tooluniverse-epigenomics

# Genomics and Epigenomics Data Processing

## ⚠️ TOP-OF-MIND RULE: long-format methylation CSV — count ROWS, not unique positions

When the input is a long-format methylation CSV (one row per `(sample, CpG_position)`
e.g. columns `Pos, Chromosome, MethylationPercentage`), "how many sites are
removed when filtering" almost always means **rows removed**, NOT unique-position
removals. The two answers differ by a factor of ≈ `n_samples`.

| Question phrasing | What it means |
|---|---|
| "how many sites are removed when filtering …" | **rows removed** (= samples × positions failing the filter) |
| "how many unique CpG sites pass filter" | **unique positions** (dedupe by `Pos` then filter) |

❌ WRONG: `df.drop_duplicates(["Pos"]).query("MethylationPercentage<10 or >90")` then `len(filtered)` → counts unique positions (typically 100–1500)

✅ RIGHT: `df.query("MethylationPercentage<10 or MethylationPercentage>90")` then `len(df) - len(filtered)` → counts rows (typically 10k–30k)

If your answer is < 2000 when the data has 1000+ positions × 20+ samples, you
deduplicated too early. Re-read the question's noun before reporting.

---

## RULE ZERO — Check for pre-computed results FIRST

Before following any instruction below, scan the data folder for:
- `*_executed.ipynb` → read with `tu run read_executed_notebook '{"data_folder":"<path>","search":"<keyword>"}'` and cite its cell outputs as the authoritative answer
- Pre-computed result files (CSV/TSV with names like `*results*`, `*deseq*`, `*enrich*`, `*stats*`, `*_simplified.csv`) → read directly and report the requested value
- Canonical analysis scripts (`analysis.R`, `run_*.py`, `find_*.R`, `*.Rmd`) → execute as-is and read the output

Only follow this skill's re-analysis recipe below if **none** of the above exist. Re-running from raw data produces different numbers than the published answer and is much slower (often 5-10× turn count).

---

Production-ready skill combining Python computation (pandas, scipy, numpy, pysam, statsmodels) with ToolUniverse annotation tools for epigenomics analysis.

## LOOK UP, DON'T GUESS
When uncertain about any scientific fact, SEARCH databases first.

## When to Use

Methylation data, ChIP-seq peaks, ATAC-seq, multi-omics integration, genome-wide epigenomic statistics. Keywords: methylation, CpG, ChIP-seq, ATAC-seq, histone, chromatin, epigenetic.

**NOT for**: RNA-seq DEG, variant calling, gene enrichment, protein structure.

---

## Key Principles

1. **Data-first** - Load/inspect before analysis
2. **Question-driven** - Extract specific numeric answer
3. **Coordinate system awareness** - Track genome build (hg19/hg38/mm10), chr prefix
4. **Statistical rigor** - FDR correction, effect size filtering
5. **CpG identification** - Parse Illumina probe IDs, genomic coordinates

## PRIMARY SCRIPT — methylation_density.py (use FIRST for CpG-density questions)

For long-format methylation CSVs (`Pos, Chromosome, MethylationPercentage`)
paired with chromosome-length CSVs, ALWAYS run the bundled script before
hand-rolling pandas. It deterministically computes every common metric in one
pass and avoids the rows-vs-sites pitfall that produces silently-wrong answers.

```bash
python skills/tooluniverse-epigenomics/scripts/methylation_density.py \
  --cpg <CpG csv> --chr-lengths <chr lengths csv> \
  --filter-meth-extremes 90 10
```

The full JSON output contains every metric. Pick the one that matches the
question's wording (NOT a similar-looking one):

| Question phrasing                                              | Script field              |
|----------------------------------------------------------------|---------------------------|
| "how many sites are removed when filtering …"                  | `rows_removed`            |
| "how many unique CpG sites pass filter"                        | `unique_pos_after_filter` |
| "genome-wide AVERAGE chromosomal density"                      | `density_avg_per_chr`     |
| "density on chromosome X"                                      | `density_chromosome` (pass `--chromosome X`) |
| "total density across the genome"                              | `density_total_over_genome` |

The two density numbers (`density_avg_per_chr` vs `density_total_over_genome`)
typically differ by ~2× because CpGs are not uniformly distributed across
chromosomes; reporting one when the question asks for the other is the most
common failure mode here.

For "sites removed" questions, the long-format CSV has multiple rows per CpG
position (one per sample), so `rows_removed` is in the tens of thousands while
`unique_pos_removed` is in the hundreds. Match the granularity to the question.

## Distinguish "rows" vs "unique sites" — methylation CSVs are usually long-format

CpG methylation CSVs typically have ONE ROW PER (sample × CpG site) — so `len(df) >> n_unique_sites`. Before computing anything, decide which axis the question is asking about:

| Question phrasing | Axis | Operation |
|-------------------|------|-----------|
| "how many sites are removed when filtering" | sample-rows | filter then count rows; do NOT dedupe by `Pos`. The CSV is in long format; "sites" here is row-shaped. Subtract `len(df_filtered)` from `len(df)`. |
| "how many unique CpG sites pass filter" | unique positions | dedupe by position (or `Pos` column), then filter |
| **"genome-wide average chromosomal density"** | per-chromosome density | MEAN of per-chromosome densities: `(n_unique_per_chr / chr_length).mean()`. NOT `total_unique / total_genome` — that gives a different answer (typically ≈ ½ of the per-chr mean for unevenly distributed CpGs). |
| **"density on chromosome X"** | single chromosome | unique positions on X / length(X). Be careful which species — check the question text for "Zebra Finch" vs "Jackdaw". |
| "chi-square for uniform distribution across chromosomes" | unique positions per chromosome | filter rows first, then dedupe by `(Chromosome, Pos)`, then count per-chromosome unique positions for chi-sq