Fibrous Dysplasia/McCune-Albright Syndrome: A Rare, Mosaic Disease of Gα s Activation.

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare disorder of striking complexity. It arises from somatic, gain-of-function mutations in GNAS, leading to mosaic Gα s activation and inappropriate production of intracellular cyclic adenosine monophosphate (cAMP). The clinical phenotype is largely determined by the location and extent of affected tissues, and the pathophysiological effects of Gα s activation within these tissues. In bone, Gα s activation results in impaired differentiation of skeletal stem cells, leading to discrete skeletal lesions prone to fracture, deformity, and pain. Extraskeletal manifestations include a variable combination of hyperpigmented macules and hyperfunctioning endocrinopathies. Distinctive age-related changes in disease development has key effects on histologic, radiographic, and clinical features. FD/MAS thus presents along a uniquely broad clinical spectrum, and the resulting challenges in diagnosis and management can be difficult for clinicians. This review presents FD/MAS in the context of a mosaic disorder of Gα s activation, providing an intellectual framework within which to understand, evaluate, and treat this interesting disease. It includes a comprehensive summary of current understanding of FD/MAS pathogenesis, and a detailed discussion of clinical presentation and management. Critical areas of unmet need are highlighted, including discussion of key challenges and potential solutions to advance research and clinical care in FD/MAS.

The Clinical Spectrum of McCune-Albright Syndrome and Its Management.

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 Gαs activation and ligand-independent signaling of the Gαs-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 Gαs activation within these tissues. Patients pre-sent clinically with a variable combination of fibrous dysplasia of bone (FD), café-au-lait skin macules, and hyperfunctioning endocrinopathies. In bone, Gαs 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.

Growth hormone-Insulin-like growth factor 1 axis hyperactivity on bone fibrous dysplasia in McCune-Albright Syndrome.

CONTEXT: In fibrous dysplasia (BFD), normal bone and bone marrow are replaced by fibro-osseous tissue, leading to fracture, deformity and pain. BFD may be isolated, or in association with cutaneous hyperpigmentation and/or hyperfunctioning endocrinopathies, termed McCune-Albright syndrome (MAS). GH hypersecretion has been described in 10%-20% of MAS-BFD patients. Aim of the study was to determine the impact of GH-insulin like growth factor 1 (IGF1) axis hyperactivity on MAS-BFD morbidities and the efficacy of GH excess therapy. DESIGN AND PATIENTS: A multicentric cross-sectional analysis was conducted on three different MAS cohorts. From 195 MAS patients, 37 subjects (19%) with GH excess were identified and compared with 34 MAS controls without GH hypersecretion. RESULTS: Mean head circumference SDS was significantly higher in GH excess: 4.025 SDS vs 0.683 SDS (P < .0001). The risk of optic neuropathy (Odds ratio 4.231; P = .039), hearing deficit (Odds ratio 2.961; P = .0481), facial asymmetry (Odds ratio 6.563; P = .0192), malignancies (Odds ratio 15.24; P = .0173) were higher in GH excess group. Overall, pharmacotherapy (octreotide alone 10-30 mg/mo or with pegvisomant 10-20 mg/d) was effective in IGF1 normalization (IGF1 Z-score between -2 and +2 SDS) in 21/29 patients (72.4%) with good compliance to the regimen. Late diagnosis and GH excess treatment after 16 years old of age was associated with an increased risk of optic neuropathy (Odds ratio 4.500; P = .0491) and growth of pituitary adenomas (Odds ratio 7.846; P = .050). CONCLUSIONS: GH-IGF1 hyperactivity increases risk of morbidities in MAS. Medical therapy is effective in normalizing IGF1 in most patients, and early treatment during paediatric age is associated with a decreased risk of optic neuropathy and GH-secreting adenomas growth.

Virilizing sclerosing-stromal tumor of the ovary in a young woman with McCune Albright syndrome: clinical, pathological, and immunohistochemical studies.

