Potent constitutive cyclic AMP-generating activity of XLαs implicates this imprinted GNAS product in the pathogenesis of McCune-Albright Syndrome and fibrous dysplasia of bone

Patients with McCune-Albright syndrome (MAS), characterized primarily by hyperpigmented skin lesions, precocious puberty, and fibrous dyslasia of bone, carry postzygotic heterozygous mutations of GNAS causing constitutive cAMP signaling. GNAS encodes the alpha-subunit of the stimulatory G protein (Gs alpha), as well as a large variant (XL alpha s) derived from the paternal allele. The mutations causing MAS affect both GNAS products, but whether XL alpha s, like Gs alpha, can be involved in the pathogenesis remains unknown. Here, we investigated biopsy samples from four previously reported and eight new patients with MAS. Activating mutations of GNAS (Arg201 with respect to the amino acid sequence of Gs alpha) were present in all the previously reported and five of the new cases. The mutation was detected within the paternally expressed XL alpha s transcript in five and the maternally expressed NESP55 transcript in four cases. Tissues carrying paternal mutations appeared to have higher XL alpha s mRNA levels than maternal mutations. The human XL alpha s mutant analogous to Gs alpha-R201 H (XL alpha s-R543H) showed markedly higher basal cAMP accumulation than wild-type XL alpha s in transfected cells. Wild-type XL alpha s demonstrated higher basal and isoproterenol-induced cAMP signaling than Gs alpha and co-purified with G beta 1 gamma 2 in transduced cells. XL alpha s mRNA was measurable in mouse calvarial cells, with its level being significantly higher in undifferentiated cells than those expressing preosteoblastic markers osterix and alkaline phosphatase. XL alpha s mRNA was also expressed in murine bone marrow stromal cells and preosteoblastic MC3T3-E1 cells. Our findings are consistent with the possibility that constitutive XL alpha s activity adds to the molecular pathogenesis of MAS and fibrous dysplasia of bone. (C) 2010 Elsevier Inc. All rights reserved.