The role of mutant UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 in regulating serum intact fibroblast growth factor 23 and matrix extracellular phosphoglycoprotein in heritable tumoral calcinosis.
BACKGROUND
Familial tumoral calcinosis (TC) results from disruptions in phosphate metabolism and is characterized by high serum phosphate with normal or elevated 1,25 dihydroxyvitamin vitamin D concentrations and ectopic and vascular calcifications. Recessive loss-of-function mutations in UDP-N-acetyl-alpha-D-galactosamine-polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3) and fibroblast growth factor-23 (FGF23) result in TC.


OBJECTIVE
The objective of the study was to determine the relationship between GALNT3 and FGF23 in familial TC.


METHODS
We assessed the major biochemical defects and potential genes involved in patients with TC.


METHODS
Combination therapy consisted of the phosphate binder Sevelamer and the carbonic anhydrase inhibitor acetazolamide.


RESULTS
We report a patient homozygous for a GALNT3 exon 1 deletion, which is predicted to truncate the encoded protein. This patient had high serum FGF23 concentrations when assessed with a C-terminal FGF23 ELISA but low-normal FGF23 levels when tested with an ELISA for intact FGF23 concentrations. Matrix extracellular phosphoglycoprotein has been identified as a possible regulator of phosphate homeostasis. Serum matrix extracellular phosphoglycoprotein levels, however, were normal in the family with GALNT3-TC and a kindred with TC carrying the FGF23 S71G mutation. The tumoral masses of the patient with GALNT3-TC completely resolved after combination therapy.


CONCLUSIONS
Our findings demonstrate that GALNT3 inactivation in patients with TC leads to inadequate production of biologically active FGF23 as the most likely cause of the hyperphosphatemic phenotype. Furthermore, combination therapy may be effective for reducing the tumoral burden associated with familial TC.
Garringer
Holly J
HJ
Department of Medical and Molecular Genetics, Indiana University School of Medicine, 975 West Walnut Street, IB130, Indianapolis, IN 46202, USA.
Fisher
Corinne
C
Larsson
Tobias E
TE
Davis
Siobhan I
SI
Koller
Daniel L
DL
Cullen
Michael J
MJ
Draman
Mohamad S
MS
Conlon
Niamh
N
Jain
Alka
A
Fedarko
Neal S
NS
Dasgupta
Bhaskar
B
White
Kenneth E
KE
D000595:Amino Acid Sequence; D001483:Base Sequence; D002114:Calcinosis / Q000097:blood / Q000235:genetics* / Q000628:therapy; D002117:Calcitriol / Q000097:blood; D016326:Extracellular Matrix Proteins / Q000097:blood*; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000097:blood* / Q000235:genetics; D006023:Glycoproteins / Q000097:blood*; D006801:Humans; D008969:Molecular Sequence Data; D009154:Mutation; D017350:N-Acetylgalactosaminyltransferases / Q000235:genetics* / Q000502:physiology; D009363:Neoplasm Proteins / Q000235:genetics* / Q000502:physiology; D010710:Phosphates / Q000097:blood*; D010750:Phosphoproteins / Q000097:blood*; D000097763:Polypeptide N-acetylgalactosaminyltransferase
D002117:Calcitriol; D016326:Extracellular Matrix Proteins; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors; D006023:Glycoproteins; D017350:N-Acetylgalactosaminyltransferases; D009363:Neoplasm Proteins; D010710:Phosphates; D010750:Phosphoproteins; D000097763:Polypeptide N-acetylgalactosaminyltransferase; C000717427:FGF23 protein, human; C486579:MEPE protein, human