Optimal bone fracture repair requires 24R,25-dihydroxyvitamin D3 and its effector molecule FAM57B2.

The biological activity of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] remains controversial, but it has been suggested that it contributes to fracture healing. Cyp24a1-/- mice, synthesizing no 24R,25(OH)2D3, show suboptimal endochondral ossification during fracture repair, with smaller callus and reduced stiffness. These defects were corrected by 24R,25(OH)2D3 treatment, but not by 1,25-dihydroxyvitamin D3. Microarrays with Cyp24a1-/- callus mRNA identified FAM57B2 as a mediator of the 24R,25(OH)2D3 effect. FAM57B2 produced lactosylceramide (LacCer) upon specific binding of 24R,25(OH)2D3. Fam57b inactivation in chondrocytes (Col2-Cre Fam57bfl/fl) phenocopied the callus formation defect of Cyp24a1-/- mice. LacCer or 24R,25(OH)2D3 injections restored callus volume, stiffness, and mineralized cartilage area in Cyp24a1-null mice, but only LacCer rescued Col2-Cre Fam57bfl/fl mice. Gene expression in callus tissue suggested that the 24R,25(OH)2D3/FAM57B2 cascade affects cartilage maturation. We describe a previously unrecognized pathway influencing endochondral ossification during bone repair through LacCer production upon binding of 24R,25(OH)2D3 to FAM57B2. Our results identify potential new approaches to ameliorate fracture healing.

[A young girl with recurrent calculosis and hypercalcemia].

Mutations of the CYP24A1 gene are associated with alterations in the activity of the enzyme 25-OH-D-24-hydroxylase, resulting in dysfunction of the metabolism of vitamin D. This enzymatic deficiency may cause hypercalcemia, low parathyroid hormone levels, hypercalciuria, nephrolithiasis and nephrocalcinosis. The clinical case of a young woman with recurrent renal lithiasis, hypercalcemia and hypercalciuria is described. These features are linked to deficiency of the enzyme 25-OH-D-24-hydroxylase, therefore to a biallelic mutation of the CYP24A1 gene.

Calcium pyrophosphate deposition disease revealing a hypersensitivity to vitamin D.

OBJECTIVE: Hypersensitivity to vitamin D (HVD) due to a loss of function mutation of the CYP24A1 gene, which encodes vitamin D catabolizing enzyme was initially described as a cause of acute hypercalcemia in children and chronic renal diseases in adults. METHODS: We describe the first case of a patient presenting a calcium pyrophosphate deposition disease (CPDD) revealing a HVD. RESULTS: An abnormality of phospho-calcic metabolism was discovered during the course of an etiological workup for CPDD in a 52-year-old patient. Laboratory tests revealed a blood calcium level at the upper limit of normal range, a markedly low parathormone level, a 25-hydroxyvitamin D level within the upper level of normal, an elevated 1,25-dihydroxyvitamin D level and an elevated urine calcium level. CYP24A1 gene sequencing analysis revealed two mutations in a heterozygous state. The study of the 25-hydroxyvitamin D3: 24,25-dihydroxyvitamin D3 ratio, two metabolites of vitamin D confirmed the enzyme deficiency in vivo. Our observation suggests that this disease could correspond to a rare cause of CPDD. CONCLUSION: In cases of CPDD associated with calcium values within the upper limit of normal range (or hypercalcemia) with an abnormally low PTH, one could suggest searching for HVD.

A Young Woman With Recurrent Gestational Hypercalcemia and Acute Pancreatitis Caused by CYP24A1 Deficiency.

The CYP24A1 gene encodes a mitochondrial 24-hydroxylase that inactivates 1,25(OH)2 D. Loss-of-function mutations in CYP24A1 cause hypercalcemia, nephrolithiasis and nephrocalcinosis. We describe a woman with CYP24A1 deficiency and recurrent gestational hypercalcemia. Her first pregnancy, at age 20, resulted with the intrauterine demise of twin fetuses. Postpartum, she developed severe hypercalcemia (14 mg/dL), altered mental status, and acute pancreatitis. Her PTH was suppressed (6 pg/mL) and her 1,25(OH)2 D was elevated (165 and 195 pg/mL on postpartum day 1 and 5, respectively). Between one and three months postpartum, her serum calcium decreased from 11.4 to 10.2 mg/dL while her 1,25(OH)2 D level decreased from 83 to 24 pg/mL. Her 24-hour urine calcium was 277 mg. Six months postpartum, she became pregnant again. At 14 weeks, her albumin-corrected calcium level was 10.4 mg/dL and her 1,25(OH)2 D level exceeded 200 pg/mL. To establish the diagnosis of CYP24A1 deficiency, we showed her 24,25(OH)2 D level to be undetectable (<2 ng/mL). Exon sequencing of the CYP24A1 gene revealed a homozygous, 8-nucleotide deletion in exon 8, causing an S334V substitution and premature termination due to a frame shift (c.999_1006del, p.Ser334Valfs*9). To prevent hypercalcemia, she was advised to discontinue prenatal vitamins, avoid sun exposure and calcium-rich foods, and start omeprazole and a calcium binder (250 mg K-Phos-neutral with meals). Despite these measures, both hypercalcemia (11.5 mg/dL) and acute pancreatitis recurred. Labor was induced and a healthy, normocalcemic boy was delivered. In the absence of lactation, maternal hypercalcemia resolved within 2 months. This report shows that CYP24A1-deficient subjects may be normocalcemic at baseline. Hypercalcemia may be unmasked by pregnancy through the routine use of calciferol-containing prenatal vitamins, increased 1-alpha hydroxylation of VitD by the placenta and maternal kidney, and production of PTHrP by the uteroplacental unit. CYP24A1 deficiency should be considered in patients with unexplained vitamin D-mediated hypercalcemia. © 2016 American Society for Bone and Mineral Research.