Rett syndrome: The Natural History Study journey
Abstract
Understanding clinical features and disease progression of Rett syndrome (RTT) and establishing clinical trial readiness was enhanced by the RTT Natural History Study (NHS). The NHS benefited from two key developments: one, the Orphan Drug Act passed by Congress in 1983 defining criteria for rare disorders in the United States and creating opportunities for pharmaceutical companies to develop products for individuals with rare disorders, and two, the Rare Diseases Act of 2002, which established the National Institutes of Health Office of Rare Diseases and provided research funding. Funding for the RTT and related disorders NHS was obtained in 2003, creating a broad network of experienced clinical investigators across the United States and producing critical results not only for RTT but also for related disorders: CDKL5 deficiency disorder, FOXG1 disorder, and MECP2 duplication syndrome. Longitudinal information from over 1800 participants (more than 1600 diagnosed with RTT) led to multiple reports describing their clinical features and natural progression and identified putative biomarkers and clinical outcome measures. Establishing clinical trial readiness assisted in evaluating the first FDA-approved medication for RTT in 2023 and continues to provide opportunities to develop potentially life-altering therapies. The experiences of the RTT NHS journey provide informative guidance for studying other rare neurological disorders. These lessons include positive features of developing productive collaborations focused on improving lives of people and families with RTT and related disorders, as well as lessons learned through retrospective analysis for improving overall conduct of natural history studies in rare disorders.
Background
Orphan (or rare) disorders, defined as conditions that affect less than one-half of 1% of individuals in the United States, collectively afflict more than 30 million individuals and their families. Previously, rare disorders received limited attention for the development of new treatments due to an incomplete understanding of the biology of these disorders, limited financial incentives for drug developers and industry, and, importantly, a lack of detailed understanding of the clinical features and natural longitudinal progression within the individual rare disorders. Prior to 1983, only 38 drugs were developed for rare disorders (Rare Disease Act of 2002). However, passage of the Orphan Drug Act by Congress in 1983 (www.fda.gov/patients/rare-diseases-fda#:~:text) helped spur development of rare disease treatment by providing incentives for tax credits, waiver of expensive Food and Drug Administration (FDA) fees, and exclusive marketing. Additionally, this act established the Orphan Product Grants Program within the FDA and together with the National Organization for Rare Disorders (NORD) encouraged and enabled individuals with rare disorders and their families in the drug development process. Success of the act is evident by the approval of over 220 new drugs by 2002 coincident with the Rare Disease Act of 2002 and a wave of new therapies on the horizon. Despite this progress, the number of rare disorders with specific FDA-approved drugs is small relative to the estimated total of 10 000 specific entities.
Although the Orphan Drug Act of 1983 enabled FDA focus on rare disorders, broader emphasis on the study of these rare disorders received little additional attention at the national level until the National Institutes of Health (NIH) Office of Rare Diseases was created in 1993. Unfortunately, that office still lacked any statutory authority or budget to promulgate further study. Even worse, funding for rare disease research did not increase in parallel with the large increases in research funding for behavioral and biomedical research in general through the 1990s. This changed in 2002 with the passage of the Rare Diseases Act, which established the Office of Rare Diseases (ORD) within the Office of the NIH Director and provided necessary funding to increase research for the diagnosis and treatment of these rare disorders (Public Law 107-280-Nov. 6, 2002: Rare Diseases Act of 2002). One of the first initiatives of ORD was to request applications for Orphan Disease Natural History Studies in 2003. Under this mechanism, a proposal was funded principally through the Eunice Kennedy Shriver National Institute of Child Health and Human Development at the NIH (HD061222) in cooperation with ORD to conduct natural history studies for three related neurodevelopmental disorders of childhood: Angelman syndrome (Beaudet, Baylor College of Medicine), Prader-Willi syndrome (Driscoll, University of Florida), and Rett (RTT) syndrome (Percy, University of Alabama at Birmingham). As outlined below, the study evolved over time and in the third round of funding focused on RTT and RTT-related disorders exclusively, no longer including Angelman or Prader-Willi syndromes.
This article focuses on the 16-year experience of the RTT and RTT-related disorders Natural History Study (NHS) component of this grant funding and describes the long and successful program to improve our understanding of the clinical features and longitudinal progression of RTT and RTT-related disorders as well as the development of clinical trial readiness for these disorders.
RTT and related disorders
In the late 1950s, Andreas Rett, a developmental pediatrician in Vienna, Austria, and Bengt Hagberg, a child neurologist in Uppsala, Sweden, recognized this unusual disorder in girls. Rett described his initial experience in 1966,1 but his report, written in German, languished in the medical literature for nearly two decades. Key differences between the individuals described in Rett's report and those seen by Hagberg explained this delay, at least in part. However, at a meeting of European child neurologists, Hagberg was encouraged to describe the features of girls from Sweden, France, and Portugal with this disorder. Prior to completing this report, Hagberg and Rett met around 1980. As a result, Hagberg decided that this disorder should be called Rett syndrome-RTT (OMIM 312750). Thus, the description of RTT in the Annals of Neurology in 19832 led to the worldwide recognition of this disorder. As RTT was subsequently reported almost exclusively in females, genetic causation focusing on the X chromosome led to Amir et al.3 in 1999 linking RTT to variations in the gene MECP2 (methyl-CpG-binding protein 2) at Xq28.
Prior to this, however, the first set of clinical criteria was generated in 1984,4 after which variant presentations were identified, including classical (typical) RTT and variant (atypical) RTT, an early-onset seizure variant,5 and a variant with very early neurodevelopmental delay.6 The early-onset seizure disorder was subsequently linked to CDKL5 variants,7 now called CDKL5 deficiency disorder (CDD), and the early developmental delay disorder was linked to FOXG1 variants,8 now called FOXG1 deficiency (FD).
A much smaller number of males was also recognized in the 1980s and subsequently shown to have MECP2 variants. These males expressed a wide variation in clinical involvement ranging from early-onset neonatal encephalopathy to significant developmental delay. Indeed, some males appeared to have features typical for RTT and have been associated with either somatic mosaicism or Klinefelter syndrome (47XXY). These males express two X chromosomes with one having an MECP2 variant.9 Finally, shortly after the association of RTT with MECP2 variants, Zoghbi demonstrated that an extra copy of MECP2 could pose significant issues,10 resulting in the identification of MECP2 duplication syndrome (MDD), an equally severe neurodevelopmental disorder seen principally in males.11
Several mouse models (and a rat model) of MECP2 deficiency have been developed, including both null and specific nucleotide-deficient strains recapitulating the features of RTT in these models.10 With these models, the possibility of effective therapeutic intervention has been investigated further. The landmark study of Guy et al.12 demonstrated that activation of an inserted MECP2 gene in a null mouse model of RTT could reverse many of the clinical features. In addition, mouse models have been used in translational studies to demonstrate the potential efficacy of various therapeutic agents in individuals with RTT.13
Significant research in RTT and RTT-related disorders by many groups has occurred aside from the NHS study. However, this report details only the efforts of the NHS.14-16
Overview and evolution of RTT NHS
Experience at Baylor College of Medicine and University of Alabama at Birmingham
In the United States, initial experience with RTT was initiated at Baylor College of Medicine (BCM) in 1983, where more than 200 individuals with this disorder were identified and followed prior to and after the association of MECP2 variants with RTT.17 One of us (AKP) moved to the University of Alabama at Birmingham (UAB) in 1992 and continued clinical studies in RTT thereafter. Thus, the RTT clinical protocol entering the NHS was based on 20 years of experience at BCM and UAB. Investigators from the Greenwood Genetic Center (GGC) traveled to UAB, where they were introduced to individuals with RTT and instructed in the clinical assessment, genetic analyses, the Clinical Severity Scale, the Motor Behavioral Assessment (MBA), and the two quality-of-life assessments (the Child Health Questionnaire for children and the SF-36v2 Health Status Questionnaire for parents or caregivers).
