A familial syndrome of decrease in sensitivity to 1,25-dihydroxyvitamin D

Differential impact of environmental factors on systemic and localized autoimmunity

The influence of environmental factors on the development of autoimmune disease is being broadly investigated to better understand the multifactorial nature of autoimmune pathogenesis and to identify potential areas of intervention. Areas of particular interest include the influence of lifestyle, nutrition, and vitamin deficiencies on autoimmunity and chronic inflammation. In this review, we discuss how particular lifestyles and dietary patterns may contribute to or modulate autoimmunity. We explored this concept through a spectrum of several autoimmune diseases including Multiple Sclerosis (MS), Systemic Lupus Erythematosus (SLE) and Alopecia Areata (AA) affecting the central nervous system, whole body, and the hair follicles, respectively. A clear commonality between the autoimmune conditions of interest here is low Vitamin D, a well-researched hormone in the context of autoimmunity with pleiotropic immunomodulatory and anti-inflammatory effects. While low levels are often correlated with disease activity and progression in MS and AA, the relationship is less clear in SLE. Despite strong associations with autoimmunity, we lack conclusive evidence which elucidates its role in contributing to pathogenesis or simply as a result of chronic inflammation. In a similar vein, other vitamins impacting the development and course of these diseases are explored in this review, and overall diet and lifestyle. Recent work exploring the effects of dietary interventions on MS showed that a balanced diet was linked to improvement in clinical parameters, comorbid conditions, and overall quality of life for patients. In patients with MS, SLE and AA, certain diets and supplements are linked to lower incidence and improved symptoms. Conversely, obesity during adolescence was linked with higher incidence of MS while in SLE it was associated with organ damage. Autoimmunity is thought to emerge from the complex interplay between environmental factors and genetic background. Although the scope of this review focuses on environmental factors, it is imperative to elaborate the interaction between genetic susceptibility and environment due to the multifactorial origin of these disease. Here, we offer a comprehensive review about the influence of recent environmental and lifestyle factors on these autoimmune diseases and potential translation into therapeutic interventions.

Hereditary Vitamin-D Dependent Rickets Type II: A Case Report

Hereditary vitamin D dependent rickets type II is a rare genetic disorder in children characterized by early onset of rickets and deranged biochemical parameters. Low serum calcium level, high alkaline phosphatase, high parathyroid hormone, and high values of 1,25-dihydroxy vitamin D are characteristic biochemical findings. We are reporting a rare case of Vitamin D Dependent Rickets and subsequent improvement after addition of cinacalcet. This is a case report of a 2.5-year-child with Hereditary Vitamin D Dependent Rickets type II receiving cinacalcet as adjunct to oral calcium and calcitriol. Oral cinacalcet (0.25mg/kg/day) was added to the regimen as an adjunct after treatment failure with high dose of oral calcium and calcitriol. A significant improvement in radiological findings and normal homeostasis of calcium, phosphate and parathyroid hormone was achieved after initiation of cinacalcet.

Rickets

Rickets is a bone disease associated with abnormal serum calcium and phosphate levels. The clinical presentation is heterogeneous and depends on the age of onset and pathogenesis but includes bowing deformities of the legs, short stature and widening of joints. The disorder can be caused by nutritional deficiencies or genetic defects. Mutations in genes encoding proteins involved in vitamin D metabolism or action, fibroblast growth factor 23 (FGF23) production or degradation, renal phosphate handling or bone mineralization have been identified. The prevalence of nutritional rickets has substantially declined compared with the prevalence 200 years ago, but the condition has been re-emerging even in some well-resourced countries; prematurely born infants or breastfed infants who have dark skin types are particularly at risk. Diagnosis is usually established by medical history, physical examination, biochemical tests and radiography. Prevention is possible only for nutritional rickets and includes supplementation or food fortification with calcium and vitamin D either alone or in combination with sunlight exposure. Treatment of typical nutritional rickets includes calcium and/or vitamin D supplementation, although instances infrequently occur in which phosphate repletion may be necessary. Management of heritable types of rickets associated with defects in vitamin D metabolism or activation involves the administration of vitamin D metabolites. Oral phosphate supplementation is usually indicated for FGF23-independent phosphopenic rickets, whereas the conventional treatment of FGF23-dependent types of rickets includes a combination of phosphate and activated vitamin D; an anti-FGF23 antibody has shown promising results and is under further study.

