McCune-Albright Syndrome

Definition

McCune-Albright Syndrome (MAS) is a rare genetic disorder characterized by the classic triad of:

1. Polyostotic fibrous dysplasia - multiple bones replaced by fibrous tissue
2. Cafe-au-lait skin pigmentation - characteristic "coast of Maine" pattern
3. Endocrine dysfunction - most commonly precocious puberty

The condition results from postzygotic activating mutations in the GNAS1 gene (guanine nucleotide-binding protein, alpha-stimulating), leading to constitutive activation of adenylyl cyclase and increased cAMP signaling in affected tissues.

Epidemiology

• Incidence: 1 in 100,000 to 1 in 1,000,000 (rare)
• Sex Distribution: More commonly diagnosed in females (due to precocious puberty presentation)
• Age at Presentation: Usually early childhood (2-10 years)
• Inheritance: NOT inherited - always arises from de novo postzygotic somatic mutation

Genetic Basis

GNAS1 Mutation:

• Located on chromosome 20q13
• Encodes the alpha subunit of the stimulatory G protein (Gs-alpha)
• Activating mutation at codon 201 (R201H or R201C) or codon 227
• Results in constitutive activation of Gs-alpha protein
• Increased adenylyl cyclase activity → elevated intracellular cAMP
• Downstream effects: enhanced cell proliferation, hormone hypersecretion

Why Postzygotic?

• Germline GNAS1 activating mutations are incompatible with life (embryonic lethal)
• Mutations occur after fertilization during early embryogenesis
• Earlier mutation = more widespread tissue involvement
• Later mutation = fewer tissues affected, milder phenotype
• This explains the mosaic distribution of lesions

McCune-Albright Syndrome is fundamentally a clinical and radiological diagnosis; molecular testing is confirmatory, not mandatory.

How the diagnosis is defined

• The classic definition requires two or more of the three cardinal features: polyostotic fibrous dysplasia, cafe-au-lait macules, and a hyperfunctioning endocrinopathy (most often gonadotropin-independent precocious puberty).
• The FD/MAS International Consortium frames the condition as a continuous spectrum rather than discrete boxes, and pragmatically accepts the label when fibrous dysplasia coexists with even a single extraskeletal (skin or endocrine) feature. Isolated monostotic or polyostotic FD without extraskeletal disease is not MAS despite sharing the same GNAS mutation.

Why GNAS testing can mislead

• The activating GNAS mutation is a post-zygotic mosaic event, so the mutant allele is confined to affected tissues and is frequently present at a very low allele fraction.
• Peripheral-blood leukocyte testing has limited sensitivity, and that sensitivity falls further with increasing age and with more limited disease burden. A negative blood result does NOT exclude MAS.
• Higher yield comes from sampling affected tissue (a fibrous dysplasia lesion or a cafe-au-lait macule) or from high-sensitivity assays able to detect low-level mosaicism, such as allele-specific or digital-droplet PCR and targeted next-generation sequencing.
• The causative substitution almost always involves arginine 201 (R201H or R201C) in exon 8; a minority involve glutamine at codon 227.

When the mosaic mutation arises very early and is widely distributed, MAS can declare itself in the first weeks to months of life with a severe, endocrine- and viscera-dominant phenotype rather than the skeletal features that dominate in older children. Recognising this pattern matters because the neonatal form carries appreciable mortality.

Key features of the neonatal phenotype

• Neonatal Cushing syndrome — ACTH-independent hypercortisolism from autonomous, often bilateral adrenal hyperplasia. It can be life-threatening, yet a distinctive feature is that hypercortisolism may spontaneously regress in some infants; medical control (metyrapone, ketoconazole) can bridge to recovery, and bilateral adrenalectomy is reserved for refractory or severe disease.
• Neonatal cholestatic hepatitis / hepatic dysfunction — usually self-limiting but can be marked, forming part of the multi-system neonatal picture.
• Cardiac involvement — tachyarrhythmias and cardiomegaly/cardiomyopathy are described and contribute to early mortality, compounded by the metabolic stress of hypercortisolism and thyrotoxicosis.
• Neonatal hyperthyroidism and hypophosphataemia frequently co-exist, adding to the metabolic burden.

Why it matters

The orthopaedic manifestations of fibrous dysplasia typically declare themselves later, so the severe neonatal presentation is easily missed by a bone-focused lens. It demands urgent multidisciplinary endocrine and paediatric intensive-care input, with the skeletal surveillance that defines childhood MAS following once the acute neonatal illness is stabilised.

