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Key Points In the 50 years since its description by Andreas Rett, we have witnessed an explosion of knowledge about Rett syndrome (RTT) in relation to its genetic basis and clinical characteristics, and their interrelationships Initially, the diagnosis of RTT was based solely on clinical criteria, but identification of its genetic cause has revolutionized this process, while presenting new challenges as we enter the era of next-generation sequencing Mutations in the methyl-CpG-binding protein 2 ( MECP2 ) gene were found to be causative of RTT, accounting for fundamentally altered neurobiological pathways, and providing the stimulus to identify pathways that can be manipulated therapeutically The type of MECP2 mutation is associated with clinical severity, and influences many aspects of the phenotype, including functional abilities, onset of scoliosis, bone health, and sleep disturbances Considerable progress has been made in understanding the natural history of RTT, leading to improvement in clinical management in selected areas, and changes in attitudes and allocation of health-care resources have increased life expectancy The advancement in knowledge about RTT has been dependent on global efforts to study this disorder, including the establishment of database infrastructures, the input of advocacy groups, and the development of international collaborations Rett syndrome (RTT) is a rare, progressive neurodevelopmental disorder that almost exclusively affects girls, and is caused by a mutation in the MECP2 gene. The authors highlight major milestones in RTT over the past 50 years, and acknowledge the international collaborations that are driving this research programme. In the 50 years since Andreas Rett first described the syndrome that came to bear his name, and is now known to be caused by a mutation in the methyl-CpG-binding protein 2 ( MECP2 ) gene, a compelling blend of astute clinical observations and clinical and laboratory research has substantially enhanced our understanding of this rare disorder. Here, we document the contributions of the early pioneers in Rett syndrome (RTT) research, and describe the evolution of knowledge in terms of diagnostic criteria, clinical variation, and the interplay with other Rett-related disorders. We provide a synthesis of what is known about the neurobiology of MeCP2, considering the lessons learned from both cell and animal models, and how they might inform future clinical trials. With a focus on the core criteria, we examine the relationships between genotype and clinical severity. We review current knowledge about the many comorbidities that occur in RTT, and how genotype may modify their presentation. We also acknowledge the important drivers that are accelerating this research programme, including the roles of research infrastructure, international collaboration and advocacy groups. Finally, we highlight the major milestones since 1966, and what they mean for the day-to-day lives of individuals with RTT and their families.

MECP2 duplication syndrome (MDS) is a rare, X-linked, neurodevelopmental disorder caused by a duplication of the methyl-CpG-binding protein 2 ( MECP2 ) gene—a gene in which loss-of-function mutations lead to Rett syndrome (RTT). MDS has an estimated live birth prevalence in males of 1/150,000. The key features of MDS include intellectual disability, developmental delay, hypotonia, seizures, recurrent respiratory infections, gastrointestinal problems, behavioural features of autism and dysmorphic features—although these comorbidities are not yet understood with sufficient granularity. This review has covered the past two decades of MDS case studies and series since the discovery of the disorder in 1999. After comprehensively reviewing the reported characteristics, this review has identified areas of limited knowledge that we recommend may be addressed by better phenotyping this disorder through an international data collection. This endeavour would also serve to delineate the clinical overlap between MDS and RTT.

