Loss-of-function mutations of CYP24A1, the vitamin D 24-hydroxylase gene, cause long-standing hypercalciuric nephrolithiasis and nephrocalcinosis

Significance of vitamin D responsiveness on the etiology of vitamin D-related diseases

Vitamin D resistance (VDRES) explains the necessity for higher doses of Vitamin D (VD) than those recommended for treatment success. VD receptor (VDR) signaling blockade, such as that caused by infections and poisons, is one basis for VDRES etiology. Mutations within genes affecting the VD system cause susceptibility to developing low VD responsiveness and autoimmunity. In contrast, VD hypersensitivity (VDHY) occurs if there is extra VD in the body; for example, as a result of an overdose of a VD supplement. Excess 1,25(OH)2D3 is produced in lymphomas and granulomatous diseases. The placenta produces excess 1,25(OH)2D3. Gene mutations regulating the production or degradation of 1,25(OH)2D3 enhance the effects of 1,25(OH)2D3. Increased 1,25(OH)2D3 levels stimulate calcium absorption in the gut, leading to hypercalcemia. Hypercalcemia can result in the calcification of the kidneys, circulatory system, or placenta, leading to kidney failure, cardiovascular disease, and pregnancy complications. The primary treatment involves avoiding exposure to the sun and VD supplements. The prevalence rates of VDRES and VDHY remain unclear. One estimate was that 25%, 51%, and 24% of the patients had strong, medium, and poor responses, respectively. Heavy-dose VD therapy may be a promising method for the treatment of autoimmune diseases; however, assessing its potential side effects is essential. To avoid VD-mediated hypercalcemia, responsiveness must be considered when treating pregnancies or cardiovascular diseases associated with VD. Furthermore, how VD is associated with the related disorders remains unclear. Investigating responsiveness to VD may provide more accurate results.

Biallelic and monoallelic pathogenic variants in CYP24A1 and SLC34A1 genes cause idiopathic infantile hypercalcemia

OBJECTIVE

Idiopathic infantile hypercalcemia (IIH) is a rare disorder of PTH-independent hypercalcemia. CYP24A1 and SLC34A1 gene mutations cause two forms of hereditary IIH. In this study, the clinical manifestations and molecular aspects of six new Chinese patients were investigated.

METHODS

The clinical manifestations and laboratory study of six patients with idiopathic infantile hypercalcemia were analyzed retrospectively.

RESULTS

Five of the patients were diagnosed with hypercalcemia, hypercalciuria, and bilateral medullary nephrocalcinosis. Their clinical symptoms and biochemical abnormalities improved after treatment. One patient presented at age 11 years old with arterial hypertension, hypercalciuria and nephrocalcinosis, but normal serum calcium. Gene analysis showed that two patients had compound heterozygous mutations of CYP24A1, one patient had a monoallelic CYP24A1 variant, and three patients had a monoallelic SLC34A1 variant. Four novel CYP24A1 variants (c.116G > C, c.287T > A, c.476G > A and c.1349T > C) and three novel SLC34A1 variants (c.1322 A > G, c.1697_1698insT and c.1726T > C) were found in these patients.

CONCLUSIONS

A monoallelic variant of CYP24A1 or SLC34A1 gene contributes to symptomatic hypercalcemia, hypercalciuria and nephrocalcinosis. Manifestations of IIH vary with onset age. Hypercalcemia may not necessarily present after infancy and IIH should be considered in patients with nephrolithiasis either in older children or adults.

Clinical heterogeneity and therapeutic options for idiopathic infantile hypercalcemia caused by CYP24A1 pathogenic variant

OBJECTIVES

Infantile hypercalcemia-1 (HCINF1) is a rare disease caused by pathogenic variants in the CYP24A1 gene, resulting in the inability to metabolize active vitamin D. This leads to hypercalcemia and severe complications.

CONTENT

On December 8th, 2022, a systematic literature search was conducted in PubMed, Wanfang, and CNKI using the keywords "hypercalcemia" and "CYP24A1". Data extraction included patient demographics, clinical presentation, treatment medications, and outcomes. The findings were synthesized to identify common patterns and variations among cases and to assess the efficacy of different therapies in reducing serum calcium. Our findings revealed two distinct peaks in the incidence of HCINF1 caused by CYP24A1 pathogenic variant. Kidney stones or renal calcifications were the most common clinical manifestations of the disease, followed by polyuria and developmental delay. Laboratory investigations showed hypercalcemia, elevated vitamin D levels, hypercalciuria, and low parathyroid hormone. Genetic analysis remains the only reliable diagnostic tool. Although there is no definitive cure for HCINF1, multiple drugs, including bisphosphonates, calcitonin, and rifampicin, have been used to control its symptoms. Blocking the production and intake of vitamin D is the preferred treatment option.

SUMMARY AND OUTLOOK

Our review highlights the basic clinical and biochemical features of HCINF1 and suggests that targeted diagnostic and therapeutic strategies are needed to address the clinical heterogeneity of the disease. The insights gained from this study may facilitate the development of innovative treatments for HCINF1.

Successful treatment of hypercalcemia in a Chinese patient with a novel homozygous mutation in the CYP24A1 gene using zoledronic acid: a case report

OBJECTIVES

To emphasize the significance of genetic mutations in idiopathic infantile hypercalcemia and the potential therapeutic effectiveness of zoledronic acid in managing hypercalcemia attributed to gene mutations.

CASE PRESENTATION

A 1-year-old female infant was referred to our hospital. The patient developed hypercalcemia despite no vitamin D prophylaxis or intake. In the acute phase, conventional calcium-lowering treatments showed limited efficacy, while the administration of zoledronic acid demonstrated effectiveness in controlling hypercalcemia. Subsequently the patient maintained normal calcium levels via a low-calcium diet and avoiding vitamin D intake. Genetic testing confirmed a homozygous mutation (c.476G>C) in the CYP24A1 gene.

CONCLUSIONS

Family screening and genetic counseling are crucial for early detection and prevention of hypercalcemia. This case emphasizes the importance of genetic mutations in disease development and the potential therapeutic efficacy of zoledronic acid in managing hypercalcemia attributed to gene mutations.

A Case of Delayed Diagnosis of Idiopathic Infantile Hypercalcemia Due to CYP24A1 Mutation: A 10-Year Journey

The parathyroid gland is responsible for the synthesis and secretion of parathyroid hormone, which is synthesized and released at an inverse relationship to the level of ionized calcium in the blood. Primary hyperparathyroidism affects women more than men. There are various causes for hyperparathyroidism-induced hypercalcemia and the most common cause is parathyroid adenoma. A less common cause of vitamin D-mediated parathyroid hormone-independent hypercalcemia is the loss of function mutation of the CYP24A1 gene. The CYP24A1 gene encodes the vitamin D 24-hydroxylase enzyme, responsible for hydroxylating the active form of vitamin D into an inactive form, and mutations in the CY24A1 gene can lead to elevated active vitamin D metabolite levels. It can result in hypercalcemia and hypercalciuria-related complications. We present a case of a 72-year-old male patient referred to the endocrine clinic, who had repeated treatments for hypercalcemia and recurrent renal calculi. He underwent ultrasound, computerized tomography, and sestamibi scans, all reported as normal. Following this, the patient underwent a positron emission tomography (PET) scan, which was also normal. He then finally underwent genetic testing and tested positive for the CYP24A1 gene. He was started on fluconazole 50mg once a day and cinacalcet 30mg twice with normalization of calcium level. Three of his family members also tested positive for the condition.

Rifampin monotherapy for children with idiopathic infantile hypercalcemia

Idiopathic Infantile Hypercalcemia (IIH) is characterized by hypercalcemia and hypercalciuria owing to PTH-independent increases in circulating concentrations of 1,25(OH)2D. At least 3 forms of IHH can be distinguished genetically and mechanistically: infantile hypercalcemia-1 (Hypercalcemia, Infantile, 1; HCINF1) due to CYP24A1 mutations results in decreased inactivation of 1,25(OH)2D; HCINF2 due to SLC34A1 mutations results in excessive 1,25(OH)2D production; and HCINF3 in which a variety of gene variants of uncertain significance (VUS) have been identified and where the mechanism for increased 1,25 (OH)2D is unclear. Conventional management with dietary calcium and vitamin D restriction has only limited success. Induction of the P450 enzyme CYP3A4 by rifampin can provide an alternate pathway for inactivation of 1,25(OH)2D that is useful in HCINF1 and may be effective in other forms of IIH. We sought to assess the efficacy of rifampin to decrease levels of serum 1,25(OH)2D and calcium, and urinary calcium concentrations in subjects with HCINF3, and to compare the response to a control subject with HCINF1. Four subjects with HCINF3 and the control subject with HCINF1 completed the study using rifampin 5 mg/kg/day and 10 mg/kg/day each for 2 months separated by a 2-month washout period. Patients had age-appropriate intake of dietary calcium plus 200 IU vitamin D/day. Primary outcome was efficacy of rifampin to lower serum concentrations of 1,25(OH)2D. The secondary outcomes included the reduction of serum calcium, urinary calcium excretion (as random urine calcium: creatinine (ca:cr) ratio) and serum 1,25(OH)2D/PTH ratio. Rifampin was well tolerated and induced CYP3A4 at both doses in all subjects. The control subject with HCINF1 showed significant response to both rifampin doses with decreases in the serum concentration of 1,25(OH)2D and the 1,25(OH)2D/PTH ratio while the serum and urine ca:cr levels were unchanged. The four patients with HCINF3 showed reductions in 1,25(OH)2D and urinary ca:cr after 10 mg/kg/d, but hypercalcemia did not improve and there were variable responses in 1,25(OH)2D/PTH ratios. These results support further longer-term studies to clarify the usefulness of rifampin as a medical therapy for IIH.

