Snakemake workflow: quaich

Quaich is a snakemake based workflow for reproducible and flexible analysis of Hi-C data. Quaich uses multi-resolution cooler (.mcool) files as its input. These files can be generated efficiently by the distiller data processing pipeline. Quaich takes advantage of the open2c ecosystem for analysis of C data, primarily making use of command line tools from cooltools. Quaich also makes use of chromosight and mustache to call Hi-C peaks (peaks, dots) as well as coolpuppy to generate lots of pileups.

Snakemake is a workflow manager for reproducible and scalable data analyses, based around the concept of rules. Rules used in Quaich are defined in the Snakefile. Quaich then uses a yaml config file to specify which rules to run, and which parameters to use for those rules.

Each rule in the Quaich Snakefile specifies inputs, outputs, resources, and threads. The best values for resources and threads depend on whether quaich is run locally or on a cluster. Outputs of one rule are often used as inputs to another. For example, the rule make_expected_cis calls cooltools compute-expected on a mcool for a set of regions at specified resolutions to output tsv. This output is then used in make_saddles, make_pileups, and call_loops_cooltools. For reprodcibility and easy setup wherever possible, the rules use snakemake wrappers instead of using shell/python code directly. This means every rule will have its own dedicated conda environment that is defined as part of the wrapper, and it is created the first time the pipeline is run.

rule make_expected_cis:
    input:
        cooler=lambda wildcards: coolfiles_dict[wildcards.sample],
        view=lambda wildcards: config["view"],
    output:
        f"{expected_folder}/{{sample}}_{{resolution,[0-9]+}}.expected.tsv",
    params:
        extra=lambda wildcards: config["expected"]["extra_args_cis"],
    threads: 4
    resources:
        mem_mb=lambda wildcards, threads: threads * 8 * 1024,
        runtime=60,
    wrapper:
        v3.12.2/bio/cooltools/expected_cis"

Quaich groups similar rules together in config.yaml, which is read as a python dictionary by the Snakefile. Parameters for individual rules are passed as indented (key, value) pairs. For example, call_dots configures three methods of calling dots in Hi-C: cooltools, chromosight, and mustache. The parameters for each specific rule are underneath, the shared parameters are below (in this case, the resolutions, and whether to generate pileups for the dot calls). Do always specifies if the workflow should attempt to produce the output for this rule. In most cases there is an extra argument, where any arbitrary command line arguments not exposed through the YAML can be passed to the underlying tool.

call_dots:
    methods:
        cooltools:
            do: True
            extra: "--max-loci-separation 10000000 --fdr 0.02"
        chromosight:
            do: True
            extra: ""
        mustache:
            do: True
            max_dist: 10000000
            extra: "-pt 0.05 -st 0.8"
    resolutions:
        - 10000
    pileup: True

quaich config.yaml has four main sections:

• genome
• samples and annotations Here you can set up what comparisons between samples to perform. The .tsv file with samples can contain any arbitrary columns, and you can provide names of those columns to fields_do_differ and fields_to_match, this will generate all combinations of samples to compare. For example, to compare different cell types the default config contains

  fields_to_match: null
  fields_to_differ:
  cell_type: hESCs

  This will not try to match any specific fields between samples, but will ensure that for comparisons the value of the cell_type column is different between samples, and the hESCs data will be used as the "reference" dataset, with the rest compared to this one.

If the samples.tsv file had a sex column, we could ensure to compare only samples on the same sex by providing

fields_to_match: sex

• i/o Here you can configure where the inputs are located and where the results are saved.

• snakemake rule configurations

The following analyses can be configured:

• expected: calculates cis and trans expected contact frequencies using cooltools
• eigenvector: calculates cis eigenvectors using cooltools for all resolutions within specified _resolutionlimits.
• saddle: calculates saddles, reflecting average interaction preferences, from cis eigenvectors for each sample using cooltools.
• pentads: calculates pentads, as another way to visualize and quantify compartment strength (https://doi.org/10.1186/s12859-022-04654-6)
• pileups: does pretty much any pileup analysis imaginable using coolpup.py :) (https://doi.org/10.1093/bioinformatics/btaa073)
• call_dots: three methods of calling dots, at specified resolutions, and postprocess output to standardized bedpe format. Implemented callers are cooltools, mustache and chromosight. Which samples are used can be defined in the samples.tsv configuration file.
• insulation: calculates diamond insulation score for specified resolutions and window sizes, using cooltools.
• compare_boundaries: generates differential boundaries, used as input for pileups.
• call_TADs: combines lists of strong boundaries for specified samples into a list across window sizes for each resolution, filtered by length, used as input for pileups. Which samples are used can be defined in the samples.tsv configuration file.

