The epialleleR User’s Guide

Introduction

Cytosine DNA methylation is an important epigenetic mechanism for regulation of gene expression. Abnormal methylation is linked to several diseases, being for example the most common molecular lesion in cancer cell.1 Multiple studies suggest that alterations in DNA methylation, despite occurring at a low mosaic level, may confer increased risk of cancer later in life.2

The cytosine methylation levels within relatively small regions of the human genome are thought to be often concordant, resulting in a limited number of distinct methylation patterns of short sequencing reads.3 Due to the cell-to-cell variations in methylation, DNA purified from tissue samples contains a mix of hyper- and hypomethylated alleles with varying ratios that depend on the genomic region and tissue type.

Unsurprisingly, when the frequencies of hypermethylated epialleles are low (e.g. 1e-02 and lower) and cytosine methylation levels are averaged and reported using conventional algorithms, the identification of such hypermethylated epialleles becomes nearly impossible. In order to increase the sensitivity of DNA methylation analysis we have developed epialleleR — an R package for calling hypermethylated variant epiallele frequencies (VEF).

Two edge cases epialleleR was designed to distinguish are presented below (more examples can be found here). While these are simplified and entirely artificial, they still give an idea of two different methylation patterns that may exist in real life, be characterised by very similar quantitative metrics (beta value, the ratio of methylated cytosines, C, to total number of cytosines, C + T, per genomic position, i.e. $\beta = \frac{C}{C+T}$), but have entirely different biological properties: non-functional scattered methylation / technical artefacts on the left, and epigenetic gene inactivation on the right. VEF values, that are essentially the ratio of methylated cytosines in hypermethylated (above threshold) reads, Ca, to total number of cytosines, C + T, per genomic position, i.e. $VEF = \frac{C^a}{C+T}$, clearly separate these cases and are thought to be more useful in detection and quantification of concordant methylation events.

epialleleR is a very fast and scalable solution for analysis of data obtained by next-generation methylation/native sequencing of DNA samples. The minimum requirement for the input is a Binary Alignment Map (BAM) file containing sequencing reads. These reads can be obtained from either deep or ultra-deep sequencing, using either narrowly targeted gene panels (amplicon sequencing), larger methylation capture panels, or even whole-genome approaches.

Current Features

  • If methylation calls are not present in BAM file, epialleleR can make and store such calls in output BAM (similar to Bismark or Illumina’s mapping/alignment solutions; short-read sequencing alignments only)
  • epialleleR can create and store sample BAM files for testing purposes by means of simulateBam method
  • In addition to conventional reporting of cytosine DNA methylation levels (beta values), epialleleR can call variant epiallele frequencies (VEF) of hypermethylated alleles at the level of individual cytosines (generateCytosineReport) or supplied genomic regions (generateBedReport)
  • Linearised Methylated Haplotype Load (lMHL, generateMhlReport) can be used instead of VEF when thresholding is not recommended (long-read sequencing)
  • DNA methylation patterns of genomic region of interest can be explored using extractPatterns and plotPatterns
  • The association of methylation with single-nucleotide variations within epialleles can be tested using generateVcfReport
  • Potential bimodality of methylation for genomic regions of interest can be assessed using generateBedEcdf method

Processing speed

Currently epialleleR runs in a single-thread mode only. Reading/writing of BAM and FASTA data is now done by means of HTSlib, therefore it is possible to speed it up significantly using additional decompression threads (nthreads option in epialleleR methods). All operations are performed using optimised C++ functions, and usually take reasonable time.

During methylation calling, human genome (hg38) loading usually takes 10-15 seconds. Using preloaded reference genome, the calling itself is performed at the speed of 200-300 thousand short reads (150 or 225 bases long) per second (25-40 MB/s of BAM data).

During methylation reporting, running time for complete task “BAM on disk -> CX report on disk” depends on the size of the BAM file, and the speed is usually within the range of 30-50 MB/s (or 250-400 thousand short reads per second) for a single core of a relatively modern CPU (Intel(R) Core(TM) i7-7700).

Major bottlenecks (in BAM loading and preprocessing) were removed in the release v1.2, full multithreading and minor improvements are expected in the future.

Sample data

The epialleleR package includes sample data, which was obtained using targeted sequencing. The description of assays and files is given below. All the genomic coordinates for external data files are according to GRCh38 reference assembly.

