Rescue of Fragile X Syndrome phenotypes in Fmr1 KO mice by the small molecule PAK inhibitor FRAX486

Abstract

Autism is a diverse and complex family of disorders, and its prevalence is on the rise: 1 in 110 children have autism. There is no effective treatment for the symptoms which often include language and communication deficits, repetitive behavior, intellectual disability, epilepsy, attention deficits, and hyperactivity. The quest for a cure is challenging due to the heterogeneity of the disorder, but also because more than 90% of cases of autism are idiopathic, meaning the cause is unknown. Fortunately, one cause of autism has been discovered: silencing of a single gene causes an autism-like disorder called Fragile X Syndrome (FXS). The knowledge of the genetic basis of FXS allowed for the development of a mouse model of autism. The fmr1 knockout (KO) mouse displays phenotypes similar to symptoms in the human condition - including hyperactivity, repetitive behaviors, and seizures. Humans and mice share not only behavioral expression of the disease, but also analogous abnormalities in the density and morphology of dendritic spines - the sites of connections between neurons and critical substrates for learning. Abnormal dendritic spines is a common feature in FXS, idiopathic autism, and intellectual disability. Thus, this neuroanatomical abnormality may contribute to disease symptoms and severity. Here we take a hypothesis-driven, mechanism-based approach to the search for an effective therapy for FXS. We hypothesize that a treatment that rescues the dendritic spine defect may also ameliorate behavioral symptoms. Thus, we targeted a protein that regulates spines through modulation of actin cytoskeleton dynamics: p21-activated kinase (PAK). In a healthy brain, PAK and FMRP - the protein product of fmr1 - antagonize one another to regulate spine number and shape. Inhibition of PAK with a strategy utilizing mouse genetics reverses spine abnormalities as well as cognitive and behavioral symptoms in fmr1 KO mice, as we demonstrated in our previous publication. This discovery highlights PAK as a potential target for drug discovery research. In this thesis work, we build on this finding to test whether the small molecule FRAX486 - selected for its ability to inhibit PAK - can rescue behavioral, morphological, and physiological phenotypes in fmr1 KO mice. Our results demonstrate that seizures and behavioral abnormalities such as hyperactivity, repetitive movements, and habituation to a novel environment can all be rescued by FRAX486. Moreover, FRAX486 reverses spine phenotypes in adult mice, thereby supporting the hypothesis that a drug treatment which reverses the spine abnormalities can also treat neurological and behavioral symptoms.