Bio Isoform Switching

Version Compatibility

Reference examples tested with: IsoformSwitchAnalyzeR 2.11+, DRIMSeq 1.34+, DEXSeq 1.52+, satuRn 1.14+, stageR 1.28+, fishpond 2.14+, tximport 1.34+, tximeta 1.24+, Salmon 1.10+

Before using code patterns, verify installed versions match. If versions differ:

• R: packageVersion('<pkg>') then ?function_name to verify parameters
• CLI: <tool> --version then <tool> --help to confirm flags

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

Isoform Switching and Differential Transcript Usage

Identify shifts in which transcript a gene predominantly uses between conditions, and predict functional consequences. Statistically distinct from DGE and DTE; biologically distinct because the same gene-level expression can hide a complete isoform switch with major protein-level consequences.

DGE vs DTE vs DTU: Which Question Is Being Asked?

DTU is statistically harder than DGE because:

1. The null is compositional (proportions sum to 1; one transcript up means another down).
2. Multi-stage testing is required: gene-level "any DTU" + transcript-level "which transcript" -> stageR formalizes this.
3. Quantification uncertainty propagates when transcripts are similar (Salmon EM ambiguity).

DTU and event-level differential splicing answer related but distinct questions: rMATS' IncLevelDifference is essentially a 1-D projection of a DTU shift onto a single event coordinate. The pragmatic 2026 default: run both an event-level tool (rMATS or leafcutter) and a DTU pipeline; reconcile.

Tool Selection for DTU

The IsoformSwitchAnalyzeR v2 default rule is: satuRn if any condition has >5 replicates; else DEXSeq. For exactly 5 replicates per condition (boundary), explicitly choose; results may differ.

Decision Tree by Research Question

IsoformSwitchAnalyzeR v2 Workflow

Goal: Identify isoform switches with functional consequences in one integrated workflow.

Approach: Import Salmon, pre-filter, run statistical test (satuRn auto-selected if any condition has >5 replicates, else DEXSeq), annotate switches with external tools (CPC2, Pfam, SignalP, IUPred2A or DeepTMHMM), then summarize consequences.

library(IsoformSwitchAnalyzeR)

salmonQuant <- importIsoformExpression(
    parentDir = 'salmon_quant/',
    addIsofomIdAsColumn = TRUE
)

design <- data.frame(
    sampleID = colnames(salmonQuant$counts)[-1],
    condition = c('control', 'control', 'control', 'treatment', 'treatment', 'treatment')
)

aSwitchList <- importRdata(
    isoformCountMatrix = salmonQuant$counts,
    isoformRepExpression = salmonQuant$abundance,
    designMatrix = design,
    isoformExonAnnoation = 'annotation.gtf',
    isoformNtFasta = 'transcripts.fa',
    showProgress = TRUE
)

aSwitchList <- preFilter(
    aSwitchList,
    geneExpressionCutoff = 1,
    isoformExpressionCutoff = 0,
    IFcutoff = 0.01,
    removeSingleIsoformGenes = TRUE,
    keepIsoformInAllConditions = TRUE
)

aSwitchList <- isoformSwitchTestSatuRn(
    aSwitchList,
    reduceToSwitchingGenes = TRUE,
    alpha = 0.05,
    dIFcutoff = 0.1
)

preFilter parameters:

• geneExpressionCutoff = 1 — minimum TPM for gene to be tested (raise for stricter)
• isoformExpressionCutoff = 0 — minimum TPM per isoform (set to 1 for stricter)
• IFcutoff = 0.01 — minimum isoform fraction; below = noise
• removeSingleIsoformGenes = TRUE — drop genes with only one detectable isoform (cannot have DTU)
• keepIsoformInAllConditions = TRUE — require expression across all conditions

For long-read input, use importRdata with long-read transcript counts directly — bypasses Salmon EM uncertainty entirely.

Functional Consequence Annotation

Goal: Predict how each switch alters protein structure, function, and stability.

Approach: Extract sequences, run external annotators outside R, then re-import results into the switchAnalyzeRlist.

aSwitchList <- extractSequence(
    aSwitchList,
    pathToOutput = 'sequences/',
    writeToFile = TRUE
)

