Mosaic AKT1 Overgrowth with Cerebriform Naevi
- PROTEUS SYNDROME is a rare mosaic overgrowth disorder caused by a somatic activating variant in AKT1 (c.49G>A, p.Glu17Lys), proved by exome sequencing of affected versus unaffected tissue from the same patients: 26 of 29 patients in the defining study carried the variant, with mutant-allele admixtures from 1% to about 50% in affected tissues. It is not inherited; no confirmed vertical transmission or sib recurrence has been reported.
- The clinical signature is PROGRESSIVE, DISPROPORTIONATE and DISTORTING overgrowth, usually asymmetric and patchy (mosaic distribution), typically with modest or no manifestations at birth, rapid progression from the toddler period, and relentless progression through childhood. The pathognomonic-weighted cutaneous feature is the CEREBRIFORM CONNECTIVE-TISSUE NAEVUS - a firm, ridged, brain-like plaque most often on the plantar surface - though it is not completely specific, having been described (histologically confirmed) in mosaic PIK3CA overgrowth as well.
- DIAGNOSIS is dyadic: all three general criteria (mosaic distribution of lesions, sporadic occurrence, progressive course) plus a weighted point score of phenotypic attributes integrated with molecular testing of AFFECTED TISSUE - a score of at least 10 points with a mosaic AKT1 pathogenic variant, or at least 15 points without one, establishes Proteus syndrome; a mosaic AKT1 variant with a score of 2-9 is designated AKT1-related overgrowth spectrum.
- ORTHOPAEDIC relevance includes asymmetric limb overgrowth with limb-length discrepancy, macrodactyly, scoliosis, exostoses, hip dysplasia, genu valgum, joint contractures and hindfoot deformity. In the dedicated leg-length-discrepancy case series (8 patients), guided growth with tension-band plates and epiphyseal arrest reduced discrepancy with few complications; deep-vein-thrombosis prophylaxis, sequential compression devices and rapid mobilisation were integral parts of the surgical protocol.
- SYSTEMIC risks dominate the natural history: a striking predisposition to deep vein thrombosis and pulmonary embolism (a significant source of mortality), progressive cystic lung disease that progresses faster in children, and a range of tumours. Targeted therapy is emerging: the pan-AKT inhibitor miransertib achieved a 50% reduction in phosphorylated AKT in affected tissue at 5 mg/m2/day in five of six patients in a phase 0/1 pharmacodynamic study, with a decrease in a cerebriform naevus and reduced pain in children.
- “Mosaic AKT1 c.49G>A (p.Glu17Lys); not inherited; test AFFECTED tissue, not blood.
- “Cerebriform connective-tissue naevus (usually plantar) is the most heavily weighted criterion - but not absolutely specific (also reported with mosaic PIK3CA).
- “Overgrowth is progressive, disproportionate and distorting; orthopaedic workload = limb-length discrepancy, macrodactyly, scoliosis, contractures.
- “Deep vein thrombosis/pulmonary embolism predisposition changes perioperative planning; avoid procoagulant drugs, growth hormone and androgenic steroids.
- “Progressive cystic lung disease, faster in children; tumour surveillance by history and examination.
Overview, Epidemiology & Historical Context
Proteus syndrome is an ultra-rare mosaic overgrowth disorder in which a somatic activating variant in AKT1 drives progressive, disproportionate and distorting overgrowth of the skeleton, skin, adipose tissue and other tissues. The variant arises after fertilisation and is confined to the cells that descend from the mutant progenitor, so the distribution is patchy and asymmetric, the severity is highly variable, and the disorder is not inherited.
How rare. An estimated prevalence of less than 1 in 1,000,000 live births is quoted in the case literature. No robust population incidence could be found, and the condition is best described as ultra-rare.
Natural history. Most affected individuals have modest or no manifestations at birth. Overgrowth develops and progresses rapidly from the toddler period and then progresses relentlessly through childhood, causing severe overgrowth and disfigurement (GeneReviews, Biesecker & Sapp).
Change is the defining behaviour. Lesions that are present evolve, enlarge and distort over time, and a static congenital asymmetry is not Proteus syndrome. That is why a progressive course is one of the general diagnostic criteria, and it helps separate Proteus syndrome from the PIK3CA-related overgrowth spectrum, where overgrowth is usually congenital and, in most patients, shows little postnatal progression.