CONTEXT: McCune-Albright syndrome (MAS) is characterized by polyostotic fibrous dysplasia, café-au-lait skin pigmentations, and gonadotropin-independent sexual precocious puberty, resulting from a somatic postzygotic activating mutation of the GNAS1 gene. SETTING: We report a virilizing sclerosing-stromal tumor of the ovary in a young female with MAS. PATIENT: She presented polyostotic fibrous dysplasia of the left upper and lower limbs and a café-au-lait skin spot in the posterior area of the neck. She had a history of precocious puberty, diagnosed at the age of 6 years and treated with cyproterone acetate until the age of 10 years; then she developed central puberty with severe oligomenorrhea. At the age of 23 years, she was hospitalized for a virilization syndrome including hirsutism, acne, deepening of the voice, amenorrhea, and clitoromegaly. Serum levels of T were dramatically increased (1293 ng/dl; normal range, 10-80). The abdominal computed tomography scan revealed a solid mass located on the left ovary. INTERVENTION: An ovariectomy was performed, and histological examination revealed a sclerosing-stromal tumor with pseudolobular pattern. RESULTS: Immunohistochemical studies revealed that the tumor cells expressed all steroidogenic enzymes involved in androgen synthesis. Molecular analysis revealed that ovarian tumor cells harbored the Arg 201 activating mutation in the GNAS1 gene. After surgery, T levels returned to normal, the patient retrieved a normal gonadal function, and she was able to become pregnant. CONCLUSION: This observation extends the clinical spectrum of ovarian pathology of women with MAS. However, the mechanisms causing this ovarian tumor remain unclear, even if the gsp oncogene has been implicated in the pathogenesis of some gonadal tumors.

Thyroid abnormalities in children and adolescents with McCune-Albright syndrome.

BACKGROUND: To date, there is no agreement about the frequency or the features of thyroid abnormalities in McCune-Albright syndrome (MAS). The aim of our study was to detect thyroid abnormalities in a cohort of MAS children and adolescents and to give indications for their treatment and follow-up. METHODS: In 36 patients, 22 females and 14 males, thyroid function and sonographic features of thyroid were evaluated every 6-12 months. RESULTS: Three males and 1 female had hyperthyroidism: 2 with nodular, 2 with diffuse goiters. They were treated with methimazole (0.2-0.5 mg/kg/day) with good clinical and biochemical responses. The remaining 32 patients were euthyroid, even if 7 displayed sonographic alterations, of whom 5 had nodular goiter with nodules >1 cm, and 2 micronodular goiter. Fine-needle aspiration biopsy was performed in 2 patients with nodules >1 cm, 1 showing hemorrhagic nodule and 1 colloid cystic nodule. CONCLUSIONS: Prevalence of thyroid alterations in the studied MAS series was 31%. 64% of 11 patients with thyroid alterations had nodular goiters, with nodules >1 cm. As the onset of thyroid disease ranged from 1 to 20 years, a strict monitoring of thyroid function is recommended every 6 months. Satisfactory treatment can be obtained and maintained with antithyroid drugs.

Wnt/ß-catenin signaling is differentially regulated by Gα proteins and contributes to fibrous dysplasia.

Skeletal dysplasias are common disabling disorders characterized by aberrant growth of bone and cartilage leading to abnormal skeletal structures and functions, often attributable to defects in skeletal progenitor cells. The underlying molecular and cellular mechanisms of most skeletal dysplasias remain elusive. Although the Wnt/ß-catenin signaling pathway is required for skeletal progenitor cells to differentiate along the osteoblastic lineage, inappropriately elevated levels of signaling can also inhibit bone formation by suppressing osteoblast maturation. Here, we investigate interactions of the four major Gα protein families (Gα(s), Gα(i/o), Gα(q/11), and Gα(12/13)) with the Wnt/ß-catenin signaling pathway and identify a causative role of Wnt/ß-catenin signaling in fibrous dysplasia (FD) of bone, a disease that exhibits abnormal differentiation of skeletal progenitor cells. The activating Gα(s) mutations that cause FD potentiated Wnt/ß-catenin signaling, and removal of Gα(s) led to reduced Wnt/ß-catenin signaling and decreased bone formation. We further show that activation of Wnt/ß-catenin signaling in osteoblast progenitors results in an FD-like phenotype and reduction of ß-catenin levels rescued differentiation defects of FD patient-derived stromal cells. Gα proteins may act at the level of ß-catenin destruction complex assembly by binding Axin. Our results indicate that activated Gα proteins differentially regulate Wnt/ß-catenin signaling but, importantly, are not required core components of Wnt/ß-catenin signaling. Our data suggest that activated Gα proteins are playing physiologically significant roles during both skeletal development and disease by modulating Wnt/ß-catenin signaling strength.

