Improved survival and reduced phenotypic severity following AAV9/MECP2 gene transfer to neonatal and juvenile male Mecp2 knockout mice

Adeno-associated virus vector delivery to the brain: Technology advancements and clinical applications

Adeno-associated virus (AAV) vectors have emerged as a promising tool in the development of gene therapies for various neurological diseases, including Alzheimer's disease and Parkinson's disease. However, the blood-brain barrier (BBB) poses a significant challenge to successfully delivering AAV vectors to the brain. Strategies that can overcome the BBB to improve the AAV delivery efficiency to the brain are essential to successful brain-targeted gene therapy. This review provides an overview of existing strategies employed for AAV delivery to the brain, including direct intraparenchymal injection, intra-cerebral spinal fluid injection, intranasal delivery, and intravenous injection of BBB-permeable AAVs. Focused ultrasound has emerged as a promising technology for the noninvasive and spatially targeted delivery of AAV administered by intravenous injection. This review also summarizes each strategy's current preclinical and clinical applications in treating neurological diseases. Moreover, this review includes a detailed discussion of the recent advances in the emerging focused ultrasound-mediated AAV delivery. Understanding the state-of-the-art of these gene delivery approaches is critical for future technology development to fulfill the great promise of AAV in neurological disease treatment.

Engineering Toxoplasma gondii secretion systems for intracellular delivery of multiple large therapeutic proteins to neurons

Delivering macromolecules across biological barriers such as the blood-brain barrier limits their application in vivo. Previous work has demonstrated that Toxoplasma gondii, a parasite that naturally travels from the human gut to the central nervous system (CNS), can deliver proteins to host cells. Here we engineered T. gondii's endogenous secretion systems, the rhoptries and dense granules, to deliver multiple large (>100 kDa) therapeutic proteins into neurons via translational fusions to toxofilin and GRA16. We demonstrate delivery in cultured cells, brain organoids and in vivo, and probe protein activity using imaging, pull-down assays, scRNA-seq and fluorescent reporters. We demonstrate robust delivery after intraperitoneal administration in mice and characterize 3D distribution throughout the brain. As proof of concept, we demonstrate GRA16-mediated brain delivery of the MeCP2 protein, a putative therapeutic target for Rett syndrome. By characterizing the potential and current limitations of the system, we aim to guide future improvements that will be required for broader application.

Focused Ultrasound-Mediated Disruption of the Blood-Brain Barrier for AAV9 Delivery in a Mouse Model of Huntington's Disease

Huntington's disease (HD) is a monogenic neurodegenerative disorder caused by a cytosine-adenine-guanine (CAG) trinucleotide repeat expansion in the HTT gene. There are no cures for HD, but the genetic basis of this disorder makes gene therapy a viable approach. Adeno-associated virus (AAV)-miRNA-based therapies have been demonstrated to be effective in lowering HTT mRNA; however, the blood-brain barrier (BBB) poses a significant challenge for gene delivery to the brain. Delivery strategies include direct injections into the central nervous system, which are invasive and can result in poor diffusion of viral particles through the brain parenchyma. Focused ultrasound (FUS) is an alternative approach that can be used to non-invasively deliver AAVs by temporarily disrupting the BBB. Here, we investigate FUS-mediated delivery of a single-stranded AAV9 bearing a cDNA for GFP in 2-month-old wild-type mice and the zQ175 HD mouse model at 2-, 6-, and 12-months. FUS treatment improved AAV9 delivery for all mouse groups. The delivery efficacy was similar for all WT and HD groups, with the exception of the zQ175 12-month cohort, where we observed decreased GFP expression. Astrocytosis did not increase after FUS treatment, even within the zQ175 12-month group exhibiting higher baseline levels of GFAP expression. These findings demonstrate that FUS can be used to non-invasively deliver an AAV9-based gene therapy to targeted brain regions in a mouse model of Huntington's disease.

Preclinical Milestones in MECP2 Gene Transfer for Treating Rett Syndrome

BACKGROUND

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2). After gene transfer in mice, exogenous MeCP2 expression must be regulated to avoid dose-dependent toxicity.

SUMMARY

The preclinical gene therapy literature for treating RTT illustrates a duly diligent progression that begins with proof-of-concept studies and advances toward the development of safer, regulated MECP2 viral genome designs. This design progression was partly achieved through international collaborative studies. In 2023, clinicians administered investigational gene therapies for RTT to patients a decade after the first preclinical gene therapy publications for RTT (clinical trial numbers NCT05606614 and NCT05898620). As clinicians take on a more prominent role in MECP2 gene therapy research, preclinical researchers may continue to test more nuanced hypotheses regarding the safety, efficacy, and mechanism of MECP2 gene transfer.

KEY MESSAGE

This review summarizes the history of preclinical MECP2 gene transfer for treating RTT and acknowledges major contributions among colleagues in the field. The first clinical injections are a shared milestone.

Synthetic dosage-compensating miRNA circuits allow precision gene therapy for Rett syndrome

A longstanding challenge in gene therapy is expressing a dosage-sensitive gene within a tight therapeutic window. For example, loss of MECP2 function causes Rett syndrome, while its duplication causes MECP2 duplication syndrome. Viral gene delivery methods generate variable numbers of gene copies in individual cells, creating a need for gene dosage-invariant expression systems. Here, we introduce a compact miRNA-based, incoherent feed-forward loop circuit that achieves precise control of Mecp2 expression in cells and brains, and improves outcomes in an AAV-based mouse model of Rett syndrome gene therapy. Single molecule analysis of endogenous and ectopic Mecp2 mRNA revealed precise, sustained expression across a broad range of gene dosages. Delivered systemically in a brain-targeting AAV capsid, the circuit strongly suppressed Rett behavioral symptoms for over 24 weeks, outperforming an unregulated gene therapy. These results demonstrate that synthetic miRNA-based regulatory circuits can enable precise in vivo expression to improve the safety and efficacy of gene therapy.

Nuclease-free precise genome editing corrects MECP2 mutations associated with Rett syndrome

Rett syndrome is an acquired progressive neurodevelopmental disorder caused by de novo mutations in the X-linked MECP2 gene which encodes a pleiotropic protein that functions as a global transcriptional regulator and a chromatin modifier. Rett syndrome predominantly affects heterozygous females while affected male hemizygotes rarely survive. Gene therapy of Rett syndrome has proven challenging due to a requirement for stringent regulation of expression with either over- or under-expression being toxic. Ectopic expression of MECP2 in conjunction with regulatory miRNA target sequences has achieved some success, but the durability of this approach remains unknown. Here we evaluated a nuclease-free homologous recombination (HR)-based genome editing strategy to correct mutations in the MECP2 gene. The stem cell-derived AAVHSCs have previously been shown to mediate seamless and precise HR-based genome editing. We tested the ability of HR-based genome editing to correct pathogenic mutations in Exons 3 and 4 of the MECP2 gene and restore the wild type sequence while preserving all native genomic regulatory elements associated with MECP2 expression, thus potentially addressing a significant issue in gene therapy for Rett syndrome. Moreover, since the mutations are edited directly at the level of the genome, the corrections are expected to be durable with progeny cells inheriting the edited gene. The AAVHSC MECP2 editing vector was designed to be fully homologous to the target MECP2 region and to insert a promoterless Venus reporter at the end of Exon 4. Evaluation of AAVHSC editing in a panel of Rett cell lines bearing mutations in Exons 3 and 4 demonstrated successful correction and rescue of expression of the edited MECP2 gene. Sequence analysis of edited Rett cells revealed successful and accurate correction of mutations in both Exons 3 and 4 and permitted mapping of HR crossover events. Successful correction was observed only when the mutations were flanked at both the 5' and 3' ends by crossover events, but not when both crossovers occurred either exclusively upstream or downstream of the mutation. Importantly, we concluded that pathogenic mutations were successfully corrected in every Rett line analyzed, demonstrating the therapeutic potential of HR-based genome editing.

