Reactome Pathways

---
name: bio-pathway-reactome
description: Reactome pathway enrichment using ReactomePA package. Use when analyzing gene lists against Reactome's curated peer-reviewed pathway database. Performs over-representation analysis and GSEA with visualization and pathway hierarchy exploration.
tool_type: r
primary_tool: ReactomePA
---

## Version Compatibility

Reference examples tested with: R stats (base), ReactomePA 1.46+, clusterProfiler 4.10+

Before using code patterns, verify installed versions match. If versions differ:
- R: `packageVersion('<pkg>')` then `?function_name` to verify parameters

If code throws ImportError, AttributeError, or TypeError, introspect the installed
package and adapt the example to match the actual API rather than retrying.

# Reactome Pathway Enrichment

## When to Use Reactome

| Scenario | Reactome? | Alternative |
|----------|-----------|-------------|
| Signaling pathway detail (reaction-level) | Yes -- best choice | KEGG (pathway-level only) |
| Metabolic pathway focus | Supplement | KEGG has stronger metabolic coverage |
| Reproducibility / open license required | Yes (CC0) | WikiPathways (CC0) |
| Non-model organism (bacteria, plants) | No (7 species only) | KEGG (8,000+ species) |
| Non-human model organism (mouse, rat, fly) | Caution | Annotations are computationally inferred via orthology from human; may contain errors |

Reactome pathways are curated by PhD-level biologists and externally peer-reviewed, making them the highest-quality curated pathway database. Human is the primary species; all others are computationally inferred.

## Core Pattern - Over-Representation Analysis

**Goal:** Identify Reactome pathways over-represented in a gene list from differential expression or other analyses.

**Approach:** Test for enrichment using the hypergeometric test via ReactomePA enrichPathway against curated peer-reviewed pathways.

**"Run pathway enrichment against Reactome"** → Test whether genes in curated Reactome pathways are over-represented among significant genes.

```r
library(ReactomePA)
library(org.Hs.eg.db)

pathway_result <- enrichPathway(
    gene = entrez_ids,         # Character vector of Entrez IDs
    organism = 'human',        # human, rat, mouse, celegans, yeast, zebrafish, fly
    pvalueCutoff = 0.05,
    pAdjustMethod = 'BH',
    readable = TRUE            # Convert to gene symbols
)

head(as.data.frame(pathway_result))
```

## Prepare Gene List from DE Results

**Goal:** Extract significant Entrez gene IDs from differential expression results for Reactome enrichment.

**Approach:** Filter by significance and fold change, then convert symbols to Entrez IDs using bitr.

```r
library(clusterProfiler)

de_results <- read.csv('de_results.csv')
sig_genes <- de_results[de_results$padj < 0.05 & abs(de_results$log2FoldChange) > 1, 'gene_symbol']

gene_ids <- bitr(sig_genes, fromType = 'SYMBOL', toType = 'ENTREZID', OrgDb = org.Hs.eg.db)
entrez_ids <- gene_ids$ENTREZID
```

## GSEA on Reactome Pathways

**Goal:** Detect coordinated expression changes in Reactome pathways using all genes ranked by a statistic.

**Approach:** Create a sorted named vector from DE results and run gsePathway for rank-based enrichment.

```r
# Create ranked gene list (named vector sorted by statistic)
gene_list <- de_results$log2FoldChange
names(gene_list) <- de_results$entrez_id
gene_list <- sort(gene_list, decreasing = TRUE)

gsea_result <- gsePathway(
    geneList = gene_list,
    organism = 'human',
    pvalueCutoff = 0.05,
    pAdjustMethod = 'BH',
    verbose = FALSE
)

head(as.data.frame(gsea_result))
```

## With Background Universe

**Goal:** Restrict enrichment testing to only genes that were actually measured in the experiment.

**Approach:** Pass all tested gene IDs as the universe parameter to enrichPathway.

```r
all_genes <- de_results$entrez_id  # All tested genes

pathway_result <- enrichPathway(
    gene = entrez_ids,
    universe = all_genes,      # Background gene set
    organism = 'human',
    pvalueCutoff = 0.05,
    readable = TRUE
)
```

