GA4GHclient
In GA4GHclient: A Bioconductor package for accessing GA4GH API data servers

Introduction

The Global Alliance for Genomics and Health (GA4GH) was formed to help accelerate the potential of genomic medicine to advance human health. It brings together over 400 leading institutions working in healthcare, research, disease advocacy, life science, and information technology. The Data Working Group of the GA4GH developed data model schemas and application program interfaces (APIs) for genomic data. These APIs are specifically designed to allow sharing of genomics data in a standardized manner and without having to exchange complete experiments. They developed a reference implementation for these APIs providing a web server for hosting genomic data.

The Bioconductor project provides tools for the analysis and comprehension of high-throughput genomic data. It uses the R statistical programming language, and is open source and open development. The Bioconductor provides stable representations for genomics data such as biological sequences and genomic variants.

We developed GA4GHclient, a Bioconductor package that provides an easy way to access public data servers through GA4GH APIs. The requested output data are converted into tidy data and presented as data frames. Our package provides methods for converting genomic variant data into VCF data allowing the creation of complex data analysis using public genomic data and well validated Bioconductor packages. This package also provides an interactive web application for interacting with genomics data through GA4GH APIs.

Public data servers that use GA4GH Genomics API:

Available request methods

The table below shows all available methods in GA4GHclient package. These methods are based on the official GA4GH schemas. Let us know if some method is missing by opening an issue at https://github.com/labbcb/GA4GHclient/issues.

knitr::kable(data.frame(
    Method = c(
        "searchDatasets",
        "searchReferenceSets",
        "searchReferences",
        "listReferenceBases",
        "searchVariantSets",
        "searchVariants",
        "searchCallSets",
        "searchVariantAnnotationSets",
        "searchVariantAnnotations",
        "searchFeatureSets",
        "searchFeatures",
        "searchReadGroupSets",
        "searchReads",
        "searchBiosamples",
        "searchIndividuals",
        "searchRnaQuantificationSets",
        "searchRnaQuantifications",
        "searchExpressionLevels",
        "searchPhenotypeAssociationSets",
        "searchPhenotypeAssociations",
        "getDataset",
        "getReferenceSet",
        "getReference",
        "getVariantSet",
        "getVariant",
        "getCallSet",
        "getVariantAnnotationSet",
        "getVariantAnnotation",
        "getFeatureSet",
        "getFeature",
        "getReadGroupSet",
        "getBiosample",
        "getIndividual",
        "getRnaQuantificationSet",
        "getRnaQuantification",
        "getExpressionLevel"
    ),
    Description = c(
        "Search for datasets",
        "Search for reference sets (reference genomes)",
        "Search for references (genome sequences, e.g. chromosomes)",
        "Get the sequence bases of a reference genome by genomic range",
        "Search for for variant sets (VCF files)",
        "Search for variants by genomic ranges (lines of VCF files)",
        "Search for call sets (sample columns of VCF files)",
        "Search for variant annotation sets (annotated VCF files)",
        "Search for annotated variants by genomic range",
        "Search for feature sets (genomic features, e.g. GFF files)",
        "Search for features (lines of genomic feature files)",
        "Search for read group sets (sequence alignement, e.g BAM files)",
        "Search for reads by genomic range (bases of aligned sequences)",
        "Search for biosamples",
        "Search for individuals",
        "Search for RNA quantification sets",
        "Search for RNA quantifications",
        "Search for expression levels",
        "Search for phenotype association sets",
        "Search for phenotype associations",
        "Get a dataset by its ID",
        "Get a reference set by its ID",
        "Get a reference by its ID",
        "Get a variant set by its ID",
        "Get a variant by its ID with all call sets for this variant",
        "Get a call set by its ID",
        "Get a variant annotation set by its ID",
        "Get an annotated variant by its ID",
        "Get a feature set by its ID",
        "Get a feature by its ID",
        "Get a read group set by its ID",
        "Get a biosample by its ID",
        "Get an individual by its ID",
        "Get an RNA quantification set by its ID",
        "Get an RNA quantification by its ID",
        "Get an expression level by its ID"
    ), stringsAsFactors = FALSE
))

Retrieving Thousand Genomes Project data through GA4GHclient package

First, load required packages for this vignette.

library(GA4GHclient)
library(org.Hs.eg.db)
library(TxDb.Hsapiens.UCSC.hg19.knownGene)
library(VariantAnnotation)

For this example we will use Hosting Thousand Genomes Project server. It contains data from the 1000 Genomes project. Set the URL of GA4GH API data server. This variable will be used by all request functions.

host <- "http://1kgenomes.ga4gh.org/"

# To avoid internet connection issues, some code of this vignette were 
# previously executed and result data stored within the package.
load(system.file(package = "GA4GHclient", "extdata/vignette.rda"))

Get basic information about data server. The dataset may contain many variant sets. A variant set represents the header of an VCF file. A collection of variants represent lines of an VCF file. The nrow attribute define the amount of entries we want to get from the server. For the dataset and variant set we want only the first entry.