Amplicon-based methylation NGS data

The samples of Human HCT116 DKO Non-Methylated (Zymo Research, cat # D5014-1), or Human HCT116 DKO Methylated (Zymo Research, cat # D5014-2) DNA,4 or their mix were bisulfite-converted, and the BRCA1 gene promoter region was amplified using four pairs of primers. Amplicons were mixed, indexed and sequenced at Illumina MiSeq system. The related files are:

Name Type Description
amplicon000meth.bam BAM a subset of reads for non-methylated DNA sample
amplicon010meth.bam BAM a subset of reads for a 1:9 mix of methylated and non-methylated DNA samples
amplicon100meth.bam BAM a subset of reads for fully methylated DNA sample
amplicon.bed BED genomic coordinates of four amplicons covering promoter area of BRCA1 gene
amplicon.vcf.gz VCF a relevant subset of sequence variations
amplicon.vcf.gz.tbi tabix tabix file for the amplicon.vcf.gz

Capture-based methylation NGS data

The tumour DNA was bisulfite-converted, fragmented and hybridized with custom-made probes covering promoter regions of 283 tumour suppressor genes (as described in 5). Libraries were sequenced using Illumina MiSeq system. The related files are:

Name Type Description
capture.bam BAM a subset of reads
capture.bed BED genomic coordinates of capture target regions
capture.vcf.gz VCF a relevant subset of sequence variations
capture.vcf.gz.tbi tabix tabix file for the capture.vcf.gz

Manually creating sample BAM files

For the purposes of testing this package’s methods or other tools for methylation calling and/or reporting, epialleleR provides a convenient way to manually create sample BAM files by specifying mandatory and optional BAM file tags. The following code will create a small BAM file that contains methylation calls and can be used for methylation reporting as described later:

bam.file <- tempfile(pattern="simulated", fileext=".bam")
simulateBam(output.bam.file=bam.file, XM=c("ZZzZZ", "zzZzz"), XG="CT")
#> Writing sample BAM [0.007s]
#> [1] 2
# one can view the resulting file using `samtools view -h <bam.file>`
# or, if desired, file can be converted to SAM using `samtools view`,
# manually corrected and converted back to BAM

Check simulateBam method help page for more information on parameters and their default values.

Typical workflow

Requirements

As mentioned earlier, epialleleR uses data stored in Binary Alignment Map (BAM) files as its input and currently allows to load both short-read (e.g., bisulfite) and long-read (native) sequencing alignments. Specific requirements for these types of data are given below. Additionally, please check the preprocessBam function help file for a full description of available parameters, as well as explanation of the function’s logic.

Short-read sequencing

It is a prerequisite that records in the BAM file contain an XG tag with a genomic strand they map to (“CT” or “GA”), and an XM tag with the methylation call string — such files are produced by mapping and alignment tools such as Bismark Bisulfite Read Mapper and Methylation Caller or state-of-the-art Illumina solutions: Illumina DRAGEN Bio-IT Platform, Illumina Cloud analysis solutions, as well as contemporary Illumina sequencing instruments with on-board read mapping/alignment (NextSeq 1000/2000, NovaSeq X). These BAM files will contain all the necessary information and can be analysed without additional steps.

If BAM files were produced by other mapping/alignment tools (e.g., bwa-meth or BSMAP) and lack XG/XM data, it is possible to call methylation using callMethylation method. This method will add absent XG/XM tags and save all data in the output BAM file that can be further analysed by epialleleR.

Long-read sequencing

For preprocessing of long reads, epialleleR requires presence of MM (Mm) and ML (Ml) tags that hold information on base modifications and related probabilities, respectively. These are standard tags described in SAM/BAM format specification, therefore relevant tools for analysis and alignment of long sequencing reads should be able to produce them.

Reading the data

All epialleleR methods can load BAM data using the file path. However, if a file is very large and several reports need to be prepared, it is advised to use the preprocessBam convenience function as shown below. This function is also used internally when a BAM file location string is supplied as an input for other epialleleR methods.

preprocessBam automatically determines if BAM file contains paired- or single-end alignments and has all the necessary tags (XM/XG) available. It is recommended to use verbose processing and check messages for correct identification of alignment endness. Otherwise, if the paired parameter is set explicitly, exception (or warning if override.check=TRUE) is thrown when expected endness differs from the auto detected one.

During preprocessing of paired-end alignments, paired reads are merged according to their base quality: nucleotide base with the highest value in the QUAL string is taken, unless its quality is less than min.baseq, which results in no information for that particular position (“-”/“N”). These merged reads are then processed as a single entity in all epialleleR methods. Due to merging, overlapping bases in read pairs are counted only once, and the base with the highest quality is taken. It is a requirement currently that paired-end BAM file must be sorted by QNAME instead of genomic location (i.e., “unsorted”) to perform merging of paired-end reads. Error message is shown if it is sorted by genomic location, in this case please sort it by QNAME using ‘samtools sort -n -o out.bam in.bam’.

During preprocessing of single-end alignments, no read merging is performed. Only bases with quality of at least min.baseq are considered. Lower base quality results in no information for that particular position (“-”/“N”).