Genomic Determinants of Vitamin D-Regulated Gene Expression

Insight into mechanisms that link the actions of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) to the regulation of gene expression has evolved extensively since the initial discovery of a nuclear protein known as the vitamin D receptor (VDR). Perhaps most important was the molecular cloning of this receptor which enabled its inclusion within the nuclear receptor gene family and further studies of both its structure and regulatory function. Current studies are now refocused on the vitamin D hormone's action at the genome, where VDR together with other transcription factors coordinates the recruitment of chromatin active coregulatory complexes that participate directly in the modification of gene output. These studies highlight the role of chromatin in the expression of genes and the dynamic impact of the epigenetic landscape that contextualizes individual gene loci thus influencing the VDR's transcriptional actions. In this chapter, we summarize advances made over the past few years in understanding vitamin D action on a genome-wide scale, focusing on overarching principles that have emerged at this level. Of particular significance is the finding that dynamic changes that occur to the genome during cellular differentiation at both genetic and epigenetic levels profoundly alter the ability of 1,25(OH)2D3 and its receptor to regulate gene expression. We address the broad impact of differentiation on specific epigenetic histone modifications that occur across the genome and the ability of the VDR to influence this activity at selected gene loci as well. These studies advance our understanding of not only vitamin D action but also of the complex and dynamic role played by the genome itself as a major determinant of VDR activity.

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.

A humanized mouse model of hereditary 1,25-dihydroxyvitamin D-resistant rickets without alopecia

The syndrome of hereditary 1,25-dihydroxyvitamin D-resistant rickets (HVDRR) is a genetic disease of altered mineral homeostasis due to mutations in the vitamin D receptor (VDR) gene. It is frequently, but not always, accompanied by the presence of alopecia. Mouse models that recapitulate this syndrome have been prepared through genetic deletion of the Vdr gene and are characterized by the presence of rickets and alopecia. Subsequent studies have revealed that VDR expression in hair follicle keratinocytes protects against alopecia and that this activity is independent of the protein's ability to bind 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. In the present study, we introduced into VDR-null mice a human VDR (hVDR) bacterial artificial chromosome minigene containing a mutation that converts leucine to serine at amino acid 233 in the hVDR protein, which prevents 1,25(OH)2D3 binding. We then assessed whether this transgene recreated features of the HVDRR syndrome without alopecia. RT-PCR and Western blot analysis in one strain showed an appropriate level of mutant hVDR expression in all tissues examined including skin. The hVDR-L233S mutant failed to rescue the aberrant systemic and skeletal phenotype characteristic of the VDR null mouse due to the inability of the mutant receptor to activate transcription after treatment with 1,25(OH)2D3. Importantly, however, neither alopecia nor the dermal cysts characteristic of VDR-null mice were observed in the skin of these hVDR-L233S mutant mice. This study confirms that we have created a humanized mouse model of HVDRR without alopecia that will be useful in defining additional features of this syndrome and in identifying potential novel functions of the unoccupied VDR.

How vitamin D works on bone

Vitamin D is important for the normal development and maintenance of bone. The elucidation of the vitamin D activation pathway and the cloning of the vitamin D receptor have advanced our understanding of the actions of vitamin D on bone. The preponderance of evidence indicates that 1,25(OH)₂D₃ enhances bone mineralization through its effects to promote calcium and phosphate absorption. Although 1,25(OH)₂D₃ stimulates bone resorption in vitro, treatment in vivo can prevent bone loss and fracture through several potential mechanisms. The development of vitamin D analogues has provided new therapeutic options for increasing bone mineral density and reducing fractures.

Genetic disorders and defects in vitamin D action

The biochemical and genetic analysis of the VDR in patients with HVDRR has yielded important insights into the structure and function of the receptor in mediating 1,25(OH)2D3 action. Similarly, study of children affected by HVDRR continues to provide a more complete understanding of the biologic role of 1,25(OH)2D3 in vivo. A concerted investigative approach to HVDRR at the clinical, cellular, and molecular levels has proved valuable in gaining knowledge of the functions of the domains of the VDR and elucidating the detailed mechanism of action of 1,25(OH)2D3. These studies have been essential to promote the well-being of the families with HVDRR and in improving the diagnostic and clinical management of this rare genetic disease.

Regulation of target gene expression by the vitamin D receptor - an update on mechanisms

Virtually all of the known biological actions of the hormonal ligand 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) are mediated by the vitamin D receptor (VDR). Following binding and activation by the ligand, the VDR localizes in the nucleus to the regulatory regions of target genes and recruits chromatin-active coregulatory complexes which, in turn, modulate transcriptional output. The failure of the VDR to function due to crippling mutations results in total hereditary resistance to 1,25(OH)(2)D(3) in both mice and humans. In this review, we summarize the structural and functional properties of the VDR and the role of 1,25(OH)(2)D(3) in receptor activation, and then describe the results of recent studies using genome-wide analyses that define the overarching principles through which the VDR modulates genes expression. We also focus on the recent analysis of a specific 1,25(OH)(2)D(3) regulated gene that provides confirmation of the principles identified through these genome-wide methodologies. Taken together, these studies suggest an unanticipated increase in the complexity of the molecular processes that govern gene regulation by hormones and other factors.

