Turner, C.; (2009) The molecular pathogenesis of Huntington’s disease. Doctoral thesis , UCL (University College London).

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Abstract

Huntington’s Disease (HD) is caused by an expansion in the CAG repeats of the huntingtin gene. This thesis describes an Ecdysone cell model which expressed inducible wild type (WT) and mutant (MT) N-terminal huntingtin (htt) in HEK 293 cells and constitutive EYFP full length (FL) htt in SH-SY5Y cells. WT and MT EYFP FL htt was diffusely localised to the cytoplasm whereas endogenous FL htt and N-terminal htt localised to the nucleus and cytoplasm suggesting that htt has a role both in the nucleus and cytoplasm and EYFP inhibited nuclear translocation. Nterminal htt partially colocalised with vesicular and mitochondrial markers suggesting that N-terminal htt may be involved in vesicle trafficking and mitochondrial function. The decrease in mitochondrial complex IV activity in MT FL htt cells supported previous reports that a complex IV defect is an early event in the pathogenesis of HD. Normal mitochondrial respiratory chain activities in cells expressing N-terminal htt contrasted with some cells models demonstrating a complex II/III defect when highly expanded CAG repeats were expressed. This suggested that a detectable complex II/III defect is not an early feature in the pathogenesis of HD. Muscle biopsies from HD patients revealed a relationship between clinical progression, CAGs and a decrease in complex II/III:CS ratio, consistent with the defect in HD brains and cell models and suggested that muscle may be a useful tissue to study the disease. Decreased aconitase activity with MT FL htt expression and increased sensitivity to paraquat with MT N-terminal htt expression demonstrated that MT htt was associated with increased oxidative stress or compromised antioxidant defences. There was evidence of proteasomal dysfunction in the MT FL htt clones and inhibition of the proteosome by lactacystin caused the formation of perinuclear "aggresome-like" inclusions in both WT and MT FL htt clones. These inclusions contained FL htt which suggested that the proteasome was necessary for processing of FL WT and MT htt. Under normal conditions there was no evidence of cleavage of WT or MT FL htt, however following treatment with lactacystin, an additional 11 kDa N-terminal htt fragment was present in most MT FL htt clones representing a novel mutation-specific cleavage product which may play an important role in the toxicity of MT htt. This thesis has demonstrated several defects in cellular function in the absence of gross cell death and htt inclusion formation. These findings expand on previous hypotheses in the pathogenesis of HD involving abnormal MT htt cleavage, oxidative stress, mitochondrial dysfunction and proteasomal inhibition.

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