Neuropathological investigations of three murine models of Huntington’s disease

Abstract

Huntington’s disease (HD) is a purely genetic neurodegenerative disorder affecting approximately 1 in 10,000 people. It is most commonly associated with excessive involuntary movement, or chorea, combined with varying degrees of other motor, psychiatric and cognitive disturbances. Identification of the mutation in the HD gene prompted the generation of several transgenic mouse models. HD is but one of a family of at least 9 triplet repeat disorders, all of which exhibit protein aggregation by a similar mechanism. The understanding of one disease is therefore of importance to the understanding of them all. This thesis aims to be a comprehensive comparative study of three very different mouse models of HD elucidating the pathological changes that precede and accompany the disease process. The work described in this thesis presents a detailed account of a longitudinal study of the pathological changes that occur within the brains of founder generations of mice transgenic for exon 1 of the HD gene, containing a highly expanded CAG repeat, the R6 lines. I have determined the intracellular sites for deposition and accumulation of the mutant protein huntingtin (htt), within both the neurons and glia of the central nervous system. The progressive accumulation of additional proteins within these aggregates has been described. The temporal evolution and spatial distribution of the neuronal intranuclear inclusion (NII) was determined using both immunohistochemical and morphometric analyses. The cellular consequences resulting from the aggregation of mutant htt were also investigated. I have conducted a detailed morphometric analysis of neurones within the cerebral cortex, striatum and cerebellum throughout the period of protein deposition, until the eventual degeneration of these cells. The dendritic and somal changes resulting from the cellular disruption associated with these NII are also described. In a further series of experiments I have investigated the changes that occur in a novel model of HD, namely the conditional, doxycycline inducible double transgenic mouse, HD94 model. It was interesting to find that the same construct when differently manipulated in two mouse lines can produce such contrasting symptoms and pathology. This was highlighted by the comparison of immunohistochemical and morphometric analyses between the HD94 and the R6 lines, where the pattern of mutant protein deposition was found to vary significantly. Lastly I have studied a more genetically accurate murine model of HD, the HD80 ‘knock-in model’. These mice develop a pathology broadly similar to that of the R6 lines but markedly different to that of the HD94, and over a much longer time frame This detailed comparative analysis of the molecular and cellular pathology of three transgenic mouse models of HD provides new insights identifying novel and unique neuropathology and suggests new approaches for therapeutic treatments for this disease