Characterisation of SCA3 mutant mice with a dominant toxic effect

Spinocerebellar Ataxia Type 3 (SCA3) is an autosomal dominant neurodegenerative disorder caused by polyglutamin-expanded ataxin-3, whose function is still unknown. In order to further evaluate the function of ataxin-3 in vivo, we analysed ataxin-3 mutant mice generated by a gene trap (GT) approach. The plasmid GT-vector betaGeo2 was integrated into the intron 8 of the ataxin-3 gene The polyglutamin repeat in exon 10 is missing in these ataxin-3 mutant mice. Homozygous mutant mice (Atx3gt/gt) survive embryogenesis and do not show obvious developmental defects.

Immunohistochemistry analysis revealed a high number of ataxin-3 positive cytoplasmatic inclusion bodies in all brain regions at the age of 3 months. Neurodegeneration of neurons in the cerebellum could be detected by different immunohistochemistal stainings.

Metabolic screens at the age of 3 months and one year, respectively, implicate an impact in the dopaminergic metabolism but exclude disturbances of the serotonin metabolism. Other screens of blood parameters, immunology, hearing and eyesight did not reveal discrepancies between the different genotypes.

Over the whole lifespan of 12 months we screened for neurological symptoms by means of the modified SHIRPA test and by footprint analysis. We further looked for motor performance and coordination on the rotarod and the beam walking test. From birth until the age of seven months we were not able to detect any behavioural differences between heterozygous and homozygous mutant mice in motor coordination and motor learning on the rotarod and beam walking test compared to their wild type littermates. Heterozygous and homozygous mutant mice developed a neurological phenotype at the age of one year. At first we could detect a dramatic reduction of the body weight followed by neurological symptoms e.g. tremor, clasping, stereotypes and ataxia. Two weeks after onset of symptoms both heterozygous and homozygous mutant mice died.

These results demonstrate that the whole ataxin-3, a widely expressed protein, does not seem to be essential for development of the ataxin-3 mutant mice. In contrast they develop a phenotype in adulthood and a premature death consistent with a neurodegenerative disease. We therefore believe that the first 8 exons of the ataxin-3 protein, containing the Josephin-domain, have a dominant toxic effect in the neuronal cells.

Detailed analysis of behaviour, RNA- and protein studies, will be investigated.