MSnSet-class {MSnbase} | R Documentation

## The "MSnSet" Class for MS Proteomics Expression Data and Meta-Data

### Description

The `MSnSet` holds quantified expression data for MS proteomics data and the experimental meta-data. The `MSnSet` class is derived from the `"eSet"` class and mimics the `"ExpressionSet"` class classically used for microarray data.

### Objects from the Class

The constructor `MSnSet(exprs, fData, pData)` can be used to create `MSnSet` instances. Argument `exprs` is a `matrix` and `fData` and `pData` must be of class `data.frame` or `"AnnotatedDataFrame"` and all must meet the dimensions and name validity constrains.

Objects can also be created by calls of the form `new("MSnSet", exprs, ...)`. See also `"ExpressionSet"` for helpful information. Expression data produced from other softwares can thus make use of this standardized data container to benefit `R` and `Bioconductor` packages. Proteomics expression data available as spreadsheets, as produced by third-party software such as Proteome Discoverer, MaxQuant, ... can be imported using the `readMSnSet` and `readMSnSet2` functions.

Coercion methods are also available to transform `MSnSet` objects to `IBSpectra`, to `data.frame` and to/from `ExpressionSet` and `SummarizedExperiment` objects. In the latter case, the metadata available in the `protocolData`, `experimentData` are completely dropped, and only the logging information of the `processingData` slot is retained. All these metadata can be subsequently be added using the `addMSnSetMetadata` (see examples below). When converting a `SummarizedExperiment` to an `MSnSet`, the respective metadata slots will be populated if available in the `SummarizedExperiment` metadata.

In the frame of the `MSnbase` package, `MSnSet` instances can be generated from `"MSnExp"` experiments using the `quantify` method).

### Slots

`qual`:

  Object of class `"data.frame"` that records peaks data for each of the reporter ions to be used as quality metrics.

`processingData`:

  Object of class `"MSnProcess"` that records all processing.

`assayData`:

  Object of class `"assayData"` containing a matrix with equal with column number equal to `nrow(phenoData)`. `assayData` must contain a matrix `exprs` with rows represening features (e.g., reporters ions) and columns representing samples. See the `"AssayData"` class, `exprs` and `assayData` accessor for more details. This slot in indirectly inherited from `"eSet"`.

`phenoData`:

  Object of class `"AnnotatedDataFrame"` containing experimenter-supplied variables describing sample (i.e the individual tags for an labelled MS experiment) (indireclty inherited from `"eSet"`). See `phenoData` and the `"eSet"` class for more details. This slot can be accessed as a `data.frame` with `pData` and be replaced by a new valid (i.e. of compatible dimensions and row names) `data.frame` with `pData()<-`.

`featureData`:

  Object of class `"AnnotatedDataFrame"` containing variables describing features (spectra in our case), e.g. identificaiton data, peptide sequence, identification score,... (inherited indirectly from `"eSet"`). See `featureData` and the `"eSet"` class for more details. This slot can be accessed as a `data.frame` with `fData` and be replaced by a new valid (i.e. of compatible dimensions and row names) `data.frame` with `fData()<-`.

`experimentData`:

  Object of class `"MIAPE"`, containing details of experimental methods (inherited from `"eSet"`). See `experimentData` and the `"eSet"` class for more details.

`annotation`:

  not used here.

`protocolData`:

  Object of class `"AnnotatedDataFrame"` containing equipment-generated variables (inherited indirectly from `"eSet"`). See `protocolData` and the `"eSet"` class for more details.

`.__classVersion__`:

  Object of class `"Versions"` describing the versions of R, the Biobase package, `"eSet"`, `"pSet"` and `MSnSet` of the current instance. Intended for developer use and debugging (inherited indirectly from `"eSet"`).

### Extends

Class `"eSet"`, directly. Class `"VersionedBiobase"`, by class "eSet", distance 2. Class `"Versioned"`, by class "eSet", distance 3.

