The epigenetic modification of DNA methylation in neurological diseases.

Methylation, a key epigenetic modification, is essential for regulating gene expression and protein function without altering the DNA sequence, contributing to various biological processes, including gene transcription, embryonic development, and cellular functions. Methylation encompasses DNA methylation, RNA methylation and histone modification. Recent research indicates that DNA methylation is vital for establishing and maintaining normal brain functions by modulating the high-order structure of DNA. Alterations in the patterns of DNA methylation can exert significant impacts on both gene expression and cellular function, playing a role in the development of numerous diseases, such as neurological disorders, cardiovascular diseases as well as cancer. Our current understanding of the etiology of neurological diseases emphasizes a multifaceted process that includes neurodegenerative, neuroinflammatory, and neurovascular events. Epigenetic modifications, especially DNA methylation, are fundamental in the control of gene expression and are critical in the onset and progression of neurological disorders. Furthermore, we comprehensively overview the role and mechanism of DNA methylation in in various biological processes and gene regulation in neurological diseases. Understanding the mechanisms and dynamics of DNA methylation in neural development can provide valuable insights into human biology and potentially lead to novel therapies for various neurological diseases.

Comprehensive High-Depth Proteomic Analysis of Plasma Extracellular Vesicles Containing Preparations in Rett Syndrome.

Backgroud: Rett syndrome is a neurodevelopmental disorder that affects 1 in 10,000 females. Various treatments have been explored; however, no effective treatments have been reported to date, except for trofinetide, a synthetic analog of glycine-proline-glutamic acid, which was approved by the FDA in 2023. Serological biomarkers that correlate with the disease status of RTT are needed to promote early diagnosis and to develop novel agents. Methods: In this study, we performed a high-depth proteomic analysis of extracellular vesicles containing preparations extracted from patient plasma samples to identify novel biomarkers. Results: We identified 33 upregulated and 17 downregulated candidate proteins among a total of 4273 proteins in RTT compared to the healthy controls. Among these, UBE3B was predominantly increased in patients with Rett syndrome and exhibited a strong correlation with the clinical severity score, indicating the severity of the disease. Conclusions: We demonstrated that the proteomics of high-depth extracellular vesicles containing preparations in rare diseases could be valuable in identifying new disease biomarkers and understanding their pathophysiology.

Associations between genotype, phenotype and behaviours measured by the Rett syndrome behaviour questionnaire in Rett syndrome.

INTRODUCTION: Rett syndrome (RTT) is a rare neurodevelopmental disorder with developmental impairments, comorbidities, and abnormal behaviours such as hand stereotypies and emotional features. The Rett Syndrome Behaviour Questionnaire (RSBQ) was developed to describe the behavioural and emotional features of RTT. Little is known how RSBQ scores are associated with genetic and clinical characteristics in RTT. This study investigated relationships between genotype, age, walking, hand function, sleep, and RSBQ total and subscale scores in RTT. METHODS: This is a cross-sectional analysis of data collected in the Australian Rett Syndrome Database and the International Rett Syndrome Phenotype Database. Parent caregivers completed the RSBQ and Sleep Disturbance Scale for Children [subscales for disorders of initiating and maintaining sleep (DIMS), disorders of excessive somnolence (DOES)], and provided information on age, variant type, functional abilities (mobility, hand function), seizure frequency and gastrointestinal problems. Associations between the RSBQ scores and the independent variables were modelled using linear regression. RESULTS: Data were available for 365 individuals with RTT [median (range) age 17.8 (2.9-51.9) years, 2 males]. Compared to adults, 2- to 12-year-old children had higher mean Total, Night-time Behaviour and Fear/Anxiety scores. Compared to individuals with a C-terminal deletion, individuals with the p.Arg255* variant had higher mean Total and Night-time Behaviours scores, whereas the p.Arg294* variant had higher mean Mood scores. Individuals with intermediate mobility and hand function abilities had a higher mean Total score. Total RSBQ and subscale scores were similar across categories for seizures, constipation, and reflux, but were higher with abnormal DIMS and abnormal DOES scores. CONCLUSION: Except for associations with sleep, the RSBQ measures the behavioural phenotype rather than clinical severity in RTT, as traditionally conceptualised in terms of functional abilities and comorbidities. When designing clinical trials, the RSBQ needs to be complemented by other outcome measures to assess specific core functions and associated comorbidities in RTT.

