r-bioc-variantannotation 1.24.2-1 / usr / lib / R / site-library / VariantAnnotation / doc / VariantAnnotation.Rnw

%\VignetteIndexEntry{1. Introduction to VariantAnnotation}

\documentclass{article}

<<style, eval=TRUE, echo=FALSE, results=tex>>=
BiocStyle::latex2()
@

\title{1. Introduction to \Biocpkg{VariantAnnotation}}
\author{Valerie Obenchain}
\date{\today}

\begin{document} 

\maketitle

\tableofcontents

<<options, echo=FALSE>>=
options(width=72)
@

\section{Introduction}
This vignette outlines a work flow for annotating and filtering 
genetic variants using the \Biocpkg{VariantAnnotation}  package. Sample data 
are in VariantCall Format (VCF) and are a subset of chromosome 22 
from 
\href{ftp://ftp-trace.ncbi.nih.gov/1000genomes/ftp/release/20110521/}{1000
Genomes}.
VCF text files contain meta-information lines, a header line with
column names, data lines with information about a position in the 
genome, and optional genotype information on samples for each
position. The 1000 Genomes page describes the 
\href{http://www.1000genomes.org/wiki/Analysis/Variant\%20Call\%20Format/vcf-variant-call-format-version-41}{VCF
format} in detail.

Data are read in from a VCF file and variants identified according 
to region such as \Rcode{coding}, \Rcode{intron}, \Rcode{intergenic}, 
\Rcode{spliceSite} etc. Amino acid coding changes are computed for the 
non-synonymous variants and SIFT and PolyPhen databases provide predictions 
of how severly the coding changes affect protein function.

\section{Variant Call Format (VCF) files}
\subsection{Data import and exploration}
Data are parsed into a \Robject{VCF} object with \Rfunction{readVcf}. 

<<readVcf>>=
library(VariantAnnotation)
fl <- system.file("extdata", "chr22.vcf.gz", package="VariantAnnotation")
vcf <- readVcf(fl, "hg19")
vcf
@

\subsubsection{Header information}
Header information can be extracted from the VCF with 
\Rfunction{header()}. We see there are 5 samples, 1 piece 
of meta information, 22 info fields and 3 geno fields. 
<<readVcf_showheader>>=
header(vcf)
@

Data can be further extracted using the named accessors.
<<headeraccessors>>=
samples(header(vcf))
geno(header(vcf))
@

\subsubsection{Genomic positions}
\Rcode{rowRanges} contains information from the CHROM, POS, and ID
fields of the VCF file, represented as a \Robject{GRanges}.
The \Rcode{paramRangeID} column is meaningful when reading
subsets of data and is discussed further below.
<<readVcf_rowRanges>>=
head(rowRanges(vcf), 3)
@

Individual fields can be pulled out with named accessors. Here
we see \Rcode{REF} is stored as a \Robject{DNAStringSet} and
\Rcode{qual} is a numeric vector. 
<<readVcf_fixed>>=
ref(vcf)[1:5]
qual(vcf)[1:5]
@

\Robject{ALT} is a \Robject{DNAStringSetList} (allows for
multiple alternate alleles per variant) or a \Rcode{DNAStringSet}.
When structural variants are present it will be a 
\Robject{CharacterList}. 
<<readVcf_ALT>>=
alt(vcf)[1:5]
@

\subsubsection{Genotype data}
Genotype data described in the \Rcode{FORMAT} fields are parsed 
into the geno slot. The data are unique to each sample and
each sample may have multiple values variable. Because of this, 
the data are parsed into matrices or arrays where the rows
represent the variants and the columns the samples. Multidimentional
arrays indicate multiple values per sample. In this file all 
variables are matrices.
<<geno_hdr>>=
geno(vcf)
sapply(geno(vcf), class)
@

Let's take a closer look at the genotype dosage (DS) variable. 
The header provides the variable definition and type.
<<explore_geno>>=
geno(header(vcf))["DS",]
@