The Elephant Man. Tibbles and Cohen (1986) marshalled the evidence that Joseph Merrick, the nineteenth-century "Elephant Man", had Proteus syndrome rather than neurofibromatosis, citing his macrocephaly, skull hyperostosis, long-bone hypertrophy, and thickened skin and subcutaneous tissue of the hands and feet including plantar hyperplasia and lipomas. The disorder had earlier been misattributed to neurofibromatosis, and misdiagnosis of Proteus syndrome remained common enough that Cohen (2005) made diagnostic criteria the centrepiece of his update.
Genetics & Pathophysiology
One variant. Lindhurst and colleagues (2011) performed exome sequencing of DNA from biopsy samples of affected and unaffected tissue from the same patients, and identified a somatic activating mutation in AKT1, c.49G>A, encoding p.Glu17Lys, in 26 of 29 patients with Proteus syndrome. Mutant-allele admixtures in affected tissues and cell lines ranged from 1% to approximately 50%, and mutant cell lines showed greater AKT phosphorylation than controls. The study concluded that Proteus syndrome is caused by this somatic activating variant, proving the long-standing somatic-mosaicism hypothesis and implicating PI3K-AKT pathway activation in the overgrowth and tumour susceptibility.
Why it has to be mosaic. The variant is lethal when ubiquitous. In a conditional mouse model, ubiquitous expression of the same Akt1 p.(E17K) allele caused embryonic lethality, with embryos showing fewer visible blood vessels, more haemorrhages, and a primitive cutaneous capillary network that failed to remodel into hierarchical vasculature (Lindhurst 2020). Survival to birth therefore requires that only a fraction of cells carry the variant, which is exactly the mosaicism seen clinically.
The pathway. AKT1 kinase sits in the PI3K-AKT-mTOR growth and survival pathway, and the p.Glu17Lys variant constitutively activates it, driving proliferation and inhibiting apoptosis in the affected cell population. Somatic PIK3CA variants activate the same pathway upstream in the PIK3CA-related overgrowth spectrum (PROS): fibroadipose overgrowth, CLOVES syndrome, megalencephaly-capillary malformation and isolated macrodactyly. That shared pathway is why these disorders overlap phenotypically and must be distinguished (Lindhurst 2012; Keppler-Noreuil 2014).
Test the affected tissue. Because the variant is confined to affected tissue, at admixtures that can be as low as 1%, molecular confirmation requires sampling affected tissue: a biopsy of overgrown or naevus tissue, or an affected surgical specimen. A negative blood result does not exclude the diagnosis.
There are no confirmed instances of vertical transmission or sib recurrence. Counselling a family about recurrence risk means saying the risks to parents and siblings are not increased over the general population.
Diagnosis: Criteria, the Cerebriform Naevus & the Differential
The general criteria. All three are required, and specific features are then scored:
- Mosaic distribution of lesions
- Sporadic occurrence
- Progressive course
The scored features. The original phenotype-only criteria were developed at an NIH workshop in March 1998, published by Biesecker and colleagues (1999) and updated in 2006. After the causal mosaic AKT1 variant was discovered and the overlapping PIK3CA-related overgrowth spectrum characterised, Sapp and colleagues (2019) re-evaluated the criteria and proposed a weighted, point-based system integrated with molecular testing, a "dyadic" gene-phenotype approach.
- Requirements
- All three general criteria plus a clinical score of at least 10 points in an individual with a mosaic AKT1 pathogenic variant, or at least 15 points without one
- Requirements
- A mosaic AKT1 pathogenic variant with a clinical score of 2-9 points
- Requirements
- An individual whose only manifestation is an AKT1 c.49G>A-positive tumour
Orthopaedic Manifestations
The workload. Reviewing the literature on 61 reported patients, Stricker (1992) found that patients with Proteus syndrome commonly develop macrodactyly, limb overgrowth, spinal deformity, hip dysplasia, genu valgum, exostoses, joint contractures and hindfoot deformities. The overgrowth distorts rather than simply enlarges: limbs become asymmetric, digits grotesque, spines progressively scoliotic, and joints stiff or malaligned.
Limb-length discrepancy. Asymmetric limb overgrowth produces a progressive discrepancy. In the dedicated surgical series (Crenshaw 2018, see Evidence), the average LLD was 3.4 cm (range 0.4-7.0) at presentation and had grown to 5.0 cm (range 1.8-10.0) by the time of first surgery, direct evidence that the discrepancy progresses and that timing matters.