The Efficacy of a Human-Ready miniMECP2 Gene Therapy in a Pre-Clinical Model of Rett Syndrome

Inactivating mutations and the duplication of methyl-CpG binding protein 2 (MeCP2), respectively, mediate Rett syndrome (RTT) and MECP2 duplication syndrome. These disorders underscore the conceptual dose-dependent risk posed by MECP2 gene therapy for mosaic RTT patients. Recently, a miRNA-Responsive Autoregulatory Element (miRARE) mitigated the dose-dependent toxicity posed by self-complementary adeno-associated viral vector serotype 9 (AAV9) miniMECP2 gene therapy (scAAV9/miniMECP2-myc) in mice. Here, we report an efficacy assessment for the human-ready version of this regulated gene therapy (TSHA-102) in male Mecp2-/y knockout (KO) mice after intracerebroventricular (ICV) administration at postnatal day 2 (P2) and after intrathecal (IT) administration at P7, P14 (±immunosuppression), and P28 (±immunosuppression). We also report qPCR studies on KO mice treated at P7-P35; protein analyses in KO mice treated at P38; and a survival safety study in female adult Mecp2-/+ mice. In KO mice, TSHA-102 improved respiration, weight, and survival across multiple doses and treatment ages. TSHA-102 significantly improved the front average stance and swing times relative to the front average stride time after P14 administration of the highest dose for that treatment age. Viral genomic DNA and miniMECP2 mRNA were present in the CNS. MiniMeCP2 protein expression was higher in the KO spinal cord compared to the brain. In female mice, TSHA-102 permitted survivals that were similar to those of vehicle-treated controls. In all, these pivotal data helped to support the regulatory approval to initiate a clinical trial for TSHA-102 in RTT patients (clinical trial identifier number NCT05606614).

Novel MECP2 gene therapy is effective in a multicenter study using two mouse models of Rett syndrome and is safe in non-human primates

The AAV9 gene therapy vector presented in this study is safe in mice and non-human primates and highly efficacious without causing overexpression toxicity, a major challenge for clinical translation of Rett syndrome gene therapy vectors to date. Our team designed a new truncated methyl-CpG-binding protein 2 (MECP2) promoter allowing widespread expression of MECP2 in mice and non-human primates after a single injection into the cerebrospinal fluid without causing overexpression symptoms up to 18 months after injection. Additionally, this new vector is highly efficacious at lower doses compared with previous constructs as demonstrated in extensive efficacy studies performed by two independent laboratories in two different Rett syndrome mouse models carrying either a knockout or one of the most frequent human mutations of Mecp2. Overall, data from this multicenter study highlight the efficacy and safety of this gene therapy construct, making it a promising candidate for first-in-human studies to treat Rett syndrome.

Rett Syndrome: A Tale of Altered Genetics, Synaptic Plasticity, and Neurodevelopmental Dynamics

Rett syndrome (RTT) is a neurodevelopmental disorder that is a leading cause of severe cognitive and physical impairment. RTT typically occurs in females, although rare cases of males with the disease exist. Its genetic cause, symptoms, and clinical progression timeline have also become well-documented since its initial discovery. However, a relatively late diagnosis and lack of an available cure signify that our understanding of the disease is incomplete. Innovative research methods and tools are thereby helping to fill gaps in our knowledge of RTT. Specifically, mouse models of RTT, video analysis, and retrospective parental analysis are well-established tools that provide valuable insights into RTT. Moreover, current and anticipated treatment options are improving the quality of life of the RTT patient population. Collectively, these developments are creating optimistic future perspectives for RTT.

Advanced genetic therapies for the treatment of Rett syndrome: state of the art and future perspectives

Loss and gain of functions mutations in the X-linked MECP2 (methyl-CpG-binding protein 2) gene are responsible for a set of generally severe neurological disorders that can affect both genders. In particular, Mecp2 deficiency is mainly associated with Rett syndrome (RTT) in girls, while duplication of the MECP2 gene leads, mainly in boys, to the MECP2 duplication syndrome (MDS). No cure is currently available for MECP2 related disorders. However, several studies have reported that by re-expressing the wild-type gene is possible to restore defective phenotypes of Mecp2 null animals. This proof of principle endorsed many laboratories to search for novel therapeutic strategies to cure RTT. Besides pharmacological approaches aimed at modulating MeCP2-downstream pathways, genetic targeting of MECP2 or its transcript have been largely proposed. Remarkably, two studies focused on augmentative gene therapy were recently approved for clinical trials. Both use molecular strategies to well-control gene dosage. Notably, the recent development of genome editing technologies has opened an alternative way to specifically target MECP2 without altering its physiological levels. Other attractive approaches exclusively applicable for nonsense mutations are the translational read-through (TR) and t-RNA suppressor therapy. Reactivation of the MECP2 locus on the silent X chromosome represents another valid choice for the disease. In this article, we intend to review the most recent genetic interventions for the treatment of RTT, describing the current state of the art, and the related advantages and concerns. We will also discuss the possible application of other advanced therapies, based on molecular delivery through nanoparticles, already proposed for other neurological disorders but still not tested in RTT.

Therapeutic strategies for autism: targeting three levels of the central dogma of molecular biology

The past decade has yielded much success in the identification of risk genes for Autism Spectrum Disorder (ASD), with many studies implicating loss-of-function (LoF) mutations within these genes. Despite this, no significant clinical advances have been made so far in the development of therapeutics for ASD. Given the role of LoF mutations in ASD etiology, many of the therapeutics in development are designed to rescue the haploinsufficient effect of genes at the transcriptional, translational, and protein levels. This review will discuss the various therapeutic techniques being developed from each level of the central dogma with examples including: CRISPR activation (CRISPRa) and gene replacement at the DNA level, antisense oligonucleotides (ASOs) at the mRNA level, and small-molecule drugs at the protein level, followed by a review of current delivery methods for these therapeutics. Since central nervous system (CNS) penetrance is of utmost importance for ASD therapeutics, it is especially necessary to evaluate delivery methods that have higher efficiency in crossing the blood-brain barrier (BBB).

State-of-the-art therapies for Rett syndrome

Rett syndrome (RTT) is an X-linked neurogenetic disorder caused by mutations of the MECP2 (methyl-CpG-binding protein 2) gene. Over two decades of work established MeCP2 as a protein with pivotal roles in the regulation of the epigenome, neuronal physiology, synaptic maintenance, and behaviour. Given the genetic aetiology of RTT and the proof of concept of its reversal in a mouse model, considerable efforts have been made to design therapeutic approaches to re-express MeCP2. By being at the forefront of the development of innovative gene therapies, research on RTT is of paramount importance for the treatment of monogenic neurological diseases. Here we discuss the recent advances and challenges of promising genetic strategies for the treatment of RTT including gene replacement therapies, gene/RNA editing strategies, and reactivation of the silenced X chromosome. WHAT THIS PAPER ADDS: Recent advances shed light on the promises of gene replacement therapy with new vectors designed to control the levels of MeCP2 expression. New developments in DNA/RNA editing approaches or reactivation of the silenced X chromosome open the possibility to re-express the native MeCP2 locus at endogenous levels. Current strategies still face limitations in transduction efficiency and future work is needed to improve brain delivery.

