Variable Bone Phenotypes in Patients with Pseudohypoparathyroidism

Pseudohypoparathyroidism (PHP) is a disorder caused by mutations and/or epigenetic changes at the complex GNAS locus. It is characterized by hypocalcemia, hyperphosphatemia, and an elevated parathyroid hormone concentration secondary to the resistance of target tissues to the biological actions of parathyroid hormone. PHP is divided into several subtypes with different yet overlapping phenotypes. Research on the bone status in patients with PHP is sparse and has yielded inconsistent results. This review was performed to summarize the current knowledge on the bone phenotypes and possible mechanisms of PHP.

Patients with PHP exhibit highly variable bone phenotypes and increased concentrations of bone turnover markers. Long-standing elevation of the parathyroid hormone concentration may lead to hyperparathyroid bone diseases, including rickets and osteitis fibrosa. Compared with normal controls, patients with PHP may exhibit similar, increased, or decreased bone mineral density. Higher bone mineral density has been found in patients with PHP type 1A than in normal controls, whereas decreased bone mass, osteosclerosis, and osteitis fibrosa cystica have been reported in patients with PHP type 1B, indicating more variable bone phenotypes in PHP type 1B. Bone tissues show partial sensitivity to parathyroid hormone in patients with PHP, leading to heterogeneous reactions to parathyroid hormone in different individuals and even in different regions of bone tissues in the same individual. Regions rich in cancellous bone are more sensitive and show more obvious improvement after therapy. Active vitamin D and calcium can significantly improve abnormal bone metabolism in patients with PHP.