Idiopathic Infantile Hypercalcemia Research Articles

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

ObjectiveIdiopathic 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.MethodsThe clinical manifestations and laboratory study of six patients with idiopathic infantile hypercalcemia were analyzed retrospectively.ResultsFive 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.ConclusionsA 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.

O-12 A rare cause of parathyroid hormone independent hypercalcemia: 1,25-(OH)2D-24 Hydroxylase (CYP24A1) deficiency

Abstract Background Common causes of parathyroid hormon (PTH)-independent hypercalcemia in adulthood are malignancy, granulomatous diseases, and endocrinopathies. Loss-of-function mutations of CYP24A1 gene, encoding for 1,25-dihydroxyvitamin D3 24-hydroxylase, can also be a rare cause of supressed intact PTH levels, usually associated with, nephrolithiasis and nephrocalcinosis. This disease, defined as idiopathic infantile hypercalcemia, classically presents with hypercalcemia in infants. However, some individuals have also been reported with late-onset clinical manifestation or late diagnosis in adulthood. Clinical Case A 59-year-old male patient with a known diagnosis of type 2 diabetes mellitus for 30 years. He had a history of recurrent kidney stones since 2006. His blood and urine tests revealed elevated creatinine level, hypercalcemia with normal phosphorus levels, normal 25-hydroxycholecalciferol levels, suppressed intact PTH level and normal 24-hour urinary calcium level (Table 1). Urinary ultrasonography was consisted with medullary nephrocalcinosis and chronic kidney disease. PTH-independent causes of hypercalcemia were investigated. Malignancies, granulomatous diseases and endocrinopathies were ruled out. The 1,25-dihydroxycholecalciferol level was within the normal range. Parathormone related peptide level was determined as 1.60 pmol/L (normal value <4.2). Genetic workup revealed a homozygous loss-of-function mutation p.E143del in the CYP24A1 gene. Thereupon, genetic screening started on the siblings of the patient which not been finalized yet. However, none of them were found to have hypercalcemia in the laboratory evaluation. Treatment was started with low-dose fluconazole, which inhibits enzymes (including 25-hydroxylases and 1-α-hydroxylase) synthesizing vitamin D. After a month of treatment at a dose of 50 mg/day, no change was observed in the patient's calcium values. Follow-up with calcium-restricted diet and adequate oral hydration therapy was planned. Conclusion Idiopathic infantile hypercalcemia may manifest later or diagnosed later in adulthood. Genetic examination should be considered in adult patients in whom other causes of PTH-independent hypercalcemia have been excluded.Table 1.Patient's laboratory parameters

Idiopathic infantile hypercalcemia with a <i>CYP24A1 </i>variant triggered by vitamin D supplementation in fortified milk: a case report

Idiopathic infantile hypercalcemia with a <i>CYP24A1 </i>variant triggered by vitamin D supplementation in fortified milk: a case report

A vitamin D receptor antagonist as a potent and safe treatment for Idiopathic Infantile Hypercalcemia

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

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.

Pharmacology of Mammalian Na+-Dependent Transporters of Inorganic Phosphate.

Inorganic phosphate (Pi) is an essential component of many biologically important molecules such as DNA, RNA, ATP, phospholipids, or apatite. It is required for intracellular phosphorylation signaling events and acts as pH buffer in intra- and extracellular compartments. Intestinal absorption, uptake into cells, and renal reabsorption depend on a set of different phosphate transporters from the SLC20 (PiT transporters) and SLC34 (NaPi transporters) gene families. The physiological relevance of these transporters is evident from rare monogenic disorders in humans affecting SLC20A2 (Fahr's disease, basal ganglia calcification), SLC34A1 (idiopathic infantile hypercalcemia), SLC34A2 (pulmonary alveolar microlithiasis), and SLC34A3 (hereditary hypophosphatemic rickets with hypercalciuria). SLC34 transporters are inhibited by millimolar concentrations of phosphonoformic acid or arsenate while SLC20 are relatively resistant to these compounds. More recently, a series of more specific and potent drugs have been developed to target SLC34A2 to reduce intestinal Pi absorption and to inhibit SLC34A1 and/or SLC34A3 to increase renal Pi excretion in patients with renal disease and incipient hyperphosphatemia. Also, SLC20 inhibitors have been developed with the same intention. Some of these substances are currently undergoing preclinical and clinical testing. Tenapanor, a non-absorbable Na+/H+-exchanger isoform 3 inhibitor, reduces intestinal Pi absorption likely by indirectly acting on the paracellular pathway for Pi and has been tested in several phase III trials for reducing Pi overload in patients with renal insufficiency and dialysis.