Unraveling autophagic imbalances and therapeutic insights in Mecp2-deficient models.

Loss-of-function mutations in MECP2 are associated to Rett syndrome (RTT), a severe neurodevelopmental disease. Mainly working as a transcriptional regulator, MeCP2 absence leads to gene expression perturbations resulting in deficits of synaptic function and neuronal activity. In addition, RTT patients and mouse models suffer from a complex metabolic syndrome, suggesting that related cellular pathways might contribute to neuropathogenesis. Along this line, autophagy is critical in sustaining developing neuron homeostasis by breaking down dysfunctional proteins, lipids, and organelles.Here, we investigated the autophagic pathway in RTT and found reduced content of autophagic vacuoles in Mecp2 knock-out neurons. This correlates with defective lipidation of LC3B, probably caused by a deficiency of the autophagic membrane lipid phosphatidylethanolamine. The administration of the autophagy inducer trehalose recovers LC3B lipidation, autophagosomes content in knock-out neurons, and ameliorates their morphology, neuronal activity and synaptic ultrastructure. Moreover, we provide evidence for attenuation of motor and exploratory impairment in Mecp2 knock-out mice upon trehalose administration. Overall, our findings open new perspectives for neurodevelopmental disorders therapies based on the concept of autophagy modulation.

Site-Blocking Antisense Oligonucleotides as a Mechanism to Fine-Tune MECP2 Expression.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by loss-of-function mutations in the methyl CpG binding protein 2 (MECP2) gene. Despite its severe phenotype, studies in mouse models suggest that restoring MeCP2 levels can reverse RTT symptomology. Nevertheless, traditional gene therapy approaches are hindered by MECP2's narrow therapeutic window, complicating the safe delivery of viral constructs without overshooting the threshold for toxicity. The 3' untranslated region (3'UTR) plays a key role in gene regulation, where factors like miRNAs bind to pre-mRNA and fine-tune expression. Given that each miRNA's contribution is modest, blocking miRNA binding may represent a potential therapeutic strategy for diseases with high dosage sensitivity, like RTT. Here, we present a series of site-blocking antisense oligonucleotides (sbASOs) designed to outcompete repressive miRNA binding at the MECP2 3'UTR. This strategy aims to increase MECP2 levels in patients with missense or late-truncating mutations, where the hypomorphic nature of the protein can be offset by increased abundance. Our results demonstrate that sbASOs can elevate MECP2 levels in a dose-dependent manner in SH-SY5Y and patient fibroblast cell lines, plateauing at levels projected to be safe. Confirming in vivo functionality, sbASO administration in wild-type mice led to significant MeCP2 upregulation and the emergence of phenotypes associated with MeCP2 overexpression. In a T158M neural stem cell model of RTT, sbASO treatment significantly increased MECP2 expression and levels of the downstream effector protein, brain-derived neurotrophic factor (BDNF). These findings highlight the potential of sbASO-based therapies for MECP2-related disorders and advocate for their continued development.

Prevalence of orthopaedic conditions in Rett syndrome: a systematic review and meta-analysis.

BACKGROUND: Rett syndrome (RTT), a developmental disorder primarily affecting girls and linked to methyl-CpG binding protein-2 (MECP2) gene mutations, presents musculoskeletal abnormalities with varying prevalence across studies and age groups. Our aim was to delineate the prevalence of orthopaedic conditions in individuals with RTT. METHOD: Three databases were searched and independently screened by two reviewers to retrieve observational studies published after 2000 that recruited 10 or more patients diagnosed with RTT and reported the prevalence of any orthopaedic conditions (scoliosis, hip displacement, knee problems or foot deformities). A random-effects meta-analysis was performed to determine the pooled prevalence based on study weight. RESULTS: Of 867 screened studies, 21 studies involving 9997 girls with RTT (mean age 14.1 years; range, 3-38.5) met the inclusion criteria. The pooled prevalence of scoliosis was 64.5% [95% confidence interval (CI) 55.4-73.6%; I2 = 99%; P < 0.01], of hip displacement was 29.6% (95% CI 8.9-50.2%; I2 = 97%; P < 0.01) and of foot deformities was 53% (95% CI 17.5-89.2%; I2 = 98%; P < 0.01). Knee problems were reported in only one study. Scoliosis prevalence increased in studies with a high percentage of genetic testing and MECP2 positivity [69.1% (95% CI 58.9-79.2%; I2 = 99%; P < 0.01)], those with a mean age over 13 years [73% (95% CI 59.1-87%; I2 = 100%; P < 0.01)], and studies combining both variables [80.13% (95% CI 70.8-89.4%; I2 = 81%; P < 0.01)]. CONCLUSIONS: This meta-analysis found that approximately two in three girls with RTT develop scoliosis, one in two exhibit foot deformities and one in three experience hip displacement. These findings enhance our understanding of the prevalence of orthopaedic conditions in RTT, which can guide the establishment of surveillance protocols, clinical guidelines and management strategies tailored to the needs of RTT patients.

