bio-pathway-gsea

name	bio-pathway-gsea
description	Tests a ranked gene vector for coordinated expression shifts in GO, KEGG, Reactome, or MSigDB gene sets with clusterProfiler's gseGO, gseKEGG, gsePathway, and GSEA (fgseaMultilevel engine), and scores per-sample pathway activity with ssGSEA and GSVA. Covers why a GSEA result is a deterministic function of three implicit choices (the ranking STATISTIC, the weight exponent p, and which LABELS are permuted), why the input must be a NAMED vector sorted DECREASING by a signed variance-calibrated metric (DESeq2 stat, limma t) not a raw p-value that erases direction, why preranked gene-permutation is anti-conservative for correlated sets (CAMERA is the fix), why nPerm is gone (eps governs tiny p), and why set.seed is required. Use when every gene carries a DE statistic, when a hard cutoff is arbitrary, or when ORA finds nothing. For gene-list ORA see go-enrichment; the ranking statistic comes from differential-expression/de-results.
tool_type	r
primary_tool	clusterProfiler

Version Compatibility

Reference examples tested with: clusterProfiler 4.18.4+, org.Hs.eg.db 3.22+, msigdbr 26+, fgsea 1.36+.

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.

gseKEGG queries the live KEGG REST API, so the same code returns different results as KEGG updates; pin the run date. gseGO/gsePathway and MSigDB GSEA use local annotation (org.*.eg.db, reactome.db, msigdbr) and are reproducible given the package version. The single source of truth for versions is this block, not headings.

Gene Set Enrichment Analysis (GSEA)

"Which pathways shift coordinately across my full ranked gene list, with no cutoff?" -> Walk a weighted running-sum down the genome-wide ranking and test whether each gene set piles up at one END - because that score reports the structure of YOUR ranking, so the ranking metric and the permutation type, not the gene sets, decide the result.

• R: gseGO(geneList, OrgDb, ont), gseKEGG(geneList, organism), GSEA(geneList, TERM2GENE)

Scope: threshold-free Functional Class Scoring (FCS) of a RANKED vector - the running-sum ES, the ranking-metric choice, the permutation null, NES/FDR, the leading edge, and per-sample ssGSEA/GSVA scores. A pre-selected unranked gene LIST -> go-enrichment (ORA). The ranking statistic source -> differential-expression/de-results. KEGG/Reactome/WikiPathways database semantics -> kegg-pathways, reactome-pathways, wikipathways. Plots -> enrichment-visualization.

The Single Most Important Modern Insight -- A GSEA Result Is a Deterministic Function of Three Usually-Implicit Choices: the Ranking Statistic, the Weight Exponent p, and the Permuted Labels

GSEA is not a discovery about biology - it is the running-sum's report on the ranking it was handed. The weighted enrichment score (Subramanian 2005; the default exponent=1 weights each hit by the gene's statistic magnitude) asks exactly one question: do the members of a set pile up at one END of YOUR ranking. So the result is fixed by three decisions tutorials usually leave silent, and Wijesooriya 2022 found most published GSEA papers report none of them.

1. The ranking statistic IS the experiment. Rank by a signed, variance-calibrated metric (DESeq2 stat, limma moderated t) - sign gives direction, variance-calibration sinks noisy low-information genes to the middle. Ranking by a RAW p-value erases the sign, so up- and down-regulated genes collapse together and NES becomes uninterpretable. Ranking by bare log2FC lets a handful of low-count genes with huge unstable fold changes hijack the leading edge. A bad ranking is faithfully reported as a ranking artifact.
2. The permutation type sets validity. Phenotype (sample-label) permutation preserves the gene-gene correlation that co-regulated pathways are made of - it is the gold standard for type-I error control because it preserves that correlation, but needs the expression matrix and adequate n per group (~>=7); below that n its validity degrades. clusterProfiler/fgsea preranked are FORCED into GENE permutation, which treats genes as independent, destroys that correlation, and is ANTI-CONSERVATIVE: it returns "significant" pathways that are nothing but co-expression. Accept it, report it, and prefer CAMERA when the design matrix is available (full competitive/self-contained theory in the category README).
3. The honesty is bounded by reporting. Log the ranking metric, the exponent p, the permutation type, the gene-set collection and its version/date, the size filters, and the multiple-testing method. Without those, the result is unfalsifiable.

