Neurobehavioral features and targeted treatments in fragile X syndrome: Current insights and future directions

Abstract

Fragile X syndrome (FXS) is the leading known monogenetic cause of autism spectrum disorder (ASD) and inherited form of intellectual disability (ID). As the major and growing public health problem worldwide, ASD is purely behaviorally defined whereas FXS is a medical/genetic disorder characterized by ID and ASD in males and learning and behavioral/emotional problems (social anxiety, attention network) in both genders. FXS is caused by a mutation in the Fragile X Mental Retardation 1 gene (FMR1) that leads to the epigenetic silencing of the gene and absence of its encoded protein, the fragile X mental retardation protein (FMRP). FMRP selectively targets approximately 4% of the transcribed mRNAs in the brain. Namely, to date, 842 such target mRNAs in mammalian brains have been identified and many of them converge on the same pathway as nonsyndromic ASD. Thus, FXS is the most studied 'disorder of synapse' model for syndromic ASD in the field of neuroscience. There are currently no effective treatments for either FXS or ASD. In this review article, we discuss recent progress and future directions in translating breakthrough preclinical findings that reveal potential therapeutic targets into clinical trials for humans with FXS of potential relevance for ASD. To date, a total of 20 double-blinded, randomized, placebo-controlled clinical trials in FXS have been identified through the FDA ClinicalTrial.gov website. The majority of these trials were completed between 2008-2015. These mostly phase II trials in adults and adolescents tested 13 compounds and have primarily targeted excitatory/inhibitory imbalance theory of FXS and ASD, namely a reversal of social/behavioral symptoms that are part of the core FXS phenotype but not the core synaptic dysfunction in FXS. Despite much progress in the field propelled by the preclinical advances, these clinical studies illustrate the gaps and challenges of translating therapies from animal models to humans with FXS and ASD ('building a bridge and walking on it'). Specifically, recent challenges in the clinical trials demonstrate the need to study for longer period of time (6-12 months), prepubertal age group(s), develop more objective clinical outcome measures (i.e., clinician-based, relevant learning paradigms, and desired biomarkers), and longer placebo run-ins to minimize the placebo impact for both FXS and ASD. Ultimately, a truly satisfactory FXS, and ASD, therapy may likely involve a combination of drugs (and learning paradigms), each targeting a different aspect of the core synaptic, cognitive and behavioral impairments in FXS.