For RRBS-like protocols, it is possible to trim alignments from one or both ends. Trimming is performed during BAM loading and will therefore influence results of all downstream epialleleR methods. Internally, trimming is performed at the level of a template (i.e., read pair for paired-end BAM or individual read for single-end BAM). This ensures that only necessary parts (real ends of sequenced fragment) are removed for paired-end sequencing reads.

Specific considerations for long-read sequencing data:

Any location not reported is implicitly assumed to contain no modification.

According to SAM format specification, MM base modification tags are allowed to list modifications observed not only on the original sequenced strand (e.g., C+m) but also on the opposite strand (e.g., G-m). The logic of their processing is as follows (with the examples given below): * if an alignment record has no methylation modifications (neither C+m, nor G-m are present), this record is, naturally, considered to be a single read with no cytosines methylated * if an alignment record has C+m modification (base modifications on the original sequenced strand), then this record is, naturally, considered to be a single read with cytosine modifications on the sequenced strand * if an alignment record has G-m modification (base modifications on the strand opposite to sequenced), then this record is treated as two reads, with the original sequenced strand having no modifications, while the opposite strand having cytosine modifications * if both C+m and G-m are present, then this record is treated as two reads, with both strands having cytosine modifications

library(epialleleR)

capture.bam <- system.file("extdata", "capture.bam", package="epialleleR")
bam.data    <- preprocessBam(capture.bam)
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.013s]

# Specifics of long-read alignment processing
out.bam <- tempfile(pattern="out-", fileext=".bam")
simulateBam(
  seq=c("ACGCCATYCGCGCCA"),
  Mm=c("C+m,0,2,0;G-m,0,0,0;"),
  Ml=list(as.integer(c(102,128,153,138,101,96))),
  output.bam.file=out.bam
  )
#> Writing sample BAM [0.003s]
#> [1] 1
generateCytosineReport(out.bam, threshold.reads=FALSE, report.context="CX")
#> Checking BAM file: long-read, single-end, unsorted alignment detected
#> Reading single-end BAM file [0.002s]
#> Preparing cytosine report [0.001s]
#>      rname strand   pos context  meth unmeth
#>     <fctr> <fctr> <int>  <fctr> <int>  <int>
#>  1:   chrS      +     2      CG     1      0
#>  2:   chrS      -     3      CG     1      0
#>  3:   chrS      +     4     CHH     0      1
#>  4:   chrS      +     5     CHH     0      1
#>  5:   chrS      +     9      CG     1      0
#>  6:   chrS      -    10      CG     1      0
#>  7:   chrS      +    11      CG     1      0
#>  8:   chrS      -    12      CG     1      0
#>  9:   chrS      +    13     CHH     0      1
#> 10:   chrS      +    14     CHH     0      1

Optional calling of cytosine methylation

If short-read BAM file lacks XG/XM tags (e.g., is an output of bwa-meth or BSMAP), preprocessing will fail with the message that cytosine methylation calling must be performed. This can be done as follows:

# bwa-meth sample output
input.bam <- system.file("extdata", "test", "bwameth-se-unsort-yd.bam", package="epialleleR")

# resulting BAM with XG/XM tags
output.bam <- tempfile(pattern="output-", fileext=".bam")

# sample reference genome
genome <- preprocessGenome(system.file("extdata", "test", "reference.fasta.gz", package="epialleleR"))
#> Reading reference genome file [0.000s]

# calls cytosine methylation and stores it in the output BAM
# Input BAM has 100 records of which 73 are mapped to the genome
callMethylation(input.bam, output.bam, genome)
#> Making methylation calls [0.022s]
#> $nrecs
#> [1] 100
#> 
#> $ncalled
#> [1] 73

# process this data further
# bam.data <- preprocessBam(output.bam)

Making cytosine reports

epialleleR can generate conventional cytosine reports in a format, which is similar to the genome-wide cytosine report produced by the coverage2cytosine Bismark module.6

Please note that generateCytosineReport produces thresholded (VEF) report by default: methylated cytosines from reads that do not pass the threshold (hypomethylated reads) are counted as being unmethylated. In order to make a conventional cytosine report, use threshold.reads=FALSE.

To produce conventional cytosine reports without thresholding by within-context methylation level though minimally affected by incomplete cytosine conversion, run this method with the following parameters: threshold.reads=TRUE, threshold.context="CG", min.context.sites=0, min.context.beta=0, max.outofcontext.beta=0.1. All cytosines within reads (read pairs) having more than 10% out-of-context cytosines methylated, will be effectively treated as unmethylated ones.