The role of vitamin D receptor mutations in the development of alopecia

Hereditary Vitamin D Resistant Rickets (HVDRR) is a rare disease caused by mutations in the vitamin D receptor (VDR). The consequence of defective VDR is the inability to absorb calcium normally in the intestine. This leads to a constellation of metabolic abnormalities including hypocalcemia, secondary hyperparathyroidism and hypophosphatemia that cause the development of rickets at an early age in affected children. An interesting additional abnormality is the presence of alopecia in some children depending on the nature of the VDR mutation. The data indicate that VDR mutations that cause defects in DNA binding, RXR heterodimerization or absence of the VDR cause alopecia while mutations that alter VDR affinity for 1,25(OH)(2)D(3) or disrupt coactivator interactions do not cause alopecia. The cumulative findings indicate that hair follicle cycling is dependent on unliganded actions of the VDR. Further research is ongoing to elucidate the role of the VDR in hair growth and differentiation.

Vitamin D metabolism and rickets in domestic animals: a review

Rickets and osteomalacia are increasing in prevalence in people because of cultural practices, breast-feeding, decreased sun exposure, and increased sunscreen usage. Several hereditary forms of rickets owing to either renal phosphate wasting or defects in vitamin D metabolism are also reported in people. Rickets is well recognized in domestic animals, but published reports are not always supported by microscopic findings, and diagnoses based on clinical signs and radiology are unreliable. Most cases in domestic animals are caused by dietary deficiency of either vitamin D or phosphorus, but occasional inherited forms are reported in pigs, sheep, cats, and dogs. There is variation between species in susceptibility to dietary vitamin D and phosphorus deficiency and in the ability to manufacture vitamin D in their skin. A number of mouse models have been discovered or created to study human skeletal diseases and skeletal homeostasis. With the discovery that vitamin D is involved in not only calcium and phosphorus homeostasis but also in the immune system and cancer, there is great potential for new and existing animal models to generate valuable information about vitamin D and its many functions. This review presents an overview of vitamin D metabolism and rickets in domestic and laboratory animals and makes comparisons where appropriate with the disease in humans.

The calcium-sensing receptor and 25-hydroxyvitamin D-1alpha-hydroxylase interact to modulate skeletal growth and bone turnover

We examined parathyroid and skeletal function in 3-month-old mice expressing the null mutation for 25-hydroxyvitamin D-1alpha-hydroxylase [1alpha(OH)ase(-/-)] and in mice expressing the null mutation for both the 1alpha(OH)ase and the calcium-sensing receptor [Casr(-/-)1alpha(OH)ase(-/-)] genes. On a normal diet, all mice were hypocalcemic, with markedly increased parathyroid hormone (PTH), increased trabecular bone volume, increased osteoblast activity, poorly mineralized bone, enlarged and distorted cartilaginous growth plates, and marked growth retardation, especially in the compound mutants. Osteoclast numbers were reduced in the Casr(-/-)1alpha(OH)ase(-/-) mice. On a high-lactose, high-calcium, high-phosphorus "rescue" diet, serum calcium and PTH were normal in the 1alpha(OH)ase(-/-) mice but increased in the Casr(-/-)1alpha(OH)ase(-/-) mice with reduced serum phosphorus. Growth plate architecture and mineralization were improved in both mutants, but linear growth of the double mutants remained abnormal. Mineralization of bone improved in all mice, but osteoblast activity and trabecular bone volume remained elevated in the Casr(-/-)1alpha(OH)ase(-/-) mice. These studies support a role for calcium-stimulated maturation of the cartilaginous growth plate and mineralization of the growth plate and bone and calcium-stimulated CaSR-mediated effects on bone resorption. PTH-mediated bone resorption may require calcium-stimulated CaSR-mediated enhancement of osteoclastic activity. (c) 2010 American Society for Bone and Mineral Research.

Genetic disorders and defects in vitamin d action

Two rare genetic diseases can cause rickets in children. The critical enzyme to synthesize calcitriol from 25-hydroxyvitamin D, the circulating hormone precursor, is 25-hydroxyvitamin D-1alpha-hydroxylase (1alpha-hydroxylase). When this enzyme is defective and calcitriol can no longer be synthesized, the disease 1alpha-hydroxylase deficiency develops. The disease is also known as vitamin D-dependent rickets type 1 or pseudovitamin D deficiency rickets. When the VDR is defective, the disease hereditary vitamin D-resistant rickets, also known as vitamin D-dependent rickets type 2, develops. Both diseases are rare autosomal recessive disorders characterized by hypocalcemia, secondary hyperparathyroidism, and early onset severe rickets. In this article, these 2 genetic childhood diseases, which present similarly with hypocalcemia and rickets in infancy, are discussed and compared.

