Package 'MsBackendMetaboLights'

Utility functions for the MetaboLights repository

Description

MetaboLights is one of the main public repositories for deposition of metabolomics experiments including (raw) mass spectrometry (MS) and NMR data files and experimental/analysis results. The experimental metadata and results are stored as plain text files in ISA-tab format. Each MetaboLights experiment must provide a file describing the samples analyzed and at least one assay file that links between the experimental samples and the (raw and processed) data files with quantification of metabolites/features in these samples.

Each experiment in MetaboLights is identified with its unique identifier, starting with MTBLS followed by a number. The data (metadata files and MS/NMR data files) of an experiment are available through the repository's ftp server.

The functions listed here allow to query and retrieve information of a data set/experiment from MetaboLights.

• mtbls_ftp_path: returns the FTP path for a provided MetaboLights ID. With mustWork = TRUE (the default) the function throws an error if the path is not accessible (either because the data set does not exist or no internet connection is available). The function returns a character(1) with the FTP path to the data set folder.

• mtbls_list_files: returns the available files (and directories) for the specified MetaboLights data set (i.e., the FTP directory content of the data set). The function returns a character vector with the relative file names to the absolute FTP path (mtbls_ftp_path()) of the data set. Parameter pattern allows to filter the file names and define which file names should be returned.

Usage

mtbls_ftp_path(x = character(), mustWork = TRUE)

mtbls_list_files(x = character(), pattern = NULL)

Arguments

x	character(1) with the ID of the MetaboLights data set (usually starting with a MTBLS followed by a number).
mustWork	for mtbls_ftp_path(): logical(1) whether the validity of the path should be verified or not. By default (with mustWork = TRUE) the function throws an error if either the data set does not exist or if the folder can not be accessed (e.g. if no internet connection is available).
pattern	for mtbls_list_files(): character(1) defining a pattern to filter the file names, such as pattern = "^a_" to retrieve the file names of all assay files of the data set (i.e., files with a name starting with "a_"). This parameter is passed to the grepl() function.

Value

• For mtbls_ftp_path(): character(1) with the ftp path to the specified data set on the MetaboLights ftp server.

• For mtbls_list_files(): character with the names of the files in the data set's base ftp directory.

Author(s)

Johannes Rainer, Philippine Louail

Examples

## Get the FTP path to the data set MTBLS2
mtbls_ftp_path("MTBLS2")

## Retrieve available files (and directories) for the data set MTBLS2
mtbls_list_files("MTBLS2")

## Retrieve the available assay files (file names starting with "a_").
afiles <- mtbls_list_files("MTBLS2", pattern = "^a_")
afiles

## Read the content of one file
a <- read.table(paste0(mtbls_ftp_path("MTBLS2"), afiles[1L]),
    header = TRUE, sep = "\t", check.names = FALSE)
head(a)

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MsBackend representing MS data from MetaboLights

Description

MsBackendMetaboLights retrieves and represents mass spectrometry (MS) data from metabolomics experiments stored in the MetaboLights repository. The backend directly extends the MsBackendMzR backend from the Spectra package and hence supports MS data in mzML, netCDF and mzXML format. Data in other formats can not be loaded with MsBackendMetaboLights. Upon initialization with the backendInitialize() method, the MsBackendMetaboLights backend downloads and caches the MS data files of an experiment locally avoiding hence repeated download of the data.

Usage

MsBackendMetaboLights()

## S4 method for signature 'MsBackendMetaboLights'
backendInitialize(
  object,
  mtblsId = character(),
  assayName = character(),
  filePattern = "mzML$|CDF$|cdf$|mzXML$",
  offline = FALSE,
  ...
)

## S4 method for signature 'MsBackendMetaboLights'
backendMerge(object, ...)

Arguments

object	an instance of MsBackendMetaboLights.
mtblsId	character(1) with the ID of a single MetaboLights data set/experiment.
assayName	character with the file names of assay files of the data set. If not provided (assayName = character(), the default), MS data files of all data set's assays are loaded. Use ⁠mtbls_list_files(<MetaboLights ID>, pattern = "^a_")⁠ to list all available assay files of a data set ⁠<MetaboLights ID>⁠.
filePattern	character with the pattern defining the supported (or requested) file types. Defaults to filePattern = "mzML$|CDF$|cdf$|mzXML$" hence restricting to mzML, CDF and mzXML files which are supported by Spectra's MsBackendMzR backend.
offline	logical(1) whether only locally cached content should be evaluated/loaded.
...	additional parameters; currently ignored.

Details

File names for data files are by default extracted from the column "Derived Spectral Data File" of the MetaboLights data set's assay table. If this column does not contain any supported file names, the assay's column "Raw Spectral Data File" is evaluated instead.

The backend uses the BiocFileCache package for caching of the data files. These are stored in the default local BiocFileCache cache along with additional metadata that includes the MetaboLights ID and the assay file name with which the data file is associated with. Note that at present only MS data files in mzML, CDF and mzXML format are supported.

The MsBackendMetaboLights backend defines and provides additional spectra variables "mtbls_id", "mtbls_assay_name" and "derived_spectral_data_file" that list the MetaboLights ID, the name of the assay file and the original data file name on the MetaboLights ftp server for each individual spectrum. The "derived_spectral_data_file" can be used for the mapping between the experiment's samples and the individual data files, respective their spectra. This mapping is provided in the MetaboLights assay file.

The MsBackendMetaboLights backend is considered read-only and does thus not support changing m/z and intensity values directly.

Also, merging of MS data of MsBackendMetaboLights is not supported and thus c() of several Spectra with MS data represented by MsBackendMetaboLights will throw an error.

Value

• For MsBackendMetaboLights(): an instance of MsBackendMetaboLights.

• For backendInitialize(): an instance of MsBackendMetaboLights with the MS data of the specified MetaboLights data set.

Initialization and loading of data

New instances of the class can be created with the MsBackendMetaboLights() function. Data is loaded and initialized using the backendInitialize() function which can be configured with parameters mtblsId, assayName and filePattern. mtblsId must be the ID of a single (existing) MetaboLights data set. Parameter assayName allows to define specific assays of the MetaboLights data set from which the data files should be loaded. If provided, it should be the file name(s) of the respective assay(s) in MetaboLights (use e.g. ⁠mtbls_list_files(<MetaboLights ID>, pattern = "^a_")⁠ to list all available assay files for a given MetaboLights ID ⁠<MetaboLights ID>⁠). By default, with assayName = character() MS data files from all assays of a data set are loaded. Optional parameter filePattern defines the pattern that should be used to filter the file names of the MS data files. It defaults to data files with file endings of supported MS data files. backendInitialize() requires an active internet connection as the function first compares the remote file content to the locally cached files and eventually synchronizes changes/updates. This can be skipped with offline = TRUE in which case only locally cached content is queried.

Author(s)

Philippine Louail, Johannes Rainer

Examples

library(MsBackendMetaboLights)

## List files of a MetaboLights data set
mtbls_list_files("MTBLS39")

## Initialize a MsBackendMetaboLights representing all MS data files of
## the data set with the ID "MTBLS39". This will download and cache all
## files and subsequently load and represent them in R.

be <- backendInitialize(MsBackendMetaboLights(), "MTBLS39")
be

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