Long Read Splicing

Version Compatibility

Reference examples tested with: FLAIR 2.0+, IsoQuant 3.5+, Bambu 3.4+, SQANTI3 5.2+, minimap2 2.26+, samtools 1.19+, rMATS-long 0.2+, IsoSeq3 4.0+

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

• Python: pip show <package> then help(module.function) to check signatures
• 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.

Long-Read Splicing Analysis

Full-length long-read sequencing solves problems that short-read AS cannot: anchor-length-limited microexon detection, complex multi-exon isoform deconvolution, recursive splicing in long introns, and transcript-quantification uncertainty in DTU. The 2024-2026 transition: long-read is becoming the splicing default for high-resolution analysis.

When Long-Read Wins

Platform Selection Matrix

Read length: PacBio HiFi cdna typically 1-10 kb; ONT cdna 0.5-50+ kb (long-tailed). Both span typical mammalian transcripts. Direct RNA on ONT preserves true 5'/3' termini and modifications.

Decision Tree by Use Case

Splice-Aware Alignment

# PacBio HiFi (Iso-Seq) -> minimap2 splice:hq preset
minimap2 -ax splice:hq -uf --secondary=no \
    -t 16 \
    reference.fa \
    isoseq.fastq.gz | \
    samtools sort -@ 8 -o isoseq_aligned.bam
samtools index isoseq_aligned.bam

# ONT direct cDNA (PCS-114, PCB-114): unstranded by default; omit -uf
minimap2 -ax splice -k14 \
    -t 16 \
    reference.fa \
    ont_cdna.fastq.gz | \
    samtools sort -@ 8 -o ont_cdna_aligned.bam
samtools index ont_cdna_aligned.bam

# ONT direct RNA (RNA004): truly stranded (RNA molecule preserves direction); -uf is correct
minimap2 -ax splice -uf -k14 \
    -t 16 \
    reference.fa \
    ont_rna.fastq.gz | \
    samtools sort -@ 8 -o ont_rna_aligned.bam
samtools index ont_rna_aligned.bam

-uf forces all reads to the forward transcript strand — correct for direct RNA (single-stranded) and stranded cDNA library preps; omit for unstranded cDNA (default ONT PCS/PCB kits) or you lose ~half the reads. --secondary=no discards secondary alignments. For genomes with poorly-annotated splice sites, supplement with --junc-bed gencode_junctions.bed. uLTRA (Sahlin & Mäkinen 2021 Bioinformatics) and deSALT (Liu 2019 Genome Biol) are alternatives with higher precision on small/cryptic exons.

Critical: splice:hq is the preset for HiFi (Q30+ reads); plain splice is for ONT regardless of cDNA vs direct RNA. Using splice on HiFi data underuses the high quality; using splice:hq on ONT misses true junctions due to error-tolerance mismatch.

FLAIR Workflow (correct → collapse → quantify → diffSplice)

Goal: Identify, quantify, and test full-length isoforms from long-read RNA-seq across conditions.

Approach: Correct splice junctions against short-read or annotation evidence, collapse isoforms, quantify per-sample expression, run diffSplice for differential isoform usage.

flair correct \
    --query aligned.bed \
    --genome reference.fa \
    --gtf gencode.v45.annotation.gtf \
    --shortread short_read_junctions.bed \
    --output flair_corrected \
    --threads 16

flair collapse \
    --query flair_corrected_all_corrected.bed \
    --reads sample.fastq.gz \
    --genome reference.fa \
    --gtf gencode.v45.annotation.gtf \
    --output flair_collapsed \
    --threads 16

flair quantify \
    --reads_manifest reads_manifest.tsv \
    --isoforms flair_collapsed.isoforms.fa \
    --output flair_quantified \
    --threads 16

flair diffSplice \
    --isoforms flair_collapsed.isoforms.bed \
    --counts_matrix flair_quantified_counts.tsv \
    --conditions_table conditions.tsv \
    --output flair_diffsplice \
    --threads 16

FLAIR (Tang 2020 Nat Commun) handles ONT and PacBio with the same workflow. Output includes per-event PSI, FDR, and visual sashimi-like plots. The --shortread flag for flair correct is strongly recommended when short-read RNA-seq is available — it dramatically improves splice junction precision.

IsoQuant for Discovery + Quantification

Goal: De novo or annotation-guided isoform discovery and quantification with high precision.