Tool Taxonomy

Method	Citation	Mechanism / role	When
Preranked GSEA (gseGO/gseKEGG/GSEA)	Subramanian 2005 PNAS 102:15545; Mootha 2003 Nat Genet 34:267	weighted running-sum ES over a ranked vector; gene-permutation null	the common case: a ranked statistic for all genes, no matrix
fgsea engine	Korotkevich 2021 bioRxiv 060012 (preprint)	fgseaMultilevel; resolves tiny p accurately down to eps	the engine under by='fgsea' (default); what gives sub-1/nperm p-values
Phenotype-permutation GSEA	Subramanian 2005 PNAS 102:15545	shuffles sample labels; preserves gene-gene correlation	matrix + phenotype + ~>=7/group; the gold-standard competitive test
CAMERA	Wu & Smyth 2012 NAR 40:e133	competitive, VIF-corrects inter-gene correlation analytically	matrix + design; want a correlation-honest competitive test
ROAST / fry	Wu 2010 Bioinformatics 26:2176	self-contained rotation test; valid at any n	matrix + design, tiny n, "is the set DE at all"
ssGSEA	Barbie 2009 Nature 462:108	per-sample rank-based enrichment score	a per-sample pathway-activity matrix (no contrast test)
GSVA	Hanzelmann 2013 BMC Bioinformatics 14:7	unsupervised per-sample, per-set kernel/CDF score	per-sample features for clustering/survival/ML

Decision Tree by Scenario

Scenario	Recommended	Why
Ranked statistic for ALL genes, cutoff would be arbitrary	preranked gseGO/gseKEGG/GSEA	threshold-free; the common case (report gene-permutation)
Pre-selected unranked list (module, GWAS hits, screen)	ORA -> go-enrichment	no genome-wide ranking exists
Function annotation, broad GO coverage	gseGO	GO is the broadest local resource
Metabolic / signaling pathways	gseKEGG -> kegg-pathways	KEGG maps (live DB)
Reaction-level, reproducible offline	gsePathway -> reactome-pathways	local reactome.db
Curated MSigDB hallmark / C2 / C5	GSEA(TERM2GENE) + msigdbr	generic-input GSEA on any collection
Matrix + design, want competitive + correlation-honest	limma::camera	VIF-corrects the inter-gene correlation gene-permutation ignores
Matrix + design, tiny n / covariates, "is set DE at all"	limma::roast/fry	self-contained rotation, valid at any n
Per-sample pathway-activity matrix for clustering/ML	ssGSEA / GSVA	scores each sample, not a contrast test
The DE statistic / ranking itself	-> differential-expression/de-results	upstream, not enrichment

Build the Ranked Vector (the Caller-Owned Step)

Goal: Turn a DE table into the named numeric vector sorted strictly decreasing that every preranked function requires, ranked by a signed variance-calibrated metric.

Approach: Pick the ranking metric to match the DE tool, name the vector by gene ID, drop NAs, deduplicate to one statistic per gene, and sort decreasing. The DE mechanics and the $padj/$adj.P.Val column conventions live in differential-expression/de-results.

