Base Editing (ABE8e) for MECP2 c.3G>A (p.Met1Ile) in Rett syndrome

Base Editing (ABE8e) via AAV9 delivery is a rationale-driven therapeutic strategy for Rett syndrome targeting the MECP2 c.3G>A (p.Met1Ile) variant (Pathogenic, missense variant, initiator_codon_variant, 5 prime UTR variant). The editing system (ABE8e-nSpCas9 (adenine base editor)) converts the pathogenic A back to G on the target strand, restoring the wild-type codon. Target tissue: CNS. Therapeutic goal: Correct loss-of-function MECP2 variants in neurons at the MECP2 locus to restore MeCP2 function and reverse or prevent severe neurodevelopmental impairment in Rett syndrome. Risk profile: off-target Medium (bystander bases in editing window), delivery complexity Medium, immunogenicity High (AAV pre-existing immunity).

1. Molecular basis: MECP2 NM_001110792.2(MECP2):c.3G>A (p.Met1Ile) is classified as Pathogenic (ClinVar variation ID 1207096). Molecular consequence: missense variant, initiator_codon_variant, 5 prime UTR variant. Protein change: M1I. 2. Epidemiology: Rett syndrome is a rare X-linked neurodevelopmental disorder, almost exclusively affecting girls, with prevalence around 1 in 10,000 female births. Onset follows 6–18 months of apparently normal development, after which regression leads to severe intellectual disability and lifelong care needs. 3. Standard of care: Management is symptomatic and supportive: multidisciplinary care with physical, occupational and speech therapy, nutritional support, and seizure control. In 2023, trofinetide (Daybue) became the first FDA-approved drug for Rett syndrome, providing statistically significant but modest improvements i 4. Pipeline: Multiple MECP2-targeted advanced therapies are in development. NGN-401 AAV-based gene therapy has entered pediatric clinical trials with preliminary reports of unexpected skill gains and manageable AAV-related safety findings, suggesting disease modification. Additional approaches include antisense 5. ABE clinical validation: ABE8e (Richter et al. 2020, Nat Biotechnol) achieves ~1.7x higher editing efficiency than ABE7.10. VERVE-101 demonstrated first-in-human LNP-ABE liver editing with 55-66% PCSK9 reduction (Raal et al. 2025, NEJM). Beam Therapeutics is advancing multiple ABE programs.

1. No published data specifically correcting MECP2 c.3G>A (p.Met1Ile) with Base Editing (ABE8e); strategy is based on general principles and must be validated preclinically.
2. PAM availability and bystander base analysis for the specific genomic context have not been performed. If no canonical NGG PAM positions the target within the editing window, PAM-flexible variants (SpRY) may be needed.
3. Delivery to CNS tissue remains a major translational bottleneck. Current vectors have limited transduction efficiency in these compartments.
4. Long-term durability, off-target genome-wide effects, and immunogenicity in the target patient population require thorough preclinical and clinical evaluation.