# Run external tools on sequences/isoformSwitchAnalyzeR_isoform_*.fasta
# Then import:

# IMPORTANT: ORF analysis must run BEFORE analyzeSwitchConsequences for NMD_status,
# ORF_seq_similarity, and coding_potential consequences to be computed.
aSwitchList <- analyzeORF(aSwitchList, orfMethod = 'longest', genomeObject = NULL)

aSwitchList <- analyzeCPC2(aSwitchList, pathToCPC2resultFile = 'cpc2_results.txt', removeNoncodinORFs = TRUE)
aSwitchList <- analyzePFAM(aSwitchList, pathToPFAMresultFile = 'pfam_results.txt')
aSwitchList <- analyzeSignalP(aSwitchList, pathToSignalPresultFile = 'signalp_results.txt')
aSwitchList <- analyzeIUPred2A(aSwitchList, pathToIUPred2AresultFile = 'iupred2_results.txt')
aSwitchList <- analyzeAlternativeSplicing(aSwitchList,

aSwitchList <- analyzeSwitchConsequences(
    aSwitchList,
    consequencesToAnalyze = c(
        'intron_retention',
        'coding_potential',
        'ORF_seq_similarity',
        'NMD_status',
        'domains_identified',
        'IDR_identified',
        'IDR_type',
        'signal_peptide_identified'
    ),
    dIFcutoff = 0.1
)

The external tools must be run outside R; IsoformSwitchAnalyzeR provides FASTA outputs and re-imports the parsed results. Plan for ~30-60 minutes of external compute on typical mammalian transcriptomes.

NMD Prediction (The 50-nt Rule)

A transcript is predicted NMD-sensitive if its premature termination codon (PTC) lies >50-55 nt upstream of the last exon-exon junction (Maquat 2004 Nat Rev Mol Cell Biol; Lykke-Andersen & Jensen 2015 Nat Rev Mol Cell Biol).

Mechanism: Spliceosome deposits the Exon Junction Complex (EJC) ~20-24 nt upstream of every exon-exon junction. During the pioneer round of translation, ribosome reading through removes EJCs upstream of the stop codon. If a stop codon precedes the last EJC by >50 nt, the EJC remains, recruits UPF1 -> SMG1 phosphorylation -> SMG6/SMG7 -> mRNA decay.

Caveats and exceptions:

• Last-exon PTCs escape NMD — can be dominant-negative or gain-of-function (e.g. MYH7 truncating variants).
• 3'UTR length matters: very long 3' UTRs (>1 kb past stop) trigger NMD via UPF1 binding even without EJCs (faux-3'UTR rule).
• Tissue-specific NMD: SMG6 vs SMG5/7 ratios vary; UPF1 stress conditions modulate.
• PTC distance must be measured on the spliced transcript, not the genomic distance.
• ~10-20% of "predicted NMD" transcripts escape NMD per orthogonal RNA-seq (Lindeboom 2016 Nat Genet; ~22% of canonical PTC-bearing transcripts escape in some tissues). Treat NMD prediction as probabilistic, not certain.

IsoformSwitchAnalyzeR's analyzeSwitchConsequences with 'NMD_status' evaluates this from the predicted ORF + transcript model.

AS-NMD as a Regulatory Layer

A large class of conserved alternative splicing events is deliberately PTC-introducing to titrate functional protein levels:

• All major SR proteins (SRSF1-12) autoregulate via poison exons (Lareau 2007 Nature; Ni 2007 Genes Dev)
• All major hnRNPs likewise
• Ribosomal protein genes use AS-NMD autoregulation (e.g. rpL3, rpL12; Cuccurese 2005 NAR)
• SCN1A poison exon -> Stoke STK-001 ASO in Phase 1/2 for Dravet syndrome (Han 2020 Sci Transl Med)

Functional implication: an increase in PSI of a poison exon decreases functional protein. Sign-of-effect in DTU output is opposite from intuition for these genes. Always check whether the alternative form is PTC-bearing before interpreting direction.

Disease examples:

• TDP-43 cryptic exons (UNC13A, STMN2) introduce PTCs -> NMD on disease-relevant transcript (Brown 2022 Nature)
• Last-exon truncating variants in TTN (and MYH7): escape NMD -> stable poison/dominant-negative protein

Manual DTU Pipeline (DRIMSeq + DEXSeq + stageR)

The canonical reference is the F1000Research "Swimming downstream" workflow (Love, Soneson, Patro 2018; Bioconductor rnaseqDTU).

library(tximeta); library(DRIMSeq); library(DEXSeq); library(stageR)

se <- tximeta(coldata)
counts <- assays(se)$counts

samples <- data.frame(
    sample_id = colnames(counts),
    condition = c('control', 'control', 'control', 'treatment', 'treatment', 'treatment')
)

txdf <- data.frame(
    gene_id = rowData(se)$gene_id,
    feature_id = rowData(se)$tx_id,
    counts
)

d <- dmDSdata(counts = txdf, samples = samples)
d <- dmFilter(d, min_samps_feature_expr = 3, min_feature_expr = 10,
              min_samps_feature_prop = 3, min_feature_prop = 0.1,
              min_samps_gene_expr = 6, min_gene_expr = 10)

design_full <- model.matrix(~ condition, data = samples(d))