Role of Coactivator Associated Arginine Methyltransferase 1 (CARM1) in the Regulation of the Biological Function of 1,25-Dihydroxyvitamin D3

1,25-Dihydroxyvitamin D3 (1,25(OH)₂D₃), the hormonally active form of vitamin D, activates the nuclear vitamin D receptor (VDR) to mediate the transcription of target genes involved in calcium homeostasis as well as in non-classical 1,25(OH)₂D₃ actions. In this study, CARM1, an arginine methyltransferase, was found to mediate coactivator synergy in the presence of GRIP1 (a primary coactivator) and to cooperate with G9a, a lysine methyltransferase, in 1,25(OH)₂D₃ induced transcription of Cyp24a1 (the gene involved in the metabolic inactivation of 1,25(OH)₂D₃). In mouse proximal renal tubule (MPCT) cells and in mouse kidney, chromatin immunoprecipitation analysis demonstrated that dimethylation of histone H3 at arginine 17, which is mediated by CARM1, occurs at Cyp24a1 vitamin D response elements in a 1,25(OH)₂D₃ dependent manner. Treatment with TBBD, an inhibitor of CARM1, repressed 1,25(OH)₂D₃ induced Cyp24a1 expression in MPCT cells, further suggesting that CARM1 is a significant coactivator of 1,25(OH)₂D₃ induction of renal Cyp24a1 expression. CARM1 was found to act as a repressor of second messenger-mediated induction of the transcription of CYP27B1 (involved in the synthesis of 1,25(OH)₂D₃), supporting the role of CARM1 as a dual function coregulator. Our findings indicate a key role for CARM1 in the regulation of the biological function of 1,25(OH)₂D₃.

Idiopathic infantile hypercalcemia in children with chronic kidney disease due to kidney hypodysplasia

BACKGROUND

Idiopathic infantile hypercalcemia (IIH) etiologies include pathogenic variants in CYP24A1, leading to increased 1,25(OH)₂ D, hypercalciuria and suppressed parathyroid hormone (PTH), and in SLC34A1 and SLC34A3, leading to the same metabolic profile via increased phosphaturia. IIH has not been previously described in CKD due to kidney hypodysplasia (KHD).

METHODS

Retrospective study of children with bilateral KHD and simultaneously tested PTH and 1,25(OH)₂D, followed in a tertiary care center between 2015 and 2021.

RESULTS

Of 295 screened patients, 139 had KHD, of them 16 (11.5%) had IIH (study group), 26 with normal PTH and any 1,25(OH)₂D were controls. There were no differences between groups' gender, obstructive uropathy rate and baseline eGFR. Study patients were younger [median (IQR) age: 5.2 (3.2-11.3) vs. 61 (13.9-158.3) months, p < 0.001], had higher 1,25(OH)₂D (259.1 ± 91.7 vs. 156.5 ± 46.4 pmol/l, p < 0.001), total calcium (11.1 ± 0.4 vs. 10.7 ± 0.3 mg/dl, p < 0.001), and lower phosphate standard deviation score (P-SDS) [median (IQR): - 1.4 (- 1.9, - 0.4) vs. - 0.3 (- 0.8, - 0.1), p = 0.03]. During 12 months of follow-up, PTH increased among the study group (8.8 ± 2.8 to 22.7 ± 12.4 pg/ml, p < 0.001), calcium decreased (11 ± 0.5 to 10.3 ± 0.6 mg/dl, p = 0.004), 1,25(OH)₂D decreased (259.5 ± 91.7 to 188.2 ± 42.6 pmol/l, p = 0.1), P-SDS increased [median (IQR): - 1.4 (- 1.9, - 0.4) vs. - 0.3 (- 0.9, 0.4), p = 0.04], while eGFR increased. Five of 9 study group patients with available urine calcium had hypercalciuria. Five patients had nephrocalcinosis/lithiasis. Genetic analysis for pathogenic variants in CYP24A1, SLC34A1 and SLC34A3 had not been performed.

CONCLUSIONS

Transient IIH was observed in infants with KHD, in association with hypophosphatemia, resembling SLC34A1 and SLC34A3 pathogenic variants' metabolic profile. A higher resolution version of the Graphical abstract is available as Supplementary information.

Vitamin D in pediatric age: Current evidence, recommendations, and misunderstandings

In recent years vitamin D has been in the spotlight of many researchers for its possible role in various disorders, including autoimmune and infectious diseases. Even if vitamin D deficiency remains a major public health problem, its symptomatic manifestations are less and less common in clinical practice, and pediatric age represents a “gray area” where vitamin D supplementation is often administered in the absence of an effective evaluation of its status. Moreover, a poor knowledge about different definitions of “deficiency,” “insufficiency,” and similar terms is spread among clinicians, while guidelines are not univocal, especially after the first year of life. The aim of this brief opinion paper is to sum up recent evidence about vitamin D status and its supplementation in pediatrics, in order to better clarify a common definition of its deficiency. The aim of this opinion article is to raise awareness on this topic among clinicians and encourage a discussion on the real need for routine 25-hydroxycholecalciferol serum evaluation and its supplementation.

How useful is an oral calcium load test for diagnosing recurrent calcium stone formers?

Hypercalciuria is the main risk factor for recurrent calcium urolithiasis. The goal of our study is to determinate how useful an oral calcium load test is for stone formers to classify different forms of hypercalciuria in pathogenetic categories defined as renal or absorptive according to the current knowledge. Between June 2013 and February 2016, a prospective study was carried out on 117 documented recurrent hypercalciuric stone formers undergoing an oral calcium load test modified from the original description by Pak. After 2 days of calcium-restricted diet, urine and blood were analyzed at baseline and 120 min after receiving orally 1 g of calcium. Total and ionized calcium, parathyroid hormone from serum and urine calcium and creatinine were assessed in order to divide patients in three groups as previously described: resorptive, absorptive, and renal hypercalciuria. This allowed the identification of 19, 39, 34 and 33 patients with normocalcemic primary hyperparathyroidism (NPHPT), renal hypercalciuria aka renal calcium leak (RCL), absorptive hypercalciuria (AH) and unidentified cause, respectively. Patients with NPHPT (who required parathyroidectomy) experienced a lower PTH decrease (41.41 ± 12.82 vs. 54.06 ± 13.84% p < 0.01), higher beta-crosslaps, as well as lower TmP/GFR and distal third radius bone mineral density. RCL resulted in increased fasting urine calcium-to-creatinine ratio (Uca/Cr), i.e., > 0.37 mmol/mmol), without hyperparathyroidism. AH was diagnosed by the presence of ΔUCa/Cr > 0.60 mmol/mmol between baseline and 120 min without any other anomaly. For all remaining patients, results were inconclusive due to the lack of sufficient increase in serum calcium or because the cause of lithogenesis could not be clearly identified. The oral calcium load test is useful in nearly 80% of patients by identifying the different forms of hypercalciuria causing urolithiasis and by guiding treatment, including parathyroid surgery.