## Visualization

**Goal:** Create publication-quality plots of Reactome enrichment results.

**Approach:** Use enrichplot functions (dotplot, barplot, emapplot, cnetplot, gseaplot2) on enrichment result objects.

```r
library(enrichplot)

# Dot plot
dotplot(pathway_result, showCategory = 15)

# Bar plot
barplot(pathway_result, showCategory = 15)

# Enrichment map (requires pairwise_termsim first)
pathway_result <- pairwise_termsim(pathway_result)
emapplot(pathway_result)

# Gene-concept network
cnetplot(pathway_result, categorySize = 'pvalue')

# GSEA plot
gseaplot2(gsea_result, geneSetID = 1:3)
```

## View Pathway in Browser

```r
# Open pathway in Reactome browser
viewPathway('R-HSA-109582', organism = 'human')  # Uses pathway ID

# Get pathway ID from results
top_pathway_id <- pathway_result@result$ID[1]
viewPathway(top_pathway_id, organism = 'human')
```

## Export Results

```r
results_df <- as.data.frame(pathway_result)
write.csv(results_df, 'reactome_enrichment.csv', row.names = FALSE)

# Key columns: ID, Description, GeneRatio, BgRatio, pvalue, p.adjust, geneID, Count
```

## Different Organisms

```r
# Mouse
pathway_mouse <- enrichPathway(gene = mouse_entrez, organism = 'mouse', readable = TRUE)

# Rat
pathway_rat <- enrichPathway(gene = rat_entrez, organism = 'rat', readable = TRUE)

# Zebrafish
pathway_zfish <- enrichPathway(gene = zfish_entrez, organism = 'zebrafish', readable = TRUE)

# Supported: human, rat, mouse, celegans, yeast, zebrafish, fly
```

## Compare Clusters

**Goal:** Compare Reactome pathway enrichment across multiple gene lists (e.g., upregulated vs downregulated).

**Approach:** Use compareCluster with enrichPathway to run enrichment per group and visualize side by side.

```r
# Compare pathways across multiple gene lists
gene_clusters <- list(
    upregulated = up_genes,
    downregulated = down_genes
)

compare_result <- compareCluster(
    geneClusters = gene_clusters,
    fun = 'enrichPathway',
    organism = 'human',
    pvalueCutoff = 0.05
)

dotplot(compare_result)
```

## Key Parameters

| Parameter | Default | Description |
|-----------|---------|-------------|
| gene | required | Vector of Entrez IDs |
| organism | human | Species name |
| pvalueCutoff | 0.05 | P-value threshold |
| pAdjustMethod | BH | Adjustment method |
| universe | NULL | Background genes |
| minGSSize | 10 | Min genes per pathway |
| maxGSSize | 500 | Max genes per pathway |
| readable | FALSE | Convert to symbols |

## Supported Organisms

| Organism | Name | OrgDb |
|----------|------|-------|
| Human | human | org.Hs.eg.db |
| Mouse | mouse | org.Mm.eg.db |
| Rat | rat | org.Rn.eg.db |
| Zebrafish | zebrafish | org.Dr.eg.db |
| Fly | fly | org.Dm.eg.db |
| C. elegans | celegans | org.Ce.eg.db |
| Yeast | yeast | org.Sc.sgd.db |

## Interpretation Notes

- Reactome is very granular -- some pathways contain only 2-3 genes. Use `minGSSize = 10` to filter these out.
- The deep hierarchy means parent pathways will often appear alongside child pathways. Look for the most specific (deepest) enriched pathway.
- Always specify a background universe (all tested genes) to avoid inflated significance.
- Examine fold enrichment (GeneRatio / BgRatio), not just p-values.
- For non-human species, note that annotations are orthology-inferred and may not capture species-specific pathway biology.

## Related Skills

- go-enrichment - Gene Ontology enrichment
- kegg-pathways - KEGG pathway enrichment
- wikipathways - WikiPathways enrichment
- gsea - Gene Set Enrichment Analysis
- enrichment-visualization - Visualization functions