PI3K / AKT / mTOR Pathway Disorders

The PI3K-AKT-mTOR pathway controls cellular growth, proliferation, autophagy, and metabolism. Germline or somatic activation produces a recognizable family of overgrowth syndromes, hamartoma syndromes, and tumor predisposition syndromes: collectively the second-most-encountered signaling-pathway family in clinical genetics after the RASopathies. The unifying mechanism is inappropriately elevated mTOR activity, which is why several disorders in this family are uniquely treatable with mTOR inhibitors (rapamycin, everolimus): a rare situation in genetics where understanding the pathway directly translates into therapy.

RTK → PI3K (catalytic: PIK3CA; regulatory: PIK3R1)
              ↓ phosphorylates PIP2 → PIP3
              ↑ PTEN dephosphorylates PIP3 ⊣ shutting OFF the pathway
                     ↓
                   AKT1/2/3 (activated by PIP3)
                     ↓ phosphorylates and inactivates TSC1/TSC2
                     ↓
               mTORC1 (mTOR + Raptor) — released from TSC1/2 inhibition
                     ↓
              Protein synthesis, cell growth, autophagy suppression

Key principles:

• PTEN is the major brake: PTEN normally turns OFF the pathway by dephosphorylating PIP3. Loss of PTEN → constitutive PI3K signaling.
• TSC1/TSC2 is the second brake: TSC1 (hamartin) + TSC2 (tuberin) form a heterodimer that inhibits mTOR. Loss → mTOR hyperactivation.
• Activating mutations in PI3K (PIK3CA), AKT, or mTOR itself push the pathway "more on" without needing brake loss.
• mTOR inhibitors (sirolimus/rapamycin, everolimus) mimic restoration of the brakes; they are useful in TSC, PHTS, and some PROS conditions.

Brake loss: TSC1 / TSC2 → Tuberous Sclerosis Complex (TSC)

• Inheritance: AD; ~2/3 are de novo. TSC2 mutations more severe than TSC1 on average.
• Mechanism: TSC1/2 normally inhibits mTOR; loss → mTOR hyperactivation → hamartoma formation in many tissues.
• Skin (often the diagnostic gateway):
  • Hypopigmented "ash-leaf" macules (Wood's lamp visualizes)
  • Facial angiofibromas (cheeks, nasolabial folds)
  • Shagreen patch (lumbar; orange-peel texture)
  • Periungual fibromas (Koenen tumors)
• Brain: cortical tubers, subependymal nodules, subependymal giant cell astrocytoma (SEGA), infantile spasms, intellectual disability, autism
• Heart: cardiac rhabdomyoma (often the prenatal/neonatal diagnostic clue; usually regresses)
• Kidney: angiomyolipomas (risk of hemorrhage when >4 cm), polycystic disease (especially with contiguous TSC2/PKD1 deletion)
• Lung: lymphangioleiomyomatosis (LAM) in adult women; can require lung transplant
• Treatment: everolimus for SEGA, renal angiomyolipoma, LAM, refractory epilepsy. Topical sirolimus for facial angiofibromas. Vigabatrin is first-line for TSC-associated infantile spasms.

Brake loss: PTEN → PHTS (PTEN Hamartoma Tumor Syndrome)

• Inheritance: AD.
• A spectrum of phenotypes united by germline PTEN mutation:
  • Cowden syndrome: adult-onset; macrocephaly, trichilemmomas, oral papillomas, GI hamartomas, cancer predisposition (breast, thyroid, endometrial, RCC, melanoma, GI)
  • Bannayan-Riley-Ruvalcaba syndrome: childhood-onset; macrocephaly, lipomas, hemangiomas, intestinal polyposis, pigmented penile macules, ID
  • PTEN-related autism with macrocephaly: autism spectrum disorder with prominent macrocephaly (head circumference >+2 SD)
  • Proteus-like overgrowth (germline PTEN): distinct from Proteus syndrome (somatic AKT1)
• Hallmark: macrocephaly is the most consistent finding across PTEN-related conditions. Test PTEN in any child with autism + macrocephaly.
• Cancer surveillance (per NCCN): annual breast MRI starting age 30, annual thyroid ultrasound starting at diagnosis, periodic endometrial sampling, colonoscopy, melanoma screening, RCC imaging.

Activating PI3K: PIK3CA-Related Overgrowth Spectrum (PROS)

• Mechanism: somatic activating PIK3CA mutations during embryonic development → mosaic tissue overgrowth in the affected lineage.
• Inheritance: not inherited (de novo somatic mosaic).
• Conditions in PROS:
  • Megalencephaly-capillary malformation (MCAP): large brain, capillary malformations on midline face, polymicrogyria, hemihyperplasia
  • CLOVES syndrome: Congenital Lipomatous Overgrowth, Vascular malformations, Epidermal nevi, Skeletal/spinal anomalies
  • Klippel-Trénaunay-Weber syndrome: capillary-venous-lymphatic malformation with limb overgrowth
  • FAVA (fibroadipose vascular anomaly), macrodactyly, isolated lymphatic anomalies
• Treatment: alpelisib (PI3K inhibitor) shows promise in PROS, FDA-approved for select indications.

Activating AKT: Proteus syndrome

• Mechanism: somatic activating AKT1 mutation; never germline (lethal in utero).
• Phenotype: severe asymmetric overgrowth that progresses postnatally, distorted limbs, cerebriform connective tissue nevi of the soles, lipomatous overgrowth, vascular malformations, lung cysts, lipoma/sarcoma risk.
• The historical Joseph Merrick ("Elephant Man") case is now believed by most authors to be Proteus.

Activating AKT3: Hemimegalencephaly / megalencephaly-polymicrogyria

• Mosaic AKT3 activating mutations → unilateral cortical overgrowth + polymicrogyria → intractable epilepsy in infancy.

Activating mTOR: Smith-Kingsmore syndrome

• Rare; germline activating MTOR mutation. Megalencephaly, ID, facial dysmorphism. Treatable in principle with mTOR inhibitors.

The pattern: germline brake loss → AD inherited disorder; somatic accelerator gain → mosaic/sporadic overgrowth syndrome. Recognizing whether a phenotype suggests germline vs. somatic mosaic mechanism predicts the testing strategy (peripheral blood for germline; affected tissue biopsy for mosaic).

• Macrocephaly + characteristic skin / overgrowth / hamartomas = think pathway disease.
• Suspected TSC: brain MRI, echocardiogram, renal imaging, dilated eye exam (retinal hamartomas), EEG if spells; TSC1/TSC2 sequencing + deletion/duplication.
• Suspected PHTS: PTEN sequencing on peripheral blood is sufficient (germline).
• Suspected PROS / Proteus: peripheral blood testing usually NEGATIVE. Need biopsy of affected tissue (high-coverage NGS to detect mosaic variant at low VAF).
• Cancer surveillance is built into long-term care for TSC, PHTS, and PROS.

This is one of the few pathway-targeted therapy stories in clinical genetics:

• Sirolimus (rapamycin) and everolimus: mTORC1 inhibitors. Active in TSC (SEGA, angiomyolipoma, LAM, facial angiofibromas), some PROS conditions, post-transplant indications.
• Alpelisib: PI3K-α-selective inhibitor; FDA-approved for PIK3CA-related overgrowth in adults.
• Vigabatrin: first-line for infantile spasms in TSC (acts via GABA, not the pathway, but TSC-specific evidence).