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.

Rickets, Vitamin D, and Ca/P Metabolism.

Rickets was a major public health problem dating from Roman times, and medical descriptions of rickets date from the 17th century. Sniadecki first advocated treatment by exposure to sunshine in 1822; contemporaneously, several British physicians advocated use of cod liver oil. Both approaches were successful. Work in 1924 showed that exposure to UV light endowed fats and other foods with antirachitic properties. Vitamins D2 and D3, the antirachitic agent in cod liver oil, were, respectively, produced by UV radiation of ergosterol and 7-dehydrocholesterol. Calcitriol (1,25[OH]2D3) was identified as the biologically active form of vitamin D in the early 1970s. The vitamin D 25-hydroxylase, 24-hydroxylase, and 1α-hydroxylase were cloned in the 1990s and their genetic defects were soon delineated. The vitamin D receptor was also cloned and its mutations identified in vitamin D-resistant rickets. Work with parathyroid hormone (PTH) began much later, as the parathyroids were not identified until the late 19th century. In 1925, James B. Collip (of insulin fame) identified PTH by its ability to correct tetany in parathyroidectomized dogs, but only in the 1970s was it clear that only a small fragment of PTH conveyed its activity. Congenital hypoparathyroidism with immune defects was described in 1968, eventually linked to microdeletions in chromosome 22q11.2. X-linked hypophosphatemic rickets was reported in 1957, and genetic linkage analysis identified the causative PHEX gene in 1997. Autosomal dominant hypophosphatemic rickets similarly led to the discovery of FGF23, a phosphate-wasting humoral factor made in bone, in 2000, revolutionizing our understanding of phosphorus metabolism.

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.

Vitamin D and Cardiovascular Disease: An Updated Narrative Review.

During the last two decades, the potential impact of vitamin D on the risk of cardiovascular disease (CVD) has been rigorously studied. Data regarding the effect of vitamin D on CVD risk are puzzling: observational data indicate an inverse nonlinear association between vitamin D status and CVD events, with the highest CVD risk at severe vitamin D deficiency; however, preclinical data and randomized controlled trials (RCTs) show several beneficial effects of vitamin D on the surrogate parameters of vascular and cardiac function. By contrast, Mendelian randomization studies and large RCTs in the general population and in patients with chronic kidney disease, a high-risk group for CVD events, largely report no significant beneficial effect of vitamin D treatment on CVD events. In patients with rickets and osteomalacia, cardiovascular complications are infrequently reported, except for an increased risk of heart failure. In conclusion, there is no strong evidence for beneficial vitamin D effects on CVD risk, either in the general population or in high-risk groups. Whether some subgroups such as individuals with severe vitamin D deficiency or a combination of low vitamin D status with specific gene variants and/or certain nutrition/lifestyle factors would benefit from vitamin D (metabolite) administration, remains to be studied.

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.

Vitamin D-Dependent Rickets Type 1B (25-Hydroxylase Deficiency): A Rare Condition or a Misdiagnosed Condition?

Vitamin D requires a two-step activation by hydroxylation: The first step is catalyzed by hepatic 25-hydroxylase (CYP2R1, 11p15.2) and the second one is catalyzed by renal 1α-hydroxylase (CYP27B1, 12q13.1), which produces the active hormonal form of 1,25-(OH)2 D. Mutations of CYP2R1 have been associated with vitamin D-dependent rickets type 1B (VDDR1B), a very rare condition that has only been reported to affect 4 families to date. We describe 7 patients from 2 unrelated families who presented with homozygous loss-of-function mutations of CYP2R1. Heterozygous mutations were present in their normal parents. We identified a new c.124_138delinsCGG (p.Gly42_Leu46delinsArg) variation and the previously published c.296T>C (p.Leu99Pro) mutation. Functional in vitro studies confirmed loss-of-function enzymatic activity in both cases. We discuss the difficulties in establishing the correct diagnosis and the specific biochemical pattern, namely, very low 25-OH-D suggestive of classical vitamin D deficiency, in the face of normal/high concentrations of 1,25-(OH)2 D. Siblings exhibited the three stages of rickets based on biochemical and radiographic findings. Interestingly, adult patients were able to maintain normal mineral metabolism without vitamin D supplementation. One index case presented with a partial improvement with 1alfa-hydroxyvitamin D3 or alfacalcidol (1α-OH-D3 ) treatment, and we observed a dramatic increase in the 1,25-(OH)2 D serum concentration, which indicated the role of accessory 25-hydroxylase enzymes. Lastly, in patients who received calcifediol (25-OH-D3 ), we documented normal 24-hydroxylase activity (CYP24A1). For the first time, and according to the concept of personalized medicine, we demonstrate dramatic improvements in patients who were given 25-OH-D therapy (clinical symptoms, biochemical data, and bone densitometry). In conclusion, the current study further expands the CYP2R1 mutation spectrum. We note that VDDR1B could be easily mistaken for classical vitamin D deficiency. © 2017 American Society for Bone and Mineral Research.