Improvement of impaired calcium and skeletal homeostasis in vitamin D receptor knockout mice by a high dose of calcitriol and maxacalcitol

Vitamin D plays a major role in mineral and skeletal homeostasis through interaction with the nuclear vitamin D receptor (VDR) of target cells. Recent reports have indicated that some cellular effects of vitamin D may occur via alternative signaling pathways, but concrete evidence for mineral homeostasis has not been shown in vivo. To investigate this issue, the actions of calcitriol (1,25D) and maxacalcitol (OCT), which were developed for treatment of uremia-induced secondary hyperparathyroidism, were analyzed in VDR knockout (VDR(-/-)) mice. The VDR(-/-) mice were fed a rescue diet immediately after weaning. 1,25D, OCT or a control solution was administered intraperitoneally to these mice three times a week for eight weeks. Biological markers and bone growth were measured and bone histomorphometric analysis of the calcein-labeled tibia was performed 24 h after the final administration. Significantly higher levels of serum Ca(2+) were observed in 1,25D- and OCT-treated mice, but the serum parathyroid hormone level was unchanged by both agents. Impaired bone growth, enlarged and distorted cartilaginous growth plates, morphological abnormalities of cancellous and cortical bones; a morbid osteoid increase, lack of calcein labeling, and thinning of cortical bone, were all significantly improved by 1,25D and OCT. The significance of these effects was confirmed by bone histomorphometrical analysis. Upregulation of the calbindin D(9k) mRNA expression level in the duodenum may explain these findings, since this protein is a major modulator of Ca transport in the small intestine. We conclude that 1,25D and OCT both at a high dose exert significant effects on Ca and skeletal homeostasis with the principal improvement of Ca status in VDR(-/-) mice, and some of these effects may occur through an alternative vitamin D signaling pathway.

Metabolic Bone Disease in Children

Metabolic bone disease in children includes many hereditary and acquired conditions of diverse etiology that lead to disturbed metabolism of the bone tissue. Some of these processes primarily affect bone; others are secondary to nutritional deficiencies, a variety of chronic disorders, and/or treatment with some drugs. Some of these disorders are rare, but some present public health concerns (for instance, rickets) that have been well known for many years but still persist. The most important clinical consequences of bone metabolic diseases in the pediatric population include reduced linear growth, bone deformations, and non-traumatic fractures leading to bone pain, deterioration of motor development and disability. In this article, we analyze primary and secondary osteoporosis, rickets, osteomalacia (nutritional and hereditary vitamin D-dependent, hypophosphatemic and that due to renal tubular abnormalities), renal osteodystrophy, sclerosing bony disorders, and some genetic bone diseases (hypophosphatasia, fibrous dysplasia, skeletal dysplasia, juvenile Paget disease, familial expansile osteolysis, and osteoporosis pseudoglioma syndrome). Early identification and treatment of potential risk factors is essential for skeletal health in adulthood. In most conditions it is necessary to ensure an appropriate diet, with calcium and vitamin D, and an adequate amount of physical activity as a means of prevention. In secondary bone diseases, treatment of the primary disorder is crucial. Most genetic disorders await prospective gene therapies, while bone marrow transplantation has been attempted in other disorders. At present, affected patients are treated symptomatically, frequently by interdisciplinary teams. The role of exercise and pharmacologic therapy with calcium, vitamin D, phosphate, bisphosphonates, calcitonin, sex hormones, growth hormone, and thiazides is discussed. The perspectives on future therapy with insulin-like growth factor-1, new analogs of vitamin D, strontium, osteoprotegerin, and calcimimetics are presented.

Genetic hair and nail disorders

Hair and nails are skin appendages that share with other ectodermal tissues a common developmental pathway. Inherited disorders affecting these two structures therefore very often involve other epithelial components and present with multiple anomalies, generating both physical and psychological distress among patients and their families. The present review briefly describes major recent advances in our understanding of hair and nail genodermatoses.

Celiac Disease Manifesting as Isolated Hypocalcemia

OBJECTIVE

To describe a patient who presented with hypocalcemia and hypocalciuria as the initial manifestations of celiac disease, despite a normal vitamin D status.

METHODS

We review the diagnostic evaluation, treatment, and biochemical and bone mineral density responses of a patient with asymptomatic celiac disease, which was initially suggested because of a low serum calcium level that became attributable to isolated malabsorption of calcium.

RESULTS

A 36-year-old woman presented with hypocalcemia in the presence of normal serum 25-hydroxyvitamin D and high serum 1,25-dihydroxyvitamin D levels. She had hypocalciuria and secondary hyperparathyroidism that were refractory to pharmacologic calcium and cholecalciferol supplementation. Fecal calcium excretion indicated malabsorption of calcium, and biopsy of the small intestine demonstrated pathologic changes characteristic of celiac disease. Bone mineral density, determined by dual-energy x-ray absorptiometry, was in the osteopenic range at the femoral neck. The initiation of a gluten-free diet resulted in correction of all biochemical abnormalities and a substantial increase in bone mineral density.

CONCLUSION

Primary intestinal malabsorption of calcium without concomitant vitamin D deficiency is possible in celiac disease because of the preferential involvement of the proximal small intestine early in the disease process. Our patient had hypocalcemia caused by celiac disease and values for serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D that were normal and elevated, respectively. Correction was demonstrated after dietary gluten withdrawal.

