RNA-seq Bioinformatics

Introduction to bioinformatics for RNA sequence analysis

Strand Settings

There are various strand-related settings for RNA-seq tools that must be adjusted to account for library construction strategy. The following table provides read orientation codes and software settings for commonly used RNA-seq analysis tools including: IGV, TopHat, HISAT2, HTSeq, Picard, Kallisto, StringTie, and others. Each of these explanations/settings is provided for several commonly used RNA-seq library construction kits that produce either stranded or unstranded data.

NOTE: A useful tool to infer strandedness of your raw sequence data is the check_strandedness tool.

Tool RF/fr-firststrand stranded FR/fr-secondstrand stranded Unstranded
check_strandedness (output) RF/fr-firststrand FR/fr-secondstrand unstranded
IGV (5p to 3p read orientation code) F2R1 F1R2 F2R1 or F1R2
TopHat (–library-type parameter) fr-firststrand fr-secondstrand fr-unstranded
HISAT2 (–rna-strandness parameter) R/RF F/FR NONE
HTSeq (–stranded/-s parameter) reverse yes no
Picard CollectRnaSeqMetrics (STRAND_SPECIFICITY parameter) SECOND_READ_TRANSCRIPTION_STRAND FIRST_READ_TRANSCRIPTION_STRAND NONE
Kallisto quant (parameter) –rf-stranded –fr-stranded NONE
StringTie (parameter) –rf –fr NONE
MGI CWL YAML (strand parameter) first second NONE
  Example kit(s): Illumina TruSeq Strand Specific Total RNA Example kit(s): NuGEN Encore Example kit(s): NuGEN OvationV2

Notes

To identify which ‘–library-type’ setting to use with TopHat, Illumina specifically documents the types in the ‘RNA Sequencing Analysis with TopHat’ Booklet. For the TruSeq RNA Sample Prep Kit, the appropriate library type is ‘fr-unstranded’. For TruSeq stranded sample prep kits, the library type is specified as ‘fr-firststrand’. These posts are also very informative: How to tell which library type to use (fr-firststrand or fr-secondstrand)? and How to determine if a library Is strand-specific. Another suggestion is to view aligned reads in IGV and determine the read orientation by one of two methods. First, you can have IGV color alignments according to strand using the ‘Color alignments’ by ‘First-of-pair strand’ setting. Second, to get more detailed information you can hover your cursor over a read aligned to an exon. ‘F2 R1’ means the second read in the pair aligns to the forward strand and the first read in the pair aligns to the reverse strand. For a positive DNA strand transcript (5’ to 3’) this would denote a fr-firststrand setting in TopHat, i.e. “the right-most end of the fragment (in transcript coordinates) is the first sequenced”. For a negative DNA strand transcript (3’ to 5’) this would denote a fr-secondstrand setting in TopHat. ‘F1 R2’ means the first read in the pair aligns to the forward strand and the second read in the pair aligns to the reverse strand. See above for the complete definitions, but its simply the inverse for ‘F1 R2’ mapping. Anything other than FR orientation is not covered here and discussion with the individual responsible for library creation would be required. Typically ‘RF’ orientation is reserved for large-insert mate-pair libraries. Other orientations like ‘FF’ and ‘RR’ seem impossible with Illumina sequence technology and suggest structural variation between the sample and reference. Additional details are provided in the TopHat manual.

For HTSeq, the htseq-count manual indicates that for the ‘–stranded’ option, ‘stranded=no’ means that a read is considered overlapping with a feature regardless of whether it is mapped to the same or the opposite strand as the feature. For ‘stranded=yes’ and single-end reads, the read has to be mapped to the same strand as the feature. For paired-end reads, the first read has to be on the same strand and the second read on the opposite strand. For ‘stranded=reverse’, these rules are reversed.

For the ‘CollectRnaSeqMetrics’ sub-command of Picard, the Picard manual indicates that one should use ‘FIRST_READ_TRANSCRIPTION_STRAND’ if the reads are expected to be on the transcription strand.