Pharmacological inhibition of the CB1 cannabinoid receptor restores abnormal brain mitochondrial CB1 receptor expression and rescues bioenergetic and cognitive defects in a female mouse model of Rett syndrome.

BACKGROUND: Defective mitochondria and aberrant brain mitochondrial bioenergetics are consistent features in syndromic intellectual disability disorders, such as Rett syndrome (RTT), a rare neurologic disorder that severely affects mainly females carrying mutations in the X-linked MECP2 gene. A pool of CB1 cannabinoid receptors (CB1R), the primary receptor subtype of the endocannabinoid system in the brain, is located on brain mitochondrial membranes (mtCB1R), where it can locally regulate energy production, synaptic transmission and memory abilities through the inhibition of the intra-mitochondrial protein kinase A (mtPKA). In the present study, we asked whether an overactive mtCB1R-mtPKA signaling might underlie the brain mitochondrial alterations in RTT and whether its modulation by systemic administration of the CB1R inverse agonist rimonabant might improve bioenergetics and cognitive defects in mice modeling RTT. METHODS: Rimonabant (0.3 mg/kg/day, intraperitoneal injections) was administered daily to symptomatic female mice carrying a truncating mutation of the Mecp2 gene and its effects on brain mitochondria functionality, systemic oxidative status, and memory function were assessed. RESULTS: mtCB1R is overexpressed in the RTT mouse brain. Subchronic treatment with rimonabant normalizes mtCB1R expression in RTT mouse brains, boosts mtPKA signaling, and restores the defective brain mitochondrial bioenergetics, abnormal peripheral redox homeostasis, and impaired cognitive abilities in RTT mice. LIMITATIONS: The lack of selectivity of the rimonabant treatment towards mtCB1R does not allow us to exclude that the beneficial effects exerted by the treatment in the RTT mouse model may be ascribed more broadly to the modulation of CB1R activity and distribution among intracellular compartments, rather than to a selective effect on mtCB1R-mediated signaling. The low sample size of few experiments is a further limitation that has been addressed replicating the main findings under different experimental conditions. CONCLUSIONS: The present data identify mtCB1R overexpression as a novel molecular alteration in the RTT mouse brain that may underlie defective brain mitochondrial bioenergetics and cognitive dysfunction.

Exploring the Clinical Utility of Targeted MECP2 Testing in Real-World Practice.

BACKGROUND: This study aimed to explore the clinical utility of targeted MECP2 testing in a large cohort of females with neurodevelopmental delays. Our aim was to identify suitable candidates for testing based on prevailing diagnostic criteria. METHODS: Eligible participants with global developmental delay/arrest or regression before age 36 months underwent MECP2 testing. MECP2-positive patients were further categorized based on Rett syndrome (RTT) diagnostic criteria, including typical, atypical, possible, and unclassified, to assess disease typicality and progression with respect to age. RESULTS: Of the 683 patients, 162 (23.7%) were diagnosed with MECP2-related RTT. Global developmental delay was the predominant initial symptom in approximately 75% of the cohort with developmental arrest/regression at testing. Symptoms emerged before age six months in 14 patients (8.6%). The average age at the time of MECP2 testing was 3.7 years, with 31.5% of the patients tested under two years. Of those under two years, 15 were initially categorized into the unclassified group; however, 12 were later reclassified into the typical/atypical RTT groups based on follow-up evaluation. Among the 119 patients monitored beyond age five years, 80% displayed typical RTT symptoms, 10 remained unclassified, and 9.8% had exonic deletions, posing challenges for detection using next-generation sequencing. CONCLUSIONS: Targeted MECP2 testing has emerged as a clinically valuable tool with a high diagnostic yield, including the identification of small deletions. Given that younger patients may not always meet the classic RTT criteria, this study recommends targeted MECP2 testing in younger patients without typical RTT features.

