methcp is a differentially methylated region
(DMR) detecting method for whole-genome bisulfite sequencing (WGBS)
data. It is applicable for a wide range of experimental designs.
In this document, we provide examples for two-group comparisons and
time-course analysis.
methcp identifies DMRs based on change point detection, which
naturally segments the genome and provides region-level
differential analysis. We direct the interested reader to our paper
here
Load packages.
library(bsseq)
library(MethCP)In this section, we use the CpG methylation data from an Arabidopsis dataset
available from GEO with accession number
GSM954584. We
take a subset of chromosome 1 and 2 from each of the samples and perform
differential analysis between the wild-type plants and the H2A.Z mutant
plants, which we refer to as treatment and control in the rest of
the document.
We use a well-developed Bioconductor package bsseq to load the store the raw
data. Below is an example of how to read raw counts using bsseq.
We provide a helper function createBsseqObject to create a bsseq object
when the data for each sample is stored in a separate text file.
For more operations regarding the bsseq object, or to create a bsseq
object customized to your file format, please refer to their
User’s Guide.
# The dataset is consist of 6 samples. 3 samples are H2A.Z mutant
# plants, and 3 samples are controls.
sample_names <- c(
paste0("control", seq_len(3)),
paste0("treatment", seq_len(3))
)
# Get the vector of file path and names
raw_files <- system.file(
"extdata", paste0(sample_names, ".txt"), package = "MethCP")
# load the data
bs_object <- createBsseqObject(
files = raw_files, sample_names = sample_names,
chr_col = 'Chr', pos_col = 'Pos', m_col = "M", cov_col = 'Cov')The bsseq object.
bs_object## An object of type 'BSseq' with
## 82103 methylation loci
## 6 samples
## has not been smoothed
## All assays are in-memoryHeader of the raw file for one of the samples.
dt <- read.table(
raw_files[1], stringsAsFactors = FALSE, header = TRUE)
head(dt)## Chr Pos M Cov
## 1 chr1 109 3 3
## 2 chr1 110 4 4
## 3 chr1 115 3 3
## 4 chr1 116 4 4
## 5 chr1 161 0 1
## 6 chr1 162 1 1We calculate the per-cytosine statistics using two different test DSS and
methylKit using the function calcLociStat. This function returns a
MethCP object that is used for the future segmentation step. We allow
parallelized computing when there are multiple chromosomes in the dataset.
# the sample names of the two groups to compare. They should be subsets of the
# sample names provided when creating the `bsseq` objects.
group1 <- paste0("control", seq_len(3))
group2 <- paste0("treatment", seq_len(3))
# Below we calculate the per-cytosine statistics using two different
# test `DSS` and `methylKit`. The users may pick one of the two for their
# application.
# obj_DSS <- calcLociStat(bs_object, group1, group2, test = "DSS")
obj_methylKit <- calcLociStat(
bs_object, group1, group2, test = "methylKit")obj_methylKit## MethCP object with 2 chromosomes, 81936 methylation loci
## test: methylKit
## group1: control1 control2 control3
## group2: treatment1 treatment2 treatment3
## has not been segmentedIn cases the user wants to use their pre-calculated test statistics for
experiments other than two-group comparison and time course data, we use the
calculated statistics and create a MethCP object.
data <- data.frame(
chr = rep("Chr01", 5),
pos = c(2, 5, 9, 10, 18),
effect.size = c(1,-1, NA, 9, Inf),
pvals = c(0, 0.1, 0.9, NA, 0.02))
obj <- MethCPFromStat(
data, test.name="myTest",
pvals.field = "pvals",
effect.size.field="effect.size",
seqnames.field="chr",
pos.field="pos"
)## Filtering out NAs and infinite values.
## Cytosine counts before filtering: 5.
## Cytosine counts after filtering: 3.obj## MethCP object with 1 chromosomes, 3 methylation loci
## test: myTest
## group1: notApplicable
## group2: notApplicable
## has not been segmentedsegmentMethCP performs segmentation on a MethCP object. We allow
parallelized computing when there are multiple chromosomes in the dataset.
Different from calcLociStat function in the previous section, we do not put
any constraint on the number of cores used. Please see the documentation for
adjusting the parameters used in the segmentation.