### Methods

MSnSet specific methods or over-riding it's super-class are described below. See also more `"eSet"` for inherited methods.

acquisitionNum

  `acquisitionNum(signature(object = "MSnSet"))`: Returns the a numeric vector with acquisition number of each spectrum. The vector names are the corresponding spectrum names. The information is extracted from the object's `featureData` slot.

fromFile

  `fromFile(signature(object = "MSnSet"))`: get the index of the file (in `fileNames(object)`) from which the raw spectra from which the corresponding feature were originally read. The relevant information is extracted from the object's `featureData` slot.

  Returns a numeric vector with names corresponding to the spectrum names.

dim

  `signature(x = "MSnSet")`: Returns the dimensions of object's assay data, i.e the number of samples and the number of features.

fileNames

  `signature(object = "MSnSet")`: Access file names in the `processingData` slot.

msInfo

  `signature(object = "MSnSet")`: Prints the MIAPE-MS meta-data stored in the `experimentData` slot.

processingData

  `signature(object = "MSnSet")`: Access the `processingData` slot.

show

  `signature(object = "MSnSet")`: Displays object content as text.

qual

  `signature(object = "MSnSet")`: Access the reporter ion peaks description.

purityCorrect

  `signature(object = "MSnSet", impurities = "matrix")`: performs reporter ions purity correction. See `purityCorrect` documentation for more details.

normalise

  `signature(object = "MSnSet")`: Performs `MSnSet` normalisation. See `normalise` for more details.

t

  `signature(x = "MSnSet")`: Returns a transposed `MSnSet` object where features are now aligned along columns and samples along rows and the `phenoData` and `featureData` slots have been swapped. The `protocolData` slot is always dropped.

as(,"ExpressionSet")

  `signature(x = "MSnSet")`: Coerce object from `MSnSet` to `ExpressionSet-class`. The `experimentData` slot is converted to a `MIAME` instance. It is also possible to coerce an `ExpressionSet` to and `MSnSet`, in which case the `experimentData` slot is newly initialised.

as(,"IBSpectra")

  `signature(x = "MSnSet")`: Coerce object from `MSnSet` to `IBSpectra` from the `isobar` package.

as(,"data.frame")

  `signature(x = "MSnSet")`: Coerce object from `MSnSet` to `data.frame`. The `MSnSet` is transposed and the `PhenoData` slot is appended.

as(,"SummarizedExperiment")

  `signature(x = "MSnSet")`: Coerce object from `MSnSet` to `SummarizedExperiment`. Only part of the metadata is retained. See `addMSnSetMetadata` and the example below for details.

write.exprs

  signature(x = "MSnSet")

Writes expression values to a tab-separated file (default is `tmp.txt`). The `fDataCols` parameter can be used to specify which `featureData` columns (as column names, column number or `logical`) to append on the right of the expression matrix. The following arguments are the same as `write.table`.

combine

  signature(x = "MSnSet", y = "MSnSet", ...)

Combines 2 or more `MSnSet` instances according to their feature names. Note that the `qual` slot and the processing information are silently dropped.

topN

  signature(object = "MSnSet", groupBy, n = 3, fun, ..., verbose = isMSnbaseVerbose())

Selects the `n` most intense features (typically peptides or spectra) out of all available for each set defined by `groupBy` (typically proteins) and creates a new instance of class `MSnSet`. If less than `n` features are available, all are selected. The `ncol(object)` features are summerised using `fun` (default is `sum`) prior to be ordered in decreasing order. Additional parameters can be passed to `fun` through `...`, for instance to control the behaviour of `topN` in case of `NA` values. (Works also with `matrix` instances.)

See also the `nQuants` function to retrieve the actual number of retained peptides out of `n`.

A complete use case using `topN` and `nQuants` is detailed in the `synapter` package vignette.

filterNA

  signature(object = "MSnSet", pNA = "numeric", pattern = "character", droplevels = "logical")

This method subsets `object` by removing features that have (strictly) more than `pNA` percent of NA values. Default `pNA` is 0, which removes any feature that exhibits missing data. The method can also be used with a character pattern composed of `0` or `1` characters only. A `0` represent a column/sample that is allowed a missing values, while columns/samples with and `1` must not have `NA`s.

This method also accepts `matrix` instances. `droplevels` defines whether unused levels in the feature meta-data ought to be lost. Default is `TRUE`. See the `droplevels` method below.

