Robust behavioral assessment of the inducible Friedreich's ataxia mouse does not show improvement with NRF2 induction

Friedreich's ataxia, a recessive disorder caused by a mutation in the frataxin (FXN) gene, has few mouse models that demonstrate a progressive behavioral decline paralleling that of patients. A mouse model of systemic frataxin deficiency, the FXNKD, was recently developed using a doxycycline-inducible method; it is thought to mimic the patient phenotype seen when frataxin levels are decreased, but it has not been determined whether it is reliable for assessment of therapeutics. FXNKD mice underwent testing for 12 weeks alongside littermates, undergoing tests of motor function, gait and sensation. Additionally, a subset underwent treatment with omaveloxolone or dimethyl fumarate, both NRF2 inducers. We identified multiple techniques that sensitively detect decline in the mice, including open field, gait analysis and Von Frey tests. Furthermore, we developed a novel Salinas-Montgomery ataxia scale, which allows for more comprehensive assessment than a four-part cerebellar ataxia scale. Despite validating multiple sensitive techniques, we did not see any benefits of NRF2-inducing therapies in any tests. This was exacerbated by the discovery of a sexual dimorphism in FXNKD mice, in which males show more significant decline and better responsiveness to NRF2-inducing therapeutics.