Mitochondrial dysfunction and increased reactive oxygen species production in MECP2 mutant astrocytes and their impact on neurons.

Studies on MECP2 function and its implications in Rett Syndrome (RTT) have traditionally centered on neurons. Here, using human embryonic stem cell (hESC) lines, we modeled MECP2 loss-of-function to explore its effects on astrocyte (AST) development and dysfunction in the brain. Ultrastructural analysis of RTT hESC-derived cerebral organoids revealed significantly smaller mitochondria compared to controls (CTRs), particularly pronounced in glia versus neurons. Employing a multiomics approach, we observed increased gene expression and accessibility of a subset of nuclear-encoded mitochondrial genes upon mutation of MECP2 in ASTs compared to neurons. Analysis of hESC-derived ASTs showed reduced mitochondrial respiration and altered key proteins in the tricarboxylic acid cycle and electron transport chain in RTT versus CTRs. Additionally, RTT ASTs exhibited increased cytosolic amino acids under basal conditions, which were depleted upon increased energy demands. Notably, mitochondria isolated from RTT ASTs exhibited increased reactive oxygen species and influenced neuronal activity when transferred to cortical neurons. These findings underscore MECP2 mutation's differential impact on mitochondrial and metabolic pathways in ASTs versus neurons, suggesting that dysfunctional AST mitochondria may contribute to RTT pathophysiology by affecting neuronal health.

Clinical-grade intranasal NGF fuels neurological and metabolic functions of Mecp2-deficient mice.

MECP2 deficiency causes a broad spectrum of neuropsychiatric disorders that can affect both genders. Rett syndrome is the most common and is characterized by an apparently normal growth period followed by a regression phase in which patients lose most of their previously acquired skills. After this dramatic period, various symptoms progressively appear, including severe intellectual disability, epilepsy, apraxia, breathing abnormalities and motor deterioration. MECP2 encodes for an epigenetic transcription factor that is particularly abundant in the brain; consequently, several transcriptional defects characterize the Rett syndrome brain. The well-known deficiency of several neurotrophins and growth factors, together with the positive effects exerted by Trofinetide, a synthetic analogue of insulin-like growth factor 1, in Rett patients and in mouse models of Mecp2 deficiency, prompted us to investigate the therapeutic potential of nerve growth factor. Initial in vitro studies demonstrated a healing effect of rhNGF on neuronal maturation and activity in cultured Mecp2-null neurons. Subsequently, we designed in vivo studies with clear translational potential using intranasally administered recombinant human GMP-grade NGF (rhNGF) already used in the clinic. Efficacy of rhNGF in vivo in Mecp2-null hemizygous male mice and heterozygous female mice was assessed. General well-being was evaluated by a conventional phenotypic score and motor performance through the Pole and Beam Walking tests, while cognitive function and interaction with the environment were measured by the Novel Object Recognition Test and the Marble Burying test, respectively. At the end of the treatment, mouse cortices were dissected and bulk RNA sequencing was performed to identify the molecular pathways involved in the protective effects of rhNGF. rhNGF exerted positive effects on cognitive and motor functions in both male and female mouse models of Rett syndrome. In male hemizygous mice, which suffer from significantly more severe and rapidly advancing symptoms, the drug's ability to slow the disease's progression was more pronounced. The unbiased research for the molecular mechanisms triggering the observed benefits revealed a strong positive effect on gene sets related to oxidative phosphorylation, mitochondrial structure and function. These results were validated by demonstrating the drug's ability to improve mitochondrial structure and respiration in Mecp2-null cerebral cortices. Furthermore, GO analyses indicated that NGF exerted the expected improvement in neuronal maturation. We conclude that intranasal administration of rhNGF is a non-invasive and effective route of administration for the treatment of Rett syndrome and possibly for other neurometabolic disorders with overt mitochondrial dysfunction.

Integrating DIA Single-Cell Proteomics Data with the DiagnoMass Proteomic Hub for Biological Insights.

