutation in exon 8 (1319-1325dupCCCACCC, Phe443Profs*24). Interestingly, one patient who was presented with severe hypocalcaemia and seizures at 4 months of age as a result of Phe443Profs*24 has improved spontaneously since 11 years of age and does not need regular treatment. Her laboratory tests showed normal serum calcium and 1,25(OH)(2) D after refusing to take medication for 12 months.

CONCLUSIONS

There is a good genotype-phenotype correlation in VDDR-IA. However, some patients may recover from the loss of CYP27B1 function, probably due to 1α-hydroxylase activity exerted by a non-CYP27B1 enzyme.

Absence of mutation in coding regions of CYP2R1 gene in apparent autosomal dominant vitamin D 25-hydroxylase deficiency rickets

CONTEXT

This is a case report of a proband and his family presenting with apparent autosomal dominant 25-hydroxylase enzyme deficiency and bone disease.

OBJECTIVE

The aim of the study was to present an alternative pattern of transmission of 25-hydroxylase enzyme deficiency other than autosomal recessive.

DESIGN

We diagnosed our patient with 25-hydroxylase enzyme deficiency, treated him, and examined his DNA for mutations. Evaluations at other institutions, clinical history of all family members, and laboratory testing for the mother, aunt, and maternal grandmother were reviewed. Family consent was obtained for all testing and record review.

SETTING

The patient was evaluated serially in an outpatient clinic. Routine laboratory studies for proband, mother, and maternal grandmother were obtained. Genetic testing was performed in another institution after obtaining family consent.

INTERVENTIONS

No research intervention was performed.

MAIN OUTCOME MEASURES

Routine clinical care included an evaluation of growth pattern. Laboratory evaluation included 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D [1,25(OH)(2)D], alkaline phosphatase, intact PTH, phosphate, calcium, ionized calcium, and magnesium levels.

RESULTS

Results showed consistently low 25-hydroxylase enzyme function. They also showed elevated alkaline phosphatase and PTH, as well as decreased phosphate and calcium. These abnormalities were corrected with 1,25(OH)(2)D therapy.

CONCLUSION

We conclude that 25-hydroxylase enzyme deficiency can be inherited in an autosomal dominant pattern. This case raises a question about different tributary pathways affecting the function of the 25-hydroxylase enzyme. Physiological dosing of 1,25(OH)(2)D was sufficient for treatment. There were no detectable mutations in any of the coding regions of the gene or its intronic junctions.

Rickets

Rickets is disorder of a growing child arising from disorders that result in impaired apoptosis of hypertrophic cells and mineralization of the growth plate. Rickets due to nutritional causes remains an important global problem. The factors responsible for resurgence of rickets among dark-skinned infants living in developed countries include the following: residence in northern or southern latitudes, voluntary avoidance of exposure to solar ultraviolet B radiation, maternal vitamin D deficiency during pregnancy, and prolonged breastfeeding without provision of vitamin D supplements. Fibroblast growth factor 23 (FGF23), secreted by osteocytes, is an important regulator of serum phosphate and 1,25(OH)(2)D(3) levels. Hypophosphatemic rickets resulting from increased synthesis or under-catabolism of FGF23 is reviewed.

Iron modifies plasma FGF23 differently in autosomal dominant hypophosphatemic rickets and healthy humans

CONTEXT

In autosomal dominant hypophosphatemic rickets (ADHR), fibroblast growth factor 23 (FGF23) resists cleavage, causing increased plasma FGF23 levels. The clinical phenotype includes variable onset during childhood or adulthood and waxing/waning of hypophosphatemia. Delayed onset after puberty in females suggests iron status may be important.

OBJECTIVE

Studies were performed to test the hypothesis that plasma C-terminal and intact FGF23 concentrations are related to serum iron concentrations in ADHR.

DESIGN AND SETTING

Cross-sectional and longitudinal studies of ADHR and a cross-sectional study in healthy subjects were conducted at an academic medical center.

PARTICIPANTS

Participants included 37 subjects with ADHR mutations from four kindreds and 158 healthy adult controls.

MAIN OUTCOME MEASURE

The relationships of serum iron concentrations with plasma C-terminal and intact FGF23 concentrations were evaluated.