Authors

• Ilya Flyamer (@phlya)

Usage

If you use this workflow in a paper, don't forget to give credits to the authors by citing the URL of this (original) repository.

Step 1: Obtain a copy of this workflow

1. Create a new github repository using this workflow as a template.
2. Clone the newly created repository to your local system, into the place where you want to perform the data analysis.

Step 2: Configure workflow

Configure the workflow according to your needs via editing the files in the config/ folder. Adjust config.yaml to configure the workflow execution, and samples.tsv to specify your sample setup. If you want to use any external bed or bedpe files for pileups, describe them in the annotations.tsv file, and pairings of samples with annotations in samples_annotations.tsv.

Step 3: Install Snakemake and other requirements

Install Snakemake and other requirements using conda:

conda env create -f workflow/envs/environment.yml

This will create an environment quaich where you can launch the pipeline. This environment includes snakemake, so no need to install it separately.

Step 4: Execute workflow

Activate the conda environment:

conda activate quaich

Test your configuration by performing a dry-run via

snakemake --use-conda --configfile config/test_config.yml -n

Execute the workflow locally via

snakemake --use-conda --configfile config/test_config.yml --cores $N

using $N cores or run it in a cluster environment via

snakemake --use-conda --configfile config/test_config.yml --cluster qsub --jobs 100

or

snakemake --use-conda --configfile config/test_config.yml --drmaa --jobs 100

For slurm, consider using the following arguments:

--cluster "sbatch -A CLUSTER_ACCOUNT -t CLUSTER_TIME -p CLUSTER_PARTITION -N CLUSTER_NODES -J JOBNAME" --jobs NUM_JOBS_TO_SUBMIT

Alternatively, you might want to look into snakemake profiles already available for your HPC scheduler online, for example, here or elsewhere.

See the Snakemake documentation for further details.

Step 5: Investigate results not available yet

After successful execution, you can create a self-contained interactive HTML report with all results via:

snakemake --report report.html

This report can, e.g., be forwarded to your collaborators. An example (using some trivial test data) can be seen here.

Step 6: Commit changes

Whenever you change something, don't forget to commit the changes back to your github copy of the repository:

git commit -a
git push

Step 7: Obtain updates from upstream

Whenever you want to synchronize your workflow copy with new developments from upstream, do the following.

1. Once, register the upstream repository in your local copy: git remote add -f upstream git@github.com:snakemake-workflows/quaich.git or git remote add -f upstream https://github.com/snakemake-workflows/quaich.git if you do not have setup ssh keys.
2. Update the upstream version: git fetch upstream.
3. Create a diff with the current version: git diff HEAD upstream/master workflow > upstream-changes.diff.
4. Investigate the changes: vim upstream-changes.diff.
5. Apply the modified diff via: git apply upstream-changes.diff.
6. Carefully check whether you need to update the config files: git diff HEAD upstream/master config. If so, do it manually, and only where necessary, since you would otherwise likely overwrite your settings and samples.

Step 8: Contribute back

In case you have also changed or added steps, please consider contributing them back to the original repository:

1. Fork the original repo to a personal or lab account.
2. Clone the fork to your local system, to a different place than where you ran your analysis.
3. Copy the modified files from your analysis to the clone of your fork, e.g., cp -r workflow path/to/fork. Make sure to not accidentally copy config file contents or sample sheets. Instead, manually update the example config files if necessary.
4. Commit and push your changes to your fork.
5. Create a pull request against the original repository.

Testing

Test cases are in the subfolder .test. They are automatically executed via continuous integration with Github Actions.