DE source	Ranking metric	Column	Why
DESeq2	Wald statistic	stat	signed + variance-calibrated; best single choice for RNA-seq
limma / voom	moderated t-statistic	t	empirical-Bayes shrinkage borrows variance; signed
edgeR (QL)	sign(logFC) * -log10(PValue)	derived	no single signed statistic column
any tool, last resort	log2 fold change	log2FoldChange	magnitude only; noisy for low-count genes

library(clusterProfiler)
library(org.Hs.eg.db)

de <- read.csv('de_results.csv')          # DE list source: differential-expression/de-results
gene_list <- de$stat                       # DESeq2 Wald stat: signed + variance-calibrated
names(gene_list) <- de$entrez_id
gene_list <- gene_list[!is.na(gene_list)]
gene_list <- gene_list[!duplicated(names(gene_list))]   # one statistic per gene; duplicates double-count hits
gene_list <- sort(gene_list, decreasing = TRUE)         # REQUIRED: unsorted input silently mis-ranks

Ranking by sign(log2FC) * -log10(pmax(pvalue, 1e-300)) (for edgeR, or when a Wald stat is unavailable) preserves direction and clamps p==0 from going to Inf. Never rank by raw p-value alone (sign erased) or by lfcShrink(type='normal') (deprecated prior distorts the ranking). apeglm/ashr-shrunk results DROP the stat column - pull stat from the unshrunk results(dds) if ranking by it.

Run Preranked GSEA on GO

Goal: Find GO terms whose members shift coordinately up or down across the full ranking, with no significance cutoff.

Approach: Set the permutation seed for reproducibility, set eps=0 for exact tiny p-values, run gseGO, then map the leading-edge IDs back to symbols and read core_enrichment as the interpretable core.

set.seed(123)                              # fixes the multilevel Monte Carlo; any fixed seed
gse_go <- gseGO(geneList = gene_list, OrgDb = org.Hs.eg.db, keyType = 'ENTREZID',
                ont = 'BP', exponent = 1, minGSSize = 10, maxGSSize = 500,
                eps = 0, pvalueCutoff = 0.05, pAdjustMethod = 'BH',
                seed = TRUE, by = 'fgsea', verbose = FALSE)
gse_go <- setReadable(gse_go, OrgDb = org.Hs.eg.db, keyType = 'ENTREZID')

gseaResult columns: ID, Description, setSize, enrichmentScore (raw ES), NES, pvalue, p.adjust (BH), qvalue, rank (ES-peak position), leading_edge, core_enrichment (/-separated leading-edge IDs). Report p.adjust/qvalue, never raw pvalue and never an invented $FDR. NES sign = direction: positive = top of the ranking (up in the contrast), negative = bottom.

Run GSEA on KEGG, Reactome, or MSigDB

Goal: Apply the same preranked engine to a pathway database with the gene-ID type that database expects.

Approach: gseKEGG/gsePathway need ENTREZ-style IDs; choose the collection, keep the seed and eps=0, and note KEGG queries the live REST API (date-dependent) while Reactome and MSigDB are local.

set.seed(123)
gse_kegg <- gseKEGG(geneList = gene_list, organism = 'hsa', keyType = 'ncbi-geneid',   # Entrez-named vector; 'kegg' keyType is the prokaryote locus-tag path
                    minGSSize = 10, maxGSSize = 500, eps = 0,
                    pvalueCutoff = 0.05, seed = TRUE, verbose = FALSE)   # live KEGG API; pin the date

library(msigdbr)
h <- msigdbr(species = 'Homo sapiens', collection = 'H')    # 26.x: collection= (was category=); gs_collection (was gs_cat)
t2g <- h[, c('gs_name', 'ncbi_gene')]                       
gse_h  GSEAgeneList  gene_list TERM2GENE  t2g exponent  
              minGSSize   maxGSSize   eps  
              pvalueCutoff   seed   verbose  

If the installed msigdbr still uses category=/entrez_gene, the old form works but warns - check ?msigdbr and names(h). ReactomePA's gsePathway(geneList, organism='human') reads the local reactome.db and also needs ENTREZ IDs.

Per-Sample Scores: ssGSEA and GSVA (Not a Contrast Test)

Goal: Convert an expression matrix into a gene-set-by-sample activity matrix to feed clustering, survival, or a classifier - there is no per-pathway p-value here.

