The Development of Adaptive Behavior in Toddlers and Preschoolers with Fragile X versus Autism

Research Gaps in Fragile X Syndrome: An Updated Literature Review to Inform Clinical and Public Health Practice

OBJECTIVE

The phenotypic impact of fragile X syndrome (FXS) has been well-documented since the discovery of the fragile X messenger ribonucleoprotein 1 gene 30 years ago. However, gaps remain in clinical and public health research. The purpose of this literature review was to determine the extent to which these gaps have been addressed and identify targeted areas of future research.

METHODS

We conducted an electronic search of several scientific databases using a variety of key words. The search focused on 5 areas identified as research gaps by an earlier review: (1) diagnosis, (2) phenotypic presentation, (3) familial impact, (4) interventions and treatments, and (5) life span perspectives. Inclusion criteria included publication between 2014 and 2020, focus on human subjects, and publication in English. A total of 480 articles were identified, 365 were reviewed, and 112 are summarized in this review.

RESULTS

Results are organized into the following categories: (1) FXS phenotype and subtypes (FXS subtypes, medical profile, cognitive/developmental profile, social and behavioral profile); (2) needs of adults; (3) public health needs (clinical diagnosis and newborn screening, health care needs, and access); (4) treatment (treatment priorities, pharmacological treatments, and behavioral and educational interventions); and (5) families (economic burden and mother-child relationship).

CONCLUSION

Despite the progress in many areas of FXS research, work remains to address gaps in clinical and public health knowledge. We pose 3 main areas of focused research, including early detection and diagnosis, determinants of health, and development and implementation of targeted interventions.

Adaptive behavior in infants and toddlers with Down syndrome and fragile X syndrome

Individuals with Down syndrome (DS) experience deficits across all domains of adaptive functioning, however little is known about the emergence and age-related changes of these impairments compared to other neurogenetic disorders with similar intellectual disability impairments, such as fragile X syndrome (FXS). Adaptive behavior is key for optimal functioning in these populations. Participants aged 5-45 months comprised three age-matched groups, DS (n = 64), FXS (n = 69), and typically developing controls (TD; n = 69). Adaptive behavior was measured on the Vineland Adaptive Behavior Scales-II. Regressions were used to examine adaptive behavior in a cross-sectional design across age. DS infants and toddlers evidenced deficits across all areas of adaptive behaviors compared to the age-matched TD group, with clear impairments present in the first year of life. Motor skills were the area of greatest weakness in children with DS with significant impairment evident at 12 months of age that remained low through 3 years. Compared to age-matched children with FXS, children with DS showed initially lower standard scores at 12 months of age, but slower declines in standard scores across age, resulting in less impaired functioning at 36 months. This is the first study to compare adaptive behavior in infants and toddlers with DS to FXS, and demonstrate the phenotypic specificity of adaptive profiles in this diagnostic group. These findings provide evidence that adaptive behavior should be a major target of intervention in children with FXS and DS, and that these differences are potentially driven by unique etiologies attributable to each disorder.

The Emergence of Effortful Control in Young Boys With Fragile X Syndrome

Effortful control, or the ability to suppress a dominant response to perform a subdominant response, is an early-emerging temperament trait that is linked with positive social-emotional development. Fragile X syndrome (FXS) is a single-gene disorder characterized by hallmark regulatory impairments, suggesting diminished effortful control. This study compared the development of effortful control in preschool boys with FXS ( n = 97) and typical development ( n = 32). Unlike their typical peers, the boys with FXS did not exhibit growth in effortful control over time, which could not be accounted for by adaptive impairments, FMR1 molecular measures, or autism symptoms. These results contribute to our understanding of the childhood phenotype of FXS that may be linked to the poor social-emotional outcomes seen in this group.

Adaptive Skill Trajectories in Infants with Fragile X Syndrome Contrasted to Typical Controls and Infants at High Risk for Autism

BACKGROUND

Adaptive behaviors are essential for optimal outcomes and independence in individuals with developmental disabilities. This study examined longitudinal trajectories of adaptive behavior in infants with fragile X syndrome (FXS), compared to typical development (TD) and infant siblings of children diagnosed with autism (ASIBs).

METHOD

Participants included 76 male infants (FXS =25, ASIBs=27, TD = 24) assessed up to 4 times between 6 and 24 months of age for a total of 215 assessments of adaptive behavior. A sample of 12 females with FXS was included for a comparative sex analysis.

RESULTS

Results indicate that infant males with FXS displayed lower initial adaptive behavior across all domains that emerged by 9 months-of-age with slower growth rates than both comparison groups. A flat profile across the domains at 24 months was evident. Increased severity of autism symptoms was related to reduced adaptive skills at 24 months-of-age. Females with FXS displayed higher scores than males on the Socialization and Motor domains and equivalent scores on the Communication and Daily Living domains at 9 months-of-age with different rates of growth across domains.

CONCLUSIONS

This is the first study to provide evidence of etiological specificity in adaptive behavior profiles during infancy across two populations at high risk for ASD. These findings support targeted adaptive behavior interventions in young children with FXS beginning as early at 9 months of age to reduce identified deficits and the cascading impacts of these early impairments.

Updated report on tools to measure outcomes of clinical trials in fragile X syndrome

OBJECTIVE

Fragile X syndrome (FXS) has been the neurodevelopmental disorder with the most active translation of preclinical breakthroughs into clinical trials. This process has led to a critical assessment of outcome measures, which resulted in a comprehensive review published in 2013. Nevertheless, the disappointing outcome of several recent phase III drug trials in FXS, and parallel efforts at evaluating behavioral endpoints for trials in autism spectrum disorder (ASD), has emphasized the need for re-assessing outcome measures and revising recommendations for FXS.

METHODS

After performing an extensive database search (PubMed, Food and Drug Administration (FDA)/National Institutes of Health (NIH)'s www.ClinicalTrials.gov, etc.) to determine progress since 2013, members of the Working Groups who published the 2013 Report evaluated the available outcome measures for FXS and related neurodevelopmental disorders using the COSMIN grading system of levels of evidence. The latter has also been applied to a British survey of endpoints for ASD. In addition, we also generated an informal classification of outcome measures for use in FXS intervention studies as instruments appropriate to detect shorter- or longer-term changes.

RESULTS

To date, a total of 22 double-blind controlled clinical trials in FXS have been identified through www.ClinicalTrials.gov and an extensive literature search. The vast majority of these FDA/NIH-registered clinical trials has been completed between 2008 and 2015 and has targeted the core excitatory/inhibitory imbalance present in FXS and other neurodevelopmental disorders. Limited data exist on reliability and validity for most tools used to measure cognitive, behavioral, and other problems in FXS in these trials and other studies. Overall, evidence for most tools supports a moderate tool quality grading. Data on sensitivity to treatment, currently under evaluation, could improve ratings for some cognitive and behavioral tools. Some progress has also been made at identifying promising biomarkers, mainly on blood-based and neurophysiological measures.

CONCLUSION

Despite the tangible progress in implementing clinical trials in FXS, the increasing data on measurement properties of endpoints, and the ongoing process of new tool development, the vast majority of outcome measures are at the moderate quality level with limited information on reliability, validity, and sensitivity to treatment. This situation is not unique to FXS, since reviews of endpoints for ASD have arrived at similar conclusions. These findings, in conjunction with the predominance of parent-based measures particularly in the behavioral domain, indicate that endpoint development in FXS needs to continue with an emphasis on more objective measures (observational, direct testing, biomarkers) that reflect meaningful improvements in quality of life. A major continuous challenge is the development of measurement tools concurrently with testing drug safety and efficacy in clinical trials.