AAV-mediated MECP2 gene replacement for Rett syndrome: dosage-sensitivity challenges and implications for c.3G>A (p.Met1Ile)

AAV-mediated MECP2 gene replacement is the leading gene therapy strategy for Rett syndrome, with TSHA-102 (Taysha Gene Therapies) and other programs in clinical development. However, MECP2 is uniquely dosage-sensitive — both loss-of-function (Rett syndrome) and gain-of-function (MECP2 duplication syndrome) cause severe neurological disease — making precise expression control a central challenge. The c.3G>A start-loss variant, which likely abolishes or severely reduces MECP2 translation, represents a complete loss-of-function allele that would benefit from exogenous MECP2 expression, provided overexpression in cells carrying the active wild-type X chromosome can be avoided.

MECP2 is X-linked, and Rett syndrome affects primarily heterozygous females due to random X-inactivation: approximately 50% of neurons express the mutant allele and 50% express wild-type MECP2. The c.3G>A variant disrupts the initiator methionine codon, likely preventing canonical translation initiation. AAV9-MECP2 gene replacement in Mecp2-null mice extended survival and reversed neurological phenotypes (Garg et al., J Neurosci 2013; PMID:23536090). TSHA-102 uses an AAV9 vector with a miniMECP2 transgene under a miR-responsive element (miR-Tight) to restrict expression in cells already expressing endogenous MECP2, attempting to solve the dosage problem. A Phase 1/2 clinical trial (NCT05606614) was initiated. The miRNA-regulated expression system aims to exploit the observation that cells expressing wild-type MECP2 from the active X have different miRNA profiles than cells expressing the mutant allele. Alternative strategies include X-chromosome reactivation to de-silence the wild-type MECP2 on the inactive X, though these remain preclinical.

The fundamental challenge is that MECP2 overexpression is toxic — MECP2 duplication syndrome causes intellectual disability, seizures, and early death. Any gene therapy must achieve expression in MECP2-deficient neurons while avoiding overexpression in the ~50% of neurons already expressing wild-type MECP2. The miR-Tight regulatory approach in TSHA-102 is elegant but unproven in human neurons at scale. Random X-inactivation patterns vary between patients, affecting the proportion of cells needing rescue. BBB penetration by AAV9 is incomplete, especially in older patients. The therapeutic window for Rett syndrome reversal remains debated — while mouse studies showed late-rescue benefit, human neuronal network maturation may limit plasticity. Taysha reported a clinical hold on TSHA-102 in 2023, and the current trial status should be verified. The c.3G>A variant specifically has not been represented in any reported trial cohort.