Rapid GDP release from Gsα in patients with gain and loss of endocrine function

LUTEINIZING hormone stimulates testicular Leydig cells to produce testosterone by binding to a receptor that activates the G protein Gs and adenylyl cyclase. Testotoxicosis is a form of precocious puberty in which the Leydig cells secrete testosterone in the absence of luteinizing hormone, often due to constitutive activation of the luteinizing hormone receptor and (indirectly) Gs (refs 1–4). Here we study two unrelated boys suffering from a paradoxical combination of testotoxicosis and pseudohypoparathyroidism type la (PHP-Ia)5, a condition marked by resistance to hormones acting through cyclic AMP (parathyroid hormone and thyroid-stimulat-ing hormone) as well as a 50% decrease in erythrocyte Gs activity (the remaining 50% is due to the normal Gs allele)5,6. In both patients, a mutation in the gene encoding the Gs α-subunit replaced alanine at position 366 with serine5. We show that this αs-A366S mutation constitutively activates adenylyl cyclase in vitro, causing hormone-independent cAMP accumulation when expressed in cul-tured cells, and accounting for the testotoxicosis phenotype (as cAMP stimulates testosterone secretion). Although as-A366S is quite stable at testis temperature, it is rapidly degraded at 37 °C, explaining the PHP-Ia phenotype caused by loss of Gs activity. In vitro experiments indicate that accelerated release of GDP causes both the constitutive activity and the thermolability of αs-A366S.