Phenotype analysis and rescue on female FVB.129-Fmr1 knockout mice

Fragile X syndrome (FXS) is the most common monogenic cause of intellectual disability and a cause for autism. FXS females report milder phenotypes and a lower rate of cognitive problems compared to males. This is most likely because most females are heterozygous, while males are hemizygous for the disease. Thus, most preclinical studies have been completed in males. As there is major interest in testing experimental drugs for FXS, it is imperative to determine whether females in animal models used for research, present behavioral alterations that might translate to humans in order to confirm that experimental drugs have an effect on both genders. In our study we describe behavioral phenotypes in homozygous FXS female mice developed on the FVB.129 background. We focused on detection of hippocampal-mediated cognitive abilities and other behaviors described for FXS. Our research shows that, while female FVB.129-Fmr1 knockout mice present normal learning, they have impaired memory, as well as susceptibility to audiogenic seizures. In agreement with previous reports in rodents and humans, significant levels of the small GTPase Rac1 were found in FXS female mice. Because Rac1 is involved in neuronal development, plasticity and behavior, we additionally aimed to pharmacologically inhibit Rac1 and determine whether observed phenotypes are rescued. Treatment of female FVB.129-Fmr1 knockout with a Rac1 inhibitor abolished behavioral deficits, bringing phenotypes to control levels. Our results suggest that female FVB.129-Fmr1 knockout mice display behavioral impairments that resemble FXS in humans. Moreover, those behavioral shortfalls might be associated with alteration of plasticity involving excessive Rac1 function, since pharmacological reduction of Rac1 normalizes previously altered phenotypes to control levels.