Investigating the Molecular and Cellular Basis of Pathogenesis in Huntington's Disease

Huntington's disease (HD) is a late onset, neurodegenerative disorder that typically progresses for 10-20 years, resulting in premature death. Symptoms include choreic movement disorder, cognitive deficits and psychiatric disturbances. The neurodegeneration associated with HD occurs most prominently in the striatum with the additional involvement of the cortex and other brain regions in later stages. Initial neuropathology within the striatum demonstrates the selective vulnerability of medium spiny neurons. HD is inherited in an autosomal dominant fashion and is caused by the expansion of a polymorphic trinucleotide (CAG) repeat within exon 1 of the ubiquitously expressed HD gene. The expansion mutation results in an elongated polyglutamine stretch within the protein product, called huntingtin. An inverse correlation between the repeat number and age at onset/extent of pathology has been observed with the longest repeats causing severe juvenile HD. A knock-in HD mouse model has been created by homologous recombination, inserting a perfect CAG repeat tract (72-80 repeats) into exon 1 of the mouse Hdh gene (Shelbourne et al, 1999), the mouse orthologue of the human HD gene. This HD knock-in mouse model provides an opportunity to explore the consequences of the HD mutation in its appropriate genomic and protein context. Previous studies of the HD knock-in mice have demonstrated several alterations reminiscent of human HD, including behavioural changes (Shelbourne et aL, 1999) and synaptic dysfunction (Usdin et al, 1999). Protein aggregates, a cellular hallmark of the HD disease process, have been detected within the nucleus and neuropil of neurons selectively vulnerable to HD (Li et al, 2000). The presence of neuropil aggregates has also been associated with axonal degeneration (Li et al, 2001). The alterations observed in HD knock-in mice occur in the absence of obvious cell loss, suggesting that this system maybe modelling the events involved in early HD pathogenesis. Movement disorder is a prominent symptom associated with human HD. Behavioural studies using sensitive, quantitative methods to assess motor function have been carried out in the HD knock-in mice. Gait abnormalities, at 9 months of age, in the form of altered weight distribution were observed and appeared to be influenced by the sex of the mouse since significant alterations were restricted to male HD knock-in mice. In an attempt to assess cognitive deficits that may result from hippocampal synaptic dysfunction previously observed in HD knock-in mice, spatial learning and memory was also investigated. Experimental design problems prevented the acquisition of robust data from these experiments and the cognitive abilities of the HD knock-in mice remain unknown. To investigate the neuronal integrity in the HD knock-in mice, neurotransmitter receptor densities were determined by quantitative ligand binding autoradiography. By examining receptor density in the striatum and cortex of 18 month old HD knock-in mice a significant reduction in striatal dopamine D2 receptors was detected, along with significant increases in striatal and cortical benzodiazepine receptor binding. Conversely, no changes were observed in dopamine Di and opioid receptor densities although a strong trend towards increased opioid receptor density was observed within the striatum. A comparison of neurotransmitter receptor densities in different congenic HD mouse lines demonstrated an influence of genetic background on neurotransmitter receptor levels in wild-type and HD knock-in mice, suggesting the presence of possible genetic modifiers of the disease process. To investigate possible molecular mechanisms responsible for the selective pattern of HD neuropathology, mutation profiles in the somatic tissues of HD knock-in mice were investigated using a small pool PCR approach. Age-dependent, tissue-specific somatic repeat instability was observed, with the striatum, the brain region most affected by HD neuropathology, displaying the greatest level and magnitude of repeat size changes. Studies also suggested that the mechanism underlying brain repeat instability might not be replication-based. Finally, the tissue-specific mutation profiles observed appeared to be influenced by the presence or absence of amino acid residues downstream of the polyglutamine stretch within the huntingtin protein. Investigations of human juvenile HD tissue confirm the presence of extensive brain region-specific repeat instability. The largest repeat size changes were observed in the cortex with lower levels of repeat instability in the striatum, perhaps reflecting the pattern of extensive cell loss noted at autopsy. The work presented in this thesis contributes to the further phenotypic characterisation of the knock-in HD mouse model and identifies early molecular and cellular mechanisms underlying HD pathogenesis. These studies will inform future discussions of potential therapeutic targets and the utility of biomarkers to evaluate therapeutic efficacy