Background Alleviating the burden of rare diseases requires research into new diagnostic and therapeutic strategies. We undertook a systematic review to identify and compare the impact of stand-alone registries, registries with biobanks, and rare disease biobanks on research outcomes in rare diseases. Methods A systematic review and meta-aggregation was conducted using the preferred reporting items for systematic reviews and meta-analyses (the PRISMA statement). English language publications were sourced from PubMed, Medline, Scopus, and Web of Science. Original research papers that reported clinical, epidemiological, basic or translational research findings derived from data contained in stand-alone registries, registries with biobanks, and rare disease biobanks were considered. Articles selected for inclusion were assessed using the critical appraisal instruments by JBI-QARI. Each article was read in its entirety and findings were extracted using the online data extraction software from JBI-QARI. Results Thirty studies including 28 rare disease resources were included in the review. Of those, 14 registries were not associated to biobank infrastructure, 9 registries were associated with biobank infrastructure, and 6 were rare disease biobank resources. Stand-alone registries had the capacity to uncover the natural history of disease and contributed to evidence-based practice. When annexed to biobank infrastructure, registries could also identify and validate biomarkers, uncover novel genes, elucidate pathogenesis at the Omics level, and develop new therapeutic strategies. Rare disease biobanks in this review had similar capacity for biological investigations, but in addition, had far greater sample numbers and higher quality laboratory techniques for quality assurance processes. Discussion We examined the research outcomes of three specific populations: stand-alone registries, registries with biobanks, and stand-alone rare disease biobanks and demonstrated that there are key differences among these resources. These differences are a function of the resources’ design, aims, and objectives, with each resource having a distinctive and important role in contributing to the body of knowledge for rare disease research. Whilst stand-alone registries had the capacity to uncover the natural history of disease, develop best practice, replace clinical trials, and improve patient outcomes, they were limited in their capacity to conduct basic research. The role of basic research in rare disease research is vital; scientists must first understand the pathways of disease before they can develop appropriate interventions. Rare disease biobanks, on the other hand (particularly larger biobanks), had the key infrastructure required to conduct basic research, making novel Omics discoveries, identify and validate biomarkers, uncover novel genes, and develop new therapeutic strategies. However, these stand-alone rare disease biobanks did not collect comprehensive data or impact on clinical observations like a rare disease registry. Rare disease research is important not only for rare diseases, but also for also common diseases. For example, research of low-density lipoprotein (LDL)-receptors in the rare disease known as familial hypercholesterolemia led to the discovery of statins, a drug therapy that is now used routinely to prevent heart disease. Conclusions Rare diseases are still under-researched worldwide. This review made the important observation that registries with biobanks had the function of both stand-alone registries (the capacity to collect comprehensive clinical and epidemiological data) and stand-alone rare disease biobanks (the ability to contribute to Omics research). We found registries with biobanks offer a unique, practical, cost-effective, and impactful solution for rare disease research. Linkage of stand-alone registries to rare disease biobanks will provide the appropriate resources required for the effective translation of basic research into clinical practice. Furthermore, facilitators such as collaboration, engagement, blended recruitment, pro-active marketing, broad consent, and “virtual biobank” online catalogues will, if utilised, add to the success of these resources. These important observations can serve to direct future rare diseases research efforts, ultimately improve patient outcomes and alleviate the significant burden associated with rare disease for clinicians, hospitals, society, and most importantly, the patients and their families.

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.

Introduction The Rett Syndrome Behaviour Questionnaire (RSBQ) describes behavioural and emotional features. This study investigated total RSBQ score trajectories and their clustering, and for trajectory groups, relationships with genotype and mobility, weight-for-age z scores, and seizure frequency. Methods Females in the Australian Rett Syndrome Database who were genetically confirmed with RSBQ data collected between 2000 and 2019 were included. The age trend was modelled with mixed-effects linear regression. Unique trajectory classes for total RSBQ scores and time-varying predictors (mobility, weight-for-age z scores, seizure frequency) were identified with group-based trajectory modelling. Associations between genotype, time-varying predictors and RSBQ total score class memberships were assessed with multinomial logistic regression. Results There were 1,034 questionnaires collected from 298 individuals (age range 1.7-37.9 years) with a median of 3 (range 1-7) per person. For individuals aged 5 to 20 years at first RSBQ response, the mean (95% CI) scores were 43.9 (40.7, 47.1) at age 5 and 42.0 (39.9, 44.2) at age 20. Total score trajectories clustered into low and middle groups where scores decreased steadily, and a high group which decreased after age 20. Compared to independent walking, assisted walking increased the probability of being in the high-scoring RSBQ group. Very low weight (<-6 z score) increased the probability of being in the medium or high-scoring group. Weak associations were found with seizure frequency and genotype. Conclusion RSBQ total scores declined with increasing age suggesting improvement in the behavioural and emotional phenotype with age and time. There were few relationships between RSBQ scores and indicators of clinical severity. These natural history data form a baseline for comparison as new treatments become available.

