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Quantify transcript expression from RNA-seq reads

Salmon is a fast and accurate tool for quantifying trancript abundances, which can then be used to also generate gene-level quantifications.

The latest release for running Salmon can be found here:

https://github.com/COMBINE-lab/salmon/releases

The Linux binaries can be downloaded and run directly on the cluster. It's a good idea to just use the latest version of Salmon at the beginning of a project. Note that you have to create a new index if you use a new version of Salmon.

Recommendation: at the bottom of this file, I provide a Snakefile which is the easiest way to run Salmon on the cluster (everything is taken care of for you). So read over the following for understanding what the steps look like, but for a project with many files you shouldn't be running the steps one-by-one or with bash scripts, but using Snakemake.

Also, a workflow from the Salmon authors on how to run Salmon can be found here:

https://combine-lab.github.io/salmon/getting_started/

Indexing the reference transcripts

First you'll need to download or generate a FASTA file of all of the reference transcript sequences. You can download a FASTA file of transcript sequence from GENCODE. For organisms other than human or mouse, you can use Ensembl FASTA (we recommend combining coding and non-coding RNA FASTA though).

(If GENCODE or Ensembl are not an option, you can also generate such a file using a tool such as rsem-prepare-reference with a GTF file and a genome FASTA file.)

It's a good idea to download the GTF file whenever you download a transcripts FASTA file.

Then, the two Salmon steps are 1) generating an index (only needs to happen once), and then 2) quantifying each sample against the index. Good to use a descriptive index name. You should put the Salmon version number on the end of the index name. The Salmon index will work for that FASTA and any sample, as long as you use the same version of Salmon for quantification. Re-indexing is no big deal, so when you upgrade Salmon, you need to re-index the transcripts FASTA.

Salmon indexing might look like this (this needs to be run either as a cluster job [best option], see below, or at least from an interactive session).

salmon index --gencode -t gencode.vABC.transcripts.fa -i gencode.vABC-salmon_x.y.z

(The --gencode is a special argument for GENCODE, which helps clean up the transcript names.)

Quantifying reads

Quantifying reads with this index might* look like this below (note that the quants dir needs to be created first):

salmon quant \
  -l A --gcBias -p 12 \
  --numGibbsSamples 20 --thinningFactor 100 \
  -i gencode.vABC-salmon_x.y.z \
  -o quants/sample1 \
  -1 fastq/sample1_1.fastq.gz \
  -2 fastq/sample1_2.fastq.gz

* Except, when working on the cluster we want to submit jobs to the cluster! Never run Salmon or any other programs on the login node, and you don't want to be sitting by your laptop during an interactive job quantifying dozens of samples. Therefore we could write a bash script to submit to the cluster, or use Snakemake. First I'll show what that bash script would look like, then I'll give the Snakemake solution.

The batch script might look like:

#!/bin/bash
#
#SBATCH --job-name=quant
#SBATCH --time=60
#SBATCH --mem=10000
#SBATCH -n 12
#SBATCH -N 1
#SBATCH [email protected]
#SBATCH --mail-type=ALL

/path/to/salmon_x.y.z/bin/salmon quant \
  -l A --gcBias -p 12 \
  --numGibbsSamples 20 --thinningFactor 100 \
  -i gencode.vABC-salmon_x.y.z \
  -o quants/sample1 \
  -1 fastq/sample1_1.fastq.gz \
  -2 fastq/sample1_2.fastq.gz

And then you can submit this to cluster with sbatch quant.sh.

However, with bash scripts, we have to run a job for each sample, and it's possible to make a mistake, e.g. if we change the sample name in the reads but we forget to change the sample name of the output directory. Therefore, it's safer and a better idea to run many jobs programmatically with Snakemake.

I therefore recommend using Snakemake to run Salmon across multiple files.

Snakemake

I have two Snakemake files for running Salmon, that can be found at the link below.

  1. Snakefile gives an example of how to run Salmon indexing and quantification over a number of files. First make the quants directory before running.
  2. Snakefile_with_QC gives an example of running both Salmon indexing/quantification as well as QC with FASTQC and MultiQC, all compiled into a single report. Note that to run this, you need to rename the file to Snakefile and load the FASTQC and MultiQC modules on the cluster. Also, first make the quants, qc, and multiqc directories before running.

https://gist.github.com/mikelove/5a8134e57f652f970f1a176efc900cbe

To run Snakemake on the cluster I use the following bash script snake.sh:

#!/bin/bash
#
#SBATCH --job-name=snake
#SBATCH --time=1440
#SBATCH --mem=1000

module load python
snakemake -j 4 --latency-wait 30 --cluster "sbatch --mem=10000 -N 1 -n 12"

The -j 4 indicates to run 4 jobs at a time, each one with 10 Gb, and with 12 threads for each job each.

Index job (if not using Snakemake)

For completeness, a bash script for indexing:

#!/bin/bash
#
#SBATCH --job-name=index
#SBATCH --time=60
#SBATCH --mem=10000
#SBATCH -n 12
#SBATCH -N 1
#SBATCH [email protected]
#SBATCH --mail-type=ALL

/path/to/salmon_x.y.z/bin/salmon index -p 12 --gencode -t gencode.vABC.transcripts.fa -i gencode.vABC-salmon_x.y.z