VRanges-class: VRanges objects

In Jiefei-Wang/VariantAnnotation: Annotation of Genetic Variants

Description

The VRanges class is a container for variant calls, including SNVs and indels. It extends GRanges to provide special semantics on top of a simple vector of genomic locations. While it is not as expressive as the VCF object, it is a simpler alternative that may be convenient for variant calling/filtering and similar exercises.

Details

VRanges extends GRanges to store the following components. Except where noted, the components are considered columns in the dataset, i.e., their lengths match the number of variants. Many columns can be stored as either an atomic vector or an Rle.

ref

(character), the reference allele. The range (start/end/width) should always correspond to this sequence.

alt

(character/Rle), the alternative allele (NA allowed). By convention there is only a single alt allele per element (row) of the VRanges. Many methods, like match, make this assumption.

refCount

(integer/Rle), read count for the reference allele (NA allowed)

altCount

(integer/Rle), read count for the alternative allele (NA allowed)

totalCount

(integer/Rle), total read count at the position, must be at least refCount+altCount (NA allowed)

sampleNames

(factor/Rle), name of the sample - results from multiple samplse can be combined into the same object (NA allowed)

softFilterMatrix

(matrix/FilterMatrix), variant by filter matrix, TRUE where variant passed the filter; use a FilterMatrix to store the actual FilterRules object that was applied

hardFilters

(FilterRules) record of hard filters applied, i.e., only the variants that passed the filters are present in this object; this is the only component that is not a column, i.e., its length does not match the number of variants

Except in the special circumstances described here, a VRanges may be treated like a GRanges. The range should span the sequence in ref. Indels are typically represented by the VCF convention, i.e., the start position is one upstream of the event. The strand is always constrained to be positive (+).

Indels, by convention, should be encoded VCF-style, with the upstream reference base prepended to the indel sequence. The ref/alt for a deletion of GCGT before A might be AGCGT/A and for an insertion might be A/AGCGT. Since the range always matches the ref sequence, this means a deletion will be the width of the deletion + 1, and an insertion is always of width 1.

VRanges and the VCF class: The VRanges and VCF classes encode different types of information and are semantically incompatible. While methods exist for converting a VCF object to a VRanges and vice versa, information is lost in the transformation. There is no way to collapse multiple rows of a VRanges at the same genomic position and accurately represent missing data. For this reason, it is not reasonable to assume that an object resulting from multiple conversions (VRanges -> VCF -> VRanges) will be equivalent to the original.

Constructors

VRanges(seqnames = Rle(), ranges = IRanges(), ref = character(), alt = NA_character_, totalDepth = NA_integer_, refDepth = NA_integer_, altDepth = NA_integer_, ..., sampleNames = NA_character_, softFilterMatrix = FilterMatrix(matrix(nrow = length(gr), ncol = 0L), FilterRules()), hardFilters = FilterRules()): Creates a VRanges object.

seqnames

Rle object, character vector, or factor containing the sequence names.

ranges

IRanges object containing the ranges.

ref

character vector, containing the reference allele.

alt

character vector/Rle, containing the alternative allele (NA allowed).

totalDepth

integer vector/Rle, containing the total read depth (NA allowed).

refDepth

integer vector/Rle, containing the reference read depth (NA allowed).

altDepth

integer vector/Rle, containing the reference read depth (NA allowed).

...

Arguments passed to the GRanges constructor.

sampleNames

character/factor vector/Rle, containing the sample names (NA allowed).

softFilterMatrix

a matrix (typically a FilterMatrix) of dimension variant by filter, with logical values indicating whether a variant passed the filter.

hardFilters

a FilterRules, containing the filters that have already been applied to subset the object to its current state.

makeVRangesFromGRanges(gr, ref.field="ref", alt.field="alt", totalDepth.field="totalDepth", refDepth.field="refDepth", altDepth.field="altDepth", sampleNames.field="sampleNames", keep.extra.columns=TRUE): Creates a VRanges object from a GRanges.

gr

A GenomicRanges object.

ref.field

The character(1) name of the GRanges metadata column to be used as the VRanges ref field.

alt.field

The character(1) name of the GRanges metadata column to be used as the VRanges alt field.

totalDepth.field

The character(1) name of the GRanges metadata column to be used as the VRanges totalDepth field.

refDepth.field

The character(1) name of the GRanges metadata column to be used as the VRanges refDepth field.

altDepth.field

The character(1) name of the GRanges metadata column to be used as the VRanges altDepth field.

sampleNames.field

The character(1) name of the GRanges metadata column to be used as the VRanges sampleNames field.

keep.extra.columns

TRUE (the default) or FALSE. If TRUE, then the columns in gr that are not used to form the VRanges are retained as metadata columns. Otherwise, they will be ignored.