# data.table::data.table object for
# CpG VEF report
cg.vef.report <- generateCytosineReport(bam.data)
#> Thresholding reads [0.001s]
#> Preparing cytosine report [0.014s]
head(cg.vef.report[order(meth+unmeth, decreasing=TRUE)])
#>     rname strand      pos context  meth unmeth
#>    <fctr> <fctr>    <int>  <fctr> <int>  <int>
#> 1:  chr17      + 61864475      CG     7      9
#> 2:  chr17      + 61864486      CG    10      6
#> 3:  chr17      + 61864504      CG     9      7
#> 4:  chr20      - 57267455      CG    13      1
#> 5:  chr17      - 61863826      CG     0     13
#> 6:  chr17      - 61863830      CG     0     13

# CpG cytosine report
cg.report <- generateCytosineReport(bam.data, threshold.reads=FALSE)
#> Preparing cytosine report [0.014s]
head(cg.report[order(meth+unmeth, decreasing=TRUE)])
#>     rname strand      pos context  meth unmeth
#>    <fctr> <fctr>    <int>  <fctr> <int>  <int>
#> 1:  chr17      + 61864475      CG     8      8
#> 2:  chr17      + 61864486      CG    10      6
#> 3:  chr17      + 61864504      CG    10      6
#> 4:  chr20      - 57267455      CG    13      1
#> 5:  chr17      - 61863826      CG     0     13
#> 6:  chr17      - 61863830      CG     0     13

# CX cytosine report
cx.report <- generateCytosineReport(bam.data, threshold.reads=FALSE,
                                    report.context="CX")
#> Preparing cytosine report [0.016s]
head(cx.report[order(meth+unmeth, decreasing=TRUE)])
#>     rname strand      pos context  meth unmeth
#>    <fctr> <fctr>    <int>  <fctr> <int>  <int>
#> 1:  chr17      + 61864338     CHG     1     25
#> 2:  chr17      + 61864348     CHH     0     24
#> 3:  chr17      + 61864364     CHH     0     24
#> 4:  chr17      + 61864365     CHH     0     24
#> 5:  chr17      + 61864373     CHH     0     24
#> 6:  chr17      + 61864324     CHG     0     23

Making VEF reports for a set of genomic regions

epialleleR allows to make reports not only for individual cytosine bases, but also for a set of genomic regions. It is especially useful when the targeted methylation sequencing was used to produce reads (such as amplicon sequencing or hybridization capture using, e.g., Agilent SureSelect Target Enrichment Probes).

The amplicon sequencing principally differs from capture-based assays in that the coordinates of reads are known. Therefore, reads can be assigned to amplicons by their exact positions, while to the capture targets — by the overlap. For this, epialleleR provides generic generateBedReport function as well as two of its aliases, generateAmpliconReport (for amplicon-based NGS) and generateCaptureReport (for capture-based NGS).

# report for amplicon-based data
# matching is done by exact start or end positions plus/minus tolerance
amplicon.report <- generateAmpliconReport(
  bam=system.file("extdata", "amplicon010meth.bam", package="epialleleR"),
  bed=system.file("extdata", "amplicon.bed", package="epialleleR")
)
#> Reading BED file [0.031s]
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.004s]
#> Thresholding reads [0.000s]
#> Preparing amplicon report [0.035s]
amplicon.report
#>    seqnames    start      end width strand amplicon nreads+ nreads-        VEF
#>      <fctr>    <int>    <int> <int> <fctr>   <char>   <int>   <int>      <num>
#> 1:    chr17 43125624 43126026   403      * CpG00-13       0     156 0.08333333
#> 2:    chr17 43125270 43125640   371      * CpG14-31       0      61 0.11475410
#> 3:    chr17 43125171 43125550   380      * CpG17-34       0      93 0.05376344
#> 4:    chr17 43124861 43125249   389      * CpG33-49       0      84 0.10714286
#> 5:     <NA>       NA       NA    NA   <NA>     <NA>      60      46 0.13207547

# report for capture-based data
# matching is done by overlap
capture.report <- generateCaptureReport(
  bam=system.file("extdata", "capture.bam", package="epialleleR"),
  bed=system.file("extdata", "capture.bed", package="epialleleR")
)
#> Reading BED file [0.007s]
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.012s]
#> Thresholding reads [0.001s]
#> Preparing capture report [0.017s]
head(capture.report)
#>    seqnames    start      end width strand     V4 nreads+ nreads-       VEF
#>      <fctr>    <int>    <int> <int> <fctr> <char>   <int>   <int>     <num>
#> 1:     chr1  3067647  3069703  2057      * PRDM16       2       1 1.0000000
#> 2:     chr1  3651039  3653096  2058      *   TP73       0       2 0.5000000
#> 3:     chr1  3689153  3691202  2050      *   TP73       0       2 1.0000000
#> 4:     chr1  3696519  3698570  2052      *   TP73       1       2 1.0000000
#> 5:     chr1  6179609  6181670  2062      *   CHD5       0       3 0.6666667
#> 6:     chr1 13698869 13699064   196      *  PRDM2      NA      NA        NA

