bio-chipseq-chromatin-state-segmentation

Version Compatibility

Reference examples tested with: ChromHMM 1.27+, Segway 3.0+, EpiSegMix 1.0+, EpiLogos (Meuleman lab), IDEAS 1.20+, samtools 1.19+, bedtools 2.31+. ChromHMM requires Java 8+; runs as java -mx<MEMORY> -jar ChromHMM.jar <command>.

Chromatin State Segmentation

"Integrate multiple histone modification ChIP-seq tracks into chromatin states" -> Learn a small set of recurring combinatorial patterns of histone marks (active promoter, active enhancer, poised enhancer, polycomb-repressed, heterochromatic, transcribed, etc.) and segment the genome by which state each region belongs to. Output: per-state genomic intervals, state-by-mark emission matrix, and state-state transition matrix.

• CLI (canonical): ChromHMM BinarizeBam -> LearnModel -> OverlapEnrichment / NeighborhoodEnrichment
• CLI (continuous signal): Segway train -> posterior -> annotate
• CLI (flexible distributions): EpiSegMix (2024)
• Visualization across biosamples: EpiLogos (Meuleman lab)
• Cell-type-aware joint: IDEAS

Chromatin state segmentation requires a panel of histone marks; minimum 4-5 marks (e.g., H3K4me3, H3K27ac, H3K4me1, H3K36me3, H3K27me3) for meaningful states. With fewer marks, simpler peak-based annotation (chipseq/peak-annotation) is more appropriate.

Tool Taxonomy

Tool	Method	Strength	Fails when
ChromHMM (Ernst & Kellis 2012; v1.27 current)	Multivariate HMM on binarized 200 bp bins	Canonical; widely used; integrated with Roadmap Epigenomics 15-state model; mature toolchain	Binarization throws away signal quantitation; default 200 bp bins may be too coarse for sharp boundaries
Segway (Hoffman 2012)	Dynamic Bayesian Network on continuous signal	Higher resolution; uses signal magnitudes not binarized	More complex setup; slower; less standardized output
EpiSegMix (Schmitz, Aggarwal, Laufer, Walter, Salhab, Rahmann 2024 Bioinformatics 40:btae178)	HMM with flexible read-count distributions + duration modeling	Modern; handles both narrow and broad mark distributions in one model	Newer; smaller user base
EpiLogos (Meuleman lab)	Multi-biosample visualization tool	Built on top of ChromHMM/Segway segmentations; compare ChromHMM states across 100s of biosamples	Visualization tool, not a segmentation method itself
IDEAS (Zhang 2016)	Cell-type-aware joint inference	Across-cell-type segmentation respecting cell-type identity	Slower; complex parameter tuning
EpiCSeg (Mammana 2015)	Negative binomial mixture	Read-count-based; doesn't need binarization	Less standardized output
GenoSTAN	HMM with various emission distributions	Flexible	Less actively developed
Roadmap 25-state model (Kundaje 2015)	ChromHMM 25-state precomputed model	Reference for cross-cell-type interpretation	Requires the Roadmap imputed 12-mark panel
Full-stack ChromHMM (Vu Ernst 2022)	100-state segmentation across 1032 datasets / 127 reference epigenomes	Comprehensive cross-tissue annotation	Computationally intensive to retrain

ChromHMM Workflow

ChromHMM is the de facto standard. The workflow has 4 stages:

Step 1: Binarize ChIP-seq signal

# Build cellMarkFileTable: cell_type<TAB>mark<TAB>file<TAB>(optional control)
cat > cellMarkFileTable.txt << EOF
GM12878	H3K4me3	gm12878_h3k4me3.bam	gm12878_input.bam
GM12878	H3K27me3	gm12878_h3k27me3.bam	gm12878_input.bam
GM12878	H3K27ac	gm12878_h3k27ac.bam	gm12878_input.bam
GM12878	H3K4me1	gm12878_h3k4me1.bam	gm12878_input.bam
GM12878	H3K36me3	gm12878_h3k36me3.bam	gm12878_input.bam
EOF

# Binarize BAMs into 200 bp bins; emission = whether mark exceeds Poisson threshold
java -mx16G -jar ChromHMM.jar BinarizeBam \
    -b 200 \
    chromsizes_hg38.txt \
    bam_dir/ \
    cellMarkFileTable.txt \
    binarized_output/

Output: per-chromosome _binary.txt files, one row per 200 bp bin, columns = marks, values 0/1.