These data are stored as a 10376 x 5 matrix. Each of the five
samples (columns) has a single value per variant location (row).
<<dim_geno>>=
DS <-geno(vcf)$DS
dim(DS)
DS[1:3,]
@

DS is also known as 'posterior mean genotypes' and range in value
from [0, 2]. To get a sense of variable distribution, we compute 
a five number summary of the minimum, lower-hinge (first quartile), 
median, upper-hinge (third quartile) and maximum.
<<fivenum>>=
fivenum(DS)
@

The majority of these values (86\%) are zero.
<<DS_zero>>=
length(which(DS==0))/length(DS)
@

View the distribution of the non-zero values.
<<DS_hist, fig=TRUE>>=
hist(DS[DS != 0], breaks=seq(0, 2, by=0.05), 
    main="DS non-zero values", xlab="DS")
@

\subsubsection{Info data}
In contrast to the genotype data, the info data are unique to 
the variant and the same across samples. All info variables
are represented in a single \Rcode{DataFrame}.
<<info>>=
info(vcf)[1:4, 1:5]
@

We will use the info data to compare quality measures between 
novel (i.e., not in dbSNP) and known (i.e., in dbSNP) variants 
and the variant type present in the file. Variants with 
membership in dbSNP can be identified by using the appropriate 
SNPlocs package for hg19.
<<examine_dbSNP>>=
library(SNPlocs.Hsapiens.dbSNP.20101109)
rd <- rowRanges(vcf)
seqlevels(rd) <- "ch22"
ch22snps <- getSNPlocs("ch22")
dbsnpchr22 <- sub("rs", "", names(rd)) %in% ch22snps$RefSNP_id
table(dbsnpchr22)
@

Info variables of interest are 'VT', 'LDAF' and 'RSQ'.
The header offers more details on these variables.
<<header_info>>=
info(header(vcf))[c("VT", "LDAF", "RSQ"),]
@

Create a data frame of quality measures of interest ...
<<examine_quality>>=
metrics <- data.frame(QUAL=qual(vcf), inDbSNP=dbsnpchr22,
    VT=info(vcf)$VT, LDAF=info(vcf)$LDAF, RSQ=info(vcf)$RSQ)
@

and visualize the distribution of qualities using \Rcode{ggplot2}.
For instance, genotype imputation quality is higher for the known
variants in dbSNP.
<<examine_ggplot2, fig=TRUE>>=
library(ggplot2)
ggplot(metrics, aes(x=RSQ, fill=inDbSNP)) +
    geom_density(alpha=0.5) +
    scale_x_continuous(name="MaCH / Thunder Imputation Quality") +
    scale_y_continuous(name="Density") +
    theme(legend.position="top")
@

\subsection{Import data subsets}
When working with large VCF files it may be more efficient to 
read in subsets of the data. This can be accomplished by
selecting genomic coordinates (ranges) or by specific fields
from the VCF file.

\subsubsection{Select genomic coordinates}
To read in a portion of chromosome 22, create a \Robject{GRanges} 
with the regions of interest.
<<subset_ranges>>=
rng <- GRanges(seqnames="22", ranges=IRanges(
           start=c(50301422, 50989541), 
           end=c(50312106, 51001328),
           names=c("gene_79087", "gene_644186")))
@

When ranges are specified, the VCF file must have an accompanying
Tabix index file. See ?\Rcode{indexTabix} for help creating an
index.
<<subset_TabixFile>>=
tab <- TabixFile(fl)
vcf_rng <- readVcf(tab, "hg19", param=rng)
@

The \Rcode{paramRangesID} column distinguishes which records came 
from which param range.
<<>>=
head(rowRanges(vcf_rng), 3)
@

\subsubsection{Select VCF fields}
Data import can also be defined by the \Rcode{fixed}, \Rcode{info}
and \Rcode{geno} fields. Fields available for import are
described in the header information. To view the header before
reading in the data, use \Robject{ScanVcfHeader}.
<<subset_scanVcfHeader>>=
hdr <- scanVcfHeader(fl)
## e.g., INFO and GENO fields
head(info(hdr), 3)
head(geno(hdr), 3)
@ 

