The present invention uncovers that mutations in GALNT3 gene encoding UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase (GalNAc-T3) cause familial tumoral calcinosis (FTC). Methods and pharmaceutical compositions useful for treating disorders associated with abnormal phosphate metabolism are provided. Specifically, inducers of GalNAc-T3 can be used to treat hyperphosphatemia related disorders such as FTC, and on the other hand, inhibitors of GalNAc-T3 can be used to treat disorders associated with hypophosphatemia, such as hypophosphatemic rickets.