DESeq

The Dataset

Our goal for this experiment is to determine which Arabidopsis thaliana genes respond to nitrate. The dataset is a simple experiment where RNA is extracted from roots of independent plants and then sequenced. Two plants were treated with the control (KCl) and two samples were treated with Nitrate (KNO3).

The sequences were processed to remove all low quality sequences, trim all low quality nucleotides, and finally aligned against the Arabidopsis genome using TopHat. Our analysis starts from here.

We have been provided the following files:

  1. 4 Bam files – An alignment file, one for each sample
  2. Arabidopsis.gtf file – which contains information about the genes in Arabidopsis and where they are located in the genome.
  3. Experiment design – a comma separated file containing meta data.
  4. Gene description – Description about the function of the genes in Arabidopsis.

Note: If one of the R packages is not loaded, you have to install it from Bioconductor

The Data Files

The Alignment Files

The alignment files are in bam format. This files will not be loaded into R, but rather simply pointed to by a reference/variable. The alignment files provided are about 15x smaller compared to an average RNA-seq sample run today. Also there will be triplicates of 3 or more different conditions resulting in much more than 4 sample. So you can imagine the amount of space and memory R would need if all that data was stored in the workspace.

To do point to the bam files we will use Rsamtools

The Annotation File

GTF file is very similar to a GFF file. Their purpose is to store the location of genome features, such as genes, exons, and CDS. It is a tab delimited flat file which looks something like this.

We will load the GTF file using GenomicFeatures and group the exons based on the gene annotation.

The Experimental Design

We will need meta information about our samples to define which sample is the KCL and which is the KNO3. A simple comma separated file can be created using a text editor where the first column has the sample file names and the second column has the categories for the samples. The file should look something like this:

Now let’s load the file into our workspace. The commands read.table, read.csv, and read.delim are all related with the exception of different default parameters. They all read a text file and load it into your workspace as a data.frame. Additonally it loads all columns that are character vectors and makes them factors. The first option for all the different versions is the neame of the file. After that, unless you have memorized the options for the commands, explicitly state which parameters you are providing.

Counting the Reads

Now we will use the GenomicAlignments package to determine where the reads are mapping. Remember we decided to map the sequences to the genome so we can make new discoveries such as novel genes, alternative splicing, and also anti-sense transcripts. In the event that the reference genome is not sequenced, one would have to assemble the RNA-seq reads first to identify all the genes that were detected in the RNA-seq samples. However assembling the transcriptome is quite a complicated process and requires a lot of time and manual curation to produce quality transcripts. We are fortunate that we are using Arabidopsis data whose genome was sequenced in 2000.

The function summarizeOverlaps takes the bam files and the gene annotation to count the number of reads that are matching a gene. The union mode means if a read matches an area where two genes overlap, then it is not counted. We can also provide other parameters such as whether sequence was single-end or pair-end.

Filtering the Counts Now, if you remember from the lecture, genes that are expressed at a very low level are extremely unreliable. We will remove all genes if neither of the groups ( KCL or KNO3 ) have a median count of 10 and call the new dataframe counts_filtered.

Differentially Expressed Genes

Now that we have the counts table filtered, we can start to determine if any of the genes are significantly differentially expressed using DESeq. DESeq performs a pairwise differential expression test by creating a negative binomial model.

Now we can create an object that DESeq needs using the function newCountDataSet. In order to create this dataset, we need the filtered data frame of read counts and the factor that will help group the data based on the condition.

Before the actual test, DESeq has to consider the difference in total reads from the different samples. This is done by using estimateSizeFactors function.

Next DESeq will estimate the dispersion ( or variation ) of the data. If there are no replicates, DESeq can manage to create a theoretical dispersion but this is not ideal.

The plot shows us that as the gene’s read count increases, dispersion decreases, which is what we expect. Now we will tell DESeq what we would like to compare. Then we will use the adjusted p-value ( p-value corrected for multiple hypothesis testing ) for our cutoff.

The sum command is normally used to add numberic values. However in logic vectors, TRUE=1 and FALSE=0, so we can use the sum function to count the number of “TRUE” in the vector. In the example we counted the number of genes that have a an adjusted p-value less than 0.05.

MA Plot

Here’s an MA plot that shows as the average count of a gene increases, a smaller fold change is needed for something to be significant. For this reason, it is often helpful to require that the log2foldchange also be greater than or less than negative of some cutoff.

Significant genes

Let’s use the same values for our cutoff to determine which genes we want to consider as significantly differentially expressed. The resSigind variable will contain genes that are induced and resSigrep will contain genes that are repressed when the plants are treated with Nitrate. To create one dataframe of differentially expressed genes, let’s combine the two dataframe. We can use the rbind command because the columns are the same in both sets. To show the name of the genes, simply look in the id column of the dataframe.

Gene Annotations

Great ! We have genes that are differentially expressed, what do we know about these genes ? The gene identifier we obtained from the GTF file is referred to as TAIR identifiers (a consortium that used to release Arabidopsis genome annotations) I managed to download the gene description for all the genes. Let’s load them into the workspace and find out what are the names of the genes.Since the set of repressed genes is smaller, let’s see what we can find out about them.

Before we do this, note that the identified in the gene_description file is slightly different ( the file contains the transcript identifier that ends with “.” and a number), Let’s replace every occurrence of . and a number with nothing. Then we will be able to use match to find where our gene is in the description file so we can only print out that row.