dxd <- DEXSeqDataSet(
    countData = round(as.matrix(counts(d)[, -c(1, 2)])),
    sampleData = samples(d),
    design = ~ sample + exon + condition:exon,
    featureID = counts(d)$feature_id,
    groupID = counts(d)$gene_id
)
dxd <- estimateSizeFactors(dxd)
dxd <- estimateDispersions(dxd, quiet = TRUE)
dxd <- testForDEU(dxd, reducedModel = ~ sample + exon)
qval <- perGeneQValue(DEXSeqResults(dxd))

dxr <- DEXSeqResults(dxd, independentFiltering = FALSE)
pConfirmation <- matrix(dxr$pvalue, ncol = 1)
rownames(pConfirmation) <- dxr$featureID
tx2gene <- as.data.frame(dxr[, c('featureID', 'groupID')])

stageRObj <- stageRTx(
    pScreen = qval,
    pConfirmation = pConfirmation,
    pScreenAdjusted = TRUE,
    tx2gene = tx2gene
)
stageRObj <- stageWiseAdjustment(stageRObj, method = 'dtu', alpha = 0.05)

results <- getAdjustedPValues(stageRObj, order = FALSE,

stageR semantics:

• Stage 1 (screening): gene-level p-value (perGeneQValue from DEXSeq, or DRIMSeq's gene-level p) is filtered at the desired Overall FDR.
• Stage 2 (confirmation): only within significant genes, individual transcripts are tested at a within-gene FWER computed to maintain global OFDR.
• Net effect: gene-level FDR is properly controlled, AND the transcript that drove the call is known.
• Without stageR: naive transcript-level BH overcounts because the gene-level multiple-testing burden is ignored.

fishpond/swish for Inferential-Uncertainty-Aware Testing

Goal: Test DTE while propagating quantification uncertainty from Salmon's Gibbs samples.

Approach: Run Salmon with --numGibbsSamples 20, import with tximeta, then use swish to average a non-parametric SAMseq-style test across inferential replicates.

library(fishpond); library(tximeta)

se <- tximeta(coldata)
y <- scaleInfReps(se)
y <- labelKeep(y)
y <- y[mcols(y)$keep, ]

set.seed(1)
y <- swish(y, x = 'condition')

dte_results <- as.data.frame(mcols(y))
sig <- subset(dte_results, qvalue < 0.05)

infRV (inferential relative variance) is a per-feature uncertainty diagnostic; high-infRV transcripts are unreliable and can be filtered before testing. Critical for genes with many similar isoforms (TTN, MAPT, NEFM) where Salmon's EM is uncertain.

Per-Tool Failure Modes

DEXSeq: Slowness at Scale

Trigger: Bulk cohort with >50 samples or single-cell DTU.

Mechanism: DEXSeq fits a NB GLM per exon-bin per gene; computational cost scales linearly with samples × bins.

Symptom: estimateDispersions takes hours; testForDEU exhausts memory.

Fix: Switch to satuRn (designed for scale, including scRNA-seq); run with parallelization (BPPARAM = MulticoreParam(8)).

DRIMSeq: Filtering Sensitivity

Trigger: Default dmFilter parameters too strict for low-expression cohort.

Mechanism: min_samps_feature_expr = 3, min_feature_expr = 10 drops transcripts seen in <=2 samples or with <10 counts.

Symptom: Most candidate genes filtered out; few testable genes.

Fix: Tune to dataset: lower thresholds for low-coverage data, raise for high-coverage. Document choice.

satuRn: Empirical-Bayes Shrinkage Limits

Trigger: Very small cohort (n=2 vs n=2) or very heterogeneous.

Mechanism: Empirical-Bayes shrinkage assumes shared dispersion across genes; collapses with too-few or too-heterogeneous samples.

Symptom: Inflated p-values; few discoveries despite real effects.

Fix: Aggregate replicates (pseudobulk), or switch to DEXSeq for small cohorts; use larger cohorts when possible.

swish: Salmon Gibbs Requirements

Trigger: Running swish on Salmon output without Gibbs samples.

Mechanism: swish averages over inferential replicates from Salmon's Gibbs sampler; requires --numGibbsSamples 20 (or bootstrap with --numBootstraps) at Salmon time.

Symptom: scaleInfReps errors about missing inferential replicates.

Fix: Re-run Salmon with --numGibbsSamples 20; this triples Salmon runtime but enables uncertainty-aware testing.

IsoformSwitchAnalyzeR: External-Annotator Failure

Trigger: Forgetting to run all 4 external annotators (CPC2, Pfam, SignalP, IUPred2A).

Mechanism: analyzeSwitchConsequences silently drops consequence types for which annotation wasn't imported.

