# Disease-modifying therapies for Rett syndrome: a review for neurologists

## TL;DR

This paper reviews disease-modifying therapies for Rett syndrome, focusing on gene therapy approaches and their implications for neurologists.

## Contribution

The paper provides a detailed review of the most advanced gene replacement therapies for Rett syndrome and highlights translational challenges.

## Key findings

- TSHA-102 and NGN-401 show early efficacy in clinical trials for Rett syndrome.
- Gene therapy for Rett syndrome faces challenges like expression toxicity and X-chromosome inactivation.
- Next-generation strategies include CRISPR-Cas9 and RNA editing for potential mutation correction.

## Abstract

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder affecting approximately 1 in 10,000–15,000 females, most often caused by loss-of-function mutations in MECP2. Until the recent approval of trofinetide, management relied exclusively on symptomatic treatment and multidisciplinary supportive care. The therapeutic landscape is now undergoing a rapid shift, driven by multiple gene therapy approaches designed to restore functional MeCP2 expression and achieve true disease modification. As these therapies progress toward potential regulatory approval, neurologists will play central roles in identifying eligible patients, counseling families, supporting clinical trial enrollment, delivering treatments, monitoring long-term outcomes, and advocating for equitable access. This review provides neurologists with the essential framework needed to understand and navigate this evolving field. We examine in detail the two most advanced gene replacement therapies currently in clinical trials. TSHA-102 uses an intrathecally delivered miniMECP2 transgene regulated by a microRNA-based autoregulatory system, whereas NGN-401 delivers full-length MECP2 via intracerebroventricular administration using a synthetic expression-feedback circuit. Both approaches have shown encouraging early efficacy, with treated children achieving developmental gains that exceed natural history expectations. However, they differ substantially in molecular design, regulatory control, delivery method, and safety considerations. We also highlight challenges unique to RTT gene therapy, including the narrow therapeutic window between insufficient expression and MeCP2 overexpression toxicity, the impact of X-chromosome inactivation mosaicism, and lessons learned from a fatal hyperinflammatory adverse event. Beyond AAV-mediated gene replacement, we review next-generation strategies in preclinical development—CRISPR-Cas9 genome editing for permanent mutation correction, ADAR-based RNA editing, translation readthrough for nonsense variants, and X-chromosome reactivation to restore endogenous MECP2 expression. Finally, we address key translational considerations such as optimal timing of intervention, dosing constraints, outcome measurement in severely impaired populations, long-term safety surveillance, and barriers to broad and equitable access. The RTT gene therapy experience serves as a model for precision medicine in other monogenic neurodevelopmental disorders, illustrating both the transformative promise and the substantial complexities of translating genetic science into meaningful clinical benefit.