With the request for applications in 2003, we were successful in advancing these natural history studies to cover three rounds of funding, the overarching goal being that of clinical trial readiness. The first two rounds included the three genetic disorders: Angelman syndrome, Prader-Willi syndrome, and RTT. During the third round of funding, studies were restricted to RTT and RTT-related disorders.
Three rounds of funding for RTT
The first RTT natural history studies were instituted at three sites (BCM—Daniel Glaze, MD, and Kathleen Motil, MD; GGC—Steven Skinner, MD; and UAB—Alan Percy, MD). Shortly thereafter investigators at Boston Children's Hospital (Omar Khwaja, MD and Walter Kaufmann, MD) joined this protocol, providing access to populations along the east and gulf coasts. A lack of availability for families across the United States was evident, and with the International Rett Syndrome Association (IRSA, now the International Rett Syndrome Foundation [IRSF]), travel clinics were established at four additional sites. The travel and costs of these clinics, which began in 2006, were funded by IRSA. The four clinic sites were Katie's Clinic at Children's Hospital Oakland in Oakland, CA, the Rush Medical Center in Chicago, the Robert Wood Johnson Medical School in New Brunswick, NJ, and a fourth site alternating between Miami and Tampa. These travel clinics allowed the funded physicians and research teams to travel to these clinical sites twice annually or for approximately 1500 person-hours per study site.
Throughout the first two rounds of NHS funding, the travel clinics were extremely effective in recruitment and enrollment, accounting for more than 40% of the total enrollment over the next 11 years, despite the cost in additional hours required. Further, these clinics created and enhanced the strong camaraderie and shared clinical expertise among the four principal research sites, stimulating enrollment and encouraging continued follow-up. In addition, the travel clinics enhanced the development of centers of excellence for RTT, reflected in the third round of funding, which continued after NHS funding ended in 2021.
In the last seven years, RTT studies alone were funded through this mechanism as the entire proposal was revamped and specific amplification of the RTT-related disorders CDD, FD, and MDD was included. Although individuals with these related disorders were enrolled in the first two iterations, specific metrics for each were only included in this third application. This third round of funding for RTT and RTT-related disorders was strengthened significantly by the additional enrollment sites (Table 1).
| Enrollment sites | Investigators |
|---|---|
| Children's Hospital of Philadelphia/University of Pennsylvania | Eric Marsh, MD, PhD |
| University of Colorado Denver | Tim Benke, MD, PhD |
| Cincinnati Children's Hospital | Shannon Standridge, DO |
| Vanderbilt Medical Center | Jeffrey L. Neul, MD, PhD, Sarika Peters, PhD, and Cary Fu, MD |
| Rush Medical Center | Peter Heydemann, MD |
| Washington University (St. Louis) | Robin Ryther, MD, PhD and Judith Weisenberg, MD |
| Children's Hospital Oakland/UCSF | Mary Jones, MD† |
| University of California San Diego | Jeffrey L. Neul, MD, PhD and Richard Haas MD |
| University of Rochester1 | Alex Paciorkowski, MD |
| Cleveland Clinic — CDD only | Elia Pestana-Knight, MD |
| Baylor College of Medicine2 | Daniel Glaze, MD and Bernhard Suter, MD, PhD |
| Greenwood Genetic Center2 | Steven Skinner, MD |
| Boston Children's Hospital2 | Walter Kaufmann, MD, David Lieberman, MD, PhD, and Heather Olson, MD |
| University of Alabama at Birmingham2 | Alan Percy, MD and Amitha Ananth, MD |
- 1 Site became inactive.
- 2 5201 sites.
RTT syndrome NHS clinical protocol 5201: 2003–2014
The RTT NHS clinical protocol initiated in 2003 (with enrollment commencing in 2006) had three overarching hypotheses: (1) phenotype–genotype analyses would reveal a close correlation between type and location of the MECP2 variation; (2) quality-of-life assessments of both the individuals with RTT and their parents/caregivers would yield a close correlation between the phenotype and X chromosome inactivation; and (3) quality-of-life assessments of both the individuals with RTT and their parents/caregivers would correlate inversely with the Clinical Severity Scores (CSS) and the MBA. Throughout this study, these two measures developed specifically for RTT were employed to assess the level of clinical involvement for each disorder. The CSS18 is a composite score based on 13 individual, ordinal categories measuring clinical features common in RTT. All scores range from 0–4 or 0–5, with 0 representing the least severe and 4 or 5 representing the most severe finding. Scores ranged from 0–58, with higher numbers representing greater severity. The MBA19 is also an ordinal score based on a 0–4 range, 0 being best and 4 being worst, and incorporates measures of behavior/social assessment (range = 0–64), orofacial/respiratory assessment (range = 0–28), and motor assessment/physical signs (range = 0–56). These measures were used to describe the relationship between the level of clinical involvement and the specific issue being analyzed.
Enrollment goals, recruitment, and periodic scheduling
The initial NHS and its subsequent successor were developed to provide comprehensive assessments of the clinical features of RTT. The goal of the initial protocol was to develop a clinical database of 1000 individuals with classic RTT and 100 individuals with atypical RTT regarding their clinical features and their MECP2 variation for the purpose of establishing phenotype-genotype correlations (Table 2). Initial recruitment was generated from the existing clinics at BCM and UAB. Additional participants were invited through the IRSA (now IRSF) website and newsletter.
| Enrollment goals | 2006–2014 | 2015–2021 |
|---|---|---|
| RTT | 1000 | 1000 |
| Atypical RTT | 100 | - |
| CDD | - | 50 |
| FD | - | 50 |
| MDD | - | 100 |
| Visit frequency: 2006–2014 | ||
| Up to age 12 years of age initially; then up to 6 years | Biannual | - |
| 13 years or greater | Annual | - |
| Visit frequency: 2015–2021 | ||
| Up to 10 years | - | Annual |
| 11–20 years | - | Biannual |
| 21 years or greater | - | Twice |
- Abbreviations: CDD, CDKL5 deficiency disorder; FD, FOXG1 disorder; MDD, MECP2 duplication disorder; NHS, natural history study; RTT, Rett syndrome.