To subset on "LDAF" and "GT" we specify them as \Rcode{character} 
vectors in the \Rcode{info} and \Rcode{geno} arguments to 
\Rcode{ScanVcfParam}. This creates a \Robject{ScanVcfParam} object 
which is used as the \Robject{param} argument to \Rfunction{readVcf}. 
<<subset_ScanVcfParam>>=
## Return all 'fixed' fields, "LAF" from 'info' and "GT" from 'geno'
svp <- ScanVcfParam(info="LDAF", geno="GT")
vcf1 <- readVcf(fl, "hg19", svp)
names(geno(vcf1))
@

To subset on both genomic coordinates and fields the
\Robject{ScanVcfParam} object must contain both. 
<<subset_ScanVcfParam_new>>=
svp_all <- ScanVcfParam(info="LDAF", geno="GT", which=rng) 
svp_all
@


\section{Locating variants in and around genes}
Variant location with respect to genes can be identified with the
\Rfunction{locateVariants} function. Regions are specified in the 
\Rcode{region} argument and can be one of the following constructors: 
CodingVariants, IntronVariants, FiveUTRVariants, ThreeUTRVariants, 
IntergenicVariants, SpliceSiteVariants or PromoterVariants. Location 
definitions are shown in Table \ref{table:location}. 

\begin{table}[h!]
\begin{center}
\begin{tabular}{l|l}
\hline
Location & Details \\
\hline
coding &  falls \emph{within} a coding region \\
fiveUTR &  falls \emph{within} a 5' untranslated region \\
threeUTR &  falls \emph{within} a 3' untranslated region \\
intron &  falls \emph{within} an intron region \\
intergenic & does not fall \emph{within} a transcript associated with a gene \\
spliceSite & overlaps any portion of the first 2 or last 2 
nucleotides of an intron \\ 
promoter & falls \emph{within} a promoter region of a transcript \\
\hline
\end{tabular}
\end{center}
\caption{Variant locations}
\label{table:location}
\end{table}

For overlap methods to work properly the chromosome names (seqlevels)
must be compatible in the objects being compared. The VCF data
chromosome names are represented by number, i.e., '22', but the TxDb
chromosome names are preceded with 'chr'. Seqlevels in the VCF can
be modified with the \Rfunction{seqlevels} function.
<<locate_rename_seqlevels>>=
library(TxDb.Hsapiens.UCSC.hg19.knownGene)
txdb <- TxDb.Hsapiens.UCSC.hg19.knownGene
seqlevels(vcf) <- "chr22"
rd <- rowRanges(vcf)
loc <- locateVariants(rd, txdb, CodingVariants())
head(loc, 3)
@ 

Locate variants in all regions with the \Rcode{AllVariants()} constructor,
<<AllVariants, eval=FALSE>>=
allvar <- locateVariants(rd, txdb, AllVariants())
@

To answer gene-centric questions data can be summarized by gene reguardless
of transcript. 
<<locate_gene_centric>>=
## Did any coding variants match more than one gene?
splt <- split(mcols(loc)$GENEID, mcols(loc)$QUERYID) 
table(sapply(splt, function(x) length(unique(x)) > 1))

## Summarize the number of coding variants by gene ID.
splt <- split(mcols(loc)$QUERYID, mcols(loc)$GENEID)
head(sapply(splt, function(x) length(unique(x))), 3)
@

\section{Amino acid coding changes}

\Rfunction{predictCoding} computes amino acid coding changes for non-synonymous 
variants. Only ranges in \Rcode{query} that overlap with a coding region in 
the \Rcode{subject} are considered. Reference sequences are retrieved from 
either a \Robject{BSgenome} or fasta file specified in \Rcode{seqSource}. 
Variant sequences are constructed by substituting, inserting or deleting values 
in the \Robject{varAllele} column into the reference sequence. Amino acid codes 
are computed for the variant codon sequence when the length is a multiple of 3. 

