Bio Read Qc Rnaseq Qc

Version Compatibility

Reference examples tested with: RSeQC 5.0+, Qualimap 2.3+, RNA-SeQC 2.4+, Picard 3.1+, salmon 1.10+, samtools 1.19+

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

• CLI: <tool> --version then <tool> --help to confirm flags
• Python: pip show <package> then help(module.function) to check signatures

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

RNA-seq QC -- post-alignment metrics that have no DNA analogue

Assess strandedness, integrity, feature distribution, and enrichment on the ALIGNED BAM, using RSeQC / Qualimap / RNA-SeQC / Picard against a gene model.

"Run RNA-seq QC" -> Infer strandedness, gene-body coverage, exonic/intronic/intergenic distribution, rRNA rate, and TIN from the BAM.

• CLI: infer_experiment.py -i aligned.bam -r genes.bed12 (strandedness)
• CLI: picard CollectRnaSeqMetrics / qualimap rnaseq / rnaseqc collapsed.gtf in.bam out/

Scope: this skill OWNS transcriptome QC on the aligned BAM. Raw-FASTQ QC (adapters, base quality) -> read-qc/quality-reports. UMI dedup -> read-qc/umi-processing. Quantification -> rna-quantification/featurecounts-counting. OUT OF SCOPE: differential expression (differential-expression/deseq2-basics).

The Single Most Important Modern Insight

1. These are POST-ALIGNMENT QC: every metric needs an aligned BAM AND a gene model (BED12 / GTF / refFlat / collapsed-GTF), which is the line that separates them from FastQC. FastQC answers "is the sequencer output clean?"; RNA-seq QC answers "did I sequence the transcriptome I think I sequenced, in the orientation I think, with the integrity I think?" The metrics below (strand, exonic rate, rRNA rate, 5'-3' bias) have no DNA analogue because DNA has no exons, no strand of transcription, and no rRNA fraction. The most common setup error is feeding the wrong gene-model format (RSeQC wants BED12; Qualimap a GTF; RNA-SeQC a COLLAPSED GTF; Picard a refFlat + ribosomal_intervals).

2. Getting strandedness wrong SILENTLY HALVES OR ZEROS the counts -- no error is thrown. dUTP (TruSeq Stranded mRNA, most rRNA-depletion kits) is fr-firststrand = REVERSE = featureCounts -s 2 = htseq reverse = salmon ISR = STAR ReadsPerGene column 4. Run it as "forward" and reads land on the antisense gene: counts collapse toward zero and the antisense neighbor inflates (running stranded data as UNSTRANDED, by contrast, roughly doubles counts). The tell is a huge "assigned to no feature" fraction or counts ~2x below the unstranded run. ALWAYS infer strandedness empirically (infer_experiment.py, salmon -l A, or how_are_we_stranded_here) before quantifying -- never assume from the kit name.

3. In standard bulk RNA-seq WITHOUT UMIs, do NOT mark or remove duplicates. A highly expressed gene legitimately produces many fragments sharing identical coordinates; at the read level a PCR duplicate and a natural duplicate are INDISTINGUISHABLE. Coordinate dedup (Picard MarkDuplicates) preferentially deletes reads from the most abundant and shortest transcripts, introducing an expression- and length-dependent bias. This is the OPPOSITE of DNA-seq. Duplication rate is a DIAGNOSTIC ("low complexity / over-sequenced / low input"), never a remove step. The only correct way to remove RNA PCR duplicates is UMIs (read-qc/umi-processing); UMI-protocol RNA-seq (QuantSeq, 10x) inverts the rule.

Integrity bonus: RIN is an electrophoresis estimate measured BEFORE library prep; gene-body coverage and TIN are the post-hoc TRUTH measured from the aligned reads. Use DV200 (% fragments >200 nt), not RIN, for FFPE/archival. In a cohort with variable quality, regress medTIN out as a covariate rather than discarding samples.

Tool Taxonomy

QC-gate order: (1) FastQC on raw FASTQ -> (2) align (STAR/HISAT2) -> (3) post-alignment QC: strandedness FIRST (it gates correct quantification), then read distribution, gene-body + TIN, rRNA/globin/MT, duplication + saturation -> (4) aggregate with MultiQC and judge each sample against the cohort.

Strandedness -- infer, then set every tool to match

infer_experiment.py -i aligned.bam -r genes.bed12     # samples reads, reports the two fractions
salmon quant -i index -l A -r sample.fq.gz -o quant/  # -l A auto-detects; see lib_format_counts.json

Single-end infer_experiment drops the read-number prefix: forward = "++,--", reverse = "+-,-+". A STAR sanity check: the ReadsPerGene column with the most counts and fewest N_noFeature is the correct strand (the wrong column makes N_noFeature blow up). Picard STRAND_SPECIFICITY is a notorious inversion: NONE / FIRST_READ_TRANSCRIPTION_STRAND (= forward/fr-secondstrand) / SECOND_READ_TRANSCRIPTION_STRAND (= dUTP/reverse/fr-firststrand, the common case).

Gene-body coverage and integrity

geneBody_coverage.py -i aligned.bam -r genes.bed12 -o coverage   # 5'->3' uniformity curve
tin.py -i aligned.bam -r genes.bed12 > tin.txt                   # per-transcript integrity; medTIN = sample score

3' bias (coverage piling at the 3' end) = RNA degradation OR oligo-dT priming of degraded/FFPE RNA -- which is why poly-A protocols fail on FFPE and rRNA-depletion + random priming is preferred there. 5' bias is rarer (5'-capture protocols / artifacts). Flat = intact RNA. RIN/DV200/TIN: RIN (1-10, pre-prep, electrophoresis) predicts degradation; DV200 (% >200 nt) is the FFPE metric because fragmented RNA has no rRNA peaks for RIN; TIN is measured from the data and can be used as a DE covariate.