Participants eligible for enrollment had the clinical diagnosis of RTT and determination of their MECP2 status but were not excluded if the genetic testing revealed no MECP2 variation. Individuals who had a MECP2 variant but did not meet the established clinical criteria were also eligible for enrollment. The initial schedule of visits was biannually for individuals through 12 years of age and annually for those greater than 12 years. This was later changed to biannually through 6 years and annually thereafter.
Inclusion criteria
The clinical diagnosis of RTT was based on meeting the established diagnostic criteria for either classic (typical) or variant (atypical) RTT, initially using those of Hagberg et al.20 from 2002 (Table 3) but later transitioning to the updated criteria of Neul et al.21 promulgated in 2010 (Table 4). The latter criteria were intended to simplify the diagnosis by streamlining the criteria and craft language that could be easily understood across the world. Data from the NHS validated these criteria.22
| Necessary criteria |
| Apparently normal prenatal and perinatal period |
| Apparently normal psychomotor development through the first 6 months |
| Normal head circumference at birth |
| Deceleration of head growth between ages 5 months and 4 years |
| Loss of acquired purposeful hand skills between ages 6 and 30 months, temporally associated with communication dysfunction and social withdrawal |
| Development of severely impaired expressive and receptive language, and presence of severe psychomotor retardation |
| Stereotypic hand movements such as hand wringing/squeezing, clapping/tapping, mouthing and washing/rubbing automatisms appearing after purposeful hand skills are lost |
| Appearance of gait apraxia and truncal apraxia/ataxia between 1 and 4 years of age |
| Diagnosis tentative until 2 to 5 years of age |
| Supportive criteria |
| Breathing dysfunction |
| Periodic apnea |
| Intermittent hyperventilation |
| Breath-holding spells |
| Forced expulsion of air or saliva |
| Electroencephalogram abnormalities |
| Slow waking background and intermittent rhythmical slowing (3-5 Hz) |
| Epileptiform discharges, with or without clinical seizures |
| Seizures |
| Spasticity, often with associated development of muscle wasting and dystonia |
| Peripheral vasomotor disturbances |
| Scoliosis |
| Growth retardation |
| Hypotrophic, small feet |
| Exclusion criteria |
| Evidence of intrauterine growth retardation |
| Organomegaly or other signs of storage disease |
| Retinopathy or optic atrophy |
| Evidence of perinatally acquired brain damage |
| Existence of identifiable metabolic or other progressive neurological disorder |
| Acquired neurological disorders resulting from severe infections or head trauma |
| Variant Rett syndrome |
| Inclusion criteria |
| A girl of at least 10 years of age with mental retardation of unexplained origin and with at least 3 of the 6 following primary criteria: |
| Loss (partial or subtotal) of acquired finger skills in late infancy/early childhood |
| Loss of acquired single words/phrases/nuanced babble |
| RS hand stereotypies, hands together or apart |
| Early deviant communication ability |
| Deceleration of head growth of 2 SD (even when still within normal limits) |
| The RS disease profile: a regression period (stage II) followed by a certain recovery of contact and communication (stage III) in contrast to slow neuromotor regression through school age and adolescence |
| And 5 of the following 11 RS supportive manifestations: |
| Breathing irregularities (hyperventilation and/or breath holding) |
| Bloating/marked air swallowing |
| Characteristic RS teeth grinding |
| Gait dyspraxia |
| Neurogenic scoliosis or high kyphosis (ambulant girls) |
| Development of lower limb abnormalities |
| Small blue/cold impaired feet, autonomic/trophic dysfunction |
| Characteristic RS electroencephalographic development |
| Unprompted sudden laughing/screaming spells |
| Impaired/delayed nociception |
| Intensive eye communication—”eye pointing” |
- Note: Hagberg et al.20 Abbreviation: RS, Rett syndrome.
| Consider diagnosis when postnatal deceleration of head growth observed |
|---|
| Required for typical or classic Rett syndrome (RTT) |
| Required for typical or classic RTT |
| A period of regression followed by recovery or stabilization |
| All main criteria and all exclusion criteria |
| Supportive criteria are not required, although often present in typical RTT |
| Development of microcephaly no longer a criterion |
| Required for atypical or variant RTT |
| A period of regression followed by recovery or stabilization |
| At least 2 of the 4 main criteria |
| At least 5 of the 11 supportive criteria |
| Main criteria |
| Partial or complete loss of acquired purposeful hand skills |
| Partial or complete loss of acquired spoken language |
| Gait abnormalities: impaired (dyspraxic) or absence of ability |
| Stereotypic hand movements such as hand wringing/squeezing, clapping/tapping, mouthing, and washing/rubbing automatisms |
| Exclusion criteria |
| Brain injury secondary to trauma (peri- or postnatally), neurometabolic disease,or severe infection that causes neurological problems |
| Grossly abnormal psychomotor development in first 6 months of life |
| Supportive criteria for atypical RTT |
| Breathing disturbances when awake |
| Bruxism when awake |
| Impaired sleep pattern |
| Abnormal muscle tone |
| Peripheral vascular disturbances |
| Scoliosis/kyphosis |
| Growth retardation |
| Small cold hands and feet |
| Inappropriate laughing/screaming spells |
| Diminished response to pain |
| Intense eye communication – “eye pointing” |
- Note: Neul et al.21
More than 300 MECP2 variants have now been identified in individuals with RTT. Eight common point variants make up approximately 60% of the total while C-terminal truncations and large deletions involving one or more exons each account for about 10%. Thus, the enrollment goal of 1000 participants was expected to provide sufficient variant types to allow the development of strong phenotype-genotype correlations. Total enrollment in 5201 was 1217, including 961 with classic RTT, 157 with atypical RTT, and 62 who were MECP2-positive but failed to meet criteria for classic or atypical RTT (Table 4).
Demographic characteristics included gender, race and ethnicity, gestational age, maternal and paternal histories, and marital status. It was recognized that most individuals enrolled in this study with RTT/MECP2 variants would be females, but males were not excluded. Significant effort was made to achieve racial and ethnic balance throughout the study. Despite these efforts, Whites made up 85% of the enrollments across each grant cycle, African Americans were 4.5%, and Hispanic-Latinos were 15%. For the RTT-related disorders, 87% were White, 3.9% were African American, and 13% were Hispanic-Latinos.
Major goals for 5201
During the first and second grant cycles, the major results were (1) the development of high-quality phenotype–genotype correlations; (2) the development of RTT specific growth charts for height, weight, body mass index, and head circumstance from birth through 20 years of age; (3) the assessment of longevity among individuals with classic RTT; and (4) the establishment of quality-of-life (QOL) data for individuals with RTT and their parents/caregivers. For the QOL assessments, we used existing instruments as no RTT-specific measures were available. For affected individuals, we used the Child Health Questionnaire and for parents/caregivers we used the SF-36 questionnaires. Despite their generic qualities, these two instruments provided valuable information.