A Pediatric Patient with a CYP24A1 Mutation: Four Years of Clinical, Biochemical, and Imaging Follow-Up.

BACKGROUND: A female infant was admitted to hospital due to failure to thrive. She presented hypercalcemia (4.09 mmol/L, normal range: 2.2-2.65 mmol/L), high 25-hydroxyvitamin D (283 nmol/L, normal range: 75-250 nmol/L), 1,25-dihydroxyvitamin D in the upper normal range, and low parathyroid hormone. Vitamin D intoxication was suspected. The patient had received routine rickets prophylaxis. METHODS: Williams-Beuren syndrome was genetically excluded. Sequencing of CYP24A1 showed 2 mutations: c.443T>C and c.1186C>T. RESULTS: The patient's clinical status improved after intravenous rehydration, cessation of supplementation, and on a low-calcium diet. 25-Hydroxyvitamin D concentrations normalized within days, while 1,25-dihydroxyvitamin D remained in the upper normal range. We also investigated our patient's bone health. CONCLUSION: The patient was hospitalized initially on suspicion of vitamin D intoxication but proved to be a case of compound heterozygosity. Data on the long-term clinical and biochemical evolution of patients with idiopathic infantile hypercalcemia are sparse. Our follow-up showed seasonal variations of vitamin D and calcium parameters, with no influence on kidney function or bone health for the investigated period.

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

UNLABELLED: 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.

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 ß-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 ß-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.

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.

Hypophosphatemic rickets with hypercalciuria due to mutation in SLC34A3/NaPi-IIc can be masked by vitamin D deficiency and can be associated with renal calcifications.

Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is caused by mutations in SLC34A3, the gene encoding the renal sodium-phosphate co-transporter NaPi-IIc. Despite increased urinary calcium excretion, HHRH is typically not associated with kidney stones prior to treatment. However, here we describe two sisters, who displayed nephrolithiasis or nephrocalcinosis upon presentation. The index patient, II-4, presented with short stature, bone pain, and knee X-rays suggestive of mild rickets at age 8.5 years. Laboratory evaluation showed hypophosphatemia, elevated 1,25(OH) (2) vitamin D levels, and hypercalciuria, later also developing vitamin D deficiency. Her sister, II-6, had a low normal serum phosphorous level, biochemically vitamin D deficiency and no evidence for osteomalacia, but had undergone left nephro-ureterectomy at age 17 because of ureteral stricture secondary to renal calculi. Nucleotide sequence analysis of DNA from II-4 and II-6 revealed a homozygous missense mutation c.586G>A (p.G196R) in SLC34A3/NaPi-IIc. Ultrasonographic examinations prior to treatment showed grade I nephrocalcinosis for II-4, while II-6 had grade I-II nephrocalcinosis in her remaining kidney. Four siblings and the mother were heterozygous carriers of the mutation, but showed no biochemical abnormalities. With oral phosphate supplements, hypophosphatemia and hypercalciuria improved in both homozygous individuals. Renal calcifications that are presumably due to increased urinary calcium excretion can be the presenting finding in homozygous carriers of G196R in SLC34A3/NaPi-IIc, and some or all laboratory features of HHRH may be masked by vitamin D deficiency.

Resurrection of vitamin D deficiency and rickets.

The epidemic scourge of rickets in the 19th century was caused by vitamin D deficiency due to inadequate sun exposure and resulted in growth retardation, muscle weakness, skeletal deformities, hypocalcemia, tetany, and seizures. The encouragement of sensible sun exposure and the fortification of milk with vitamin D resulted in almost complete eradication of the disease. Vitamin D (where D represents D2 or D3) is biologically inert and metabolized in the liver to 25-hydroxyvitamin D [25(OH)D], the major circulating form of vitamin D that is used to determine vitamin D status. 25(OH)D is activated in the kidneys to 1,25-dihydroxyvitamin D [1,25(OH)2D], which regulates calcium, phosphorus, and bone metabolism. Vitamin D deficiency has again become an epidemic in children, and rickets has become a global health issue. In addition to vitamin D deficiency, calcium deficiency and acquired and inherited disorders of vitamin D, calcium, and phosphorus metabolism cause rickets. This review summarizes the role of vitamin D in the prevention of rickets and its importance in the overall health and welfare of infants and children.

Rickets in osteopetrosis--a paradoxical association.

Osteopetrosis is a hereditary bone disease with intense positive balance of body calcium. Infantile variety is often associated with rickets--a paradoxical association. Two siblings with osteopetro rickets are reported in the article. The pathophysiologic mechanism of the paradoxical association has been explained and various management options have been discussed. Both cases were treated with high dose calcitriol and calcium supplements.

Novel regulators of vitamin D action and metabolism: Lessons learned at the Los Angeles zoo.