Highlights This is the largest study to date to investigate the acquisition, retention and loss of functional skills in MECP2 duplication syndrome (MDS). Females were more likely than males to acquire gross and fine motor skills. Use of words was the most common parent-reported skill regression. Those with seizures had lower functional ability than those without seizures. There is a need for better understanding of the role of interventional therapy for functional skill retention in MDS. MECP2 duplication syndrome (MDS) is an ultrarare, X-linked neurodevelopmental disorder that is poorly understood in terms of its natural history and phenotypic variability. There is limited information on how individuals with MDS acquire, retain or lose fundamental functional skills (gross motor, purposeful hand function and communication) – that of which this study aimed to better characterise in the largest case series to date. For 160 individuals with MDS (median age 9.06 y, range: 0.57–51.63 y; 84% male), we report that phenotypic penetrance in females can, in some, result in a similar functional skill deficits to males. However, a higher proportion of females acquired gross motor and fine motor skills compared to males. Use of words was the most common parent-reported skill regression (34/90 [38%]) followed by fine motor/hand function (26/90 [29%]), independent walking (25/90 [28%]) and feeding (25/90 [28%]). Additionally, lower proportions of functional ability were present in those with seizures compared to those without. A general trend was also observed for decreasing functional skills with increasing age. Additionally, those with a larger duplication length (1 + Mb) were less likely to be able to acquire independent walking compared with those with less than a 1 + Mb duplication ( p  < 0.001). This is the first study to comprehensively map the developmental trajectory of functional skills in MDS and provides a seminal baseline for better characterising the natural history of this disorder. Further investigations are required to understand the importance of interventional therapy on the retainment of functional skills.

Background Initially described as an early onset seizure variant of Rett syndrome, the CDKL5 disorder is now considered as an independent entity. However, little is currently known about the full spectrum of comorbidities that affect these patients and available literature is limited to small case series. This study aimed to use a large international sample to examine the prevalence in this disorder of comorbidities of epilepsy, gastrointestinal problems including feeding difficulties, sleep and respiratory problems and scoliosis and their relationships with age and genotype. Prevalence and onset were also compared with those occurring in Rett syndrome. Methods Data for the CDKL5 disorder and Rett syndrome were sourced from the International CDKL5 Disorder Database (ICDD), InterRett and the Australian Rett syndrome Database (ARSD). Logistic regression (multivariate and univariate) was used to analyse the relationships between age group, mutation type and the prevalence of various comorbidities. Binary longitudinal data from the ARSD and the equivalent cross-sectional data from ICDD were examined using generalized linear models with generalized estimating equations. The Kaplan-Meier method was used to estimate the failure function for the two disorders and the log-rank test was used to compare the two functions. Results The likelihood of experiencing epilepsy, GI problems, respiratory problems, and scoliosis in the CDKL5 disorder increased with age and males were more vulnerable to respiratory and sleep problems than females. We did not identify any statistically significant relationships between mutation group and prevalence of comorbidities. Epilepsy, GI problems and sleep abnormalities were more common in the CDKL5 disorder than in Rett syndrome whilst scoliosis and respiratory problems were less prevalent. Conclusion This study captured a much clearer picture of the CDKL5 disorder than previously possible using the largest sample available to date. There were differences in the presentation of clinical features occurring in the CDKL5 disorder and in Rett syndrome, reinforcing the concept that CDKL5 is an independent disorder with its own distinctive characteristics.

Rett syndrome is associated with severe functional impairments and many comorbidities, each in urgent need of treatments. Mutations in the  MECP2  gene were identified as causing Rett syndrome in 1999. Over the past 20 years there has been an abundance of preclinical research with some studies leading to human clinical trials. Despite this, few viable therapeutic options have emerged from this investment of effort. Reasons for this lack of success as they relate both to preclinical research and the clinical trial landscape are discussed. Considering what needs to be done to promote further success in the field, we take a positive and constructive approach and introduce the concept of clinical trial readiness and its necessary ingredients for Rett syndrome. These include: listening to the needs of families; support from advocacy groups; optimising use of existing clinic infrastructures and available natural history data; and, finally, the validation of existing outcome measures and/or the development and validation of new measures. We conclude by reiterating the need for a collaborative and coordinated approach amongst the many different stakeholder groups and the need to engage in new types of trial design which could be much more efficient, less costly and much less burdensome on families.

