callVariants {VariantTools} | R Documentation

## Call Variants

### Description

Calls variants from either a BAM file or a `VRanges` object. The variants are called using a binomial likelihood ratio test. Those calls are then subjected to a post-filtering step.

### Usage

    ## S4 method for signature 'BamFile'
    callVariants(x, tally.param,
                                     calling.filters = VariantCallingFilters(...),
                                     post.filters = FilterRules(),
                                     ...)
    ## S4 method for signature 'character'
    callVariants(x, ...)
    ## S4 method for signature 'VRanges'
    callVariants(x,
         calling.filters = VariantCallingFilters(...),
         post.filters = FilterRules(),
         ...)
    VariantCallingFilters(read.count = 2L, p.lower = 0.2, p.error = 1/1000)

### Arguments

`x` | Either a path to an indexed bam, a `BamFile` object, or a `VRanges` as returned by `tallyVariants`.
`tally.param` | Parameters controlling the variant tallying step, as typically constructed by `TallyVariantsParam`.
`calling.filters` | Filters used in the calling step, typically constructed with `VariantCallingFilters`, see arguments listed below.
`post.filters` | Filters that are applied after the initial calling step. These consider the set of variant calls as a whole and remove those with suspicious patterns.

`...`Arguments for `VariantCallingFilters`, listed below.

`read.count` | Require at least this many high quality reads with the alternate base. The default value is designed to catch sequencing errors where coverage is too low to rely on the LRT. Increasing this value has a significant negative impact on power.
`p.lower` | The lower bound on the binomial probability for a true variant.
`p.error` | The binomial probability for a sequencing error (default is reasonable for Illumina data with the default quality cutoff).
`...` | Arguments to pass to `VariantCallingFilters`.

### Details

There are two steps for calling variants: the actual statistical test that decides whether a variant exists in the data, and a post-filtering step. By default, the initial calling is based on a binomial likelihood ratio test (P(D|p=`p.lower`) / P(D|p=`p.error`) > 1). The test amounts to excluding putative variants with less than ~4% alt frequency. A variant is also required to be represented by at least 2 alt reads. The post-filtering stage considers the set of variant calls as a whole and removes variants with suspicious patterns. Currently, there is a single post-filter, disabled by default, that removes variants that are clumped together on the chromosome (see the `max.nbor.count` parameter).

### Value

For `callVariants`, a `VRanges` of the called variants (the tallies that pass the calling filters). See the documentation of `bam_tally` for complete details.

For `VariantCallingFilters`, a `FilterRules` object with the filters for calling the variants.

### Author(s)

Michael Lawrence, Jeremiah Degenhardt

### Examples

    bams <- LungCancerLines::LungCancerBamFiles()
    if (requireNamespace("gmapR")) {
        tally.param <- TallyVariantsParam(gmapR::TP53Genome(),
                                          high_base_quality = 23L,
                                          which = gmapR::TP53Which())

        ## simple usage
        variants <- callVariants(bams$H1993, tally.param)
    }
    ## customize
    data(vignette)
    calling.filters <- VariantCallingFilters(p.error = 1/1000)
    callVariants(tallies_H1993, calling.filters)

[Package VariantTools version 1.26.0 Index]