# generateBedReport is a generic function for BED-guided reports
bed.report <- generateBedReport(
  bam=system.file("extdata", "capture.bam", package="epialleleR"),
  bed=system.file("extdata", "capture.bed", package="epialleleR"),
  bed.type="capture"
)
#> Reading BED file [0.008s]
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.013s]
#> Thresholding reads [0.001s]
#> Preparing capture report [0.016s]
identical(capture.report, bed.report)
#> [1] TRUE

Linearized MHL reports

VEF values are extremely useful for detection of mosaic epimutations. However, default thresholding parameters might not fit with the nature of regions of interest. In this case, it is advised to learn the characteristics of these regions with extractPatterns and generateBedEcdf methods as described below. Alternatively, epialleleR provides a method to calculate a metric that is similar to VEF in its ability to highlight hypermethylated regions but does not require thresholding — linearised Methylated Haplotype Load (lMHL).

lMHL is a modified version of MHL (MHL was first described by Guo et al., 2017 7) sought to be faster and applicable for a wider range of sequencing data. More information on this is given in the help page for the generateMhlReport as well as in the values vignette.

# lMHL report can be generated using
mhl.report <- generateMhlReport(
  bam=system.file("extdata", "capture.bam", package="epialleleR")
)
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.011s]
#> Preparing lMHL report [0.022s]

Exploring DNA methylation patterns

Individual epialleles can be extracted and plotted in order to visualize methylation patters within a genomic region of interest. For this, epialleleR provides methods extractPatterns and plotPatterns which can be used as follows:

# First, let's extract base methylation information for sequencing reads
# of 1:9 mix of methylated and non-methylated control DNA
patterns <- extractPatterns(
  bam=system.file("extdata", "amplicon010meth.bam", package="epialleleR"),
  bed=as("chr17:43125200-43125600","GRanges")
)
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.004s]
#> Extracting methylation patterns [0.019s]

# that many read pairs overlap genomic region of interest
nrow(patterns)
#> [1] 238

# now we can plot the most abundant them of them using default parameters
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


# now let's explore methylation patterns in RAD51C gene promoter using
# methylation capture data
capture.patterns <- extractPatterns(
  bam=system.file("extdata", "capture.bam", package="epialleleR"),
  bed=as("chr17:58691673-58693108", "GRanges"),
  verbose=FALSE
)

# let's plot all the patterns using discrete genomic scale
plotPatterns(capture.patterns, npatterns.per.bin=Inf,
             genomic.scale="discrete", context.size=1)
#> 59 patterns supplied
#> 56 unique
#> 56 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]
#>      22         3         1         2         3         1         2         0        11      11

Exploring sequence variants in epialleles

It is known that sequence variants can affect the methylation status of a DNA.8 The generateVcfReport function calculates frequencies of single nucleotide variants (SNVs) within epialleles and tests for the association between SNV and epiallelic status using Fisher Exact test. Base counts and the test’s p-values are included in the returned value.

In addition to BAM file location string or preprocessed BAM object, the function requires a location string for the Variant Call Format (VCF) file or the VCF object that was obtained using VariantAnnotation::readVcf function. As VCF files can be extremely large, it is strongly advised to prefilter the VCF object by the relevant set of genomic regions, or specify such relevant set of regions as a bed parameter when vcf points to a VCF file location.

Please note, that the output report is currently limited to SNVs only. Also, the default (min.baseq=0) output of generateVcfReport is equivalent to the one of samtools mplieup -Q 0 ..., and therefore may result in false SNVs caused by misalignments. Remember to increase min.baseq (samtools mplieup -Q default value is 13) to obtain results of a higher quality.

# VCF report
vcf.report <- generateVcfReport(
  bam=system.file("extdata", "amplicon010meth.bam", package="epialleleR"),
  bed=system.file("extdata", "amplicon.bed", package="epialleleR"),
  vcf=system.file("extdata", "amplicon.vcf.gz", package="epialleleR"),
  # thresholds on alignment and base quality
  min.mapq=30, min.baseq=13,
  # when VCF seqlevels are different from BED and BAM it is possible
  # to convert them internally
  vcf.style="NCBI"
)
#> Loading required namespace: VariantAnnotation
#> Reading BED file [0.026s]
#> Reading VCF file [5.236s]
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.003s]
#> Thresholding reads [0.000s]
#> Extracting base frequences [0.037s]

