Unusual clinical presentation and possible rescue of a novel claudin-16 mutation.
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.
Müller
Dominik
D
Department of Pediatric Nephrology, Charite Children's Hospital and Center for Cardiovascular Research, 12200 Berlin, Germany.
Kausalya
P Jaya
PJ
Bockenhauer
Detlef
D
Thumfart
Julia
J
Meij
Iwan C
IC
Dillon
Michael J
MJ
van't Hoff
William
W
Hunziker
Walter
W
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
D057167:Claudins; D008565:Membrane Proteins; C502010:claudin 16; D002118:Calcium