RESULTS

Serum phosphate and 1,25-dihydroxyvitamin D correlated negatively with C-terminal FGF23 and intact FGF23 in ADHR but not in controls. Serum iron was negatively correlated to both C-terminal FGF23 (r = -0.386; P < 0.05) and intact FGF23 (r = -0.602; P < 0.0001) in ADHR. However, control subjects also demonstrated a negative relationship of serum iron with C-terminal FGF23 (r = -0.276; P < 0.001) but no relationship with intact FGF23. Longitudinally in ADHR subjects, C-terminal FGF23 and intact FGF23 concentrations changed negatively with iron concentrations (P < 0.001 and P = 0.055, respectively), serum phosphate changed negatively with C-terminal FGF23 and intact FGF23 (P < 0.001), and there was a positive relationship between serum iron and phosphate (P < 0.001).

CONCLUSIONS

Low serum iron is associated with elevated FGF23 in ADHR. However, in controls, low serum iron was also associated with elevated C-terminal FGF23, but not intact FGF23, suggesting cleavage maintains homeostasis despite increased FGF23 expression.

Mutation prediction by PolyPhen or functional assay, a detailed comparison of CYP27B1 missense mutations

Vitamin D-dependent rickets type 1 (VDDR-I) is caused by mutation in CYP27B1. The glycine residue at codon 102 is not conserved between human (G(102)) and rodent (S(102)). G102E mutation results in 80% reduction in its enzymatic activity but PolyPhen predicts benign change. It is not known whether G102S has any damaging effect on 1α-hydroxylase activity. We investigated the effect of CYP27B1 (G102S) on its enzymatic activity and compared mutation prediction accuracy for all known CYP27B1 mutations among three free online protein prediction programs: PolyPhen, PolyPhen-2, and PSIPRED. G102S has no damaging effect on 1α-hydroxylase activity. G102D retained 30% enzymatic activity. All three programs correctly predicted damaging change for G102D. PolyPhen predicted benign change for G102S, whereas PolyPhen-2 and PSIPRED indicated possible damaging effect. Among 24 reported damaging mutations, PSIPRED, PolyPhen-2, and PolyPhen achieved 100%, 91.7% (22/24), and 75% (18/24) accuracy rate, respectively. The residues of incorrectly predicted mutations were not conserved. We conclude that G102D resulted in a significant reduction in 1α-hydroxylase activity, whereas G102S did not. PSIPRED and PolyPhen-2 are superior to PolyPhen in predicting damaging mutations.

A novel nonsense mutation in the DMP1 gene identified by a genome-wide association study is responsible for inherited rickets in Corriedale sheep

Inherited rickets of Corriedale sheep is characterized by decreased growth rate, thoracic lordosis and angular limb deformities. Previous outcross and backcross studies implicate inheritance as a simple autosomal recessive disorder. A genome wide association study was conducted using the Illumina OvineSNP50 BeadChip on 20 related sheep comprising 17 affected and 3 carriers. A homozygous region of 125 consecutive single-nucleotide polymorphism (SNP) loci was identified in all affected sheep, covering a region of 6 Mb on ovine chromosome 6. Among 35 candidate genes in this region, the dentin matrix protein 1 gene (DMP1) was sequenced to reveal a nonsense mutation 250C/T on exon 6. This mutation introduced a stop codon (R145X) and could truncate C-terminal amino acids. Genotyping by PCR-RFLP for this mutation showed all 17 affected sheep were "T T" genotypes; the 3 carriers were "C T"; 24 phenotypically normal related sheep were either "C T" or "C C"; and 46 unrelated normal control sheep from other breeds were all "C C". The other SNPs in DMP1 were not concordant with the disease and can all be ruled out as candidates. Previous research has shown that mutations in the DMP1 gene are responsible for autosomal recessive hypophosphatemic rickets in humans. Dmp1_knockout mice exhibit rickets phenotypes. We believe the R145X mutation to be responsible for the inherited rickets found in Corriedale sheep. A simple diagnostic test can be designed to identify carriers with the defective "T" allele. Affected sheep could be used as animal models for this form of human rickets, and for further investigation of the role of DMP1 in phosphate homeostasis.

An outbreak of rickets in Corriedale sheep: Evidence for a genetic aetiology

CASE HISTORY

A skeletal disease characterised by lameness, limb deformities and reduced growth rate occurred over two successive years in lambs born on a commercial sheep farm in Marlborough. A genetic aetiology was considered likely following exclusion of other known causes of rickets and because of the progressive nature of the disease, even after affected animals were transferred to another property.

