Context: Familial tumoral calcinosis (TC) is a rare autosomal recessive disorder characterized by metastatic calcifications, often periarticular. Biochemical findings include hyperphosphatemia, high 1,25-dihydroxyvitamin D levels, and elevated tubular maximum for phosphate reabsorption per deciliter of glomerular filtrate (TmP/GFR). TC is caused by biallelic mutations of the genes encoding either fibroblast growth factor 23 (FGF23) or uridine diphosphate-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GalNAc transferase 3 or GALNT3).
Objective: The objective was to identify mutations in FGF23 or GALNT3 responsible for a mild TC phenotype by DNA sequencing and to determine serum FGF23 levels by ELISA.
Patients or other participants: The subject was a 25-yr-old Caucasian woman with eyelid calcifications and biochemical features of TC.
Results: Eyelid biopsy revealed superficial dermis calcifications. There was no history of metastatic calcifications, mineral homeostasis abnormalities, or renal dysfunction. Biochemistry revealed normal levels of calcium, creatinine, PTH, and 25-hydroxyvitamin D, with elevated phosphorous, TmP/GFR, and high normal 1,25-dihydroxyvitamin D levels. Intact FGF23 was undetectable (< 3 pg/ml), whereas C-terminal FGF23 was elevated (698.2 RU/ml). Mutation detection revealed compound heterozygosity for two novel mutations in the glycosyl transferase domain of the GALNT3 gene.
Conclusion: Previously reported GALNT3 mutations in TC have been null mutations. This study shows that missense mutations affecting the glycosyl transferase domain of GalNAc transferase 3 also cause TC. Elevated C-terminal FGF23 fragments with undetectable intact FGF23 suggest that the mutant enzyme lacks the ability to glycosylate FGF23 and that glycosylation by GalNAc transferase 3 is necessary for secretion of functional full-length FGF23.