Epilepsy and Cognitive Impairment in Childhood and Adolescence: A Mini-Review

Managing epilepsy in people with an intellectual disability remains a therapeutic challenge and must take into account additional issues such as diagnostic difficulties and frequent drug resistance. Advances in genomic technologies improved our understanding of epilepsy and raised the possibility to develop patients-tailored treatments acting on the key molecular mechanisms involved in the development of the disease. In addition to conventional antiseizure medications (ASMs), ketogenic diet, hormone therapy and epilepsy surgery play an important role, especially in cases of drugresistance. This review aims to provide a comprehensive overview of the mainfactors influencing cognition in children and adolescents with epilepsy and the main therapeutic options available for the epilepsies associated with intellectual disability.

Gene editing in monogenic autism spectrum disorder: animal models and gene therapies

Autism spectrum disorder (ASD) is a lifelong neurodevelopmental disease, and its diagnosis is dependent on behavioral manifestation, such as impaired reciprocal social interactions, stereotyped repetitive behaviors, as well as restricted interests. However, ASD etiology has eluded researchers to date. In the past decades, based on strong genetic evidence including mutations in a single gene, gene editing technology has become an essential tool for exploring the pathogenetic mechanisms of ASD via constructing genetically modified animal models which validates the casual relationship between genetic risk factors and the development of ASD, thus contributing to developing ideal candidates for gene therapies. The present review discusses the progress in gene editing techniques and genetic research, animal models established by gene editing, as well as gene therapies in ASD. Future research should focus on improving the validity of animal models, and reliable DNA diagnostics and accurate prediction of the functional effects of the mutation will likely be equally crucial for the safe application of gene therapies.

Reversibility and therapeutic development for neurodevelopmental disorders, insights from genetic animal models

Neurodevelopmental Disorders (NDDs) encompass a broad spectrum of conditions resulting from atypical brain development. Over the past decades, we have had the fortune to witness enormous progress in diagnosis, etiology discovery, modeling, and mechanistic understanding of NDDs from both fundamental and clinical research. Here, we review recent neurobiological advances from experimental models of NDDs. We introduce several examples and highlight breakthroughs in reversal studies of phenotypes using genetically engineered models of NDDs. The in-depth understanding of brain pathophysiology underlying NDDs and evaluations of reversibility in animal models paves the foundation for discovering novel treatment options. We discuss how the expanding property of cutting-edge technologies, such as gene editing and AAV-mediated gene delivery, are leveraged in animal models for the therapeutic development of NDDs. We envision opportunities and challenges toward faithful modeling and fruitful clinical translation.

Selective Xi reactivation and alternative methods to restore MECP2 function in Rett syndrome

The human X-chromosome harbors only 4% of our genome but carries over 20% of genes associated with intellectual disability. Given that they inherit only one X-chromosome, males are more frequently affected by X-linked neurodevelopmental genetic disorders than females. However, despite inheriting two X-chromosomes, females can also be affected because X-chromosome inactivation enables only one of two X-chromosomes to be expressed per cell. For Rett syndrome and similar X-linked disorders affecting females, disease-specific treatments have remained elusive. However, a cure may be found within their own cells because every sick cell carries a healthy copy of the affected gene on the inactive X (Xi). Therefore, selective Xi reactivation may be a viable approach that would address the root cause of various X-linked disorders. Here, we discuss Rett syndrome and compare current approaches in the pharmaceutical pipeline to restore MECP2 function. We then focus on Xi reactivation and review available methods, lessons learned, and future directions.

Genetics behind Cerebral Disease with Ocular Comorbidity: Finding Parallels between the Brain and Eye Molecular Pathology

Cerebral visual impairments (CVIs) is an umbrella term that categorizes miscellaneous visual defects with parallel genetic brain disorders. While the manifestations of CVIs are diverse and ambiguous, molecular diagnostics stand out as a powerful approach for understanding pathomechanisms in CVIs. Nevertheless, the characterization of CVI disease cohorts has been fragmented and lacks integration. By revisiting the genome-wide and phenome-wide association studies (GWAS and PheWAS), we clustered a handful of renowned CVIs into five ontology groups, namely ciliopathies (Joubert syndrome, Bardet–Biedl syndrome, Alstrom syndrome), demyelination diseases (multiple sclerosis, Alexander disease, Pelizaeus–Merzbacher disease), transcriptional deregulation diseases (Mowat–Wilson disease, Pitt–Hopkins disease, Rett syndrome, Cockayne syndrome, X-linked alpha-thalassaemia mental retardation), compromised peroxisome disorders (Zellweger spectrum disorder, Refsum disease), and channelopathies (neuromyelitis optica spectrum disorder), and reviewed several mutation hotspots currently found to be associated with the CVIs. Moreover, we discussed the common manifestations in the brain and the eye, and collated animal study findings to discuss plausible gene editing strategies for future CVI correction.

Exploration of group II metabotropic glutamate receptor modulation in mouse models of Rett syndrome and MECP2 Duplication syndrome

Rett syndrome (RTT) and MECP2 Duplication syndrome (MDS) have opposing molecular origins in relation to expression and function of the transcriptional regulator Methyl-CpG-binding protein 2 (MeCP2). Several clinical and preclinical phenotypes, however, are shared between these disorders. Modulation of MeCP2 levels has recently emerged as a potential treatment option for both of these diseases. However, toxicity concerns remain with these approaches. Here, we focus on pharmacologically modulating the group II metabotropic glutamate receptors (mGlu), mGlu₂ and mGlu₃, which are two downstream targets of MeCP2 that are bidirectionally affected in expression in RTT patients and mice (Mecp2^Null/+) versus an MDS mouse model (MECP2^Tg1/o). Mecp2^Null/+ and MECP2^Tg1/o animals also exhibit contrasting phenotypes in trace fear acquisition, a form of temporal associative learning and memory, with trace fear deficiency observed in Mecp2^Null/+ mice and abnormally enhanced trace fear acquisition in MECP2^Tg1/o animals. In Mecp2^Null/+ mice, treatment with the mGlu_2/3 agonist LY379268 reverses the deficit in trace fear acquisition, and mGlu_2/3 antagonism with LY341495 normalizes the abnormal trace fear learning and memory phenotype in MECP2^Tg1/o mice. Altogether, these data highlight the role of group II mGlu receptors in RTT and MDS and demonstrate that both mGlu₂ and mGlu₃ may be potential therapeutic targets for these disorders.

Mechanisms for Cognitive Impairment in Epilepsy: Moving Beyond Seizures

There has been a major emphasis on defining the role of seizures in the causation of cognitive impairments like memory deficits in epilepsy. Here we focus on an alternative hypothesis behind these deficits, emphasizing the mechanisms of information processing underlying healthy cognition characterized as rate, temporal and population coding. We discuss the role of the underlying etiology of epilepsy in altering neural networks thereby leading to both the propensity for seizures and the associated cognitive impairments. In addition, we address potential treatments that can recover the network function in the context of a diseased brain, thereby improving both seizure and cognitive outcomes simultaneously. This review shows the importance of moving beyond seizures and approaching the deficits from a system-level perspective with the guidance of network neuroscience.

