PHEX Protein in the Parathyroid Gland Contributes to Phosphate Sensing

Area of Science:

• Endocrinology
• Mineral Metabolism
• Genetics

Background:

• Loss-of-function variants in the PHEX gene cause X-linked hypophosphatemia (XLH), characterized by abnormal fibroblast growth factor 23 (FGF23) secretion.
• The PHEX protein is implicated in phosphate (Pi) sensing in osteocytes, but its role in parathyroid gland Pi sensing remains unclear.
• Parathyroid glands regulate parathyroid hormone (PTH) secretion in response to serum Pi fluctuations, a mechanism not fully elucidated.

Purpose of the Study:

• To investigate the role of PHEX in parathyroid gland phosphate sensing.
• To compare PTH secretion patterns following phosphate loading in patients with XLH and tumor-induced osteomalacia (TIO).

Main Methods:

• Retrospective analysis of serum phosphate (Pi), intact PTH (iPTH), and corrected calcium (cCa) levels.
• Patients received oral phosphate loads ranging from 300 mg to 1,500 mg.
• Comparison of iPTH trends relative to Pi levels between XLH and TIO patient groups.

Main Results:

• Six XLH and 13 TIO patients were analyzed.
• Serum Pi levels increased significantly post-phosphate load, while serum cCa remained stable.
• The slope of iPTH versus Pi was significantly steeper in XLH patients (median 41.4) compared to TIO patients (median 7.1), indicating a heightened PTH response to Pi changes.

Conclusions:

• Parathyroid gland PHEX may influence the serum Pi-sensing threshold and mediate PTH secretion during rapid Pi fluctuations.
• The exaggerated PTH response in XLH patients suggests a potential mechanism for the high incidence of secondary and tertiary hyperparathyroidism in this condition.