Coercion

These functions/methods coerce objects to and from VRanges:

asVCF(x, info = character(), filter = character(), meta = character()): Creates a VCF object from a VRanges object. The following gives the mapping from VRanges components to VCF:

seqnames(x)

CHROM column

start(x)

POS column

names(x)

ID column

ref(x)

REF column

alt(x)

ALT column

totalDepth(x)

DP in FORMAT column

altDepth(x), refDepth(x)

AD in FORMAT column

sampleNames(x)

Names the sample columns

softFilterMatrix(x)

FT in FORMAT column, except filters named in filter argument, which are considered per-position and placed in the FILTER column

hardFilters(x)

Not yet exported

mcols(x)

Become fields in the FORMAT column; unless they are named in the info argument, in which case they are considered per-position and placed in the INFO column

metadata(x)

If named in the meta argument, output in the VCF header; a component is required to be coercible to a character vector of length one.

Note that identical(x, as(as(x, "VCF"), "VRanges")) generally return FALSE. During coercion to VCF, the "geno" components are reshaped into matrix form, with NAs filling the empty cells. The reverse coercion will not drop the NA values, so rows are added to the new VRanges. All logical values will become integers in VCF, and there is no automatic way of regenerating the logical column with the reverse coercion. There are many other cases of irreversibility.

as(from, "VCF"): Like calling asVCF(from).

as(from, "VRanges"): When from is a VCF this coercion is essentially the inverse of asVCF. Information missing in the VCF is imputed as NA.

When from is a GRanges, metadata columns of ref, alt, refDepth, altDepth, totalDepth and sampleNames are transfered to the VRanges object. Additional metadata columns in the GRanges can be retained or dropped with keep.extra.columns. See also makeVRangesFromGRanges.

Accessors

In addition to all of the GRanges accessors, VRanges provides the following, where x is a VRanges object.

alt(x), alt(x) <- value: Get or set the alt allele (character).

ref(x), ref(x) <- value: Get or set the ref allele (character).

altDepth(x), altDepth(x) <- value: Get or set the alt allele read depth (integer).

refDepth(x), refDepth(x) <- value: Get or set the ref allele read depth (integer).

totalDepth(x), totalDepth(x) <- value: Get or set the total read depth (integer).

altFraction(x): Returns altDepth(x)/totalDepth(x) (numeric).

sampleNames(x), sampleNames(x) <- value: Get or set the sample names (character/factor).

softFilterMatrix(x), softFilterMatrix(x) <- value: Gets or sets the soft filter matrix (any matrix, but ideally a FilterMatrix).

resetFilter(x): Removes all columns from softFilterMatrix.

called(x): Returns whether all filter results in softFilterMatrix(x) are TRUE for each variant.

hardFilters(x), hardFilters(x) <- value: Gets or sets the hard filters (those applied to yield the current subset).

Utilities and Conveniences

match(x): Like GRanges match, except matches on the combination of chromosome, start, width, and alt.

tabulate(bin): Finds unique(bin) and counts how many times each unique element occurs in bin. The result is stored in mcols(bin)$sample.count.

softFilter(x, filters, ...): applies the FilterRules in filters to x, storing the results in softFilterMatrix.

Input/Output to/from VCF

writeVcf(obj, filename, ...): coerces to a VCF object and writes it to a file; see writeVcf.

readVcfAsVRanges(x, genome, param = ScanVcfParam(), ...): Reads a VCF x directly into a VRanges; see readVcf for details on the arguments. readVcfAsVRanges is an alternative syntax to

  as(readVcf(), "VRanges") 

NOTE: By default all INFO and FORMAT fields are read in with ScanVcfParam(). The minimal information needed to create the VRanges can be specified as follows:

  ScanVcfParam(fixed = "ALT", info = NA, geno = "AD")) 

Variant Type

Functions to identify variant type include isSNV, isInsertion, isDeletion, isIndel, isSubstitution and isTransition. See the ?isSNV man page for details.

Author(s)

Michael Lawrence. makeVRangesFromGRanges was contributed by Thomas Sandmann.

See Also

VRangesList, a list of VRanges; bam_tally in the gmapR package, which generates a VRanges.

Examples

## construction
vr <- VRanges(seqnames = c("chr1", "chr2"),
              ranges = IRanges(c(1, 10), c(5, 20)),
              ref = c("T", "A"), alt = c("C", "T"),
              refDepth = c(5, 10), altDepth = c(7, 6),
              totalDepth = c(12, 17), sampleNames = letters[1:2],
              hardFilters =
                FilterRules(list(coverage = function(x) totalDepth > 10)),
              softFilterMatrix =
                FilterMatrix(matrix = cbind(depth = c(TRUE, FALSE)),
                             FilterRules(depth = function(x) altDepth(x) > 6)),
              tumorSpecific = c(FALSE, TRUE))

## simple accessors
ref(vr)
alt(vr)
altDepth(vr)
vr$tumorSpecific
called(vr)

## coerce to VCF and write
vcf <- as(vr, "VCF")
## writeVcf(vcf, "example.vcf")
## or just
## writeVcf(vr, "example.vcf")

## other utilities
match(vr, vr[2:1])

Jiefei-Wang/VariantAnnotation documentation built on March 19, 2020, 12:03 a.m.