Output

This document describes the output produced by the pipeline. Most of the plots are taken from the MultiQC report, which summarises results at the end of the pipeline.

Pipeline overview

The pipeline is built using Nextflow. See main README.md for a condensed overview of the steps in the pipeline, and the bioinformatics tools used at each step.

See Illumina website for more information regarding the ATAC-seq protocol, and for an extensive list of publications.

The directories listed below will be created in the output directory after the pipeline has finished. All paths are relative to the top-level results directory.

Library-level analysis

The initial QC and alignments are performed at the library-level e.g. if the same library has been sequenced more than once to increase sequencing depth. This has the advantage of being able to assess each library individually, and the ability to process multiple libraries from the same sample in parallel.

1. Raw read QC

   Software:
   FastQC

   Description:
   FastQC gives general quality metrics about your reads. It provides information about the quality score distribution across your reads, the per base sequence content (%A/C/G/T). You get information about adapter contamination and other overrepresented sequences. For further reading and documentation see the FastQC help.

   Output directories:

   • fastqc/
     FastQC *.html files for read 1 (and read2 if paired-end) before adapter trimming.
   • fastqc/zips/
     FastQC *.zip files for read 1 (and read2 if paired-end) before adapter trimming.

2. Adapter trimming

   Software:
   Trim Galore!

   Description:
   Trim Galore! is a wrapper tool around Cutadapt and FastQC to consistently apply quality and adapter trimming to FastQ files. By default, Trim Galore! will automatically detect and trim the appropriate adapter sequence. For most ATAC-seq datasets this will be the Nextera adapter sequence ‘CTGTCTCTTATA’. See usage.md for more details about the trimming options.

   Output directories:

   • trim_galore/
     If --saveTrimmed is specified *.fastq.gz files after adapter trimming will be placed in this directory.
   • trim_galore/logs/
*.log files generated by Trim Galore!.
   • trim_galore/fastqc/
     FastQC *.html files for read 1 (and read2 if paired-end) after adapter trimming.
   • trim_galore/fastqc/zips/
     FastQC *.zip files for read 1 (and read2 if paired-end) after adapter trimming.

   Plots:
   MultiQC - Cutadapt trimmed sequence plot

3. Alignment

   Software:
   BWA, SAMtools

   Description:
   Adapter-trimmed reads are mapped to the reference assembly using BWA. A genome index is required to run BWA so if this isnt provided explicitly using the --bwa_index_dir and --bwa_index_base parameters then it will be created automatically from the genome fasta input. The index creation process can take a while for larger genomes so it is possible to use the --saveGenomeIndex parameter to save the indices for future pipeline runs, reducing processing times.

   File names in the resulting directory (i.e. bwa/library/) will have the ‘.Lb.’ (Library) suffix.

   Output directories:

   • bwa/library/
     The files resulting from the alignment of individual libraries are not saved by default so this directory will not be present in your results. You can override this behaviour with the use of the --saveAlignedIntermediates flag in which case it will contain the coordinate sorted alignment files in *.bam format.
   • bwa/library/samtools_stats/
     SAMtools *.flagstat, *.idxstats and *.stats files generated from the alignment files.

   Plots:
   MultiQC - SAMtools idxstats plot

Merged library-level analysis

The library-level alignments associated with the same sample are merged and subsequently used for the downstream analyses.

1. Alignment merging, duplicate marking and filtering

   Software:
   picard, SAMtools, BEDTools, BAMTools, Pysam

   Description:
   Picard MergeSamFiles and MarkDuplicates are used in combination to merge the alignments, and for the marking of duplicates, respectively. If you only have one library for any given replicate then the merging step isnt carried out because the library-level and merged library-level BAM files will be exactly the same.

   Read duplicate marking is carried out using the Picard MarkDuplicates command. Duplicate reads are generally removed from the aligned reads to mitigate for fragments in the library that may have been sequenced more than once due to PCR biases. There is an option to keep duplicate reads with the --keepDups parameter but its generally recommended to remove them to avoid the wrong interpretation of the results. A similar option has been provided to keep reads that are multi-mapped - --keepMultiMap.

   Certain cell types and tissues yield an enormous fraction (typically 20–80%) of unusable sequences of mitochondrial origin. This is a known problem that is specific to ATAC-seq library preps - see Montefiori et al. 2017. There is an option to keep these reads using the --keepMito parameter but its generally recommended to remove these in order to get a more reliable assessment of the duplication rate from the rest of the genome, and to avoid any biases in the downstream analyses.

   Other steps have been incorporated into the pipeline to clean the resulting alignments - see main README.md for a more comprehensive listing, and the tools used at each step.