Genetic therapies for neurological disorders

In recent years, it has become increasingly apparent that many neurological disorders are underpinned by a genetic aetiology. This has resulted in considerable efforts to develop therapeutic strategies which can treat the disease-causing mutation, either by supplying a functional copy of the mutated gene or editing the genomic sequence. In this review, we will discuss the main genetic strategies which are currently being explored for the treatment of monogenic neurological disorders, as well as some of the challenges they face. In addition, we will address some of the ethical difficulties which may arise.

TAT-MeCP2 protein variants rescue disease phenotypes in human and mouse models of Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder caused by pathogenic variants leading to functional impairment of the MeCP2 protein. Here, we used purified recombinant MeCP2e1 and MeCP2e2 protein variants fused to a TAT protein transduction domain (PTD) to evaluate their transduction ability into RTT patient-derived fibroblasts and the ability to carry out their cellular function. We then assessed their transduction ability and therapeutic effects in a RTT mouse model. In vitro, TAT-MeCP2e2-eGFP reversed the pathological hyperacetylation of histones H3K9 and H4K16, a hallmark of abolition of MeCP2 function. In vivo, intraperitoneal administration of TAT-MeCP2e1 and TAT-MeCP2e2 extended the lifespan of Mecp2^-/y mice by >50%. This was accompanied by rescue of hippocampal CA2 neuron size in animals treated with TAT-MeCP2e1. Taken together, these findings provide a strong indication that recombinant TAT-MeCP2 can reach mouse brains following peripheral injection and can ameliorate the phenotype of RTT mouse models. Thus, our study serves as a first step in the development of a potentially novel RTT therapy.

Chromatin Structure and Dynamics: Focus on Neuronal Differentiation and Pathological Implication

Chromatin structure is an essential regulator of gene expression. Its state of compaction contributes to the regulation of genetic programs, in particular during differentiation. Epigenetic processes, which include post-translational modifications of histones, DNA methylation and implication of non-coding RNA, are powerful regulators of gene expression. Neurogenesis and neuronal differentiation are spatio-temporally regulated events that allow the formation of the central nervous system components. Here, we review the chromatin structure and post-translational histone modifications associated with neuronal differentiation. Studying the impact of histone modifications on neuronal differentiation improves our understanding of the pathophysiological mechanisms of chromatinopathies and opens up new therapeutic avenues. In addition, we will discuss techniques for the analysis of histone modifications on a genome-wide scale and the pathologies associated with the dysregulation of the epigenetic machinery.

AAV-delivered suppressor tRNA overcomes a nonsense mutation in mice

Gene therapy is a potentially curative medicine for many currently untreatable diseases, and recombinant adeno-associated virus (rAAV) is the most successful gene delivery vehicle for in vivo applications^1-3. However, rAAV-based gene therapy suffers from several limitations, such as constrained DNA cargo size and toxicities caused by non-physiological expression of a transgene^4-6. Here we show that rAAV delivery of a suppressor tRNA (rAAV.sup-tRNA) safely and efficiently rescued a genetic disease in a mouse model carrying a nonsense mutation, and effects lasted for more than 6 months after a single treatment. Mechanistically, this was achieved through a synergistic effect of premature stop codon readthrough and inhibition of nonsense-mediated mRNA decay. rAAV.sup-tRNA had a limited effect on global readthrough at normal stop codons and did not perturb endogenous tRNA homeostasis, as determined by ribosome profiling and tRNA sequencing, respectively. By optimizing the AAV capsid and the route of administration, therapeutic efficacy in various target tissues was achieved, including liver, heart, skeletal muscle and brain. This study demonstrates the feasibility of developing a toolbox of AAV-delivered nonsense suppressor tRNAs operating on premature termination codons (AAV-NoSTOP) to rescue pathogenic nonsense mutations and restore gene function under endogenous regulation. As nonsense mutations account for 11% of pathogenic mutations, AAV-NoSTOP can benefit a large number of patients. AAV-NoSTOP obviates the need to deliver a full-length protein-coding gene that may exceed the rAAV packaging limit, elicit adverse immune responses or cause transgene-related toxicities. It therefore represents a valuable addition to gene therapeutics.

Rett Syndrome and MECP2 Duplication Syndrome: Disorders of MeCP2 Dosage

Rett syndrome (RTT) is a neurodevelopmental disorder caused predominantly by loss-of-function mutations in the gene Methyl-CpG-binding protein 2 (MECP2), which encodes the MeCP2 protein. RTT is a MECP2-related disorder, along with MECP2 duplication syndrome (MDS), caused by gain-of-function duplications of MECP2. Nearly two decades of research have advanced our knowledge of MeCP2 function in health and disease. The following review will discuss MeCP2 protein function and its dysregulation in the MECP2-related disorders RTT and MDS. This will include a discussion of the genetic underpinnings of these disorders, specifically how sporadic X-chromosome mutations arise and manifest in specific populations. We will then review current diagnostic guidelines and clinical manifestations of RTT and MDS. Next, we will delve into MeCP2 biology, describing the dual landscapes of methylated DNA and its reader MeCP2 across the neuronal genome as well as the function of MeCP2 as a transcriptional modulator. Following this, we will outline common MECP2 mutations and genotype-phenotype correlations in both diseases, with particular focus on mutations associated with relatively mild disease in RTT. We will also summarize decades of disease modeling and resulting molecular, synaptic, and behavioral phenotypes associated with RTT and MDS. Finally, we list several therapeutics in the development pipeline for RTT and MDS and available evidence of their safety and efficacy.

iPSC toolbox for understanding and repairing disrupted brain circuits in autism

Over the past decade, tremendous progress has been made in defining autism spectrum disorder (ASD) as a disorder of brain connectivity. Indeed, whole-brain imaging studies revealed altered connectivity in the brains of individuals with ASD, and genetic studies identified rare ASD-associated mutations in genes that regulate synaptic development and function. However, it remains unclear how specific mutations alter the development of neuronal connections in different brain regions and whether altered connections can be restored therapeutically. The main challenge is the lack of preclinical models that recapitulate important aspects of human development for studying connectivity. Through recent technological innovations, it is now possible to generate patient- or mutation-specific human neurons or organoids from induced pluripotent stem cells (iPSCs) and to study altered connectivity in vitro or in vivo upon xenotransplantation into an intact rodent brain. Here, we discuss how deficits in neurodevelopmental processes may lead to abnormal brain connectivity and how iPSC-based models can be used to identify abnormal connections and to gain insights into underlying cellular and molecular mechanisms to develop novel therapeutics.

Pharmacological read-through of R294X Mecp2 in a novel mouse model of Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder primarily caused by mutations in Methyl-CpG-binding Protein 2 (MECP2). More than 35% of affected individuals have nonsense mutations in MECP2. For these individuals, nonsense suppression has been suggested as a possible therapeutic approach. To assess the viability of this strategy, we created and characterized a mouse model with the common p.R294X mutation introduced into the endogenous Mecp2 locus (Mecp2R294X). Mecp2R294X mice exhibit phenotypic abnormalities similar to those seen in complete null mouse models; however, these occur at a later time point consistent with the reduced phenotypic severity seen in affected individuals containing this specific mutation. The delayed onset of severe phenotypes is likely due to the presence of truncated MeCP2 in Mecp2R294X mice. Supplying the MECP2 transgene in Mecp2R294X mice rescued phenotypic abnormalities including early death and demonstrated that the presence of truncated MeCP2 in these mice does not interfere with wild-type MeCP2. In vitro treatment of a cell line derived from Mecp2R294X mice with the nonsense suppression agent G418 resulted in full-length MeCP2 protein production, demonstrating feasibility of this therapeutic approach. Intraperitoneal administration of G418 in Mecp2R294X mice was sufficient to elicit full-length MeCP2 protein expression in peripheral tissues. Finally, intracranial ventricular injection of G418 in Mecp2R294X mice induced expression of full-length MeCP2 protein in the mouse brain. These experiments demonstrate that translational read-through drugs are able to suppress the Mecp2 p.R294X mutation in vivo and provide a proof of concept for future preclinical studies of nonsense suppression agents in RTT.