See also the `is.na.MSnSet` and `plotNA` methods for missing data exploration.

filterZero

  signature(object = "MSnSet", pNA = "numeric", pattern = "character", droplevels = "logical")

As `filterNA`, but for zeros.

filterMsLevel

  signature(object = "MSnSet", msLevel. = "numeric", fcol = "character")

Keeps only spectra with level `msLevel.`, as defined by the `fcol` feature variable (default is `"msLevel"`).

log

  signature(object = "MSnSet", base = "numeric")

Log transforms `exprs(object)` using `base::log`. `base` (defaults is `e='exp(1)'`) must be a positive or complex number, the base with respect to which logarithms are computed.

droplevels

  signature(x = "MSnSet", ...)

Drops the unused factor levels in the `featureData` slot. See `droplevels` for details.

exprsToRatios

  `signature(object = "MSnSet", log = "logical")`

calculates all possible ratios between `object`'s columns/samples. See `exprsToRatios` for more details.

impute

  `signature(object = "MSnSet", ...)`

Performs data imputation on the `MSnSet` object. See `impute` for more details.

trimws

  signature(object = "MSnSet", ...)

Trim leading and/or trailing white spaces in the feature data slot. Also available for `data.frame` objects. See `?base::trimws` for details.

Additional accessors for the experimental metadata (`experimentData` slot) are defined. See `"MIAPE"` for details.

### Plotting

meanSdPlot

  `signature(object = "MSnSet")`

Plots row standard deviations versus row means. See `meanSdPlot` (`vsn` package) for more details.

image

  `signature(x = "MSnSet", facetBy = "character", sOrderBy = "character", legend = "character", low = "character", high = "character", fnames = "logical", nmax = "numeric")`

Produces an heatmap of expression values in the `x` object. Simple horizontal facetting is enabled by passing a single character as `facetBy`. Arbitrary facetting can be performed manually by saving the return value of the method (see example below). Re-ordering of the samples is possible by providing the name of a phenotypic variable to `sOrderBy`. The title of the legend can be set with `legend` and the colours with the `low` and `high` arguments. If any negative value is detected in the data, the values are considered as log fold-changes and a divergent colour scale is used. Otherwise, a gradient from low to high is used. To scale the quantitative data in `x` prior to plotting, please see the `scale` method.

When there are more than `nmax` (default is 50) features/rows, these are not printed. This behaviour can be controlled by setting `fnames` to `TRUE` (always print) or `FALSE` (never print). See examples below.

The code is based on Vlad Petyuk's `vp.misc::image_msnset`. The previous version of this method is still available through the `image2` function.

plotNA

  `signature(object = "MSnSet", pNA = "numeric")`

Plots missing data for an `MSnSet` instance. `pNA` is a `numeric` of length 1 that specifies the percentage of accepted missing data values per features. This value will be highlighted with a point on the figure, illustrating the overall percentage of NA values in the full data set and the number of proteins retained. Default is 1/2. See also `plotNA`.

MAplot

  `signature(object = "MSnSet", log.it = "logical", base = "numeric", ...)`

Produces MA plots (Ratio as a function of average intensity) for the samples in `object`. If `ncol(object) == 2`, then one MA plot is produced using the `ma.plot` function from the `affy` package. If `object` has more than 2 columns, then `mva.pairs`. `log.it` specifies is the data should be log-transformed (default is `TRUE`) using `base`. Further `...` arguments will be passed to the respective functions.

addIdentificationData

  `signature(object = "MSnSet", ...)`: Adds identification data to a `MSnSet` instance. See `addIdentificationData` documentation for more details and examples.

removeNoId

  `signature(object = "MSnSet", fcol = "pepseq", keep = NULL)`: Removes non-identified features. See `removeNoId` documentation for more details and examples.

removeMultipleAssignment

  `signature(object = "MSnSet", fcol = "nprot")`: Removes protein groups (or feature belong to protein groups) with more than one member. The latter is defined by extracting a feature variable (default is `"nprot"`). Also removes non-identified features/

idSummary

  `signature(object = "MSnSet", ...)`: Prints a summary that lists the percentage of identified features per file (called `coverage`).

