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The authors describe a germ-line mutation in the gene for PRKAR1A in three unrelated patients with acrodysostosis and resistance to multiple hormones. The mutated protein subunit impairs the response of protein kinase A to cyclic-AMP stimulation. Numerous hormones activate heterotrimeric G-protein–coupled receptors, which then activate G protein and adenylyl cyclase, generating intracellular cAMP. 1 In turn, cAMP activates protein kinase A, resulting in the phosphorylation of specific proteins that mediate the physiological effects of these hormones. 2 – 4 Loss-of-function mutations in the gene encoding GNAS cause pseudohypoparathyroidism type 1a, a disease with characteristic developmental and skeletal abnormalities (collectively called Albright's hereditary osteodystrophy, i.e., short stature, brachydactyly most frequently affecting the fourth and fifth metacarpals, rounded facies, obesity, and heterotopic subcutaneous ossification) that are associated with hormone resistance. 1 , 5 Acrodysostosis (Online Mendelian Inheritance in Man number 101800) is a . . .

Chinese hamster ovary transfectants that express insulin receptors in which tyrosine residues 1162 and 1163 were replaced by phenylalanine exhibit a total inhibition of the insulin-mediated tyrosine kinase activity toward exogenous substrates [histone, casein, and poly(Glu/Tyr)]; this latter activity is associated with total inhibition of the hypersensitivity reported for insulin in promoting 2-deoxyglucose uptake. We now present evidence that the twin tyrosines also control the insulin-mediated stimulation of glycogen synthesis. Surprisingly, this type of Chinese hamster ovary transfectant is as hypersensitive to insulin for its mitogenic effect as are Chinese hamster ovary cells expressing many intact insulin receptors. Such data suggest that (i) the insulin mitogenic effect routes through a different pathway than insulin uses to activate the transport and metabolism of glucose and (ii) the mitogenic effect of insulin is not controlled by the twin tyrosines. At the molecular level, the solubilized mutated receptor has no insulin-dependent tyrosine kinase activity, whereas this receptor displays measurable insulin-stimulated phosphorylation of its β subunit in 32P-labeled cells. We therefore propose that the autocatalytic phosphorylating activity of the receptor reports a cryptic tyrosine kinase activity that cannot be visualized by the use of classical exogenous substrates.