LBSAT130 Hypercalcemia In Pregnancy Due To CYP24A1 Mutation

Abstract Background The active form of vitamin D (1,25 OH2 vitamin D) is catabolized by a key enzyme 1,25 OH vitamin D-24 hydroxylase, which is encoded by the CYP24A1 gene. Loss of function mutation in the CYP24A1 leads to elevated levels of active vitamin D, causing PTH independent hypercalcemia with nephrolithiasis and nephrocalcinosis. Two phenotypic variants have been identified: idiopathic infantile hypercalcemia, which is a more severe form presenting in infancy, and a milder form with onset in adulthood as is this case. Pregnancy is known to initiate or worsen the hypercalcemia. Clinical Case A 27-year-old woman presented at 11 weeks gestation with hyperemesis, noted to have hypercalcemia calcium 15.7mg/dl (8.2 to 10.1 pg/ml), low PTH 2pg/ml (24 -85 pg/ml) with high serum 1,25 OHD 309pg/ml (nl 19.9-79.3 pg/ml). She had low PTHrp, normal ACE level, and negative TB testing. Pre-pregnancy calcium was normal (9.8mg/dl). She had a history of hypercalcemia in previous pregnancy (Calcium 13mg/dl) that ended in IUFD. Family history revealed nephrolithiasis in her sister, mother, and paternal aunt as well as a consanguineous marriage in our patient and her parents. Vitamin D metabolites evaluated via liquid chromatography showed low serum 24,25(OH)2D (0.29ng/ml nl N/A) and elevated 25OHD 138 ng/mL (20-80 ng/ml). Ratio of 25 (OH) Vit D to 24,25(OH)2D was 475.86 (values >80 indicate probable bi-allelic CYP24A1 deletion or mutation. Genetic test pending. Conclusion This case highlights the fact that patients with 24 hydroxylase deficiency may be normocalcemic with hypercalcemia unveiled during pregnancy due to increased 1alpha hydroxylase production by the placenta and kidneys as well as intake of prenatal vitamins containing calciferol. An index of suspicion for CYP24A1 mutation is needed in patients with high active vitamin D metabolites, high or normal calcium, low PTH, family history of kidney stones or worsening hypercalcemia in pregnancy. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.

Idiopathic Infantile Hypercalcemia in Children With CKD due to Kidney Hypodysplasia

Idiopathic Infantile Hypercalcemia in Children With CKD due to Kidney Hypodysplasia

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

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). 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. 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 ± 46.4pmol/l, p < 0.001), total calcium (11.1 ± 0.4 vs. 10.7 ± 0.3mg/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 12months of follow-up, PTH increased among the study group (8.8 ± 2.8 to 22.7 ± 12.4pg/ml, p < 0.001), calcium decreased (11 ± 0.5 to 10.3 ± 0.6mg/dl, p = 0.004), 1,25(OH)2D decreased (259.5 ± 91.7 to 188.2 ± 42.6pmol/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. 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 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-R396w/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.

Idiopathic infantile hypercalcemia with severe nephrocalcinosis, associated with CYP24A1 mutations: a case report

Nephrocalcinosis often occurs in infants and is caused by excessive calcium or vitamin D supplementation, neonatal primary hyperparathyroidism, and genetic disorders. Idiopathic infantile hypercalcemia (IIH), a rare cause of nephrocalcinosis, results from genetic defects in CYP24A1 or SLC34A1. Mutations in CYP24A1, which encodes 25-hydroxyvitamin D 24-hydroxylase, disrupt active vitamin D degradation. IIH clinically manifests as failure to thrive and hypercalcemia within the first year of life and usually remits spontaneously. Herein, we present a case of IIH wih CYP24A1 mutations.An 11-month-old girl visited our hospital with incidental hypercalcemia. She showed failure to thrive, and her oral intake had decreased over time since the age of 6 months. Her initial serum parathyroid hormone level was low, 25-OH vitamin D and 1,25-OH vitamin D levels were normal, and renal ultrasonography showed bilateral nephrocalcinosis. Whole-exome sequencing revealed compound heterozygous variants in CYP24A1 (NM_000782.4:c.376C&gt;T [p.Pro126Ser] and c.1310C&gt;A [p.Pro437His]). Although her hypercalcemia and poor oral intake spontaneously resolved in approximately 8 months, we suggested that her nephrocalcinosis and renal function be regularly checked in consideration of potential asymptomatic renal damage. Hypercalcemia caused by IIH should be suspected in infants with severe nephrocalcinosis, especially when presenting with failure to thrive.

