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Which genes are highly expressed, based on my RNA-seq data? Are any of the highly expressed genes also differentially expressed?
This recipe provides an outline of one method to identify and visualize genes and isoforms that are highly expressed in RNA-seq data. In particular, this recipe utilizes an analysis pipeline, allowing a user to chain together multiple analysis steps into one workflow that can be run in one step. An example use of this recipe is a case where an investigator wants to process several datasets in the same way, in which case the pipeline will allow the investigator to re-use the same modules and parameters, over and over again.
Given a set of raw RNA-seq reads, the goal is to align the reads to a reference genome, estimate expression abundance levels for reference genes and isoforms, filter out low-expressed genes and isoforms, and visualize the read alignments and their expression levels. In particular, this recipe uses the UCSC Table Browser to retrieve a reference genome to align RNA-seq reads against. We also uses several modules in GenePattern to align the reads against the reference genome, and to identify differentially expressed genes when comparing two conditions. Finally, we use IGV to visualize the differentially expressed genes.
Why differential expression analysis? We assume that most genes are not expressed all the time, but rather are expressed in specific tissues, stages of development, or under certain conditions. Genes which are expressed in one condition, such as cancerous tissue, are said to be differentially expressed when compared to normal conditions. To identify which genes change in response to specific conditions (e.g. cancer), we must filter or process the dataset to remove genes which are not informative.