Pre-clinical Investigation of Rett Syndrome Using Human Stem Cell-Based Disease Models

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder, mostly caused by mutations in MECP2. The disorder mainly affects girls and it is associated with severe cognitive and physical disabilities. Modeling RTT in neural and glial cell cultures and brain organoids derived from patient- or mutation-specific human induced pluripotent stem cells (iPSCs) has advanced our understanding of the pathogenesis of RTT, such as disease-causing mechanisms, disease progression, and cellular and molecular pathology enabling the identification of actionable therapeutic targets. Brain organoid models that recapitulate much of the tissue architecture and the complexity of cell types in the developing brain, offer further unprecedented opportunity for elucidating human neural development, without resorting to conventional animal models and the limited resource of human neural tissues. This review focuses on the new knowledge of RTT that has been gleaned from the iPSC-based models as well as limitations of the models and strategies to refine organoid technology in the quest for clinically relevant disease models for RTT and the broader spectrum of neurodevelopmental disorders.

Expanding the MECP2 network using comparative genomics reveals potential therapeutic targets for Rett syndrome

Inactivating mutations in the Methyl-CpG Binding Protein 2 (MECP2) gene are the main cause of Rett syndrome (RTT). Despite extensive research into MECP2 function, no treatments for RTT are currently available. Here, we used an evolutionary genomics approach to construct an unbiased MECP2 gene network, using 1028 eukaryotic genomes to prioritize proteins with strong co-evolutionary signatures with MECP2. Focusing on proteins targeted by FDA-approved drugs led to three promising targets, two of which were previously linked to MECP2 function (IRAK, KEAP1) and one that was not (EPOR). The drugs targeting these three proteins (Pacritinib, DMF, and EPO) were able to rescue different phenotypes of MECP2 inactivation in cultured human neural cell types, and appeared to converge on Nuclear Factor Kappa B (NF-κB) signaling in inflammation. This study highlights the potential of comparative genomics to accelerate drug discovery, and yields potential new avenues for the treatment of RTT.

Investigating Immune Responses to the scAAV9-HEXM Gene Therapy Treatment in Tay-Sachs Disease and Sandhoff Disease Mouse Models

GM2 gangliosidosis disorders are a group of neurodegenerative diseases that result from a functional deficiency of the enzyme β-hexosaminidase A (HexA). HexA consists of an α- and β-subunit; a deficiency in either subunit results in Tay–Sachs Disease (TSD) or Sandhoff Disease (SD), respectively. Viral vector gene transfer is viewed as a potential method of treating these diseases. A recently constructed isoenzyme to HexA, called HexM, has the ability to effectively catabolize GM2 gangliosides in vivo. Previous gene transfer studies have revealed that the scAAV9-HEXM treatment can improve survival in the murine SD model. However, it is speculated that this treatment could elicit an immune response to the carrier capsid and “non-self”-expressed transgene. This study was designed to assess the immunocompetence of TSD and SD mice, and test the immune response to the scAAV9-HEXM gene transfer. HexM vector-treated mice developed a significant anti-HexM T cell response and antibody response. This study confirms that TSD and SD mouse models are immunocompetent, and that gene transfer expression can create an immune response in these mice. These mouse models could be utilized for investigating methods of mitigating immune responses to gene transfer-expressed “non-self” proteins, and potentially improve treatment efficacy.

Profiling beneficial and potential adverse effects of MeCP2 overexpression in a hypomorphic Rett syndrome mouse model

De novo loss-of-function mutations in methyl-CpG-binding protein 2 (MeCP2) lead to the neurodevelopmental disorder Rett syndrome (RTT). Despite promising results from strategies aimed at increasing MeCP2 levels, additional studies exploring how hypomorphic MeCP2 mutations impact the therapeutic window are needed. Here, we investigated the consequences of genetically introducing a wild-type MECP2 transgene in the Mecp2 R133C mouse model of RTT. The MECP2 transgene reversed the majority of RTT-like phenotypes exhibited by male and female Mecp2 R133C mice. However, three core symptom domains were adversely affected in female Mecp2^R133C/+ animals; these phenotypes resemble those observed in disease contexts of excess MeCP2. Parallel control experiments in Mecp2^Null/+ mice linked these adverse effects to the hypomorphic R133C mutation. Collectively, these data provide evidence regarding the safety and efficacy of genetically overexpressing functional MeCP2 in Mecp2 R133C mice and suggest that personalized approaches may warrant consideration for the clinical assessment of MeCP2-targeted therapies.

Safety and efficacy of genetic MECP2 supplementation in the R294X mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disorder caused predominantly by loss-of-function mutations in MECP2, encoding transcriptional modulator methyl-CpG-binding protein 2 (MeCP2). Although no disease-modifying therapies exist at this time, some proposed therapeutic strategies aim to supplement the mutant allele with a wild-type allele producing typical levels of functional MeCP2, such as gene therapy. Because MECP2 is a dosage-sensitive gene, with both loss and gain of function causing disease, these approaches must achieve a narrow therapeutic window to be both safe and effective. While MeCP2 supplementation rescues RTT-like phenotypes in mouse models, the tolerable threshold of MeCP2 is not clear, particularly for partial loss-of-function mutations. We assessed the safety of genetically supplementing full-length human MeCP2 in the context of the R294X allele, a common partial loss-of-function mutation retaining DNA-binding capacity. We assessed the potential for adverse effects from MeCP2 supplementation of a partial loss-of-function mutant and the potential for dominant negative interactions between mutant and full-length MeCP2. In male hemizygous R294X mice, MeCP2 supplementation rescued RTT-like behavioral phenotypes and did not elicit behavioral evidence of excess MeCP2. In female heterozygous R294X mice, RTT-specific phenotypes were similarly rescued. However, MeCP2 supplementation led to evidence of excess MeCP2 activity in a motor coordination assay, suggesting that the underlying motor circuitry is particularly sensitive to MeCP2 dosage in females. These results show that genetic supplementation of full-length MeCP2 is safe in males and largely so females. However, careful consideration of risk for adverse motor effects may be warranted for girls and women with RTT.

Engineered microRNA-based regulatory element permits safe high-dose miniMECP2 gene therapy in Rett mice

MECP2 gene transfer has been shown to extend the survival of Mecp2-/y knockout (KO) mice modeling Rett syndrome (RTT), an X-linked neurodevelopmental disorder. However, controlling deleterious overexpression of MeCP2 remains the critical unmet obstacle towards a safe and effective gene therapy approach for RTT. A recently developed truncated miniMECP2 gene has also been shown to be therapeutic after AAV9-mediated gene transfer in KO neonates. We show that AAV9/miniMECP2 has a similar dose-dependent toxicity profile to that of a published second-generation AAV9/MECP2 vector after treatment in adolescent mice. To overcome that toxicity, we developed a risk-driven viral genome design strategy rooted in high-throughput profiling and genome mining to rationally develop a compact, synthetic miRNA target panel (miR-Responsive Auto-Regulatory Element, "miRARE") to minimize the possibility of miniMECP2 transgene overexpression in the context of RTT gene therapy. The goal of miRARE is to have a built-in inhibitory element responsive to MeCP2 overexpression. The data provided herein show that insertion of miRARE into the miniMECP2 gene expression cassette greatly improved the safety of miniMECP2 gene transfer without compromising efficacy. Importantly, this built-in regulation system does not require any additional exogenous drug application, and no miRNAs are expressed from the transgene cassette. Although broad applications of miRARE have yet to be determined, the design of miRARE suggests a potential use in gene therapy approaches for other dose-sensitive genes.