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 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 Gß1γ2 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.

Clinical, radiological and endocrinological findings in a case of McCune-Albright syndrome.

The McCune-Albright syndrome was described as a syndrome of polyostotic fibrous dysplasia, café au lait skin pigmentation, and autonomous endocrine hyperfunction in 1937. We report a 17-year girl with early menarche and accelerated growth for the past three years. The endocrinological examination showed slight rise of growth hormone with other hormones in normal range. The CT showed an expansive bony lesion over the left parietal area. The bone mass was excised with bone cement cranioplasty performed for the defect. Histology confirmed it was fibrous dysplasia. Although uncommon, this syndrome must be kept in mind in cases with bony abnormalities and extensive endocrinological workup done and treatment given for best results.

The journey from vitamin D-resistant rickets to the regulation of renal phosphate transport.

In 1937, Fuller Albright first described two rare genetic disorders: Vitamin D resistant rickets and polyostotic fibrous dysplasia, now respectively known as X-linked hypophosphatemic rickets (XLH) and the McCune-Albright syndrome. Albright carefully characterized and meticulously analyzed one patient, W.M., with vitamin D-resistant rickets. Albright subsequently reported additional carefully performed balance studies on W.M. In this review, which evaluates the journey from the initial description of vitamin D-resistant rickets (XLH) to the regulation of renal phosphate transport, we (1) trace the timeline of important discoveries in unraveling the pathophysiology of XLH, (2) cite the recognized abnormalities in mineral metabolism in XLH, (3) evaluate factors that may affect parathyroid hormone values in XLH, (4) assess the potential interactions between the phosphate-regulating gene with homology to endopeptidase on the X chromosome and fibroblast growth factor 23 (FGF23) and their resultant effects on renal phosphate transport and vitamin D metabolism, (5) analyze the complex interplay between FGF23 and the factors that regulate FGF23, and (6) discuss the genetic and acquired disorders of hypophosphatemia and hyperphosphatemia in which FGF23 plays a role. Although Albright could not measure parathyroid hormone, he concluded on the basis of his studies that showed calcemic resistance to parathyroid extract in W.M. that hyperparathyroidism was present. Using a conceptual approach, we suggest that a defect in the skeletal response to parathyroid hormone contributes to hyperparathyroidism in XLH. Finally, at the end of the review, abnormalities in renal phosphate transport that are sometimes found in patients with polyostotic fibrous dysplasia are discussed.

Regulation of spermatogenesis in McCune-Albright syndrome: lessons from a 15-year follow-up.

CONTEXT: McCune-Albright syndrome (MAS) is a disorder caused by a post-zygotic gain-of-function mutation in the gene encoding the Gs-alpha protein. Sexual precocity, common in girls, has been reported in only 15% of boys, and little is known on the long-term evolution of MAS in males. OBJECTIVE: In a boy with MAS, we studied spermatogenesis, testis histology, and immunohistochemistry with the aim to shed light on seminiferous tubule activity. DESIGN: A boy who presented at the age of 2.9 years with sexual precocity, monolateral macroorchidism, increased testosterone levels, and suppressed gonadotropins was followed up until the age of 18. RESULTS: Throughout follow-up testicular asymmetry persisted and gonadotropin and testosterone pattern did not change. At the age of 18, inhibin B was undetectable while alpha-immunoreactive inhibin was within normal range. Anti-Mullerian hormone level was slightly subnormal. Sperm cells were 3,900,000 per ejaculate. Histology of both testes showed spermatogonia, spermatocytes, and, in some tubes, matured spermatozoa. Sertoli cells were markedly stained with anti-inhibin alpha-subunit antibody in both the testes. There was no immunostaining of Sertoli, Leydig, or germ cells with anti-betaA or anti-betaB antibody. MAS R201H mutation was identified in both the testes. CONCLUSION: The 15-year follow-up in this boy with MAS demonstrated that autonomous testicular activation and gonadotropin suppression persisted over time. This provides an interesting model of active spermatogenesis despite long-term FSH suppression. It also suggests that FSH is needed for the full expression of the inhibin betaB-subunit gene, an expression previously reported in the germ and Leydig cells of normal adult subjects.

