bio-chipseq-differential-binding

name	bio-chipseq-differential-binding
description	Differential binding analysis using DiffBind. Compare ChIP-seq peaks between conditions with statistical rigor. Requires replicate samples. Outputs differentially bound regions with fold changes and p-values. Use when comparing ChIP-seq binding between conditions.
tool_type	r
primary_tool	DiffBind

Differential Binding with DiffBind

Create Sample Sheet

# Create sample sheet as data frame or CSV
samples <- data.frame(
    SampleID = c('ctrl_1', 'ctrl_2', 'treat_1', 'treat_2'),
    Tissue = c('cell', 'cell', 'cell', 'cell'),
    Factor = c('H3K4me3', 'H3K4me3', 'H3K4me3', 'H3K4me3'),
    Condition = c('control', 'control', 'treatment', 'treatment'),
    Replicate = c(1, 2, 1, 2),
    bamReads = c('ctrl1.bam', 'ctrl2.bam', 'treat1.bam', 'treat2.bam'),
    Peaks = c('ctrl1_peaks.narrowPeak', 'ctrl2_peaks.narrowPeak',
              'treat1_peaks.narrowPeak', 'treat2_peaks.narrowPeak'),
    PeakCaller = c('macs', 'macs', 'macs', 'macs')
)

write.csv(samples, 'samples.csv', row.names = FALSE)

Load Data

library(DiffBind)

# From sample sheet
dba_obj <- dba(sampleSheet = 'samples.csv')

# View summary
dba_obj

Count Reads in Peaks

# Count reads in consensus peaks (DiffBind 3.0+ defaults)
# summits=250 and bUseSummarizeOverlaps=TRUE are now defaults
dba_obj <- dba.count(dba_obj)

# With specific parameters
dba_obj <- dba.count(
    dba_obj,
    summits = 250,         # Re-center peaks around summits (default in 3.0)
    minOverlap = 2         # Peak must be in at least 2 samples
)

Normalize Data

# Normalize (required before analysis)
dba_obj <- dba.normalize(dba_obj)

# Check normalization
dba.normalize(dba_obj, bRetrieve = TRUE)

Set Up Contrast (DiffBind 3.0+)

# Recommended: design formula approach (DiffBind 3.0+)
dba_obj <- dba.contrast(dba_obj, design = '~ Condition')

# Or use categories for automatic contrast
dba_obj <- dba.contrast(dba_obj, categories = DBA_CONDITION)

# Legacy approach (retained for backward compatibility, not recommended)
# dba_obj <- dba.contrast(dba_obj, group1 = dba_obj$masks$control,
#                         group2 = dba_obj$masks$treatment)

Run Differential Analysis

# Analyze with DESeq2 (default)
dba_obj <- dba.analyze(dba_obj, method = DBA_DESEQ2)

# Or with edgeR
dba_obj <- dba.analyze(dba_obj, method = DBA_EDGER)

View Results

# Summary of differential peaks
dba.show(dba_obj, bContrasts = TRUE)

# Retrieve differential binding results
db_results <- dba.report(dba_obj)
db_results

Filter Results

# Get significant peaks (FDR < 0.05, |FC| > 2)
db_sig <- dba.report(dba_obj, th = 0.05, fold = 2)

# Get all results for custom filtering
db_all <- dba.report(dba_obj, th = 1)

Export Results

# To data frame
results_df <- as.data.frame(dba.report(dba_obj, th = 1))

# Export to CSV
write.csv(results_df, 'differential_binding.csv', row.names = FALSE)

# Export to BED
library(rtracklayer)
export(db_sig, 'diff_peaks.bed', format = 'BED')

Visualization

# PCA plot
dba.plotPCA(dba_obj, DBA_CONDITION, label = DBA_ID)

# Correlation heatmap
dba.plotHeatmap(dba_obj)

# MA plot
dba.plotMA(dba_obj)

# Volcano plot
dba.plotVolcano(dba_obj)

# Heatmap of differential peaks
dba.plotHeatmap(dba_obj, contrast = 1, correlations = FALSE)

Venn Diagram of Peaks

# Overlap between conditions
dba.plotVenn(dba_obj, dba_obj$masks$control)
dba.plotVenn(dba_obj, dba_obj$masks$treatment)

Profile Plots

# Average signal profile
profiles <- dba.plotProfile(dba_obj)

Get Consensus Peaks

# Export consensus peakset
consensus <- dba.peakset(dba_obj, bRetrieve = TRUE)
export(consensus, 'consensus_peaks.bed', format = 'BED')

Multi-Factor Design

# With blocking factor (e.g., batch correction)
dba_obj <- dba.contrast(dba_obj, design = '~ Batch + Condition')
dba_obj <- dba.analyze(dba_obj)

DiffBind 3.0 Notes

DiffBind 3.0+ introduced significant changes:

• dba.normalize() is now required before analysis
• Default summits=250 recenters peaks (was FALSE in older versions)
• Use design formulas instead of group1/group2 for contrasts
• Blacklist filtering is applied by default

Sample Sheet Columns

Column	Required	Description
SampleID	Yes	Unique identifier
Tissue	No	Tissue/cell type
Factor	No	ChIP target
Condition	Yes	Experimental condition
Treatment	No	Additional grouping
Replicate	Yes	Replicate number
bamReads	Yes	Path to BAM file
Peaks	Yes	Path to peak file
PeakCaller	Yes	macs, bed, narrow
bamControl	No	Path to input BAM

Key Functions

Function	Purpose
dba	Create DBA object
dba.count	Count reads in peaks
dba.normalize	Normalize counts
dba.contrast	Set up comparison
dba.analyze	Run differential analysis
dba.report	Get results
dba.plotPCA	PCA visualization
dba.plotMA	MA plot
dba.plotHeatmap	Heatmap

Related Skills

• peak-calling - Generate input peak files
• peak-annotation - Annotate differential peaks
• differential-expression - Compare with RNA-seq
• pathway-analysis - Functional enrichment