Quantitative MS QFeatures

Conceptually, a QFeatures object holds a set of assays, each composed of a matrix (or array) containing quantitative data and row annotations (meta-data). The number and the names of the columns (samples) must always be the same across the assays, but the number and the names of the rows (features) can vary. The assays are typically defined as SummarizedExperiment objects. In addition, a QFeatures object also uses a single DataFrame to annotate the samples (columns) represented in all the matrices.

The QFeatures class extends the MultiAssayExperiment::MultiAssayExperiment and inherits all the functionality of the MultiAssayExperiment::MultiAssayExperiment class.

A typical use case for such QFeatures object is to represent quantitative proteomics (or metabolomics) data, where different assays represent quantitation data at the PSM (the main assay), peptide and protein level, and where peptide values are computed from the PSM data, and the protein-level data is calculated based on the peptide-level values. The largest assay (the one with the highest number of features, PSMs in the example above) is considered the main assay.

The recommended way to create QFeatures objects is the use the readQFeatures() function, that creates an instance from tabular data. The QFeatures constructor can be used to create objects from their bare parts. It is the user's responsability to make sure that these match the class validity requirements.

Usage

QFeatures(..., assayLinks = NULL)

# S4 method for class 'QFeatures'
show(object)

# S3 method for class 'QFeatures'
plot(x, interactive = FALSE, ...)

# S4 method for class 'QFeatures,ANY,ANY,ANY'
x[i, j, ..., drop = TRUE]

# S4 method for class 'QFeatures,character,ANY,ANY'
x[i, j, k, ..., drop = TRUE]

# S4 method for class 'QFeatures'
c(x, ...)

# S4 method for class 'QFeatures'
dims(x, use.names = TRUE)

# S4 method for class 'QFeatures'
nrows(x, use.names = TRUE)

# S4 method for class 'QFeatures'
ncols(x, use.names = TRUE)

# S4 method for class 'QFeatures'
rowData(x, use.names = TRUE, ...)

# S4 method for class 'QFeatures,DataFrameList'
rowData(x) <- value

# S4 method for class 'QFeatures,ANY'
rowData(x) <- value

rbindRowData(object, i)

selectRowData(x, rowvars)

rowDataNames(x)

# S4 method for class 'QFeatures,character'
names(x) <- value

addAssay(x, y, name, assayLinks)

removeAssay(x, i)

replaceAssay(x, y, i)

# S4 method for class 'QFeatures,ANY,ANY'
x[[i, j, ...]] <- value

# S4 method for class 'QFeatures'
updateObject(object, ..., verbose = FALSE)

dropEmptyAssays(object, dims = 1:2)

Arguments

...: See MultiAssayExperiment for details. For plot, further arguments passed to igraph::plot.igraph.
assayLinks: An optional AssayLinks.
object: An instance of class QFeatures.
x: An instance of class QFeatures.
interactive: A logical(1). If TRUE, an interactive graph is generated using plotly. Else, a static plot using igraph is generated. We recommend interactive exploration when the QFeatures object contains more than 50 assays.
i: An indexing vector. See the corresponding section in the documentation for more details.
j: character(), logical(), or numeric() vector for subsetting by colData rows.
drop: logical (default TRUE) whether to drop empty assay elements in the ExperimentList.
k: character(), logical(), or numeric() vector for subsetting by assays
use.names: A logical(1) indicating whether the rownames of each assay should be propagated to the corresponding rowData.
value: The values to use as a replacement. See the corresponding section in the documentation for more details.
rowvars: A character() with the names of the rowData variables (columns) to retain in any assay.
y: An object that inherits from SummarizedExperiment or a named list of assays. When y is a list, each element must inherit from a SummarizedExperiment and the names of the list are used as the names of the assays to add. Hence, the list names must be unique and cannot overlap with the names of the assays already present in x.
name: A character(1) naming the single assay. Ignored if y is a list of assays.
verbose: logical (default FALSE) whether to print extra messages
dims: numeric() that defines the dimensions to consider to drop empty assays. 1 for rows (i.e. assays without any features) and 2 for columns (i.e. assays without any samples). Default is 1:2. Any value other that 1 and/or 2 will trigger an error.

Constructors

QFeatures(..., assayLinks) allows the manual construction of objects. It is the user's responsability to make sure these comply. The arguments in ... are those documented in MultiAssayExperiment::MultiAssayExperiment(). For details about assayLinks, see AssayLinks. An example is shown below.
The readQFeatures() function constructs a QFeatures object from text-based spreadsheet or a data.frame used to generate an assay. See the function manual page for details and an example.

Accessors

The QFeatures class extends the MultiAssayExperiment::MultiAssayExperiment class and inherits all its accessors and replacement methods.
The rowData method returns a DataFrameList containing the rowData for each assay of the QFeatures object. On the other hand, rowData can be modified using rowData(x) <- value, where value is a list of tables that can be coerced to DFrame tables. The names of value point to the assays for which the rowData must be replaced. The column names of each table are used to replace the data in the existing rowData. If the column name does not exist, a new column is added to the rowData.
The rbindRowData functions returns a DFrame table that contains the row binded rowData tables from the selected assays. In this context, i is a character(), integer() or logical() object for subsetting assays. Only rowData variables that are common to all assays are kept.
The rowDataNames accessor returns a list with the rowData variable names.
The longForm() accessor takes a QFeatures instance and returns it in a long tidy DataFrame, where each quantitative value is reported on a separate line.

