QFeatures-aggregate: Aggregate an assay's quantitative features
In rformassspectrometry/QFeatures: Quantitative features for mass spectrometry data

aggregateFeatures

R Documentation

Aggregate an assay's quantitative features

Description

This function aggregates the quantitative features of an assay, applying a summarisation function (fun) to sets of features. The fcol variable name points to a rowData column that defines how to group the features during aggregate. This variable can eigher be a vector (we then refer to an aggregation by vector) or an adjacency matrix (aggregation by matrix).

The rowData of the aggregated SummarizedExperiment assay contains a .n variable that provides the number of parent features that were aggregated.

When aggregating with a vector, the newly aggregated SummarizedExperiment assay also contains a new aggcounts assay containing the aggregation counts matrix, i.e. the number of features that were aggregated for each sample, which can be accessed with the aggcounts() accessor.

Usage

## S4 method for signature 'QFeatures'
aggregateFeatures(
  object,
  i,
  fcol,
  name = "newAssay",
  fun = MsCoreUtils::robustSummary,
  ...
)

## S4 method for signature 'SummarizedExperiment'
aggregateFeatures(object, fcol, fun = MsCoreUtils::robustSummary, ...)

## S4 method for signature 'QFeatures'
adjacencyMatrix(object, i, adjName = "adjacencyMatrix")

adjacencyMatrix(object, i, adjName = "adjacencyMatrix") <- value

## S4 method for signature 'SummarizedExperiment'
aggcounts(object, ...)

Arguments

`object`	An instance of class `SummarizedExperiment` or `QFeatures`.
`i`	When adding an adjacency matrix to an assay of a `QFeatures` object, the index or name of the assay the adjacency matrix will be added to. Ignored when `x` is an `SummarizedExperiment`.
`fcol`	A `character(1)` naming a rowdata variable (of assay `i` in case of a `QFeatures`) defining how to aggregate the features of the assay. This variable is either a `character` or a (possibly sparse) matrix. See below for details.
`name`	A `character(1)` naming the new assay. Default is `newAssay`. Note that the function will fail if there's already an assay with `name`.
`fun`	A function used for quantitative feature aggregation. See Details for examples.
`...`	Additional parameters passed the `fun`.
`adjName`	`character(1)` with the variable name containing the adjacency matrix. Default is `"adjacencyMatrix"`.
`value`	An adjacency matrix with row and column names. The matrix will be coerced to compressed, column-oriented sparse matrix (class `dgCMatrix`) as defined in the `Matrix` package, as generaled by the `sparseMatrix()` constructor.

Details

Aggregation is performed by a function that takes a matrix as input and returns a vector of length equal to ncol(x). Examples thereof are

MsCoreUtils::medianPolish() to fits an additive model (two way decomposition) using Tukey's median polish_ procedure using stats::medpolish();
MsCoreUtils::robustSummary() to calculate a robust aggregation using MASS::rlm() (default);
base::colMeans() to use the mean of each column;
colMeansMat(x, MAT) to aggregate feature by the calculating the mean of peptide intensities via an adjacency matrix. Shared peptides are re-used multiple times.
matrixStats::colMedians() to use the median of each column.
base::colSums() to use the sum of each column;
colSumsMat(x, MAT) to aggregate feature by the summing the peptide intensities for each protein via an adjacency matrix. Shared peptides are re-used multiple times.

See MsCoreUtils::aggregate_by_vector() for more aggregation functions.

Value

A QFeatures object with an additional assay or a SummarizedExperiment object (or subclass thereof).

Missing quantitative values

Missing quantitative values have different effects based on the aggregation method employed:

The aggregation functions should be able to deal with missing values by either ignoring or propagating them. This is often done with an na.rm argument, that can be passed with .... For example, rowSums, rowMeans, rowMedians, ... will ignore NA values with na.rm = TRUE, as illustrated below.
Missing values will result in an error when using medpolish, unless na.rm = TRUE is used. Note that this option relies on implicit assumptions and/or performes an implicit imputation: when summing, the values are implicitly imputed by 0, assuming that the NA represent a trully absent features; when averaging, the assumption is that the NA represented a genuinely missing value.
When using robust summarisation, individual missing values are excluded prior to fitting the linear model by robust regression. To remove all values in the feature containing the missing values, use filterNA().

More generally, missing values often need dedicated handling such as filtering (see filterNA()) or imputation (see impute()).

