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

<b><i>Objectives:</i></b> Biallelic pathogenic variants in <i>CYPA24A1</i> and <i>SLC34A1</i> 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 <i>CYP24A1</i> or <i>SLC34A1</i> variants in children with early hypercalcemia or late-onset hypercalciuria. <b><i>Method:</i></b> 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 <i>CYP24A1</i> enzyme. Mutational analyses were undertaken. Family cascade screening was performed if pathogenic variants were detected in probands. <b><i>Results:</i></b> 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 <i>CYP24A1.</i> Four <i>CYP24A1</i> 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 <i>SLC34A1</i> pathogenic variants were detected. <b><i>Conclusion:</i></b> In Canada, pathogenic variants in <i>CYP24A1</i> 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 <i>CYP24A1</i>. We confirm that cascade testing is important for these variants.

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 4-6 months. Results Median age at initial diagnosis was 4·5 months. Median levels of serum calcium (2·82 mmol/l) and 1,25 (OH)2 D (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(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.

Mild Idiopathic Infantile Hypercalcemia – Part 1: Biochemical and Genetic Findings

Context Idiopathic Infantile Hypercalcemia (IIH), an uncommon disorder characterized by elevated serum concentrations of 1,25(OH)2D and low PTH levels, may present with mild to severe hypercalcemia during the first months of life. Biallelic variants in the CYP24A1 or SLC34A1 genes are associated with severe IIH. Little is known about milder forms. Objective To characterize the genetic associations and biochemical profile of mild IIH. Methods This is a cross-sectional study including children between 6 months and 17 years of age with IIH who were followed in the Calcium Clinic at the Hospital for Sick Children (SickKids), Toronto, Canada. 20 children with mild IIH on calcium-restricted diets were evaluated. We performed a dietary assessment and analysed biochemical measures including vitamin D metabolites and performed a stepwise molecular genetic analysis. Complementary biochemical assessments and renal ultrasounds were offered to first-degree family members of positive probands. Results The median age was 16 months. Median serum levels of calcium (2·69 mmol/L), urinary Calcium:Creatinine ratio (0·72 mmol/mmol) and 1,25(OH)2D (209 pmol/L) were elevated while intact PTH was low normal (22·5 ng/L). Mean1,25(OH)2D/PTH and 1,25(OH)2D/25(OH)D ratios were increased by comparison to healthy controls. Eleven subjects (55%) had renal calcification. Genetic variants were common (65%) with the majority being heterozygous variants in SLC34A1 and SLC34A3 while a minority showed variants of CYP24A1 and other genes related to hypercalciuria. Conclusion The milder form of IIH has a distinctive vitamin D metabolite profile and is primarily associated with heterozygous SLC34A1 and SLC34A3 variants.

Duplex high resolution melting analysis (dHRMA) to detect two hot spot CYP24A1 pathogenic variants (PVs) associated to idiopathic infantile hypercalcemia (IIH)

Pathogenic variants (PVs) in CYP24A1 gene are associated with Idiopathic Infantile Hypercalcemia disease (IIH). The identification of CYP24A1 PVs can be a useful tool for the improvement of target therapeutic strategies. Aim of this study is to set up a rapid and inexpensive High Resolution Melting Analysis (HRMA)-based method for the simultaneous genotyping of two hot spot PVs in CYP24A1 gene, involved in IIH. A duplex-HRMA (dHRMA) was designed in order to detect simultaneously CYP24A1 c.428_430delAAG, p.(Glu143del) (rs777676129) and c.1186C > T, p.(Arg396Trp) (rs114368325), in peculiar cases addressed to our Laboratory. dHRMA was able to identify clearly and simultaneously both hot spot CYP24A1 PVs evaluating melting curve shape and melting temperature (Tm). This is the first dHRMA approach to rapidly screen the two most frequent CYP24A1 PVs in peculiar case, providing useful information for diagnosis and patient management in IIH disease.