Fibrous dysplasia, phosphate wasting and fibroblast growth factor 23.

Fibrous dysplasia (FD) is a classic feature of McCune-Albright syndrome (MAS). Renal phosphate wasting commonly occurs in FD, contributing to the mineralization defect in FD lesions and in non-FD bones, potentially increasing bone deformity. The presence of phosphate wasting correlates with measures of FD disease activity. Hypophosphatemia and phosphate wasting in FD are accompanied by inappropriately normal or low 1,25-dihydroxyvitamin D3 concentrations, similar to X-linked hypophosphatemic rickets. Recently, fibroblast growth factor 23 (FGF23) has emerged as an important humoral factor regulating phosphate and vitamin D homeostasis. FGF23 inhibits renal tubular phosphate reabsorption and decreases 1,25-dihydroxyvitamin D3. Interestingly, FGF23 is produced by normal osteoblasts as well as the abnormal osteogenic precursors present in FD lesions. However, FD lesions likely produce FGF23 in an unregulated fashion. Elevated circulating FGF23 correlates with total body FD disease burden and the presence of phosphate wasting. MAS mutations increase immature osteoblast lineage cells causing FD, resulting in dysregulated FGF23 production and, hence, phosphate wasting.

McCune-Albright syndrome and disorders due to activating mutations of GNAS1.

It has been more than seven decades since Drs. Fuller Albright and Donovan McCune published the first reports on individuals with McCune-Albright syndrome (MAS). Since then, the classic triad of precocious puberty, café-aulait spots, and polyostotic bone dysplasia continues to define the syndrome. However, having gathered a better picture of the pathophysiology of MAS, the way this condition is understood has changed. Isolated activating mutations of the alpha subunit of the G protein (GNAS1) have been found in different tissues, including pituitary adenomas, thyroid adenomas, ovarian cysts, monostotic bone dysplasia, and the adrenal glands, to name a few. For this reason, we have added 'and disorders due to activating mutations of GNAS1' to the title of this review. We discuss here the clinical consequences of GNAS1 activating mutations in different body systems and organs, the diagnostic approach to MAS, and the current therapeutic recommendations.

Patients with mutations in Gsalpha have reduced activation of a downstream target in epithelial tissues due to haploinsufficiency.

CONTEXT: Patients with Albright hereditary osteodystrophy (AHO) have defects in stimulatory G protein signaling due to loss of function mutations in GNAS. The mechanism by which these mutations lead to the AHO phenotype has been difficult to establish due to the inaccessibility of the affected tissues. OBJECTIVE: The objective of the study was to gain insight into the downstream consequences of abnormal stimulatory G protein signaling in human epithelial tissues. PATIENTS AND DESIGN: We assessed transcription of GNAS and Gsalpha-stimulated activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in AHO patients, compared with normal controls and patients with cystic fibrosis. MAIN OUTCOME MEASURES: Relative expression of Gsalpha transcripts from each parental GNAS allele and cAMP measurements from nasal epithelial cells were compared among normal controls and AHO patients. In vivo measurements of CFTR function, pulmonary function, and pancreatic function were assessed in AHO patients. RESULTS: GNAS was expressed equally from each allele in normals and two of five AHO patients. cAMP generation was significantly reduced in nasal respiratory epithelial cells from AHO patients, compared with normal controls (0.4 vs. 0.6, P = 0.0008). Activation of CFTR in vivo in nasal (P = 0.0065) and sweat gland epithelia (P = 0.01) of AHO patients was significantly reduced from normal. In three patients, the reduction in activity was comparable with patients with cystic fibrosis due to mutations in CFTR. Yet no AHO patients had pulmonary or pancreatic disease consistent with cystic fibrosis. CONCLUSIONS: In humans, haploinsufficiency of GNAS causes a significant reduction in the activation of the downstream target, CFTR, in vivo.

Letrozole treatment of precocious puberty in girls with the McCune-Albright syndrome: a pilot study.