The human pathogenic 91del7 mutation in SLC34A1 has no effect in mineral homeostasis in mice

Kidneys are key regulators of phosphate homeostasis. Biallelic mutations of the renal Na+/phosphate cotransporter SLC34A1/NaPi-IIa cause idiopathic infantile hypercalcemia, whereas monoallelic mutations were frequently noted in adults with kidney stones. Genome-wide-association studies identified SLC34A1 as a risk locus for chronic kidney disease. Pathogenic mutations in SLC34A1 are present in 4% of the general population. Here, we characterize a mouse model carrying the 91del7 in-frame deletion, a frequent mutation whose significance remains unclear. Under normal dietary conditions, 12 weeks old heterozygous and homozygous males have similar plasma and urinary levels of phosphate as their wild type (WT) littermates, and comparable concentrations of parathyroid hormone, fibroblast growth factor 23 (FGF-23) and 1,25(OH)2 vitamin D3. Renal phosphate transport, and expression of NaPi-IIa and NaPi-IIc cotransporters, was indistinguishable in the three genotypes. Challenging mice with low dietary phosphate did not result in differences between genotypes with regard to urinary and plasma phosphate. Urinary and plasma phosphate, plasma FGF-23 and expression of cotransporters were similar in all genotypes after weaning. Urinary phosphate and bone mineral density were also comparable in 300 days old WT and mutant mice. In conclusion, mice carrying the 91del7 truncation do not show signs of impaired phosphate homeostasis.

Hypercalcemia due to CYP24A1 mutations: a systematic descriptive review.

CYP24A1 encodes a 24-hydroxylase involved in vitamin D catabolism, whose loss-of-function results in vitamin D-dependent hypercalcemia. Since the identification of CYP24A1 variants as a cause of idiopathic infantile hypercalcemia, a large body of literature has emerged indicating heterogeneity in penetrance, symptoms, biochemistry, and treatments. The objectives of the present research work were to investigate the clinical heterogeneity of the disease, the possibility of a relevant phenotype for monoallelic carriers, and to compare the hypocalcemic effect of the available therapies. Two reviewers searched different databases for studies published between the identification of CYP24A1 variants and December 31, 2020. Eligible studies included clinical trials and reports describing carriers of CYP24A1 variants. Fifty eligible studies were identified, accounting for 221 patients. Genetic data were retrieved and allele frequencies were calculated. Acute hypercalcemia was the typical presentation during the first year of life (76%, P = 0.0005), and nephrocalcinosis was more frequent in infancy (P < 0.0001). Pregnancy was associated with symptomatic hypercalcemia in 81.8% and high rates of obstetric complications. Monoallelic carriers displayed significant rates of nephrolithiasis (19.4%), nephrocalcinosis (4.9%), and symptomatic hypercalcemia (5.6%). CYP24A1 loss-of-function results in an age-dependent phenotype, which can be exacerbated by triggering factors, such as pregnancy. Although biallelic carriers present more significant clinical and biochemical features, monoallelic carriers have an increased risk of calcium-related conditions. The highly variable tested therapeutic approaches did not allow to draw conclusions on preferable therapeutic regime.