All data were submitted to the Data Technology and Coordinating Center at the University of South Florida as established for these studies by the NIH. Data sharing across all consortium sites was provided through agreement with the NIH. As a result, data sharing occurred beginning five years after acquisition through dbGaP (database of Genotypes and Phenotypes; National Library of Medicine—National Center for Biotechnology Information).
During the initial phases of the study, we also completed analysis of CSF 5-methyltetrahydrofolate levels,23 the presence of osteopenia,24 and a retrospective analysis of gall bladder dysfunction in individuals with RTT.25
Rett syndrome NHS protocol 5211: 2015–2021
Enrollment goals
The third round of funding only included individuals with RTT or the three RTT-related disorders: CDD,7, 26 FD,8 and MDD.27, 28 Individuals with each disorder were actively enrolled through collaboration with the specific organizations/foundations that exist for each syndrome (IRSF; International FOXG1 Foundation, International Foundation for CDKL5 Research).
The enrollment goals were 1000 individuals with RTT, 100 with MDD, and 50 each with CDD and FD (Table 1). As in the initial phase of the NHS, specific measures were assessed at each visit, but the visit intervals were changed. Assessments were scheduled annually for the first 10 years, biannually from 11 through 2 years, and twice for those older than twenty years.
Major revision of enrollment forms
During the last seven years of the NHS, data gathering expanded to include specific elements such as diagnostic testing, developmental features, mother's pregnancy, sibling information, diet, abnormal movements, chronic medical diagnoses, specific features of RTT, seizures, infections and allergies, surgery, laboratory studies, imaging studies, electroencephalogram (EEG), electrocardiogram, and involvement in other research. In addition, we developed an interval history form to gather information on family characteristics including education, occupation, and income; overall function of the individual in the study including hand use, communication, sitting, standing, and walking; RTT features (hand stereotypies, breathing issues, bruxism, sleep, and toilet training); behavior; specific issues (pain, constipation, gastroesophageal reflux, hand/foot temperature, and motor issues); and scoliosis (presence and progression). In addition, we assessed the parent/caregiver top three concerns for their child developed from a list of 24 items plus an opportunity to provide free-text answers.
The efforts to obtain socioeconomic data that were broadly representative of socioeconomic status were compromised as individuals who responded to this question were overrepresented in the average or above income ranges, suggesting a bias against those with lesser means or ability to reach the enrollment sites.
Major goals
Four specific areas of study were accomplished: (1) 5211—longitudinal assessments for RTT, CDD, FD, and MDD; (2) 5212—neurophysiological assessments (EEG, visual evoked potentials, and auditory evoked potentials); (3) biomarker assessment of various biochemical features; and (4) 5213—biobanking of DNA, RNA, and serum. Total enrollment for protocol 5211 was 1035, including 662 with classic RTT and 89 with atypical RTT, 77 with MDD, 73 with CDD, and 65 with FD; 187 for 5212, including 45 normal controls; and for 5213, 305 probands, 392 parents, and 44 siblings (Table 5).
| Diagnosis | All | 5201 | 5211 (total) | 5211 (unique) | ||
|---|---|---|---|---|---|---|
| Classic RTT | 1282 | 961 | 662 | 321 | ||
| Atypical RTT | 218 | 157 | 89 | 61 | ||
| MECP2 positive, non-RTT | 115 | 62 | 65 | 53 | ||
| MDD | 100 | 36 | 77 | 64 | ||
| CDD | 73 | 0 | 73 | 73 | ||
| FD | 65 | 1 | 65 | 64 | ||
| FD Duplication | 3 | 0 | 3 | 3 | ||
| Other | 1 | 0 | 1 | 1 | ||
| Total | 1857 | 1217 | 1035 | 640 | ||
| Diagnosis | 5212 | |||||
| Classic RTT | 85 | |||||
| MDD | 17 | |||||
| CDD | 28 | |||||
| FD | 12 | |||||
| Controls | 45 | |||||
| Total | 187 | |||||
| Diagnosis | 5213 | |||||
| Probands | Parents | Siblings | ||||
| Classic RTT | 191 | 255 | 34 | |||
| Atypical RTT | 22 | 31 | 2 | |||
| MECP2 positive, non-RTT | 25 | 26 | 2 | |||
| MDD | 26 | 31 | 3 | |||
| CDD | 22 | 22 | 1 | |||
| FD | 19 | 27 | 2 | |||
| Total | 305 | 392 | 44 | |||
- Abbreviations: CDD, CDKL5 deficiency disorder; FD, FOXG1 disorder; MDD, MECP2 duplication disorder; NHS, Natural History Study; RTT, Rett syndrome.
Visit compliance throughout the NHS
In 5201 (2003–2014), overall participant compliance through the nine years of enrollment was 91.0%. In 5211 (2015–2021), overall compliance for seven years of enrollment was 91.8%. Both figures are considered good to excellent. Inasmuch as the period from 2020 through early 2021 encompassed the COVID pandemic, this is regarded as exceptionally good. The average number of visits across both studies was 5.2 (range 1.0–18).
Review of major findings for RTT-specific outcomes
Over the course of the 16 years covering both protocols, the overall enrollment was more than 1600 females with RTT and only 30 males who had MECP2 variants. For CDD, FD, and MDD, we had 70 individuals each. Thus, data analysis is heavily weighted to RTT.
Data from the RTT and RTT-related disorders NHS have yielded many publications over the past 20 years. Continued assessment of the accumulated data is expected to yield additional publications. Nearly 60 peer-reviewed publications have appeared with several others under review or in varying stages of preparation. These reports involve core outcome measures anticipated from the NHS, including clinical features, genetics, clinical trial readiness information, biomarkers, and specific features as well as comparisons derived from the RTT-related disorders: CDD, FD, and MDD. More than a dozen chapters and reviews also significantly explore and expand on information from the NHS. In addition, the recent and ongoing clinical trials have resulted in publications that derive much of their background information from the NHS, including the paper describing the first pharmaceutical product approved by the FDA specifically for individuals with RTT.29, 30
Clinical features
Developmental delay
Early development appears normal in RTT but may be more apparent than real. Developmental delays in four realms, gross and fine motor skills and receptive and expressive communication, were examined in classic and atypical RTT and linked to phenotype-genotype correlations in classic RTT.19
Epilepsy prevalence and trends
Epilepsy has been reported in 50%–80% of individuals with RTT. Reports from the NHS study characterized the clinical spectrum and natural history of RTT in advance of clinical trials.31 A subsequent study of epilepsy in RTT examined factors not previously analyzed, namely, the longitudinal course of epilepsy and the patterns of seizure onset and remission.32 Lifetime prevalence of epilepsy was reported in 90%, with point prevalence being about 30%–44%. While daily seizures are uncommon in RTT, prolonged remission is less common than in other causes of childhood-onset epilepsy. Complete remission off antiseizure medications is possible.