CLINICAL FINDINGS

Affected lambs appeared normal at birth but developed clinical signs during the first 2 months of life. The most severely affected animals either died or were euthanised within the first year of life, but some survived to breeding age. Serum biochemistry revealed hypocalcaemia, hypophosphataemia and increased concentrations of 1,25 dihydroxyvitamin D. The mean serum 25 hydroxyvitamin D concentration was similar to that of control lambs.

PATHOLOGICAL FINDINGS

Gross lesions included enlarged costochondral junctions, bilateral irregularity of articular surfaces on humeral heads due to collapse of subchondral bone, thickened cortices in long bones and irregular thickening of physeal cartilages. Microscopically, tongues of hypertrophic chondrocytes extended from physes into metaphyseal regions; metaphyseal trabeculae were thick, disorganised and often lined by wide osteoid seams. Osteoclastic activity was excessive both in cortical and trabecular bone.

DIAGNOSIS

Inherited rickets in Corriedale sheep. CLINICAL RELEVANCE AND CONCLUSIONS: This disease is likely to be present in several Corriedale sheep flocks in New Zealand and may have been misdiagnosed as arthritis or other diseases causing lameness and/or poor growth. A defect in end-organ responsiveness to 1,25 dihydroxyvitamin D is the likely mechanism. This disease of sheep may be a useful model for studying vitamin D metabolism and the treatment of inherited forms of rickets in human beings.

The receptor-dependent actions of 1,25-dihydroxyvitamin D are required for normal growth plate maturation in NPt2a knockout mice

Rickets is a growth plate abnormality observed in growing animals and humans. Rachitic expansion of the hypertrophic chondrocyte layer of the growth plate, in the setting of hypophosphatemia, is due to impaired apoptosis of these cells. Rickets is observed in humans and mice with X-linked hypophosphatemia that is associated with renal phosphate wasting secondary to elevated levels of fibroblast growth factor-23. Rickets is also seen in settings of impaired vitamin D action, due to elevated PTH levels that increase renal phosphate excretion. However, mice with hypophosphatemia secondary to ablation of the renal sodium-dependent phosphate transport protein 2a (Npt2a), have not been reported to develop rickets. Because activation of the mitochondrial apoptotic pathway by phosphate is required for hypertrophic chondrocyte apoptosis in vivo, investigations were undertaken to address this paradox. Analyses of the Npt2a null growth plate demonstrate expansion of the hypertrophic chondrocyte layer at 2 wk of age, with resolution of this abnormality by 5 wk of age. This is temporally associated with an increase in circulating levels of 1,25-dihydroxyvitamin D. To address whether the receptor-dependent actions of this steroid hormone are required for normalization of the growth plate phenotype, the Npt2a null mice were mated with mice lacking the vitamin D receptor or were rendered vitamin D deficient. These studies demonstrate that the receptor-dependent actions of 1,25-dihydroxyvitamin D are required for maintenance of a normal growth plate phenotype in the Npt2a null mice.

A novel G102E mutation of CYP27B1 in a large family with vitamin D-dependent rickets type 1

CONTEXT

Mutations in the CYP27B1 gene, which encodes vitamin D 1alpha-hydroxylase, are the genetic basis for vitamin D-dependent rickets type 1 (VDDR-I).

OBJECTIVE

The aim of this study was to investigate the CYP27B1 mutation in a large family with VDDR-I and characterize the genotype-phenotype correlation.

PATIENTS AND METHODS

The index patient was a 23-yr-old female who had a progressive form of rickets and growth retardation since the age of 9 months. Laboratory data showed hypocalcemia, low urine calcium, hypophosphatemia, high serum alkaline phosphatase, elevated PTH, and low serum 1,25-dihydroxyvitamin D(3). Her parents were healthy first-degree cousins, and two of her 12 siblings were affected with similar but milder rickets. Three other siblings were asymptomatic but had biochemical evidence of the disease. The entire coding region of the CYP27B1 gene was sequenced, and the mutation was characterized by functional studies.

RESULTS

We found a novel biallelic c.305G>A sequence variation at codon 102, changing amino acid from glycine to glutamic acid (G102E) in the patient and five affected siblings, whereas a monoallelic c.305G>A variation was present in the mother and five nonaffected siblings. This variation was not present in 100 population controls. Expression of this mutant in CHO cells revealed an 80% reduction in the 1alpha-hydroxylase activity as compared to wild-type activity.

CONCLUSIONS

A novel mutation in the CYP27B1 gene was found in patients with VDDR-I. This mutation resulted in a significant reduction in 1alpha-hydroxylase activity. The residual enzymatic activity may account for the mild phenotype presentation in some affected members.