CONTEXT: Girls with McCune-Albright syndrome (MAS) and related disorders have gonadotropin-independent precocious puberty due to estrogen secretion from ovarian cysts. Their puberty does not respond to GnRH agonist therapy, and short-acting aromatase inhibitors have had limited effectiveness. OBJECTIVE: Our objective was to assess the effectiveness of the potent, third-generation aromatase inhibitor letrozole in decreasing pubertal progression in girls with MAS and to assess the response of indices of bone turnover associated with the patients' polyostotic fibrous dysplasia. DESIGN: Subjects were evaluated at baseline and every 6 months for 12-36 months while on treatment with letrozole 1.5-2.0 mg/m(2).d. SETTING: This was an open-label therapeutic trial at a single clinical center. PATIENTS: Patients included nine girls aged 3-8 yr with MAS and/or gonadotropin-independent puberty. MAIN OUTCOME MEASURES: Measures included rates of linear growth, bone age advance, mean ovarian volume, estradiol, episodes of vaginal bleeding, and levels of the indices of bone metabolism: serum osteocalcin, alkaline phosphatase, urinary hydroxyproline, pyridinoline, deoxypyridinoline, and N-telopeptides. RESULTS: Girls had decreased rates of growth (P < or = 0.01) and bone age advance (P < or = 0.004) and cessation or slowing in their rates of bleeding over 12-36 months of therapy. Mean ovarian volume, estradiol, and indices of bone metabolism fell after 6 months (P < or = 0.05) but tended to rise by 24-36 months. Uterine volumes did not change. One girl had a ruptured ovarian cyst after 2 yr of treatment. CONCLUSIONS: This preliminary study suggests that letrozole may be effective therapy in some girls with MAS and/or gonadotropin-independent precocious puberty. Possible adverse effects include ovarian enlargement and cyst formation.

A rare case of metabolic bone disease.

A 26-year-old female overseas student was admitted to hospital with a fracture of her left humerus following minimal trauma. Biochemical abnormalities included hypercalcaemia, hypophosphataemia, raised alkaline phosphatase, raised parathyroid hormone and undetectable 25-hydroxy-vitamin D. Skeletal X-rays revealed multiple osteolytic lesions in the humerus as well as similar lesions in the femora and pelvis. Magnetic resonance imaging of her left shoulder showed a large soft tissue mass in the proximal humerus. Bone biopsy was reported as consistent with a brown tumour of primary hyperparathyroidism and a sestamibi scan confirmed the presence of a parathyroid adenoma. However, the isotope bone scan was reported as showing features typical of fibrous dysplasia involving multiple sites. The patient subsequently fractured her right femoral shaft, and a femoral nail was inserted. Parathyroidectomy was performed at the same time. Postoperatively she exhibited increased calcium and vitamin D requirements. Coexistence of primary hyperparathyroidism and polyostotic fibrous dysplasia is very rare.

Identification of a second-site suppressor mutation of the GTPase defect associated with McCune-Albright syndrome: a model using the yeast heterotrimeric G protein, GPA1.

McCune-Albright syndrome (MAS) causes a variety of bone and endocrine abnormalities due to the post-zygotic mutation of the alpha subunit of the stimulatory G-protein Gsalpha. This mutation causes signal-independent activity of the G-protein in the affected cells. We report the development of a system to study the effects of MAS mutations using Saccharomyces cerevisiae, wherein activation of the yeast G-protein pathway results in growth arrest in a genetically recessive fashion. We introduced the MAS mutation into the analogous site in the yeast Galpha gene, GPA1 and randomly mutated the gene to produce intragenic suppressors. Yeast with normal and mutated G-protein genes were induced to lose the normal gene, and mutations able to intragenically suppress the constitutive activity of the MAS mutation were identified based on their ability to form colonies. We report one mutation in GPA1, also in the active site, that is an intragenic suppressor of the MAS defect.

The "pirate sign" in fibrous dysplasia.

Fibrous dysplasia commonly involves the skull in both its monostotic and polyostotic variants. We present two cases of fibrous dysplasia involving the sphenoid wing, which were strikingly similar in their bone scan appearance. Both patients demonstrated intense increased uptake of Tc-99m MDP in a pattern reminding us of a "pirate wearing an eyepatch." We propose that this characteristic appearance of fibrous dysplasia of the sphenoid wing be called the "pirate sign." A review of the literature revealed several other pathologic conditions that have been reported to involve the sphenoid bone and should be considered in the differential diagnosis of abnormal bone tracer uptake in this region.