Hand issues
Hand function and hand stereotypies (HS) are considered separately as they reflect different functional underpinnings. Hand function refers to the apraxia and specific fine motor capabilities. HS represent a movement disorder characterized by repetitive, nonpurposeful movements that are quite different from fine motor function. It is also evident that these stereotypies may involve, to a far lesser extent, the orofacial muscles and the feet. Most importantly, longitudinal progress for hand function and HS differs.
Hand function
Representing the continued progression of RTT features, hand function continues to decline through childhood and adolescence correlating with MECP2 variant groupings.33 Those with milder variants showed steeper declines compared with more severe variants through age 18 years.
Hand stereotypies
Hand stereotypies were analyzed in 1123 girls and women enrolled in the RTT NHS.34 HS were reported in 922 participants with classic RTT (100%), 73 with atypical severe RTT (97.3%), 74 with atypical mild RTT (96.1%), and 17 females with MECP2 variants without RTT (34.7%). Individuals with RTT who had classic presentation or severe MECP2 variants had higher frequency and earlier onset of HS. While hand function decreased over time, prevalence and frequency of HS remained relatively unchanged.
Growth and development
Growth charts
Prominent growth failure typifies RTT. Analysis of growth data allowed the development of RTT-specific growth charts (height, weight, body mass index, and head circumference) to compare growth in children with RTT with that of unaffected children, and to compare growth patterns among RTT genotypes and phenotypes.35 Poor growth was associated with worse development and greater disease severity and correlated with genotype.
Anthropometry
To characterize growth and anthropometric measurements in females with classic and atypical RTT and to correlate these measurements with functional motor outcomes, we systematically obtained longitudinal growth measurements from 1154 girls and women with classic and atypical RTT.36 Arm and leg circumference, skinfold thickness, and muscle area measurements were significantly lower in females with classic and severe atypical RTT compared with mild atypical RTT and deviated from normative patterns of growth. These growth patterns and anthropometric measures in females with RTT differ significantly from normal while demonstrating clear differences between classic and mild or severe atypical RTT. These results also correlate with functional motor outcomes and possibly could provide biologic markers that may support efficacy outcomes in clinical trials.
Sexual maturation
Pubertal trajectories in females with RTT deviate from the general female population with early pubertal onset and delayed menarche.37 Compared with the general female population, more than 25% initiated puberty early yet entered menarche six months later than normal (median age 13.0 years).
Gastrointestinal (GI) issues
Impaired GI function
A nationwide survey was conducted to determine the prevalence of common GI and nutritional disorders related to age and MECP2 gene status. This survey revealed the following levels of involvement: GI dysmotility including constipation (92%), chewing and swallowing difficulties (81%), weight deficits or excess (47%), growth deficits (45%), low bone mineral content or fractures (37%), and biliary tract disorders (3%).38 A separate study determined the prevalence of vitamin D deficiency and identified the relation between 25-hydroxyvitamin d (25-(OH) D) levels and the consumption of dietary sources of vitamin D or exposure to anticonvulsants in girls and women with RTT.39
Biliary tract disease
Sixty-two individuals with RTT (median age was 21 years; range was 3 to >40 years) and biliary tract disease were identified, a prevalence of biliary tract disease of 4.4%.25 Presenting symptoms included abdominal pain (94%), irritability (88%), weight loss (64%), and vomiting (52%). Biliary dyskinesia, cholecystitis, and cholelithiasis were identified in 90% by cholescintigraphy.
Autonomic/physiological
Awake breathing patterns and trends
RTT is associated with a peculiar breathing disturbance occurring most impressively during wakefulness that is distressing and can even prompt emergency resuscitation. Through the RTT NHS, cross-sectional and longitudinal characteristics of awake breathing abnormalities in RTT were examined.40 Awake breathing dysfunction is common in RTT (>90%), more so than seizures, and correlates with overall function, quality of life, and risk for cardiac dysrhythmia.
Cardiac arrhythmias
One quarter of deaths associated with RTT are sudden and unexpected.41 RTT is associated with prolonged QTc interval (long QT syndrome), and cardiac arrhythmias associated with long QT syndrome are a potential cause of unexpected death.
Urinary tract
In a separate study, a subset of individuals with RTT presenting with urological dysfunction including frequent urinary tract infections, kidney stones, and urine retention requiring frequent catheterization for bladder voiding were identified and correlated with evidence of urological dysfunction in a mouse model of RTT.42 Taken together, these findings identify additional phenotypes caused by loss of MECP2.
Scoliosis
Scoliosis prevalence and surgical impact
Scoliosis was identified in most individuals with RTT, with surgical correction in 12%.43 Two common MECP2 variants, R294X and R306C, had reduced risk for scoliosis. These findings corroborated previous reports on scoliosis and extended understanding of comorbidities, clinical severity, and relative risk reduction for specific variants. A subsequent study confirmed these findings and reported that surgical correction increased to more than 18%.44
Sleep
Sleep behavior was characterized in Rett, Angelman, and Prader-Willi syndromes to identify effective approaches for treating sleep problems in these populations.45 Inasmuch as individuals with RTT, Angelman syndrome, and Prader-Willi syndrome frequently experience sleep problems, including sleep-disordered breathing, screening for sleep problems in these and other neurogenetic disorders should be considered in treatment strategies as well as future clinical trials.
Behavior
Behavioral profiles
RTT is a complex neurodevelopmental disorder with known behavioral abnormalities, both internalizing (e.g., anxiety and social withdrawal) and externalizing (e.g., aggression and self-injurious behavior). The evaluation of behavioral features could guide future treatment strategies.46 Anxiety-like behavior and anxiolytic treatments along with associated clinical features occur in most (77.5%) individuals with RTT.47 Increased anxiolytic use, including a selective serotonin reuptake inhibitor (SSRI), were related to more frequent anxiety-like behaviors (p < 0.001), older age (p < 0.001), and mild MECP2 variants (p = 0.002).
Age at diagnosis and risk factors for late recognition
Diagnosis of RTT is often delayed.48 Since the most recent elaboration of diagnostic criteria,21 median age of diagnosis decreased from greater than 3.7 years to 2.5 years. The diagnosis among subspecialists represented 89.7% of the total; the diagnosis among pediatricians was 5.2%.48 Strategies for educating diagnosticians should include specific factors that account for delayed diagnosis.