Approach: Run isoquant.py with reference + reads + data type; output is GTF + counts.

isoquant.py \
    --reference reference.fa \
    --genedb gencode.v45.annotation.gtf \
    --fastq sample1.fastq.gz sample2.fastq.gz \
    --data_type pacbio_ccs \
    --output isoquant_output \
    --threads 16 \
    --model_construction_strategy default_pacbio

--data_type accepts pacbio_ccs (HiFi), nanopore (ONT), or assembly. As of v3.0+, --genedb is optional for de novo discovery. IsoQuant (Prjibelski 2023 Nat Biotech) is current SOTA for novel transcript reconstruction; pairs well with SQANTI3 for downstream classification.

Memory requirement: >=64 GB for atlas-scale runs.

Bambu for Annotation-Aware Discovery + Quantification

Goal: Joint discovery and quantification with statistical filtering of novel isoforms.

Approach: R Bioconductor package; takes BAM + reference annotation + genome; outputs ranged SE objects of known + novel transcripts.

library(bambu)

bam_files <- c('sample1.bam', 'sample2.bam', 'sample3.bam')
genome <- 'reference.fa'
gtf <- 'gencode.v45.annotation.gtf'

bambuAnnotations <- prepareAnnotations(gtf)

se <- bambu(
    reads = bam_files,
    annotations = bambuAnnotations,
    genome = genome,
    NDR = 0.1,
    ncore = 8
)

writeBambuOutput(se, path = 'bambu_output/')

tx_counts <- as.data.frame(assays(se)$counts)
gene_counts <- transcriptToGeneExpression(se)

Bambu (Chen 2023 Nat Methods 20:1187-1195) uses NDR (Novel Discovery Rate) as a single, calibrated parameter replacing per-sample heuristics:

Excellent for combined discovery + quantification when statistical filtering matters.

SQANTI3 Classification

Goal: Classify discovered isoforms relative to reference; flag artifacts (intra-priming, RT-switching).

Approach: Run sqanti3_qc.py on the isoform GTF; review classification (FSM/ISM/NIC/NNC/antisense/genic/intergenic/fusion) and quality flags.

sqanti3_qc.py \
    isoforms.gtf \
    gencode.v45.annotation.gtf \
    reference.fa \
    --output sqanti3_qc \
    --aligner_choice minimap2 \
    --cage_peak refTSS_v3.3_human_coordinate.hg38.bed \
    --polyA_motif_list mouse_and_human.polyA_motif.txt \
    --cpus 8

sqanti3_filter.py rules \
    sqanti3_qc_classification.txt \
    --isoforms isoforms.fa \
    --gtf isoforms.gtf \
    --output sqanti3_filtered

SQANTI-LR (Pardo-Palacios 2024 Nat Methods) is the long-read-specific branch with QC tailored to ONT/PacBio error patterns. Filter intra-priming and RT-switching flags before reporting.

rMATS-long for Differential Isoform Analysis on Long-Read Data

Goal: Apply differential isoform analysis to long-read transcript abundance with classification and visualization.

Approach: rMATS-long is a multi-script Python pipeline distributed via bioconda; entry point is rmats-long followed by the script name. It supports two modes: abundance-based (using ESPRESSO-style abundance estimates) and ASM-based (Alternative Splicing Modules — sets of isoforms sharing exon-junction structure). Run preprocessing scripts in order before rmats_long.py.

conda install -c conda-forge -c bioconda rmats-long

# Preprocessing pipeline (ASM mode); per-script flag names verified vs Xinglab/rmats-long
rmats-long organize_gene_info_by_chr.py --gtf annotation.gtf --out-dir gene_info_by_chr/

# simplify_alignment_info processes one BAM at a time -> one TSV
for bam in *.bam; do
    rmats-long simplify_alignment_info.py --in-file "$bam" --out-tsv "alignment_info/${bam%.bam}.tsv"
done

# organize_alignment_info_by_gene_and_chr requires a samples-tsv (sample_id<TAB>tsv_path)
rmats-long organize_alignment_info_by_gene_and_chr.py \
    --gtf-dir gene_info_by_chr/ \
    --out-dir organized/ \
    --samples-tsv samples.tsv

rmats-long detect_splicing_events.py --align-dir organized/ --out-dir events/
rmats-long create_gtf_from_asm_definitions.py --event-dir events/ --out-gtf asm.gtf
rmats-long count_reads_for_asms.py --align-dir organized/ --event-dir events/ --out-dir asm_counts/

