Complex Operative Unit of Medicine, Hospital of Asissi-USL Umbria 1, Assisi, Italy.

Laboratory of Applied Biotechnology in Orthopedics, IRCCS Galeazzi Orthopedic Institute, Milan, Italy.

IRCCS Galeazzi Orthopedic Institute, Milan; Department of Biomedical Sciences for Health, University of Milan, Italy.

Rheumatology Unit, Integrated University Hospital of Verona, University of Verona, Italy.

In this case report, a novel N-acetylgalactosaminyltransferase 3 homozygous mutation (c.782 G>A; p.R261Q) associated with hyperphosphatemic familial tumoral calcinosis/hyperostosis-hyperphosphatemia syndrome is described. The patient had elbow, pelvis, and lower limb pain and a hard mass in the hip and olecranon regions. Increased levels of inorganic phosphorus (Pi) and C-reactive protein were observed. After treating the patient with conventional drugs, we tested denosumab, which reduced but did not normalize the Pi.

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