Rett syndrome: MECP2 biology, multisystem pathophysiology, and the evolving therapeutic landscape.

UNLABELLED: Rett syndrome (RTT) is an X-linked neurodevelopmental disorder arising predominantly from de novo loss-of-function mutations in MECP2, affecting approximately 1 in 10,000 live female births. MECP2 is a dosage-sensitive epigenetic regulator acting through its methyl-CpG-binding (MBD) and transcriptional-repression (TRD) domains; isoform-specific expression and X-chromosome-inactivation mosaicism drive the wide phenotypic variability, with mutation type broadly correlating with severity. Although historically viewed as neuron-centric, RTT is a multisystem disorder: beyond psychomotor regression, stereotypic hand movements, gait apraxia, and loss of spoken language, MECP2 deficiency contributes to brainstem-mediated breathing dysrhythmia, QTc prolongation, enteric dysmotility, low bone density, and mitochondrial/metabolic deficits, amplified by glial-neuronal crosstalk. Preclinical models, MECP2 rodents, patient iPSC-derived neurons, and cerebral organoids, reveal synaptic and dendritic deficits that improve substantially when MECP2 is reactivated in these systems, indicating that MECP2-deficient neurons remain viable rather than degenerating; whether comparable improvement can be achieved in affected individuals is not yet established. The therapeutic landscape now spans multidisciplinary supportive care, the first approved pharmacotherapy trofinetide, AAV-mediated MECP2 gene replacement (now in pivotal trials, with an important gene-therapy safety signal), antisense oligonucleotides for dosage normalisation, and emerging metabolic agents; validated biomarkers and objective outcome measures remain a major unmet need.

CONCLUSION: RTT is a multisystem monogenic disorder whose management is shifting from symptom control toward disease modification. Phenotypic rescue after MECP2 restoration in rodent models provides the rationale for this shift but does not establish that RTT is reversible in patients; progress now depends on validated biomarkers, dose-controlled disease-modifying therapies, long-term outcome data, and equitable global access.

WHAT IS KNOWN: • More than 90-95% of classic Rett syndrome is caused by de novo loss-of-function mutations in MECP2, a dosage-sensitive epigenetic reader whose isoform distribution, residual function and X-chromosome-inactivation mosaicism together account for much of the phenotypic variability. • Reactivating MECP2 in symptomatic adult rodents substantially improves many disease features, establishing that MECP2-deficient neurons are functionally impaired rather than lost and providing the rationale for disease-modifying therapy.

WHAT IS NEW: • This review integrates isoform-specific and non-neuronal (glial, respiratory, cardiac, gastrointestinal, skeletal and metabolic) mechanisms with a literature base updated to 15 August 2026, and identifies the continuing absence of any validated biomarker or surrogate endpoint, rather than target identification, as the principal obstacle to efficient trials. • It critically appraises the post-trofinetide landscape - divergent FDA and EMA assessments of the same trial data, a fatal high-dose AAV hyperinflammatory event, and single-arm registrational gene-therapy programmes reported largely outside peer review - and concludes that the demonstrated outcome to date is gain or re-acquisition of developmental milestones, not reversal of established disease.