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Genetic neurodevelopmental disorders (NDDs) encompass a heterogeneous group of conditions characterized by impaired cognitive, behavioral, and neurological development resulting from pathogenic variants affecting brain development and synaptic function. Advances in molecular genetics and next-generation sequencing have significantly expanded the understanding of the genetic architecture underlying disorders such as Rett syndrome (RTT), Fragile X syndrome (FXS), Angelman syndrome (AS), and autism spectrum disorders. Beyond these classical neurodevelopmental disorders, spinal muscular atrophy (SMA) is included as a paradigmatic example of successful RNA-based therapeutic translation. Concurrently, RNA-based therapeutics have emerged as promising precision medicine strategies capable of modulating gene expression at the transcriptional and post-transcriptional levels. These approaches include antisense oligonucleotides (ASOs), small interfering RNAs (siRNAs), messenger RNA (mRNA) therapies, RNA editing technologies, and splice-modulating agents. Recent clinical successes, particularly in spinal muscular atrophy, have demonstrated the transformative potential of RNA therapeutics in neurological disease. However, substantial challenges remain, including BBB penetration, long-term safety, immune activation, and genotype-specific variability in therapeutic response. This review summarizes current advances in RNA-based therapeutics for genetic NDDs, highlighting molecular mechanisms, disease-specific therapeutic strategies, translational progress, delivery challenges, and future directions. Overall, continued progress will depend on the integration of disease biology, rational RNA therapeutic design, and effective CNS-targeted delivery, supporting the broader implementation of precision RNA medicine for genetic neurodevelopmental disorders.

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.

BACKGROUND AND AIMS: Rett syndrome (RTT) is a severe neurological disorder caused by pathogenic variants in the MECP2 gene associated with gastrointestinal (GI) motility disorders strongly impacting patients' quality of life. Intestinal motility relies on coordinated neuronal activity in the enteric nervous system (ENS) where MECP2 is highly expressed. Here, we sought to determine whether cholinergic-mediated excitatory neuromuscular transmission could be affected in the ENS of a RTT mouse model and the functional consequences on intestinal motility. METHODS: In this study, 55 days-old wild type (WT) and Mecp2-knock-out (KO) mice were used. After confirming the increase in GI transit time in Mecp2-KO mice in vivo, intestinal mechanical activity was assessed in vitro by evaluating contractile responses of small intestinal muscle to electrical field stimulation (EFS) of the ENS. The function of the cholinergic pathway was evaluated using gene expression, protein, and enzymatic activity quantifications of different factors of the acetylcholine (ACh) metabolism. RESULTS: Our in vivo results confirm that the slowed GI transit previously identified in RTT patients and in the mouse model. In vitro, successive EFS lead to a progressive and rapid decrease in the amplitude of cholinergic contractions in Mecp2-KO mice contrary to WT mice. Levels of ACh are decreased in the ENS of Mecp2-KO mice (-47%, P-value<0.01) due to decreased activity of the synthesizing enzyme choline-acetyltransferase (-39%, P-value = 0.04). In addition to the deficit of ACh, we found that both nicotinic and muscarinic cholinergic receptors are also significantly impacted. CONCLUSION: This is the first report of a major dysfunction of the enteric cholinergic pathway in a mouse model of RTT supporting its direct involvement in GI disorders in this disease. We propose that the rhythmic GI contractions occurring during fed conditions may cause a rapid depletion of the stock of enteric ACh that could largely depress the propulsive efficiency of, or even suppress, peristalsis during digestion.

Rett syndrome (RTT) is a severe neurodevelopmental disorder characterized by multiple neurological impairments, which affects predominantly females. It is associated, in most cases, with pathogenic variants in MECP2, the gene encoding the methyl-CpG-binding protein 2. In the four decades since its description in the English literature, the RTT field has seen major progress in the understanding of its genetics, neurobiology, and clinical evolution. These have led to the first drug approved for core symptoms of a neurodevelopmental disorder, trofinetide, and gene replacement programs at advanced development. These achievements have brought new challenges and highlighted unresolved issues. These include diagnosis before developmental regression and of individuals without core RTT features, including males; the role of MECP2 testing in RTT diagnosis; the boundaries of RTT; instruments needed for assessing clinical evolution in research and practice; the definition of meaningful clinical improvement; and the development, availability, and affordability of new treatments. These topics are reviewed in terms of their implications for RTT and for other genetic neurodevelopmental disorders.