Discovery and validation of relevant markers of Huntington’s disease progression using a transgenic sheep model

Abstract

Huntington’s disease (HD) is a progressive neurodegenerative disorder caused by the inheritance of a single defect in the huntingtin gene, called a polyglutamine coding (CAG) repeat expansion. In unaffected individuals the average CAG repeat length is ~19 units but in HD patients this is expanded to beyond 36 repeats, and this is the causative mutation. At the moment, there is no effective treatment for HD. In an effort to fast-track treatment strategies for HD into clinical trials, allow long term safety studies and to gain a better understanding of early molecular pathophysiological mechanisms, our group developed a large-animal HD transgenic ovine model carrying a full length human HTT with 73 CAG repeats (OVT73). The OVT73 line of sheep demonstrates an ideally situated insertion locus and early circadian abnormality. The results of this thesis describe neuropathological anayses of the OVT73 sheep. A progressive loss of GABAA α1 receptor immunoreactivity was observed in the striatum from 6 month to 5 year old OVT73 cohorts. In addition, progressive time point histopathological analyses in OVT73 reveal mutant huntingtin aggregates mainly in cortex and this distinctive cortical distribution closely resembles the pattern observed in early stage HD patients. In addition, this thesis describes a large scale proteomic screen to identify markers to track HD disease progression for use in therapeutic testing. This screen made use of advanced mass spectrometry techniques to quantify all detectable proteins within OVT73 brain samples. We identified and quantified approximately 3000 proteins in each brain region. There were more proteins altered in abundance between OVT73 and control sheep in striatum compared to motor cortex and cerebellum. These alterations included proteins involved in energy metabolism, synaptic function, mitochondrial activity and oxidative stress. OVT73 sheep appear to recapitulate the late onset phenotype of the human HD condition, thus providing an extended presymptomatic window through which experimental therapies can be tested. Ultimately, it is hoped that this model will serve as a useful platform to rapidly develop therapies for HD to translate into clinic.