Step 2: Learn model

# Train HMM with N states; common choices: 15, 18, 25
# 15 states: Ernst & Kellis 2011 model; canonical
# 18 states: extends with additional regulatory states
# 25 states: Roadmap Epigenomics extended model
java -mx16G -jar ChromHMM.jar LearnModel \
    -p 8 \
    binarized_output/ \
    model_15state/ \
    15 \
    hg38

# Output: model_15state.txt (emission + transition matrices),
# emissions_15.png (visualization), transitions_15.png,
# per-chromosome _segments.bed (state assignments)
# AND automatically runs OverlapEnrichment + NeighborhoodEnrichment

Step 3: Interpret states from emission matrix

Roadmap 15-state assignments (canonical)
1_TssA — Active TSS (high H3K4me3, H3K27ac)
2_TssAFlnk — Flanking TSS (H3K4me3, H3K27ac, lower)
3_TxFlnk — Transcript flanking
4_Tx — Strong transcription (H3K36me3, H3K79me2 if available)
5_TxWk — Weak transcription
6_EnhG — Enhancer in gene body (H3K4me1, H3K27ac)
7_Enh — Generic enhancer (H3K4me1, H3K27ac)
8_ZNF/Rpts — Zinc-finger / repeats
9_Het — Heterochromatin (H3K9me3)
10_TssBiv — Bivalent TSS (H3K4me3 + H3K27me3)
11_BivFlnk — Bivalent flanking
12_EnhBiv — Bivalent enhancer (H3K4me1 + H3K27me3)
13_ReprPC — Polycomb-repressed (H3K27me3)
14_ReprPCWk — Weak Polycomb
15_Quies — Quiescent (no signal)

Step 4: Functional enrichment of states

# OverlapEnrichment: enrichment of each state for external feature sets.
# Options (e.g. -labels) MUST precede the three positional args.
java -mx16G -jar ChromHMM.jar OverlapEnrichment \
    -labels \
    model_15state/GM12878_15_segments.bed \
    /path/to/anchor_files/ \
    enrichment_output/GM12878

# NeighborhoodEnrichment: enrichment relative to anchor positions (e.g., TSS)
java -mx16G -jar ChromHMM.jar NeighborhoodEnrichment \
    -labels \
    model_15state/GM12878_15_segments.bed \
    /path/to/tss_anchors.txt \
    enrichment_output/GM12878_TSS

Anchor files: BED files of features (CGIs, repeats, conserved elements, etc.) for OverlapEnrichment; position files for NeighborhoodEnrichment.

Choosing State Count

States	Use case	Mark panel size
8-10	Initial exploration; small mark panel (3-4 marks)	3-5 marks
15	Roadmap Epigenomics canonical	5 core (H3K4me3, H3K4me1, H3K36me3, H3K27me3, H3K9me3)
18	Roadmap extended (adds H3K27ac -> fine enhancer subtypes)	6 marks (core 5 + H3K27ac)
25	Roadmap Epigenomics extended; cross-cell-type compatibility	12 imputed marks
50+	Full-stack model (Vu Ernst 2022)	Many marks across many cell types

Practical workflow: Train at N=15, 18, 25; compare emission matrices; choose the smallest N where biology is interpretable. Higher N risks over-segmentation (state splitting random variation).

Segway Workflow

# Segway reads only the Genomedata format, so first pack the signal tracks
# (one track per bigWig) plus the genome sequence into an archive.
genomedata-load \
    -s hg38.fa \
    -t h3k4me3=h3k4me3.bw \
    -t h3k27ac=h3k27ac.bw \
    -t h3k4me1=h3k4me1.bw \
    -t h3k36me3=h3k36me3.bw \
    -t h3k27me3=h3k27me3.bw \
    signal.genomedata

# Train Segway model; GENOMEDATA and TRAINDIR are positional
segway train \
    --num-labels=25 \
    --num-instances=3 \
    --resolution=100 \
    signal.genomedata traindir/

# Posterior probabilities + hard-call annotation (GENOMEDATA TRAINDIR OUTDIR, positional)
segway posterior signal.genomedata traindir/ posteriordir/
segway annotate signal.genomedata traindir/ identifydir/

# Output: identifydir/segway.bed.gz (state assignments)

Segway uses continuous signal (from the genomedata archive) vs ChromHMM's binarized bins. Trade-off: more information per region (continuous) but more complex training.

EpiLogos Visualization

EpiLogos doesn't perform segmentation; it visualizes existing ChromHMM/Segway segmentations across many biosamples (epilogos.org).

# Use precomputed ChromHMM segmentations across multiple cell types
# Web interface: https://epilogos.altius.org/
# Local: github.com/meuleman/epilogos

Useful for: cross-cell-type comparison; identifying tissue-specific regulatory states; cohort-level chromatin landscape summaries.

Full-Stack ChromHMM (Vu Ernst 2022)

The full-stack model trained on 1032 datasets / 127 reference epigenomes:

# Use precomputed model from Ernst lab
# github.com/ernstlab/full_stack_ChromHMM_annotations
# Annotate new sample by applying model to binarized data
java -mx16G -jar ChromHMM.jar MakeSegmentation \
    full_stack_model_100states.txt \
    binarized_sample/ \
    full_stack_output/

Useful for: applying a comprehensive cross-tissue annotation to a new sample; comparing to canonical Roadmap states.

Per-Tool Failure Modes

ChromHMM -- Bin size 200 bp too coarse for sharp boundaries

Trigger: Studying TF binding boundaries or sharp enhancer transitions at 200 bp resolution.

Mechanism: ChromHMM default 200 bp bins; biology may shift within a bin.

Fix: Reduce to -b 100 or -b 50 (smaller bin); increases memory and compute time but improves boundary resolution. Re-train model at finer resolution.

