🧬 Epigenomics Analysis

Peak calling and chromatin accessibility analysis for ATAC-seq and ChIP-seq data.

Core Capabilities

1. MACS3 peak calling: ATAC-specific and ChIP-seq peak detection
2. Motif analysis: Homer, MEME motif enrichment in open chromatin regions
3. Quality control: Fragment size distribution, TSS enrichment, FRiP scores
4. Differential accessibility: Condition comparison of chromatin regions
5. IDR replicate analysis: Irreproducible Discovery Rate for peak reproducibility

CLI Reference

python omicsclaw.py run epigenomics --demo
python omicsclaw.py run epigenomics --input <data.bam> --output <dir>

Algorithm / Methodology

MACS3 for ATAC-seq

Goal: Identify open chromatin regions from ATAC-seq data using ATAC-specific parameters.

Approach: Run MACS3 in paired-end mode with Tn5 shift correction, no model building, and duplicate retention.

macs3 callpeak \
    -t sample.bam \
    -f BAMPE \
    -g hs \
    -n sample \
    --outdir peaks/ \
    -q 0.05 \
    --nomodel \
    --shift -75 \
    --extsize 150 \
    --keep-dup all \
    -B \
    --call-summits

Why These Parameters?

--nomodel

--shift -75

--extsize 150

--keep-dup all

-f BAMPE

Call Peaks on NFR Only

# Filter to nucleosome-free reads (<100bp fragments)
samtools view -h sample.bam | \
    awk 'substr($0,1,1)=="@" || ($9>0 && $9<100) || ($9<0 && $9>-100)' | \
    samtools view -b > nfr.bam

# Call peaks on NFR
macs3 callpeak -t nfr.bam -f BAMPE -g hs -n sample_nfr \
    --nomodel --shift -37 --extsize 75 --keep-dup all -q 0.01

Broad Peaks (Optional)

macs3 callpeak -t sample.bam -f BAMPE -g hs -n sample_broad \
    --nomodel --shift -75 --extsize 150 --broad --broad-cutoff 0.1

Batch Processing

#!/bin/bash
GENOME=hs  # hs for human, mm for mouse
OUTDIR=peaks

mkdir -p $OUTDIR

for bam in *.bam; do
    sample=$(basename $bam .bam)
    macs3 callpeak -t $bam -f BAMPE -g $GENOME -n $sample --outdir $OUTDIR \
        --nomodel --shift -75 --extsize 150 --keep-dup all -q 0.05 -B --call-summits
done

IDR for Replicate Consistency

# Call peaks on each replicate separately
macs3 callpeak -t rep1.bam -f BAMPE -g hs -n rep1 ...
macs3 callpeak -t rep2.bam -f BAMPE -g hs -n rep2 ...

# Run IDR
idr --samples rep1_peaks.narrowPeak rep2_peaks.narrowPeak \
    --input-file-type narrowPeak --output-file idr_peaks.txt --plot

# Filter by IDR threshold
awk '$5 >= 540' idr_peaks.txt > reproducible_peaks.bed

Convert to BigWig

sort -k1,1 -k2,2n sample_treat_pileup.bdg > sample.sorted.bdg
bedGraphToBigWig sample.sorted.bdg chrom.sizes sample.bw

Output Files

_peaks.narrowPeak

_summits.bed

_peaks.xls

_treat_pileup.bdg

narrowPeak Format

Columns: chrom, start, end, name, score, strand, signalValue, pValue, qValue, summit_offset

Parameters

--method

macs3

--mode

atac

--genome

hs

--qvalue

0.05

Why This Exists

• Without it: Open chromatin peaks are modeled with poor shifting logic or non-reproducible parameters
• With it: ATAC-specific logic correctly infers true transposase insertions vs nucleosomes
• Why OmicsClaw: Exposes high-level biology queries directly mapped to strictly modeled peak calling routines

Workflow

1. Calculate: Prepare cross-correlation and fragment histograms.
2. Execute: Test local enrichment over poisson distribution null.
3. Assess: Perform IDR replicates and FDR significance tests.
4. Generate: Output structured summit maps and bedGraph.
5. Report: Tabulate core peaks and TF motifs.

Example Queries

• "Run ATAC peak calling with MACS3 on this bam file"
• "Perform motif analysis on epigenomics data BED"

Output Structure

output_directory/
├── report.md
├── result.json
├── peaks.narrowPeak
├── figures/
│   └── footprint_plot.png
├── tables/
│   └── motif_enrichment.csv
└── reproducibility/
    ├── commands.sh
    ├── environment.yml
    └── checksums.sha256

Safety

• Local-first: Strict offline processing without external upload.
• Disclaimer: Requires OmicsClaw reporting structures and disclaimers.
• Audit trail: Hyperparameters and operational flow states are logged fully.

Integration with Orchestrator

Trigger conditions:

• Automatically invoked dynamically based on tool metadata and user intent matching.

Chaining partners:

• align

  — Upstream generation of BAM

• genomics-qc

  — Upstream filtering

Version Compatibility

Reference examples tested with: MACS3 3.0+, samtools 1.19+

Dependencies

Required: MACS3 (macs3), samtools Optional: Homer, Genrich, IDR, bedtools, pyGenomeTracks

Citations

• MACS3 — Zhang et al., Genome Biology 2008
• Homer — Heinz et al., Molecular Cell 2010
• IDR — Li et al., Annals of Applied Statistics 2011
• ATAC-seq — Buenrostro et al., Nature Methods 2013

Related Skills

• genomics-qc

  — QC before peak calling

• align

  — Read alignment upstream