Adding, removing and replacing assays

The aggregateFeatures() function creates a new assay by aggregating features of an existing assay.
addAssay(x, y, name, assayLinks): Adds one or more new assay(s) y to the QFeatures instance x. name is a character(1) naming the assay if only one assay is provided, and is ignored if y is a list of assays. assayLinks is an optional AssayLinks. The colData(y) is automatically added to colData(x) by matching sample names, that is colnames(y). If the samples are not present in x, the rows of colData(x) are extended to account for the new samples. Be aware that conflicting information between the colData(y) and the colData(x) will result in an error.
removeAssay(x, i): Removes one or more assay(s) from the QFeatures instance x. In this context, i is a character(), integer() or logical() that indicates which assay(s) to remove.
replaceAssay(x, y, i): Replaces one or more assay(s) from the QFeatures instance x. In this context, i is a character(), integer() or logical() that indicates which assay(s) to replace. The AssayLinks from or to any replaced assays are automatically removed, unless the replacement has the same dimension names (columns and row, order agnostic). Be aware that conflicting information between colData(y) and colData(x) will result in an error.
x[[i]] <- value: a generic method for adding (when i is not in names(x)), removing (when value is null) or replacing (when i is in names(x)). Note that the arguments j and ... from the S4 replacement method signature are not allowed.

Subsetting

QFeatures object can be subset using the x[i, j, k, drop = TRUE] paradigm. In this context, i is a character(), integer(), logical() or GRanges() object for subsetting by rows. See the argument descriptions for details on the remaining arguments.
The subsetByFeature() function can be used to subset a QFeatures object using one or multiple feature names that will be matched across different assays, taking the aggregation relation between assays.
The selectRowData(x, rowvars) function can be used to select a limited number of rowData columns of interest named in rowvars in the x instance of class QFeatures. All other variables than rowvars will be dropped. In case an element in rowvars isn't found in any rowData variable, a message is printed.
The dropEmptyAssays(object, dims) function removes empty assays from a QFeatures. Empty assays are defined as having 0 rows and/or 0 columns, as defined by the dims argument.

Author

Laurent Gatto

Examples

## ------------------------
## An empty QFeatures object
## ------------------------

QFeatures()
#> An empty instance of class QFeatures (type: bulk)

## -----------------------------------
## Creating a QFeatures object manually
## -----------------------------------

## two assays (matrices) with matching column names
m1 <- matrix(1:40, ncol = 4)
m2 <- matrix(1:16, ncol = 4)
sample_names <- paste0("S", 1:4)
colnames(m1) <- colnames(m2) <- sample_names
rownames(m1) <- letters[1:10]
rownames(m2) <- letters[1:4]

## two corresponding feature metadata with appropriate row names
df1 <- DataFrame(Fa = 1:10, Fb = letters[1:10],
                 row.names = rownames(m1))
df2 <- DataFrame(row.names = rownames(m2))

(se1 <- SummarizedExperiment(m1, df1))
#> class: SummarizedExperiment 
#> dim: 10 4 
#> metadata(0):
#> assays(1): ''
#> rownames(10): a b ... i j
#> rowData names(2): Fa Fb
#> colnames(4): S1 S2 S3 S4
#> colData names(0):
(se2 <- SummarizedExperiment(m2, df2))
#> class: SummarizedExperiment 
#> dim: 4 4 
#> metadata(0):
#> assays(1): ''
#> rownames(4): a b c d
#> rowData names(0):
#> colnames(4): S1 S2 S3 S4
#> colData names(0):

## Sample annotation (colData)
cd <- DataFrame(Var1 = rnorm(4),
                Var2 = LETTERS[1:4],
                row.names = sample_names)

el <- list(assay1 = se1, assay2 = se2)
fts1 <- QFeatures(el, colData = cd)
fts1
#> An instance of class QFeatures (type: bulk) with 2 sets:
#> 
#>  [1] assay1: SummarizedExperiment with 10 rows and 4 columns 
#>  [2] assay2: SummarizedExperiment with 4 rows and 4 columns 
fts1[[1]]
#> class: SummarizedExperiment 
#> dim: 10 4 
#> metadata(0):
#> assays(1): ''
#> rownames(10): a b ... i j
#> rowData names(2): Fa Fb
#> colnames(4): S1 S2 S3 S4
#> colData names(0):
fts1[["assay1"]]
#> class: SummarizedExperiment 
#> dim: 10 4 
#> metadata(0):
#> assays(1): ''
#> rownames(10): a b ... i j
#> rowData names(2): Fa Fb
#> colnames(4): S1 S2 S3 S4
#> colData names(0):

## Rename assay
names(fts1) <- c("se1", "se2")

## Add an assay
fts1 <- addAssay(fts1, se1[1:2, ], name = "se3")

## Get the assays feature metadata
rowData(fts1)
#> DataFrameList of length 3
#> names(3): se1 se2 se3

## Keep only the Fa variable
selectRowData(fts1, rowvars = "Fa")
#> An instance of class QFeatures (type: bulk) with 3 sets:
#> 
#>  [1] se1: SummarizedExperiment with 10 rows and 4 columns 
#>  [2] se2: SummarizedExperiment with 4 rows and 4 columns 
#>  [3] se3: SummarizedExperiment with 2 rows and 4 columns 

## -----------------------------------
## See ?readQFeatures to create a
## QFeatures object from a data.frame
## or spreadsheet.
## -----------------------------------