ChromHMM -- Binarization throws away signal quantitation

Trigger: Distinguishing low- from high-signal regions of the same state.

Mechanism: ChromHMM binarizes each 200 bp bin to 0/1 per mark; state assignment uses combinatorial pattern, not magnitude.

Fix: Use Segway (continuous signal) or EpiSegMix (flexible distributions) for magnitude-aware segmentation.

ChromHMM / Segway -- Wrong state count

Trigger: Training with N=50 states on a 4-mark panel; or N=10 on a 7-mark panel.

Mechanism: Excess states fragment biology; insufficient states force unrelated regions into the same state.

Symptom: Emission matrix shows redundant states (multiple states with same emission profile) at high N; or biologically distinct regions lumped together at low N.

Fix: Train at N=15, 18, 25; inspect emission matrix similarity; choose the smallest N where states are interpretable as distinct biology.

Mark panel mismatch with model

Trigger: Applying Roadmap 25-state model to a sample with different mark panel.

Mechanism: Model was trained on specific marks; emission probabilities are mark-specific. Applying to different mark panel produces nonsensical state assignments.

Fix: Either train a new model on the available mark panel; OR ensure the exact same marks (and ordering) as used in the model.

IgG control instead of input

Trigger: Using IgG controls in BinarizeBam for histone marks.

Mechanism: ChromHMM's binarization compares mark signal to control; histone mark biology assumes input (sonicated chromatin) as background, not IgG.

Fix: Use sonicated input as control for histone mark ChIP. IgG is not appropriate for ChromHMM binarization of histone marks.

Cross-cell-type state mapping

Trigger: Training separate models per cell type and trying to compare state assignments.

Mechanism: State 5 in cell type A may not correspond to state 5 in cell type B if trained independently.

Fix: Train one model on concatenated data from all cell types (joint segmentation); or apply a single precomputed model (Roadmap 15-state, full-stack) to all samples for consistent state labels.

Reconciliation

Pattern	Likely cause	Action
ChromHMM and Segway segments differ	Different bin sizes / binarization vs continuous	Both can be valid; inspect emission matrices; pick the tool matching the resolution needs
State assignment varies wildly between replicates	Insufficient marks; over-binned	Increase mark panel; reduce state count
Active TSS state overlaps polycomb state at promoters	Bivalent biology (Bernstein 2006)	Expected for ESC-like cells; not an error; consider bivalent-specific state in N=18 model
Roadmap 25-state model annotates unknown cell type	Cross-cell-type generalization	Use cautiously; verify against tissue-specific tracks
Heterochromatin (H3K9me3) state has too many bins	H3K9me3 covers large fraction of genome	Expected; heterochromatin is genome-wide

Common Errors

Error / symptom	Cause	Solution
java.lang.OutOfMemoryError	Insufficient JVM heap	java -mx32G -jar ChromHMM.jar ...
BinarizeBam very slow	Large BAMs without index	samtools index all BAMs first
All states have similar emissions	Mark panel too small	Need at least 5 marks for canonical 15-state model
Segments file empty for some chromosomes	Chromosome not in chromsizes file	Add or use -chrom flag to restrict
State labels don't match Roadmap	Trained model independently	Use Roadmap precomputed model OR map states by emission similarity
ChromHMM "no signal in marks"	All bins binarized to 0	Check signal quality; verify control normalization

References

• Ernst J & Kellis M 2012 Nat Methods 9:215 (ChromHMM v1)
• Ernst J & Kellis M 2017 Nat Protoc 12:2478 (ChromHMM protocol)
• Hoffman MM et al 2012 Nat Methods 9:473 (Segway)
• Schmitz JE, Aggarwal N, Laufer L, Walter J, Salhab A, Rahmann S 2024 Bioinformatics 40:btae178 (EpiSegMix)
• Meuleman W et al 2020 Nature 584:244 (EpiLogos / DHS index)
• Zhang Y & Hardison 2016 Nucleic Acids Res 44:6721 (IDEAS)
• Mammana A & Chung HR 2015 Genome Biol 16:151 (EpiCSeg)
• Roadmap Epigenomics Consortium 2015 Nature 518:317 (Roadmap 25-state model)
• Vu H & Ernst J 2022 Genome Biol 23:9 (full-stack ChromHMM)
• Kundaje A et al 2015 Nature 518:317 (Roadmap integrative analysis)

Related Skills

• chip-seq/peak-calling - Peak calling per mark before segmentation
• chip-seq/chipseq-qc - Replicate concordance per mark; QC before integration
• chip-seq/peak-annotation - cCRE classification (PLS/pELS/dELS) complementary to chromatin states
• chip-seq/spike-in-normalization - Spike-in normalize per-mark BAMs before binarization for cross-condition state comparison
• atac-seq/single-cell-atac - scATAC + ChIP integration via multimodal methods
• machine-learning/model-validation - Model selection (state count); cross-validation
• data-visualization/genome-tracks - Visualize state segmentations
• gene-regulatory-networks/coexpression-networks - Cross-reference chromatin states with co-expression modules