Single-cell proteomics has emerged as a powerful technology for unraveling the complexities of cellular heterogeneity, enabling insights into individual cell functions and pathologies. One of the primary challenges in single-cell proteomics is data generation, where low mass spectral signals often preclude the triggering of MS2 events. This challenge is addressed by Data Independent Acquisition (DIA), a data acquisition strategy that does not depend on peptide ion isotopic signatures to generate an MS2 event. In this study, we present data generated from the integration of DIA single-cell proteomics with a version of the DiagnoMass Proteomic Hub that was adapted to handle DIA data. DiagnoMass employs a hierarchical clustering methodology that enables the identification of tandem mass spectral clusters that are discriminative of biological conditions, thereby reducing the reliance on search engine biases for identifications. Nevertheless, a search engine (in this work, DIA-NN) can be integrated with DiagnoMass for spectral annotation. We used single-cell proteomic data from iPSC-derived neuroprogenitor cell cultures as a test study of this integrated approach. We were able to differentiate between control and Rett Syndrome patient cells to discern the proteomic variances potentially contributing to the disease's pathology. Our research confirms that the DiagnoMass-DIA synergy significantly enhances the identification of discriminative proteomic signatures, highlighting critical biological variations such as the presence of unique spectra that could be related to Rett Syndrome pathology.

Involvement of extracellular vesicle microRNA clusters in developing healthy and Rett syndrome brain organoids.

Rett syndrome (RTT) is a neurodevelopmental disorder caused by de novo mutations in the MECP2 gene. Although miRNAs in extracellular vesicles (EVs) have been suggested to play an essential role in several neurological conditions, no prior study has utilized brain organoids to profile EV-derived miRNAs during normal and RTT-affected neuronal development. Here we report the spatiotemporal expression pattern of EV-derived miRNAs in region-specific forebrain organoids generated from female hiPSCs with a MeCP2:R255X mutation and the corresponding isogenic control. EV miRNA and protein expression profiles were characterized at day 0, day 13, day 40, and day 75. Several members of the hsa-miR-302/367 cluster were identified as having a time-dependent expression profile with RTT-specific alterations at the latest developmental stage. Moreover, the miRNA species of the chromosome 14 miRNA cluster (C14MC) exhibited strong upregulation in RTT forebrain organoids irrespective of their spatiotemporal location. Together, our results suggest essential roles of the C14MC and hsa-miR-302/367 clusters in EVs during normal and RTT-associated neurodevelopment, displaying promising prospects as biomarkers for monitoring RTT progression.

Ex vivo disease modelling of Rett syndrome: the transcriptomic and metabolomic implications of direct neuronal conversion.

BACKGROUND: Rett syndrome (RTT) is a rare neurodevelopmental disorder that primarily affects females and is characterized by a period of normal development followed by severe cognitive, motor, and communication impairment. The syndrome is predominantly caused by mutations in the MECP2. This study aimed to use comprehensive multi-omic analysis to identify the molecular and metabolic alterations associated with Rett syndrome. METHODS AND RESULTS: Transcriptomic and metabolomic profiling was performed using neuron-like cells derived from the fibroblasts of 3 Rett syndrome patients with different MECP2 mutations (R168X, P152R, and R133C) and 1 healthy control. Differential gene expression, alternative splicing events, and metabolite changes were analyzed to identify the key pathways and processes affected in patients with Rett syndrome. Transcriptomic analysis showed there was significant down-regulation of genes associated with the extracellular matrix (ECM) and cytoskeletal components, which was particularly notable in patient P3 (R133C mutation), who had non-random X chromosome inactivation. Additionally, significant changes in microtubule-related gene expression and alternative splicing events were observed, especially in patient P2 (P152R mutation). Metabolomic profiling showed that there were alterations in metabolic pathways, particularly up-regulation of ketone body synthesis and degradation pathways, in addition to an increase in free fatty acid levels. Integrated analysis highlighted the interplay between structural gene down-regulation and metabolic shifts, underscoring the adaptive responses to cellular stress in Rett neurons. CONCLUSION: The present findings provide valuable insights into the molecular and metabolic landscape of Rett syndrome, emphasizing the importance of combining omic data to enlighten the molecular pathophysiology of this syndrome.

[Rett syndrome: from pathophysiology to developments in treatment]. / Síndrome de Rett: desde la fisiopatología a las novedades en el tratamiento.

Rett Syndrome (RTT) is a neurodevelopment disorder which primarily affects females and is caused by pathogenic variants in the MECP2 gene. The disease has a characteristic developmental regression resulting in impairment of expressive language, hand skills, and ambulation that is accompanied by hand stereotypies. The goal of this article it to provide an overview of the diagnosis, natural history, and treatment.