Longevity
Analysis of the RTT NHS and the extensive North American RTT Database allowed the examination of longevity in a large cohort of individuals from the United State and Canada and indicated that 50% survival exceeded 50 years of age.49 These results provide strong evidence for significant longevity in RTT and indicates the need for careful planning for long-term care of these women. Additionally, the overall survival is at least double that described for the original group evaluated by Andreas Rett in Vienna beginning around 1960.50
Consensus clinical management guidelines
The absence of a peer-reviewed, consensus-based therapeutic guidance for care in RTT led to the development of a consensus guidance of best practice.51 Separately, a review of the literature allowed for comorbidities in RTT to be identified.52
Revised diagnostic criteria in collaboration with international experts
The accumulation of clinical and molecular information in recent years has generated considerable confusion regarding the diagnosis of RTT. Thus, the 2002 consensus criteria were revised in anticipation of treatment trials21 to reinforce the concept that RTT is a clinical diagnosis based on distinct clinical criteria, independent of molecular findings. Further, the analysis of 819 participants enrolled in the RTT Natural History Study allowed validation of the revised criteria.22
Genetics
Genotype/phenotype
A strong relationship exists between the clinical features of RTT and specific variants in MECP2.18 The R133C, R294X, R306C, and late carboxy-terminal (3′) truncating variants demonstrated less severe involvement than R106W, R168X, R255X, and R270X (p < 0.05). Clinical differences were notable in ambulation, hand use, and language (p < 0.004), three cardinal features of RTT. Thus, specific variants in MECP2 confer different severity and should be considered in clinical intervention trials.
Expanding on these results, Cuddapah et al.53 confirmed the above findings using a larger database of 1052 participants assessed in more than 4940 unique visits and added new information demonstrating that clinical severity increased with age regardless of mutation type.
X chromosome inactivation
The MECP2 gene is subject to X chromosome inactivation (XCI).54 Highly or moderately skewed XCI patterns were observed in 36% (n = 45/125) of classic RTT probands and 20% (n = 3/15) of CDD probands, much higher than that of the moderately/highly skewed XCI in phenotypically unaffected newborns and adults. These data demonstrate that X chromosome skewing is relatively common in classic RTT compared with unaffected individuals. The paternal allele is preferentially inactivated in classic RTT individuals. In contrast, the maternal allele is preferentially inactivated in CDD individuals. This is the first report documenting the parental origin of preferential inactivation for individuals with CDD and may explain the lack of clear phenotypic severity differences in boys and girls with CDD.
Parental age
Parental age was not associated with increased risk for occurrence of RTT, nor is it related to clinical severity of those with RTT.55
Other genes producing RTT
Twenty-two RTT females without apparent MECP2, CDKL5, and FOXG1 variants were evaluated using both whole-exome sequencing and single-nucleotide polymorphism array–based copy number variant analyses.56 Three had MECP2 variants initially missed by clinical testing. Of the remaining 19, 17 (89.5%) had 29 other likely pathogenic intragenic variants and/or copy number variants (10 individuals had two or more).
MECP2 variants in males
It had been suspected that variants in MECP2 led to embryonic lethality in males. However, males have been reported in numerous small series. We identified 30 males with MECP2 variants in the RTT NHS.9 A wide phenotypic spectrum was observed, ranging from severe neonatal encephalopathy to cognitive impairment. Two males with somatic mosaicism and a variation in MECP2 in one X chromosome had classic RTT.
Gene reference panel
In a collaboration with the Centers for Disease Control and Prevention's Genetic Testing Reference Materials Coordination Program, the genetic testing community, and the Coriell Cell Repositories, 27 new cell lines were established and the MECP2 variants characterized in these and in eight previously available cell lines.57 These MECP2 and other characterized genomic DNA samples are publicly available from the NIGMS Repository at the Coriell Cell Repositories.
Recommendation for gene variant interpretation
The advent of widely available large-scale clinical sequencing has amplified the need for accurate variant interpretation. To create a professional standard for variant interpretation, the American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) established a framework for variant classification.58 These guidelines provide an evidence-based, albeit generalized, framework to capture and evaluate evidence consistently from a variety of sources used for variant interpretation (see the Clinical Genome Resource—ClinGen; http://www.clinicalgenome.org).
Establishing clinical trials
Quality-of-life (QOL) assessments
Examination of QOL was assessed in those with RTT.59 Severe clinical impairment was highly associated with poor physical QOL, but worse motor function and earlier age at onset of RTT stereotypies were associated with better psychosocial QOL; conversely, better motor function was associated with poorer psychosocial QOL. This suggested that improvement of motor function in clinical trials could have adverse consequences on behavior. Subsequently, caregiver quality of life was examined.60 The clinical features of RTT clearly impact caregivers’ quality of life, with physical component scores being better than mental component scores.
Top caregiver concerns
The top caregiver concerns for classic RTT were lack of effective communication, seizures, walking/balance issues, lack of hand use, and constipation.61 These concerns did vary by age, clinical severity, and specific mutations, which is consistent with the known variation throughout these domains. Across disorders, caregivers rated communication to be a greater or similar concern to epilepsy. Comparison of the concerns across the four disorders (RTT, CDD, FD, and MDD) revealed significant commonalities but also specific differences related to the impact of core features for each.
Caregiver burden inventory
RTT requires total caregiver attention and leads to potential difficulties throughout life. The Caregiver Burden Inventory, designed originally for Alzheimer disease,62 was modified to a RTT Caregiver Inventory Assessment, providing a needed metric of caregiver burden in this disorder.63
Clinical global impression anchors
The development of RTT-specific anchors for the CGI Scales represents a concerted effort to have high-quality severity and improvement outcome measures, specific to the core symptoms of RTT.64 The CGI-S Scale is a measure of global measure of severity and the CGI-I Scale is a measure of improvement to a given agent. As such, we developed key anchors specific for RTT for the CGI-S Scale and used a similar 7-point Likert scale for the CGI-I. These have been used in clinical trials leading to FDA approval of an RTT-specific medication—trofinetide.29, 30
Revised Motor Behavioral Assessment (MBA)
The need to develop clinical trial outcome measures in RTT led us to examine the factor structure, internal consistency, and validity of the clinician-reported MBA scale using data from the RTT NHS (n = 1075 participants).65 This analysis resulted in a five-factor model: (1) motor dysfunction, (2) functional skills, (3) social skills, (4) aberrant behavior, and (5) respiratory behaviors and the development of a revised MBA. Subsequent validation of the revised MBA could provide an effective outcome measure.
Rett Syndrome Caregiver Assessment of Symptom Severity (RCASS)
The RCASS is a newly developed outcome measure that has strong factor analysis, both exploratory and confirmatory, reliability, and validity in a four-factor model. It also has convergent validity with the revised MBA, the CSS, the CGI Scale, and the Child Health Questionnaire. Future testing is needed to evaluate sensitivity and reliability.66
Biomarkers
Metabolomics
A significant challenge in RTT is the lack of biomarkers of disease state, disease severity, or treatment response. Using a nontargeted metabolomic approach,67 66 significantly altered metabolites were identified. These cluster broadly into amino acid, nitrogen handling, and exogenous substance pathways pointing to oxidative stress, mitochondrial dysfunction, and alterations in gut microflora, offering a basis for identifying disease severity biomarkers.
