importIsoformExpression: Import expression data from Kallisto, Salmon, RSEM or...

In kvittingseerup/IsoformSwitchAnalyzeR: Identify, Annotate and Visualize Isoform Switches with Functional Consequences from both short- and long-read RNA-seq data

Import expression data from Kallisto, Salmon, RSEM or StringTie into R.

Description

A general-purpose import function which imports isoform expression data from Kallisto, Salmon, RSEM or StringTie into R. This is a wrapper for the tximport package with some extra functionalities and is meant to be used to import the data and afterwards a switchAnalyzeRlist can be created with importRdata. It is highly recommended that both the imported TxPM and counts values are used both in the creation of the switchAnalyzeRlist with importRdata (through the "isoformCountMatrix" and "isoformRepExpression" arguments). Importantly this import function also enables (and per default performs) inter-library normalization (via edgeR) of the abundance estimates. Note that the pattern argument allows import of only a subset of files. Can be used together with isoformToGeneExp() to get gene expression.

Usage

importIsoformExpression(
    ### Core arguments
    parentDir = NULL,
    sampleVector = NULL,

    ### Advanced arguments
    calculateCountsFromAbundance=TRUE,
    addIsofomIdAsColumn=TRUE,
    interLibNormTxPM=TRUE,
    normalizationMethod='TMM',
    pattern='',
    invertPattern=FALSE,
    ignore.case=FALSE,
    ignoreAfterBar = TRUE,
    ignoreAfterSpace = TRUE,
    ignoreAfterPeriod = FALSE,
    readLength = NULL,
    showProgress = TRUE,
    quiet = FALSE
)

Arguments

parentDir	Parent directory where each quantified sample is in a sub-directory. The function will then look for files containing the (suffix) of the default files names for the quantification tools. The suffixes identified are 'abundance.tsv' for Kallisto, 'quant.sf' for Salmon, 'isoforms.results' for RSEM and 't_data.ctab' for StringTie. This is an alternative to sampleVector (aka only one of them should be used).
sampleVector	A vector with the path to each quantification file to import. If the vector has names assigned (via the names function) these names will be used as the column name of the resulting tables. Else This is an alternative to parentDir (aka only one of them should be used). See example.
calculateCountsFromAbundance	A logic indicating whether to generate estimated counts using the estimated abundances. Recommended as it will incorporate the bias correction algorithms into the analysis. Default is TRUE.
addIsofomIdAsColumn	A logic indicating whether to add isoform id as a separate column (necessary for use with isoformSwitchAnalyzeR) or not (resulting in a data.frame ready for many other functions for exploratory data analysis (EDA) or clustering). Default is TRUE.
interLibNormTxPM	A logic indicating whether to apply an inter-library normalization (via edgeR) to the imported abundances. Recommended as it allow better comparison of abundances between samples. Will not affect the returned counts - even if calculateCountsFromAbundance=TRUE. Default is TRUE.
normalizationMethod	A string indicating the method used for the inter-library normalization. Must be one of "TMM", "RLE", "upperquartile". See ?edgeR::calcNormFactors for more details. Default is "TMM".
pattern	Only used in combination with parentDir. A character string containing a regular expression for which files to import (applied to full path). Default is "" corresponding to all. See base::grepl for more details.
invertPattern	Only used in combination with parentDir. A Logical. If TRUE only use files which do not match the pattern argument.
ignore.case	Only used in combination with parentDir. A logical. If TRUE case is ignored duing matching with the pattern argument. If FALSE the matching with the pattern argument is case sensitive.
ignoreAfterBar	A logic indicating whether to subset the isoform ids by ignoring everything after the first bar ("|"). Useful for analysis of GENCODE data. Default is TRUE.
ignoreAfterSpace	A logic indicating whether to subset the isoform ids by ignoring everything after the first space (" "). Useful for analysis of gffutils generated GTF files. Default is TRUE.
ignoreAfterPeriod	A logic indicating whether to subset the gene/isoform is by ignoring everything after the first period ("."). Should be used with care. Default is FALSE.
readLength	Only necessary when importing from StringTie. Must be the number of base pairs sequenced. e.g. if the data quantified is single end 75bp use readLength=75 and if 75bp paired end use readLength=150.
showProgress	A logic indicating whether to make a progress bar (if TRUE) or not (if FALSE). Default is FALSE.
quiet	A logic indicating whether to avoid printing progress messages (incl. progress bar). Default is FALSE

Details

This function requires all data that should be imported is in a directory (as indicated by parentDir) where each quantified sample is in a separate sub-directory.