# NA values are shown for the C->T variants on the "+" and G->A on the "-"
# strands, because bisulfite conversion makes their counting impossible
head(vcf.report)
#>           name seqnames    range    REF    ALT M+Ref U+Ref M-Ref U-Ref M+Alt U+Alt M-Alt U-Alt
#>         <char>   <fctr>    <int> <char> <char> <num> <num> <num> <num> <num> <num> <num> <num>
#> 1: rs546660277    chr17 43124874      A      C     0     0     9    74     0     0     0     0
#> 2: rs574263814    chr17 43124891      G      A     0     0    NA    NA     0     0    NA    NA
#> 3:   rs8176076    chr17 43124935      G      A     0     0    NA    NA     0     0    NA    NA
#> 4: rs535977743    chr17 43125016      C      T    NA    NA     9    73    NA    NA     0     0
#> 5: rs191784032    chr17 43125050      C      A     0     0     9    73     0     0     0     1
#> 6: rs111956204    chr17 43125083      C      A     0     0     9    74     0     0     0     0
#>    SumRef SumAlt  FEp+  FEp-
#>     <num>  <num> <num> <num>
#> 1:     83      0     1     1
#> 2:      0      0     1    NA
#> 3:      0      0     1    NA
#> 4:     82      0    NA     1
#> 5:     82      1     1     1
#> 6:     83      0     1     1

# let's sort the report by increasing Fisher's exact test's p-values.
# the p-values are given separately for reads that map to the "+"
head(vcf.report[order(`FEp-`, na.last=TRUE)])
#>           name seqnames    range    REF    ALT M+Ref U+Ref M-Ref U-Ref M+Alt U+Alt M-Alt U-Alt
#>         <char>   <fctr>    <int> <char> <char> <num> <num> <num> <num> <num> <num> <num> <num>
#> 1: rs546660277    chr17 43124874      A      C     0     0     9    74     0     0     0     0
#> 2: rs535977743    chr17 43125016      C      T    NA    NA     9    73    NA    NA     0     0
#> 3: rs191784032    chr17 43125050      C      A     0     0     9    73     0     0     0     1
#> 4: rs111956204    chr17 43125083      C      A     0     0     9    74     0     0     0     0
#> 5:  rs55680227    chr17 43125086      A      C     0     0     8    64     0     0     0     0
#> 6: rs539733232    chr17 43125088      C      A     0     0     8    71     0     0     0     0
#>    SumRef SumAlt  FEp+  FEp-
#>     <num>  <num> <num> <num>
#> 1:     83      0     1     1
#> 2:     82      0    NA     1
#> 3:     82      1     1     1
#> 4:     83      0     1     1
#> 5:     72      0     1     1
#> 6:     79      0     1     1

# and to the "-" strand
head(vcf.report[order(`FEp+`, na.last=TRUE)])
#>           name seqnames    range    REF    ALT M+Ref U+Ref M-Ref U-Ref M+Alt U+Alt M-Alt U-Alt
#>         <char>   <fctr>    <int> <char> <char> <num> <num> <num> <num> <num> <num> <num> <num>
#> 1: rs546660277    chr17 43124874      A      C     0     0     9    74     0     0     0     0
#> 2: rs574263814    chr17 43124891      G      A     0     0    NA    NA     0     0    NA    NA
#> 3:   rs8176076    chr17 43124935      G      A     0     0    NA    NA     0     0    NA    NA
#> 4: rs191784032    chr17 43125050      C      A     0     0     9    73     0     0     0     1
#> 5: rs111956204    chr17 43125083      C      A     0     0     9    74     0     0     0     0
#> 6:  rs55680227    chr17 43125086      A      C     0     0     8    64     0     0     0     0
#>    SumRef SumAlt  FEp+  FEp-
#>     <num>  <num> <num> <num>
#> 1:     83      0     1     1
#> 2:      0      0     1    NA
#> 3:      0      0     1    NA
#> 4:     82      1     1     1
#> 5:     83      0     1     1
#> 6:     72      0     1     1

# and finally, let's plot methylation patterns overlapping one of the most
# covered SNPs in the methylation capture test data set - rs573296191
# (chr17:61864584) in BRIP1 gene
brip1.patterns <- extractPatterns(
  bam=system.file("extdata", "capture.bam", package="epialleleR"),
  bed=as("chr17:61864583-61864585", "GRanges"),
  highlight.positions=61864584,
  verbose=FALSE
)
plotPatterns(brip1.patterns)
#> 24 patterns supplied
#> 17 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         0         0         1         0         1         2         1         0       2

Plotting the distribution of per-read beta values

As stated in the introduction, human genomic DNA regions often show bimodal methylation patterns. epialleleR allows to visualize this information by plotting empirical cumulative distribution functions (eCDFs) for within- and out-of-context beta values.

