NM_001110792.2(MECP2):c.74C>G (p.Ser25Ter)

For MECP2 c.74C>G (p.Ser25Ter), an early nonsense variant abolishing MeCP2 protein expression, AAV9-mediated MECP2 gene replacement with miRNA-based expression regulation represents the most clinically advanced therapeutic approach. NGN-401 (Neurogene) uses a self-regulating AAV9 vector encoding MECP2 with endogenous miRNA target sites to constrain expression within a narrow therapeutic range — critical because both MeCP2 deficiency (Rett syndrome) and overexpression (MECP2 duplication syndrome) cause severe neurological disease. Phase 1/2 data from the initial pediatric cohort have reported unexpected developmental skill gains, supporting proof-of-concept that gene replacement can modify the Rett phenotype even after symptom onset.

Neurogene NGN-401 clinical program (NCT06387563) represents a second-generation AAV9-MECP2 approach with built-in miR-regulated expression cassette to prevent toxic overexpression. Preclinical studies in Mecp2-null mice demonstrate that AAV9-mediated MECP2 redelivery to the CNS extends survival and rescues neurological phenotypes (Gadalla et al., Mol Ther 2017; PMID:28222895). The critical challenge of MECP2 dosage sensitivity — loss causes Rett, duplication causes MECP2 duplication syndrome — has been addressed by miRNA-responsive elements (miR-regulatory cassettes) that attenuate transgene expression in cells with adequate MeCP2 levels. For p.Ser25Ter specifically, this variant creates a stop codon at position 25 of the 498-amino-acid MeCP2 protein, resulting in functional null and complete loss of MeCP2-mediated transcriptional regulation in neurons. ClinVar classifies this variant as Pathogenic. The variant is X-linked, but in females, random X-inactivation creates mosaic expression with ~50% neurons expressing the mutant allele, providing a partial baseline that gene therapy aims to supplement.

MECP2 gene therapy faces the fundamental dosage problem: the therapeutic window between insufficient and toxic MeCP2 levels is narrow. Whether miRNA-regulated cassettes can maintain expression within this window across diverse cell types and brain regions long-term is unproven in humans. AAV9 CNS delivery at high doses carries risks of dorsal root ganglion toxicity and hepatotoxicity observed in other AAV9 programs. X-inactivation mosaicism in female patients means ~50% of neurons already express wild-type MeCP2 — delivering additional MeCP2 to these cells risks overexpression toxicity. The optimal age for intervention is uncertain; whether gene therapy can reverse established neuronal circuit dysfunction rather than merely prevent further deterioration is unknown. Immune responses to AAV9 capsid may limit retreatment. No Phase 3 data exist yet, and long-term transgene expression durability in the human CNS remains to be established.