Vitamin D and Diseases of Mineral Homeostasis: A Cyp24a1 R396W Humanized Preclinical Model of Infantile Hypercalcemia Type 1

Infantile hypercalcemia type 1 (HCINF1), previously known as idiopathic infantile hypercalcemia, is caused by mutations in the 25-hydroxyvitamin D 24-hydroxylase gene, CYP24A1. The R396W loss-of-function mutation in CYP24A1 is the second most frequent mutated allele observed in affected HCINF1 patients. We have introduced the site-specific R396W mutation within the murine Cyp24a1 gene in knock-in mice to generate a humanized model of HCINF1. On the C57Bl6 inbred background, homozygous mutant mice exhibited high perinatal lethality with 17% survival past weaning. This was corrected by crossbreeding to the CD1 outbred background. Mutant animals had hypercalcemia in the first week of life, developed nephrolithiasis, and had a very high 25(OH)D₃ to 24,25(OH)₂D₃ ratio which is a diagnostic hallmark of the HCINF1 condition. Expression of the mutant Cyp24a1 allele was highly elevated while Cyp27b1 expression was abrogated. Impaired bone fracture healing was detected in CD1-R396^w/w mutant animals. The augmented lethality of the C57Bl6-R396W strain suggests an influence of distinct genetic backgrounds. Our data point to the utility of unique knock-in mice to probe the physiological ramifications of CYP24A1 variants in isolation from other biological and environmental factors.

Vitamin D resistant genes - promising therapeutic targets of chronic diseases

Vitamin D is an essential vitamin indispensable for calcium and phosphate metabolism, and its deficiency has been implicated in several extra-skeletal pathologies, including cancer and chronic kidney disease. Synthesized endogenously in the layers of the skin by the action of UV-B radiation, the vitamin maintains the integrity of the bones, teeth, and muscles and is involved in cell proliferation, differentiation, and immunity. The deficiency of Vit-D is increasing at an alarming rate, with nearly 32% of children and adults being either deficient or having insufficient levels. This has been attributed to Vit-D resistant genes that cause a reduction in circulatory Vit-D levels through a set of signaling pathways. CYP24A1, SMRT, and SNAIL are three genes responsible for Vit-D resistance as their activity either lowers the circulatory levels of Vit-D or reduces its availability in target tissues. The hydroxylase CYP24A1 inactivates analogs and prohormonal and/or hormonal forms of calcitriol. Elevation of the expression of CYP24A1 is the major cause of exacerbation of several diseases. CYP24A1 is rate-limiting, and its induction has been correlated with increased prognosis of diseases, while loss of function mutations cause hypersensitivity to Vit-D. The silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and its corepressor are involved in the transcriptional repression of VDR-target genes. SNAIL1 (SNAIL), SNAIL2 (Slug), and SNAIL3 (Smuc) are involved in transcriptional repression and binding to histone deacetylases and methyltransferases in addition to recruiting polycomb repressive complexes to the target gene promoters. An inverse relationship between the levels of calcitriol and the epithelial-to-mesenchymal transition is reported. Studies have demonstrated a strong association between Vit-D deficiency and chronic diseases, including cardiovascular diseases, diabetes, cancers, autoimmune diseases, infectious diseases, etc. Vit-D resistant genes associated with the aforementioned chronic diseases could serve as potential therapeutic targets. This review focuses on the basic structures and mechanisms of the repression of Vit-D regulated genes and highlights the role of Vit-D resistant genes in chronic diseases.

Amelioration of Fructus Ligustri Lucidi and its phenol glycosides on hypercalciuria via stimulating PTH1R/PKA/TRPV5 signaling

BACKGROUND

Our early studies performed on aged rats, ovariectomized (OVX) rats and diabetic mice, indicated the calciotropic role of Fructus Ligustri Lucidi (FLL), the fruit of Ligustrum lucidum Ait., in mediating calcium homeostasis which was partially attributed to its stimulation on renal calcium reabsorption.

PURPOSE

This study aimed to explicate the underlying molecular mechanism and explore the potential bioactive ingredients in FLL.

STUDY DESIGN AND METHODS

The OVX C57BL/6 J mice were orally administered with low (FL, 75 mg/kg), middle (FM, 225 mg/kg) or high (FH, 675 mg/kg) dose of extract of Fructus Ligustri Lucidi for 10 weeks. The biological properties of trabecular bone were measured by micro-CT and H&E staining. The molecular expression was assessed by immunoblotting and immunostaining. The potential active components were identified by cell membrane chromatography (CMC) and explored in renal tubular cells with Fluo-3/AM fluorescent staining to indicate intracellular calcium level. The male mice fed with high calcium diet (1.2% Ca) and orally treated with active components for 3 weeks.

RESULTS

Treatment of OVX mice with FLL extract suppressed the elevation in urinary calcium level (FH, 0.081 ± 0.012, vs. OVX, 0.189 ± 0.038 mg/mg), and increased bone mineral density (FH, 62.41 ± 2.57, vs. OVX, 43.72 ± 8.43 mg/ccm) and percentage of trabecular bone area. It also decreased circulating PTH level (FH, 66.69 ± 10.94, vs. OVX, 303.50 ± 26.56 pg/ml) and up-regulated TRPV5 expression in renal cortex of OVX mice as well as enhanced the expression of PTH receptor (PTH1R) and the ratio of p-PKA/PKA. The PKA inhibitor H89 abolished the induction of serum, prepared from rats treated with FLL extract, on PKA/TRPV5 signaling in renal tubular cells. The CMC identified phenol glycosides, including salidroside and oleuropein, which increased intracellular calcium content, promoted expression of PTH1R and TRPV5 and ratio of p-PKA/PKA as well as decreased calcium excretion in urine of mice fed with high calcium diet.

CONCLUSION

Salidroside and oleuropein are major ingredients contributing to the anti-hypercalciuria effects of FLL via acting on PTH1R/PKA/TRPV5 signaling in kidney. Further translational research would be required.

CYP24A1 and SLC34A1 Pathogenic Variants Are Uncommon in a Canadian Cohort of Children with Hypercalcemia or Hypercalciuria

OBJECTIVES

Biallelic pathogenic variants in CYPA24A1 and SLC34A1 are causes of idiopathic infantile hypercalcemia. Pathogenic variants in both may also give rise to hypercalciuria with nephrocalcinosis or nephrolithiasis without previous hypercalcemia (renal group). Our objective was to examine the frequency of CYP24A1 or SLC34A1 variants in children with early hypercalcemia or late-onset hypercalciuria.

METHOD

Forty-one children from 7 centers across Canada were recruited. Local investigations were undertaken. The serum was evaluated by liquid chromatography tandem-mass spectrometry for the ratio of 25-hydroxyvitamin D3 to 24,25-dihydroxyvitamin D3, (25-OH-D3:24,25-(OH)2D3), an elevation pathognomonic for the loss of function of the CYP24A1 enzyme. Mutational analyses were undertaken. Family cascade screening was performed if pathogenic variants were detected in probands.

RESULTS

Twenty-nine children had early-onset hypercalcemia; none had elevated 25-OH-D3:24,25-(OH)2D3 or variants. Interestingly, 2 of 12 in the renal group had elevated 25-OH-D3:24,25-(OH)2D3 and presented as preadolescents. In case 1, cascade testing revealed a sibling and parent with asymptomatic pathogenic variants in CYP24A1. Four CYP24A1 pathogenic variants were identified in these 2 probands: 3 have been described in European populations, and 1 is a rare variant in exon 7 (c931delC) that is likely pathogenic. No SLC34A1 pathogenic variants were detected.

CONCLUSION

In Canada, pathogenic variants in CYP24A1 appear to manifest with late-onset hypercalciuria and its sequelae. The 25-OH-D3:24,25-(OH)2D3 ratio is an excellent tool for screening for biallelic pathogenic variants in CYP24A1. We confirm that cascade testing is important for these variants.

Genetic causes of neonatal and infantile hypercalcaemia

The causes of hypercalcaemia in the neonate and infant are varied, and often distinct from those in older children and adults. Hypercalcaemia presents clinically with a range of symptoms including failure to thrive, poor feeding, constipation, polyuria, irritability, lethargy, seizures and hypotonia. When hypercalcaemia is suspected, an accurate diagnosis will require an evaluation of potential causes (e.g. family history) and assessment for physical features (such as dysmorphology, or subcutaneous fat deposits), as well as biochemical measurements, including total and ionised serum calcium, serum phosphate, creatinine and albumin, intact parathyroid hormone (PTH), vitamin D metabolites and urinary calcium, phosphate and creatinine. The causes of neonatal hypercalcaemia can be classified into high or low PTH disorders. Disorders associated with high serum PTH include neonatal severe hyperparathyroidism, familial hypocalciuric hypercalcaemia and Jansen's metaphyseal chondrodysplasia. Conditions associated with low serum PTH include idiopathic infantile hypercalcaemia, Williams-Beuren syndrome and inborn errors of metabolism, including hypophosphatasia. Maternal hypocalcaemia and dietary factors and several rare endocrine disorders can also influence neonatal serum calcium levels. This review will focus on the common causes of hypercalcaemia in neonates and young infants, considering maternal, dietary, and genetic causes of calcium dysregulation. The clinical presentation and treatment of patients with these disorders will be discussed.