Approach: GSVA >= 1.50 uses a PARAMETER-OBJECT API: build gsvaParam(...) or ssgseaParam(...) and pass it to gsva(). The old gsva(expr, gset.idx.list, method=) signature is defunct.

# GSVA >= 1.50 / Bioc 3.18 parameter-object API (older method= signature errors)
library(GSVA)
gsva_scores  <- gsva(gsvaParam(expr_matrix, gene_sets))      # unsupervised per-sample set scores
ssgsea_scores <- gsva(ssgseaParam(expr_matrix, gene_sets))   # ssGSEA via the same dispatch

Use GSEA (preranked or phenotype) for a CONTRAST and a pathway-level p-value; use ssGSEA/GSVA for a per-sample activity matrix for downstream modeling. GSVA is not installed in the reference environment - verify the installed signature with ?gsva before running.

Per-Method Failure Modes

Ranking by raw p-value

Trigger: gene_list <- -log10(de$pvalue) with no sign(). Mechanism: the magnitude is symmetric, so up- and down-regulated genes both land at the top. Symptom: NES signs are meaningless; "enriched" sets mix directions. Fix: rank by sign(log2FC) * -log10(pmax(p, 1e-300)), or use DESeq2 stat / limma t.

Preranked p-values treated as correlation-honest

Trigger: reporting FDR 0.001 from gseGO/fgsea on a co-regulated set. Mechanism: gene permutation assumes gene independence; correlated sets inflate the set-statistic variance, so p is too small. Symptom: "significant" pathways that are co-expression and do not replicate. Fix: state the permutation type; for type-I control with a design matrix use CAMERA (Wu & Smyth 2012).

Unsorted or duplicated geneList

Trigger: an un-sorted vector, or duplicate gene names after ID conversion. Mechanism: clusterProfiler assumes pre-sorting and uses names to map into sets; duplicates double-count a gene in the hit increments. Symptom: silently wrong ES, or an fgsea ties warning. Fix: sort(gl[!duplicated(names(gl))], decreasing=TRUE); prefer a continuous metric (Wald stat / moderated t rarely tie).

Bare log2FC ranking

Trigger: ranking by log2FoldChange from raw counts. Mechanism: a gene with 2 vs 8 counts shows a huge unstable LFC. Symptom: the leading edge is one or two low-count outliers, not a coordinated shift. Fix: rank by stat/t; if LFC is unavoidable use apeglm/ashr-shrunk LFC (never type='normal').

No set.seed

Trigger: running gseGO without fixing the seed. Mechanism: the multilevel Monte Carlo is stochastic. Symptom: p-values and the significant-set list drift across identical reruns. Fix: set.seed(123) AND seed=TRUE in the call.

Tiny leading edge believed

Trigger: trusting a high |NES| without inspecting core_enrichment. Mechanism: a 1-2 gene leading edge is outlier-driven, not a pathway shift; large sets reach high |NES| by chance. Symptom: an unreplicated headline pathway. Fix: FDR first, then leading-edge size/concentration, then NES for prioritization.

Stale nPerm / FDR<0.25 lore

Trigger: copying nPerm=10000 and "FDR < 0.25" from a Broad-desktop tutorial. Mechanism: nPerm was REMOVED at the fgsea/multilevel switch; clusterProfiler p.adjust is BH, not the Broad empirical-null FDR that 0.25 was calibrated for. Symptom: an argument error (nPerm) or a mis-transplanted threshold. Fix: drop nPerm, govern tiny-p with eps, treat p.adjust as BH and pick a defensible cutoff (often 0.05).

Stale GSVA call

Trigger: gsva(expr, gene_sets, method='ssgsea'). Mechanism: GSVA >= 1.50 dispatches on a parameter object's class. Symptom: the old signature errors. Fix: gsva(gsvaParam(expr, gene_sets)) / ssgseaParam(...).