The \Robject{query} argument to \Rfunction{predictCoding} can be a 
\Robject{GRanges} or \Robject{VCF}. When a \Robject{GRanges} is supplied 
the \Rcode{varAllele} argument must be specified. In the case of a 
\Robject{VCF}, the alternate alleles are taken from \Robject{alt(<VCF>)} and 
the \Robject{varAllele} argument is not specified.

The result is a modified \Rcode{query} containing only variants that fall 
within coding regions. Each row represents a variant-transcript match so more 
than one row per original variant is possible.

<<predictCoding>>=
library(BSgenome.Hsapiens.UCSC.hg19)
coding <- predictCoding(vcf, txdb, seqSource=Hsapiens)
coding[5:7]
@

Using variant rs114264124 as an example, we see varAllele \Rcode{A} 
has been substituted into the \Rcode{refCodon} \Rcode{CGG} to produce 
\Rcode{varCodon} \Rcode{CAG}. The \Rcode{refCodon} is the sequence of 
codons necessary to make the variant allele substitution and therefore 
often includes more nucleotides than indicated in the range (i.e. the 
range is 50302962, 50302962, width of 1). Notice it is the second position 
in the \Rcode{refCodon} that has been substituted. This position in the 
codon, the position of substitution, corresponds to genomic position 
50302962. This genomic position maps to position 698 in coding region-based 
coordinates and to triplet 233 in the protein. This is a non-synonymous 
coding variant where the amino acid has changed from \Rcode{R} (Arg) 
to \Rcode{Q} (Gln).

When the resulting \Rcode{varCodon} is not a multiple of 3 it cannot be 
translated. The consequence is considered a \Rcode{frameshift} and 
\Robject{varAA} will be missing. 

<<predictCoding_frameshift>>=
## CONSEQUENCE is 'frameshift' where translation is not possible
coding[mcols(coding)$CONSEQUENCE == "frameshift"]
@

\section{SIFT and PolyPhen Databases}

From \Rfunction{predictCoding} we identified the amino acid coding changes for 
the non-synonymous variants. For this subset we can retrieve predictions of how 
damaging these coding changes may be. SIFT (Sorting Intolerant From 
Tolerant) and PolyPhen (Polymorphism Phenotyping) are methods that predict the 
impact of amino acid substitution on a human protein. The SIFT method uses 
sequence homology and the physical properties of amino acids to make predictions 
about protein function. PolyPhen uses sequence-based features and structural 
information characterizing the substitution to make predictions about the 
structure and function of the protein. 

Collated predictions for specific dbSNP builds are available as downloads from 
the SIFT and PolyPhen web sites. These results have been packaged into
\Rpackage{SIFT.Hsapiens.dbSNP132.db} and \Rpackage{PolyPhen.Hapiens.dbSNP131.db} 
and are designed to be searched by rsid. Variants that are in dbSNP can be 
searched with these database packages. When working with novel variants, SIFT
and PolyPhen must be called directly. See references for home pages.

Identify the non-synonymous variants and obtain the rsids.
<<nonsynonymous>>=
nms <- names(coding)
idx <- mcols(coding)$CONSEQUENCE == "nonsynonymous"
nonsyn <- coding[idx]
names(nonsyn) <- nms[idx]
rsids <- unique(names(nonsyn)[grep("rs", names(nonsyn), fixed=TRUE)])
@

Detailed descriptions of the database columns can be found with
\Rcode{?SIFTDbColumns} and \Rcode{?PolyPhenDbColumns}. Variants in these
databases often contain more than one row per variant. The variant may have
been reported by multiple sources and therefore the source will differ as well
as some of the other variables. 

It is important to keep in mind the pre-computed predictions in the
SIFT and PolyPhen packages are based on specific gene models. SIFT is
based on Ensembl and PolyPhen on UCSC Known Gene. The \Rcode{TxDb}
we used to identify the coding snps was based on UCSC Known Gene so we
will use PolyPhen for predictions. PolyPhen provides predictions using two 
different training datasets and has considerable information about 3D protein 
structure. See \Rcode{?PolyPhenDbColumns} or the PolyPhen web site listed in 
the references for more details.