   File names in the resulting directory (i.e. bwa/mergedLibrary/) will have the ‘.mLb.’ suffix to denote merged Libraries.

   Output directories:

   • bwa/mergedLibrary/
     Merged library-level, coordinate sorted *.bam files after the marking of duplicates, and filtering based on various criteria. The file suffix for the final filtered files will be *.mLb.clN.*. If you specify the --saveAlignedIntermediates parameter then two additional sets of files will be present. These represent the unfiltered alignments with duplicates marked (*.mLb.mkD.*), and in the case of paired-end datasets the filtered alignments before the removal of orphan read pairs (*.mLb.flT.*).
   • bwa/mergedLibrary/samtools_stats/
     SAMtools *.flagstat, *.idxstats and *.stats files generated from the alignment files.
   • bwa/mergedLibrary/picard_metrics/
     Alignment QC files from picard CollectMultipleMetrics and the metrics file from MarkDuplicates: *_metrics and *.metrics.txt, respectively.
   • bwa/mergedLibrary/picard_metrics/pdf/
     Alignment QC plot files in *.pdf format from picard CollectMultipleMetrics.

   Plots:
   MultiQC - Picard insert size plot
   MultiQC - Picard deduplication stats plot

2. Normalised bigWig files

   Software:
   BEDTools, wigToBigWig

   Description:
   The bigWig format is in an indexed binary format useful for displaying dense, continuous data in Genome Browsers such as the UCSC and IGV. This removes the need to load the much larger BAM files for data visualisation purposes which will be slower and result in memory issues. The coverage values represented in the bigWig file can also be normalised in order to be able to compare the coverage across multiple samples - this is not possible with BAM files. The bigWig format is also supported by various bioinformatics software for downstream processing such as meta-profile plotting.

   Output directories:

   • bwa/mergedLibrary/bigwig/
     Normalised *.bigWig files scaled to 1 million mapped reads.

3. Call peaks

   Software:
   MACS2, HOMER, R

   Description:
   MACS2 is one of the most popular peak-calling algorithms for ChIPSeq data. For ATAC-seq data we are also looking for genome-wide regions of enrichment but in this case without comparison to a standard control sample (e.g. input DNA).

   By default, the peaks are called with the MACS2 --broad parameter as this is recommended for ATAC-seq data. If, however, you would like to call narrow peaks then please provide the --narrowPeak parameter when running the pipeline.

   HOMER annotatePeaks.pl is used to annotate the peaks relative to known genomic features. HOMER is able to use the --gtf annotation file which is provided to the pipeline. Please note that some of the output columns will be blank because the annotation isnt provided using HOMER’s in-built database format. However, the more important fields required for downstream analysis will be populated i.e. Annotation, Distance to TSS and Nearest Promoter ID.

   Various QC plots per sample including number of peaks, fold-change distribution, FRiP score and peak-to-gene feature annotation are also generated by the pipeline. Where possible these have been integrated into the MultiQC report.

   See MACS2 outputs for a description of the output files generated by MACS2.

   Output directories:

   • bwa/mergedLibrary/macs2/
     • MACS2 output files: *.xls, *.broadPeak or *.narrowPeak, *.gappedPeak and *summits.bed.
       The files generated will depend on whether MACS2 has been run in narrowPeak or broadPeak mode.
     • HOMER peak-to-gene annotation file: *.annotatePeaks.txt.
   • bwa/mergedLibrary/macs2/qc
     • QC plots for MACS2 peaks: macs_peak.mLb.clN.plots.pdf
     • QC plots for peak-to-gene feature annotation: macs_annotatePeaks.mLb.clN.plots.pdf
     • MultiQC custom-content files for FRiP score, peak count and peak-to-gene ratios: *.FRiP_mqc.tsv, *.count_mqc.tsv and macs_annotatePeaks.mLb.clN.summary_mqc.tsv respectively.

   Plots:
   MultiQC - MACS2 total peak count plot
   MultiQC - MACS2 peaks FRiP score plot
   MultiQC - HOMER annotatePeaks peak-to-gene feature ratio plot

4. Create consensus set of peaks

   Software:
   BEDTools

   Description:
   In order to perform the differential accessibility analysis we need to be able to carry out the read quantification for the same intervals across all of the samples in the experiment. To this end, the individual peaksets called per sample have to be merged together in order to create a consensus set of peaks.

   Using the consensus peaks it is also possible to assess the degree of overlap between the peaks from a set of samples e.g. Which consensus peaks contain peaks that are common to a given set of samples?. This may be useful for downstream filtering of peaks based on whether they are called in multiple replicates/conditions. Please note that it is possible for a consensus peak to contain multiple peaks from the same sample. Unfortunately, this is sample-dependent but the files generated by the pipeline do have columns that report such instances and allow you to factor them into any further analysis.