# Main differential analysis (ASM mode)
# Note: in ASM mode, --group-1 / --group-2 take sample IDs (matching the BAM basenames
# you organized into --align-dir); the BAM-to-counts step is done by count_reads_for_asms.py above.
rmats-long rmats_long.py \
    --group-1 ctrl1,ctrl2,ctrl3 \
    --group-2 trt1,trt2,trt3 \
    --event-dir events/ \
    --asm-counts-dir asm_counts/ \
    --align-dir organized/ \
    --gtf-dir gene_info_by_chr/ \
    --out-dir rmats_long_output/ \
    --adj-pvalue 0.05 \
    --delta-proportion 0.05 \
    --average-reads-per-group 10

# Alternative: abundance-based mode (when you already have ESPRESSO-style estimates)
rmats-long rmats_long.py \
    --abundance abundance.esp \
    --updated-gtf updated.gtf \
    --group-1 sample1,sample2,sample3 \
    --group-2 sample4,sample5,sample6 \
    --out-dir rmats_long_output/ \
    --no-splice-graph-plot

Key flags: --adj-pvalue (default 0.05), --delta-proportion (default 0.05), --average-reads-per-group (default 10), --no-splice-graph-plot (skip expensive splice-graph rendering).

rMATS-long is a separate tool from short-read rMATS-turbo. The predecessor lr2rmats used long reads only to augment the short-read rMATS GTF. The ASM framework treats AS as a set-of-isoforms problem, more natural for long-read data than rMATS-turbo's pre-defined event categories.

DTU on Long-Read Counts

Goal: Apply DRIMSeq + DEXSeq + stageR DTU pipeline to long-read transcript counts (no quantification uncertainty).

Approach: Use FLAIR or Bambu transcript counts as input; long-read counts are read-level identities, so no Salmon Gibbs samples needed.

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

counts <- read.table('flair_quantified_counts.tsv', header=TRUE, sep='\t')

samples <- data.frame(
    sample_id = c('s1', 's2', 's3', 's4', 's5', 's6'),
    condition = c('ctrl', 'ctrl', 'ctrl', 'trt', 'trt', 'trt')
)

d <- dmDSdata(counts = counts, samples = samples)
d <- dmFilter(
    d,
    min_samps_feature_expr = 3, min_feature_expr = 5,
    min_samps_feature_prop = 3, min_feature_prop = 0.1,
    min_samps_gene_expr = 6, min_gene_expr = 10
)

Then proceed with the standard DEXSeq + stageR DTU pipeline (see isoform-switching skill). IsoformSwitchAnalyzeR v2 has explicit long-read input support.

Single-Cell Long-Read for Splicing

Goal: Combine cell typing (10X 5' short read) with full-length isoform structure (Kinnex / MAS-Iso-seq).

Approach: Split 10X library; sequence half short-read for cell typing, half PacBio Kinnex for isoforms; recover cell barcodes from long reads via FLAMES or skera (Kinnex de-array).

# Demultiplex MAS-Iso-seq reads
skera split \
    raw_kinnex.bam \
    mas12_primers.fasta \
    demuxed.bam

# Then proceed with lima -> isoseq3 refine -> isoseq3 cluster pipeline
# For barcode rescue from FLAMES:
match_cell_barcode \
    --bam demuxed.bam \
    --barcodes 10x_barcodes.tsv \
    --output flames_demuxed.bam

Joglekar and colleagues used this approach for the mouse cortex isoform atlas (consult most recent publication for exact venue/year). See single-cell-splicing for tools that work on the demultiplexed data.

Per-Tool Failure Modes

minimap2: Wrong Preset

Trigger: Using -ax splice for PacBio HiFi (instead of -ax splice:hq) or -ax splice:hq for ONT.

Mechanism: Presets configure k-mer size, error tolerance, and indel scoring; mismatched preset is sub-optimal.

Symptom: Lower alignment rate; missed junctions on HiFi, false novel junctions on ONT.

Fix: splice:hq for HiFi; splice -k14 for ONT cDNA (unstranded); add -uf only for ONT direct RNA or stranded cDNA preps.

IsoQuant: Memory Pressure

Trigger: Atlas-scale cohort or low-RAM environment.

Mechanism: IsoQuant builds graph structures across all reads simultaneously.

Symptom: OOM kill; very slow runtime.