Background Rett syndrome is caused by a pathogenic mutation in the MECP2 gene with major consequences for motor and cognitive development. One of the effects of impaired MECP2 function is reduced production of Brain Derived Neurotrophic Factor (BDNF), a protein required for normal neuronal development. When housed in an enriched environment, MECP2 null mice improved motor abilities and increased levels of BDNF in the brain. We investigated the effects of environmental enrichment on gross motor skills and blood BDNF levels in girls with Rett syndrome. Methods A genetically variable group of 12 girls with a MECP2 mutation and younger than 6 years participated in a modified individually randomised stepped wedge design study. Assessments were conducted on five occasions, two during the baseline period and three during the intervention period. Gross motor function was assessed using the Rett Syndrome Gross Motor Scale (maximum score of 45) on five occasions, two during the baseline period and three during the intervention period. Blood levels of BDNF were measured at the two baseline assessments and at the end of the intervention period. The intervention comprised motor learning and exercise supplemented with social, cognitive and other sensory experiences over a six-month period. Results At the first assessment, the mean (SD) age of the children was 3 years (1 year 1 month) years ranging from 1 year 6 months to 5 years 2 months. Also at baseline, mean (SD) gross motor scores and blood BDNF levels were 22.7/45 (9.6) and 165.0 (28.8) ng/ml respectively. Adjusting for covariates, the enriched environment was associated with improved gross motor skills (coefficient 8.2, 95%CI 5.1, 11.2) and a 321.4 ng/ml (95%CI 272.0, 370.8) increase in blood BDNF levels after 6 months of treatment. Growth, sleep quality and mood were unaffected. Conclusions Behavioural interventions such as environmental enrichment can reduce the functional deficit in Rett syndrome, contributing to the evidence-base for management and further understanding of epigenetic mechanisms. Environmental enrichment will be an important adjunct in the evaluation of new drug therapies that use BDNF pathways because of implications for the strengthening of synapses and improved functioning. Trial registration ACTRN12615001286538 .

Background Appropriate support for the health of children with an intellectual disability by parents and healthcare professionals is pivotal, given the high risk of chronic conditions. However, there is limited research that has collected important insights from parents on their learnings for supporting their child's evolving healthcare needs. Aim This study focuses on parents' experiences and learnings from managing and supporting the health of their child with intellectual disability. It aims to understand what parents wish they had known earlier, the essential knowledge and skills they needed to manage their child's health. Method A qualitative study was carried out using semi‐structured interviews with 21 parents of adolescents and young people with intellectual disability. The children had chronic health conditions that fell into six health domains, including (1) difficulties with movement and physical activity, (2) epilepsy, (3) dental care, (4) respiratory health and infection, (5) behaviour, mental health or sleep and (6) gastrointestinal health. Results Thematic analysis yielded five themes: (1) optimising mutual engagement between healthcare professionals and families; (2) planning and practising effective healthcare; (3) having the right information at the right time; (4) finding the support that was needed and (5) navigating healthcare and disability systems. Over time, parents developed specific skills and knowledge for managing their child's health effectively. Some parents expressed regret for not seeking support and information about their child's health conditions earlier. Parents described how mutual engagement between healthcare professionals and parents optimised the management of their child's healthcare. Conclusion The study found that managing the health of a child with intellectual disability is complex. The themes were consistent across health comorbidities, indicating important common experiences. The themes aligned with conceptualisations of health literacy, suggesting that improving health literacy skills can help parents better manage their children's health conditions. Patient or Public Contribution We developed the project in consultation with members of the public who have lived experience of parenting a child with intellectual disability. They commented on the study aims, interview schedule, participant recruitment and provided feedback on the analysis and discussion.