Neurophysiology
The neurophysiology protocol was modified to focus on both visual and auditory event-related potentials (ERP) and quantitative EEG from resting/background recordings. This project was conducted at five sites, all of which provided similar testing methodology that were synchronized with a human phantom (Dr. Tim Roberts from the University of Pennsylvania). Human phantom results demonstrated similar quality of data acquisition at all five sites. This project was led by Dr. Eric Marsh at Children's Hospital of Philadelphia/University of Pennsylvania, the remaining sites being at Boston Children's Hospital (Harvard), Vanderbilt University, Cincinnati Children's Hospital, and the University of Colorado-Denver. Dr. Marsh and colleagues at Children's Hospital of Philadelphia were responsible for data analysis. This project was delayed by two years due to the U54 review structure such that sufficient time remained for the initial evaluation of participants and age-matched sibling controls and a smaller cohort of this initial group based on one or two repeated studies.
ERPs were evaluated as potential biomarkers of cortical function in RTT.68 These studies indicated that ERPs could provide unbiased assessment of disease staging and potentially aid in prognosis and response to therapy, providing foundational information for their use as biomarkers for future clinical trial assessments. Quantitative EEG, particularly alpha/delta frequency ratios, also demonstrated correlations to RTT severity and potential use as a biomarker for clinical trials.69-71
Additional studies examined the same parameters across all populations evaluated in the 5212 protocol.70, 71
Other disorders
Cross-comparisons
Comparisons of core clinical features across RTT, MDD, CDD, and FD
RTT, CDD, FD, and MDD share clinical features with distinct differences. A head-to-head comparison of clinical features in these conditions72 provided distinct patterns of clinical severity, seizure onset age, and regression. Individuals with CDD were the most severely affected and had the youngest age at seizure onset (2 months), whereas children with MDD had the oldest median age at seizure onset (64 months) and lowest severity scores. RTT and FD were intermediate in both features. Smaller head circumference correlated with increased severity in all disorders and earlier age at seizure onset in MDD. Developmental regression occurred in all RTT participants (median = 18 months) but in only 23%–34% of the other disorders. Seizure incidence was highest for CDD (96.2%) and lowest for RTT (47.5%). These developmental encephalopathies share many clinical features but have clear differences in severity, regression, and seizures.
Comparison of evoked potentials across RTT, CDD, FD, and MDD
Consistent abnormalities across these four disorders were reported for both visual and auditory evoked potentials.70 Specific differences were documented that require further refinement and validation.
CDKL5/CDD
Current treatment and emerging therapies
CDD is associated with refractory infantile onset epilepsy, global developmental delay, and variable features that include sleep issues, behavioral disturbances, and movement disorders. Current treatment is primarily symptom-based. We described medication and nonmedication approaches to treatment of epilepsy and three additional key neurological symptoms (sleep disturbances, behavioral issues, and movement disorders) in a cohort of 180 individuals meeting criteria for CDD, 157 evaluated at three CDD Centers of Excellence in the United States and 40 identified through the NIH NHS of Rett and related disorders.73
CDD neurophysiologic biomarkers
Promising therapeutics are under development for CDD, but the lack of validated biomarkers of brain function and disease severity may limit the ability to assess the efficacy of new treatments objectively. To address this need, the current study quantified electrophysiological measures in individuals with CDD and the association between these parameters and disease severity.69 Group level comparisons revealed reduced visual, but not auditory, evoked potential amplitude and a variety of altered qEEG features in CDD versus controls. Auditory evoked potential amplitudes correlated with clinical severity in CDD.
CDKL5 clinical severity assessment
Pathologic variations in cyclin-dependent kinase-like 5 cause CDD, a genetic syndrome associated with severe epilepsy and cognitive, motor, visual, and autonomic disturbances. A severity assessment was developed based on clinical and research experience from the International Foundation for CDKL5 Research Centers of Excellence consortium and the NHS.74 The NHS and the CDD outcome measure were foundational for the current U01 (NS114312-04, NCT05558371).
MDD
Characterizing effects of Xq28 duplication size in MDD
Individuals with MDD have varying degrees of severity in their mobility, hand use, developmental skills, and susceptibility to infections. Other genes typically duplicated within Xq28 (e.g., GDI1, RAB39B, and FLNA) are associated with distinct clinical features independent of MECP2. Using existing indices of clinical severity in RTT syndrome, we found that larger duplication size correlated with greater severity in total CSS (r = 0.36; p = 0.02) and in total MBA inventory scores (r = 0.31; p = 0.05).75 Greater severity was also associated with having the RAB39B gene duplicated.
Phenotypic features of MDD related to age
Although phenotypic features in MDD have been described, we have a limited understanding of the range of severity of these features and how they evolve with age. The cross-sectional results of 69 participants (ages 6 months through 33 years) with MDD enrolled in the NHS used clinical severity as a clinician-report measure.76 Overall CSS, motor dysfunction, and functional skills were significantly worse with increasing age.
Other items under review
Additional data from the NHS are in varying stages of review for potential publication. These include the course of ambulation across the lifespan, the patterns of dystonia with advancing age, an analysis of therapies for specific medical issues such as seizures, GI difficulties, bone health, sleep, individuals who have MECP2 variants but do not have RTT, older women with RTT, and males with MECP2 variants. Other matters of interest may arise that can be explored through the extensive database.
NHS database
The NHS database has been migrated to IRSF, where it may be accessed through a contractual arrangement by professional organizations or individual researchers with specific questions for which access to the data may be facilitated.
Clinical trial readiness
Clinical trials are the essential endpoint of the NHS and have become increasingly emphasized by basic and clinical research. They are also the goal of families affected by these neurodevelopmental disorders. During the past 18 years, the NHS has promoted the creation of clinical trial readiness for RTT and, with the expansion under 5211 to include MDD, CDD, and FD, for these disorders as well. In keeping with this effort, members of the NHS modified the existing CGI-S and the accompanying CGI-I as global measures to assess and assign the severity of involvement as well as potential improvement or decline during a clinical trial.64, 77
Clinical trial efforts
Prior clinical trials in RTT were limited to investigator-initiated studies involving one or only a few clinical sites. Three principal industry-sponsored trials have been conducted within the past 10 years in this country. The repurposed drug sarizotan was evaluated under the direction of Newron Pharmaceuticals, the principal target being the significant issues associated with periodic breathing. Unfortunately, this trial did not achieve clinically significant results. Two additional trials have involved drugs tested specifically in animal (mouse) models for RTT.