We undertook an investigation of an outbreak of rachitic bone disease in the Emperor Tamarin New World primate colony at the Los Angeles Zoo in the mid-1980s. The disease phenotype resembled that observed in humans with an inactivating mutation of the vitamin D receptor (VDR), hypocalcemia, high 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) levels, and rickets in rapidly growing adolescent primates. In contrast to the human disease, the New World primate VDR was functionally normal in all respects. The proximate cause of vitamin D hormone resistance in New World primates was determined to be the constitutive overexpression of a heterogeneous nuclear ribonucleoprotein in the A family which we coined the vitamin D response element binding protein (VDRE-BP). VDRE-BP competed in trans with the VDR-retinoid X receptor (RXR) for binding to the vitamin D response element. VDRE-BP-legislated resistance to 1,25-(OH)(2)D was antagonized (i.e., compensated) by another set of constitutively overexpressed proteins, the hsp-70-related intracellular vitamin D binding proteins (IDBPs). IDBPs, present but expressed at much lower levels in Old World primates including man, exhibited a high capacity for 25-hydroxylated vitamin D metabolites and functioned to traffic vitamin Ds to specific intracellular destinations to promote their action and metabolism.

The autosomal dominant hypophosphatemic rickets R176Q mutation in fibroblast growth factor 23 resists proteolytic cleavage and enhances in vivo biological potency.

Missense mutations in fibroblast growth factor 23 (FGF23) are the cause of autosomal dominant hypophosphatemic rickets (ADHR). The mutations (R176Q, R179W, and R179Q) replace Arg residues within a subtilisin-like proprotein convertase (SPC) cleavage site (RXXR motif), leading to protease resistance of FGF23. The goals of this study were to examine in vivo the biological potency of the R176Q mutant FGF23 form and to characterize alterations in homeostatic mechanisms that give rise to the phenotypic presentation of this disorder. For this, wild type and R176Q mutant FGF23 were overexpressed in the intact animals using a tumor-bearing nude mouse system. At comparable circulating levels, the mutant form was more potent in inducing hypophosphatemia, in decreasing circulating concentrations of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), and in causing rickets and osteomalacia in these animals compared with wild type FGF23. Parameters of calcium homeostasis were also altered, leading to secondary hyperparathyroidism and parathyroid gland hyperplasia. However, the raised circulating levels of parathyroid hormone were ineffective in normalizing the reduced 1,25(OH)(2)D(3) levels by increasing renal expression of 25(OH)D(3)-1alpha-hydroxylase (Cyp40) to promote its synthesis and by decreasing that of 25(OH)D(3)-24-hydroxylase (Cyp24) to prevent its catabolism. The findings provide direct in vivo evidence that missense mutations from ADHR kindreds are gain-of-function mutations that retain and increase the protein's biological potency. Moreover, for the first time, they define a potential role for FGF23 in dissociating parathyroid hormone actions on mineral fluxes and on vitamin D metabolism at the level of the kidney.

Adolescent rickets in Saudi Arabia: a rich and sunny country.

In this retrospective study from Saudi Arabia, which is a rich and sunny country, we report our experience with 34 adolescents (20 females, 10 males) with rickets. The commonest cause was vitamin D deficiency (58.8%) followed by rickets due to low calcium intake (11.8%) and genetic causes, including possible 25-hydroxylase deficiency (8.8%). The etiology of nutritional rickets is multifactorial, including lack of sun exposure and inadequate calcium intake. The clinical symptoms were nonspecific and therefore cases in this country are either underdiagnosed or missed. Vitamin D deficient patients needed an average of 19 months of treatment before recovery. High dose vitamin D plus calcium supplementation are recommended for treatment. Measures to prevent rickets in all age groups including adolescents are suggested. Further studies on nutritional and genetic forms of rickets are recommended.

Dietary phosphorus restriction reverses the impaired bone mineralization in vitamin D receptor knockout mice.

Deficiency of vitamin D, which is required for calcium homeostasis, causes rickets with hypocalcemia and hypophosphatemia, resulting in growth retardation and impaired bone formation. Mice lacking the vitamin D receptor (VDR) develop the typical features of rickets, establishing that VDR plays a role in controlling the actions of vitamin D. Normalization of impaired mineral homeostasis in VDR KO mice fed a diet supplemented with high concentrations of calcium (2%) and phosphorus (1.25%) is reported to reverse the malformation of bone and the growth retardation as well. However, the relationship between mobilization of phosphorus and calcium and nuclear control of vitamin D actions remains unclear. The present study was undertaken to determine the effect of dietary phosphorus on mineral mobilization and bone mineralization. We report here that feeding a diet supplemented with a restricted amount of phosphorus (0.25%) and a normal amount of calcium (0.5%) for 4 weeks reverses the growth retardation and the impaired mineralization in VDR KO mice, as does a high-calcium and high-phosphorus diet (Ca: 2%; P: 1.25%). Thus, the present study suggests that mobilization of calcium and mobilization of phosphorus are differentially regulated through vitamin D-dependent and -independent systems, and that intake of calcium and phosphorus in the proper ratio is important for mineral homeostasis and bone mineralization.