Unlocking the Mechanisms of Cutaneous Adverse Drug Reactions: Activation of the Phosphatidylinositol 3-Kinase/Protein Kinase B Pathway by EGFR Inhibitors Triggers Keratinocyte Differentiation and Polarization of Epidermal Immune Responses

EGFR inhibitors used in oncology therapy modify the keratinocyte differentiation processes, impairing proper skin barrier formation and leading to cutaneous adverse drug reactions. To uncover the molecular signatures associated with cutaneous adverse drug reactions, we applied phosphoproteomic and transcriptomic assays on reconstructed human epidermis tissues exposed to a therapeutically relevant concentration of afatinib, a second-generation EGFR inhibitor. After drug exposure, we observed activation of the phosphatidylinositol 3-kinase/protein kinase B pathway associated with an increased expression of gene families involved in keratinocyte differentiation, senescence, oxidative stress, and alterations in the epidermal immune-related markers. Furthermore, our results show that afatinib may interfere with vitamin D3 metabolism, acting via CYP27A1 and CYP24A1 to regulate calcium concentration through the phosphatidylinositol 3-kinase/protein kinase B pathway. Consequently, basal layer keratinocytes switch from a pro-proliferating to a prodifferentiative program, characterized by upregulation of biomarkers associated with increased keratinization, cornification, T helper type 2 response, and decreased innate immunity. Such effects may increase skin susceptibility to cutaneous penetration of irritants and pathogens. Taken together, these findings demonstrate a molecular mechanism of EGFR inhibitor–induced cutaneous adverse drug reactions.

Low serum 1,25(OH)2D3 in end-stage renal disease: is reduced 1α-hydroxylase the only problem?

Low serum 1,25-dihydroxyvitamin D (1,25(OH)2D) in end-stage renal disease (ESRD) is considered a consequence of elevated fibroblast growth factor 23 (FGF23) and concomitant reduced activity of renal 1α-hydroxylase (CYP27B1). Current ESRD treatment strategies to increase serum calcium and suppress secondary hyperparathyroidism involve supplementation with vitamin D analogues that circumvent 1α-hydroxylase. This overlooks the potential importance of 25-hydroxyvitamin D (25(OH)D) deficiency as a contributor to low serum 1,25(OH)2D. We investigated the effects of vitamin D (cholecalciferol) supplementation (40,000 IU for 12 weeks and maintenance dose of 20,000 IU fortnightly), on multiple serum vitamin D metabolites (25(OH)D, 1,25(OH)2D3 and 24,25(OH)2D3) in 55 haemodialysis patients. Baseline and 12 month data were compared using related-samples Wilcoxon signed rank test. All patients remained on active vitamin D analogues as part of routine ESRD care. 1,25(OH)2D3 levels were low at baseline (normal range: 60-120 pmol/L). Cholecalciferol supplementation normalised both serum 25(OH)D and 1,25(OH)2D3. Median serum 25(OH)D increased from 35.1 nmol/L (IQR: 23.0-47.5 nmol/L) to 119.9 nmol/L (IQR: 99.5-143.3 nmol/L) (P < 0.001). Median serum 1,25(OH)2D3 and 24,25(OH)2D3 increased from 48.3 pmol/L (IQR: 35.9-57.9 pmol/L) and 3.8 nmol/L (IQR: 2.3-6.0 nmol/L) to 96.2 pmol/L (IQR: 77.1-130.6 pmol/L) and 12.3 nmol/L (IQR: 9-16.4 nmol/L), respectively (P < 0.001). A non-significant reduction in daily active vitamin D analogue dose occurred, 0.94 µmcg at baseline to 0.77 µmcg at 12 months (P = 0.73). The ability to synthesise 1,25(OH)2D3 in ESRD is maintained but is substrate dependent, and serum 25(OH)D was a limiting factor at baseline. Therefore, 1,25(OH)2D3 deficiency in ESRD is partly a consequence of 25(OH)D deficiency, rather than solely due to reduced 1α-hydroxylase activity as suggested by current treatment strategies.

Mild Idiopathic Infantile Hypercalcemia-Part 2: A Longitudinal Observational Study

CONTEXT

Idiopathic infantile hypercalcemia (IIH) is an uncommon disorder with variable clinical features. The natural history and response to dietary calcium and vitamin D restriction in IIH remains unclear.

OBJECTIVE

The aim of this study is to describe the clinical and biochemical response to dietary calcium and vitamin D restriction in a genetically characterized cohort of mild IIH.

METHODS

This is a longitudinal, observational cohort study of 20 children with mild IIH monitored for a median of 21months. Biochemical measures, dietary assessment, and yearly renal ultrasound results, since the time of diagnosis, were obtained and assessed prospectively every 4 to 6 months.

RESULTS

Median age at initial diagnosis was 4.5 months. Median levels of serum calcium (2.82 mmol/L) and 1,25 (OH)2D (192 pmol/L) were elevated, whereas serum PTH was reduced (10 ng/L). Urinary calcium:creatinine ratio was elevated for some, but not all individuals (median 1.49 mmol/mmol). All patients who were managed with a low-calcium diet showed an improvement in serum and urinary calcium measures, but the serum concentration of 1,25 dihydroxyvitamin D (1,25(OH)2D) and 1,25(OH)2D/PTH ratio remained elevated. In 2 of the 11 subjects, renal calcification worsened. There were no differences in response between individuals with CYP24A1 or SLC34A1/A3 variants.

CONCLUSION

The clinical presentation of mild IIH is variable, and dietary calcium and vitamin D restriction does not consistently normalize elevated 1,25(OH)2D concentrations or prevent worsening of renal calcification in all cases. Therapeutic options should target the defect in vitamin D metabolism.

Long-term outcome of the survivors of infantile hypercalcaemia with CYP24A1 and SLC34A1 mutations

Background

Infantile hypercalcaemia (IH) is a vitamin D₃ metabolism disorder. The molecular basis for IH is biallelic mutations in the CYP24A1 or SLC34A1 gene. These changes lead to catabolism disorders (CYP24A1 mutations) or excessive generation of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] (SLC34A1 mutations). The incidence rate of IH in children and the risk level for developing end-stage renal disease (ESRD) are still unknown. The aim of this study was to analyse the long-term outcome of adolescents and young adults who suffered from IH in infancy.

Design

Forty-two children (23 girls; average age 10.7 ± 6.3 years) and 26 adults (14 women; average age 24.2 ± 4.4 years) with a personal history of hypercalcaemia with elevated 1,25(OH)₂D₃ levels were included in the analysis. In all patients, a genetic analysis of possible IH mutations was conducted, as well as laboratory tests and renal ultrasonography.

Results

IH was confirmed in 20 studied patients (10 females). CYP24A1 mutations were found in 16 patients (8 females) and SLC34A1 in 4 patients (2 females). The long-term outcome was assessed in 18 patients with an average age of 23.8 years (age range 2–34). The average glomerular filtration rate (GFR) was 72 mL/min/1.73 m² (range 15–105). Two patients with a CYP24A1 mutation developed ESRD and underwent renal transplantation. A GFR <90 mL/min/1.73 m² was found in 14 patients (77%), whereas a GFR <60 mL/min/1.73 m² was seen in 5 patients (28%), including 2 adults after renal transplantation. Three of 18 patients still had serum calcium levels >2.6 mmol/L. A renal ultrasound revealed nephrocalcinosis in 16 of 18 (88%) patients, however, mild hypercalciuria was detected in only one subject.

Conclusions

Subjects who suffered from IH have a greater risk of progressive chronic kidney disease and nephrocalcinosis. This indicates that all survivors of IH should be closely monitored, with early implementation of preventive measures, e.g. inhibition of active metabolites of vitamin D₃ synthesis.

24-Hydroxylase Deficiency Due to CYP24A1 Sequence Variants: Comparison With Other Vitamin D-mediated Hypercalcemia Disorders

Context

CYP24A1 encodes 24-hydroxylase, which converts 25(OH)D3 and 1,25(OH)₂D₃ to inactive metabolites. Loss-of-function variants in CYP24A1 are associated with 24-hydroxylase deficiency (24HD), characterized by hypercalcemia, nephrolithiasis, and nephrocalcinosis. We retrospectively reviewed laboratory, imaging, and clinical characteristics of patients with suspected or confirmed 24HD and patients with other vitamin D−mediated hypercalcemia disorders: sarcoidosis, lymphoma, and exogenous vitamin D toxicity (EVT).

Objective

To identify features that differentiate 24HD from other vitamin D-mediated hypercalcemia disorders.