Query the PolyPhen database,
<<polyphen>>=
library(PolyPhen.Hsapiens.dbSNP131)

pp <- select(PolyPhen.Hsapiens.dbSNP131, keys=rsids,
          cols=c("TRAININGSET", "PREDICTION", "PPH2PROB"))
head(pp[!is.na(pp$PREDICTION), ])
@

\section{Other operations}
\subsection{Create a SnpMatrix}

The 'GT' element in the \Rcode{FORMAT} field of the VCF represents the 
genotype. These data can be converted into a \Robject{SnpMatrix} object 
which can then be used with the functions offered in \Rpackage{snpStats} 
and other packages making use of the \Robject{SnpMatrix} class. 

The \Rfunction{genotypeToSnpMatrix} function converts the genotype calls in
\Rcode{geno} to a \Robject{SnpMatrix}. No dbSNP package is used in this
computation. The return value is a named list where 'genotypes' is a 
\Robject{SnpMatrix} and 'map' is a \Robject{DataFrame} with SNP names and 
alleles at each loci. The \Rcode{ignore} column in 'map' indicates which 
variants were set to NA (missing) because they met one or more of the following 
criteria,
\begin{itemize}
\item{variants with >1 ALT allele are set to NA}
\item{only single nucleotide variants are included; others are set to NA}
\item{only diploid calls are included; others are set to NA}
\end{itemize}

See ?\Rfunction{genotypeToSnpMatrix} for more details.

<<snpMatrix>>=
res <- genotypeToSnpMatrix(vcf)
res
@

In the map \Rcode{DataFrame}, allele.1 represents the reference allele
and allele.2 is the alternate allele.
<<snpMatrix_ALT>>=
allele2 <- res$map[["allele.2"]]
## number of alternate alleles per variant
unique(elementNROWS(allele2))
@

In addition to the called genotypes, genotype likelihoods or
probabilities can also be converted to a \Robject{SnpMatrix}, using
the \Rpackage{snpStats} encoding of posterior probabilities as byte
values.  To use the values in the 'GL' or 'GP' \Rcode{FORMAT} field
instead of the called genotypes, use the \Rcode{uncertain=TRUE} option
in \Rcode{genotypeToSnpMatrix}.

<<>>=
fl.gl <- system.file("extdata", "gl_chr1.vcf", package="VariantAnnotation") 
vcf.gl <- readVcf(fl.gl, "hg19")
geno(vcf.gl)

## Convert the "GL" FORMAT field to a SnpMatrix
res <- genotypeToSnpMatrix(vcf.gl, uncertain=TRUE)
res
t(as(res$genotype, "character"))[c(1,3,7), 1:5]

## Compare to a SnpMatrix created from the "GT" field
res.gt <- genotypeToSnpMatrix(vcf.gl, uncertain=FALSE)
t(as(res.gt$genotype, "character"))[c(1,3,7), 1:5]

## What are the original likelihoods for rs58108140?
geno(vcf.gl)$GL["rs58108140", 1:5]
@ 

For variant rs58108140 in sample NA06989, the "A/B" genotype is much
more likely than the others, so the \Rcode{SnpMatrix} object displays
the called genotype.

\subsection{Write out VCF files}

A VCF file can be written out from data stored in a \Rcode{VCF} class.

<<writeVcf>>=
fl <- system.file("extdata", "ex2.vcf", package="VariantAnnotation")

out1.vcf <- tempfile()
out2.vcf <- tempfile()
in1 <- readVcf(fl, "hg19")
writeVcf(in1, out1.vcf)
in2 <- readVcf(out1.vcf, "hg19")
writeVcf(in2, out2.vcf)
in3 <- readVcf(out2.vcf, "hg19")

identical(rowRanges(in1), rowRanges(in3))
identical(geno(in1), geno(in2))
@

\section{Performance}

Targeted queries can greatly improve the speed of data input.
When all data from the file are needed define a \Rcode{yieldSize} 
in the \Rcode{TabixFile} to iterate through the file in chunks.

