extractPatterns

This function extracts methylation patterns (epialleles) for a given genomic region of interest.

Usage

extractPatterns(
  bam,
  bed,
  bed.row = 1,
  zero.based.bed = FALSE,
  match.min.overlap = 1,
  extract.context = c("CG", "CHG", "CHH", "CxG", "CX"),
  min.context.freq = 0.01,
  clip.patterns = FALSE,
  strand.offset = c(CG = 1, CHG = 2, CHH = 0, CxG = 0, CX = 0)[extract.context],
  highlight.positions = c(),
  ...,
  verbose = TRUE
)

Arguments

bam

BAM file location string OR preprocessed output of preprocessBam function. Read more about BAM file requirements and BAM preprocessing at preprocessBam.

bed

Browser Extensible Data (BED) file location string OR object of class GRanges holding genomic coordinates for regions of interest. It is used to match sequencing reads to the genomic regions for pattern extraction. The style of seqlevels of BED file/object must match the style of seqlevels of the BAM file/object used. The BED/GRanges rows are not sorted internally. As of now, the strand information is ignored and patterns matching both strands are extracted.

bed.row

single non-negative integer specifying what `bed` region should be included in the output (default: 1).

zero.based.bed

boolean defining if BED coordinates are zero based (default: FALSE).

match.min.overlap

integer for the smallest overlap between read's and BED/GRanges start or end positions during matching of capture-based NGS reads (default: 1).

extract.context

string defining cytosine methylation context used to report:

• "CG" (the default) – CpG cytosines (called as zZ)

• "CHG" – CHG cytosines (xX)

• "CHH" – CHH cytosines (hH)

• "CxG" – CG and CHG cytosines (zZxX)

• "CX" – all cytosines

min.context.freq

real number in the range [0;1] (default: 0.01). Genomic positions that are covered by smaller fraction of patterns (e.g., with erroneous context) won't be included in the report.

clip.patterns

boolean if patterns should not extend over the edge of `bed` region (default: FALSE).

strand.offset

single non-negative integer specifying the offset of bases at the reverse (-) strand compared to the forward (+) strand. Allows to "merge" genomic positions when methylation is symmetric (in CG and CHG contexts). By default, equals 1 for `extract.context`=="CG", 2 for "CHG", or 0 otherwise.

highlight.positions

integer vector with genomic positions of bases to include in every overlapping pattern. Allows to visualize the distribution of single-nucleotide variations (SNVs) among methylation patterns. `highlight.positions` takes precedence if any of these positions overlap with within-the-context positions of methylation pattern.

...

other parameters to pass to the preprocessBam function. Options have no effect if preprocessed BAM data was supplied as an input.

verbose

boolean to report progress and timings (default: TRUE).

Value

data.table object containing per-read (pair) base methylation information for the genomic region of interest. The report columns are:

• seqnames – read (pair) reference sequence name

• strand – read (pair) strand

• start – start of the read (pair)

• end – end of the read (pair)

• nbase – number of within-the-context bases for this read (pair)

• beta – beta value of this read (pair), calculated as a ratio of the number of methylated within-the-context bases to the total number of within-the-context bases

• pattern – hex representation of 64-bit FNV-1a hash calculated for all reported base positions and bases in this read (pair). This hash value depends only on included genomic positions and their methylation call string chars (hHxXzZ) or nucleotides (ACGT, for highlighted bases only), thus it is expected to be unique for every methylation pattern, although equal for identical methylation patterns independently on read (pair) start, end, or strand (when correct `strand.offset` is given)

• ... – columns for each genomic position that hold corresponding methylation call string char, or NA if position is not present in the read (pair)

Details

The function matches reads (for paired-end sequencing alignment files - read pairs as a single entity) to the genomic region provided in a BED file/GRanges object, extracts methylation statuses of bases within those reads, and returns a data frame which can be used for further analysis and/or plotting of DNA methylation patterns by plotPatterns function.

See also

plotPatterns for pretty plotting of the output, preprocessBam for preloading BAM data, generateCytosineReport for methylation statistics at the level of individual cytosines, generateBedReport for genomic region-based statistics, generateVcfReport for evaluating epiallele-SNV associations, generateBedEcdf for analysing the distribution of per-read beta values, and `epialleleR` vignettes for the description of usage and sample data.

Examples

  # amplicon data
  amplicon.bam <- system.file("extdata", "amplicon010meth.bam",
                              package="epialleleR")
  amplicon.bed <- system.file("extdata", "amplicon.bed",
                              package="epialleleR")
  
  # extract patterns
  patterns <- extractPatterns(bam=amplicon.bam, bed=amplicon.bed, bed.row=3)
#> Reading BED file 
#> [0.030s]
#> Checking BAM file: 
#> short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file 
#> [0.008s]
#> Extracting methylation patterns 
#> [0.026s]
  
  # and then plot them
  plotPatterns(patterns)
#> 238 patterns supplied
#> 45 unique
#> 9 most frequent unique patterns were selected for plotting using 10 beta value bins:
#> [0,0.1) [0.1,0.2) [0.2,0.3) [0.3,0.4) [0.4,0.5) [0.5,0.6) [0.6,0.7) [0.7,0.8) [0.8,0.9) [0.9,1]
#>       2         1         1         0         0         0         0         1         2       2