Methods

Patients seen at the Mayo Clinic (Rochester, MN) from January 1, 2008, to 31 December, 2016, with the following criteria were retrospectively identified: serum calcium ≥9.6 mg/dL, parathyroid hormone <30 pg/mL, and 1,25(OH)₂D₃ >40 pg/mL. Patients were considered to have 24HD if they had (1) confirmed CYP24A1 gene variant or (2) 25(OH)D₃:24,25(OH)₂D ratio ≥50. Patients with sarcoidosis, lymphoma, and EVT were also identified. Groups were compared using the Fisher exact test (categorical variables) or the Wilcoxon rank sum test (continuous variables).

Results

We identified 9 patients with 24HD and 28 with other vitamin D−mediated disorders. Patients with 24HD were younger at symptom onset (median 14 vs 63 years; P = .001) and had positive family history (88.9% vs 20.8%; P < .001), nephrocalcinosis (88.9% vs 6.3%; P < .001), lower lumbar spine Z-scores (median −0.50 vs 1.20; P = .01), higher peak serum phosphorus (% of peak reference range, median 107 vs 84; P = .01), and higher urinary calcium:creatinine ratios (median 0.24 vs 0.17; P = .047).

Conclusion

Patients with 24HD had clinical and laboratory findings that differed from other vitamin D−mediated hypercalcemia disorders. 24HD should be suspected in patients with hypercalcemia who present at younger age, have positive family history, and have nephrocalcinosis.

Do the Heterozygous Carriers of a CYP24A1 Mutation Display a Different Biochemical Phenotype Than Wild Types?

CONTEXT

Human cytochrome P450 24 subfamily A member 1 (CYP24A1) loss-of-function mutations result in impaired activity of the 24-hydroxylase involved in vitamin D catabolism, thus inducing a vitamin D-dependent hypercalcemia. Homozygotes often present an overt clinical phenotype named idiopathic infantile hypercalcemia (IIH), whereas it is debated whether heterozygotes display an abnormal phenotype.

OBJECTIVE

To compare the clinical and biochemical features of heterozygous carriers of CYP24A1 variant and healthy wild-type controls sharing the same genetic and environmental exposure.

METHODS

A large family harboring the nonsense c.667A>T, p.Arg223* pathogenic variant in the CYP24A1 gene was evaluated. All subjects underwent clinical and biochemical evaluation and complete analysis of vitamin D metabolites using mass spectroscopy including 1,24,25(OH)3D3. Subjects were divided into 2 groups according to their genotype: heterozygotes and wild-type for the CYP24A1 variant.

RESULTS

The proband, a 40-year-old man, homozygous for p.Arg223* pathogenic variant, had a history of mild hypercalcemia with a seasonal trend, recurrent nephrolithiasis, and no episodes of acute hypercalcemia. He showed the highest serum levels of fibroblast growth factor 23, the highest 25(OH)D3/24,25(OH)2D3 ratio and undetectable levels of 1,24,25(OH)3D3, which represent indicators of a loss-of-function CYP24A1. Compared with the wild-types, heterozygotes had higher serum calcium and 25(OH)D3 concentrations (P = .017 and P = .025, respectively), without any difference in the other biochemical parameters and in the rate of nephrolithiasis.

CONCLUSION

Heterozygotes exhibit a biochemical phenotype different from that of wild-type subjects. In clinical practice, these individuals might require surveillance because of the potential risk of developing hypercalcemia and related clinical manifestations if exposed to triggering factors.

Childhood Hypercalciuric Hypercalcemia With Elevated Vitamin D and Suppressed Parathyroid Hormone: Long-Term Follow Up

Purpose: Hypercalcemia with low parathyroid hormone (PTH) level, hypercalciuria, nephrocalcinosis, or nephrolithiasis, was recently reported as caused by mutations in CYP24A1 and SLC34A genes. These encode for vitamin D-24A-hydroxylase and for the renal phosphate transporters NaPiIIa and NaPiIIc, respectively. We aimed to describe the clinical course of these monogenic disorders in patients with and without found mutations during long-term follow-up. Methods: Ten patients with hypercalcemia, hypercalciuria, elevated 1,25-(OH)₂D levels and suppressed PTH were followed in our center during 1998-2019. Relevant laboratory and imaging data and results of genetic evaluation were retrieved from medical files. Results: The median age at presentation was 9.5 months (range 1 month-11 years), six were males, and the median follow-up time was 3.8 (1.1-14) years. Mutations in CYP24A1 and SLC34A3 were identified in three and one patients, respectively. Five patients presented with nephrocalcinosis, three with nephrolithiasis, and two had normal renal ultrasound. High blood calcium and 1,25-(OH)₂D levels at presentation decreased during follow-up [11.1 ± 1 vs. 9.9 ± 0.5 mg/dl (p = 0.012), and 307 ± 130 vs. 209 ± 65 pmol/l (p = 0.03), respectively]; this paralleled an increase in suppressed PTH levels (5.8 ± 0.9 vs. 11.8 ± 7.3 pg/ml, p = 0.2). Substantial improvements in hypercalciuria and renal sonography findings were not observed. Two patients had impaired renal function (eGFR 84-88 ml/min/1/73 m²) at the last follow up. Interventions included appropriate diet, citrate supplementation, and thiazides. Conclusion: Despite improvement in hypercalcemia and 1,25-(OH)₂D levels, not all the patients showed improvements in hypercalciuria and nephrocalcinosis. Deterioration of renal function was also observed. Long-term follow up and intervention to prevent nephrocalcinosis and nephrolithiasis are recommended in these children.

Overlapping Phenotypes Associated With CYP24A1, SLC34A1, and SLC34A3 Mutations: A Cohort Study of Patients With Hypersensitivity to Vitamin D

Mutations in CYP24A1 (vitamin D 24-hydroxylase) and SLC34A1 (renal phosphate transporter NPT2a) cause autosomal recessive Infantile Hypercalcemia type 1 and 2, illustrating links between vitamin D and phosphate metabolism. Patients may present with hypercalciuria and alternate between chronic phases with normal serum calcium but inappropriately high 1,25-(OH)₂D and appropriately low PTH, and acute phases with hypercalcemia with suppressed PTH. Mutations in SLC34A3 and SLC9A3R1 have been associated with phosphate wasting without hypercalcemia. The aims of this study were to evaluate the frequency of mutations in these genes in patients with a medical history suggestive of CYP24A1 mutation to search for a specific pattern. Using next generation sequencing, we screened for mutations in 185 patients with PTH levels < 20 pg/mL, hypercalcemia and/or hypercalciuria, and relatives. Twenty-eight (15%) patients harbored biallelic mutations in CYP24A1 (25) and SLC34A3 (3), mostly associated with renal disease (lithiasis, nephrocalcinosis) (86%). Hypophosphatemia was found in 7 patients with biallelic mutations in CYP24A1 and a normal phosphatemia was reported in 2 patients with biallelic mutations in SLC34A3. Rare variations in SLC34A1 and SLC34A3 were mostly of uncertain significance. Fifteen patients (8%) carried only one heterozygous mutation. Heterozygous relatives carrying SLC34A1 or SLC34A3 variation may present with biochemical changes in mineral metabolism. Two patients' genotype may suggest digenism (heterozygous variations in different genes). No variation was found in SLC9A3R1. As no specific pattern can be found, patients with medical history suggestive of CYP24A1 mutation should benefit from SLC34A1 and SLC34A3 analysis.

CYP24A1 and SLC34A1 genetic defects associated with idiopathic infantile hypercalcemia: from genotype to phenotype

Loss of function mutations in the CYP24A1 gene, involved in vitamin D catabolism and in calcium homeostasis, are known to be the genetic drivers of both idiopathic infantile hypercalcemia (IIH) and adult renal stone disease. Recently, also defects in the SLC34A1 gene, encoding for the renal sodium-phosphate transporter NaPi-IIa, were associated with the disease. IIH typically affects infants and pediatric patients with a syndrome characterized by severe hypercalcemia, hypercalciuria, suppressed parathyroid hormone level and nephrolithiasis. In SLC34A1 mutated carriers, hypophosphatemia is also a typical biochemical tract. IIH may also persist undiagnosed into adulthood, causing an increased risk of nephrocalcinosis and renal complication. To note, a clinical heterogeneity characterizes IIH manifestation, principally due to the controversial gene-dose effect and, to the strong influence of environmental factors. The present review is aimed to provide an overview of the current molecular findings on the IIH disorder, giving a comprehensive description of the association between genotype and biochemical and clinical phenotype of the affected patients. We also underline that patients may benefit from genetic testing into a targeted diagnostic and therapeutic workflow.