El síndrome de Rett (SR) es un desorden del neurodesarrollo que afecta principalmente a mujeres y es causado por una variante patogénica en el gen MECP2. Esta enfermedad se caracteriza por una regresión del desarrollo que resulta en el deterioro del lenguaje expresivo, habilidades manuales, y deambulación, y está acompañado de estereotipias manuales. El objetivo de este artículo es proporcionar una visión general del diagnóstico, la historia natural y el tratamiento.

Neurotrophins and Their Receptors: BDNF's Role in GABAergic Neurodevelopment and Disease.

Neurotrophins and their receptors are distinctly expressed during brain development and play crucial roles in the formation, survival, and function of neurons in the nervous system. Among these molecules, brain-derived neurotrophic factor (BDNF) has garnered significant attention due to its involvement in regulating GABAergic system development and function. In this review, we summarize and compare the expression patterns and roles of neurotrophins and their receptors in both the developing and adult brains of rodents, macaques, and humans. Then, we focus on the implications of BDNF in the development and function of GABAergic neurons from the cortex and the striatum, as both the presence of BDNF single nucleotide polymorphisms and disruptions in BDNF levels alter the excitatory/inhibitory balance in the brain. This imbalance has different implications in the pathogenesis of neurodevelopmental diseases like autism spectrum disorder (ASD), Rett syndrome (RTT), and schizophrenia (SCZ). Altogether, evidence shows that neurotrophins, especially BDNF, are essential for the development, maintenance, and function of the brain, and disruptions in their expression or signaling are common mechanisms in the pathophysiology of brain diseases.

Epigenetics in rare neurological diseases.

Rare neurological diseases include a vast group of heterogenous syndromes with primary impairment(s) in the peripheral and/or central nervous systems. Such rare disorders may have overlapping phenotypes, despite their distinct genetic etiology. One unique aspect of rare neurological diseases is their potential common association with altered epigenetic mechanisms. Epigenetic mechanisms include regulatory processes that control gene expression and cellular phenotype without changing the composition of the corresponding DNA sequences. Epigenetic factors include three types of proteins, the "readers, writers, and erasers" of DNA and DNA-bound proteins. Thus, epigenetic impairments of many neurological diseases may contribute to their pathology and manifested phenotypes. Here, we aim to provide a comprehensive review on the general etiology of selected rare neurological diseases, that include Rett Syndrome, Prader-Willi Syndrome, Rubinstein-Taybi Syndrome, Huntington's disease, and Angelman syndrome, with respect to their associated aberrant epigenetic mechanisms.

Drug repurposing in Rett and Rett-like syndromes: a promising yet underrated opportunity?

Rett syndrome (RTT) and Rett-like syndromes [i.e., CDKL5 deficiency disorder (CDD) and FOXG1-syndrome] represent rare yet profoundly impactful neurodevelopmental disorders (NDDs). The severity and complexity of symptoms associated with these disorders, including cognitive impairment, motor dysfunction, seizures and other neurological features significantly affect the quality of life of patients and families. Despite ongoing research efforts to identify potential therapeutic targets and develop novel treatments, current therapeutic options remain limited. Here the potential of drug repurposing (DR) as a promising avenue for addressing the unmet medical needs of individuals with RTT and related disorders is explored. Leveraging existing drugs for new therapeutic purposes, DR presents an attractive strategy, particularly suited for neurological disorders given the complexities of the central nervous system (CNS) and the challenges in blood-brain barrier penetration. The current landscape of DR efforts in these syndromes is thoroughly examined, with partiuclar focus on shared molecular pathways and potential common drug targets across these conditions.

Burden of illness in Rett syndrome: initial evaluation of a disorder-specific caregiver survey.