Fix: Increase RAM to >=64 GB; or batch by chromosome.

Bambu: NDR Mistuning

Trigger: NDR=0.5+ or NDR=0.01.

Mechanism: NDR controls the precision-recall tradeoff for novel transcripts.

Symptom: Too many spurious novel transcripts (high NDR) or missing real novel transcripts (low NDR).

Fix: Default NDR=0.1 is balanced; adjust based on validation expectations.

SQANTI3: RT-Switching Flags

Trigger: PacBio/ONT cDNA libraries with template switching artifacts.

Mechanism: RT-switching produces chimeric reads spanning two unrelated transcripts; SQANTI3 flags these.

Symptom: Many "fusion" transcripts in non-cancer samples; biologically implausible.

Fix: Filter out RT-switching flags via sqanti3_filter.py; investigate library prep if rate >5%.

FLAIR: Short-Read Augmentation Missing

Trigger: Running flair correct without --shortread.

Mechanism: FLAIR uses short-read junctions to correct long-read junction calls; without them, long-read errors persist as junction calls.

Symptom: Many false novel junctions; junction precision low.

Fix: Always include --shortread short_read_junctions.bed when short-read RNA-seq is available; generate with regtools junctions.

rMATS-long: GTF-Only Input

Trigger: Trying to give rMATS-long raw long-read BAMs.

Mechanism: rMATS-long expects per-sample isoform GTFs (from FLAIR/IsoQuant collapse), not raw alignments.

Symptom: Confusing parsing errors.

Fix: Run FLAIR/IsoQuant per sample first; pass the resulting GTFs.

Reconciliation: When Long-Read Tools Disagree

Quality Control for Long-Read Splicing

Pre-R10 ONT (R9.4.1) had ~85-90% junction concordance and is no longer recommended for splicing.

Common Errors

Quality Thresholds

Common Pitfalls

• Ignoring reference annotation completeness — SQANTI3 NNC categorization differs by GENCODE version; report version with results.
• Not running isoseq3 refine — concatemers and polyA artifacts inflate isoform counts.
• Confusing FLAIR's 'collapse' with 'cluster' — collapse merges similar isoforms post-alignment; cluster (in isoseq3) merges raw reads pre-alignment.
• Treating ONT R9.x splice calls as reliable — pre-R10.4.1 error patterns generate false novel junctions.
• Skipping CAGE / polyA validation in SQANTI3 — TSS / TTS hallucination is common in long-read isoforms.
• DTU on too few replicates — long-read is expensive; n=2 vs n=2 is common but underpowered.
• PacBio HiFi alignment with -ax splice (not splice:hq) — use the HQ preset for HiFi data; default splice is for ONT.
• Skipping --shortread in FLAIR correct — long-read junction precision is much higher with short-read augmentation.

Related Skills

• splicing-quantification - Short-read PSI for cross-validation
• isoform-switching - DTU framework on long-read counts
• single-cell-splicing - MAS-Iso-seq + 10X integration
• long-read-sequencing/isoseq-analysis - PacBio Iso-Seq general pipeline (CCS, lima, refine, cluster)
• long-read-sequencing/long-read-alignment - minimap2 splice:hq details
• long-read-sequencing/long-read-qc - QC for long-read data
• splice-variant-prediction - Cross-reference variant predictions with full isoforms

References

• Tang et al 2020 Nat Commun - FLAIR
• Prjibelski et al 2023 Nat Biotech - IsoQuant
• Chen et al 2023 Nat Methods 20:1187-1195 - Bambu
• Tardaguila et al 2018 Genome Res - SQANTI (original)
• Pardo-Palacios et al 2024 Nat Methods - SQANTI3 / LRGASP benchmark
• Wyman et al 2020 bioRxiv - TALON (note: not formally peer-reviewed)
• Li 2018 / 2021 Bioinformatics - minimap2
• Sahlin & Makinen 2021 Bioinformatics - uLTRA
• Sibley et al 2015 Nature - recursive splicing
• Al'Khafaji et al 2024 Nat Biotech - MAS-Iso-seq / Kinnex
• Joglekar et al - scISOr-Seq2 mouse cortex (consult most recent publication for venue/year)
• Tian et al 2021 Genome Biology 22:310 - FLAMES
• Brown et al 2022 Nature - UNC13A cryptic exon (TDP-43)
• Klim et al 2019 Nat Neurosci - STMN2 cryptic splicing