Skeletal Complications

Endocrine Complications

Craniofacial Complications

Surgical Complications

• Nonunion/delayed union - more common through FD lesions
• Recurrent deformity - progressive disease during growth
• Hardware failure - abnormal bone biology
• Refracture - at implant ends if not spanning lesion
• Blood loss - FD lesions can be vascular

Global Epidemiology

• Prevalence: estimated between 1 in 100,000 and 1 in 1,000,000; true figure uncertain because mild/monostotic and subclinical disease is under-ascertained.
• No sex predilection in skeletal disease, but girls present earlier and more often because of GnRH-independent precocious puberty.
• Mosaic, sporadic worldwide with no recognised geographic, ethnic or familial clustering, consistent with a postzygotic GNAS mutation.
• Disease burden tracks with mutation timing — earlier embryonic mutation produces more widespread, severe, multi-system disease.

Side-by-Side Guidance

Registry & Network Considerations

There is no implant-survival registry specific to FD/MAS (it is rare and managed with custom constructs rather than catalogued arthroplasty implants). Evidence is instead consolidated through:

• National rare-bone-disease networks and reference centres (for example RUDY in the UK, NIH natural-history cohorts in the US) which pool longitudinal outcome data.
• The FD/MAS International Consortium, which functions as the de facto global standard-setting and data-sharing body.

High- vs Limited-Resource Practice Variation

• Well-resourced settings: coordinated multidisciplinary teams (paediatric orthopaedics, endocrinology, craniofacial surgery, ophthalmology, genetics, metabolic bone), IV bisphosphonate programmes, GNAS molecular testing, whole-body MRI mapping, and access to burosumab (anti-FGF23) for refractory hypophosphatemia where licensed.
• Limited-resource settings: diagnosis rests on clinical triad plus plain radiographs; management prioritises fracture and deformity stabilisation and oral phosphate/calcitriol for phosphate wasting; molecular confirmation, advanced imaging and newer biologics are often unavailable. Radiotherapy must still be avoided regardless of setting.
• Denosumab and burosumab show promise in refractory disease but remain off-label/restricted for FD/MAS in most jurisdictions and are not first-line.

Transition of Care

Adolescents require structured transfer to adult endocrinology, adult orthopaedics with a bone-dysplasia interest, and ophthalmology/audiology as needed, with genetic counselling that emphasises the non-heritable nature of the GNAS mutation.

Bisphosphonates: pain relief without disease modification

IV pamidronate reliably reduces bone pain and resorption markers, but no randomised evidence shows reduced fracture rate, halted lesion progression, or restored normal bone. Oral bisphosphonates have failed to beat placebo for pain. They are therefore a symptomatic tool, not disease-modifying, and routine use for asymptomatic disease is not justified.

Prophylactic optic-nerve decompression

Historically advocated for radiographic optic-canal encasement, this is now discouraged. Cohort data show most fully encased nerves never develop neuropathy and remain stable, while prophylactic surgery has caused vision loss. Decompression is reserved for objective, progressive optic neuropathy. GH excess (which worsens encasement) should be actively sought and treated.

Timing of craniofacial contouring

Lesions may reactivate and regrow during growth and pregnancy, so cosmetic contouring is generally deferred toward skeletal maturity. Aggressive resection risks recurrence and functional harm; conservative shaving with long-term follow-up is preferred, but optimal timing remains debated.

Emerging biologics (denosumab, burosumab)

Denosumab (anti-RANKL) can reduce pain and lesion activity but raises concerns over rebound hypercalcaemia and unknown long-term effects on growing bone; it remains experimental in FD/MAS. Burosumab (anti-FGF23) is rational for FGF23-driven hypophosphatemia but is licensed for X-linked hypophosphatemia, not FD/MAS, and evidence is preliminary.

Surgical construct in the growing femur

Choice between flexible (elastic) nails and rigid/locked nails, and the role of valgus versus multilevel ("shish-kebab") osteotomy for shepherd's crook deformity, is individualised. Plates and curettage-with-grafting are widely discouraged in extensive disease, but no randomised trial defines the single best construct.

Malignant transformation risk

Quoted at well under 1% (rising with prior radiotherapy), the true baseline rate is uncertain because of small cohorts and selection bias. The consistent message is unambiguous: never irradiate FD, and investigate any rapid growth, escalating pain, or soft-tissue mass.