In Vivo Repair of a Protein Underlying a Neurological Disorder by Programmable RNA Editing

Programmable RNA editing is gaining momentum as an approach to repair mutations, but its efficiency in repairing endogenous mutant RNA in complex tissue is unknown. Here we apply this approach to the brain and successfully repair a guanosine-to-adenosine mutation in methyl CpG binding protein 2 RNA that causes the neurodevelopmental disease Rett syndrome. Repair is mediated by hippocampal injections of juvenile Mecp2^317G>A mice with an adeno-associated virus expressing the hyperactive catalytic domain of adenosine deaminase acting on RNA 2 and Mecp2 guide. After 1 month, 50% of Mecp2 RNA is recoded in three different hippocampal neuronal populations. MeCP2 protein localization to heterochromatin is restored in neurons to 50% of wild-type levels. Whole-transcriptome RNA analysis of one neuronal population indicates that the majority of off-target editing sites exhibit rates of 30% or less. This study demonstrates that programmable RNA editing can be utilized to repair mutations in mouse models of neurological disease.

Sinnamon et al. show that the Mecp2 guide-targeted “editase,” introduced by an adeno-associated virus into the hippocampus of a Rett syndrome mouse model, repairs a large fraction of pathological Mecp2^G>A RNA. MeCP2 function, visualized by chromatin association in neurons, is repaired to similar levels. The results point toward a potential therapy for Rett syndrome.

The Molecular Functions of MeCP2 in Rett Syndrome Pathology

MeCP2 protein, encoded by the MECP2 gene, binds to DNA and affects transcription. Outside of this activity the true range of MeCP2 function is still not entirely clear. As MECP2 gene mutations cause the neurodevelopmental disorder Rett syndrome in 1 in 10,000 female births, much of what is known about the biologic function of MeCP2 comes from studying human cell culture models and rodent models with Mecp2 gene mutations. In this review, the full scope of MeCP2 research available in the NIH Pubmed (https://pubmed.ncbi.nlm.nih.gov/) data base to date is considered. While not all original research can be mentioned due to space limitations, the main aspects of MeCP2 and Rett syndrome research are discussed while highlighting the work of individual researchers and research groups. First, the primary functions of MeCP2 relevant to Rett syndrome are summarized and explored. Second, the conflicting evidence and controversies surrounding emerging aspects of MeCP2 biology are examined. Next, the most obvious gaps in MeCP2 research studies are noted. Finally, the most recent discoveries in MeCP2 and Rett syndrome research are explored with a focus on the potential and pitfalls of novel treatments and therapies.

Gene Transfer Therapy for Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) include a broad spectrum of disorders that disrupt normal brain development. Though some NDDs are caused by acquired insults (i.e., toxic or infectious encephalopathy) or may be cryptogenic, many NDDs are caused by variants in a single gene or groups of genes that disrupt neuronal development or function. In this review, we will focus on those NDDs with a genetic etiology. The exact mechanism, timing, and progression of the molecular pathology are seldom well known; however, the abnormalities in development typically manifest in similar patterns such as delays or regression in motor function, social skills, and language or cognitive abilities. Severity of impairment can vary widely. At present, only symptomatic treatments are available to manage seizures and behavioral problems commonly seen in NDDs. In recent years, there has been a rapid expansion of research into gene therapy using adeno-associated viruses (AAVs). Using AAVs as vectors to replace the non- or dysfunctional gene in vivo is a relatively simple model which has created an unprecedented opportunity for the future of NDD treatment. Advances in this field are of paramount importance as NDDs lead to a massive lifelong burden of disease on the affected individuals and families. In this article, we review the unique advantages and challenges of AAV gene therapies. We then look at potential applications of gene therapy for 3 of the more common NDDs (Rett syndrome, fragile X syndrome, and Angelman syndrome), as well as 2 less common NDDs (SLC13A5 deficiency disorder and SLC6A1-related disorder). We will review the available natural history of each disease and current state of preclinical studies including a discussion on the application of AAV gene therapies for each disease.

Rett Syndrome: A Timely Review From Recognition to Current Clinical Approaches and Clinical Study Updates

Since the discovery of the genetic basis of Rett syndrome in 1999, our understanding has grown considerably both in the scientific and the clinical realms. In the last two decades, we have learned about the far-reaching effects of the aberrant MeCP2 protein, the growing list of involved genetic factors, and the genotype-phenotype clinical expression of common MECP2 mutations. This knowledge has led to several basic science research and clinical trials, focusing specifically on emerging treatments of Rett syndrome. As the pathophysiology behind the disease is better understood, treatments aimed at specific molecular targets will become available for clinicians to improve the life of individuals with Rett syndrome.

Mind the translational gap: using iPS cell models to bridge from genetic discoveries to perturbed pathways and therapeutic targets

Autism spectrum disorder (ASD) comprises a group of neurodevelopmental disorders characterized by impaired social interactions as well as the presentation of restrictive and repetitive behaviors. ASD is highly heritable but genetically heterogenous with both common and rare genetic variants collaborating to predispose individuals to the disorder. In this review, we synthesize recent efforts to develop human induced pluripotent stem cell (iPSC)-derived models of ASD-related phenotypes. We firstly address concerns regarding the relevance and validity of available neuronal iPSC-derived models. We then critically evaluate the robustness of various differentiation and cell culture protocols used for producing cell types of relevance to ASD. By exploring iPSC models of ASD reported thus far, we examine to what extent cellular and neuronal phenotypes with potential relevance to ASD can be linked to genetic variants found to underlie it. Lastly, we outline promising strategies by which iPSC technology can both enhance the power of genetic studies to identify ASD risk factors and nominate pathways that are disrupted across groups of ASD patients that might serve as common points for therapeutic intervention.