Vitamin D-dependent rickets Type II with alopecia: two case reports and review of the literature

Vitamin D-dependent rickets Type II is a rare autosomal recessive disorder. It usually presents with rachitic changes not responsive to Vitamin D treatment with elevated circulating levels of 1,25-Dihydroxyvitamin D3, thus differentiating it from Vitamin D-dependent rickets Type I. Alopecia of the scalp or the body is seen in some families with Vitamin D-dependent rickets Type II. This is usually associated with more severe resistance to Vitamin D. We report two Saudi brothers with this disease, and review the salient features of this disease with emphasis on the associated alopecia.

Molecular basis for pseudo vitamin D-deficiency rickets in the Hannover pig

The molecular basis for pseudo vitamin D deficiency rickets (PDDR) in the Hannover pig model was determined in the current study. Consistent with the inability of Hannover PDDR pigs to maintain ambient levels of 1,25-dihydroxyvitamin D (i.e., 1,25D), the bioactivation enzyme cytochrome P450C1 (or CYP27B1) was determined to contain coding-region deletions that rendered the enzyme ineffective due to frame-shift mutations and expression of a premature termination codon. Expression levels of P450C1mRNA were up-regulated in response to the low-1,25D high-parathyroid hormone state of the PDDR animals. In a complementary manner, cytochrome P450C24 mRNA was not detectable in PDDR pigs. Two different deletions were detected within the Hannover pig strain in which the P450C1 coding region contained either 173 bp or 329 bp deletions that resulted in the expression of non-sense products beginning within the I-helix region and extending through the truncated C-terminal domains. The boundaries for the deletion segments aligned with derived mRNA processing sites. This observation was consistent with an mRNA processing error as the causative factor for the coding-region deletions. Based upon the expression of a non-functional P450C1 enzyme, the Hannover pig model for PDDR was determined to be identical to the human disease in which enzyme-inhibitory mutations are the molecular basis for the calcium disorder.

Vitamin D dependent rickets type II and normal vitamin D receptor cDNA sequence. A cluster in a rural area of Cauca, Colombia, with more than 200 affected children

Vitamin D dependent rickets type II is an autosomal recessive disease caused by the vitamin D defective receptor. More than 200 patients with different types of lower limb deformities were detected in a rural area of the Cauca department in the southwest part of Colombia. Patients were well nourished and in good physical condition in spite of their deformities. None of them presented alopecia, myopathy, seizures or aminoaciduria. Serum analysis showed significantly lower serum calcium as compared to normal relatives, though in the normal low range, normal phosphorus, high alkaline phosphatase, normal 25-hydroxyvitamin D3 and high 1,25-dihydroxyvitamin D3, indicating target organ resistance. The cDNA analysis showed normal nucleotide sequence. We suggest that our patients represent a distinct form of receptor-positive resistance to vitamin D. This report is the first extensive study on this class of patients.

Calcitriol-resistant rickets due to vitamin D receptor defects

Calcitriol-resistant rickets (CRR) is an autosomal recessive disease due to a defect in the vitamin D receptor (VDR) or a site distal to it. The main characteristics are extreme rickets, with growth attenuation, osteomalacia, secondary hyperparathyroidism, severe dental caries, and alopecia. Serum studies reveal hypocalcemia, hypophosphatemia, very high calcitriol, and increased alkaline phosphatase levels. The clinical and chemical abnormalities do not respond to therapy with high-dose vitamin D, indicating target organ unresponsiveness. Eleven different mutations in the gene-encoding VDR have thus far been reported. They affect either the C-terminal ligand-binding region or the N-terminal DNA binding zinc-fingers sequences, with mutation hot spots identified at conserved sequences among the steroid-thyroid receptors superfamily. These result in impaired calcitriol binding to target organs, signified in vitro as failure of fibroblasts to bind [(3)H]calcitriol or to respond to calcitriol by 24-hydroxylase activity enhancement. Receptor studies and mutational analyses are used for prenatal diagnosis of CRR. Therapy with high-dose calcium overcomes the VDR defect, normalizes serum calcium, and maintains bone remodeling and mineral apposition. These responses to therapy have interesting implications upon our understanding of the potential role of calcium alone and that of vitamin D in bone physiology. Like other hormone-resistant diseases, CRR, with its various mutations, provides the opportunity for investigating the nature of vitamin D and of VDR physiology, which has been only partially explored to date.