   Output directories:

   • bwa/mergedLibrary/macs2/consensus/
     • Consensus peakset across all samples in *.bed format.
     • Consensus peakset across all samples in *.saf format. Required by featureCounts for read quantification.
     • HOMER *.annotatePeaks.txt peak-to-gene annotation file for consensus peaks.
     • Spreadsheet representation of consensus peakset across samples with gene annotation columns: *.boolean.annotatePeaks.txt.
       The columns from individual peak files are included in this file along with the ability to filter peaks based on their presence or absence in multiple replicates/conditions.
     • Spreadsheet representation of consensus peakset across samples without gene annotation columns: *.boolean.txt.
       Same as file above but without annotation columns.
     • UpSetR files to illustrate peak intersection: *.boolean.intersect.plot.pdf and *.boolean.intersect.txt.

   Plots:
   R - UpSetR peak intersection plot

5. Read counting and differential accessibility analysis

   Software:
   featureCounts, DESeq2, R

   Description:
   The featureCounts tool is used to count the number of reads relative to the consensus peakset across all of the samples. This essentially generates a file containing a matrix where the rows represent the consensus intervals, the columns represent all of the samples in the experiment, and the values represent the raw read counts.

   DESeq2 is more commonly used to perform differential expression analysis for RNA-seq datasets. However, it can also be used for ATAC-seq differential accessibility analysis, in which case you can imagine that instead of counts per gene for RNA-seq data we now have counts per accessible region.

   This pipeline uses a standardised DESeq2 analysis script to get an idea of the reproducibility within the experiment, and to assess the overall differential accessibility. Please note that this will not suit every experimental design, and if there are other problems with the experiment then it may not work as well as expected. By default, the peak sets are not filtered, therefore, the consensus peaks will be generated across all peaks. However, it is possible to filter the consensus peaks and the corresponding read counts based on user-defined criteria (outlined in the section above), and then to use the same scripts to re-generate the results for a more refined analysis. In future iterations of the pipeline more formal analyses such as IDR may be implemented to obtain reproducible and high confidence peak sets with which to perform this sort of analysis.

   By default, all possible pairwise comparisons across the groups within the experiment are performed. The DESeq2 results are outputted by the pipeline in various ways. You can load up the results across all of the comparisons in a single spreadsheet, or individual folders will also be created that contain the results specific to a particular comparison. For the latter, additional files will also be generated where the intervals have been pre-filtered based on a couple of standard FDR thresholds.

   Please see DESeq2 output for a description of the columns generated by DESeq2.

   Output directories:

   • bwa/mergedLibrary/macs2/consensus/deseq2/
     • .featureCounts.txt file for read counts across all samples relative to consensus peak set.
     • Differential accessibility *.results.txt spreadsheet containing results across all consensus peaks and all comparisons.
     • *.plots.pdf file for PCA and hierarchical clustering.
     • *.log file with information for number of differentially bound intervals at different FDR and fold-change thresholds for each comparison.
     • *.dds.rld.RData file containing R dds and rld objects generated by DESeq2.
     • R_sessionInfo.log file containing information about R, the OS and attached or loaded packages.
   • bwa/mergedLibrary/macs2/consensus/<COMPARISON>/
     • *.results.txt spreadsheet containing comparison-specific DESeq2 output for differential accessibility results across all peaks.
     • Subset of above file for peaks that pass FDR <= 0.01 (*FDR0.01.results.txt) and FDR <= 0.05 (*FDR0.05.results.txt).
     • BED files for peaks that pass FDR <= 0.01 (*FDR0.01.results.bed) and FDR <= 0.05 (*FDR0.05.results.bed).
     • MA, Volcano, clustering and scatterplots at FDR <= 0.01 and FDR <= 0.05: *deseq2.plots.pdf.
   • bwa/mergedLibrary/macs2/consensus/sizeFactors/
     Files containing DESeq2 sizeFactors per sample: *.txt and *.RData.

   Plots:
   MultiQC - featureCounts consensus peak read assignment plot
   MultiQC - DESeq2 PCA plot
   MultiQC - DESeq2 sample similarity plot
   R - DESeq2 MA plot
   R - DESeq2 Volcano plot

6. ataqv

   Software:
   ataqv

   Description:
   ataqv is a toolkit for measuring and comparing ATAC-seq results. It was written to help understand how well ATAC-seq assays have worked, and to make it easier to spot differences that might be caused by library prep or sequencing.