Symptom: extractConsequenceSummary shows fewer types than requested; specific consequence reports missing.

Fix: Verify all 4 result files exist before analyzeSwitchConsequences; check aSwitchList$AlternativeSplicingAnalysis slot for completeness.

Reconciliation: When DTU and Event-Level Disagree

For high-confidence reporting: concordant DTU + event-level + sashimi visualization.

Single-Cell DTU

For scRNA-seq, satuRn scales where DEXSeq does not. IsoformSwitchAnalyzeR v2 supports single-cell input via importRdata with single-cell count matrices, and the underlying satuRn test has explicit single-cell calibration (Gilis 2022 F1000Research).

Strong recommendation: pseudobulk by cell type first; per-cell DTU is rarely powered with droplet 3' chemistry. See single-cell-splicing for chemistry-specific limitations.

Visualization

extractTopSwitches(
    aSwitchList,
    filterForConsequences = TRUE,
    n = 25,
    sortByQvals = TRUE
)

switchPlot(
    aSwitchList,
    gene = 'TARGET_GENE',
    condition1 = 'control',
    condition2 = 'treatment',
    localTheme = theme_bw(base_size = 12)
)

extractConsequenceSummary(aSwitchList, consequencesToAnalyze = 'all', plotGenes = FALSE)
extractConsequenceEnrichment(aSwitchList, consequencesToAnalyze = 'all')
extractSplicingSummary(aSwitchList, asFractionTotal = FALSE)

Significance Thresholds

Common Errors

Common Pitfalls

• Skipping stageR -> inflated transcript-level FDR; gene-level multiple-testing burden ignored.
• Forgetting NMD direction -> sign-of-effect on protein opposite to sign-of-effect on transcript when alternative form is a PTC-bearer. Always check.
• Treating short-read-derived isoform calls as ground truth -> Salmon EM is uncertain; use Gibbs samples + swish if quantification uncertainty matters.
• Comparing across annotations -> GENCODE basic vs comprehensive, RefSeq, Ensembl all have different transcript catalogs; switches "appear" or "disappear" with annotation choice. Document version.
• Not running long-read where possible -> Iso-Seq / ONT removes ambiguity for genes with many similar isoforms (TTN, MAPT, NEFM, DSCAM).
• Choosing satuRn or DEXSeq blindly at the n=5 boundary -> IsoformSwitchAnalyzeR v2 auto-selects based on >5 vs <=5; results may differ. Document choice.
• Reporting a "switch" without a sashimi plot -> reviewers will demand it; do it upfront.
• Forgetting stageR also corrects gene-level p when starting from DRIMSeq -> DRIMSeq's gene_p should be passed as pScreen, not raw transcript p-values.

Related Skills

• differential-splicing - Event-level (rMATS, leafcutter, MAJIQ) complementary to DTU
• splicing-quantification - PSI is a 1D projection of DTU shifts
• splicing-qc - Verify upstream library, depth, alignment before DTU
• sashimi-plots - Required visualization for switch validation and reporting
• splice-variant-prediction - Connects SpliceAI variant predictions to specific isoforms
• long-read-splicing - Full-isoform DTU bypasses transcript-quant uncertainty; preferred for many-isoform genes
• pathway-analysis/go-enrichment - Pathway enrichment of switching genes
• rna-quantification/alignment-free-quant - Salmon with --numGibbsSamples is upstream

References

• Han et al 2025 bioRxiv 10.64898/2025.12.08.693027 - IsoformSwitchAnalyzeR v2
• Vitting-Seerup & Sandelin 2019 Bioinformatics 35:4469-4471 - IsoformSwitchAnalyzeR original
• Anders et al 2012 Genome Res - DEXSeq
• Nowicka & Robinson 2016 F1000Research - DRIMSeq
• Gilis et al 2022 F1000Research - satuRn
• Zhu et al 2019 NAR - swish / fishpond
• Van den Berge et al 2017 Genome Biol - stageR
• Love, Soneson, Patro 2018 F1000Research - Swimming downstream DTU workflow
• Maquat 2004 Nat Rev Mol Cell Biol - NMD review
• Lykke-Andersen & Jensen 2015 Nat Rev Mol Cell Biol - NMD update
• Lindeboom et al 2016 Nat Genet - NMD escape rates from RNA-seq
• Lareau et al 2007 Nature - SR protein AS-NMD autoregulation
• Ni et al 2007 Genes Dev - ultraconserved-element AS-NMD in splicing regulators
• Cuccurese et al 2005 NAR 33:5965-5977 - ribosomal protein AS-NMD autoregulation
• Brown et al 2022 Nature - UNC13A cryptic exon (TDP-43)
• Han et al 2020 Sci Transl Med - SCN1A poison exon ASO