Idiopathic infantile hypercalcemia: mutations in SLC34A1 and CYP24A1 in two siblings and fathers

ObjectivesBoth CYP24A1 and SLC34A1 gene mutations are responsible for idiopathic infantile hypercalcemia, whereas loss-of-function mutations in CYP24A1 (25-OH-vitamin D-24-hydroxylase) lead to a defect in the inactivation of active 1.25(OH)2D; mutations in SLC34A1 encoding renal sodium phosphate cotransporter NaPi-IIa lead to primary renal phosphate wasting combined with an inappropriate activation of vitamin D. The presence of mutations in both genes has not been reported in the same patient until today.Case presentationHypercalcemia was incidentally detected when a 13-month-old boy was being examined for urinary tract infection. After 21 months, hypercalcemia was detected in his six-month-old sister. High dose of vitamin D was not given to both siblings. Both of them also had hypophosphatemia and decreased tubular phosphate reabsorption. Intensive hydration, furosemide and oral phosphorus treatment were given. Bilateral medullary nephrocalcinosis was detected in both siblings and their father. Serum Ca and P levels were within normal limits at follow-up in both siblings. Siblings and their parents all carry a homozygous stop codon mutation (p.R466*) in CYP24A1. Interestingly, both siblings and the father also have a heterozygous splice-site mutation (IVS6(+1)G>A) in SLC34A1. The father has nephrocalcinosis.ConclusionsA biallelic loss-of-function mutation in the CYP24A1 gene was identified as responsible for hypercalcemia, hypercalciuria and nephrocalcinosis. In addition, a heterozygous mutation in the SLC34A1 gene, although not being the main pathogenic factor, might contribute to the severe phenotype of both patients.

SUN-096 Incidentally Found Severe Hypercalcemia in a Pediatric Patient, Diagnostic Challenge

Introduction: Idiopathic infantile hypercalcemia is an intriguing feature of Williams syndrome (WS), occurring in ~15% of diagnoses and is typically not clinically severe. Symptomatic hypercalcemia usually resolves during childhood, but lifelong abnormalities of calcium(Ca) and vitamin D metabolism may persist. The cause of the abnormality in Ca metabolism is still unknown. Hypercalciuria generally accompanies hypercalcemia, but isolated hypercalciuria, especially after infancy, can also occur. Nephrocalcinosis is relatively rare, found in less than 10% of patients undergoing renal ultrasonography. We report a 13-month-old female infant with a history of peripheral pulmonary stenosis and constipation, who presented with severe hypercalcemia that led to a new diagnosis of WS. Case presentation: A 13-month-old girl with a history of peripheral pulmonary stenosis, global developmental delay, and constipation presented to the neurology clinic for evaluation of gross motor delay. She was found to have upper body part hypotonia, decreased reflexes, and on laboratory evaluation, severe hypercalcemia with Ca level of 15.0 mg/dL (8.7 - 10.7). The patient was admitted for management of severe hypercalcemia. Physical exam was also remarkable for subtle features of WS: a happy baby, very social, with prominent eyes, full cheeks, flat nasal bridge, round nasal tip, full lips, and a wide smile. Repeated Ca level on admission was 15.9 mg/dL, with normal albumin level of 4.6 g/dL (2.9-5.5), elevated ionized calcium (iCal) of 1.99 mmol/L (0.95 - 1.32), and intact parathyroid hormone (PTH) of &lt;4.0 pg/mL (8.0 - 85.0). Further evaluation revealed a normal 25 hydroxy-vitamin D:41 ng/mL (30-80) and low 1,25-dihydroxy vitamin D:10pg/mL (31-87). Further evaluation revealed elevated urine calcium to creatinine ratio of 0.7 (normal for age &lt;0.56) and renal ultrasound remarkable for medullary nephrocalcinosis. The patient had a complete blood count within normal limits and a PTH related protein of 26 pg/mL (14-27), ruling out malignancy. Hypercalcemia responded well to intravenous fluids and diuretics, the patient being discharged home after two days on furosemide and potassium supplements with close electrolytes monitoring. The patient required calcium reducing therapy for four months to maintain Ca levels within 9-12 range. The medication was decreased gradually based on calcium and ical levels. The patient is currently doing well, with a normal calcium level, and has being off medication for the past three months. Conclusion: This is a rare case of severe hypercalcemia, which led to the diagnosis of WS. Although idiopathic infantile hypercalcemia occurs in 15% of patients with WS, usually the presentation is mild, and the patients do not require medical interventions. Our patient presented with severe hypercalcemia and subtle physical features of WS that led to genetic testing and final diagnosis.

