Defects in the G (guanine nucleotide-binding)protein subunit (G(α)s) which stimulates adenylyl cyclase may result in either loss or gain of endocrine function. Reduced G(α)s activity is found in the hormone resistance syndrome, pseudohypoparathyroidism type Ia (PHP-Ia), while constitutive activation of G(α)s is associated with endocrine organ overactivity, including the gonadotropin-independent sexual precocity seen in patients with McCune-Albright syndrome. We identified two unrelated boys presenting with concurrent PHP-Ia and gonadotropin-independent sexual precocity (testotoxicosis). Mutational screening by denaturing gradient gel electrophoresis and sequencing of PCR-amplified exons of the G(α)s gene revealed a point mutation which generates an alanine-to-serine substitution in codon 366 of one G(α)s allele (A366S), an alanine present at the homologous position in all G-proteins. We have previously shown in transfected testis cells that the A366S mutation activates G(α)s by decreasing affinity for GDP, thereby increasing the rate of nucleotide exchange in a receptor-independent fashion. In contrast to differential stability of the activated mutant G(α)s protein in Leydig cells, with stability at 32°C but not at 37°C, skin fibroblasts with the mutation had the same reduced G(α)s levels at both temperatures. Our findings explain the limitation of clinical manifestations of G(α)s overactivity to testis, without involvement of other body appendages which are generally at lower than core body temperature. This unique mutation at a critically conserved residue of G(α)s is the first mutant G-protein which affects guanine nucleotide affinity and is associated with human disease, producing widely divergent and tissue-specific effects.
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