# obj_DSS <- segmentMethCP(
# obj_DSS, bs_object, region.test = "weighted-coverage")
obj_methylKit <- segmentMethCP(
obj_methylKit, bs_object, region.test = "fisher")obj_methylKit## MethCP object with 2 chromosomes, 81936 methylation loci
## test: methylKit
## group1: control1 control2 control3
## group2: treatment1 treatment2 treatment3
## has been segmentedUse function getSigRegion on a MethCP object to get the list of DMRs.
# region_DSS <- getSigRegion(obj_DSS)
# head(region_DSS)region_methylKit <- getSigRegion(obj_methylKit)
head(region_methylKit)## seqnames start end nC.valid nC mean.diff mean.cov region.pval
## 1.2 chr1 26853 27083 10 10 -0.3126 6.8500 6.306067e-14
## 1.5 chr1 30691 30768 11 11 -0.1776 8.3788 4.028997e-05
## 1.13 chr1 47817 47966 16 16 -0.2058 7.1354 4.314515e-08
## 1.21 chr1 65562 65735 10 10 -0.5742 5.2167 0.000000e+00
## 1.36 chr1 92672 92891 10 10 0.4067 8.9667 0.000000e+00
## 1.38 chr1 94712 95021 12 12 0.1304 7.9444 1.295822e-10MethCP is flexible for a wide variety of experimental designs. We apply MethCP on an Arabidopsis thaliana seed germination dataset available from the GEO with accession number https://www.ncbi.nlm.nih.gov/geo/query/acc.cgi?acc=GSE94712 The data is generated from two replicates of dry seed and germinating seeds of wild-type plants (Col-0) and ros1 dml2 dml3 (rdd) triple demethylase mutant plants at 0-4 days after imbibition for 4 days (DAI). For cytosine-based statistics, we fit linear models on the methylation ratios and test the differences between the time coefficient of condition Col-0 and condition rdd.
Read the meta data.
meta_file <- system.file(
"extdata", "meta_data.txt", package = "MethCP")
meta <- read.table(meta_file, sep = "\t", header = TRUE)
head(meta)## Condition TimeName Time Replicate SampleName
## 1 Col-0 DS 0 1 GSM2481288_mC_Col-0_DS_r1
## 2 Col-0 DS 0 2 GSM2481289_mC_Col-0_DS_r2
## 3 Col-0 0DAI 1 1 GSM2481290_mC_Col-0_0DAI_r1
## 4 Col-0 0DAI 1 2 GSM2481291_mC_Col-0_0DAI_r2
## 5 Col-0 1DAI 2 1 GSM2481292_mC_Col-0_1DAI_r1
## 6 Col-0 1DAI 2 2 GSM2481293_mC_Col-0_1DAI_r2Read the counts data.
# Get the vector of file path and names
raw_files <- system.file(
"extdata", paste0(meta$SampleName, ".tsv"), package = "MethCP")
# read files
bs_object <- createBsseqObject(
files = raw_files, sample_names = meta$SampleName,
chr_col = 1, pos_col = 2, m_col = 4, cov_col = 5, header = TRUE)Apply coverage filter to make sure each loci has total coverage (summed across samples) more than 3 for each condition.
groups <- split(seq_len(nrow(meta)), meta$Condition)
coverages <- as.data.frame(getCoverage(bs_object, type = "Cov"))
filter <- rowSums(coverages[, meta$SampleName[groups[[1]]]] != 0) >= 3 &
rowSums(coverages[, meta$SampleName[groups[[2]]]] != 0) >= 3
bs_object <- bs_object[filter, ]Calculate the statistics. A dataframe of the meta data will be passed to
function calcLociStatTimeCourse. Note that there must be columns named
Condition, Time and SampleName in the dataframe.
obj <- calcLociStatTimeCourse(bs_object, meta)obj## MethCP object with 1 chromosomes, 26001 methylation loci
## test: TimeCourse
## group1: GSM2481288_mC_Col-0_DS_r1 GSM2481289_mC_Col-0_DS_r2 GSM2481290_mC_Col-0_0DAI_r1 GSM2481291_mC_Col-0_0DAI_r2 GSM2481292_mC_Col-0_1DAI_r1 GSM2481293_mC_Col-0_1DAI_r2 GSM2481294_mC_Col-0_2DAI_r1 GSM2481295_mC_Col-0_2DAI_r2 GSM2481296_mC_Col-0_3DAI_r1 GSM2481297_mC_Col-0_3DAI_r2 GSM2481298_mC_Col-0_4DAI_r1 GSM2481299_mC_Col-0_4DAI_r2
## group2: GSM2481300_mC_rdd_DS_r1 GSM2481301_mC_rdd_DS_r2 GSM2481302_mC_rdd_0DAI_r1 GSM2481303_mC_rdd_0DAI_r2 GSM2481304_mC_rdd_1DAI_r1 GSM2481305_mC_rdd_1DAI_r2 GSM2481306_mC_rdd_2DAI_r1 GSM2481307_mC_rdd_2DAI_r2 GSM2481308_mC_rdd_3DAI_r1 GSM2481309_mC_rdd_3DAI_r2 GSM2481310_mC_rdd_4DAI_r1 GSM2481311_mC_rdd_4DAI_r2
## has not been segmentedSegmentation.
obj <- segmentMethCP(obj, bs_object, region.test = "stouffer")Get the DMRs.
