Razak et al. Journal of Neurodevelopmental Disorders (2020) 12:13 https://doi.org/10.1186/s11689-020-09310-9 REVIEW Open Access Developmental studies in fragile X syndrome Khaleel A. Razak1, Kelli C. Dominick2,3 and Craig A. Erickson2,3* Abstract Fragile X syndrome (FXS) is the most common single gene cause of autism and intellectual disabilities. Humans with FXS exhibit increased anxiety, sensory hypersensitivity, seizures, repetitive behaviors, cognitive inflexibility, and social behavioral impairments. The main purpose of this review is to summarize developmental studies of FXS in humans and in the mouse model, the Fmr1 knockout mouse. The literature presents considerable evidence that a number of early developmental deficits can be identified and that these early deficits chart a course of altered developmental experience leading to symptoms well characterized in adolescents and adults. Nevertheless, a number of critical issues remain unclear or untested regarding the development of symptomology and underlying mechanisms. First, what is the role of FMRP, the protein product of Fmr1 gene, during different developmental ages? Does the absence of FMRP during early development lead to irreversible changes, or could reintroduction of FMRP or therapeutics aimed at FMRP-interacting proteins/pathways hold promise when provided in adults? These questions have implications for clinical trial designs in terms of optimal treatment windows, but few studies have systematically addressed these issues in preclinical and clinical work. Published studies also point to complex trajectories of symptom development, leading to the conclusion that single developmental time point studies are unlikely to disambiguate effects of genetic mutation from effects of altered developmental experience and compensatory plasticity. We conclude by suggesting a number of experiments needed to address these major gaps in the field. Introduction affecting synthesis of many proteins including those Fragile X syndrome (FXS) is the most common inher- involved in synaptic pruning during development. ited cause of intellectual disability and most common Additional functions such as modulation of ion chan- single gene cause of autism. Common symptoms of nels have also been proposed [15, 29, 39]. A recent FXS include hyperactivity, repetitive behaviors and meta-analysis estimates the frequencies of individuals cognitive inflexibility, hypersensitivity to sensory stim- with the full mutation FXS allele to be approximately uli and seizures, social and language impairments, in- 1 in 7000 males and 1 in 11,000 females [75]. As an tellectual disability, and increased anxiety. FXS occurs X-linked disorder, the phenotypic expression is quite as a result of Fmr1 gene hypermethylation due to an variable in females because of random inactivation unstable CGG triplet repeat expansion (> 200 repeats) and potential compensation by the normal X chromo- in the 5′ untranslated region of the Fmr1 gene some [108]. Therefore, the prevalence of females who located on the X chromosome, which leads to gene carry the full mutation allele and display the pheno- methylation, inactivation, and resultant loss of fragile typic features of FXS is less than 1 in 11,000 [75]. X mental retardation protein expression (FMRP; The Fmr1 knockout (KO) mouse has been the most [147]). FMRP functions as a translational regulator, widely studied animal model of FXS, but other model systems such as drosophila and Fmr1 KO rats have also contributed valuable information regarding basic bio- * Correspondence: [email protected] 2Department of Psychiatry and Behavioral Neuroscience, University of logical functions of FMRP and mechanisms of patho- Cincinnati, Cincinnati, OH, USA physiology when FMRP is removed. Over the past two 3 ’ Division of Child and Adolescent Psychiatry, Cincinnati Children s Hospital decades, considerable progress has been made in terms Medical Center, 3333 Burnet Avenue MLC 4002, Cincinnati, OH 45229, USA Full list of author information is available at the end of the article of our understanding of symptomology and underlying © The Author(s). 2020 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Razak et al. Journal of Neurodevelopmental Disorders (2020) 12:13 Page 2 of 15 neurobiology in humans with FXS and animal models. literature on animal models. The major point made Because of the monogenic origin of the disorder and sig- bythesestudiesisthatmanyofthephenotypicdiffer- nificant overlap with autism, the interest in FXS is at ences between KO and WT mice are developmentally least partly due to the potential for discoveries about transient, suggesting the importance of studying tra- FXS to serve as a paradigm for studies of neurodevelop- jectories. This is all the more relevant if altered brain mental disorders more broadly. responses attributed to an imbalance in excitatory While several potential therapeutics show efficacy in and inhibitory function reflects compensatory plasti- reversing symptoms in the Fmr1 KO mice, they have city, and is not necessarily the primary defect driven failed clinical trials [12]. A number of reasons may by the genetic mutation causing neurodevelopmental explain these failures including the validity of the pre- disorders [4, 102, 136]. clinical models, validity of outcome measures, insuffi- cient stratification of patients, optimality of dose and Developmental regulation of FMRP expression duration, and finally, appropriate treatment age. To FMRP levels are developmentally regulated [46] in the address optimal treatment ages, we focused on devel- mouse brain. Peak FMRP expression during develop- opmental studies of both humans and mice to gener- ment occurs between P3 and P12 in different brain re- ate hypotheses based on published data. FXS is a gions and is reduced in adulthood. This age coincides neurodevelopmental disorder. However, there is little with the onset of sensory stimulation and developmental understanding of whether and how functions of critical period plasticity windows, suggesting an import- FMRP change during development. Only a few studies ant role for FMRP in experience-dependent plasticity. In have systematically examined the roles of this protein the auditory and somatosensory cortex, peak FMRP for normal brain development over time. The devel- levels are seen between P7 and P12. In the hippocampus, opmental trajectories of symptoms and their relation- cerebellum, striatum, and brainstem, the peak FMRP ex- ship to changes in underlying mechanisms are only pression is between P3 and P7. FMRP is expressed beginning to be understood. In human work, only a across cell types in multiple brain regions in adults. handful of studies have examined early childhood de- Whether different cell types in these regions show differ- velopment (age < 5). Only a few longitudinal studies ent developmental profiles of FMRP expression has not with large sample sizes have examined trajectories of been studied. developmental change in FXS phenotypes. There is Such consistently high levels of FMRP expression in little data-driven guidance on optimal time points in early development suggest that the function of FMRP is development for clinical tests of potential therapeu- essential during this time period. FMRP regulates pro- tics. While studies point to early detection of many tein translation by association with a vast array of phenotypes, whether developmental treatment has mRNAs [16, 148]. A well-known phenotype in the Fmr1 longer lasting benefits than adult treatments remains KO mouse brain is increased protein synthesis [93, 109]. unknown. In the following sections, we summarize However, it remains unclear if this phenotype is ob- the animal and human studies that have bearing on served across development and if different brain regions these issues and suggest a number of future experi- have similar changes in protein synthesis across develop- mental approaches that are needed to fill major gaps ment. Most studies examine a single or a few time in our knowledge of FXS development. points in specific brain regions. This issue is further complicated in that different cell types may show dif- Animal Models ferent proteins whose levels are increased or de- Development of mechanisms and phenotypes in animal creased. Recent studies of protein synthesis in models of FXS humans with FXS show that at least some of the pa- There is an extensive literature on preclinical animal tients show decreased protein synthesis [81, 114], op- models of FXS, mostly in mice. Many of these studies posite to that seen in the mouse model. While this have examined phenotypes during early development naturally leads to questions on the utility of the either due to convenience of methodology (e.g., mouse model, this paradox can be potentially resolved in vitro slices are easier to record from in very young if developmental profiles of protein synthesis in FXS mice) or to examine developmental trajectory. Almost in different brain regions are