Severe offtarget effects following intravenous delivery of AAV9-MECP2 in a female mouse model of Rett syndrome

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder that is primarily caused by mutations in the methyl CpG binding protein 2 gene (MECP2). RTT is the second most prevalent genetic cause of intellectual disability in girls, and there is currently no cure for the disease. We have previously shown that gene therapy using a self-complementary AAV9 viral vector expressing a codon-optimized Mecp2 version (AAV9-MCO) significantly improved symptoms and increased survival in male Mecp2-deficient mice. Here, we pursued our studies and investigated the safety and efficacy of long-term gene therapy in the genetically relevant RTT mouse model: the heterozygous (HET) Mecp2 deficient female mouse. These mice were injected with the AAV9-MCO vector through the tail vein and an array of behavioral tests was performed. At 16- and 30-weeks post-injection, this treatment was able to rescue apneas and improved the spontaneous locomotor deficits and circadian locomotor activity in Mecp2 HET mice treated with AAV9-MCO at a dose of 5 × 10¹¹ vg/mouse. To examine whether a higher dose of vector could result in increased improvements, we injected Mecp2 HET mice with a higher MCO vector dose (10¹² vg/mouse), which resulted in some severe, sometimes lethal, side effects. In order to confirm these effects, a new cohort of Mecp2 HET mice were administered increasing doses of MCO vector (10¹¹, 5 × 10¹¹ and 10¹² vg/mouse). Again, two weeks after vector administration, some Mecp2 HET mice were found dead while others displayed severe side effects and had to be euthanized. These deleterious effects were not observed in Mecp2 HET mice injected with a high dose of AAV9-GFP and were directly proportionate to vector dosage (0, 23 or 54% mortality at an AAV9-MCO dose of 10¹¹, 5 × 10¹¹, 10¹² vg/mouse, respectively), and no such lethality was observed in wild-type (WT) mice. In the Mecp2 HET mice treated with the high and medium AAV9-MCO doses, blood chemistry analysis and post-mortem histology showed liver damage with drastically elevated levels of liver transaminases and disorganized liver architecture. Apoptosis was confirmed by the presence of TUNEL- and cleaved-caspase 3-positive cells in the Mecp2 HET mice treated with the higher doses of AAV9-MCO. We then studied the involvement of the unfolded protein response (UPR) in triggering apoptosis since it can be activated by AAV vectors. Increased expression of the C/EBP homologous protein (CHOP), one of UPR downstream effectors, was confirmed in Mecp2 HET mice after vector administration. The toxic reaction seen in some treated mice indicates that, although gene therapy for RTT improved breathing deficits observed in Mecp2 HET mice, further studies are needed to better understand the underlying mechanisms and caution must be exercised before similar attempts are undertaken in female Rett patients.

Drug Studies on Rett Syndrome: From Bench to Bedside

Drug studies on Rett syndrome (RTT) have drastically increased over the past few decades. This review aims to provide master data on bench-to-bedside drug studies involving RTT. A comprehensive literature review was performed by searching in PUBMED, MEDLINE and Google Scholar, international, national and regional clinical trial registries and pharmaceutical companies using the keywords "Rett syndrome treatment and/or drug or compound or molecule". Seventy drugs were investigated in non-clinical (N = 65 animal/cell line-based studies; N = 5 iPSC-based study) and clinical trials (N = 34) for ameliorating the symptoms of RTT. Though there is good progress in both clinical and non-clinical studies, none of these drugs entered phase III/IV for being launched as a therapeutic agent for RTT.

Emerging Insights into the Distinctive Neuronal Methylome

The genomes of mammalian neurons are enriched for unique forms of DNA methylation, including exceptionally high levels of non-CG methylation. Here, we review recent studies defining how non-CG methylation accumulates in neurons and is read out by the critical regulator of neuronal transcription, MeCP2. We discuss the role of gene expression and genome architecture in establishing non-CG methylation and highlight emerging mechanistic insights into how non-CG methylation and MeCP2 control transcription. Further, we describe the cell type-specific functions of this methylation and explore growing evidence that disruption of this regulatory pathway contributes to neurodevelopmental disorders. These findings uncover how the distinctive epigenome in neurons facilitates the development and function of the complex mammalian brain.

Recent advances in gene therapy for neurodevelopmental disorders with epilepsy

Neurodevelopmental disorders can be caused by mutations in neuronal genes fundamental to brain development. These disorders have severe symptoms ranging from intellectually disability, social and cognitive impairments, and a subset are strongly linked with epilepsy. In this review, we focus on those neurodevelopmental disorders that are frequently characterized by the presence of epilepsy (NDD + E). We loosely group the genes linked to NDD + E with different neuronal functions: transcriptional regulation, intrinsic excitability and synaptic transmission. All these genes have in common a pivotal role in defining the brain architecture and function during early development, and when their function is altered, symptoms can present in the first stages of human life. The relationship with epilepsy is complex. In some NDD + E, epilepsy is a comorbidity and in others seizures appear to be the main cause of the pathology, suggesting that either structural changes (NDD) or neuronal communication (E) can lead to these disorders. Furthermore, grouping the genes that cause NDD + E, we review the uses and limitations of current models of the different disorders, and how different gene therapy strategies are being developed to treat them. We highlight where gene replacement may not be a treatment option, and where innovative therapeutic tools, such as CRISPR‐based gene editing, and new avenues of delivery are required. In general this group of genetically defined disorders, supported increasing knowledge of the mechanisms leading to neurological dysfunction serve as an excellent collection for illustrating the translational potential of gene therapy, including newly emerging tools.

AAV-mediated FOXG1 gene editing in human Rett primary cells

Variations in the Forkhead Box G1 (FOXG1) gene cause FOXG1 syndrome spectrum, including the congenital variant of Rett syndrome, characterized by early onset of regression, Rett-like and jerky movements, and cortical visual impairment. Due to the largely unknown pathophysiological mechanisms downstream the impairment of this transcriptional regulator, a specific treatment is not yet available. Since both haploinsufficiency and hyper-expression of FOXG1 cause diseases in humans, we reasoned that adding a gene under nonnative regulatory sequences would be a risky strategy as opposed to a genome editing approach where the mutated gene is reversed into wild-type. Here, we demonstrate that an adeno-associated viruses (AAVs)-coupled CRISPR/Cas9 system is able to target and correct FOXG1 variants in patient-derived fibroblasts, induced Pluripotent Stem Cells (iPSCs) and iPSC-derived neurons. Variant-specific single-guide RNAs (sgRNAs) and donor DNAs have been selected and cloned together with a mCherry/EGFP reporter system. Specific sgRNA recognition sequences were inserted upstream and downstream Cas9 CDS to allow self-cleavage and inactivation. We demonstrated that AAV serotypes vary in transduction efficiency depending on the target cell type, the best being AAV9 in fibroblasts and iPSC-derived neurons, and AAV2 in iPSCs. Next-generation sequencing (NGS) of mCherry⁺/EGFP⁺ transfected cells demonstrated that the mutated alleles were repaired with high efficiency (20–35% reversion) and precision both in terms of allelic discrimination and off-target activity. The genome editing strategy tested in this study has proven to precisely repair FOXG1 and delivery through an AAV9-based system represents a step forward toward the development of a therapy for Rett syndrome.

Intellectual and Developmental Disabilities Research Centers: A Multidisciplinary Approach to Understand the Pathogenesis of Methyl-CpG Binding Protein 2-related Disorders

Disruptions in the gene encoding methyl-CpG binding protein 2 (MECP2) underlie complex neurodevelopmental disorders including Rett Syndrome (RTT), MECP2 duplication disorder, intellectual disabilities, and autism. Significant progress has been made on the molecular and cellular basis of MECP2-related disorders providing a new framework for understanding how altered epigenetic landscape can derail the formation and refinement of neuronal circuits in early postnatal life and proper neurological function. This review will summarize selected major findings from the past years and particularly highlight the integrated and multidisciplinary work done at eight NIH-funded Intellectual and Developmental Disabilities Research Centers (IDDRC) across the US. Finally, we will outline a path forward with identification of reliable biomarkers and outcome measures, longitudinal preclinical and clinical studies, reproducibility of results across centers as a synergistic effort to decode and treat the pathogenesis of the complex MeCP2 disorders.