Vitamin D supplementation in pregnancy: A word of caution. Familial hypercalcaemia due to disordered vitamin D metabolism

Disorders of vitamin D metabolism have only recently become more widely recognized. In 2011, a series reported six children with familial idiopathic infantile hypercalcaemia, a condition in which patients develop hypercalcaemia following bolus vitamin D supplementation due to mutations in CYP24A1, formerly known as 24-hydroxylase. This is the chief enzyme in the catabolism of 1,25-dihydroxyvitamin D3 (calcitriol) to inactive 1,24,25-(OH)3D3. Isolated cases of loss of function CYP24A1 mutations have been reported across a wide spectrum of ages, including three cases first identified during pregnancy in the context of vitamin D supplementation. We describe a family in which two siblings had hypercalcaemia due to a disorder of calcitriol catabolism as a result of compound heterozygous loss of function mutations of CYP24A1, including a novel mutation K351Nfs*21. The index case, who has kindly given written informed consent for this report, was a female in her mid-20s presenting with symptomatic hypercalcaemia precipitated by vitamin D supplementation in her first pregnancy. In a subsequent pregnancy, she remained normocalcaemic in the absence of supplementation. Her asymptomatic brother was identified through cascade screening. Upregulation of calcitriol production in pregnancy, particularly when combined with vitamin D supplementation, can unmask previously unidentified disorders of vitamin D metabolism. This report emphasizes the importance of screening of family members and the need for caution with vitamin D supplementation in pregnancy.

Nephrocalcinosis: A Review of Monogenic Causes and Insights They Provide into This Heterogeneous Condition

The abnormal deposition of calcium within renal parenchyma, termed nephrocalcinosis, frequently occurs as a result of impaired renal calcium handling. It is closely associated with renal stone formation (nephrolithiasis) as elevated urinary calcium levels (hypercalciuria) are a key common pathological feature underlying these clinical presentations. Although monogenic causes of nephrocalcinosis and nephrolithiasis are rare, they account for a significant disease burden with many patients developing chronic or end-stage renal disease. Identifying underlying genetic mutations in hereditary cases of nephrocalcinosis has provided valuable insights into renal tubulopathies that include hypercalciuria within their varied phenotypes. Genotypes affecting other enzyme pathways, including vitamin D metabolism and hepatic glyoxylate metabolism, are also associated with nephrocalcinosis. As the availability of genetic testing becomes widespread, we cannot be imprecise in our approach to nephrocalcinosis. Monogenic causes of nephrocalcinosis account for a broad range of phenotypes. In cases such as Dent disease, supportive therapies are limited, and early renal replacement therapies are necessitated. In cases such as renal tubular acidosis, a good renal prognosis can be expected providing effective treatment is implemented. It is imperative we adopt a precision-medicine approach to ensure patients and their families receive prompt diagnosis, effective, tailored treatment and accurate prognostic information.

New developments in our understanding of vitamin metabolism, action and treatment

Although vitamin D has been reported to have pleiotropic effects including effects on the immune system and on cancer progression, the principal action of vitamin D is the maintenance of calcium and phosphate homeostasis. The importance of vitamin D in this process is emphasized by the consequences of vitamin D deficiency which includes rickets in children and osteomalacia in adults. Vitamin D deficiency has also been reported to increase the risk of falls and osteoporotic fractures. Although vitamin D fortification of foods (including dairy products) has contributed to a marked decrease in rickets in the Western world, vitamin D deficiency in children and adults is still prevalent world-wide. This review summarizes new developments in our understanding of vitamin D endocrine system and addresses clinical syndromes related to abnormalities in vitamin D metabolism and action. In addition, the current understanding of the evaluation of vitamin D deficiency and sufficiency and recommendations for achieving vitamin D sufficiency are discussed.

Hereditary Hypercalcemia Caused by a Homozygous Pathogenic Variant in the CYP24A1 Gene: A Case Report and Review of the Literature

Introduction

Loss of function mutations of CYP24A1 gene, which is involved in vitamin D catabolism, cause vitamin D-mediated PTH-independent hypercalcemia. The phenotype varies from life-threatening forms in the infancy to milder forms in the adulthood.

Case Presentation

We report a case of a 17-year-old woman with a history of nephrolithiasis, mild PTH-independent hypercalcemia (10,5mg/dL), and high serum 1,25(OH)₂D concentrations (107pg/mL). Other causes of hypercalcemia associated with the above biochemical signature were excluded. Family history revealed nephrolithiasis in the sister. Blood testing in first-degree relatives showed serum PTH in the low-normal range and 1,25(OH)₂D at the upper normal limit or slightly elevated. The CYP24A1 gene analysis revealed a known homozygous loss-of-function pathogenic variant (c.428_430delAAG, rs777676129, p.Glu143del). The panel of vitamin D metabolites evaluated by liquid chromatography showed the typical profile of CYP24A1 mutations, namely, low 24,25(OH)₂D₃, elevated 25(OH)D₃:24,25(OH)₂D₃ ratio, and undetectable 1,24,25(OH)₃D₃. The parents and both the siblings harbored the same variant in heterozygosis. We decided for a watchful waiting approach and the patient remained clinically and biochemically stable over a 24-month followup.

Conclusion

CYP24A1 gene mutations should be considered in cases of PTH-independent hypercalcemia, once that more common causes (hypercalcemia of malignancy, granulomatous diseases, and vitamin D intoxication) have been ruled out.

Vitamin D metabolism in dogs with and without hypercalciuric calcium oxalate urolithiasis

Background

There are abnormalities in vitamin D metabolism in people with calcium nephrolithiasis, but limited data are available on vitamin D status in dogs with calcium oxalate (CaOx) urolithiasis.

Objective

To compare serum concentrations of vitamin D metabolites in dogs with and without hypercalciuric CaOx urolithiasis.

Animals

Thirty‐eight dogs with (n = 19) and without (n = 19) a history of CaOx urolithiasis and hypercalciuria.

Methods

Retrospective cross‐sectional study. Serum 25‐hydroxyvitamin D [25(OH)D], 1,25‐dihydroxyvitamin D [1,25(OH)₂D], and 24,25‐dihydroxyvitamin D [24,25(OH)₂D] were measured. The ratios of 25(OH)D/24,25(OH)₂D and 1,25(OH)₂D/25(OH)D were compared between cases and controls.

Results

There were no significant differences between cases and controls when comparing 25(OH)D, 24,25(OH)₂D, 1,25(OH)₂D, or 1,25(OH)₂D/25(OH)D. Cases had higher 25(OH)D/24,25(OH)₂D (median = 1.40, range = 0.98‐1.58) compared to controls (median = 1.16, range = 0.92‐2.75; P = .01). There was overlap in the ranges for 25(OH)D/24,25(OH)₂D between cases and controls, but 6 cases (32%) had ratios above the control dog range. There was a moderate positive correlation between the ratio of 25(OH)D/24,25(OH)₂D and urinary calcium‐to‐creatinine ratios (r = 0.40, 95% confidence interval = 0.10‐0.64; P = .01).

Conclusions and Clinical Importance

These data suggest that decreased conversion of 25(OH)D to 24,25(OH)₂D occurs in a subset of dogs with CaOx urolithiasis. Abnormalities in vitamin D metabolism might contribute to stone risk in dogs.

CYP24A1 Variants in Two Chinese Patients with Idiopathic Infantile Hypercalcemia

BACKGROUND

Biallelic pathogenic variants in CYP24A1 can cause idiopathic infantile hypercalcemia (HCINF).

METHODS

We report 2 additional molecular abnormalities in 2 Chinese children with CHINF1.

RESULTS

Biallelic variants in CYP24A1 were found in two patients. Patient One was compound heterozygous for c.449 + 1G > T and c.1426_1427delCT. Patient Two was compound heterozygous for c.1310C > A and c.1426_1427delCT. The c.1310C > A and c.449 + 1G > T were two different novel CYP24A1 variants. Multiple computational tools predicted that both impact protein function. A total of 36 variants have been previously reported in patients with HCINF1, of which 27 were classified as pathogenic or likely pathogenic and nine as uncertain clinical significance.

CONCLUSION

Genetic tests are helpful in order to counsel the susceptible individuals to avoid vitamin D and take preventive measures in order to avoid complications.

Prenatal hyperechogenic kidneys in three cases of infantile hypercalcemia associated with SLC34A1 mutations

BACKGROUND

Prenatal diagnosis of hyperechogenic kidneys is associated with a wide range of etiologies and prognoses. The recent advances in fetal ultrasound associated with the development of next-generation sequencing for molecular analysis have enlarged the spectrum of etiologies, making antenatal diagnosis a very challenging discipline. Of the various known causes of hyperechogenic fetal kidneys, calcium and phosphate metabolism disorders represent a rare cause. An accurate diagnosis is crucial for providing appropriate genetic counseling and medical follow-up after birth.

