hicer-nf is a bioinformatics analysis pipeline used for HiC data.

The pipeline is built using Nextflow, a workflow tool to run tasks across multiple compute infrastructures in a very portable manner. It comes with docker containers making installation trivial and results highly reproducible.

1. Raw read QC (FastQC)
2. Adapter trimming (Trim Galore!)
3. Alignment, Filtering and Deduplication (HiCUP)
4. Generating an indexed pairs file from the alignments (pairix)
5. Build base resolution contact matrix (i.e. the smallest resolution given; default 5kb) (cooler)
6. Aggregate bins to a range of default resolutions including optional custom resolutions (cooler)
7. Compute matrix normalization vectors for all aggregated resolutions with the KR and the IC algorithm (HiCExplorers C++ implementation of the KR algorithm and cooler Out-Of-Core balancing)
8. Present QC for raw reads, alignment and filtering MultiQC

The generated mcool files are compatible with cooltools for downstream analysis and Higlass for visualization. KR balancing weights are stored in the 'weight', IC balancing weights are stored in the 'ICE' column of the bins table of each cooler. A range of possible downstream analyses can be found in our hic_analysis repo. In addition the pipeline also generates hic files with juicer_tools pre for visualization in juicebox and/or loop calling with juicer's HICCUPS algorithm.

i. Install nextflow

ii. Install one of docker, singularity or conda

iii. Clone repository

nextflow pull pavrilab/hicer-nf

iv. Read the Tips for a smooth user experience right below this section

v. Start running your own analysis!

# if you have specified an igenomes directory from or run a profile from any of the institutions
# listed here https://raw.githubusercontent.com/nf-core/configs/master/nfcore_custom.config
nextflow run pavrilab/hicer-nf --input samples.txt --genome mm9 --re ^GATC,MboI

# else you can specify the most essential things manually
# if HICUP digest and bowtie2 index are not available
nextflow run pavrilab/hicer-nf --input samples.txt --genome mm9 --re ^GATC,MboI --fasta genome.fa --chromSizes chrom.sizes

# if micro-C or similar protocols with non-specific cutters are used
nextflow run pavrilab/hicer-nf --input samples.txt --genome mm9 --resolutions 500 --chromSizes chrom.sizes

These invocations compute cooler and hic files for a default resolution list of 5kb, 10kb, 25kb, 50kb, 100kb, 250kb, 500kb and 1Mb (except for the micro-C run). If you want resolutions that are not listed here you could use the --resolutions parameter (see below). In addition to this, the pipeline parallelizes the hicup workflow in a more flexible way than the hicup control script by splitting the sample reads into chunks of specific length and threads these through the hicup scripts. Per default, the fastq chunks have a size of 25M reads, but this can be changed using the --readsPerSplit parameter to suit you sample size. Be aware that in case of the micro-C mode, the pipeline alters the way it processes the read pairs. In particular the RE truncation is skipped and the filtering is solely based on mapping proximity of two reads of a pair which is 500 bp per default but can be altered by the --minMapDistance argument. Be aware that because the restiction fragments are used to calculate the insert size distribution, this is not available in the micro-C invokation. Please also not that you might have to modify the memory of the balancing process due to the high resolution of micro-C. Additionally, the HICUP QC report will not contain an insert size distribution and only display the "Same fragment - internal" category for filtering.

The current resource configuration was tested for a 2.6B read Hi-C data set and ran in approximately a day. However, if you are using bigger samples you might have to change the settings in terms of job duration. Additionally, the pipeline automatically adjusts for genome size in terms of memory for some critical processes. This behaviour was tested with the axolotl genome and runs smoothly with it.

The pipeline comes with a prebuilt Docker container which contains all the software you need to run right away. To skip the process (and frustration of installing everything manually) we would recommend using Singularity or Docker with the respective profile for running the pipeline.

