chompR

The chompR package is designed to perform peak-calling and differential analysis of ChIP or ATAC-seq data. It provides tools to:

  • perform quality control of raw reads in fastq format and generate HTML reports (using FASTQC and MULTIQC)
  • perform read alignment using HISAT2 and sort and filter alignment files
  • perform quality control of aligned reads
  • perform peak-calling using MACS2
  • run differential analysis using DESeq2, and to export the results into easily readable tab-delimited files
  • perform pathway analysis

The chompR package does not attempt to replace the tools it uses, but instead provides an automated environment in which to use them.

The website for chompR may be found here.

Installation

Command line tools

The workflow used by chompR requires the installation of several command-line tools: Fastqc (v0.11.7), MultiQC (v1.6), fastp (v0.19.4), SAMtools (v1.9), Sambamba (v0.6.7), HISAT2 (v2.1.0), MACS (v2.1.0), and featureCounts (v1.6.2). Installation instructions for these may be found at their respective websites, but a guide is given below for convenience.

If using a Unix-based system, open up a terminal and follow the commands as is. If using the windows subsystem for linux (WSL) on Windows 10 then WSL must first be set up as detailed here. Once WSL is up and running, then the tools may be installed as on any Unix-based system.

It is assumed that root priviliges are available (if necessary) - if not, then a system administrator may need to install these for you.

MultiQC

To run MultiQC ensure there is a working python distribution.

R package dependencies

There are a number of dependencies to the chompR package detailed in the DESCRIPTION file. These are either CRAN or Bioconductor packages. These dependencies will automatically install when installing chompR.

Install chompR

Once all dependencies are installed, then chompR may be installed as follows:

devtools::install_github("anilchalisey/chompR", build_vignettes = TRUE)

Usage

The entire workflow of chompR may be run by a call to a single function run_chip() or run_atac(), as shown below.

The arguments to the run_chip() and run_atac() functions are:

sample.info:

This is the path to a tab-delimited file with at least the columns:

  • condition: treatment or condition labels on the basis of which the differential analysis will be performed (e.g. “CONTROL”, “TREATED”; or “WILD-TYPE”, “KNOCK-OUT1”, “KNOCK-OUT2”).
  • sample: names of the samples - these labels will be used in plots and as sample column headers in the output.
  • file1: absolute or relative path to fastq files.

Optional columns include:

  • file2: if fastq files and PE reads, then this column should also be present, specifying the absolute or relative paths to the second pair of fastq reads.
    # batch: if a batch effect is to be included in the design, then this should be identified under this column (e.g. litter number or sequencing run). # input1: if ChIP-seq and input samples were used then the path to the fastq file for the input matching file1 in the same row should be included here. # input2: same as input1, but if PE reads, then the second pair should be specified here.

Dummy examples of the tab-delimited files accepted by chompR may be found at system.file("extdata", "paired-example.txt", pkg = "chompR"), system.file("extdata", "sinle-example.txt", pkg = "chompR"), and system.file("extdata", "quants-example.txt", pkg = "chompR").

reference:

The order in which the condition labels should be evaluated when performing differential analysis. For example, c(“A”, “B”, “C”, “D”) would mean “A” is the reference condition to which “B”, “C” and “D” are compared; in addition, “C” and “D” will be compared to “B”, and “D” will be compared to “C”. If then the comparisons will be arranged alphabetically.

species:

May be one of “human” or “mouse” - other options will be added in later versions.

output.dir:

Directory to which results should be saved.

threads:

Number of threads to be used when parallelisation is possible. If NULL then one less than the maximum numbre of threads available will be used.

index.dir:

The path to the hisat2 index.

Other arguments are also possible, and details for these may be found in the manual, but for most users, the default settings are satisfactory.

Importantly, if the user has already run the analysis once and read QC and alignment has already occurred, then this will be detected and these steps will be skipped (provided the same output directory is specified).

Other functions

The package also contains several functions to directly run several tools from within R. These all begin with and include , , , , , , , , . Additional functions include those to import MACS2 format peak files into R () and to annotate peaks ().

Output

Once complete, all the results will be saved in the specified output directory with the following structure:

#>                  levelName
#> 1  results                
#> 2   ¦--alignment_stats.txt
#> 3   ¦--bam                
#> 4   ¦--consensus          
#> 5   ¦--cross-correlation  
#> 6   ¦--differential       
#> 7   ¦--fastqc             
#> 8   ¦--index              
#> 9   ¦--multiqc            
#> 10  ¦--filteredbam        
#> 11  ¦--shifted            
#> 12  ¦--trimmed            
#> 13  ¦--peaks              
#> 14  ¦--DESeq2             
#> 15  °--fragmentLength

The directories DESeq2, fastqc, and multiqc contain the results of the respective analyses including relevant plots. The index directory contains the hisat2 index for human or mouse; it will only be creeated if a pre-existing index was not specified beforehand. The bam, filteredbam, trimmedbam, and shifted directories contain BAM files. The shifted directory only exists for ATAC-seq analysis and contains BAM files whose reads have been shifted to take account of transposon insertion. The peaks and consensus directories contain MACS2 called peaks and consensus peaks respectively. The differential directory contains the results of the a binary differential analysis i.e. peaks present or absent in each sample. The cross-correlation and fragmentlength directories are only created for ChIP_seq or ATAC-seq respectively (i.e. they are mutually exclusive) and contain QC plots following alignment. The alignment_stats.txt file contains simple alignment statistics.

Running only part of the chompR pipeline

Importantly, if the user has already run the analysis once and read QC and alignment has already occurred, then this will be detected and these steps will be skipped (provided the same output directory is specified).

Pathway analysis

GREAT analysis (using the GREAT website) is performed using the package rGREAT. A wrapper script, which converts regions into hg19 co-ordinates (if using “human”) and then runs the rGREAT package, is included.

peaks <- peak2Granges("peaks/results.narrowPeak")
pathway <- run_great(regions = peaks, species = "human")

This may be repeated as necessary for the other comparisons.

About chompR

The chompR package has been developed in the Chris O’Callaghan Group at the Centre for Cellular and Molecular Biology, University of Oxford by Anil Chalisey and Chris O’Callaghan.