Investigation of mRNA Oxidation in Alzheimer's Disease

Oxidative modifications to vital cellular components have been described in association with Alzheimer’s disease (AD). Cytoplasmic RNA oxidation is a prominent feature of vulnerable neurons in AD affected regions. However, the role of RNA oxidation in the pathogenesis of AD is unknown. This dissertation examined the magnitude of oxidized mRNAs in AD, identified oxidized mRNA species, and characterized the consequence of mRNA oxidation. An immunoprecipitation method was developed to isolate, characterize and quantify oxidized mRNAs in AD brain. The data showed that 30-70% of mRNAs are oxidized in frontal cortices of AD as compared to age-matched normal control. Identification of oxidized mRNA species revealed that mRNA oxidation in AD is not random but selective. Many identified oxidized mRNA species have been implicated in the pathogenesis of AD. Quantitative analysis revealed that some mRNA species are more susceptible to oxidative damage for which the relative amounts of oxidized transcripts reach 50-70%. To understand whether oxidation of mRNAs is involved in the pathogenesis of AD, the consequence of mRNA oxidation was studied using in vitro translation system and cell lines. mRNA oxidation may have deleterious effects by interfering with normal translational process or by a toxic gain-of-function. Examination of the protein expression from oxidized mRNAs revealed that mRNA oxidation causes a decrease of protein level and associated activity, which may result from the abnormal association of polyribosomes with oxidized mRNAs during the process of translation. Furthermore, we observed that microinjection of oxidized mRNAs into neuronal cells caused cell death, suggesting that increased mRNA oxidation could be lethal to cells. Studies in primary neuronal cultures revealed that RNA oxidation is an early event far preceding cell death induced by insults associated with AD. Neurons are specifically susceptible to RNA oxidation. Some mRNA species highly oxidized in AD brain are also present in oxidized mRNA pool of oxidative insult-treated cultures. A decrease of protein level was observed to oxidized mRNA species in cultures. Taken together, these studies indicate the possibility of oxidized mRNAs to interfere with physiological functions of cells and also implicate the potential contribution of RNA oxidation in the pathogenesis of AD.