Genotype and Phenotype Characterisation of an Enhanced Friedreich Ataxia GAA Repeat Expansion Mouse Model Fxn_null::YG8s(GAA)>800

Abstract

Our lab has previously generated YG8R and YG22R FRDA mouse models that are based upon human FXNYAC transgenic mice containing a large FXNhuman genomic transgene with GAA repeat expansions crossed with Fxn- knockout mice. These mouse models have exhibited a rather mild, late-onset FRDA-like phenotype. Therefore, over the last few years we have performed selective breeding of these mice to produce larger GAA repeat expansion containing FRDA mouse models with a more representative earlier onset FRDA-like phenotype. We now have YG8- derived FRDA mice lines with GAA repeat expansion sizes of approximately ~300 repeats (YG8sR), ~400bp repeats (YG8LR) and ~600bp repeats (YG8XLR). More recently, we have obtained the latest FRDA humanised model that was derived from our previous YG8sR mice by Jackson laboratory (Fxnnull::YG8s(GAA)>800). These mice contain a larger expansion of 800 GAA repeats, designated YG8JR. The YG8JR mice underwent a partially different genetic manipulation in which the Fxn-knockout in these models was generated by CRISPR/Cas9 and Cre loxP-mediated deletion of exon 2. These models have the largest GAA repeat sizes of all the current FRDA mouse models. Phe- notypically, these mice exhibit a degree of hair loss and have reduced weight compared with Y47JR control mice. In addition, these mice have shown further decreases in frataxin expression levels compared to all our previous FRDA mouse models. We have also detected increased somatic GAA repeat instability in the brain and cerebellum of YG8JR mice, together with reduced aconitase activity and altered FXN histone modifications and DNA methylation, compared with the control Y47JR mice. Coordination ability of YG8JR mice, together with Y47JR control mice, was assessed using accelerating rotarod analysis. The results indicated a decline in the motor coordination of YG8JR mice at the older age (6-9m) compared to Y47JR controls. We aim to further characterise these mouse models for neurobehavioral deficits and other biochemical and molecular analysis.