regions <- getSigRegion(obj)head(regions)## seqnames start end nC.valid nC mean.diff mean.cov region.pval
## 31 1 238367 239326 69 69 -0.1042 5.7144 0.0012861236
## 66 1 394175 395220 50 50 -0.2490 3.9292 0.0026789957
## 70 1 405983 408116 52 52 -0.1079 6.3197 0.0082183067
## 92 1 531037 532534 36 36 -0.1900 4.3507 0.0007397345
## 187 1 962256 966028 145 145 -0.1049 5.9529 0.0029463070
## 241 1 1258866 1259519 30 30 -0.1454 5.6153 0.0028433185sessionInfo() ## R version 4.0.3 (2020-10-10)
## Platform: x86_64-pc-linux-gnu (64-bit)
## Running under: Ubuntu 18.04.5 LTS
##
## Matrix products: default
## BLAS: /home/biocbuild/bbs-3.12-bioc/R/lib/libRblas.so
## LAPACK: /home/biocbuild/bbs-3.12-bioc/R/lib/libRlapack.so
##
## locale:
## [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C
## [3] LC_TIME=en_US.UTF-8 LC_COLLATE=C
## [5] LC_MONETARY=en_US.UTF-8 LC_MESSAGES=en_US.UTF-8
## [7] LC_PAPER=en_US.UTF-8 LC_NAME=C
## [9] LC_ADDRESS=C LC_TELEPHONE=C
## [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C
##
## attached base packages:
## [1] stats4 parallel stats graphics grDevices utils datasets
## [8] methods base
##
## other attached packages:
## [1] MethCP_1.4.0 bsseq_1.26.0
## [3] SummarizedExperiment_1.20.0 Biobase_2.50.0
## [5] MatrixGenerics_1.2.0 matrixStats_0.57.0
## [7] GenomicRanges_1.42.0 GenomeInfoDb_1.26.0
## [9] IRanges_2.24.0 S4Vectors_0.28.0
## [11] BiocGenerics_0.36.0 BiocStyle_2.18.0
##
## loaded via a namespace (and not attached):
## [1] splines_4.0.3 DelayedMatrixStats_1.12.0
## [3] R.utils_2.10.1 gtools_3.8.2
## [5] BiocManager_1.30.10 BSgenome_1.58.0
## [7] GenomeInfoDbData_1.2.4 Rsamtools_2.6.0
## [9] yaml_2.2.1 pillar_1.4.6
## [11] numDeriv_2016.8-1.1 lattice_0.20-41
## [13] glue_1.4.2 limma_3.46.0
## [15] bbmle_1.0.23.1 digest_0.6.27
## [17] XVector_0.30.0 qvalue_2.22.0
## [19] colorspace_1.4-1 htmltools_0.5.0
## [21] Matrix_1.2-18 R.oo_1.24.0
## [23] plyr_1.8.6 pkgconfig_2.0.3
## [25] XML_3.99-0.5 emdbook_1.3.12
## [27] DSS_2.38.0 bookdown_0.21
## [29] zlibbioc_1.36.0 purrr_0.3.4
## [31] mvtnorm_1.1-1 scales_1.1.1
## [33] HDF5Array_1.18.0 BiocParallel_1.24.0
## [35] tibble_3.0.4 generics_0.0.2
## [37] ggplot2_3.3.2 ellipsis_0.3.1
## [39] mclust_5.4.6 magrittr_1.5
## [41] crayon_1.3.4 evaluate_0.14
## [43] R.methodsS3_1.8.1 MASS_7.3-53
## [45] tools_4.0.3 data.table_1.13.2
## [47] lifecycle_0.2.0 stringr_1.4.0
## [49] Rhdf5lib_1.12.0 munsell_0.5.0
## [51] locfit_1.5-9.4 DelayedArray_0.16.0
## [53] Biostrings_2.58.0 compiler_4.0.3
## [55] fastseg_1.36.0 rlang_0.4.8
## [57] rhdf5_2.34.0 grid_4.0.3
## [59] RCurl_1.98-1.2 rhdf5filters_1.2.0
## [61] methylKit_1.16.0 bitops_1.0-6
## [63] rmarkdown_2.5 DNAcopy_1.64.0
## [65] gtable_0.3.0 reshape2_1.4.4
## [67] R6_2.4.1 GenomicAlignments_1.26.0
## [69] dplyr_1.0.2 knitr_1.30
## [71] rtracklayer_1.50.0 bdsmatrix_1.3-4
## [73] permute_0.9-5 stringi_1.5.3
## [75] Rcpp_1.0.5 vctrs_0.3.4
## [77] tidyselect_1.1.0 xfun_0.18
## [79] sparseMatrixStats_1.2.0 coda_0.19-4