CYP24A1 and SLC34A1 Mutations in Five Cases with Idiopathic Infantile Hypercalcemia

CYP24A1 and SLC34A1 Mutations in Five Cases with Idiopathic Infantile Hypercalcemia

Идиопатическая гиперкальциемия у ребенка: клинический случай гипервитаминоза D и гиперкальциемии у ребенка раннего возраста

На фоне практически мировой пандемии недостаточности витамина D среди населения планеты, и особенно у детей, необходимо привлечь внимание врачей к состояниям, сопровождающимся гипервитаминозом D и тяжелыми нарушениями фосфорно-кальциевого обмена, которые приводят к опасным последствиям. К ним относится идиопатическая инфантильная гиперкальциемия. Идиопатическая инфантильная гиперкальциемия — редкостное наследственное аутосомно-рецессивное заболевание, связанное с нарушением инактивации активных метаболитов витамина D вследствие инактивирующих мутаций в гене CYP24A1. В статье приведен клинический случай гипервитаминоза D, гиперкальциемии, гипопаратиреоза, развития нефрокальциноза у ребенка раннего возраста на фоне инактивирующих мутаций гена CYP24A1.

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.

Ultra-rare renal diseases diagnosed with whole-exome sequencing: Utility in diagnosis and management

BackgroundThis study aimed to use whole-exome sequencing (WES) to diagnose ultra-rare renal diseases and the clinical impact of such an approach on patient care.MethodsClinical, radiological, pathological, and genetic findings were reviewed in the patients and their family members.ResultsNine patients from nine unrelated Korean families were included in the study and evaluated. WES identified eight different conditions in these patients, i.e., autosomal dominant tubulointerstitial kidney disease associated with UMOD mutation; recurrent urinary stones associated with APRT deficiency; Ayme-Gripp syndrome associated with MAF mutation; short rib-thoracic dysplasia associated with IFT140 mutation; renal coloboma syndrome associated with PAX2 mutations; idiopathic infantile hypercalcemia associated with CYP24A1 mutation; and hypomagnesemia associated with TRPM mutation. Eleven different mutations, including seven novel mutations, were identified, i.e., four truncating mutations, six missense mutations, and one splice-acceptor variant. After genetic confirmation, strategies for the management of the following: medications, donor selection for renal transplantation, and surveillance for extra-renal manifestations were altered. In addition, genetic counseling was provided for the patients and their family members with respect to family member screening for affected but yet unidentified patients and future reproductive planning.ConclusionAs WES can effectively identify ultra-rare genetic renal diseases, facilitate the diagnosis process, and improve patient care, it is a good approach to enable a better understanding of ultra-rare conditions and for the establishment of appropriate counseling, surveillance, and management strategies.

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

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. 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. 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. 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. 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.

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.

Deciphering allosteric coupling between substrate binding and redox partner recognition in rat CYP24A1, a Vitamin‐D C‐24 hydroxylase

The mitochondrial cytochrome P450 24A1 (CYP24A1) is necessary for the deactivation of bioactive vitamin-D (1,25(OH)2D3) by side-chain hydroxylation. The enzyme requires molecular recognition by its cognate redox partner adrenodoxin (Adx), which houses an iron-sulfur cluster and provides two electrons for the hydroxylation step to achieve the reduction of heme iron in CYP24A1. Dysregulation of CYP24A1 may lead to idiopathic infantile hypercalcemia, characterized by elevated calcium, failure to thrive, vomiting, dehydration, and nephrocalcinosis. We have previously used solution nuclear magnetic resonance (NMR) to demonstrate the long-range allostery of CYP24A1 in response to redox partner recognition. However, the details of this allosteric connection are not clear. In this study, we utilize chemical cross-linking coupled to mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy, and CYP24A1 functional assays to inform a working model of a CYP24A1–Adx complex. We report that differential chemical cross-linking internal to CYP24A1 points toward an Adx-induced conformational change that perturbs the F and G α-helices, which are required for substrate binding. The combined presence of Adx with 1,25(OH)2D3 induces a change in this cross-linking pattern, consistent with an open-to-closed change in the enzyme. Both outcomes suggest conformational coupling in the CYP24A1 structure in response to Adx and 1,25(OH)2D3. Moreover, the modeled complex suggests that a semi-conserved nonpolar interaction at the interface may influence CYP24A1 regioselectivity. This insight leads us to a revised hypothesis in which conformational coupling is also important in determining site-specific hydroxylation of the CYP24A1 substrate. Taken together, these findings contribute toward our understanding of Adx recognition in a critical vitamin-D inactivating enzyme and provide broader insight regarding the variability inherent in CYP–Adx interactions.