Macrodactyly and local gigantism. Fingers and toes enlarge disproportionately. Debulking, osteotomy, ray resection or amputation may be needed for function, shoe wear or pain.
Spinal deformity. Progressive scoliosis and kyphoscoliosis are well recognised and can be driven by vertebral overgrowth and chest-wall asymmetry. Surveillance and standard deformity principles apply, with the caveat that progression can be unpredictable.
Beyond the limbs and spine. Craniofacial bony overgrowth and testicular and parotid masses also appear in the modern case literature.
The mistake is to treat the deformity as static. Measure the discrepancy serially, project it to skeletal maturity, and time guided growth or epiphyseal arrest accordingly. Build the thromboprophylaxis protocol into every surgical plan, because the thromboembolic risk is part of the disease, not an incidental anaesthetic consideration.
Management
Multidisciplinary, staged and built around the risks. Confirm the diagnosis dyadically, integrating the general criteria and weighted score with mosaic AKT1 testing of affected tissue, and classify AKT1-variant patients with low scores as AKT1-related overgrowth spectrum rather than forcing the label. The thromboembolic, pulmonary and tumour measures above run through every stage, alongside:
- Orthopaedics - procedures to delay or halt linear bone growth (guided growth, epiphyseal arrest), correction of skeletal deformities such as scoliosis, and debulking or ray resection/amputation for macrodactyly, with rehabilitation medicine, physiotherapy and occupational therapy throughout
- Skin - dermatological management, especially pedorthic intervention for the cerebriform plantar naevus (footwear, pressure distribution, skin care)
- Lungs - pulmonology surveillance of cystic lung disease
- Tumours - treatment by the surgeon or oncologist
- Development and psychosocial - developmental intervention, special education and psychosocial counselling are warranted in most individuals
Systemic Complications: Thromboembolism, Lung Disease & Tumours
A significant source of mortality. GeneReviews lists a "striking predisposition to deep vein thrombosis and pulmonary embolism" among the defining clinical characteristics, and the pulmonary literature identifies pulmonary embolism as a significant source of mortality. The substrate is anatomical: capillary, venous and lymphatic malformations with prominent varicosities, and dilated conducting veins (an enlarged inferior vena cava, saphenous varicosities and enlarged leg arteries and veins are documented in imaged cases), compounded by orthopaedic surgery and immobilisation.
What to avoid. GeneReviews advises avoiding medications that increase the risk of deep vein thrombosis or are procoagulant, and medications that increase growth (androgenic steroids, growth hormone).
Around surgery. The orthopaedic LLD series built DVT prophylaxis, sequential compression devices and rapid mobilisation into its surgical protocol as integral elements.
When it happens. Deep vein thrombosis and pulmonary embolism are evaluated and treated conventionally. A published adult case of pulmonary embolism in Proteus syndrome underlines that the risk does not stop at skeletal maturity. Chronic thromboembolic disease can also produce pulmonary hypertension, and precapillary pulmonary arterial hypertension without chronic emboli has been reported; dyspnoea in these patients warrants detailed investigation.
Guidelines, Registries & Global Practice
There is no disease-specific guideline, registry or validated surveillance schedule for Proteus syndrome; no randomised trial exists for any intervention. The authoritative synthesis used worldwide is the peer-reviewed GeneReviews chapter (Biesecker & Sapp, 2012, updated 2023), maintained by the group that defined the diagnostic criteria, and the 2019 dyadic diagnostic framework (Sapp et al.), which is designed to be applied anywhere that clinical assessment and molecular testing of affected tissue are available. Management is delivered by multidisciplinary teams - clinical genetics, paediatrics, orthopaedics, dermatology, pulmonology, vascular medicine and rehabilitation - and case reports from every inhabited continent (including the Kenyan and Indian cases underpinning this page's sourced images) describe the same pattern of progressive distorting overgrowth, cerebriform naevi and thromboembolic risk, with care constrained mainly by access to molecular diagnostics and to orthopaedic and pulmonary surveillance. The global messages are uniform: the disorder is mosaic and not inherited, the overgrowth is progressive, test affected tissue, and plan every operation around the thromboembolic risk.