In general, the pipeline was designed to run with a predefined reference genome database such as the igenomes, which location can be specified via the -c option (detailed below in the section Customizing the resource requirements, igenomes_base and other parameters). However, if you do not want to use this feature you can just specify the whole genome fasta and the chrom.sizes file of the genome via the command line parameters --fasta and --chromSizes respectively. The pipeline than computes the bowtie2 index automatically. If you do not use the igenomes database please make sure to omit the --genome parameter or set --igenomes_ignore T or the pipeline will otherwise try to access it

We developed the pipeline with the aim to facilitate a smooth integration of the results into the two most widely used postprocessing frameworks cooltools and juicer_tools. This also included the possibility to directly view results on their respective browser right away. Thus, we prespecified a range of resolutions which will be computed by default (5000, 10000, 25000, 50000, 100000, 250000, 500000, 1000000) to meet this aim. The --resolutions parameter does not overwrite this default set but rather adds the specified resolutions to this list (duplicates are ignored). You can customize the default set as described below in Customizing the resource requirements, igenomes_base and other parameters. Please be aware of this behaviour

Please make sure that the format of these files meet exactly the specifications detailed below. This means the paths to the fastq files must be given as full paths otherwise the pipeline will not find the respective files on your harddrive (see --input section for more details). The chrom.sizes fill must exactly contain two tab-separated columns containing chromosome name and its size (see --chromSizes section for more details). This format is crucial as juicer_tools pre will crash if there are more than these two columns.

The resource requirements of this pipeline were mostly tweaked for use with the CBE cluster at the Vienna Biocenter. Thus you might want to customize these settings accorting to your infrastructure. In order to do so for individual processes please make use of the way nextflow handles configuration overrides as described here. This will prevent any complication arising from directly editing the resource.config file. In brief, do the following:

1. create a new nextflow.config file somewhere on your disc
2. copy the respective process resource configurations you want to change from the resources.config file to the new nextflow.config file like this

process {
   withName: processname {
      cpus = 2
      memory = { 20.GB * task.attempt }
      time = { 4.h * task.attempt }
   }
   .
   .
   .
}

3. Use this file via nextflows -c parameter to override the default configuration

nextflow run pavrilab/hicer-nf -c /path/to/custom/nextflow.config [further parameters]

In addition to this you can also customize parameter settings such as the default resolutions list, the location of the igenomes database or the igenomes_ignore parameter by simply appending the nextflow.config file generated above with

params {
   defaultResolutions = '1000,10000,100000,1000000'
   igenomes_base      = '/path/to/igenomes/base/directory'
   .
   .
   .
}

Although we are using the KR implementation of the HiCExplorer in our pipeline, we do not perform rescaling of the balancing weights as their hicCorrectMatrix tool does. As described here this rescaling mainly has the purpose to subvert any issues the HiCExplorer tools may encounter with small values resulting from balancing rows and cols to a sum of 1. However, it has a major drawback, namely the reintroduction of the coverage bias. Since matrix balancing is intended to remove any coverage bias from the Hi-C data, two matrices from different conditions are assumed to be comparable after balancing. If we now recale with the factor sqrt(sum of balanced matrix / sum of unbalanced matrix) we effectively reintroduce the coverage bias and thus abolish comparability. Thus, if you intend to use tools other than cooltools and especially HiCExplorer keep this in mind if you need to have the balancing weights rescaled.

Use this parameter to choose a configuration profile. Profiles can give configuration presets for different compute environments. The pipeline uses the nf-core custom_configs repo to load config files for compute clusters of different institutions. So please check with this or make your own config file if you want to use the igenomes reference database. Note that multiple profiles can be loaded, for example: -profile docker cbe - the order of arguments is important!

If -profile is not specified at all the pipeline will be run locally and expects all software to be installed and available on the PATH.

• docker
  • A generic configuration profile to be used with Docker
  • Pulls software from dockerhub: zuberlab/hicer-nf
• singularity
  • A generic configuration profile to be used with Singularity
  • Pulls software from DockerHub: zuberlab/hicer-nf

You will need to create a design file with information about the samples in your experiment before running the pipeline. Use this parameter to specify its location. It has to be a comma-separated file with 3 columns, and a header row as shown in the examples below. Specified input samples will be grouped by samplename and analyzed as a whole (i.e. fastq files will be concatenated)

--input '[path to samples file]'

sample,fastq_1,fastq_2
WT,/path/to/WT_1.fastq.gz,/path/to/WT_2.fastq.gz
KD,/path/to/KD_1.fastq.gz,/path/to/KD_2.fastq.gz

The genome argument has two properties. Firstly, it is used to document the name of the reference genome of the organism the Hi-C data originates from and secondly it can be used to retrieve prespecified reference data from a local igenomes database, where the pipeline automatically takes the files it requires for processing the Hi-C data (i.e. a bowtie2 index, a genome fasta and a chromSizes file).

There are 31 different species supported in the iGenomes references. To run the pipeline, you must specify which to use with the --genome flag.

