title: Genetic defect in CYP24A1, the vitamin D 24-hydroxylase gene, in a patient with severe infantile hypercalcemia.
abstract: BACKGROUND
  Idiopathic infantile hypercalcemia (IIH) is a disorder the genetic etiology and physiological basis of which are not well understood.
  
  
  OBJECTIVE
  The objective of the study was to describe the underlying physiology and genetic cause of hypercalcemia in an infant with severe IIH and to extend these genetic findings into an additional cohort of children with IIH.
  
  
  METHODS
  This was an inpatient study of a single patient with consanguineous parents at an academic medical center with follow-up in a specialty clinic cohort.
  
  
  METHODS
  The patient population was one patient with severe IIH for gene discovery and physiological testing and 27 patients with idiopathic infantile hypercalcemia in the replication cohort.
  
  
  METHODS
  Interventions included a calcium isotopic absorption study as well as homozygosity mapping and whole-exome sequencing in a single patient followed up by gene sequencing in replication cohort.
  
  
  METHODS
  Fractional absorption of calcium and genetic variants causing hypercalcemia were measured.
  
  
  RESULTS
  Intestinal calcium absorption was extremely elevated (∼90%). A rare homozygous deletion in the CYP24A1 gene was found, leading to the loss of a single highly conserved amino acid. In vivo functional studies confirmed decreased 24-hydroxylase activity because the subject had undetectable levels of 24,25-dihydroxyvitamin D. No coding variants in CYP24A1 were found in the 27 additional patients with IIH.
  
  
  CONCLUSIONS
  Our study confirms that CYP24A1 plays a causal role in some but not all cases of IIH via markedly increased intestinal absorption of calcium, suggesting that genetic diagnosis could be helpful in a subset of IIH patients. This case demonstrates the power of an unbiased, genome-wide approach accompanied by informative physiological studies to provide new insights into human biology.
authors: Division of Endocrinology, Children's Hospital Boston, Clinical Investigator Training Program, Boston, Massachusetts 02115, USA. andrew.dauber@childrens.harvard.edu
authors: Andrew
authors: A
authors: Dauber
authors: Thutrang T
authors: TT
authors: Nguyen
authors: Etienne
authors: E
authors: Sochett
authors: David E C
authors: DE
authors: Cole
authors: Ronald
authors: R
authors: Horst
authors: Steven A
authors: SA
authors: Abrams
authors: Thomas O
authors: TO
authors: Carpenter
authors: Joel N
authors: JN
authors: Hirschhorn
mesh_terms: D017668:Age of Onset; D001483:Base Sequence; D002118:Calcium / Q000493:pharmacokinetics; D003241:Consanguinity; D005500:Follow-Up Studies; D006801:Humans; D006934:Hypercalcemia / Q000453:epidemiology / Q000235:genetics*; D007223:Infant; D007408:Intestinal Absorption / Q000235:genetics / Q000502:physiology; D008297:Male; D010375:Pedigree; D012720:Severity of Illness Index; D013250:Steroid Hydroxylases / Q000235:genetics*; D065668:Vitamin D3 24-Hydroxylase
chemical_list: D013250:Steroid Hydroxylases; C585549:CYP24A1 protein, human; D065668:Vitamin D3 24-Hydroxylase; D002118:Calcium