Selective resistance to parathyroid hormone caused by a novel uncoupling mutation in the carboxyl terminus of G alpha(s) - A cause of pseudohypoparathyroidism type Ib

G(s) is a heterotrimeric (alpha, beta, and gamma chains) G protein that cou ples heptahelical plasma membrane receptors to stimulation of adenylyl cycl ase, Inactivation of one GNAS1 gene allele encoding the a chain of G(s) (G alpha (s)) causes pseudohypoparathyroidism type Ia. Affected subjects have resistance to parathyroid hormone (PTH) and other hormones that activate ad enylyl cyclase plus somatic features termed Albright hereditary ostesdystro phy. By contrast, subjects with pseudohspoparathyroidism type Ib have hormo ne resistance that is limited to PTH and lack Albright hereditary osteodyst rophy, The molecular basis for pseudohypoparathyroidism type Ib is unknown. We analyzed the GNAS1 gene for mutations using polymerase chain reaction t o amplify genomic DNA from three brothers with pseudohypoparathyroidism typ e Ib. We identified a novel heterozygous 3-base pair deletion causing loss of isoleucine 382 in the three affected boys and their clinically unaffecte d mother and maternal grandfather, This mutation was absent in other family members and 15 additional unrelated subjects with pseudohypoparathyroidism type Ib. To characterize the signaling properties of the mutant G alpha (s ), we used site-directed mutagenesis to introduce the isoleucine 382 deleti on into a wild type G alpha (s) cDNA, transfected HEK293 cells with either wild type or mutant G alpha (s) cDNA, plus cDNAs encoding heptahelical rece ptors for PTH, thyrotropic hormone, or luteinizing hormone, and we measured cAMP production in response to hormone stimulation. The mutant G alpha (s) protein was unable to interact with the receptor for PTH but showed normal coupling to the other coexpressed heptahelical receptors, These results pr ovide evidence of selective uncoupling of the mutant G alpha (s) from PTH r eceptors and explain PTH-specific hormone resistance in these three brother s with pseudohypoparathyroidism type Ib. The absence of PTH resistance in t he mother and maternal grandfather who carry the same mutation is consisten t with current models of paternal imprinting of time GNAS1 gene.