The code below produces plots for the sequencing reads of control DNA samples. Note that within-the-context eCDF(0.5) values are very close to the expected 1-VEF values for the corresponding control DNA samples:

  • non-methylated DNA — expected VEF = 0, observed 1-eCDF(0.5) ≈ 0
  • 1:9 mix of methylated and non-methylated DNA — expected VEF = 0.1, observed 1-eCDF(0.5) ≈ 0.1
  • and fully methylated DNA — expected VEF = 1, observed 1-eCDF(0.5) ≈ 1
# First, let's visualise eCDFs for within- and out-of-context beta values
# for all four amplicons and unmatched reads. Note that within-the-context eCDF
# of 0.5 is very close to the expected 1-VEF value (0.1) for all amplicons
# produced from this 1:9 mix of methylated and non-methylated control DNA

# let's compute eCDF
amplicon.ecdfs <- generateBedEcdf(
  bam=system.file("extdata", "amplicon010meth.bam", package="epialleleR"),
  bed=system.file("extdata", "amplicon.bed", package="epialleleR"),
  bed.rows=NULL
)
#> Reading BED file [0.008s]
#> Checking BAM file: short-read, paired-end, name-sorted alignment detected
#> Reading paired-end BAM file [0.003s]
#> Computing ECDFs for within- and out-of-context per-read beta values [0.007s]

# there are 5 items in amplicon.ecdfs, let's plot all of them
par(mfrow=c(1,length(amplicon.ecdfs)))

# cycle through items
for (x in 1:length(amplicon.ecdfs)) {
  # four of them have names corresponding to genomic regions of amplicon.bed
  # fifth - NA for all the reads that don't match to any of those regions
  main <- if (is.na(names(amplicon.ecdfs[x]))) "unmatched"
          else names(amplicon.ecdfs[x])
  
  # plotting eCDF for within-the-context per-read beta values (in red)
  plot(amplicon.ecdfs[[x]]$context, col="red", verticals=TRUE, do.points=FALSE,
       xlim=c(0,1), xlab="per-read beta value", ylab="cumulative density",
       main=main)
  
  # adding eCDF for out-of-context per-read beta values (in blue)
  plot(amplicon.ecdfs[[x]]$out.of.context, add=TRUE, col="blue",
       verticals=TRUE, do.points=FALSE)
}



# Second, let's compare eCDFs for within-the-context beta values for only one
# amplicon but all three sequenced samples: pure non-methylated DNA, 1:9 mix of
# methylated and non-methylated DNA, and fully methylated DNA

# our files
bam.files <- c("amplicon000meth.bam", "amplicon010meth.bam",
               "amplicon100meth.bam")

# let's plot all of them
par(mfrow=c(1,length(bam.files)))

# cycle through items
for (f in bam.files) {
  # let's compute eCDF
  amplicon.ecdfs <- generateBedEcdf(
    bam=system.file("extdata", f, package="epialleleR"),
    bed=system.file("extdata", "amplicon.bed", package="epialleleR"),
    # only the second amplicon
    bed.rows=2, verbose=FALSE
  )
  
  # plotting eCDF for within-the-context per-read beta values (in red)
  plot(amplicon.ecdfs[[1]]$context, col="red", verticals=TRUE, do.points=FALSE,
       xlim=c(0,1), xlab="per-read beta value", ylab="cumulative density",
       main=f)
  
   # adding eCDF for out-of-context per-read beta values (in blue)
  plot(amplicon.ecdfs[[1]]$out.of.context, add=TRUE, col="blue",
       verticals=TRUE, do.points=FALSE)
}

Other information

Citing the epialleleR package

Oleksii Nikolaienko, Per Eystein Lønning, Stian Knappskog, epialleleR: an R/Bioconductor package for sensitive allele-specific methylation analysis in NGS data. GigaScience, Volume 12, 2023, giad087, https://doi.org/10.1093/gigascience/giad087

The data underlying epialleleR manuscript

Replication Data for: “epialleleR: an R/BioC package for quantifying and analysing low-frequency DNA methylation”, https://doi.org/10.18710/2BQTJP

NCBI GEO dataset GSE201690: “Methylation analysis of promoter regions for selected tumour suppressor genes in DNA from white blood cells”, https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE201690

Our experimental studies that use the package

Per Eystein Lonning, Oleksii Nikolaienko, Kathy Pan, Allison W. Kurian, Hans Petter Petter Eikesdal, Mary Pettinger, Garnet L Anderson, Ross L Prentice, Rowan T. Chlebowski, and Stian Knappskog. Constitutional BRCA1 methylation and risk of incident triple-negative breast cancer and high-grade serous ovarian cancer. JAMA Oncology 2022. https://doi.org/10.1001/jamaoncol.2022.3846

Oleksii Nikolaienko, Hans P. Eikesdal, Elisabet Ognedal, Bjørnar Gilje, Steinar Lundgren, Egil S. Blix, Helge Espelid, Jürgen Geisler, Stephanie Geisler, Emiel A.M. Janssen, Synnøve Yndestad, Laura Minsaas, Beryl Leirvaag, Reidun Lillestøl, Stian Knappskog, Per E. Lønning. Prenatal BRCA1 epimutations contribute significantly to triple-negative breast cancer development. Genome Medicine 2023. https://doi.org/10.1186/s13073-023-01262-8. Data: GSE243966