The searchVariants function requests genomic variant data from the server. It uses genomic intervals to retrieve these varants. As R programming language, the genomic indice is 1-based. If you want to see what reference names (e.g. chromosomes) are available run the searchReferences function.

datasets <- searchDatasets(host, nrows = 1)
datasetId <- datasets$id
variantSets <- searchVariantSets(host, datasetId, nrows = 1)
variantSetId <- variantSets$id
variants <- searchVariants(host, variantSetId, referenceName = "1",
    start = 15000, end = 16000, nrows = 10)

variants

Almost search functions will return an DataFrame object from r Biocpkg("S4Vector") package. The searchVariants and getVariant functions will return an VCF object with header from r Biocpkg("VariantAnnotation") package. The getVariantSet function will return an VCFHeader object. These three functions will return DataFrame object when asVCF or asVCFHeader parameters be FALSE.

variants.df <- searchVariants(host, variantSetId, referenceName = "1",
    start = 15000, end = 16000, nrows = 10, asVCF = FALSE)

DT::datatable(as.data.frame(variants.df), options = list(scrollX = TRUE))

Due to internet connection, there is a limit of amount for response data imposed by the server (or the responseSize parameter). By default the function will make requests until get all response data. Increasing the value of the responseSize parameter will reduce the number os requests to server.

Search Variants by genomic location of genes

Bioconductor has annotation packages for many genomes. For example, the r Biocpkg("TxDb.Hsapiens.UCSC.hg19.knownGene") pakage exposes an annotation databases generated from UCSC by exposing these as TxDb objects. Before using annotation packages, it is very important to check what version of reference genome was used by the GA4GH API-based data server. In other words, what reference genome was used to align sequencing reads and call for variants. This information can be accessed via searchReferenceSets function.

referenceSets <- searchReferenceSets(host, nrows = 1)
referenceSets$name

In this case, the version of reference genome is NCBI37, which is compatible with r Biocpkg("TxDb.Hsapiens.UCSC.hg19.knownGene").

Get name of all available genes.

head(keys(org.Hs.eg.db, keytype = "SYMBOL"))

Get genomic location of all genes.

head(genes(TxDb.Hsapiens.UCSC.hg19.knownGene))

Use case: search for variants located in SCN1A gene.

df <- select(org.Hs.eg.db, keys = "SCN1A", columns = c("SYMBOL", "ENTREZID"), keytype = "SYMBOL")
gr <- genes(TxDb.Hsapiens.UCSC.hg19.knownGene, filter=list(gene_id=df$ENTREZID))
seqlevelsStyle(gr) <- "NCBI"

variants.scn1a <- searchVariantsByGRanges(host, variantSetId, gr, asVCF = FALSE)

DT::datatable(as.data.frame(variants.scn1a[[1]]), options = list(scrollX = TRUE))

Use case: locate annotation data for a group of variants located at SCN1A gene.

variants.scn1a.gr <- makeGRangesFromDataFrame(variants.scn1a[[1]],
    seqnames.field = "referenceName")
seqlevelsStyle(variants.scn1a.gr) <- "UCSC"
locateVariants(variants.scn1a.gr, TxDb.Hsapiens.UCSC.hg19.knownGene,
    CodingVariants())

VariantAnnotation classes

The searchVariants and getVariant functions return VCF objects. These VCF objects contains the VCF header. This is an example of how to get variants with call data. We can get calls running the searchCallSets function. After convertion, it can be written into disk as VCF file using writeVcf.

callSetIds <- searchCallSets(host, variantSetId, nrows = 5)$id
vcf <- searchVariants(host, variantSetId, referenceName = "1",
    start = 15000, end = 16000, callSetIds = callSetIds, nrows = 10)

vcf
header(vcf)

The vcf object works as expected. We can get the genotype data for example. More information about working with VCF data see vignette("VariantAnnotation").

geno(vcf)$GT

# The code below saves objects obtained from data server in package.
# It is only necessary during development.
save(referenceSets, variantSetId, variants, variants.df, variants.scn1a,
    vcf, file = "inst/extdata/vignette.rda")