\begin{verbatim}
    readVcf(TabixFile(fl, yieldSize=10000))
\end{verbatim}

\Rfunction{readVcf} can be used with a \Rcode{ScanVcfParam} to
select any combination of INFO and GENO fields, samples or genomic 
positions.

\begin{verbatim}
    readVcf(TabixFile(fl), param=ScanVcfParam(info='DP', geno='GT'))
\end{verbatim}

While \Rfunction{readvcf} offers the flexibility to define combinations
of INFO, GENO and samples in the \Rcode{ScanVcfParam}, sometimes
only a single field is needed. In this case the lightweight \Rcode{read} 
functions (\Rfunction{readGT}, \Rfunction{readInfo} and 
\Rfunction{readGeno}) can be used. These functions return the single field 
as a matrix instead of a \Rcode{VCF} object.

\begin{verbatim}
    readGT(fl)
\end{verbatim}

The table below highlights the speed differences of targeted queries
vs reading in all data. The test file is from 1000 Genomes and has 
494328 variants, 1092 samples, 22 INFO, and 3 GENO fields and is located at
\url{ftp://ftp-trace.ncbi.nih.gov/1000genomes/ftp/release/20101123/}.
\Rcode{yieldSize} is used to define chunks of 100, 1000, 10000 and 100000
variants. For each chunk size three function calls are compared:
\Rfunction{readGT} reading only \Rcode{GT}, \Rfunction{readVcf} reading 
both \Rcode{GT} and \Rcode{ALT} and finally \Rfunction{readVcf} reading 
in all the data.

\begin{verbatim}
library(microbenchmark)
fl <- "ALL.chr22.phase1_release_v3.20101123.snps_indels_svs.genotypes.vcf.gz"
ys <- c(100, 1000, 10000, 100000)

## readGT() input only 'GT':
fun <- function(fl, yieldSize) readGT(TabixFile(fl, yieldSize))
lapply(ys, function(i) microbenchmark(fun(fl, i), times=5))

## readVcf() input only 'GT' and 'ALT':
fun <- function(fl, yieldSize, param) 
           readVcf(TabixFile(fl, yieldSize), "hg19", param=param)
param <- ScanVcfParam(info=NA, geno="GT", fixed="ALT")
lapply(ys, function(i) microbenchmark(fun(fl, i, param), times=5))

## readVcf() input all variables:
fun <- function(fl, yieldSize) readVcf(TabixFile(fl, yieldSize), "hg19")
lapply(ys, function(i) microbenchmark(fun(fl, i), times=5))
\end{verbatim}

\begin{table}[h!]
\begin{center}
\begin{tabular}{l|l|l|l}
\hline
n records & readGT & readVcf (GT and ALT) & readVcf (all) \\
\hline
100    &  0.082  &   0.128  &  0.501  \\
1000   &  0.609  &   0.508  &  5.878  \\
10000  &  5.972  &   6.164  & 68.378  \\
100000 & 78.593  &  81.156  & 693.654 \\
\hline
\end{tabular}
\end{center}
\caption{Targeted queries (time in seconds)}
\label{table:performance}
\end{table}


\section{References}
Wang K, Li M, Hakonarson H, (2010), ANNOVAR: functional annotation of genetic 
variants from high-throughput sequencing data. Nucleic Acids Research, Vol 38,
No. 16, e164.\\

\noindent McLaren W, Pritchard B, RiosD, et. al., (2010), Deriving the consequences of
genomic variants with the Ensembl API and SNP Effect Predictor. Bioinformatics,
Vol. 26, No. 16, 2069-2070.\\

\noindent SIFT home page :
\url{http://sift.bii.a-star.edu.sg/}\\

\noindent PolyPhen home page :
\url{http://genetics.bwh.harvard.edu/pph2/}

\section{Session Information}
<<sessionInfo, echo=FALSE>>=
sessionInfo()
@

\end{document}