Coverage: Calls SpliceWiz's C++ function to retrieve coverage from a...

In alexchwong/SpliceWiz: interactive analysis and visualization of alternative splicing in R

Calls SpliceWiz's C++ function to retrieve coverage from a COV file

Description

This function returns an RLE / RLEList or data.frame containing coverage data from the given COV file

COV files are generated by SpliceWiz's processBAM and BAM2COV functions. It records alignment coverage for each nucleotide in the given BAM file. It stores this data in "COV" format, which is an indexed BGZF-compressed format specialised for the storage of unstranded and stranded alignment coverage in RNA sequencing.

Unlike BigWig files, COV files store coverage for both positive and negative strands.

These functions retrieves coverage data from the specified COV file. They are computationally efficient as they utilise random-access to rapidly search for the requested data from the COV file.

Usage

getCoverage(file, seqname = "", start = 0, end = 0, strand = c("*", "+", "-"))

getCoverage_DF(
  file,
  seqname = "",
  start = 0,
  end = 0,
  strand = c("*", "+", "-")
)

getCoverageRegions(
  file,
  regions,
  strandMode = c("unstranded", "forward", "reverse")
)

getCoverageBins(
  file,
  region,
  bins = 2000,
  strandMode = c("unstranded", "forward", "reverse"),
  bin_size
)

Arguments

file	(Required) The file name of the COV file
seqname	(Required for getCoverage_DF) A string denoting the chromosome name. If left blank in getCoverage, retrieves RLEList containing coverage of the entire file.
start, end	1-based genomic coordinates. If start = 0 and end = 0, will retrieve RLE of specified chromosome.
strand	Either "*", "+", or "-"
regions	A GRanges object for a set of regions to obtain mean / total coverage from the given COV file.
strandMode	The stranded-ness of the RNA-seq experiment. "unstranded" means that an unstranded protocol was used. Stranded protocols can be either "forward", where the first read is the same strand as the expressed transcript, or "reverse" where the second strand is the same strand as the expressed transcript.
region	In getCoverageBins, a single query region as a GRanges object
bins	In getCoverageBins, the number of bins to divide the given region. If bin_size is given, overrides this parameter
bin_size	In getCoverageBins, the number of nucleotides per bin

Value

For getCoverage: If seqname is left as "", returns an RLEList of the whole BAM file, with each RLE in the list containing coverage data for one chromosome. Otherwise, returns an RLE containing coverage data for the requested genomic region

For getCoverage_DF: Returns a two-column data frame, with the first column coordinate denoting genomic coordinate, and the second column value containing the coverage depth for each coordinate nucleotide.

For getCoverageRegions: Returns a GRanges object with an extra metacolumn: cov_mean, which gives the mean coverage of each of the given ranges.

For getCoverageBins: Returns a GRanges object which spans the given region, divided by the number of bins or by width as given by bin_size. Mean coverage in each bin is calculated (returned by the cov_mean metadata column). This function is useful for retrieving coverage of a large region for visualisation, especially when the size of the region vastly exceeds the width of the figure.

Functions

• getCoverage(): Retrieves alignment coverage as an RLE or RLElist

• getCoverage_DF(): Retrieves alignment coverage as a data.frame

• getCoverageRegions(): Retrieves total and mean coverage of a GRanges object from a COV file

• getCoverageBins(): Retrieves coverage of a single region from a COV file, binned by the given number of bins or bin_size

Examples

se <- SpliceWiz_example_NxtSE()

cov_file <- covfile(se)[1]

# Retrieve Coverage as RLE

cov <- getCoverage(cov_file, seqname = "chrZ",
  start = 10000, end = 20000,
  strand = "*"
)

# Retrieve Coverage as data.frame

cov.df <- getCoverage_DF(cov_file, seqname = "chrZ",
  start = 10000, end = 20000,
  strand = "*"
)

# Retrieve mean coverage of 100-nt window regions as defined
# in a GRanges object:

gr <- GenomicRanges::GRanges(
    seqnames = "chrZ",
    ranges = IRanges::IRanges(
        start = seq(1, 99901, by = 100),
        end = seq(100, 100000, by = 100)
    ), strand = "-"
)

gr.unstranded <- getCoverageRegions(cov_file,
    regions = gr,
    strandMode = "unstranded"
)

gr.stranded <- getCoverageRegions(cov_file,
    regions = gr,
    strandMode = "reverse"
)

# Retrieve binned coverage of a large region

gr.fetch <- getCoverageBins(
    cov_file,
    region = GenomicRanges::GRanges(seqnames = "chrZ",
        ranges = IRanges::IRanges(start = 100, end = 100000),
        strand = "*"
    ),
    bins = 2000
)

# Plot coverage using ggplot:

require(ggplot2)

ggplot(cov.df, aes(x = coordinate, y = value)) +
    geom_line() + theme_white

ggplot(as.data.frame(gr.unstranded),
    aes(x = (start + end) / 2, y = cov_mean)) +
    geom_line() + theme_white

ggplot(as.data.frame(gr.fetch), 
    aes(x = (start + end)/2, y = cov_mean)) +
    geom_line() + theme_white

# Export COV data as BigWig

cov_whole <- getCoverage(cov_file)
bw_file <- file.path(tempdir(), "sample.bw")
rtracklayer::export(cov_whole, bw_file, "bw")

alexchwong/SpliceWiz documentation built on March 17, 2024, 3:16 a.m.