In mdeber/BRGenomics: Tools for the Efficient Analysis of High-Resolution Genomics Data

Importing Annotations with rtracklayer

Importing genomics files is accomplished using the rtracklayer package, which contains a variety of functions and options for importing and exporting.

# import bed file
genelist <- import.bed("~/data/genelists/genes.bed")

# import gff
genelist <- import.gff("~/data/genelists/genes.gff")

# export a bed file after modifying
export.bed(genelist, "~/data/genelists/filtered_genes.bed")

Defining Regions Using the genebodies Function

One of the more useful GenomicRanges functions is the promoters() function, which returns ranges centered on the strand-specific start of the input ranges:

library(BRGenomics)

data("txs_dm6_chr4")
tx4 <- txs_dm6_chr4[c(1, 10, 200, 300)]
tx4

tx4_pr <- promoters(tx4, upstream = 50, downstream = 100)
tx4_pr
width(tx4_pr)

BRGenomics ships with a more flexible alternative function called genebodies(). While promoters() has the arguments upstream and downstream, which take only positive values, the genebodies() function uses start and end arguments that can be positive or negative, and arguments fix.start and fix.end for determining whether to define the positions in relation to the (strand-specific) beginning or ends of genes.

Below, we demonstrate several uses of the genebodies() function, using a list of transcripts which start at a transcription start site (TSS) and end at a cleavage and polyadenylation site (CPS).

---

Original regions:

tx4

\

Genebody regions from 300 bp downstream of the TSS to 300 bp upstream of the CPS:

genebodies(tx4, start = 300, end = -300)

By default, fix.start = "start" and fix.end = "end". But we can change either of them to define ranges based solely on the beginnings or ends of the input regions.

Get promoter regions from 50 bp upstream to 100 bp downstream of the TSS:

genebodies(tx4, -50, 100, fix.end = "start")

\

Regions from 100 bp upstream of to 50 bp upstream of the TSS:

genebodies(tx4, -100, -50, fix.end = "start")

\

Regions from 1kb upstream of the CPS to 1kb downstream of the CPS

genebodies(tx4, -1000, 1000, fix.start = "end")

\

Regions within the first 10kb downstream of the CPS:

genebodies(tx4, 0, 10000, fix.start = "end")

\

Modify-By-Gene

The reduceByGene() and intersectByGene() are two other useful functions, which perform two common tasks very efficiently.

reduceByGene

reduceByGene() takes all ranges that share the same gene name (e.g. different transcript isoforms) and combines them such that all positions are represented.

txs <- txs_dm6_chr4[order(txs_dm6_chr4$gene_id)] # sort by gene_id
txs[1:10]

reduceByGene(txs, gene_names = txs$gene_id)

By default, the gene names are maintained as the names of the rows (ranges) in the output. To set them into metadata again, we could run:

txs_redux <- reduceByGene(txs, gene_names = txs$gene_id)
txs_redux$gene_id <- names(txs_redux)
names(txs_redux) <- NULL
txs_redux

Note that reduceByGene() is not guaranteed to produce a single range per gene, but will produce the fewest number of ranges required to represent all input positions.

Also note that while the output ranges for a given gene are disjoint, it is possible for ranges from different genes to overlap one another.

To make all ranges disjoint (no position overlapped more than once), set disjoin = TRUE.

intersectByGene

While reduceByGene() creates a comprehensive representation of all input ranges (e.g. a "union" of the set of input ranges), intersectByGene() outputs only the consensus region, i.e. the region that is shared across all the ranges of a particular gene.

txs[1:10]
txs_insxt <- intersectByGene(txs, gene_names = txs$gene_id)
txs_insxt[order(names(txs_insxt))]

Unlike reduceByGene(), intersectByGene() is guaranteed to return no more than 1 range per gene. However, genes for which no consensus is possible (i.e. no single range can overlap every input range) are dropped from the genelist.

mdeber/BRGenomics documentation built on March 4, 2024, 9:46 a.m.