Hormone-nuclear receptor interactions in health and disease. Vitamin D resistance

Tissue resistance to vitamin D, or vitamin D-dependent rickets (VDDR), can be classified as two separate conditions--VDDR type I and VDDR type II--both of which present with the classical clinical, radiological and biochemical features of rickets despite adequate vitamin D intake. VDDR II can also be associated with alopecia, for reasons that are not clear. The two syndromes result from distinct disorders of vitamin D metabolism or action. Both are inherited in an autosomal recessive fashion. VDDR I is caused by decreased production of the active form of vitamin D, 1,25-dihydroxycholecalciferol, with the proposed defect being in the gene encoding the enzyme 1 alpha-hydroxylase. VDDR II results from mutations in the gene for the intracellular receptor for 1,25-dihydroxycholecalciferol (vitamin D receptor), resulting in changes in hormone or DNA binding, depending on the mutation. These mutations are analogous to those affecting receptors for other steroid-thyroid hormones, which have also been shown to cause resistance to hormone action.

Tissue resistance to 1,25-dihydroxyvitamin D without a mutation of the vitamin D receptor gene

OBJECTIVE

Hereditary vitamin D resistant rickets (HVDRR) is characterized by severe rickets and is often accompanied by alopecia. Mutations in the gene encoding the vitamin D receptor have been found in this condition. In a patient with the characteristic phenotype we have investigated the functional defect and sequenced the gene to seek a mutation.

DESIGN

Patient and control cell lines prepared from skin fibroblasts and peripheral blood lymphocytes were used to measure binding of 1,25(OH)2D3 and to isolate vitamin D receptor mRNA. VDR cDNA was sequenced and transfected into receptor defective cells.

PATIENT

A child with alopecia diagnosed as having rickets due to resistance to 1,25(OH)2D3.

MEASUREMENTS

Cytosolic binding and nuclear association of 1,25(OH)2D3 were determined in patient and control cells, and functional response to 1,25(OH)2D3 assessed by measurement of 24-hydroxylase activity. VDR mRNA was prepared, reverse transcribed, and cDNA sequenced. VDR cDNA was also transfected into VDR-deficient CV-1 cells and functional response to 1,25(OH)2D3 assessed by co-transfection with a chloramphenicol acetyltransferase (CAT) reporter plasmid.

RESULTS

VDR from the patient were able to bind 1,25(OH)2D3 but showed no nuclear localization resulting in an absence of functional response to 1,25(OH)2D3. Sequencing revealed that the VDR coding region was normal. Expression studies of the patient's VDR showed functionally normal VDR as evidenced by normal transactivation in the presence of 1,25(OH)2D3.

CONCLUSION

These data indicate a new cause of tissue resistance to 1,25(OH)2D3 which occurs in the absence of mutations in the coding region of VDR gene and which is characterized by defective nuclear localization of this receptor.

Calcium therapy for calcitriol-resistant rickets

Ten patients with calcitriol-resistant rickets caused by a defect in the ligand-binding domain of the vitamin D receptor are described. Eight patients, 1.7 to 13.8 years of age, received high doses of elemental calcium (range, 0.4 to 1.4 gm/m2) through indwelling intracaval catheters for periods of 1.8 to 3.8 years. Two other patients, aged 1.1 and 2.2 years, were given oral calcium therapy as the sole mode of treatment. In five of the intravenously treated patients, oral calcium therapy was initiated after radiologic evidence of healing of the rickets. To maintain normal serum calcium concentration, the patients required daily doses of elemental calcium of 3.5 to 9 gm/m2 body surface area. Clinical improvement was observed within a week of the start of intravenous therapy, with disappearance of bone pain; several of the younger patients started to walk for the first time. Growth velocity increased within 2 to 3 months, from a pretreatment rate of -0.8 to -6.3 standard deviation score (SDS), to a posttreatment rate of +0.1 to +5.1 (SDS). Serum calcium, parathyroid hormone, phosphorus, and alkaline phosphatase values returned to normal within a year. Radiologic signs of healing occurred more rapidly in the intravenous treatment groups and in younger patients. Episodes of septicemia occurred frequently in those receiving parenteral therapy and required replacement of the catheter. We recommend that in the treatment of calcitriol-resistant rickets, oral calcium therapy be started at the youngest possible age, in doses to the limit of intestinal tolerance. When rickets is present, calcium should be infused through a large vessel in doses high enough to produce normocalcemia, normophosphatemia, and suppression of parathyroid hormone. Only after radiologic healing has been observed can oral calcium therapy be introduced.

Molecular genetics of mineral metabolic disorders

The recent advances in molecular biology and cytogenetics have made it possible to localize, clone and characterize some of the genetic abnormalities which result in disorders of phosphate and calcium homeostasis. Thus, the genes causing X-linked hypophosphataemic rickets, Lowe syndrome, X-linked recessive hypoparathyroidism, Di George syndrome, multiple endocrine neoplasia type 1, multiple endocrine neoplasia type 2 and vitamin D-dependent rickets type I have been mapped. In addition the genes involved in the pathogenesis of the humoral hypercalcaemia of malignancy, vitamin D-dependent rickets type II, pseudohypoparathyroidism, and some of the autosomal forms of hypoparathyroidism have been cloned and the mutations characterized. The molecular and genetic studies which have mapped and identified these disease genes are described and the implications of these developments in clinical practice and in further elucidation of the mineral metabolic defects are discussed.