The first, based on animal studies by Tropea et al.,78 was sponsored by Neuren Pharmaceuticals, who conducted FDA-approved phase 2 studies in adults and then children in brief clinical exposures of four and six weeks, respectively. Both trials demonstrated positive clinical improvement. As a result, Neuren contracted with Acadia Pharmaceuticals to conduct a phase 3 study in individuals ages 5–20 years using the CGI-I as the primary clinician-reported measure and the Rett Syndrome Behavioral Questionnaire as the primary caregiver reported measure.29, 79 This trial resulted in FDA approval of trofinetide in March 2023.29, 30
The second trial, sponsored by Anavex Pharmaceuticals, has been completed in adults and children in the United States, Europe, and Australia. Unfortunately, the pediatric trial in the United Kingdom, Canada, and Australia failed to show significance in the primary outcome marker, the Rett Syndrome Behavioral Questionnaire.
An investigator-initiated phase 2 trial with the FDA-approved agent ketamine was completed, the results of which are being prepared for publication.
Future directions
The NHS efforts provided the groundwork for development of clinical trial readiness by defining the clinical features and disease progression of RTT, establishing a network of clinical research sites capable of conducting clinical research in RTT, and developing putative biomarkers and outcome measures. These research efforts have promoted several completed and ongoing clinical trials involving pharmaceutical agents, and clinical trials evaluating gene therapy have recently been initiated by Taysha Gene Therapies and Neurogene. Importantly, the completed, ongoing, and proposed trials rely heavily on the clinical trial readiness established over the life of the NHS.
Lessons learned
RTT was recognized more than 60 years ago by Andreas Rett in Austria1 and Bengt Hagberg in Sweden. Andreas Rett was responsible for the early efforts to explore this unique and fascinating disorder. In 1983, Hagberg et al.2 provided the first widely read paper in which Hagberg termed "Rett syndrome" for the first time. Since then, significant international attention has developed for RTT and RTT-related disorders, including the identification of unique genetic signatures not only for RTT but also for CDD, FD, and FDD.
Through the efforts of Congress and federal agencies at the FDA and NIH, rare disorder research has dramatically accelerated in the last 20 years. As part of this process, the natural history of RTT and its related disorders was explored, leading to remarkable progress in our understanding of these disorders, expansion of clinical expertise, and in this country the creation of Centers of Excellence through the International Rett Syndrome Foundation. The long-term goal of specific treatments for RTT is now within reach. The approach taken with this NHS journey has many lessons than can be extended to other rare neurological disorders. The most important lesson has been the intangible satisfaction of developing productive collaborations and relationships among clinical investigators and patient-affiliated groups that are focused on a common good of improving the lives of people and families with RTT and related disorders.
Would we do it differently? By all means. In hindsight, several issues could have been improved. First, we should have insisted on a database that would have made the outcomes available more widely, both to families and to the broader research community. Second, improved geographic coverage and participant diversity, especially for underrepresented racial, ethnic, and socioeconomic status groups, is an important lesson learned. Although efforts were made throughout this study, such as expanding the number of enrollment sites, the diversity was unfortunately limited, which may relate to the requirement for participants to travel to enrolling sites without compensation. Potential mechanisms to address these limitations include incorporation of more sites, reimbursement of travel expenses for participants, and the development and systematic use of remote assessment methods to allow for broader participation. Third, although we were successful in developing outcome measures and putative biomarkers, much of this work was conducted later within the study period. This should have had greater emphasis earlier in the process. Unfortunately, the use of more formal psychometric evaluations was limited by funding. Fourth, we could have used remote data capture, such as via virtual assessments, at-home caregiver surveys, and wearable devices. Lastly, we should have broadened the collection of data earlier in the study to include biomarkers, neurophysiologic parameters, and systematic capture and inclusion of clinical electronic medical records. Importantly, the methodology to address the limitation and lessons learned has dramatically improved in the two decades since the initiation of the NHS, and future natural history studies in rare disorders such as RTT should consider including these methods or methodologies being developed. Despite the limitations and lessons learned, the NHS represents a leap forward in this group of rare disorders and provides not only valuable knowledge on RTT and related disorders, but also a guide for the design and implementation of future natural history studies in other rare disorders.
Author contributions
Alan K. Percy: Conceptualization; data curation; formal analysis; funding acquisition; investigation; methodology; project administration; resources; supervision; validation; writing—original draft; writing—review & editing. Timothy A. Benke: Conceptualization; formal analysis; investigation; resources; validation; writing—review & editing. Eric D. Marsh: Conceptualization; formal analysis; investigation; resources; validation; writing—review & editing. Jeffrey L. Neul: Conceptualization; data curation; formal analysis; funding acquisition; investigation; project administration; resources; supervision; validation; writing—review & editing.
Acknowledgment
The US NHS was funded by the NIH and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (HD061222). No funding was involved in the publication.
Conflict of interest statement
Alan K. Percy has a consulting relationship with Acadia Pharmaceuticals, Anavex, Ionis, Neurogene, and Taysha and has previously had a contract researcher relationship with Acadia Pharmaceuticals. Timothy A. Benke has received research funding from GRIN2B Foundation, the International Foundation for CDKL5 Research, Loulou Foundation, the National Institutes of Health, and Simons Foundation; consultancy for Alcyone, AveXis, GRIN Therapeutics, GW Pharmaceuticals, the International Rett Syndrome Foundation, Marinus Pharmaceuticals, Neurogene, Ovid Therapeutics, and Takeda Pharmaceutical Company Limited; clinical trials with Acadia Pharmaceuticals Inc., GW Pharmaceuticals, Marinus Pharmaceuticals, Neurogene, Ovid Therapeutics, and Rett Syndrome Research Trust; all remuneration has been made to his department. Eric D. Marsh is site PI for Stoke Therapeutics, Zogenix Pharmaceuticals, Acadia Pharmaceuticals, Marinus Pharmaceuticals, Takeda Pharmaceuticals, Epygenix Pharmaceuticals, SK Life Pharmaceuticals, and Ionis Pharmaceuticals. He has received research support from the NIH, Penn Orphan Disease Center, International Rett Syndrome Foundation, International Foundation for CDKL5 Research, and Curaleaf Inc.; is a consultant for Acadia Pharmaceuticals and Novartis Pharmaceuticals; and has prepared an educational program for Medscape and France Foundation. Jeffrey L. Neul has received research funding from the National Institutes of Health, the International Rett Syndrome Foundation, and Rett Syndrome Research Trust; served as investigator for clinical trials conducted by Acadia Pharmaceuticals, GW Pharmaceuticals, Neuren Pharmaceuticals, and Newron Pharmaceuticals; received personal consultancy for Acadia Pharmaceuticals Inc, Analysis Group, Anavex, AveXis, GW Pharmaceuticals, Hoffmann-La Roche, Ionis Pharmaceuticals, Myrtelle, Neurogene, Newron Pharmaceuticals, Signant Health, and Taysha Gene Therapies, and the preparation of CME activities for the France Foundation, MedEdicus, Medscape, PeerView Institute, and TotalCME; served on the scientific advisory board of Alcyone Lifesciences; was a scientific cofounder of LizarBio Therapeutics; and a member of a data safety monitoring board for clinical trials conducted by Ovid Therapeutics and Ultragenix.
