StructuralVariantAnnotation Quick Overview"

In StructuralVariantAnnotation: Variant annotations for structural variants

knitr::opts_chunk$set(#echo = TRUE,
  collapse = TRUE,
  comment = "#>")

Introduction

This vignette outlines a workflow of parsing and plotting structural variants from Variant Call Format (VCF) using the StructuralVariantAnnotation package. StructuralVariantAnnotation contains useful helper functions for reading and interpreting structural variant calls. The package contains functions for parsing VCFs from a number of popular callers as well as functions for dealing with breakpoints involving two separate genomic loci encoded as GRanges objects.

Installation

The StructuralVariationAnnotation package can be installed from Bioconductor as follows:

if (!requireNamespace("BiocManager", quietly = TRUE))
    install.packages("BiocManager")
BiocManager::install("StructuralVariantAnnotation")

Breakpoint notation

The VCF standard describes two types of SV notations. One is by SV types, i.e. insertions, deletions, inversions, translocations, etc. The other is by breakend notations, often labelled with SVTYPE=BND. To describe a SV with breakend notations, each SV has two positions, each captured by one breakend (except for inversions, which have 4 separate records). Each breakend includes a genomic locus, as well as a half interval extending out to the partner breakend. In VCF BND notations, the ALT field encodes directional information of the partner breakend.

• 1 200 . N N[5:500[ partner breakend immediately after chr1:200, starting from chr5:500 and extending rightwards
• 1 200 . N ]5:500]N partner breakend immediately before chr1:200, extending from the left and ending at chr5:500
• 1 200 . N [5:500[N partner breakend immediately before chr1:200, starting from chr5:500 and extending rightwards
• 1 200 . N N]5:500] partner breakend immediately after chr1:200, extending from the left and ending at chr5:500

Using GRanges for structural variants: a breakend-centric data structure

Unlike breakpoint-centric data structures such as the Pairs object that rtracklayer uses to load BEDPE files, this package uses a breakend-centric notation. Breakends are stored in a GRanges object with strand used to indicate breakpoint orientation. Consistent with how breakends are encoded in VCF + indicates that the breakpoint occurs immediately after the given position, with - indicating the breakpoint occurs immediately before the given position. Breakpoints are represented using a partner field containing the name of the breakend at the other side of the breakend. Both single breakends and breakpoints are supported but many-to-many breakend partner mappings supported by the VCF MATEID field are not: each breakend must have 0 (single breakend) or 1 (breakpoint) partner breakends.

This notation was chosen as it simplifies many common operations, and annotations are breakend-level annotations. These include annotation associated with genomic positions (e.g. genes, repeats, mappability), as well as breakend-level attributes of a breakpoint such as variant allele fractions (e.g. a structural variant can be homozygous at one breakend, but heterzygous at the other breakend).

Workflow

Loading data from VCF

VCF data is parsed into a VCF object using the readVCF function from the Bioconductor package VariantAnnotation. Simple filters could be applied to a VCF object to remove unwanted calls. More information about VCF objects can be found by consulting the vignettes in the VariantAnnotation package with browseVignettes("VariantAnnotation").

StructuralVariantAnnotation supports structural variants reported in the following VCF notations:

• Non-symbolic allele
• Symbolic allele with SVTYPE of DEL, INS, and DUP.
• Breakpoint notation SVTYPE=BND
• Single breakend notation

In addition to parsing spec-compliant VCFs, additional logic has been added to enable parsing of non-compliant variants for the following callers:

• Pindel (SVTYPE=RPL)
• manta (INv3, INV5 fields)
• Delly (SVTYPE=TRA, CHR2, CT fields)
• TIGRA (SVTYPE=CTX)

Breakpoint ambiguity reported using the spec-defined CIPOS is, by default, incorporated into the GRanges breakend intervals.

suppressPackageStartupMessages(require(StructuralVariantAnnotation))
suppressPackageStartupMessages(require(VariantAnnotation))
vcf.file <- system.file("extdata", "gridss.vcf", package = "StructuralVariantAnnotation")
vcf <- VariantAnnotation::readVcf(vcf.file, "hg19")
gr <- breakpointRanges(vcf)

partner() returns the breakpoint GRanges object with the order rearranged such that the partner breakend on the other side of each breakpoint corresponds with the local breakend.

