Output file formats

Although most output files include headers that describe the data, a brief explanation of the output files is provided below.

Read to isoform assignment

Tab-separated values, the columns are:

• read_id - read id;
• chr - chromosome id;
• strand - strand of the assigned isoform (not to be confused with read mapping strand);
• isoform_id - isoform id to which the read was assigned;
• gene_id - gene id to which the read was assigned;
• assignment_type - assignment type, can be:
  • unique - reads was unambiguously assigned to a single known isoform;
  • unique_minor_difference - read was assigned uniquely but has alignment artifacts;
  • inconsistent - read was matched with inconsistencies, closest match(es) are reported;
  • inconsistent_non_intronic - read was matched with inconsistencies, which do not affect intron chain (e.g. olly TSS/TES);
  • inconsistent_ambiguous - read was matched with inconsistencies equally well to two or more isoforms;
  • ambiguous - read was assigned to multiple isoforms equally well;
  • noninfomative - reads is intronic or has an insignificant overlap with a known gene;
  • intergenic - read is intergenic.
• assignment_events - list of detected inconsistencies; for each assigned isoform a list of detected inconsistencies relative to the respective isoform is stored; values in each list are separated by + symbol, lists are separated by comma, the number of lists equals to the number of assigned isoforms; possible events are (see graphical representation below):
  • consistent events:
    • none / . / undefined - no special event detected;
    • mono_exon_match mono-exonic read matched to mono-exonic transcript;
    • fsm - full splice match;
    • ism_5/3 - incomplete splice match, truncated on 5'/3' side;
    • ism_internal - incomplete splice match, truncated on both sides;
    • mono_exonic - mono-exonic read matching spliced isoform;
    • tss_match / tss_match_precise - 5' read is located less than 50 / delta bases from the TSS of the assigned isoform
    • tes_match / tes_match_precise - 3' read is located less than 50 / delta bases from the TES of the assigned isoform (can be reported without detecting polyA sites)
  • alignment artifacts:
    • intron_shift - intron that seems to be shifted due to misalignment (typical for Nanopores);
    • exon_misalignment - short exon that seems to be missed due to misalignment (typical for Nanopores);
    • fake_terminal_exon_5/3 - short terminal exon at 5'/3' end that looks like an alignment artifact (typical for Nanopores);
    • terminal_exon_misalignment_5/3 - missed reference short terminal exon;
    • exon_elongation_5/3 - minor exon extension at 5'/3' end (not exceeding 30bp);
    • fake_micro_intron_retention - short annotated introns are often missed by the aligners and thus are not considered as intron retention;
  • intron retentions:
    • intron_retention - intron retention;
    • unspliced_intron_retention - intron retention by mono-exonic read;
    • incomplete_intron_retention_5/3 - terminal exon at 5'/3' end partially covers adjacent intron;
  • significant inconsistencies (each type end with _known if all resulting read introns are annotated and _novel otherwise):
    • major_exon_elongation_5/3 - significant exon extension at 5'/3' end (exceeding 30bp);
    • extra_intron_5/3 - additional intron on the 5'/3' end of the isoform;
    • extra_intron - read contains additional intron in the middle of exon;
    • alt_donor_site - read contains alternative donor site;
    • alt_acceptor_site - read contains alternative annotated acceptor site;
    • intron_migration - read contains alternative annotated intron of approximately the same length as in the isoform;
    • intron_alternation - read contains alternative intron, which doesn't fall intro any of the categories above;
    • mutually_exclusive_exons - read contains different exon(s) of the same total length comparing to the isoform;
    • exon_skipping - read skips exon(s) comparing to the isoform;
    • exon_merge - read skips exon(s) comparing to the isoform, but a sequence of a similar length is attached to a neighboring exon;
    • exon_gain - read contains additional exon(s) comparing to the isoform;
    • exon_detach - read contains additional exon(s) comparing to the isoform, but a neighboring exon looses a sequnce of a similar length;
    • terminal_exon_shift - read has alternative terminal exon;
    • alternative_structure - reads has different intron chain that does not fall into any of categories above;
  • alternative transcription start / end (reported when poly-A tails are present):
    • alternative_polya_site - read has alternative polyadenylation site;
    • internal_polya_site - poly-A tail detected but seems to be originated from A-rich intronic region;
    • correct_polya_site - poly-A site matches reference transcript end;
    • aligned_polya_tail - poly-A tail aligns to the reference;
    • alternative_tss - alternative transcription start site.
• exons - list of coordinates for normalized read exons (1-based, indels and polyA exons are excluded);
• additional - field for supplementary information, which may include:
  • gene_assignment - Gene assignment classification; possible values are the same as for transcript classification.
  • PolyA - True if poly-A tail is detected;
  • Canonical - True if all read introns are canonical, Unspliced is used for mono-exon reads; (use --check_canonical);
  • Classification - SQANTI-like assignment classification.

Note, that a single read may occur more than once if assigned ambiguously.

Expression table format

Non-grouped counts/TPM values are stored in a simple TSV file, the columns are:

• feature_id - genomic feature ID;
• TPM or count - expression value (float).

Grouped counts are stored in linear format by default - a TSV file with 3 columns:

• feature_id - genomic feature ID;
• group_id - group name;
• count - read count of the feature in this group.

By default, IsoQuant converts grouped counts with small number of groups/samples (<=100) to standard matrix format; larger matrices (e.g. for single-cell experiments) will be saved to MTX format, which is compatible with the Seurat package. In standard matrix rows represent features, columns represent groups. While being more human-readable, this file make take substantial disk space when the number of groups is large. In MTX format, .matrix.mtx represents counts, .features.tsv contains the feature list and .barcodes.tsv contains group list (typically, barcodes are used as groups in single-cell and spatial experiments). See options to tune your output.

Exon and intron count format

Tab-separated values, the columns are:

• chr - chromosome ID;
• start - feature leftmost 1-based positions;
• end - feature rightmost 1-based positions;
• strand - feature strand;
• flags - symbolic feature flags, can contain the following characters:
  • X - terminal feature;
  • I - internal feature;
  • T - feature appears as both terminal and internal in different isoforms;
  • S - feature has similar positions to some other feature;
  • C - feature is contained in another feature;
  • U - unique feature, appears only in a single known isoform;
  • M - feature appears in multiple different genes.
• gene_ids - list if gene ids feature belong to;
• group_id - read group if provided (NA by default);
• include_counts - number of reads that include this feature;
• exclude_counts - number of reads that span, but do not include this feature;

Transcript models format

Constructed transcript models are stored in usual GTF format. Contains exon, transcript and gene features.

Known genes and transcripts are reposted with their reference IDs. Novel genes IDs have format novel_gene_XXX_### and novel transcript IDs are formatted as transcript###.XXX.TYPE, where ### is the unique number (not necessarily consecutive), XXX is the chromosome name and TYPE can be one of the following:

• nic - novel in catalog, new transcript that contains only annotated introns;
• nnic - novel not in catalog, new transcript that contains unannotated introns.

Each exon also has a unique ID stored in exon_id attribute.

In addition, each transcript contains canonical property if --check_canonical is set.

If --sqanti_output option is set, each novel transcript also has a similar_reference_id field containing ID of a most similar reference isoform and alternatives attribute, which indicates the exact differences between this novel transcript and the similar reference transcript.