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Production and characterization of YAC transgenic mice expressing the normal and mutant Huntington’s disease gene Hodgson, John Graeme


Insights into how polyglutamine (Q) expansion in the Huntington's disease (HD) protein, huntingtin (htt), results in neurotoxicity in vivo can be achieved with the development of an animal model. No such model exists for HD with accurate spatial and temporal expression of a full-length mutant human protein. To address this, a Yeast Artificial Chromosome (YAC) transgenic approach was adopted to produce mice that express normal (18Q) and mutant (46Q and 72Q) htt. The normal and mutant human proteins are expressed in a temporal and spatial manner identical to that of the endogenous mouse protein. Mice expressing 46Q htt show no obvious neurological impairment up to 20 months. Behavior in home cages, coordination, and activity, were no different to controls, and no obvious pathological abnormalities were noted in the brain up to 20 months. Electrophysiological analyses in the hippocampal CA1 region at 6 and 10 months revealed a significant reduction in LTP, a form of synaptic plasticity involved in learning and memory. Two transgenic lines expressing 72Q htt were generated, 1 of which has integrated a higher number of YAC copies (3-5X). This increase in YAC copy number was associated with a 2-3X increase in protein expression levels. The high expressing founder (2498) exhibited a neurological phenotype at 6 wks, characterized by hyperactivity, lateralized circling, and impaired motor control, which were not seen in the lower expressing founder (2511). Analyses of brain tissue showed a dramatic increase in htt nuclear staining in the lateral striatum of mouse 2498 with accumulation of neuronal nuclear htt microaggregates and evidence for neurodegeneration. Neurodegeneration was also evident in founder 2511 without obvious presence of nuclear htt aggregates. These results demonstrate that a behavioral, neurophysiological and neuropathological phenotype can be achieved in YAC transgenic mice expressing full-length mutant htt consistent with that observed in HD patients. Additionally, CAG trinucleotide instability was assessed in the YAC transgenic lines as the trinucleotide repeat was passed from parent to offspring. These results show that size of the CAG repeat was a significant factor in influencing trinucleotide instability with more unstable meioses occurring as CAG size increased. Sex of transmitting parent and age of transmission did not significantly affect instability rates. Transmission of the repeat in a DNA mismatch repair (MSH2) deficient background also significantly increased repeat instability. The YAC transgenic mice presented in this thesis represent an important milestone in the development of an animal model for HD that closely represent the human condition. They provide an important tool for gaining insight into the in vivo mechanisms involved in polyglutamine mediated neuronal cell death and CAG trinucleotide instability, as well as a model on which to test therapeutic strategies designed to alleviate or eliminate disease progression in HD.

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