Mnemonics & Memory Aids
PROTEUS
Hook:PROTEUS: Progressive overgrowth, Ridged plantar naevus, mOsaic AKT1, Thromboembolism, E17K, Unusual tumours, Skeleton.
SCORE-10/15Dyadic Diagnosis
Hook:Sporadic, Course progressive, One mosaic pattern, Result integrated, Eligibility 10/15 - all three general criteria plus the weighted score, married to the gene.
MCQ Practice Points
- The causal lesion is a somatic (mosaic) AKT1 c.49G>A, p.Glu17Lys gain-of-function variant - found in 26 of 29 patients in the defining study; it is not inherited, and ubiquitous expression is embryonic-lethal in a mouse model, which is why the disorder is always mosaic.
- Molecular confirmation requires affected tissue; mutant-allele admixtures range from 1% to ~50%, so blood can be negative.
- Diagnosis is dyadic: three general criteria (mosaic, sporadic, progressive) plus a weighted point score integrated with molecular testing - ≥10 points with a mosaic AKT1 variant or ≥15 without; 2-9 points with the variant = AKT1-related overgrowth spectrum.
- The cerebriform connective-tissue naevus (usually plantar) is the most heavily weighted criterion but not absolutely specific - it has been reported, histologically confirmed, with mosaic PIK3CA.
- Orthopaedic manifestations: macrodactyly, limb overgrowth, limb-length discrepancy, spinal deformity, hip dysplasia, genu valgum, exostoses, contractures, hindfoot deformity (Stricker 1992, 61-patient review).
- LLD surgery evidence: Crenshaw 2018 - tension-band-plate guided growth and epiphyseal arrest; mean LLD 3.4 cm at presentation → 5.0 cm at first surgery → 2.6 cm at last follow-up; DVT prophylaxis integral.
- Systemic risks: DVT/PE (a significant mortality source), progressive cystic lung disease (faster in children: Cystic Lung Score +5.6/year vs +1.6 in adults), and a range of tumours (ovarian cystadenoma, parotid adenoma, meningioma, immature teratoma).
- Avoid: procoagulant/DVT-risk medications, growth hormone and androgenic steroids.
- Targeted therapy: miransertib (oral allosteric pan-AKT inhibitor) - 5 mg/m2/day gave ≥50% pAKT reduction in affected tissue in 5/6 patients (phase 0/1); early sirolimus case report exists. Neither is standard of care.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A 7-year-old boy presents with progressive asymmetric overgrowth of the right leg and a thickened, folded, brain-like plaque on the sole of the foot. What is the likely diagnosis, how do you confirm it, and what must you think about before any operation?”
“An 11-year-old with molecularly confirmed Proteus syndrome has a leg-length discrepancy that has increased from 3 cm to 6 cm over two years, progressive scoliosis and macrodactyly of two toes. Outline your orthopaedic strategy and the evidence behind it.”
Genetics
- Mosaic (somatic) AKT1 c.49G>A, p.Glu17Lys - 26/29 patients (Lindhurst 2011)
- Allele admixture 1% to ~50% in affected tissue - TEST AFFECTED TISSUE, not blood
- Not inherited; no confirmed vertical transmission; ubiquitous expression embryonic-lethal (mouse)
Diagnosis
- General criteria (all three): mosaic distribution, sporadic occurrence, progressive course
- Dyadic score: >=10 points with mosaic AKT1 variant, or >=15 without; 2-9 with variant = AKT1-related overgrowth spectrum
- Cerebriform connective-tissue naevus (usually plantar) = most heavily weighted criterion, NOT absolutely specific (mosaic PIK3CA overlap)
- Differential: PROS/CLOVES (PIK3CA), Klippel-Trénaunay, PTEN hamartoma tumour syndrome, hemihyperplasia
Orthopaedic
- Macrodactyly, limb overgrowth, LLD, scoliosis, hip dysplasia, genu valgum, exostoses, contractures, hindfoot deformity
- LLD: serial measurement, project to maturity; tension-band-plate guided growth → epiphyseal arrest (Crenshaw 2018: 3.4→5.0 cm pre-op, 2.6 cm at final)
- Older mixed series: mixed surgical results - conservative/supportive care is the default; operate for defined functional goals
Systemic risks
- DVT/PE predisposition - significant mortality source; prophylaxis + rapid mobilisation around every operation
- Progressive cystic lung disease - faster in children (CLS +5.6/yr vs +1.6/yr adults)
- Tumours: ovarian cystadenoma, parotid adenoma, meningioma, immature teratoma - surveillance by history/examination, not blanket imaging
- Avoid procoagulant drugs, growth hormone, androgenic steroids
Targeted therapy
- Miransertib (pan-AKT inhibitor): 5 mg/m2/day → >=50% pAKT reduction in 5/6 (phase 0/1, Keppler-Noreuil 2019)
- One-year case report: improved mobility, reduced cerebriform naevus, stable MRI (Biesecker 2020)
- Early sirolimus case report (Weibel 2021); no targeted agent is standard of care
Evidence & Key Studies
A mosaic activating mutation in AKT1 associated with the Proteus syndrome
- Exome sequencing of DNA from biopsy samples of affected tissue compared with unaffected tissue from the same patients identified a somatic activating AKT1 mutation (c.49G>A, p.Glu17Lys).