Session Info

sessionInfo()
#> R version 4.4.2 (2024-10-31)
#> Platform: x86_64-pc-linux-gnu
#> Running under: Ubuntu 24.04.1 LTS
#> 
#> Matrix products: default
#> BLAS:   /usr/lib/x86_64-linux-gnu/openblas-pthread/libblas.so.3 
#> LAPACK: /usr/lib/x86_64-linux-gnu/openblas-pthread/libopenblasp-r0.3.26.so;  LAPACK version 3.12.0
#> 
#> locale:
#>  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8       
#>  [4] LC_COLLATE=C               LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
#>  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
#> [10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       
#> 
#> time zone: Etc/UTC
#> tzcode source: system (glibc)
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] ggplot2_3.5.1     epialleleR_1.15.1 rmarkdown_2.29   
#> 
#> loaded via a namespace (and not attached):
#>  [1] KEGGREST_1.47.0             SummarizedExperiment_1.37.0 gtable_0.3.6               
#>  [4] rjson_0.2.23                xfun_0.49                   bslib_0.8.0                
#>  [7] Biobase_2.67.0              lattice_0.22-6              vctrs_0.6.5                
#> [10] tools_4.4.2                 bitops_1.0-9                generics_0.1.3             
#> [13] curl_6.0.1                  stats4_4.4.2                parallel_4.4.2             
#> [16] RSQLite_2.3.9               tibble_3.2.1                fansi_1.0.6                
#> [19] AnnotationDbi_1.69.0        blob_1.2.4                  pkgconfig_2.0.3            
#> [22] Matrix_1.7-1                BSgenome_1.75.0             data.table_1.16.4          
#> [25] S4Vectors_0.45.2            lifecycle_1.0.4             GenomeInfoDbData_1.2.13    
#> [28] compiler_4.4.2              farver_2.1.2                Rsamtools_2.23.1           
#> [31] Biostrings_2.75.3           munsell_0.5.1               codetools_0.2-20           
#> [34] GenomeInfoDb_1.43.2         htmltools_0.5.8.1           sys_3.4.3                  
#> [37] buildtools_1.0.0            sass_0.4.9                  RCurl_1.98-1.16            
#> [40] yaml_2.3.10                 pillar_1.9.0                crayon_1.5.3               
#> [43] jquerylib_0.1.4             BiocParallel_1.41.0         cachem_1.1.0               
#> [46] DelayedArray_0.33.3         abind_1.4-8                 digest_0.6.37              
#> [49] restfulr_0.0.15             VariantAnnotation_1.53.0    labeling_0.4.3             
#> [52] maketools_1.3.1             fastmap_1.2.0               grid_4.4.2                 
#> [55] colorspace_2.1-1            cli_3.6.3                   SparseArray_1.7.2          
#> [58] magrittr_2.0.3              S4Arrays_1.7.1              GenomicFeatures_1.59.1     
#> [61] XML_3.99-0.17               utf8_1.2.4                  withr_3.0.2                
#> [64] scales_1.3.0                UCSC.utils_1.3.0            bit64_4.5.2                
#> [67] XVector_0.47.0              httr_1.4.7                  matrixStats_1.4.1          
#> [70] bit_4.5.0.1                 png_0.1-8                   memoise_2.0.1              
#> [73] evaluate_1.0.1              knitr_1.49                  BiocIO_1.17.1              
#> [76] GenomicRanges_1.59.1        IRanges_2.41.2              rtracklayer_1.67.0         
#> [79] rlang_1.1.4                 Rcpp_1.0.13-1               glue_1.8.0                 
#> [82] DBI_1.2.3                   BiocGenerics_0.53.3         jsonlite_1.8.9             
#> [85] R6_2.5.1                    GenomicAlignments_1.43.0    MatrixGenerics_1.19.0      
#> [88] zlibbioc_1.52.0

References

  1. https://doi.org/10.1146/annurev.pharmtox.45.120403.095832↩︎

  2. https://doi.org/10.1101/2020.12.01.403501↩︎

  3. https://doi.org/10.1093/bib/bbx077↩︎

  4. https://www.zymoresearch.com/products/human-methylated-non-methylated-dna-set-dna-w-primers↩︎

  5. https://doi.org/10.1186/s13148-020-00920-7↩︎

  6. https://www.bioinformatics.babraham.ac.uk/projects/bismark/↩︎

  7. https://doi.org/10.1038/ng.3805↩︎

  8. https://doi.org/10.1038/modpathol.2009.130↩︎