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UBC Theses and Dissertations

Methods for transcript variant discovery and alternative expression analysis : application to the study of fluorouracil resistance in colorectal cancer Griffith, Malachi


RNA transcripts are expressed from tens of thousands of loci across the human genome. Several studies have suggested that many genes are alternatively expressed to produced multiple mRNA isoforms and many of these remain undiscovered. Identifying specific isoforms associated with human diseases such as cancer has potential to lead to improved treatments. The scale and complexity of the transcriptome present significant barriers to (1) identifying isoforms and (2) applying knowledge to human disease research. Recent advances in genome-wide microarray and sequencing platforms have begun to provide the capacity and resolution to address these challenges. The goal of this thesis was to develop novel methods that allow genome-wide identification and quantification of mRNA isoforms. I first approached this problem by creating a microarray design platform for alternative expression analysis called 'ALEXA-array' (www.AlexaPlatform.org). To evaluate the ALEXA-array approach I used it to generate a microarray design that I then used to measure differential expression of mRNA isoforms in 5-fluorouracil (5-FU) sensitive and resistant colorectal cancer cell lines. This approach identified several isoforms potentially involved in 5-FU resistance. While the ALEXA-array approach was successful, I identified several limitations of the method. For example, the approach was insensitive to isoforms with small differences in sequence content and limited by both the transcriptome annotations and the number of microarray features available at design time. I developed a second method, ‘ALEXA-seq’, to take advantage of advances in massively parallel sequencing. Applying this method to the same cell lines I showed that the approach was able to overcome many limitations of the microarray approach. Several additional candidate 5-FU resistance isoforms were identified. Both the ALEXA-array and ALEXA-seq approaches identified expression of an aberrant isoform of the uridine monophosphate synthetase as a top candidate. Interestingly, this gene was suspected to function in the conversion of 5-FU to active anti-cancer metabolites. Additional characterization was performed to elucidate the expression pattern, transcript diversity and sequence variation of this gene in a panel of cell lines and tumours. The methods presented here should help to identify mRNA isoforms with potential utility as therapeutic targets or as prognostic or diagnostic markers.

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