Quantitative Thresholds

Threshold	Source	Rationale
exponent = 1	Subramanian 2005 PNAS 102:15545	weights each hit by
minGSSize = 10	clusterProfiler default	drops tiny sets that overfit on one outlier
maxGSSize = 500	clusterProfiler default	drops overly broad sets that always 'enrich'
eps = 0 (default 1e-10)	clusterProfiler / fgsea	replaces nPerm: eps=0 resolves exact tiny p-values; 1e-10 is the default floor
pAdjustMethod = 'BH'	clusterProfiler default	Benjamini-Hochberg FDR; NOT the Broad empirical-null FDR, so do not reflex to 0.25
pvalueCutoff = 0.05	clusterProfiler default	filters on p.adjust by default; defensible BH cutoff
~>=7 samples/group	Broad GSEA docs	minimum for a non-degenerate phenotype-permutation null
set.seed(123)	reproducibility	any fixed seed; the point is to fix the multilevel Monte Carlo

Common Errors

Error / symptom	Cause	Solution
Error about names / wrong ES	geneList not named or not sorted decreasing	sort(setNames(v, ids), decreasing=TRUE)
--> No gene can be mapped	wrong keyType/OrgDb, or non-ENTREZ IDs	bitr to the expected ID type first
gseKEGG returns 0 terms	ENSEMBL/SYMBOL passed, wrong organism code, or KEGG API down	convert to kegg-id/ENTREZ; check the organism code; retry (live API)
Different results each run	no set.seed, or live KEGG DB changed	fix the seed; pin the KEGG run date
nPerm argument error	copied from a pre-4.0 tutorial	remove nPerm; use eps
GSVA method= error	pre-1.50 signature	gsva(gsvaParam(expr, sets))
core_enrichment is NA / all-ID	setReadable not applied	setReadable(gse, OrgDb, keyType='ENTREZID')

References

• Mootha VK, Lindgren CM, Eriksson KF, et al. 2003. PGC-1alpha-responsive genes involved in oxidative phosphorylation are coordinately downregulated in human diabetes. Nat Genet 34:267-273.
• Subramanian A, Tamayo P, Mootha VK, et al. 2005. Gene set enrichment analysis: a knowledge-based approach for interpreting genome-wide expression profiles. PNAS 102:15545-15550.
• Korotkevich G, Sukhov V, Budin N, et al. Fast gene set enrichment analysis. bioRxiv 060012 (preprint). DOI 10.1101/060012.
• Wu D, Smyth GK. 2012. Camera: a competitive gene set test accounting for inter-gene correlation. Nucleic Acids Res 40:e133.
• Wu D, Lim E, Vaillant F, et al. 2010. ROAST: rotation gene set tests for complex microarray experiments. Bioinformatics 26:2176-2182.
• Barbie DA, Tamayo P, Boehm JS, et al. 2009. Systematic RNA interference reveals that oncogenic KRAS-driven cancers require TBK1. Nature 462:108-112.
• Hanzelmann S, Castelo R, Guinney J. 2013. GSVA: gene set variation analysis for microarray and RNA-seq data. BMC Bioinformatics 14:7.
• Reimand J, Isserlin R, Voisin V, et al. 2019. Pathway enrichment analysis and visualization of omics data using g:Profiler, GSEA, Cytoscape and EnrichmentMap. Nat Protoc 14:482-517.
• Wijesooriya K, Jadaan SA, Perera KL, et al. 2022. Urgent need for consistent standards in functional enrichment analysis. PLoS Comput Biol 18:e1009935.

Related Skills

• go-enrichment - Gene-list ORA alternative when no ranking exists
• kegg-pathways - KEGG pathway and module enrichment and ID conventions
• reactome-pathways - Reactome curated-pathway enrichment (local DB)
• wikipathways - WikiPathways community-pathway enrichment
• enrichment-visualization - gseaplot2, ridgeplot, and dotplot of GSEA results
• differential-expression/de-results - Source of the ranking statistic and the padj column conventions
• workflows/expression-to-pathways - End-to-end DE-to-enrichment pipeline