partner(gr)

Single breakends are loaded using the breakendRanges() function. The GRanges object is of the same form as breakpointRanges() but as the breakend partner is not specified, the partner is NA. A single GRanges object can contain both breakend and breakpoint variants.

colo829_vcf <- VariantAnnotation::readVcf(system.file("extdata", "COLO829T.purple.sv.ann.vcf.gz", package = "StructuralVariantAnnotation"))
colo829_bpgr <- breakpointRanges(colo829_vcf)
colo829_begr <- breakendRanges(colo829_vcf)
colo829_gr <- sort(c(colo829_begr, colo829_bpgr))
colo829_gr[seqnames(colo829_gr) == "6"]

Ensuring breakpoint consistency

Functions such as findBreakpointOverlaps() require the GRanges object to be composed entirely of valid breakpoints. Subsetting a breakpoint GRanges object can result in one side of a breakpoint getting filtered with the remaining orphaned record no longer valid as its partner no longer exists. Such record can be filtered

colo828_chr6_breakpoints <- colo829_gr[seqnames(colo829_gr) == "6"]
# A call to findBreakpointOverlaps(colo828_chr6_breakpoints, colo828_chr6_breakpoints)
# will fail as there are a) single breakends, and b) breakpoints with missing partners
colo828_chr6_breakpoints <- colo828_chr6_breakpoints[colo828_chr6_breakpoints$partner %in% names(colo828_chr6_breakpoints)]
# As expected, each call on chr6 only overlaps with itself
countBreakpointOverlaps(colo828_chr6_breakpoints, colo828_chr6_breakpoints)

Note that if you did want to include inter-chromosomal breakpoints involving chromosome 6, you would need to update the filtering criteria to include records with chr6 on either side. In such cases, the filtering logic can be simplified by the selfPartnerSingleBreakends parameter of partner(). When selfPartnerSingleBreakends=TRUE, the partner of single breakend events is considered to be the single breakend itself.

colo828_chr6_breakpoints <- colo829_gr[
  seqnames(colo829_gr) == "6" |
    seqnames(partner(colo829_gr, selfPartnerSingleBreakends=TRUE)) == "6"]
# this way we keep the chr3<->chr6 breakpoint and don't create any orphans
head(colo828_chr6_breakpoints, 1)

Breakpoint Overlaps

findBreakpointOverlaps() and countBreakpointOverlaps() are functions for finding and counting overlaps between breakpoint objects. All breakends must have their partner breakend included in the GRanges. A valid overlap requires that breakends on boths sides overlap.

To demonstrate the countBreakpointOverlaps() function, we use a small subset of data from our structural variant caller benchmarking paper to construct precision recall curves for a pair of callers.

truth_vcf <- readVcf(system.file("extdata", "na12878_chr22_Sudmunt2015.vcf", package = "StructuralVariantAnnotation"))
truth_svgr <- breakpointRanges(truth_vcf)
truth_svgr <- truth_svgr[seqnames(truth_svgr) == "chr22"]
crest_vcf <- readVcf(system.file("extdata", "na12878_chr22_crest.vcf", package = "StructuralVariantAnnotation"))
# Some SV callers don't report QUAL so we need to use a proxy
VariantAnnotation::fixed(crest_vcf)$QUAL <- info(crest_vcf)$left_softclipped_read_count + info(crest_vcf)$left_softclipped_read_count
crest_svgr <- breakpointRanges(crest_vcf)
crest_svgr$caller <- "crest"
hydra_vcf <- readVcf(system.file("extdata", "na12878_chr22_hydra.vcf", package = "StructuralVariantAnnotation"))
hydra_svgr <- breakpointRanges(hydra_vcf)
hydra_svgr$caller <- "hydra"
svgr <- c(crest_svgr, hydra_svgr)
svgr$truth_matches <- countBreakpointOverlaps(svgr, truth_svgr,
  # read pair based callers make imprecise calls.
  # A margin around the call position is required when matching with the truth set
  maxgap=100,
  # Since we added a maxgap, we also need to restrict the mismatch between the
  # size of the events. We don't want to match a 100bp deletion with a 
  # 5bp duplication. This will happen if we have a 100bp margin but don't also
  # require an approximate size match as well
  sizemargin=0.25,
  # We also don't want to match a 20bp deletion with a 20bp deletion 80bp away
  # by restricting the margin based on the size of the event, we can make sure
  # that simple events actually do overlap
  restrictMarginToSizeMultiple=0.5,
  # HYDRA makes duplicate calls and will sometimes report a variant multiple
  # times with slightly different bounds. countOnlyBest prevents these being
  # double-counted as multiple true positives.
  countOnlyBest=TRUE)