You can find the keys to specify the genomes in the iGenomes config file. Common genomes that are supported are:

• Human
  • --genome GRCh37
• Mouse
  • --genome GRCm38
• Drosophila
  • --genome BDGP6
• S. cerevisiae
  • --genome 'R64-1-1'

Note that you can use the same configuration setup to save sets of reference files for your own use, even if they are not part of the iGenomes resource. See the Nextflow documentation for instructions on where to save such a file.

The syntax for this reference configuration is as follows:

params {
  genomes {
    'GRCh37' {
      fasta      = "/path/to/genome/fasta/file" // Used if no bowtie2 index or hicupDigest given
      bowtie2    = "/path/to/bowtie2/index/basename"
      chromSizes = "/path/to/chrom/sizes/file"
    }
    // Any number of additional genomes, key is used with --genome
  }
}

Hi-C experiments typically involve digestion of the with restriction enzymes after cross-linking. To be able to computationally identify artifacts in the sequence data arising from this process the HICUP pipeline requires the sequence motif the used enzyme cuts including its name. This information is given via the --re parameter in the form specified by HICUP which is ^GATC,MboI, where ^ indicates the cutsite, GATC is the sequence motif the enzyme recognizes and MboI is the name of the enzyme. Due to some specificities of HICUP the current version of the pipeline does not support the double digestion protocol. In case of micro-C or protocols with non-specific cutters, this parameter can be omitted.

This parameter is used to specify the genome fasta file and is only required if no igenomes database is available or hicup digestion file or bowtie2 index is not available locally. The file is used for in-silico restriction digestion for HICUP (if the file is not specified manually with --hicupDigest, see below) and bowtie2 index computation (if not specified manually). In case of omitting --re this can also be omitted if you already have a bowtie2 index specified.

This parameter is used to specify file containing the chromosome names and their size. This information is used by cooler for binning the genome. The file can either be a tab-separated file with two columns (no header and empty lines)

cat chrom.sizes.tsv

chr1 10000
chr2 30499

or an XML file as given by the igenomes database

cat chrom.sizes.xml

<sequenceSizes genomeName="genome">
        <chromosome fileName="genome.fa" contigName="chr1" totalBases="10000" isCircular="false" md5="be7e6a13cc6b9da7c1da7b7fc32c5506" ploidy="2" knownBases="126847849" />
        <chromosome fileName="genome.fa" contigName="chr2" totalBases="30499" isCircular="false"
</sequenceSizes>

The XML will be converted to TSV in-situ where the pipeline uses the contigName and totalBases keys for generating the TSV file. Thus at least these fields have to be present in the XML. This option is for compatibility with older igenomes databases.

This parameter specifies the number of reads each split fastq file should contain for parallel processing. The default is 25M reads per split file.

By default, the pipeline computes matrices and normalizations thereof for resolutions 5kb, 10kb, 25kb, 50kb, 100kb, 250kb, 500kb and 1Mb for both cooler and hic files. However, if you want to add additional resolutions which are not present in this list you can use the --resolutions parameter by passing either a single value or multiple ones separated by commas in Bp. The specified resolutions will then be added. However, be aware that the the specified resolutions have to be an integer multiple of the smallest resolution in the list i.e. a multiple of 5kb when using the default resolutions or a multiple of any resolution specified that is smaller than 5kb. In this sense any smaller resolution should also be a divisor of all default resolutions (e.g. 500bp, 1kb would all be fine however 1.5kb will not work because 5kb is not an integer multiple of 1.5kb). Defaults are 5000, 10000, 25000, 50000, 100000, 250000, 500000, 1000000.

--resolution 20000 / --resolution 20000,75000,200000

This parameter specifies the minimum mapping distance of two reads of a pair and is only used when processing micro-C or similar data to filter out closely mapping pairs that are thought to belong to the same or adjacent fragments. By default this parameter is set to 500bp (roughly two nucleosomes).

Name of the folder to which the output will be saved (default: results)

--outdir '[directory name]'

The pipeline was developed by Tobias Neumann and Daniel Malzl for use at the IMP, Vienna.

The nf-core/rnaseq and nf-core/chipseq pipelines developed by Phil Ewels were initially used as a template for this pipeline. Many thanks to Phil for all of his help and advice, and the team at SciLifeLab.

Many thanks to others who have helped out along the way too, including (but not limited to): @apeltzer, @micans, @pditommaso, @golobor.

• Nextflow

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• FastQC

• Trim Galore!

• pairix

• pandas

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• cooler

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• numpy

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• scipy

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• pysam

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• Singularity

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• Docker