Genotype and phenotype characterisation of Friedreich ataxia mouse models and cells

Abstract

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder, caused by a GAA repeat expansion mutation within intron 1 of the FXN gene, resulting in reduced level of frataxin protein. Normal individuals have 5 to 40 GAA repeat sequences, whereas affected individuals have approximately 70 to more than 1000 GAA triplets. Frataxin is a mitochondrial protein involved in iron-sulphur cluster and heme biosynthesis. The reduction in frataxin expression leads to oxidative stress, mitochondrial iron accumulation and consequential cell death with the primary sites of neurons of the dorsal root ganglia and the dentate nucleus of the cerebellum. FRDA, which is the most common inherited ataxia, affecting 1:50,000 Caucasians, is characterised by neurodegeneration, cardiomyopathy, diabetes mellitus and skeletal deformities. To investigate FRDA molecular disease mechanisms and therapy, several human FXN YAC transgenic mouse models have been established: Y47R, containing normal-sized (GAA)9 repeats; YG8R and YG22R, which initially contained expanded GAA repeats of 90-190 units and 190 units, respectively, but which have subsequently been bred to now contain expanded GAA repeats of 120-220 units and 170-260 units, respectively, and YG8sR (YG8R with a small GAA band) that was recently generated from YG8R breeding. To determine the FXN transgene copy number in the enhanced GAA repeat expansion-based FRDA mouse lines, a TaqMan qPCR assay was developed. The results demonstrated that the YG22R and Y47R lines had a single copy of the FXN transgene while the YG8R line had two copies. The YG8s lines showed less than one copy of the target gene, suggesting potential deletion of the FXN gene. Single integration sites of all transgenes were confirmed by fluorescence in situ hybridisation (FISH) analysis of metaphase and interphase chromosomes. However, in the YG8s line, at least 25% of the YG8s cells had no signals, while the remaining cells showed one signal corresponding to the transgenic FXN gene. In addition, the analysis of FXN exons in YG8s rescue mice by PCR confirmed the presence of all FXN exons in these lines, suggesting the incidence of somatic mosaicism in these lines. Extended functional analysis was carried out on these mice from 4 to 12 months of age. Coordination ability of YG8R, YG8sR and YG22R ‘FRDA-like’ mice, together with Y47R and C57BL6/J wild-type control mice, was assessed using accelerating rotarod analysis. The results indicated a progressive decrease in the motor coordination of YG8R, YG22R and YG8sR mice compared to Y47R or C57BL6/J controls. Locomotor activity was also assessed using an open field beam-breaker apparatus followed by four additional functional analyses including beam-walk, hang wire, grip strength and foot print tests. The results indicated significant functional deficits in the FRDA mouse models. Glucose and insulin tolerance tests were also conducted in the FRDA mouse models, indicating glucose intolerance and insulin hypersensitivity in the aforementioned lines. To investigate the correlation between the FRDA-like pathological phenotype and frataxin deficiency in the FRDA mouse models, frataxin mRNA and protein levels as well as somatic GAA repeat instability were examined. The results indicated that somatic GAA repeats increased in the cerebellum and brain of YG22R, YG8R and YG8sR mice, together with significantly reduced levels of FXN mRNA and protein in the liver of YG8R and YG22R compared to Y47R. However, YG8sR lines showed a significant decrease in FXN mRNA in all of the examined tissues compared to Y47R human FXN and C57BL6/J mouse Fxn mRNA. Protein expression levels were also considerably reduced in all the tissues of YG8sR mice compared to Y47R. Subsequently, the telomere length of human and mouse FRDA and control fibroblasts was assessed using qPCR and Q-FISH. The results indicated that the FRDA cells had chromosomes with relatively longer telomeric repeats in comparison to the controls. FRDA cells were screened for expression of telomerase activity using the TRAP assay and a quantitative assay for hTERT mRNA expression using TaqMan qRT-PCR. The results indicated that telomerase activity was not present in the FRDA cells. To investigate whether FRDA cells maintained their telomeres by ALT associated PML bodies (APBs), co-localisation of PML bodies with telomeres was assessed in these cells using combined immunofluorescence to PML and Q-FISH for telomere detection. The results demonstrated that the FRDA cells had significantly higher co-localised PML foci with telomeric DNA compared to the normal cells. Moreover, telomere sister chromatid exchange (T-SCE) frequencies were analysed in the human FRDA cell lines using chromosome orientation FISH (CO-FISH). The results indicated a significant increase in T-SCE levels of the FRDA cell lines relative to the controls. Furthermore, growth curve and population doubling analysis of the human FRDA and control fibroblasts was carried out. The results showed that the FRDA fibroblast cell cultures underwent growth arrest with higher cumulative population doubling compared to the controls. Though, further analysis of telomere length at different passage numbers revealed that the FRDA cells lost telomeres faster than the controls. Finally, the telomere dysfunction-induced foci (TIF) assay was performed to detect DNA damage in the human FRDA fibroblast cells using an antibody against DNA damage marker γ-H2AX and a synthetic PNA probe for telomeres. The frequency of γ-H2AX foci was significantly higher in the FRDA cells compared to the controls. Similarly, the FRDA cells had greater frequencies of TIFs in comparison to the controls, suggesting induced telomere dysfunction in the FRDA cells.