Effect of long-term treatment with massive doses of 1 alpha-hydroxyvitamin D3 on calcium-phosphate balance in patients with vitamin D-dependent rickets type II

Three patients with vitamin D-dependent rickets type II were given massive doses of 1 alpha-hydroxyvitamin D3 for 29 to 36 months and their calcium-phosphate balance was studied during treatment and one month after cessation of treatment. During treatment fasting hypercalciuria was observed in patient 1 and an increased rate of calcium excretion after calcium loading in patients 1 and 2. In these patients, calcium excretion was parallel with the serum 24,25-dihydroxyvitamin D concentration, which may reflect the level of receptor for 1,25-dihydroxyvitamin D. These findings suggested that the responsiveness to 1,25-dihydroxyvitamin D improved during long-term treatment of these two patients with vitamin D-dependent rickets type II.

Point mutations in the human vitamin D receptor gene associated with hypocalcemic rickets

Hypocalcemic vitamin D-resistant rickets is a human genetic disease resulting from target organ resistance to the action of 1,25-dihydroxyvitamin D3. Two families with affected children homozygous for this autosomal recessive disorder were studied for abnormalities in the intracellular vitamin D receptor (VDR) and its gene. Although the receptor displays normal binding of 1,25-dihydroxyvitamin D3 hormone, VDR from affected family members has a decreased affinity for DNA. Genomic DNA isolated from these families was subjected to oligonucleotide-primed DNA amplification, and each of the nine exons encoding the receptor protein was sequenced for a genetic mutation. In each family, a different single nucleotide mutation was found in the DNA binding domain of the protein; one family near the tip of the first zinc finger (Gly----Asp) and one at the tip of the second zinc finger (Arg----Gly). The mutant residues were created in vitro by oligonucleotide directed point mutagenesis of wild-type VDR complementary DNA and this cDNA was transfected into COS-1 cells. The produced protein is biochemically indistinguishable from the receptor isolated from patients.

Long-term intracaval calcium infusion therapy in end-organ resistance to 1,25-dihydroxyvitamin D

Two boys aged six and four with the syndrome of hereditary resistance to 1,25-dihydroxyvitamin D3 with rickets alopecia and growth retardation are presented. After unsuccessful therapeutic trials with pharmacologic doses of vitamin D or its active metabolites, the patients were treated by long-term intracaval infusions of calcium through an implantable catheter. A total of 0.5 to 0.9 g of elemental calcium was infused daily for 18 months and the serum calcium concentration was maintained at 9 to 10 mg/dl. Bone pain subsided within one week of treatment. Serum phosphorus, immunoreactive parathyroid hormone, and 1,25-dihydroxyvitamin D concentrations and alkaline phosphatase activity were normalized within four to nine months. Radiographs of the knees and hands revealed progressive healing of rickets with complete resolution after one year of treatment. The patients gained 12 cm and 8 cm per year in height as compared with 3 cm and 2 cm, respectively, in the previous year. A transilial bone biopsy obtained from one patient prior to treatment revealed severe osteomalacia associated with osteitis fibrosa. A follow-up biopsy examined after 12 months of therapy showed almost complete healing of osteomalacia and normal mineralization. These observations indicate the following: (1) Long-term intracaval calcium infusions are an effective mode of therapy for these patients, and (2) When adequate serum calcium and phosphorus concentrations are maintained, healing of rickets and normal growth rate could be achieved even in the absence of a normal 1,25-dihydroxyvitamin D3 receptor-effector system.

Analysis of the relation between alopecia and resistance to 1,25-dihydroxyvitamin D

Alopecia is a frequent feature in hereditary resistance to (1,25(OH)2D). We sought insight into this feature by analysing data from affected members of 30 kindreds. We assessed indices of mineral metabolism in one group with normal hair compared with a group with alopecia. Hereditary resistance to 1,25(OH)2D was diagnosed at an earlier age in alopecic patients (0.9 vs 3.3 years, P less than 0.05); this reflected late presentation of metabolic bone disease in some cases with normal hair and could not be attributed to early diagnosis resulting from the striking feature of alopecia. For untreated subjects, serum concentrations of calcium and 1,25(OH)2D were similar in both groups of patients. During calciferol therapy, however, the cases with alopecia showed lower serum calcium (1.9 vs 2.4 mmol/l, P less than 0.005), but higher serum 1,25(OH)2D (2900 v 340 pg/ml, P less than 0.005). Hair status did not predict the type of defect identified with cultured skin fibroblasts but did correlate with responsiveness of 25(OH)D 24-hydroxylase to 1,25(OH)2D3 in those cells. Cells from seven of eight kindreds with alopecia showed no 24-hydroxylase response to high doses of 1,25(OH)2D3 while cells from five of six kindreds with normal hair showed a 24-hydroxylase response to high doses of 1,25(OH)2D3. We conclude that in cases with hereditary resistance to 1,25(OH)2D alopecia reflects the more severe grades of this resistance based upon earlier age at time of diagnosis, lower potential for calcaemic response to calciferols, and lower potential for 24-hydroxylase response to 1,25(OH)2D3 by cultured skin fibroblasts.