Whole brain delivery of an instability-prone Mecp2 transgene improves behavioral and molecular pathological defects in mouse models of Rett syndrome

Rett syndrome is an incurable neurodevelopmental disorder caused by mutations in the gene encoding for methyl-CpG binding-protein 2 (MeCP2). Gene therapy for this disease presents inherent hurdles since MECP2 is expressed throughout the brain and its duplication leads to severe neurological conditions as well. Herein, we use the AAV-PHP.eB to deliver an instability-prone Mecp2 (iMecp2) transgene cassette which, increasing RNA destabilization and inefficient protein translation of the viral Mecp2 transgene, limits supraphysiological Mecp2 protein levels. Intravenous injections of the PHP.eB-iMecp2 virus in symptomatic Mecp2 mutant mice significantly improved locomotor activity, lifespan and gene expression normalization. Remarkably, PHP.eB-iMecp2 administration was well tolerated in female Mecp2 mutant or in wild-type animals. In contrast, we observed a strong immune response to the transgene in treated male Mecp2 mutant mice that was overcome by immunosuppression. Overall, PHP.eB-mediated delivery of iMecp2 provided widespread and efficient gene transfer maintaining physiological Mecp2 protein levels in the brain.

AAV-PHP.B Administration Results in a Differential Pattern of CNS Biodistribution in Non-human Primates Compared with Mice

The ability of recombinant adeno-associated virus (AAV) to deliver transgenes to the CNS has allowed for several advancements in the field of gene therapy to treat brain disorders. Although most AAVs do not readily cross the blood-brain barrier and transduce the CNS following peripheral administration, AAV-PHP.B has recently been shown to transduce brains of mice with higher efficiency compared with its parent serotype, AAV9, following injection into the retro-orbital sinus. Here, we extended this foundational work by comparing AAV-PHP.B transduction efficiency in wild-type C57BL/6J mice using four clinically applicable delivery strategies including two intravascular (intra-jugular vein and intra-carotid artery) and two intra-cerebral spinal fluid (CSF) routes (intra-cisterna magna and intra-lateral ventricle). We scaled up these comparisons in a larger-animal model and evaluated transduction efficiency of AAV-PHP.B in the rhesus macaque. We found widespread and largely equal CNS transduction in mice following all four injection strategies, whereas we observed a differential pattern of transduction in macaques with broad cortical and spinal cord transduction seen after intrathecal administration and only very low transduction following intravascular administration. Taken together, these results suggest that AAV-PHP.B may be a useful gene therapy vector for neurological disorders, particularly those stemming from broad cortical or spinal cord neuropathology.

Efficient and Precise CRISPR/Cas9-Mediated MECP2 Modifications in Human-Induced Pluripotent Stem Cells

Patients with Rett syndrome (RTT) have severe mental and physical disabilities. The majority of RTT patients carry a heterozygous mutation in methyl-CpG binding protein 2 (MECP2), an X-linked gene encoding an epigenetic factor crucial for normal nerve cell function. No curative therapy for RTT syndrome exists, and cellular mechanisms are incompletely understood. Here, we developed a CRISPR/Cas9-mediated system that targets and corrects the disease relevant regions of the MECP2 exon 4 coding sequence. We achieved homologous recombination (HR) efficiencies of 20% to 30% in human cell lines and iPSCs. Furthermore, we successfully introduced a MECP2^R270X mutation into the MECP2 gene in human induced pluripotent stem cells (iPSCs). Consequently, using CRISPR/Cas9, we were able to repair such mutations with high efficiency in human mutant iPSCs. In summary, we provide a new strategy for MECP2 gene targeting that can be potentially translated into gene therapy or for iPSCs-based disease modeling of RTT syndrome.

Cannabidivarin completely rescues cognitive deficits and delays neurological and motor defects in male Mecp2 mutant mice

BACKGROUND

Recent evidence suggests that 2-week treatment with the non-psychotomimetic cannabinoid cannabidivarin (CBDV) could be beneficial towards neurological and social deficits in early symptomatic Mecp2 mutant mice, a model of Rett syndrome (RTT).

AIM

The aim of this study was to provide further insights into the efficacy of CBDV in Mecp2-null mice using a lifelong treatment schedule (from 4 to 9 weeks of age) to evaluate its effect on recognition memory and neurological defects in both early and advanced stages of the phenotype progression.

METHODS

CBDV 0.2, 2, 20 and 200 mg/kg/day was administered to Mecp2-null mice from 4 to 9 weeks of age. Cognitive and neurological defects were monitored during the whole treatment schedule. Biochemical analyses were carried out in brain lysates from 9-week-old wild-type and knockout mice to evaluate brain-derived neurotrophic factor (BDNF) and insulin-like growth factor-1 (IGF-1) levels as well as components of the endocannabinoid system.

RESULTS

CBDV rescues recognition memory deficits in Mecp2 mutant mice and delays the appearance of neurological defects. At the biochemical level, it normalizes BDNF/IGF1 levels and the defective PI3K/AKT/mTOR pathway in Mecp2 mutant mice at an advanced stage of the disease. Mecp2 deletion upregulates CB1 and CB2 receptor levels in the brain and these changes are restored after CBDV treatment.

CONCLUSIONS

CBDV administration exerts an enduring rescue of memory deficits in Mecp2 mutant mice, an effect that is associated with the normalization of BDNF, IGF-1 and rpS6 phosphorylation levels as well as CB1 and CB2 receptor expression. CBDV delays neurological defects but this effect is only transient.

Treating Rett syndrome: from mouse models to human therapies

Rare diseases are very difficult to study mechanistically and to develop therapies for because of the scarcity of patients. Here, the rare neuro-metabolic disorder Rett syndrome (RTT) is discussed as a prototype for precision medicine, demonstrating how mouse models have led to an understanding of the development of symptoms. RTT is caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2). Mecp2-mutant mice are being used in preclinical studies that target the MECP2 gene directly, or its downstream pathways. Importantly, this work may improve the health of RTT patients. Clinical presentation may vary widely among individuals based on their mutation, but also because of the degree of X chromosome inactivation and the presence of modifier genes. Because it is a complex disorder involving many organ systems, it is likely that recovery of RTT patients will involve a combination of treatments. Precision medicine is warranted to provide the best efficacy to individually treat RTT patients.

Intrathecal Adeno-Associated Viral Vector-Mediated Gene Delivery for Adrenomyeloneuropathy

Mutations in the gene encoding the peroxisomal ATP-binding cassette transporter (ABCD1) cause elevations in very long-chain fatty acids (VLCFAs) and the neurodegenerative disease adrenoleukodystrophy (ALD). In most adults, this manifests as the spinal cord axonopathy adrenomyeloneuropathy (AMN). A challenge in virus-based gene therapy in AMN is how to achieve functional gene correction to the entire spinal cord while minimizing leakage into the systemic circulation, which could contribute to toxicity. In the present study, we used an osmotic pump to deliver adeno-associated viral (AAV) vector into the lumbar cerebrospinal fluid space in mice. We report that slow intrathecal delivery of recombinant AAV serotype 9 (rAAV9) achieves efficient gene transfer across the spinal cord and dorsal root ganglia as demonstrated with two different transgenes, GFP and ABCD1. In the Abcd1^-/- mouse, gene correction after continuous rAAV9-CBA-hABCD1 delivery led to a 20% decrease in VLCFA levels in spinal cord compared with controls. The major cell types transduced were astrocytes, vascular endothelial cells, and neurons. Importantly, rAAV9 delivered intrathecally by osmotic pump, in contrast to bolus injection, reduced systemic leakage into peripheral organs, particularly liver and heart tissue.