The actual import of data is done with tximport using "countsFromAbundance='scaledTPM'" to extract counts.

For Kallisto the bias estimation is enabled by adding '–bias' to the function call. For Salmon the bias estimation is enabled by adding '–seqBias' and '–gcBias' to the function call. For RSEM the bias estimation is enabled by adding '–estimate-rspd' to the function call. For StringTie the bias corrections are always enabled (and cannot be turned off by the user).

Inter library normalization is (almost always) necessary due to small changes in the RNA composition between cells and is highly recommended for all analysis of RNAseq data. For more information please refer to the edgeR user guide.

The inter-library normalization of FPKM/TxPM values is performed as a 3/4 step process: If calculateCountsFromAbundance=TRUE the effective counts are calculated from the abundances using the library specific effective isoform lengths, else the original counts are used. The count matrix is then subsetted to the isoforms expressed more than 1 TxPM/RPKM in more than one sample. The count matrix supplied to edgeR which calculates the normalization factors necessary. Lastly the calculated normalization factors are applied to the imported FPKM/TxPM values.

This function expects the files produced by Kallisto/Salmon/RSEM/StringTie to be called their default names (with possible custom prefix): Kallisto files are called 'abundance.tsv', Salmon files are called 'quant.sf', RSEM files are called 'isoforms.results' and StringTie files are called 't_data.ctab'.

Importantly StringTie must be run with the -B option to produce the quantified file: An example could be: "StringTie -eB -G transcripts.gtf <source_file.bam>"

Value

A list containing an abundance matrix, a count matrix and a matrix with the effective lengths for each isoform quantified (rows) in each sample (col) where the first column contains the isoform_ids. The options used for import are stored under the "importOptions" entry). The abundance estimates are in the unit of Transcripts Per Million (TPM) and measuring the relative abundance of a specific transcript.

Transcripts Per Million values are abbreviated to TPM by RSEM, Kallisto and Salmon but will here referred to as TxPM to avoid confusion with the commonly used Tags Per Million (which have been around for way longer). TxPM is an equivalent to RPKM/FPKM except it has been adjusted for as all the biases being modeled by the tools used for the quantification including the fragment length distribution and sequence-specific bias as well as GC-fragment bias (this is specific to each tool and how it was run so you need to look up the specific tool). The TxPM is optimal for expression comparison of abundances since most biases will be taking into account.

Author(s)

Kristoffer Vitting-Seerup

References

Vitting-Seerup et al. The Landscape of Isoform Switches in Human Cancers. Mol. Cancer Res. (2017). Soneson et al. Differential analyses for RNA-seq: transcript-level estimates improve gene-level inferences. F1000Research 4, 1521 (2015). Robinson et al. A scaling normalization method for differential expression analysis of RNA-seq data. Genome Biology (2010)

See Also

importRdata
createSwitchAnalyzeRlist
preFilter

Examples

### Please note
# The way of importing files in the following example with
# "system.file('pathToFile', package="IsoformSwitchAnalyzeR") is
# specialized way of accessing the example data in the IsoformSwitchAnalyzeR package
# and not something you need to do - just supply the string e.g.
# parentDir = "/path/to/mySalmonQuantifications/" or
# sampleVector = c('mySalmonQuantifications/file1.sf', 'mySalmonQuantifications/file2.sf') to the function

### Import all quantifications stored in a folder
salmonQuant <- importIsoformExpression(
    parentDir = system.file("extdata/", package="IsoformSwitchAnalyzeR")
)

names(salmonQuant)
head(salmonQuant$abundance, 2)


### Import individual quantificaiton files
myFiles <- c(
    system.file("extdata/Fibroblasts_0/quant.sf.gz", package="IsoformSwitchAnalyzeR"),
    system.file("extdata/Fibroblasts_1/quant.sf.gz", package="IsoformSwitchAnalyzeR")
)
names(myFiles) <- c('Fibroblasts_0','Fibroblasts_1')

salmonQuant <- importIsoformExpression(
    sampleVector = myFiles
)

names(salmonQuant)
head(salmonQuant$abundance, 2)


### Get gene expression/count from isoform expression/count
geneRepCount <- isoformToGeneExp(
    isoformRepExpression  = salmonQuant$counts, # just change to "salmonQuant$abundance" to get gene abundances
    isoformGeneAnnotation = system.file("extdata/example.gtf.gz", package="IsoformSwitchAnalyzeR")
)

head(geneRepCount, 2)

kvittingseerup/IsoformSwitchAnalyzeR documentation built on June 28, 2024, 5:41 p.m.