Lecture 16 Part I: Rett Syndrome

What are the four core clinical features required for diagnosis?

• Stereotypic hand movements.

• Loss of acquired purposeful hand skills.

• Loss of acquired spoken language.

• Gait abnormalities.

What is the hallmark developmental pattern?

• Infants appear completely normal at birth, often described as "too good."

• Regression begins ~6–18 months when neuronal MeCP2 demand is highest.

• RTT is a problem of neuronal maintenance, not formation.

What are the four clinical stages and their key features?

• Stage 1 (6–18 months to 5 yrs): Seizures, microcephaly, abnormal gait, breathing dysregulation, QTc monitoring.

• Stage 2 (5–12 yrs): GI dysmotility, feeding difficulties, urinary retention, anxiety/depression.

• Stage 3 (12–21 yrs): Scoliosis, dystonia, hip/joint issues, self-injury.

• Stage 4 (21+ yrs): Stabilization, social withdrawal; ~70% survive into their 50s.

What does postmortem brain pathology show?

• Brain weight reduced ~12–34%; smaller, more densely packed neurons with fewer branches and synapses.

• RTT is not neurodegenerative: synaptic dysfunction, not cell death, so changes are potentially reversible.

What is the MECP2 gene and where is it located?

• Located on X chromosome (Xq28).

• Has 4 exons; encodes two isoforms (MECP2E1, MECP2E2).

• Key protein domains: MBD (methyl-CpG binding), TRD, NTD, CTD.

What are the pathogenic MECP2 variants?

• Over 300 loss-of-function variants; arise from C to T transitions at methylated CpG sites.

• ~69% missense, ~13% nonsense; also frameshift and splice site mutations.

• Eight recurrent hotspots account for >60% of cases: R106W, R133C, T158M, R168X, R255X, R270X, R294X, R306C.

  • Have genotype-phenotype correlations.

What are the functions of MeCP2 protein?

• Primary (repressor): Binds methylated CpG sites, recruits NCoR/SMRT, closes chromatin, silences genes.

• Secondary (activator): Interacts with RNA Pol II and CREB1 to drive BDNF and IGF1 expression.

• Also regulates miRNA processing and higher-order chromatin architecture.

What happens to MeCP2 functions in Rett Syndrome?

• Repression fails: chromatin stays open, so silenced genes become abnormally expressed.

• Activation fails: BDNF and IGF1 expression is reduced.

• Downstream: synaptic function, metabolism, and neurotransmitter balance all disrupted; MeCP2-deficient astrocytes also indirectly harm dendrites.

What are the multisystem consequences of MeCP2 loss?

• Heart: prolonged QT, major cause of sudden death.

• Respiratory: breathing problems in >90% of individuals.

• GI: constipation nearly universal; ~1/3 need a feeding tube.

• Orthopaedic: progressive scoliosis; ~30% fractures from osteoporosis.

Why does Rett Syndrome primarily affect females, and why are males more severe?

• Females: random X-inactivation creates ~50% normal / ~50% mutant cells, allowing partial compensation; severity depends on skewing.

• Males: one X chromosome means all cells carry the mutation, causing severe brain dysfunction at birth, typically not surviving to adulthood.

Why do symptoms not appear at birth?

• MeCP2 expression is low during early development, so mutant cells can compensate.

• Expression rises sharply as neurons mature, and regression begins when demand peaks (~6–18 months).

• Restoring MeCP2 after birth in mice still improves symptoms, so treatment is possible at any stage.

What is the MECP2 dosage sensitivity "Goldilocks" problem?

• Too little: Rett Syndrome (loss-of-function).

• Too much: MECP2 Duplication Syndrome, a distinct disorder with severe intellectual disability and seizures.

• Precise dosage control is the central challenge in gene replacement therapy.

What are the current treatments for Rett Syndrome?

• Supportive: therapy (PT/OT/speech), seizure management, GI care, orthopaedic monitoring, AAC/eye-gaze technology.

• Trofinetide (Daybue): Only FDA-approved medication (March 2023, ages 2+); synthetic IGF-1 peptide that supports neuronal signaling and reduces neuroinflammation; improves communication, hand function, and social interaction.

What are TSHA-102 and NGN-401?

• Both use AAV9 vectors and entered clinical trials in late 2025.

• TSHA-102: "mini-MECP2" with miRARE to prevent overexpression; delivered into spinal fluid.

• NGN-401: Full-length MECP2 with EXACT system for dosage control; delivered into brain ventricles.

What was the key finding and approach of Lou et al. (2025)?

• Hypothesis: blocking miR106a disrupts XCI, reactivating the healthy MECP2 allele on the inactive X.

• Used PARIS2 (freezes RNA interactions mid-cell to reveal bonding sites) to show miR106a binds Xist at 4 MREs in the RepA region, stabilizing it.

• Blocking miR106a with a synthetic miR106sp sponge destabilizes Xist, reactivates the inactive X, and restores healthy MECP2 expression.

• Key advantage over gene therapy: uses the patient's own allele, so no dosage-control problem.

What were the key results and limitations of Lou et al. (2025)?

• RTT mice: ~32% MECP2 restoration; median survival 29.6 vs. 12.1 weeks (144% increase); improved motor function, breathing, and brain volume.

• Human iPSC neurons: increased MECP2 expression, larger soma, more dendritic branching, more calcium signaling.

• Limitations: affects all cells (not just mutant); some off-target X-linked gene upregulation; not applicable to males; clinical trials ~3–5 years away.