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

INSERM, Université Paris Descartes, U986, Hôpital Saint Vincent de Paul, 75014 Paris, France.
Bone (Impact Factor: 3.97). 09/2010; 48(2):312-20. DOI: 10.1016/j.bone.2010.09.032
Source: PubMed


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 α-subunit of the stimulatory G protein (Gsα), as well as a large variant (XLαs) derived from the paternal allele. The mutations causing MAS affect both GNAS products, but whether XLαs, like Gsα, 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 Gsα) were present in all the previously reported and five of the new cases. The mutation was detected within the paternally expressed XLαs transcript in five and the maternally expressed NESP55 transcript in four cases. Tissues carrying paternal mutations appeared to have higher XLαs mRNA levels than maternal mutations. The human XLαs mutant analogous to Gsα-R201H (XLαs-R543H) showed markedly higher basal cAMP accumulation than wild-type XLαs in transfected cells. Wild-type XLαs demonstrated higher basal and isoproterenol-induced cAMP signaling than Gsα and co-purified with Gβ1γ2 in transduced cells. XLαs 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. XLαs mRNA was also expressed in murine bone marrow stromal cells and preosteoblastic MC3T3-E1 cells. Our findings are consistent with the possibility that constitutive XLαs activity adds to the molecular pathogenesis of MAS and fibrous dysplasia of bone.

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    • "McCune-Albright syndrome is a complex inborn disorder due to early embryonal postzygotic somatic activating mutations in the GNAS1 gene, which codes for a number of different transcripts by alternative promoters and alternative splicing. Among these, the Gs-alpha, the XLAS, the NESP55 and the A/B transcripts are involved in the regulation of bone metabolism and endocrine functions [1-4]. "
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    ABSTRACT: Murine models indicate that Gαs and its extra-long variant XLαs, both of which are derived from GNAS, markedly differ regarding their cellular actions, but these differences are unknown. Here we investigated activation-induced trafficking of Gαs and XLαs, using immunofluorescence microscopy, cell fractionation, and total internal reflection fluorescence microscopy. In transfected cells, XLαs remained localized to the plasma membrane, whereas Gαs redistributed to the cytosol after activation by GTPase-inhibiting mutations, cholera toxin treatment, or G protein-coupled receptor agonists (isoproterenol or parathyroid hormone (PTH)(1-34)). Cholera toxin treatment or agonist (isoproterenol or pituitary adenylate cyclase activating peptide-27) stimulation of PC12 cells expressing Gαs and XLαs endogenously led to an increased abundance of Gαs, but not XLαs, in the soluble fraction. Mutational analyses revealed two conserved cysteines and the highly charged domain as being critically involved in the plasma membrane anchoring of XLαs. The cAMP response induced by M-PTH(1-14), a parathyroid hormone analog, terminated quickly in HEK293 cells stably expressing the type 1 PTH/PTH-related peptide receptor, whereas the response remained maximal for at least 6 min in cells that co-expressed the PTH receptor and XLαs. Although isoproterenol-induced cAMP response was not prolonged by XLαs expression, a GTPase-deficient XLαs mutant found in certain tumors and patients with fibrous dysplasia of bone and McCune-Albright syndrome generated more basal cAMP accumulation in HEK293 cells and caused more severe impairment of osteoblastic differentiation of MC3T3-E1 cells than the cognate Gαs mutant (gsp oncogene). Thus, activated XLαs and Gαs traffic differently, and this may form the basis for the differences in their cellular actions.
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    ABSTRACT: GNAS mutations have been implicated in the development of fibrous dysplasia and multiple endocrinopathies of the Albright-McCune syndrome. To investigate the diagnostic utility of GNAS mutations in patients with fibrous dysplasia, we performed mutational analyses of histologically confirmed fibrous dysplasia and conducted a meta-analysis of the literature. We collected 48 cases of fibrous dysplasia from 3 institutions from 2002 to 2011 and performed polymerase chain reaction and direct bidirectional sequencing of exons 8 and 9 of GNAS using paraffin-embedded tissues. We searched MEDLINE, PubMed, and the KoreaMed databases from 1997 to 2011 and included an additional 155 cases of fibrous dysplasia from 8 representative studies to conduct a meta-analysis. In our sample, 28 (58.3%) of 48 cases showed point mutations of codon 201 at exon 8. Twenty-five cases had a substitution of arginine at codon 201 for histidine (p.R201H), and 3 cases had a substitution for cysteine (p.R201C). One case had a new mutation at codon 224 (p.V224A). The incidence of GNAS mutations was significantly greater in cases that involved long bones than in cases that involved flat bones (P = .017) and was higher in polyostotic cases than in monostotic cases (P = .067). In meta-analysis, 9 studies and 203 patients were included. The overall positive rate of GNAS mutation in fibrous dysplasia was 71.9% (146/203). The major types of mutations were missense mutations such as R201H (66.4%) and R201C (30.8%). As a result, the detection of GNAS mutation could be a valuable adjunct to conventional histopathologic diagnosis of fibrous dysplasia.
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