Clinical and Functional Characterization of Novel GALNT3 Mutations in a Chinese Child with Hyperphosphatemic Familial Tumoral Calcinosis

Hyperphosphatemic familial tumoral calcinosis (HFTC) is a rare autosomal recessive disorder characterized by hyperphosphatemia and ectopic calcifications. Mutations in GALNT3, which encodes a key enzyme responsible for O-glycosylation of FGF23, represent a major genetic cause of HFTC. This modification is essential for the stability and secretion of FGF23. We investigated a 4-year and 6-month-old Chinese girl with HFTC to characterize the clinical features, identify the causative variants, and explore the underlying pathogenic mechanism. Whole-exome sequencing followed by Sanger validation identified novel compound heterozygous variants in GALNT3 (c.659T>A, p.Ile220Asn and c.1850C>A, p.Ser617*). The patient exhibited hyperphosphatemia with a biochemical profile consistent with FGF23 deficiency, including extremely low intact FGF23 and elevated C-terminal fragments. Functional studies using Western blotting and wheat germ agglutinin affinity chromatography demonstrated that the mutant GALNT3 caused a severe defect in FGF23 O-glycosylation, leading to impaired secretion of intact FGF23. Glycosylated FGF23 was detected only in the medium of cells expressing wild-type GALNT3. These findings indicate that defective O-glycosylation results in failure of FGF23 secretion and functional inactivation. This study expands the mutational spectrum of GALNT3 and provides mechanistic insight into the role of GALNT3 in phosphate homeostasis.