Read distribution and enrichment

read_distribution.py -i aligned.bam -r genes.bed12 > distribution.txt

• High INTRONIC = pre-mRNA / nuclear RNA or gDNA contamination (in snRNA-seq it is SIGNAL, not a fail).
• High INTERGENIC = gDNA contamination or annotation gaps. gDNA drives intronic AND intergenic up together; an annotation gap drives only intergenic.
• rRNA rate = the readout of poly-A-selection / rRNA-depletion efficiency (high = wasted reads, failed depletion).
• Globin (HBA/HBB) crowds whole-blood PAXgene libraries -- deplete (GLOBINclear); globin% is the readout.
• Mitochondrial %: high = degradation (bulk) or dying cells / ambient contamination (single-cell; in snRNA-seq it should be LOW).

Duplication and saturation -- diagnostic, not a remove step

# Duplication as a DIAGNOSTIC only -- do NOT remove duplicates in non-UMI bulk RNA-seq
read_duplication.py -i aligned.bam -o dup                         # sequence- and mapping-based curves
junction_saturation.py -i aligned.bam -r genes.bed12 -o junc_sat  # enough depth for splicing?

Complete QC pipeline

Goal: Produce a per-sample RNA-seq QC summary covering strandedness, distribution, integrity, and Picard metrics.

Approach: Infer strandedness first, run the RSeQC suite, then Picard with STRAND_SPECIFICITY set to the inferred protocol, and append to one report (do NOT dedup).

#!/bin/bash
set -euo pipefail
SAMPLE=$1; BAM=$2; BED12=$3; REFFLAT=$4; RRNA_INTERVALS=$5
STRAND=${6:-SECOND_READ_TRANSCRIPTION_STRAND}   # SECOND = dUTP/reverse (common); FIRST = forward; NONE = unstranded

REPORT="${SAMPLE}_rnaseq_qc.txt"
echo "=== RNA-seq QC: $SAMPLE ===" > "$REPORT"

echo "--- Strandedness (set downstream tools to match) ---" >> "$REPORT"
infer_experiment.py -i "$BAM" -r "$BED12" >> "$REPORT"

echo "--- Read distribution ---" >> "$REPORT"
read_distribution.py -i "$BAM" -r "$BED12" >> "$REPORT"

geneBody_coverage.py -i "$BAM" -r "$BED12" -o "${SAMPLE}_genebody"
tin.py -i "$BAM" -r "$BED12"                       # writes <bam>.summary.txt (mean/median TIN) + <bam>.tin.xls
echo "--- TIN (medTIN = median column of the summary) ---" >> "$REPORT"
cat *.summary.txt >> "$REPORT" 2>/dev/null

echo "--- Picard RNA-seq metrics (STRAND=$STRAND) ---" >> "$REPORT"
picard CollectRnaSeqMetrics I="$BAM" O="${SAMPLE}_picard.txt" \
    REF_FLAT="$REFFLAT" STRAND_SPECIFICITY="$STRAND" RIBOSOMAL_INTERVALS="$RRNA_INTERVALS"

cat "$REPORT"

The collapsed gene model

A standard GTF lists many overlapping isoforms per gene, so a read that is exonic in isoform A but intronic in B is ambiguous and overlapping isoforms double-count the same base. RNA-SeQC 2 REQUIRES a COLLAPSED model (one flattened transcript per gene, inter-gene overlaps excluded), built with GTEx collapse_annotation.py. Mismatched or un-collapsed models are a leading cause of "my exonic rate looks wrong". Picard PCT_* metrics are FRACTIONS (0-1), not percentages, despite the name.

Quantitative Thresholds

Thresholds are protocol-specific: an intronic rate that fails a poly-A bulk sample is normal/required for snRNA-seq (nuclei are >50% intronic); a 3' bias that condemns fresh poly-A is expected for FFPE. Apply cohort-relative outlier logic on top.

Common Errors

References

Wang L, Wang S, Li W. 2012. RSeQC: quality control of RNA-seq experiments. Bioinformatics 28(16):2184-2185. Okonechnikov K, Conesa A, Garcia-Alcalde F. 2016. Qualimap 2: advanced multi-sample quality control for high-throughput sequencing data. Bioinformatics 32(2):292-294. Graubert A, Aguet F, Ravi A, Ardlie KG, Getz G. 2021. RNA-SeQC 2: efficient RNA-seq quality control and quantification for large cohorts. Bioinformatics 37(18):3048-3050. Schroeder A, Mueller O, Stocker S, et al. 2006. The RIN: an RNA integrity number for assigning integrity values to RNA measurements. BMC Molecular Biology 7:3. Wang L, Nie J, Sicotte H, et al. 2016. Measure transcript integrity using RNA-seq data. BMC Bioinformatics 17:58. Smith T, Heger A, Sudbery I. 2017. UMI-tools: modeling sequencing errors in Unique Molecular Identifiers to improve quantification accuracy. Genome Research 27(3):491-499.

Related Skills

read-qc/quality-reports - Raw-FASTQ QC before alignment read-qc/umi-processing - Molecule-accurate dedup for UMI RNA-seq read-qc/contamination-screening - rRNA and cross-species contamination read-alignment/star-alignment - Aligner that emits ReadsPerGene strandedness columns rna-quantification/featurecounts-counting - Strand-aware quantification after QC differential-expression/deseq2-basics - Use medTIN as a covariate in the design