--- name: bio-chipseq-peak-calling source: https://app.decimal.ai/s/bio-chipseq-peak-calling@1/SKILL.md source_sha256: d305464639e0 --- ## Version Compatibility Reference examples tested with: MACS2 2.2+, MACS3 3.0+ Before using code patterns, verify installed versions match. If versions differ: - CLI: ` --version` then ` --help` to confirm flags If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying. # Peak Calling with MACS3 **"Call peaks from my ChIP-seq data"** → Identify significantly enriched regions (narrow peaks for TFs, broad peaks for histone marks) by comparing IP signal to input control. - CLI: `macs3 callpeak -t chip.bam -c input.bam -f BAM -g hs -n sample` MACS3 is the actively developed successor to MACS2. Commands are identical except the binary name. MACS2 is in maintenance mode. ## Basic Peak Calling **Goal:** Call enriched regions from ChIP-seq alignments with input control normalization. **Approach:** Compare treatment BAM signal against input control using MACS3 local Poisson model. ```bash # Call peaks with input control (recommended) macs3 callpeak -t chip.bam -c input.bam -f BAM -g hs -n sample --outdir peaks/ # For MACS2 (legacy), replace 'macs3' with 'macs2' - syntax is identical ``` ## Without Input Control **Goal:** Call peaks without a matched input/control sample. **Approach:** Use MACS3 with genomic background estimation only (less accurate than with control). ```bash # Not recommended, but possible macs3 callpeak -t chip.bam -f BAM -g hs -n sample --outdir peaks/ ``` ## Narrow Peaks (TF, H3K4me3, H3K27ac) **Goal:** Call sharp, well-defined peaks typical of transcription factors and active histone marks. **Approach:** Use default narrow peak mode with q-value filtering and genome size correction. ```bash macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ # hs=human, mm=mouse, ce=worm, dm=fly -n sample_narrow \ --outdir peaks/ \ -q 0.05 # q-value threshold ``` ## Broad Peaks (H3K36me3, H3K27me3, H3K9me3) **Goal:** Call diffuse, broad enrichment domains typical of repressive or elongation-associated histone marks. **Approach:** Enable broad peak mode which links nearby enriched regions into broader domains. ```bash macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ -n sample_broad \ --outdir peaks/ \ --broad \ # Broad peak mode --broad-cutoff 0.1 # Broad peak q-value ``` ## Paired-End Data **Goal:** Call peaks from paired-end sequencing using actual fragment sizes instead of modeled estimates. **Approach:** Use BAMPE format so MACS3 calculates fragment size from mate pairs directly. ```bash # MACS3 uses BAMPE format for paired-end macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAMPE \ # Paired-end BAM -g hs \ -n sample_pe \ --outdir peaks/ ``` ## Multiple Replicates **Goal:** Call peaks from multiple biological replicates pooled together for increased statistical power. **Approach:** Provide all replicate BAMs to MACS3, which internally pools reads before peak calling. ```bash # Pool replicates (MACS3 handles internally) macs3 callpeak \ -t rep1.bam rep2.bam rep3.bam \ -c input.bam \ -f BAM \ -g hs \ -n pooled \ --outdir peaks/ ``` ## Custom Genome Size **Goal:** Call peaks for non-model organisms without a built-in genome size shortcut. **Approach:** Provide the effective genome size as a numeric value instead of a species abbreviation. ```bash # For non-model organisms or custom genomes macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g 2.7e9 \ # Effective genome size in bp -n sample \ --outdir peaks/ ``` ## Common Genome Sizes | Genome | Flag | Effective Size | |--------|------|----------------| | Human | hs | 2.7e9 | | Mouse | mm | 1.87e9 | | C. elegans | ce | 9e7 | | D. melanogaster | dm | 1.2e8 | ## Fixed Fragment Size **Goal:** Call peaks when fragment size modeling fails or a specific extension size is needed. **Approach:** Bypass model building and specify a fixed read extension size manually. ```bash # If modeling fails or for ATAC-seq macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ --nomodel \ # Skip model building --extsize 200 \ # Fixed extension size -n sample \ --outdir peaks/ ``` ## Generate Signal Tracks **Goal:** Produce normalized signal tracks for genome browser visualization alongside peak calls. **Approach:** Enable bedGraph output with signal-per-million-reads normalization, then convert to bigWig. ```bash # Generate bedGraph and bigWig files macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ -n sample \ --outdir peaks/ \ -B \ # Generate bedGraph --SPMR # Signal per million reads # Convert to bigWig (requires UCSC tools) sort -k1,1 -k2,2n peaks/sample_treat_pileup.bdg > peaks/sample.sorted.bdg bedGraphToBigWig peaks/sample.sorted.bdg chrom.sizes peaks/sample.bw ``` ## Local Lambda for Broad Marks **Goal:** Improve broad peak calling by disabling the genome-wide lambda estimate. **Approach:** Use --nolambda to rely solely on local background estimation for very broad domains. ```bash # Recommended for very broad marks macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ --broad \ --nolambda \ # Use local lambda only -n sample \ --outdir peaks/ ``` ## Cutoff Analysis **Goal:** Evaluate how different significance thresholds affect the number of called peaks. **Approach:** Run MACS3 cutoff analysis mode to generate a table of peak counts at various q-value cutoffs. ```bash # Test different q-value cutoffs macs3 callpeak \ -t chip.bam \ -c input.bam \ -f BAM \ -g hs \ --cutoff-analysis \ # Generate cutoff analysis file -n sample \ --outdir peaks/ ``` ## Output Files | File | Description | |------|-------------| | *_peaks.narrowPeak | Peak coordinates (BED6+4) | | *_peaks.broadPeak | Broad peak coordinates | | *_summits.bed | Peak summit positions | | *_model.r | R script for model visualization | | *_treat_pileup.bdg | Treatment signal (with -B) | | *_control_lambda.bdg | Control signal (with -B) | ## narrowPeak Format ``` chr1 100 200 peak_1 100 . 5.2 10.5 8.3 50 ``` Columns: chr, start, end, name, score, strand, signalValue, pValue, qValue, peak ## Filter Peaks **Goal:** Post-filter called peaks by statistical significance or signal strength. **Approach:** Use awk on narrowPeak columns to apply q-value or signal-value cutoffs. ```bash # Filter by q-value awk '$9 > 2' peaks.narrowPeak > peaks.filtered.narrowPeak # -log10(q) > 2 means q < 0.01 # Sort by signal strength sort -k7,7nr peaks.narrowPeak > peaks.sorted.narrowPeak ``` ## Key Parameters | Parameter | Default | Description | |-----------|---------|-------------| | -t | required | Treatment BAM file(s) | | -c | none | Control BAM file(s) | | -f | AUTO | Format (BAM, BAMPE, BED) | | -g | hs | Genome size | | -n | NA | Output prefix | | -q | 0.05 | Q-value cutoff | | -p | none | P-value cutoff (overrides -q) | | --broad | false | Broad peak calling | | --nomodel | false | Skip model building | | --extsize | 200 | Extension size (with --nomodel) | | -B | false | Generate bedGraph | | --SPMR | false | Signal per million reads | ## Related Skills - peak-annotation - Annotate peaks to genes - differential-binding - Compare peaks between conditions - alignment-files - Prepare BAM files - chipseq-visualization - Visualize peaks