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UBC Theses and Dissertations

Genetic analysis of huntington disease Andrew, Susan E.


Huntington disease (lID) is an autosomal dominant neurodegenerative disease characterized by progressive dementia and chorea. The initial aim of this thesis was to identify candidate regions for the HD gene. Markers separated by 3 Mb were found to be in strong allelic association with HI), thus identifying two mutually exclusive candidate regions. A screen for genomic rearrangements in affected individuals using exonic clones from the proximal candidate region was undertaken. One auspicious clone showed a genomic rearrangement, cosegregating with HI) in two families. Subsequent cloning and sequencing of this region demonstrated an Alu retrotransposition event associated with Huntington disease in the two families. During this work, the mutation causing Huntington disease was identified by the HD Collaborative Research Group as an expanded CAG trinucleotide repeat in a novel gene. Analysis of new polymorphic markers in the gene permitted a retrospective analysis of linkage disequilibrium in a 300kb region harbouring the CAG repeat. Analysis of HI) haplotypes showed that multiple haplotypes underlie CAG expansion. Mean CÁO length on control chromosomes with haplotypes identical to those most frequently observed in HI) were significantly larger than CAG lengths on other control chromosomes, consistent with the hypothesis that these chromosomes are a reservoir for further expansions, resulting in Hi) chromosomes. The nature of the dynamic trinucleotide repeat mutation resolved several previously confusing issues of HI). There is a highly significant correlation between the age of onset of Huntington disease and CAG repeat length which accounts for approximately 50% of the variation in the age of onset. The instability of the repeat and the tendency to expand further over the generations accounts for the occurrences of new mutations and observed anticipation. CAG expansion is the basis of the worldwide occurrence of HD and was shown to be a sensitive and specific marker for inheritance of the ND gene. A small proportion of clinically affected patients tested lacked CAG expansion, representing misdiagnosis, sample mix-up, clerical error or possible genetic heterogeneity. CAG analysis also allowed for the resolution of recombinant ND chromosomes that previously had suggested the gene was located in the distal candidate region.

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