| Summary: | 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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