SUN-LB13 Idiopathic Infantile Hypercalcemia Secondary to CYP24A1 Mutation: A Rare Case Without Exogenous Vitamin D Supplementation

Background: Mutations in CYP24A1, which encodes 24-hydroxylase, the key enzyme for Vitamin D breakdown, cause symptomatic hypercalcemia and nephrocalcinosis in infants on Vitamin D supplementation. New, symptomatic diagnoses of idiopathic infantile hypercalcemia without exogenous supplementation are rare. Previous case reports describe a seasonal effect with worsening hypercalcemia and hypercalciuria during summertime, attributed to increased sun exposure and endogenous Vitamin D production. Clinical Case: A 10-month-old female presented to endocrine care with hypercalcemia and nephrocalcinosis, detected on renal ultrasound (US) due to history of UTI. Her first renal US and serum calcium (Ca) at 3mo of age were normal. Subsequent renal US at 6mo and 9mo of age demonstrated nephrocalcinosis, prompting nephrology and endocrine evaluation. History was significant for failure to thrive. She was born in the fall, with worsening hypercalcemia and nephrocalcinosis during the summer. Diet consisted of standard infant formula and age appropriate solid foods with no added Vitamin D supplementation (~300 IU/day in her formula). She had no family history of nephrocalcinosis, nephrolithiasis, bone disease, or disorders of Ca regulation. Initial labs were notable for Ca corrected for albumin 11.5 (7.8-11.1 mg/dL), PTH &lt;4 (8.7-77.1 pg/mL), 25-OH-Vitamin D 81 (30-96 ng/mL), 1,25-OH-Vitamin D 23.1 (26.1-95 pg/mL), Urine Ca/creatinine ratio of 0.9 mg/mg (&lt;0.81), normal chromosomal microarray, and normal thyroid function tests. She was started on reduced mineral formula PM 60/40. One week later, repeat Ca level increased to Ca corrected 14.2 (7.8-11.1 mg/dL). She was admitted for IV fluids and pamidronate, and was transitioned to a low Ca and Vitamin D formula (Calcilo), with improvement in Ca levels. Testing revealed an increased ratio of 25-OH-Vitamin D to 24,25-OH-Vitamin D of 192 (normal &lt;25), and genetic testing showed 2 pathogenic missense mutations in CYP24A1 genes: c.1226T&gt;C p.(Leu409Ser) and c.1186C&gt;T p.(Arg396Trp). The Leu409Ser mutation has shown a small amount of 24-hydroxylase activity in previous in vitro analysis. She has continued a low Ca diet with stable Ca corrected of 10.7-10.8 (8.7-9.8 mg/dL) and significantly improved weight gain. Conclusion: This is one of the few documented cases of symptomatic idiopathic infantile hypercalcemia secondary to CYP24A1 mutation in an infant without exogeneous Vitamin D supplementation. Her nephrocalcinosis and hypercalcemia worsened over the summer, suggesting increased sun exposure may have been a contributing factor. This case demonstrates that 1,25-OH-Vitamin D levels may be normal or low in this condition, particularly for individuals with the Leu409Ser mutation who may retain partial 24-hydroxylase function.

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

Loss of function mutations in theCYP24A1gene, 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 theSLC34A1gene, 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. InSLC34A1mutated 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.