Once we know which calls match the truth set, we can generate Precision-Recall and ROC curves for each caller using one of the many ROC R packages, or directly with dplyr.

suppressPackageStartupMessages(require(dplyr))
suppressPackageStartupMessages(require(ggplot2))
ggplot(as.data.frame(svgr) %>%
  dplyr::select(QUAL, caller, truth_matches) %>%
  dplyr::group_by(caller, QUAL) %>%
  dplyr::summarise(
    calls=dplyr::n(),
    tp=sum(truth_matches > 0)) %>%
  dplyr::group_by(caller) %>%
  dplyr::arrange(dplyr::desc(QUAL)) %>%
  dplyr::mutate(
    cum_tp=cumsum(tp),
    cum_n=cumsum(calls),
    cum_fp=cum_n - cum_tp,
    Precision=cum_tp / cum_n,
    Recall=cum_tp/length(truth_svgr))) +
  aes(x=Recall, y=Precision, colour=caller) +
  geom_point() +
  geom_line() +
  labs(title="NA12878 chr22 CREST and HYDRA\nSudmunt 2015 truth set")

Converting between BEDPE, Pairs, and breakpoint GRanges

The package supports converting GRanges objects to BEDPE files. The BEDPE format is defined by bedtools. This is achieved using breakpointgr2pairs and pairs2breakpointgr functions to convert to and from the GRanges Pairs notation used by rtracklayer

suppressPackageStartupMessages(require(rtracklayer))
# Export to BEDPE
rtracklayer::export(breakpointgr2pairs(gr), con="gridss.bedpe")

# Import to BEDPE
bedpe.gr  <- pairs2breakpointgr(rtracklayer::import("gridss.bedpe"))

Visualising breakpoint pairs via circos plots

One way of visualising paired breakpoints is by circos plots. Here we use the package circlize to demonstrate breakpoint visualisation. The bedpe2circos function takes BEDPE-formatted dataframes (see breakpointgr2bedpe()) and plotting parameters for the circos.initializeWithIdeogram() and circos.genomicLink() functions from circlize.

To generate a simple circos plot of paired breakpoints:

suppressPackageStartupMessages(require(circlize))
colo829_bpgr_with_chr_prefix <- colo829_bpgr
seqlevelsStyle(colo829_bpgr_with_chr_prefix) <- "UCSC"
pairs <- breakpointgr2pairs(colo829_bpgr_with_chr_prefix)
circos.initializeWithIdeogram()
circos.genomicLink(as.data.frame(S4Vectors::first(pairs)), as.data.frame(S4Vectors::second(pairs)))
circos.clear()

Alternatively, the plotting package ggbio provides flexible track functions which bind with ggplot2 objects. It takes GRanges objects as input and supports circos plots. To plot structural variant breakpoints in a circos plot using ggbio, we need to first prepare the breakpoint GRanges. The function requires a special column, indicating the end of the link using GRanges format, which we can add to gr using plyranges.

suppressPackageStartupMessages(require(ggbio))
gr.circos <- colo829_bpgr[seqnames(colo829_bpgr) %in% seqlevels(biovizBase::hg19sub)]
seqlevels(gr.circos) <- seqlevels(biovizBase::hg19sub)
mcols(gr.circos)$to.gr <- granges(partner(gr.circos))

We can then plot the breakpoints against reference genomes.

p <- ggbio() +
    circle(gr.circos, geom="link", linked.to="to.gr") +
    circle(biovizBase::hg19sub, geom='ideo', fill='gray70') +
    circle(biovizBase::hg19sub, geom='scale', size=2) +
    circle(biovizBase::hg19sub, geom='text', aes(label=seqnames), vjust=0, size=3)
p

SessionInfo

sessionInfo()

StructuralVariantAnnotation documentation built on Nov. 8, 2020, 5:43 p.m.