   Please see ataqv homepage for documentation and an example report.

   Output directories:

   • bwa/mergedLibrary/ataqv/
.json files containing ATAC-seq specific metrics for each sample.
   • bwa/mergedLibrary/ataqv/html/
     Folder containing ataqv results aggregated across all samples for visualisation via an internet browser.

Merged replicate-level analysis

The alignments associated with all of the replicates from the same experimental condition can also be merged. This can be useful to increase the coverage for peak-calling and for other analyses that require high sequencing depth such as motif footprinting. The analysis steps and directory structure for bwa/mergedLibrary/ and bwa/mergedReplicate/ are almost identical.

File names in the resulting directory (i.e. bwa/mergedReplicate/) will have the ‘.mRp.’ suffix to denote merged Replicates.

You can skip this portion of the analysis by specifying the --skipMergeReplicate parameter.

NB: Merged library-level alignments will be used for read counting relative to the consensus merged replicate-level peakset. This is the only way in which differential analysis can be performed at the merged replicate-level.

Aggregate analysis

1. Present QC for the raw read, alignment, peak and differential accessibility results

   Software:
   MultiQC

   Description:
   MultiQC is a visualisation tool that generates a single HTML report summarising all samples in your project. Most of the pipeline QC results are visualised in the report and further statistics are available within the report data directory.

   Results generated by MultiQC collate pipeline QC from FastQC, TrimGalore, samtools flagstat, samtools idxstats, samtools stats, picard CollectMultipleMetrics, picard MarkDuplicates and featureCounts. The default multiqc config file also contains the provision for loading custom-content to report peak counts, FriP scores, peak-to-gene annnotation proportions, sample-similarity heatmaps and PCA plots.

   The pipeline has special steps which also allow the software versions to be reported in the MultiQC output for future traceability.

   For more information about how to use MultiQC reports, see http://multiqc.info.

   Output directories:

   • multiqc/
     • multiqc_report.html - a standalone HTML file that can be viewed in your web browser.
     • multiqc_data/ - directory containing parsed statistics from the different tools used in the pipeline.
     • multiqc_plots/ - directory containing static images from the report in various formats.

2. Create IGV session file

   Software:
   IGV

   Description:
   An IGV session file will be created at the end of the pipeline containing the normalised bigWig tracks, peaks and differential sites. This avoids having to load all of the data individually into IGV for visualisation.

   The genome fasta file required for the IGV session will be the same as the one that was provided to the pipeline. This will be copied into reference_genome/ to overcome any loading issues. If you prefer to use another path or an in-built genome provided by IGV just change the genome entry in the second-line of the session file.

   The file paths in the IGV session file will only work if the results are kept in the same place on your storage. If the results are moved or for example, if you prefer to load the data over the web then just replace the file paths with others that are more appropriate.

   Once installed, open IGV, go to File > Open Session and select the igv_session.xml file for loading.

   NB: If you arent using an in-built genome provided by IGV you will need to load the annotation yourself e.g. in .gtf and/or .bed format.

   Output directories:

   • igv/
     • igv_session.xml file.
     • igv_files.txt file containing a listing of the files used to create the IGV session, and their associated colours.

   Plots:
   IGV screenshot

Other results

1. Reference genome files

   Software:
   BWA, BEDTools, SAMtools

   Description: Reference genome-specific files can be useful to keep for the downstream processing of the results.

   Output directories:

   • reference_genome/
     A number of genome-specific files are generated by the pipeline in order to aid in the filtering of the data, and because they are required by standard tools such as BEDTools. These can be found in this directory along with the genome fasta file which is required by IGV.
   • reference_genome/BWAIndex/
     If they dont exist already and if the --saveGenomeIndex parameter is provided then the alignment indices generated by the pipeline will be saved in this directory. This can be quite a time-consuming process so it permits their reuse for future runs of the pipeline or for other purposes.

2. Pipeline information

   Software:
   Nextflow!

   Description:
   Nextflow provides excellent functionality for generating various reports relevant to the running and execution of the pipeline. This will allow you to trouble-shoot errors with the running of the pipeline, and also provide you with other information such as launch commands, run times and resource usage.

   See Nextflow Tracing & visualisation.

   Output directories:

   • pipeline_info/
     Reports generated by the pipeline - pipeline_report.html, pipeline_report.txt and software_versions.csv.
   • pipeline_info/nf-core/
     Default reports generated by Nextflow - execution_report.html, execution_timeline.html, execution_trace.txt and pipeline_dag.svg.
   • Documentation/
     Documentation for interpretation of results in HTML format - results_description.html.