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McCune-Albright Syndrome

Recent research efforts aimed at curing McCune-Albright Syndrome.

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McCune-Albright Syndrome

Overview

McCune-Albright syndrome (MAS) is a rare, non-inherited condition caused by a post-fertilization (mosaic) activating change in the GNAS gene. Because only some cells carry the change, severity varies widely: people may have fibrous dysplasia (FD), in which abnormal fibrous tissue replaces normal bone; café-au-lait skin patches; and overactive hormone-producing tissues, including precocious puberty, thyroid disease, growth-hormone excess, phosphate wasting, or—rarely—newborn hypercortisolism. GeneReviews: FD/MAS

Prognosis depends on which tissues are affected and how extensive the mosaicism is. Some people have a limited, stable bone lesion, while others develop fractures, deformity, scoliosis, chronic pain, impaired mobility, or craniofacial disease that can affect vision or hearing. Current care is multidisciplinary and symptom-directed: orthopedic and rehabilitation care for skeletal complications; treatment of endocrine overactivity; monitoring of vision, hearing, skeletal burden, and phosphate; and pain treatment, including intravenous bisphosphonates when appropriate. No established medicine currently changes the underlying course of FD lesions. GeneReviews: management of FD/MAS

Scope of Recent Research (2020–present)

The field is active but remains small, with most recent work focused on the skeletal component of MAS/FD: identifying why lesions grow, interrupting abnormal bone remodeling, measuring mosaic GNAS variants without invasive biopsies, and finding drugs that reduce signaling downstream of mutant Gαs. The leading near-term candidates are disease-modifying rather than curative—especially RANKL blockade with denosumab—while direct correction of mosaic GNAS mutations by gene editing, RNA therapy, or cell replacement has not yet reached clinical testing. State-of-the-art FD/MAS research review

Major Breakthroughs and Emerging Therapies

The most consequential clinical advance has been targeting RANKL, a signaling protein that promotes formation of osteoclasts, the cells that resorb bone. In a 2023 NIH Phase 2 study, high-dose denosumab—a monoclonal antibody against RANKL—substantially reduced bone-turnover markers and lesion activity in adults with FD. Follow-up analysis of treated human lesions and mouse models found reduced lesion cellularity, more mature bone formation, and lower expression of the pathogenic Gαs variant in lesion tissue. This suggests denosumab may affect the lesion environment more deeply than a conventional anti-resorptive drug, although it does not eliminate the underlying mosaic mutation. Phase 2 denosumab report Mechanistic RANKL-inhibition study

A central safety issue is the “rebound” after denosumab withdrawal: bone turnover can rise sharply, and severe hypercalcemia has occurred in patients with high skeletal disease burden. In mouse models, RANKL blockade could arrest lesion growth and increase mineralized bone, but disease activity resumed after treatment withdrawal. These findings support denosumab as a potentially important disease-modifying approach, not a durable cure unless future research establishes safer dosing, discontinuation, or combination strategies. Phase 2 denosumab report State-of-the-art FD/MAS research review

Researchers are also pursuing therapies closer to the root molecular defect. Activating GNAS variants keep Gαs signaling switched on, increasing cyclic AMP (cAMP), a cellular messenger. A 2022 chemical-biology study identified cell-permeable macrocyclic peptides that selectively modulate Gαs; one inhibited several oncogenic Gαs mutants, including R201C and R201H, the same hotspot variants that cause most MAS cases. This is an important proof of principle, but these compounds have not yet been tested as MAS treatments in people or shown to reach affected bone safely. Gαs macrocyclic-peptide study

Other preclinical work points to Wnt/β-catenin signaling, adenylyl cyclase/cAMP inhibition, and pain-related pathways as possible targets. Genetic or small-molecule inhibition of Wnt signaling improved long-bone and craniofacial features in mouse FD models, while FD/MAS-focused seed grants have supported testing of compounds that inhibit cAMP signaling and GNAS-driven pathways. These are scientifically promising directions, but they remain preclinical and are not evidence of a current curative therapy. State-of-the-art FD/MAS research review FD/MAS Alliance research grants

For phosphate wasting caused by excess fibroblast growth factor 23 (FGF23), burosumab—a monoclonal antibody against FGF23—is being studied to normalize blood phosphate. This approach may improve osteomalacia and related symptoms, but it treats a downstream complication rather than the mutant cells or FD lesions themselves. GeneReviews: therapies under investigation

