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      The Clinical Spectrum of McCune-Albright Syndrome and Its Management

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          McCune-Albright syndrome (MAS) is a rare, mosaic disorder presenting along a broad clinical spectrum. Disease arises from somatic-activating GNAS mutations, leading to constitutive Gα<sub>s</sub> activation and ligand-independent signaling of the Gα<sub>s</sub>-coupled protein receptor. The phenotype is largely determined by location and extent of tissues in which the GNAS mutation is expressed, as well as the pathophysiologic effects of Gα<sub>s</sub> activation within these tissues. Patients pre­sent clinically with a variable combination of fibrous dysplasia of bone (FD), café-au-lait skin macules, and hyperfunctioning endocrinopathies. In bone, Gα<sub>s</sub> leads to impaired differentiation of skeletal stem cells and formation of discrete, expansile FD lesions, resulting in fractures, pain, and functional impairment. A systematic approach to diagnosis and management is critically important to optimize outcomes for patients with FD/MAS. There are no medical therapies capable of altering the disease course in FD; however, screening and treatment for endocrinopathies can mitigate some skeletal morbidities. This review summarizes current understanding of MAS pathophysiology, describes the spectrum of clinical features, and includes a detailed discussion of the recommended approach to diagnosis and management.

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          Most cited references 68

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          Activating mutations of the stimulatory G protein in the McCune-Albright syndrome.

          The McCune-Albright syndrome is a sporadic disease characterized by polyostotic fibrous dysplasia, café au lait spots, sexual precocity, and hyperfunction of multiple endocrine glands. These manifestations may be explained by a somatic mutation in affected tissues that results in activation of the signal-transduction pathway generating cyclic AMP (cAMP). We analyzed DNA from tissues of patients with the McCune-Albright syndrome for the presence of activating mutations of the gene for the alpha subunit of the G protein (Gs alpha) that stimulates cAMP formation. Genomic DNA fragments encompassing regions (exons 8 and 9) previously found to contain activating missense mutations of the Gs alpha gene (gsp mutations) in sporadically occurring pituitary tumors were amplified in tissues from four patients with the McCune-Albright syndrome by the polymerase chain reaction. The amplified DNA was analyzed for mutations by denaturing gradient gel electrophoresis and allele-specific oligonucleotide hybridization. We detected one of two activating mutations within exon 8 of the Gs alpha gene in tissues from all four patients, including affected endocrine organs (gonads, adrenal glands, thyroid, and pituitary) and tissues not classically involved in the McCune-Albright syndrome. In two of the patients, histidine was substituted for arginine at position 201 of Gs alpha, and in the other two patients cysteine was substituted for the same arginine residue. In each patient the proportion of cells affected varied from tissue to tissue. In two endocrine organs, the highest proportion of mutant alleles was found in regions of abnormal cell proliferation. Mutations within exon 8 of the Gs alpha gene that result in increased activity of the Gs protein and increased cAMP formation are present in various tissues of patients with the McCune-Albright syndrome. Somatic mutation of this gene early in embryogenesis could result in the mosaic population of normal and mutant-bearing tissues that may underlie the clinical manifestations of this disease.
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            The emerging mutational landscape of G proteins and G-protein-coupled receptors in cancer.

            Aberrant expression and activity of G proteins and G-protein-coupled receptors (GPCRs) are frequently associated with tumorigenesis. Deep sequencing studies show that 4.2% of tumours carry activating mutations in GNAS (encoding Gαs), and that oncogenic activating mutations in genes encoding Gαq family members (GNAQ or GNA11) are present in ~66% and ~6% of melanomas arising in the eye and skin, respectively. Furthermore, nearly 20% of human tumours harbour mutations in GPCRs. Many human cancer-associated viruses also express constitutively active viral GPCRs. These studies indicate that G proteins, GPCRs and their linked signalling circuitry represent novel therapeutic targets for cancer prevention and treatment.
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              FGF-23 in fibrous dysplasia of bone and its relationship to renal phosphate wasting.

              FGF-23, a novel member of the FGF family, is the product of the gene mutated in autosomal dominant hypophosphatemic rickets (ADHR). FGF-23 has been proposed as a circulating factor causing renal phosphate wasting not only in ADHR (as a result of inadequate degradation), but also in tumor-induced osteomalacia (as a result of excess synthesis by tumor cells). Renal phosphate wasting occurs in approximately 50% of patients with McCune-Albright syndrome (MAS) and fibrous dysplasia of bone (FD), which result from postzygotic mutations of the GNAS1 gene. We found that FGF-23 is produced by normal and FD osteoprogenitors and bone-forming cells in vivo and in vitro. In situ hybridization analysis of FGF-23 mRNA expression identified "fibrous" cells, osteogenic cells, and cells associated with microvascular walls as specific cellular sources of FGF-23 in FD. Serum levels of FGF-23 were increased in FD/MAS patients compared with normal age-matched controls and significantly higher in FD/MAS patients with renal phosphate wasting compared with those without, and correlated with disease burden bone turnover markers commonly used to assess disease activity. Production of FGF-23 by FD tissue may play an important role in the renal phosphate-wasting syndrome associated with FD/MAS.

                Author and article information

                Horm Res Paediatr
                Hormone Research in Paediatrics
                S. Karger AG
                June 2020
                19 December 2019
                : 92
                : 6
                : 347-356
                Skeletal Disorders and Mineral Homeostasis Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, Maryland, USA
                Author notes
                *Alison M. Boyce, National Institutes of Health, Building 30 Room 218 MSC 4320, Bethesda, MD 20892 (USA), E-Mail boyceam@mail.nih.gov
                504802 Horm Res Paediatr 2019;92:347–356
                © 2019 S. Karger AG, Basel

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