METHODS

We report on three cases of fetal hyperechogenic kidneys corresponding to postnatal diagnosis of nephrocalcinosis. In all cases, antenatal ultrasound showed hyperechogenic kidneys of normal to large size from 22 gestational weeks, with a normal amount of amniotic fluid. Postnatal ultrasound follow-up showed nephrocalcinosis associated with hypercalcemia, hypercalciuria, elevated 1,25(OH)₂-vitamin D, and suppressed parathyroid hormone levels.

RESULTS

Molecular genetic analysis by next-generation sequencing performed after birth in the three newborns revealed biallelic pathogenic variants in the SLC34A1 gene, encoding the sodium/phosphate cotransporter type 2 (Npt2a), confirming the diagnosis of infantile hypercalcemia.

CONCLUSIONS

Nephrocalcinosis due to infantile hypercalcemia can be a cause of fetal hyperechogenic kidneys, which suggests early antenatal anomaly of calcium and phosphate metabolism. This entity should be considered in differential diagnosis. Postnatal follow-up of infants with hyperechogenic kidneys should include evaluation of calcium and phosphate metabolism.

Update of the tolerable upper intake level for vitamin D for infants

Following a request from the European Commission, the Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to revise the tolerable upper intake level (UL) for vitamin D for infants (≤ 1 year) set in 2012. From its literature review, the Panel concluded that the available evidence on daily vitamin D intake and the risk of adverse health outcomes (hypercalciuria, hypercalcaemia, nephrocalcinosis and abnormal growth patterns) cannot be used alone for deriving the UL for infants. The Panel conducted a meta-regression analysis of collected data, to derive a dose-response relationship between daily supplemental intake of vitamin D and mean achieved serum 25(OH)D concentrations. Considering that a serum 25(OH)D concentration of 200 nmol/L or below is unlikely to pose a risk of adverse health outcomes in infants, the Panel estimated the percentage of infants reaching a concentration above this value at different intakes of vitamin D. Based on the overall evidence, the Panel kept the UL of 25 μg/day for infants aged up to 6 months and set a UL of 35 μg/day for infants 6-12 months. The Panel was also asked to advise on the safety of the consumption of infant formulae with an increased maximum vitamin D content of 3 μg/100 kcal (Commission Delegated Regulation (EU) 2016/127 repealing Directive 2006/141/EC in 2020). For infants aged up to 4 months, the intake assessment showed that the use of infant formulae containing vitamin D at 3 μg/100 kcal may lead some infants to receive an intake above the UL of 25 μg/day from formulae alone without considering vitamin D supplemental intake. For infants aged 4-12 months, the 95th percentile of vitamin D intake (high consumers) estimated from formulae and foods fortified or not with vitamin D does not exceed the ULs, without considering vitamin D supplemental intake.

Juvenile onset IIH and CYP24A1 mutations

The term Idiopathic infantile hypercalcemia (IIH) was first introduced almost 70 years ago when symptomatic hypercalcemia developed in children after receiving high doses of vitamin D for the prevention of rickets. The underlying pathophysiology remained unknown until recessive mutations in CYP24A1 encoding Vitamin D₃-24-hydroxylase were discovered. The defect in vitamin D degradation leads to an accumulation of active 1,25(OH)₂D₃ with subsequent hypercalcemia. Enhanced renal calcium excretions lead to hypercalciuria and nephrocalcinosis. Meanwhile, the phenotypic spectrum associated with CYP24A1 mutations has significantly broadened. Patients may present at all age groups with symptoms originating from increased serum calcium levels as well as from increased urinary calcium excretions, i.e. kidney stones. Possible long term sequelae comprise chronic renal failure as well as cardiovascular disease. Here, we present a family with two affected siblings with differing clinical presentation as an example for the phenotypic variability of CYP24A1 defects.

•

CYP24A1 mutations result in increased vitamin D sensitivity.

•

Associated phenotypes range from infantile hypercalcemia to kidney stone disease.

•

Potential long-term sequelae include chronic renal failure.

•

Future research needs to focus on potential treatments to limit vitamin D activation.

Regulation of vitamin D metabolizing enzymes in murine renal and extrarenal tissues by dietary phosphate, FGF23, and 1,25(OH)2D3

Background

The 1,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) together with parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) regulates calcium (Ca²⁺) and phosphate (Pi) homeostasis, 1,25(OH)₂D₃ synthesis is mediated by hydroxylases of the cytochrome P450 (Cyp) family. Vitamin D is first modified in the liver by the 25-hydroxylases CYP2R1 and CYP27A1 and further activated in the kidney by the 1α-hydroxylase CYP27B1, while the renal 24-hydroxylase CYP24A1 catalyzes the first step of its inactivation. While the kidney is the main organ responsible for circulating levels of active 1,25(OH)₂D₃, other organs also express some of these enzymes. Their regulation, however, has been studied less.

Methods and results

Here we investigated the effect of several Pi-regulating factors including dietary Pi, PTH and FGF23 on the expression of the vitamin D hydroxylases and the vitamin D receptor VDR in renal and extrarenal tissues of mice. We found that with the exception of Cyp24a1, all the other analyzed mRNAs show a wide tissue distribution. High dietary Pi mainly upregulated the hepatic expression of Cyp27a1 and Cyp2r1 without changing plasma 1,25(OH)₂D₃. FGF23 failed to regulate the expression of any of the studied hydroxylases at the used dosage and treatment length. As expected, renal mRNA expression of Cyp27b1 was reduced and Cyp24a1 was increased in response to 1,25(OH)₂D₃ treatment. However, the 25-hydroxylases were rather unaffected by 1,25(OH)₂D₃ treatment.

Conclusions

The analyzed vitamin D hydroxylases are regulated in a tissue and treatment-specific manner.

The When, What & How of Measuring Vitamin D Metabolism in Clinical Medicine

We now have the ability to measure a number of different vitamin D metabolites with very accurate methods. The most abundant vitamin D metabolite, 25-hydroxyvitamin D, is currently the best marker for overall vitamin D status and is therefore most commonly measured in clinical medicine. The added value of measuring metabolites beyond 25-hydroxyvitamin D, like 1,25-, and 24,25-dihydroxyvitamin D is not broadly appreciated. Yet, in some more complicated cases, these metabolites may provide just the information needed for a legitimate diagnosis. The problem at present, is knowing when to measure, what to measure and how to measure. For 25-hydroxyvitamin D, the most frequently used automated immunoassays do not meet the requirements of today’s standards for certain patient groups and liquid chromatography-tandem mass spectrometry is the desired method of choice in these individuals. The less frequently measured 1,25-dihydroxyvitamin D metabolite enables us to identify a number of conditions, including 1α-hydroxylase deficiency, hereditary vitamin D-resistant rickets and a number of granulomatous diseases or lymphoproliferative diseases accompanied by hypercalcaemia. Furthermore, it discriminates between the FGF23-mediated and non-FGF23-mediated hypophosphatemic syndromes. The 24,25-dihydroxyvitamin D metabolite has proven its value in the diagnosis of idiopathic infantile hypercalcaemia and has the potential of having value in identifying other diseases. For both metabolites, the understanding of the origin of differences between assays is limited and requires further attention. Nonetheless, in every way, appropriate measurement of vitamin D metabolism in the clinical laboratory hinges eminently on the comprehension of the value of the different metabolites, and the importance of the choice of method.

A rapid screening of a recurrent CYP24A1 pathogenic variant opens the way to molecular testing for Idiopathic Infantile Hypercalcemia (IIH)

INTRODUCTION

Loss-of-function mutations in cytochrome P450 family 24 subfamily A member 1 (CYP24A1) gene are associated with Idiopathic Infantile Hypercalcemia (IIH) and adult kidney stone disease. The enzyme deficiency leads to an impaired vitamin D catabolism pathway, resulting in a syndrome characterized by recurrent hypercalcemia, hypercalciuria and suppressed parathyroid hormone (PTH) levels. In these patients, the genetic evaluation of CYP24A1 is an important diagnostic tool, allowing the definitive diagnosis of IIH.

METHODS

A rapid CYP24A1 gene testing based on High Resolution Melting Analysis (HRMA) was designed in order to detect the CYP24A1 c.428_430delAAG (p.Glu143del), a recurrent IIH-associated variant.

RESULTS

HRMA method was able to identify c.428_430delAAG genotypes evaluating melting curve shape and melting temperature (T_m). Heterozygous samples exhibited a typical melting profile while homozygous samples showed a specific T_m shift.

CONCLUSIONS

We provide evidence about application of HRMA in unambiguous genotyping of the CYP24A1 c.428_430delAAG variant, making this method useful in clinical molecular diagnostics. This approach opens the way to a helpful molecular analysis of CYP24A1 gene in IIH diagnosis, to an improved pharmacological treatment strategy and to a reduced risk of recurrent stones and worsening nephrocalcinosis.