### Functions

updateFvarLabels

  signature(object, label, sep)

This function updates `object`'s featureData variable labels by appending `label`. By default, `label` is the variable name and the separator `sep` is `.`.

updateSampleNames

  signature(object, label, sep)

This function updates `object`'s sample names by appending `label`. By default, `label` is the variable name and the separator `sep` is `.`.

updateFeatureNames

  signature(object, label, sep)

This function updates `object`'s feature names by appending `label`. By default, `label` is the variable name and the separator `sep` is `.`.

ms2df

  signature(x, fcols)

Coerces the `MSnSet` instance to a `data.frame`. The direction of the data is retained and the feature variable labels that match `fcol` are appended to the expression values. See also `as(x, "data.frame")` above.

addMSnSetMetadata

  signature(x, y)

When coercing an `MSnSet` `y` to a `SummarizedExperiment` `x` with `x <- as(y, "SummarizedExperiment")`, most of `y`'s metadata is lost. Only the file names, the processing log and the MSnbase version from the `processingData` slots are passed along. The `addMSnSetMetadata` function can be used to add the complete `processingData`, `experimentData` and `protocolData` slots. The downside of this is that MSnbase is now required to use the `SummarizedExperiment` object.

### Author(s)

Laurent Gatto <lg390@cam.ac.uk>

### See Also

`"eSet"`, `"ExpressionSet"` and `quantify`. `MSnSet` quantitation values and annotation can be exported to a file with `write.exprs`. See `readMSnSet` to create and `MSnSet` using data available in a spreadsheet or `data.frame`.

### Examples

    data(msnset)
    msnset <- msnset[10:15]

    exprs(msnset)[1, c(1, 4)] <- NA
    exprs(msnset)[2, c(1, 2)] <- NA
    is.na(msnset)
    featureNames(filterNA(msnset, pNA = 1/4))
    featureNames(filterNA(msnset, pattern = "0110"))

    M <- matrix(rnorm(12), 4)
    pd <- data.frame(otherpdata = letters[1:3])
    fd <- data.frame(otherfdata = letters[1:4])
    x0 <- MSnSet(M, fd, pd)
    sampleNames(x0)

    M <- matrix(rnorm(12), 4)
    colnames(M) <- LETTERS[1:3]
    rownames(M) <- paste0("id", LETTERS[1:4])
    pd <- data.frame(otherpdata = letters[1:3])
    rownames(pd) <- colnames(M)
    fd <- data.frame(otherfdata = letters[1:4])
    rownames(fd) <- rownames(M)
    x <- MSnSet(M, fd, pd)
    sampleNames(x)

    ## Visualisation

    library("pRolocdata")
    data(dunkley2006)
    image(dunkley2006)
    ## Changing colours
    image(dunkley2006, high = "darkgreen")
    image(dunkley2006, high = "darkgreen", low = "yellow")
    ## Forcing feature names
    image(dunkley2006, fnames = TRUE)
    ## Facetting
    image(dunkley2006, facetBy = "replicate")
    p <- image(dunkley2006)
    library("ggplot2") ## for facet_grid
    p + facet_grid(replicate ~ membrane.prep, scales = 'free', space = 'free')
    p + facet_grid(markers ~ replicate)
    ## Fold-changes
    dd <- dunkley2006
    exprs(dd) <- exprs(dd) - 0.25
    image(dd)
    image(dd, low = "green", high = "red")
    ## Feature names are displayed by default for smaller data
    dunkley2006 <- dunkley2006[1:25, ]
    image(dunkley2006)
    image(dunkley2006, legend = "hello")

    ## Coercion
    if (require("SummarizedExperiment")) {
        data(msnset)
        se <- as(msnset, "SummarizedExperiment")
        metadata(se) ## only logging
        se <- addMSnSetMetadata(se, msnset)
        metadata(se) ## all metadata
        msnset2 <- as(se, "MSnSet")
        processingData(msnset2)
    }

    as(msnset, "ExpressionSet")

[Package MSnbase version 2.10.1 Index]