- 26 of 29 patients with Proteus syndrome carried the variant; confirmation used a custom restriction-enzyme assay across 158 samples from 29 patients.
- Mutant-allele admixtures in tissues and cell lines ranged from 1% to approximately 50% - the quantitative signature of mosaicism.
- Mutant cell lines showed greater AKT phosphorylation than controls; paired single-cell clones differing only in mutation status differed in AKT phosphorylation.
- The authors concluded the syndrome is caused by this somatic activating AKT1 mutation, proving the somatic-mosaicism hypothesis and implicating PI3K-AKT pathway activation in the overgrowth and tumour susceptibility.
A dyadic genotype-phenotype approach to diagnostic criteria for Proteus syndrome
- Phenotype-based criteria dated from 1999 and were updated in 2006; the causal mosaic AKT1 c.49G>A p.E17K variant and the PIK3CA-related overgrowth spectrum required their re-evaluation.
- Proposes a weighted, point-based system for phenotypic attributes integrated with molecular results.
- Designations: AKT1-related Proteus syndrome (score >=10 with, or >=15 without, a mosaic AKT1 variant) versus AKT1-related overgrowth spectrum (mosaic AKT1 variant with score 2-9).
- A patient whose only manifestation is an AKT1 c.49G>A-positive tumour receives neither designation.
Proteus syndrome: diagnostic criteria, differential diagnosis, and patient evaluation
- Report of the March 1998 NIH workshop that produced the original recommendations for diagnostic criteria, differential diagnosis and patient evaluation.
- Recognised the disorder as highly variable and apparently mosaic, with diagnostic confusion aggravated by a lack of longitudinal natural-history data.
- Established the general criteria (mosaic distribution, sporadic occurrence, progressive course) and specific criteria structure on which later systems built.
Molecular heterogeneity of the cerebriform connective tissue nevus in mosaic overgrowth syndromes
- The cerebriform connective-tissue naevus was the most heavily weighted specific criterion in the pre-molecular diagnostic criteria.
- Two individuals with connective-tissue naevi and general attributes of Proteus syndrome had mosaic PIK3CA variants, not AKT1.
- Their plantar lesions showed the firm ridges-and-furrows morphology of a CCTN, histologically confirmed in one.
- Conclusion: CCTNs are not specific to Proteus syndrome; diagnosis should not rest on the CCTN alone but on the full criteria plus genetic analysis of affected tissue.
References
- Lindhurst MJ, Sapp JC, Teer JK, Johnston JJ, Finn EM, Peters K, et al. A mosaic activating mutation in AKT1 associated with the Proteus syndrome. N Engl J Med. 2011;365(7):611-619. doi:10.1056/NEJMoa1104017. PMID: 21793738
- Biesecker LG, Sapp JC. Proteus Syndrome. In: Adam MP, Bick S, Mirzaa GM, et al., editors. GeneReviews [Internet]. Seattle (WA): University of Washington, Seattle; 2012 Aug 9 [updated 2023 May 25]. PMID: 22876373
- Sapp JC, Buser A, Burton-Akright J, Keppler-Noreuil KM, Biesecker LG. A dyadic genotype-phenotype approach to diagnostic criteria for Proteus syndrome. Am J Med Genet C Semin Med Genet. 2019;181(4):565-570. doi:10.1002/ajmg.c.31744. PMID: 31692258
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