Transcript Abundance Quantification Import

transcript-abundance-quantification-import

Summary

Import transcript-level abundance quantifications from tools like Salmon, kallisto, or RSEM and aggregate them to gene-level counts using tximport, producing a list object suitable for downstream differential expression analysis with DESeq2.

When to use

You have transcript abundance files (e.g., Salmon quant.sf.gz, kallisto abundance.h5, RSEM .isoforms.results) from a quantification tool and need to construct a gene-level count matrix for DESeq2 analysis. Use this skill when you want to leverage transcript-to-gene mapping to aggregate transcript-level estimates rather than re-aligning to gene annotations.

When NOT to use

• Your quantification files are already aggregated to gene level (e.g., HTSeq-count output) — use DESeqDataSetFromHTSeq() or DESeqDataSetFromMatrix() instead.
• You have already imported quantifications via tximeta and obtained a SummarizedExperiment with automatic metadata — construct DESeqDataSet directly from the SummarizedExperiment rather than calling tximport again.
• Your input is a raw count matrix in tabular format — use DESeqDataSetFromMatrix() directly.

Inputs

• transcript quantification files (Salmon quant.sf.gz, kallisto abundance.h5, RSEM .isoforms.results, or equivalent)
• sample metadata table (data frame with condition, batch, or other covariates; row names as run IDs)
• tx2gene mapping table (two-column data frame: transcript ID → gene ID)

Outputs

• tximport list object (containing $abundance, $counts, $length, $countsFromAbundance)
• DESeqDataSet object (SummarizedExperiment subclass with un-normalized count assay)

How to apply

First, assemble sample metadata (condition labels, run IDs) and construct file paths to transcript quantification outputs. Load the tx2gene mapping table (a two-column data frame linking transcript IDs to gene IDs), which defines the aggregation target. Call tximport() with the quantification file paths, type parameter matching your quantification tool (e.g., type='salmon'), and the tx2gene mapping to produce a list object containing un-normalized estimated gene counts, transcript lengths, and offsets. Pass this list directly to DESeqDataSetFromTximport() along with sample metadata (colData) and a design formula to construct a DESeqDataSet. The resulting counts remain un-normalized (not scaled by library size), allowing DESeq2 to apply its own normalization internally.

Related tools

• tximport (Primary function to import transcript-level quantifications and aggregate to gene level using tx2gene mapping) — http://bioconductor.org/packages/tximport
• DESeq2 (Accepts tximport output via DESeqDataSetFromTximport() to construct DESeqDataSet for differential expression analysis) — https://github.com/thelovelab/DESeq2
• Salmon (Quantification tool producing quant.sf.gz files imported with type='salmon' in tximport)
• kallisto (Quantification tool producing abundance.h5 files imported with type='kallisto' in tximport)
• RSEM (Quantification tool producing .isoforms.results files imported with type='rsem' in tximport)
• tximportData (R package providing example transcript quantification files and tx2gene mapping for testing and vignettes) — http://bioconductor.org/packages/tximportData
• tximeta (Alternative import function that produces SummarizedExperiment with automatic metadata, bypassing need for manual tx2gene specification in some cases)

Examples

txi <- tximport(files, type="salmon", tx2gene=tx2gene); ddsTxi <- DESeqDataSetFromTximport(txi, colData=samples, design=~condition)

Evaluation signals

• The tximport list object contains keys $abundance, $counts, and $length with numeric values; $counts contains integer-like estimated counts (not scaled by library size).
• The DESeqDataSet assay(dds) contains un-normalized count values consistent with the tximport $counts matrix, verifiable by comparing ranges and sums across samples.
• Gene counts in the DESeqDataSet match the union of genes represented in the tx2gene mapping; no transcript-level IDs remain in row names.
• DESeq() can be executed on the resulting DESeqDataSet without errors regarding missing or malformed count data; the design formula matches the sample metadata structure.
• Independent filtering and variance stabilization downstream proceed normally, indicating the count matrix is properly formatted and free of structural defects.

Limitations

• tximport depends on a correctly formatted tx2gene mapping table; misaligned or incomplete mappings will result in missing or incorrect gene aggregations.
• The method requires that transcript IDs in the quantification files exactly match those in the tx2gene table; ID format mismatches (e.g., ENST vs transcript_id) will cause silent loss of data.
• tximport produces un-normalized counts; further preprocessing (filtering, normalization) is the responsibility of downstream methods like DESeq2 and should not be assumed automatic.
• For organisms or transcriptomes without pre-built tx2gene resources, manual construction is error-prone; tximeta with automatic metadata retrieval (when available) can mitigate this risk.
• Aggregation from transcript to gene level loses fine-grained transcript-level analysis potential; if differential transcript usage or isoform switching is the research goal, preserve transcript-level counts separately.

Evidence

• [other] tximport produces transcript-to-gene aggregation: "you could import the data with tximport, which produces a list, and then you can use DESeqDataSetFromTximport()"
• [other] tximport ingests quantification files and aggregates by tx2gene: "txi <- tximport(files, type="salmon", tx2gene=tx2gene)"
• [other] tximport output contains un-normalized gene-level counts: "tximport ingests transcript-level quantification files and produces a list object that contains un-normalized counts aggregated to the gene level"
• [other] DESeqDataSet construction from tximport: "ddsTxi <- DESeqDataSetFromTximport(txi, colData = samples, design = ~ condition)"
• [other] Supported quantification tools: "If you have performed transcript quantification (with Salmon, kallisto, RSEM, etc.) you could import the data with tximport"