BACKGROUND: Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder associated with multiple neurologic impairments. Previous studies have shown challenges to the quality of life of individuals with RTT and their caregivers. However, instruments applied to quantify disease burden have not adequately captured the impact of these impairments on affected individuals and their families. Consequently, an international collaboration of stakeholders aimed at evaluating Burden of Illness (BOI) in RTT was organized. METHODS: Based on literature reviews and qualitative interviews with parents of children and adults with RTT, a caregiver questionnaire was constructed to evaluate 22 problems (inclusive of core characteristics, functional impairments, and comorbidities) often experienced with RTT, rated mainly with a 5-level Likert scale. The questionnaire was administered anonymously online to an international sample of 756 caregivers (predominantly parents) of girls and women with RTT. Descriptive statistics were used to identify problems of high frequency and impact on affected individuals and caregivers. Chi-square tests characterized the relationship between problem severity and impact responses, while nonparametric ANOVAs of raw and z-score adjusted scores identified agreement between severity and impact on individual and caregiver. Secondary inferential tests were used to determine the roles of age, clinical type, and country of residence on BOI in RTT. RESULTS: There was variability in reported frequency of problems, with the most prevalent, severe and impactful being those related to the core features of RTT (i.e., communication and fine and gross motor impairments). Chi-square analyses demonstrated interdependence between severity and impact responses, while ANOVAs showed that many problems had disproportionately greater impact than severity, either on affected individuals (e.g., hand stereotypies) or their caregivers (e.g., sleep difficulties, seizures, pain, and behavioral abnormalities). With certain exceptions (e.g., breath-holding, seizures), age, clinical type, or country of residence did not influence these BOI profiles. CONCLUSIONS: Our data demonstrate that core features and related impairments are particularly impactful in RTT. However, problems with mild severity can also have disproportionate impact on affected individuals and, particularly, on their caregivers. Future analyses will examine the role of factors such as treatment outcomes, healthcare services, and healthcare provider's perspectives, in these BOI profiles.

Using Precision Medicine to Disentangle Genotype-Phenotype Relationships in Twins with Rett Syndrome: A Case Report.

Rett syndrome (RTT) is a paediatric neurodevelopmental disorder spanning four developmental stages. This multi-system disorder offers a unique window to explore genotype-phenotype relationships in a disease model. However, genetic prognosticators of RTT have limited clinical value due to the disorder's heterogeneity on multiple levels. This case report used a precision medicine approach to better understand the clinical phenotype of RTT twins with an identical pathogenic MECP2 mutation and discordant neurodevelopmental profiles. Targeted genotyping, objective physiological monitoring of heart rate variability (HRV) parameters, and clinical severity were assessed in a RTT twin pair (5 years 7 months old) with an identical pathogenic MECP2 mutation. Longitudinal assessment of autonomic HRV parameters was conducted using the Empatica E4 wristband device, and clinical severity was assessed using the RTT-anchored Clinical Global Impression Scale (RTT-CGI) and the Multi-System Profile of Symptoms Scale (MPSS). Genotype data revealed impaired BDNF function for twin A when compared to twin B. Twin A also had poorer autonomic health than twin B, as indicated by lower autonomic metrics (autonomic inflexibility). Hospitalisation, RTT-CGI-S, and MPSS subscale scores were used as measures of clinical severity, and these were worse in twin A. Treatment using buspirone shifted twin A from an inflexible to a flexible autonomic profile. This was mirrored in the MPSS scores, which showed a reduction in autonomic and cardiac symptoms following buspirone treatment. Our findings showed that a combination of a co-occurring rs6265 BDNF polymorphism, and worse autonomic and clinical profiles led to a poorer prognosis for twin A compared to twin B. Buspirone was able to shift a rigid autonomic profile to a more flexible one for twin A and thereby prevent cardiac and autonomic symptoms from worsening. The clinical profile for twin A represents a departure from the disorder trajectory typically observed in RTT and underscores the importance of wider genotype profiling and longitudinal objective physiological monitoring alongside measures of clinical symptoms and severity when assessing genotype-phenotype relationships in RTT patients with identical pathogenic mutations. A precision medicine approach that assesses genetic and physiological risk factors can be extended to other neurodevelopmental disorders to monitor risk when genotype-phenotype relationships are not so obvious.

Clinical Features and Disease Progression in Older Individuals with Rett Syndrome.

Although long-term survival in Rett syndrome (RTT) has been observed, limited information on older people with RTT exists. We hypothesized that increased longevity in RTT would be associated with genetic variants in MECP2 associated with milder severity, and that clinical features would not be static in older individuals. To address these hypotheses, we compared the distribution of MECP2 variants and clinical severity between younger individuals with Classic RTT (under 30 years old) and older individuals (over 30 years old). Contrary to expectation, enrichment of a severe MECP2 variant (R106W) was observed in the older cohort. Overall severity was not different between the cohorts, but specific clinical features varied between the cohorts. Overall severity from first to last visit increased in the younger cohort but not in the older cohort. While some specific clinical features in the older cohort were stable from the first to the last visit, others showed improvement or worsening. These data do not support the hypothesis that mild MECP2 variants or less overall severity leads to increased longevity in RTT but demonstrate that clinical features change with increasing age in adults with RTT. Additional work is needed to understand disease progression in adults with RTT.