Vitamin D-dependent rickets type II: extreme end organ resistance to 1,25-dihydroxy vitamin D3 in a patient without alopecia

Vitamin D-dependent rickets type II (VDDR II) is a rare syndrome resulting in severe rickets and is resistant to treatment with vitamin D and its derivatives. Patient with this disease, who are frequently the children of consanguinous marriages, present with elevated circulating concentrations of 1,25-dihydroxy vitamin D, the active metabolite of vitamin D, and in vitro studies have indicated a failure of intracellular binding of the hormone. Alopecia has been noted in many of these patients and it has been suggested that this feature may indicate a more marked resistance to treatment. However we describe a 3-year-old boy with this disease who, although having normal hair growth, displayed extreme resistance to treatment with active vitamin D metabolites. In vitro studies of skin fibroblasts disclosed not only an absence of hormone binding or 1,25(OH)2D3-induced 24-hydroxylase activity but reduced metabolism of 1,25(OH)2D3 itself. In this child, treatment with exogenous 1,25-dihydroxy vitamin D3 at doses of up to 24 micrograms/day, which increased the circulating concentration of the metabolite to greater than 100 times the normal adult mean, failed to alleviate his condition and he died at the age of 39 months. This would therefore suggest that absence of alopecia, in this condition, cannot be regarded as a constant predictive sign of a lesser resistance and of responsiveness to Vitamin D treatment.

Long-term nocturnal calcium infusions can cure rickets and promote normal mineralization in hereditary resistance to 1,25-dihydroxyvitamin D

We report the beneficial effects of calcium infusions in a child with hereditary resistance to 1,25(OH)2D and alopecia. This patient after transient responsiveness to vitamin D derivatives became unresponsive to all therapy despite serum 1,25(OH)2D concentrations maintained at levels approximately 100-fold normal. A 7-mo trial with calcium infusions led to correction of biochemical abnormalities and healing of rickets. Bone biopsies (n = 3) showed a normal mineralization and the disappearance of the osteomalacia. Cultures of bone-derived cells demonstrated a lack of activation of 25-hydroxyvitamin D 24-hydroxylase and osteocalcin synthesis by 1,25(OH)2D3 (10(-9) and 10(-6) M). These results demonstrate that even in the absence of a normal 1,25(OH)2D3 receptor-effector system in bone cells, normal mineralization can be achieved in humans if adequate serum calcium and phosphorus concentrations are maintained; and calcium infusions may be an efficient alternative for the management of patients with this condition who are unresponsive to large doses of vitamin D derivatives.

The physiology and pathophysiology of vitamin D

The vitamin D endocrine system plays an important role in the maintenance of normal calcium homeostasis. Abnormalities of this system occur in many conditions, such as rickets, osteomalacia, hypoparathyroidism, and hyperparathyroidism. The diagnosis and treatment of these disorders will be facilitated if the clinician understands the general mechanisms by which defects in vitamin D metabolism and action occur. We review this information and discuss the use and limitations of vitamin D metabolite assays for diagnosis of clinical disorders of mineral metabolism.

The mechanism of end-organ resistance to 1 alpha,25-dihydroxycholecalciferol in the common marmoset

The common marmoset, a New World monkey, requires a large amount of cholecalciferol (110 i.u./day per 100g body wt.) to maintain its normal growth. In a previous report, we demonstrated that the circulating levels of 1 alpha, 25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] in the marmosets are much higher than those in rhesus monkeys and humans, but the marmosets are not hypercalcaemic [Shinki, Shiina, Takahashi, Tanioka, Koizumi & Suda (1983) Biochem. Biophys. Res. Commun. 14, 452-457]. To compare the effect of the daily intake of cholecalciferol, two rhesus monkeys were given a large amount of cholecalciferol (900 i.u./day per 100g body wt). Their serum levels of calcium, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol were markedly elevated, but the serum 1 alpha,25(OH)2D3 levels remained within a range similar to those in the rhesus monkeys fed the normal diet (intake of cholecalciferol 5 i.u./day per 100g body wt). Intestinal cytosols prepared from both monkeys contained similar 3.5 S macromolecules to which 1 alpha,25(OH)2D3 was bound specifically. However, the cytosols from the marmosets contained only one-sixth as many 1 alpha,25(OH)2D3 receptors as those from the rhesus monkeys. Furthermore, the activity of the 1 alpha,25(OH)2D3-receptor complex in binding to DNA-cellulose was very low in the marmosets. These results suggest that the marmoset possesses an end-organ resistance to 1 alpha,25(OH)2D3 and is a useful animal model for studying the mechanism of vitamin D-dependent rickets, type II.