Missing values in the row data

Missing values in the row data of an assay will also impact the resulting (aggregated) assay row data, as illustrated in the example below. Any feature variables (a column in the row data) containing NA values will be dropped from the aggregated row data. The reasons underlying this drop are detailed in the reduceDataFrame() manual page: only invariant aggregated rows, i.e. rows resulting from the aggregation from identical variables, are preserved during aggregations.

The situation illustrated below should however only happen in rare cases and should often be imputable using the value of the other aggregation rows before aggregation to preserve the invariant nature of that column. In cases where an NA is present in an otherwise variant column, the column would be dropped anyway.

Using an adjacency matrix

When considering non-unique peptides explicitly, i.e. peptides that map to multiple proteins rather than as a protein group, it is convenient to encode this ambiguity explicitly using a peptide-by-proteins (sparse) adjacency matrix. This matrix is typically stored in the rowdata and set/retrieved with the adjacencyMatrix() function. It can be created manually (as illustrated below) or using PSMatch::makeAdjacencyMatrix().

Examples


## ---------------------------------------
## An example QFeatures with PSM-level data
## ---------------------------------------
data(feat1)
feat1

## Aggregate PSMs into peptides
feat1 <- aggregateFeatures(feat1, "psms", "Sequence", name = "peptides")
feat1

## Aggregate peptides into proteins
feat1 <- aggregateFeatures(feat1, "peptides", "Protein", name = "proteins")
feat1

assay(feat1[[1]])
assay(feat1[[2]])
aggcounts(feat1[[2]])
assay(feat1[[3]])
aggcounts(feat1[[3]])

## --------------------------------------------
## Aggregation with missing quantitative values
## --------------------------------------------
data(ft_na)
ft_na

assay(ft_na[[1]])
rowData(ft_na[[1]])

## By default, missing values are propagated
ft2 <- aggregateFeatures(ft_na, 1, fcol = "X", fun = colSums)
assay(ft2[[2]])
aggcounts(ft2[[2]])

## The rowData .n variable tallies number of initial rows that
## were aggregated (irrespective of NAs) for all the samples.
rowData(ft2[[2]])

## Ignored when setting na.rm = TRUE
ft3 <- aggregateFeatures(ft_na, 1, fcol = "X", fun = colSums, na.rm = TRUE)
assay(ft3[[2]])
aggcounts(ft3[[2]])

## -----------------------------------------------
## Aggregation with missing values in the row data
## -----------------------------------------------
## Row data results without any NAs, which includes the
## Y variables
rowData(ft2[[2]])

## Missing value in the Y feature variable
rowData(ft_na[[1]])[1, "Y"] <- NA
rowData(ft_na[[1]])

ft3 <- aggregateFeatures(ft_na, 1, fcol = "X", fun = colSums)
## The Y feature variable has been dropped!
assay(ft3[[2]])
rowData(ft3[[2]])

## --------------------------------------------
## Using a peptide-by-proteins adjacency matrix
## --------------------------------------------

## Let's use assay peptides from object feat1 and
## define that peptide SYGFNAAR maps to proteins
## Prot A and B

se <- feat1[["peptides"]]
rowData(se)$Protein[3] <- c("ProtA;ProtB")
rowData(se)

## This can also be defined using anadjacency matrix, manual
## encoding here. See PSMatch::makeAdjacencyMatrix() for a
## function that does it automatically.
adj <- matrix(0, nrow = 3, ncol = 2,
              dimnames = list(rownames(se),
                              c("ProtA", "ProtB")))
adj[1, 1] <- adj[2, 2] <- adj[3, 1:2] <- 1
adj

adjacencyMatrix(se) <- adj
rowData(se)
adjacencyMatrix(se)

## Aggregation using the adjacency matrix
se2 <- aggregateFeatures(se, fcol = "adjacencyMatrix",
                         fun = MsCoreUtils::colMeansMat)

## Peptide SYGFNAAR was taken into account in both ProtA and ProtB
## aggregations.
assay(se2)


## Aggregation by matrix on a QFeature object works as with a
## vector
ft <- QFeatures(list(peps = se))
ft <- aggregateFeatures(ft, "peps", "adjacencyMatrix", name = "protsByMat",
                        fun = MsCoreUtils::colMeansMat)
assay(ft[[2]])
rowData(ft[[2]])

rformassspectrometry/QFeatures documentation built on Sept. 18, 2024, 10:39 p.m.