Clinical Trials and Experimental Approaches

The completed NIH/NIDCR-sponsored Phase 2 denosumab trial in adults with FD, including people within the FD/MAS spectrum, enrolled eight women who received denosumab for six months followed by eight months of observation. Treatment markedly suppressed bone turnover and was associated with reduced lesion activity and reported improvements in selected complications; however, severe rebound hypercalcemia after discontinuation required intensified monitoring. Phase 2 denosumab report Denosumab trial record

A pediatric NIDCR Phase 2 study, NCT05419050, is evaluating denosumab for prevention of skeletal disease progression in children with FD/MAS. The trial is listed as completed, but peer-reviewed efficacy and long-term safety results were not identified for this report. Pediatric denosumab trial record

A separate NIDCR Phase 2 study, NCT05509595, evaluates burosumab for FGF23-mediated hypophosphatemia in FD. Its goal is to assess safety and the ability to normalize serum phosphate; it is not designed to remove FD lesions or correct mosaic GNAS mutations. Burosumab trial record GeneReviews: burosumab in FD

Methodologies and Scientific Approaches

Researchers combine patient tissue with several complementary models. These include primary bone-marrow stromal cells and skeletal stem cells from FD lesions, induced pluripotent stem-cell models, engineered human bone “ossicles” implanted in mice, and multiple transgenic mouse strains that express disease-causing GNAS variants in specific bone-cell lineages. These systems allow investigators to test how mutant and non-mutant cells interact, how lesions form, and whether therapies restore bone maturation or prevent lesion expansion. State-of-the-art FD/MAS research review

Modern molecular methods are improving both diagnosis and trial measurement. In a 2023 NIH study, digital-droplet PCR and allele-specific PCR detected mutant GNAS DNA in circulating cell-free DNA from plasma in 68.2% of 66 FD/MAS patients when methods were combined; detection was more likely in younger people and those with higher skeletal burden. Researchers also use lesion histology, RNA sequencing, PET/CT lesion activity, bone-turnover markers, and skeletal-burden scoring to evaluate potential therapies. Circulating cell-free DNA study Mechanistic RANKL-inhibition study

Leading Institutions and Funding

The NIH National Institute of Dental and Craniofacial Research (NIDCR) and NIH Clinical Center are central to FD/MAS translational research, including natural-history studies, the adult denosumab trial, pediatric denosumab research, and burosumab testing. Major academic contributors include Boston Children’s Hospital and Harvard-affiliated hospitals, the University of California at San Francisco and Davis, Sapienza University of Rome, Boston College, the University of Copenhagen, and the University of California San Diego. State-of-the-art FD/MAS research review GeneReviews: FD/MAS

Patient-led funding is unusually important in this small field. Through the University of Pennsylvania Orphan Disease Center’s Million Dollar Bike Ride, the FD/MAS Alliance reported more than $120,000 raised in 2024, supporting a $80,468 project at UC San Diego on GNAS-driven precision therapy and a $40,234 mouse-pain project. In 2023, the program awarded $80,642 for cAMP-inhibitor testing and two $40,321 grants for pain and spatial-transcriptomics studies. FD/MAS Alliance research grants

Strengths, Limitations, and Challenges

The strongest recent evidence is for RANKL inhibition: it has human Phase 2 data, measurable biological effects, and supporting mechanistic studies in lesion tissue and mice. However, denosumab is not yet an established FD/MAS therapy because rebound bone turnover and hypercalcemia can be dangerous, particularly in younger patients and those with extensive disease. It also requires specialized monitoring and does not directly repair the mosaic genetic change across bone, endocrine tissues, and skin. GeneReviews: denosumab cautions Mechanistic RANKL-inhibition study

The core scientific challenge is mosaicism. Each person has a different distribution and proportion of mutant cells, so one systemic therapy may have different effects across bones and organs. FD lesions contain both mutant and genetically typical cells, and their interactions appear necessary for disease behavior. This complexity complicates gene delivery, gene editing, biomarker interpretation, trial enrollment, and prediction of whether a treatment can permanently stop lesion growth. State-of-the-art FD/MAS research review Circulating cell-free DNA study

Outlook and Future Directions

As of October 22, 2026, a true cure for McCune-Albright syndrome is not close: no therapy has been shown to safely eliminate or permanently correct mosaic GNAS mutations across affected tissues. The most meaningful milestones to watch are publication of pediatric denosumab outcomes; longer-term data on preventing rebound after RANKL blockade; results of burosumab for phosphate wasting; validation of circulating cell-free GNAS DNA as a biomarker; and conversion of Gαs-, cAMP-, or Wnt-targeting discoveries into bone-directed medicines with acceptable safety. The realistic near-term goal is durable disease modification—preventing lesion progression, fractures, deformity, and pain—while direct molecular correction remains an earlier-stage ambition. State-of-the-art FD/MAS research review

References

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