title: Unusual clinical presentation and possible rescue of a novel claudin-16 mutation.
abstract: BACKGROUND
  Familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC) is caused by a dysfunction of Claudin-16 (CLDN16) and characterized by renal wasting of Mg(2+) and Ca(2+).
  
  
  OBJECTIVE
  The objectives of this study were to study the clinical parameters in suspected FHHNC patients, identify mutations in the CLDN16 gene, and analyze molecular defects associated with the mutant protein.
  
  
  METHODS
  CLDN16 genes from two siblings diagnosed with FHHNC were sequenced. Expression and characterization of the mutant protein in renal MDCK cells were studied.
  
  
  METHODS
  Standard urine and serum parameters to diagnose FHHNC were determined. Mutations in the CLDN16 gene were identified. The subcellular distribution of the mutant protein was analyzed by immunofluorescence microscopy.
  
  
  RESULTS
  Urine and blood analysis showed signs typical for FHHNC. One patient, in addition, presented with hypocalcemic tetany, a phenomenon so far not described for FHHNC. Both siblings carry a novel mutation in CLDN16, Y207X. The review of medical records showed that hypocalcemia is not uncommon in the early childhood of FHHNC patients. Expressed in MDCK cells, the Y207X mutant is not detected at tight junctions but instead is found in lysosomes and, to a lesser extent, the endoplasmic reticulum. Surface expression can be rescued by inhibiting clathrin-mediated internalization.
  
  
  CONCLUSIONS
  We propose that mutations in CLDN16 are considered in childhood hypocalcemia. CLDN16 Y207X is transiently delivered to the plasma membrane but not retained and is rapidly retrieved by internalization. Inhibitors of endocytosis may provide novel therapeutic strategies.
authors.affiliation: Department of Pediatric Nephrology, Charite Children's Hospital and Center for Cardiovascular Research, 12200 Berlin, Germany.
authors.forename: Dominik
authors.initials: D
authors.lastname: Müller
authors.forename: P Jaya
authors.initials: PJ
authors.lastname: Kausalya
authors.forename: Detlef
authors.initials: D
authors.lastname: Bockenhauer
authors.forename: Julia
authors.initials: J
authors.lastname: Thumfart
authors.forename: Iwan C
authors.initials: IC
authors.lastname: Meij
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Dillon
authors.forename: William
authors.initials: W
authors.lastname: van't Hoff
authors.forename: Walter
authors.initials: W
authors.lastname: Hunziker
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002118:Calcium / Q000652:urine*; D002460:Cell Line; D002648:Child; D057167:Claudins; D004285:Dogs; D005260:Female; D005455:Fluorescent Antibody Technique; D015870:Gene Expression; D006720:Homozygote; D006801:Humans; D007223:Infant; D007668:Kidney; D008275:Magnesium Deficiency / Q000150:complications / Q000235:genetics*; D008297:Male; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics*; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D009154:Mutation; D009397:Nephrocalcinosis / Q000150:complications / Q000235:genetics*; D017422:Sequence Analysis, DNA; D014162:Transfection
chemical_list: D057167:Claudins; D008565:Membrane Proteins; C502010:claudin 16; D002118:Calcium