Pseudohypoparathyroidism: inheritance and expression of deficient receptor-cyclase coupling protein activity

Pseudohypoparathyroidism, Type I (PSP-I) is a familial disorder characterized by secondary hyperparathyroidism, resistance of urinary cyclic adenosine-3', 5'-monophosphate (cAMP) excretion to exogenous parathyroid hormone (PTH), and by effects upon other hormones, including thyrotrophin (TSH) hyperresponsiveness to thyroliberin (TRH). In the present study, 12 PSP-I patients in five families exhibited partial deficiency of receptor-cyclase coupling protein (N protein) in blood cells, in association with the skeletal findings of Albright's hereditary osteodystrophy. In one father and six mothers of PSP-I patients, deficient N protein activity was associated with normal urinary cAMP responses to PTH. In this group of seven parents, five had Albright's osteodystrophy, two exhibited secondary hyperparathyroidism, and two had TSH hyperresponsiveness to TRH. In a sixth family with none of the features of Albright's osteodystrophy, N protein deficiency did not correlate with urinary cAMP responsiveness to PTH. In this kindred, one mother with N protein deficiency, but normal urinary cAMP responsiveness to PTH had raised serum levels of immunoreactive PTH. We conclude that in the majority of families with PSP-I the urinary cAMP response to PTH is an inadequate indicator of the genetic defect. In such families, deficiency of N activity more consistently points to metabolic defects, including secondary hyperparathyroidism and TSH hyperresponsiveness, even when urinary cAMP responses are normal.