estimateOffTargets {TAPseq} | R Documentation

## Estimate primer off-targets using BLAST

### Description

Functions to use BLAST to align TAP-seq primers against a genome and chromosome reference to estimate potential off-target binding sites.

### Usage

    createBLASTDb(
      genome,
      annot,
      blastdb,
      standard_chromosomes = TRUE,
      tx_id = "transcript_id",
      tx_name = "transcript_name",
      gene_name = "gene_name",
      gene_id = "gene_id",
      title = "TAP-seq_GT_DB",
      verbose = FALSE,
      makeblastdb = getOption("TAPseq.makeblastdb")
    )

    blastPrimers(
      object,
      blastdb,
      max_mismatch = 0,
      min_aligned = 0.75,
      primer_targets = c("transcript_id", "transcript_name", "gene_id", "gene_name"),
      tmpdir = tempdir(),
      blastn = getOption("TAPseq.blastn")
    )

    ## S4 method for signature 'TsIO'
    blastPrimers(
      object,
      blastdb,
      max_mismatch = 0,
      min_aligned = 0.75,
      primer_targets = c("transcript_id", "transcript_name", "gene_id", "gene_name"),
      tmpdir = tempdir(),
      blastn = getOption("TAPseq.blastn")
    )

    ## S4 method for signature 'TsIOList'
    blastPrimers(
      object,
      blastdb,
      max_mismatch = 0,
      min_aligned = 0.75,
      primer_targets = c("transcript_id", "transcript_name", "gene_id", "gene_name"),
      tmpdir = tempdir(),
      blastn = getOption("TAPseq.blastn")
    )

### Arguments

`genome` | A `BSgenome` (or `DNAStringSet`) object containing the sequences of all chromosomes to obtain genome and transcript sequences.
`annot` | A `GRanges` object containing all exons of transcripts to be considered.
`blastdb` | TAP-seq BLAST database created with `createBLASTDb`.
`standard_chromosomes` | (logical) Specifies whether only standard chromosomes should be included in output genome sequences (e.g. chr1-22, chrX, chrY, chrM for homo sapiens).
`tx_id, tx_name, gene_name, gene_id` | (character) Column names in annot metadata containing transcript id, transcript name, gene name and gene id information.
`title` | Optional title for BLAST database.
`verbose` | (logical) If `TRUE`, additional information from `makeblastdb` is printed to the console. Default: `FALSE`.
`makeblastdb` | Path to the `makeblastdb` executable. Usually this is inferred when loading/attaching the package.
`object` | A `TsIO` or `TsIOList` object containing designed primers.
`max_mismatch` | Maximum number of mismatches allowed for off-target hits (default: 0).
`min_aligned` | Minimum portion of the primer sequence starting from the 3' end that must align for off-target hits (default: 0.75).
`primer_targets` | Specifies what should be used to identify primer targets for off-target identification. I.e. to what does the `sequence_id` in TsIO objects refer? Can be a subset of `transcript_id`, `transcript_name`, `gene_id` or `gene_name`. By default all 4 are checked. Set to `NULL` to disable any off-target identification. See Details for more information.
`tmpdir` | Directory needed to store temporary files.
`blastn` | Path (character) to the `blastn` executable. Usually this is inferred when loading/attaching the package.

### Details

`createBLASTDb` creates a BLAST database containing genome and transcriptome sequences, which is required by `blastPrimers`. The created database contains both sequence files for BLAST and annotations to process the results.

Use `blastPrimers` to align designed TAP-seq primers against the created database to estimate off-target priming potential. Only hits where at least a specified portion of the sequence involving the 3' end of the primer aligns with not more than a certain number of mismatches are considered.

`blastPrimers` counts the number of genes in which a primer has 1) exonic hits or 2) intronic hits, or 3) the number of hits in intergenic regions of the genome. The exonic and intronic counts should be interptreted as: "In how many genes does a primer have exonic (or intronic) hits?".

If a BLAST hit falls in both intronic and exonic regions of a given gene (i.e. exonic for one transcript, intronic for another transcript), only the exonic hit is counted for that gene. If a primer has for instance 3 BLAST hits in one gene, 2 exonic and 1 intronic, then one exonic hit and one intronic hit is counted for that gene.

If sequence IDs of the designed primers (`sequence_id`) refer to the target gene/transcripts and can be found in the BLAST database annotations via `primer_targets`, then only off-target hits are counted. This is usually the case if input for primer design was produced from target gene annotations.

### Value

For `createBLASTDb` a directory containing the BLAST database. For `blastPrimers` a `TsIO` or `TsIOList` object with the number of potential off-targets added to the TAP-seq primer metadata.

### Functions

- `createBLASTDb`: Create a genome and transcriptome TAP-seq BLAST database

- `blastPrimers,TsIO-method`: BLAST primers in a `TsIO` object

- `blastPrimers,TsIOList-method`: BLAST primers in a `TsIOList` object

### Examples

    ## Not run:
    library(BSgenome)

    # human genome (hg38) BSgenome object
    hg38 <- getBSgenome("BSgenome.Hsapiens.UCSC.hg38")

    # get annotations for BLAST
    annot_url <- paste0("ftp://ftp.ebi.ac.uk/pub/databases/gencode/Gencode_human/release_32/",
                        "gencode.v32.annotation.gtf.gz")
    annot <- import(annot_url, format = "gtf")
    blast_exons <- annot[annot$type == "exon" & annot$gene_type == "protein_coding"]

    # build BLAST database
    blastdb <- file.path(tempdir(), "blastdb")
    createBLASTDb(genome = hg38, annot = blast_exons, blastdb = blastdb)

    # chr11 primers example data (already contains off-targets, but we can overwrite them)
    data("chr11_primers")
    chr11_primers <- chr11_primers[1:3]  # only use a small subset for this example

    # run blast to identify potential off-targets
    chr11_primers <- blastPrimers(chr11_primers, blastdb = blastdb)
    tapseq_primers(chr11_primers)

    # allow 1 mismatch between primer and off-target
    chr11_primers <- blastPrimers(chr11_primers, blastdb = blastdb, max_mismatch = 1)
    tapseq_primers(chr11_primers)

    ## End(Not run)

[Package TAPseq version 1.4.0 Index]