New mouse model better mimics FA features in humans: Study
Model created by further suppressing frataxin protein in standard mouse model
Researchers have developed a new mouse model of Friedreich’s ataxia (FA) that captures coordination and muscle strength impairment seen in patients, a study reports.
The model was created by worsening symptoms of a standard mouse model by further suppressing production of the frataxin protein, which is deficient in FA patients.
The study, “Finding an Appropriate Mouse Model to Study the Impact of a Treatment for Friedreich Ataxia on the Behavioral Phenotype,” was published in the journal Genes.
FA is caused by excessive repeats of three DNA building blocks (nucleotides), one guanine (G), and two adenines (A), called GAA, in the FXN gene sequence.
Normally, GAA is repeated between five and 68 times, but in FA patients, GAA can be repeated from 66 to 1,700 times. This leads to a deficiency in the production of frataxin, a protein essential for the function of energy-producing mitochondria, and its loss has a severe impact on the nervous system and muscles.
Generally, more GAA repeats correspond to a younger age at FA symptom onset, with more severe and rapidly progressing symptoms.
Models that accurately reflect human disease needed to test potential therapies
Animal models that accurately reflect human disease are needed to understand underlying biological mechanisms and to monitor the effect of potential therapies.
Mouse models with the entire FXN gene removed develop severe complications and die too early. The YG8sR strain, which carries a human version of FXN with 300 GAA repeats, has lower frataxin levels but mild symptoms.
Recently, a team of scientists at Université Laval, in Canada, characterized a new strain of FA mice with 800 GAA repeats, called YG8-800, and showed it captured many disease features, such as neurological dysfunction and heart disease.
Now, the team has tested an FA mouse model derived from the YG8sR strain, but its frataxin production was further decreased by small hairpin RNAs (shRNA) designed to lower FXN gene activity. The YG8-800 strain was also included as a comparison, as well as Y47, a strain that harbored the human FXN gene but without any disease-related genetic defects.
Compared to the Y47 strain, frataxin concentration in the tissues of YG8sR mice ranged from 13% to 20%, and in YG8-800 mice, it was between 0.9% and 16.3%. YG8-800 mice also had significantly lower body weight at 5 and 8 months than control mice.
Motor function of Y47 and YG8sR mice were similar for many tests from 2 to 11 months of age. While YG8sR mice had mildly impaired movement coordination, the YG8-800 mice had more severe coordination problems.
The team designed and tested four shRNAs in cells and found two that suppressed frataxin production to a concentration of 28% of the controls. These shRNAs were then delivered to YG8sR mice using a harmless virus (AAV) via intravenous (into-the-vein) infusion.
One shRNA, named shRNA3, significantly worsened the coordination of the YG8sR mice five weeks after AAV infusion, as indicated by longer time crossing balance beams with more faulty footsteps. shRNA3 also significantly reduced the muscle strength of YG8sR with a lower time hanging on an inverted grid.
When compared, at 3-4 months after infusion, the YG8sR mice plus shRNA3 needed an average of 35 seconds to cross the balance beam and make 12 foot faults. This was more severe than the YG8-800 mice, with 20 seconds required to cross the beam and seven foot faults at five months.
Features of new mouse model ‘comparable to the human patients’
Tissue analysis found a 60% reduction in the levels of FXN messenger RNA (mRNA), the molecule derived from DNA that guides frataxin production, in the liver with shRNA3. FXN mRNA was reduced by 29.3% in the cerebellum, a brain region important for coordinating movements. No effects were observed in the cerebrum, the largest part of the brain.
Finally, co-injection of shRNA3 and one coding for frataxin reversed coordination deficits and improved muscle strength.
“In our study, we were able to rapidly increase the severity of the YG8sR phenotype with an AAV coding for a sh-RNA against frataxin,” the researchers concluded. “Indeed, only 5 weeks after this AAV injection, the mice demonstrated significantly more severe locomotion and balance symptoms.”
“The complete characterization of this model has been carried out in our laboratory showing that the phenotype of the YG8-800 mice is comparable to the human patients,” the researchers noted. “When compared, the YG8sR mice injected with the shRNA3 have a phenotype comparable to the YG8-800 mice.”