Single Cell Epigenomics Peak Analysis

single-cell-epigenomics-peak-analysis

Summary

Identifies and characterizes chromatin accessibility peaks in single-cell ATAC-seq data using spectral embedding and peak-calling algorithms. This skill enables discovery of cell-type-specific regulatory regions and their enrichment for transcription factor motifs.

When to use

Apply this skill when you have preprocessed single-cell ATAC-seq fragment files or count matrices and need to identify open chromatin regions (peaks) to support downstream differential accessibility analysis, motif discovery, or regulatory network inference. Use it after BAM-to-fragment conversion and cell filtering but before comparing accessibility between cell populations.

When NOT to use

• Input is already a curated set of consensus peaks from bulk ATAC-seq or ChIP-seq; skip to motif enrichment or annotation.
• Single-cell data lacks sufficient sequencing depth (~5,000 fragments per cell minimum); peak calling will be unreliable.
• Analyzing bulk ATAC-seq or RNA-seq data; use bulk peak callers (MACS2, ENCODE pipeline) instead.

Inputs

• BAM or fragment files (TSV or gzipped format)
• Cell barcodes and metadata (cell-type annotations or cluster assignments)
• Reference genome (optional; for peak annotation)

Outputs

• Peak count matrix (.h5ad AnnData object with peaks × cells)
• Peak coordinates (BED format or interval table)
• Spectral embedding coordinates (low-dimensional representation)
• Differential accessibility results (peak IDs, log2-fold-change, p-values)

How to apply

Begin by constructing a tile matrix or peak matrix from fragment files using pp.make_tile_matrix or pp.make_peak_matrix. Apply dimension reduction via matrix-free spectral embedding (tl.spectral) to embed cells in a low-dimensional space, which enables clustering and visualization without materializing the full count matrix. Perform peak calling using tl.macs3 (or merge peaks across cell types with tl.merge_peaks) to define consensus peak sets. For peaks identified as differentially accessible via tl.diff_test, validate peak quality by checking for non-zero counts and reasonable distribution of peak widths. The spectral embedding is scalable to >10 million cells and supports integration with downstream tools (Scanpy, peak annotation) via AnnData format.

Related tools

• SnapATAC2 (Python/Rust framework for spectral embedding, peak calling, and differential analysis of single-cell ATAC-seq data) — https://github.com/scverse/SnapATAC2
• pp.make_tile_matrix (Generate fixed-width tile matrix from fragment files for spectral embedding) — https://github.com/scverse/SnapATAC2
• pp.make_peak_matrix (Generate peak count matrix from fragment files using predefined peak coordinates) — https://github.com/scverse/SnapATAC2
• tl.spectral (Matrix-free spectral embedding for dimension reduction without materializing full count matrix) — https://github.com/scverse/SnapATAC2
• tl.macs3 (Peak calling to identify chromatin accessibility regions within single-cell ATAC-seq data) — https://github.com/scverse/SnapATAC2
• tl.merge_peaks (Merge and consolidate peaks across cell types or clusters to define consensus peak set) — https://github.com/scverse/SnapATAC2
• tl.diff_test (Differential accessibility testing to identify peaks enriched in specific cell populations) — https://github.com/scverse/SnapATAC2
• Scanpy (Integration framework for downstream single-cell analysis (clustering, annotation) with SnapATAC2 outputs)

Examples

import snapatac2 as snap; adata = snap.pp.make_tile_matrix(snap.io.read_h5ad('fragments.h5ad')); snap.tl.spectral(adata, random_state=0); snap.tl.macs3(adata); snap.tl.diff_test(adata, groupby='cell_type')

Evaluation signals

• Peak count matrix is non-empty and sparse (median non-zero entries per cell >100 and <0.5% density), consistent with ATAC-seq sparsity.
• Spectral embedding coordinates have low reconstruction error and embed cells such that k-nearest neighbors within a cluster have higher accessibility correlation than cross-cluster pairs.
• Peak width distribution is reasonable (median 300–500 bp for mammalian data); peaks should not be extremely narrow (<50 bp) or broad (>10 kb).
• Differentially accessible peaks (tl.diff_test output) show expected p-value distribution with enrichment at small p-values and minimal bias toward specific regions or cell types.
• AnnData .h5ad file contains all required slots (X, obs, var) with no null values in peak ID, chromosome, start, and end columns.

Limitations

• Peak calling accuracy depends on sequencing depth; low-coverage cells (<5,000 fragments) may fail or produce spurious peaks.
• Spectral embedding scales well to >10 million cells but requires tuning of neighborhood parameters (n_neighbors, n_components) for optimal results.
• SnapATAC2 outputs are in AnnData format; integration with non-scverse tools (e.g., Seurat, Cell Ranger) requires format conversion.
• Motif enrichment analysis (tl.motif_enrichment) is performed post-hoc on differential peaks; it does not directly inform peak calling and may miss rare TF binding sites.

Evidence

• [readme] End-to-end analysis pipeline for single-cell ATAC-seq data, including preprocessing, dimension reduction, clustering, data integration, peak calling, differential analysis, motif analysis, regulatory network analysis.: "End-to-end analysis pipeline for single-cell ATAC-seq data, including preprocessing, dimension reduction, clustering, data integration, peak calling, differential analysis, motif analysis, regulatory"
• [methods] Matrix-free spectral embedding algorithm that is applicable to a wide range of single-cell omics data, including single-cell ATAC-seq, single-cell RNA-seq, single-cell Hi-C, and single-cell methylation.: "Matrix-free spectral embedding algorithm that is applicable to a wide range of single-cell omics data, including single-cell ATAC-seq, single-cell RNA-seq, single-cell Hi-C, and single-cell"
• [intro] Scale to more than 10 million cells.: "Scale to more than 10 million cells."
• [methods] tl.macs3, tl.merge_peaks for peak calling: "tl.macs3, tl.merge_peaks for peak calling"
• [methods] tl.marker_regions, tl.diff_test for differential analysis: "tl.marker_regions, tl.diff_test for differential analysis"
• [readme] Blazingly fast preprocessing tools for BAM to fragment files conversion and count matrix generation.: "Blazingly fast preprocessing tools for BAM to fragment files conversion and count matrix generation."