CYP24A1 Mutation in a Girl Infant with Idiopathic Infantile Hypercalcemia

Idiopathic infantile hypercalcemia (IIH) was associated with vitamin-D supplementation in the 1950's. Fifty years later, mutations in the CYP241A gene, involved in the degradation of vitamin-D, have been identified as being a part of the etiology. We report a case of a 21-month old girl, initially hospitalized due to excessive consumption of water and behavioral difficulties. Blood tests showed hypercalcemia and borderline high vitamin-D levels. Renal ultrasound revealed medullary nephrocalcinosis. An abnormality in vitamin-D metabolism was suspected and genetic testing was performed. This revealed the patient to be compound heterozygous for a common (p.E143del) and a novel (likely) disease-causing mutation (p.H83D) in the CYP24A1 gene. The hypercalcemia normalized following a calcium depleted diet and discontinuation of vitamin-D supplementation. Increased awareness of the typical symptoms of hypercalcemia, such as anorexia, polydipsia, vomiting and failure to thrive, is of utmost importance in diagnosing IHH early and preventing long-term complications such as nephrocalcinosis. Further identification of as many disease-causing mutations in the CYP24A1 gene as possible can help identification of predisposed individuals in whom vitamin-D supplementation should be reconsidered.

Biology and Mechanisms of Action of the Vitamin D Hormone

The central role of hormonal 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] is to regulate calcium and phosphorus homeostasis via actions in intestine, kidney, and bone. These and other actions in many cell types not involved in mineral metabolism are mediated by the vitamin D receptor. Recent studies using genome-wide scale techniques have extended fundamental ideas regarding vitamin D-mediated control of gene expression while simultaneously revealing a series of new concepts. This article summarizes the current view of the biological actions of the vitamin D hormone and focuses on new concepts that drive the understanding of the mechanisms through which vitamin D operates.

Hypercalcemic Disorders in Children

Hypercalcemia is defined as a serum calcium concentration that is greater than two standard deviations above the normal mean, which in children may vary with age and sex, reflecting changes in the normal physiology at each developmental stage. Hypercalcemic disorders in children may present with hypotonia, poor feeding, vomiting, constipation, abdominal pain, lethargy, polyuria, dehydration, failure to thrive, and seizures. In severe cases renal failure, pancreatitis and reduced consciousness may also occur and older children and adolescents may present with psychiatric symptoms. The causes of hypercalcemia in children can be classified as parathyroid hormone (PTH)-dependent or PTH-independent, and may be congenital or acquired. PTH-independent hypercalcemia, ie, hypercalcemia associated with a suppressed PTH, is commoner in children than PTH-dependent hypercalcemia. Acquired causes of PTH-independent hypercalcemia in children include hypervitaminosis; granulomatous disorders, and endocrinopathies. Congenital syndromes associated with PTH-independent hypercalcemia include idiopathic infantile hypercalcemia (IIH), William's syndrome, and inborn errors of metabolism. PTH-dependent hypercalcemia is usually caused by parathyroid tumors, which may give rise to primary hyperparathyroidism (PHPT) or tertiary hyperparathyroidism, which usually arises in association with chronic renal failure and in the treatment of hypophosphatemic rickets. Acquired causes of PTH-dependent hypercalcemia in neonates include maternal hypocalcemia and extracorporeal membrane oxygenation. PHPT usually occurs as an isolated nonsyndromic and nonhereditary endocrinopathy, but may also occur as a hereditary hypercalcemic disorder such as familial hypocalciuric hypercalcemia, neonatal severe primary hyperparathyroidism, and familial isolated primary hyperparathyroidism, and less commonly, as part of inherited complex syndromic disorders such as multiple endocrine neoplasia (MEN). Advances in identifying the genetic causes have resulted in increased understanding of the underlying biological pathways and improvements in diagnosis. The management of symptomatic hypercalcemia includes interventions such as fluids, antiresorptive medications, and parathyroid surgery. This article presents a clinical, biochemical, and genetic approach to investigating the causes of pediatric hypercalcemia. © 2017 American Society for Bone and Mineral Research.

Vitamin D Activity and Metabolism in Bone

PURPOSE OF REVIEW

In addition to the actions of the endocrine hormone, 1alpha,25-dihydroxyvitamin D (1,25(OH)₂D) in stimulating intestinal calcium absorption, the regulation of bone mineral metabolism by 1,25(OH)₂D is also considered an important contributor to calcium homeostasis. However, recent evidence suggest that 1,25(OH)₂D acting either via endocrine or autocrine pathways plays varied roles in bone, which suggests that vitamin D contributes to the maintenance of bone mineral in addition to its catabolic roles. This review highlights the contrasting evidence for the direct action for vitamin D metabolism and activity in bone.

RECENT FINDINGS

Numerous cells within bone express vitamin D receptor (VDR), synthesise and catabolise 1,25(OH)₂D via 25-hydroxyvitamin D 1alpha-hydroxylase (CYP27B1), and 25-hydroxyvitamin D 24-hydroxylase (CYP24A1) enzymes, respectively. Recent evidence suggests that all three genes are required to regulate processes of bone resorption, mineralization and fracture repair. The actions of vitamin D in bone appear to negatively or positively regulate bone mineral depending on the physiological and pathological circumstances, suggesting that vitamin D plays pleiotropic roles in bone.

CYP24A1 loss of function: Clinical phenotype of monoallelic and biallelic mutations

CYP24A1, encoding the vitamin D-24-hydroxylase, is of major clinical and physiologic importance, serving to regulate the catabolism of 1,25-(OH)₂D, the physiologically active vitamin D metabolite. In addition to facilitating catabolism of 1,25-(OH)₂D, CYP24A1 also enhances the turnover and elimination of 25-OHD, the abundant precursor metabolite and storage form of the vitamin. CYP24A1 can be stimulated hormonally by 1,25-(OH)₂D and by FGF23, whereas CYP27B1, encoding the vitamin D-1α-hydroxylase, is stimulated hormonally by parathyroid hormone (PTH) and downregulated by FGF23. Thus CYP24A1 and CYP27B1, together, provide for alternate and regulated fates of 25-OHD, and control the availability of the active metabolite, 1,25-(OH)₂D, depending upon physiologic needs. These two enzymes, are therefore central to the homeostatic control of vitamin D metabolism, and as a result affect calcium metabolism in critical ways. Disruption of CYP24A1 in mice results in elevated circulating 1,25-(OH)₂D, substantiating the importance of the enzyme in the maintenance of vitamin D metabolism. The consequential skeletal phenotype in these mice further demonstrates the biologic sequelae of the disruption of the vitamin D pathway, and illustrates a specific developmental pathology mediated largely by oversupply of 1,25-(OH)₂D. More recent evidence has identified loss of function mutations in CYP24A1 in association with hypercalcemia, hypercalciuria and nephrolithiasis in humans. Initial reports described certain variant mutations in CYP24A1 as an unrecognized cause of "Idiopathic Infantile Hypercalcemia," and more recently older children and adults have been identified with a similar phenotype. Over 25 likely disease-causing variants are described. Homozygous and compound heterozygote mutations account for the overwhelming majority of cases, however the heterozygous loss-of-function mutations of CYP24A1 do not appear to consistently result in symptomatic hypercalcemia. Considerations ripe for exploration include the potential role for such mutations in the tolerance to challenges to the calcium homeostatic system, such as changes in dietary calcium intake, vitamin D supplementation, sunlight exposure or pregnancy.

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.

A variant in a cis-regulatory element enhances claudin-14 expression and is associated with pediatric-onset hypercalciuria and kidney stones

The greatest risk factor for kidney stones is hypercalciuria, the etiology of which is largely unknown. A recent genome-wide association study (GWAS) linked hypercalciuria and kidney stones to a claudin-14 (CLDN14) risk haplotype. However, the underlying molecular mechanism was not delineated. Recently, renal CLDN14 expression was found to increase in response to increased plasma calcium, thereby inducing calciuria. We hypothesized therefore that some children with hypercalciuria and kidney stones harbor a CLDN14 variant that inappropriately increases gene expression. To test this hypothesis, we sequenced the CLDN14 risk haplotype in a cohort of children with idiopathic hypercalciuria and kidney stones. An intronic SNP was more frequent in affected children. Dual luciferase and cell-based assays demonstrated increased reporter or CLDN14 expression when this polymorphism was introduced. In silico studies predicted the SNP introduced a novel insulinoma-associated 1 (INSM1) transcription factor binding site. Consistent with this, repeating the dual luciferase assay in the presence of INSM1 further increased reporter expression. Our data suggest that children with the INSM1 binding site within the CLDN14 risk haplotype have a higher likelihood of hypercalciuria and kidney stones. Enhanced CLDN14 expression may play a role in the pathophysiology of their hypercalciuria.

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.