In perishky/dmrff: Identifying differentially methylated regions efficiently with power and control

Meta-analysis using dmrff

Download and prepare DNA methylation datasets

We'll use a couple of small cord blood DNA methylation datasets available on GEO: GSE79056, GSE62924.

Loading each dataset takes about 1 minute.

accessions <- c("GSE79056", "GSE62924")
library(geograbi) ## https://github.com/yousefi138
samples <- sapply(accessions, geograbi.get.samples, simplify=F)
vars <- sapply(samples, geograbi.extract.characteristics, simplify=F)
system.time(methylation <- sapply(accessions, geograbi.get.data, simplify=F))

Remove non-CpG probes from the datasets.

for (acc in accessions)
    methylation[[acc]] <- methylation[[acc]][grepl("^c", rownames(methylation[[acc]])),]

Extract the variable of interest from each dataset.

variable <- sapply(accessions, function(acc) {
    idx <- grep("(ga weeks|gestational_age|gestational age)", colnames(vars[[acc]]))
    stopifnot(length(idx) == 1)
    as.numeric(vars[[acc]][,idx])
}, simplify=F)

Remove samples with incomplete data.

for (acc in accessions) {
    idx <- which(!is.na(variable[[acc]]))
    samples[[acc]] <- samples[[acc]][idx,]
    vars[[acc]] <- vars[[acc]][idx,]
    variable[[acc]] <- variable[[acc]][idx]
    methylation[[acc]] <- methylation[[acc]][,idx]
}

Calculate surrogate variables to handle unknown confounding

SVA takes a few seconds for each dataset.

library(sva, quietly=T)
set.seed(20191029)
covariates <- sapply(accessions, function(acc) {
    mod <- model.matrix(~ var, data.frame(var=variable[[acc]]))
    mod0 <- mod[,1,drop=F]
    random.idx <- sample(1:nrow(methylation[[acc]]), 5000)
    meth.sva <- methylation[[acc]][random.idx,]
    meth.mean <- rowMeans(meth.sva, na.rm=T)
    idx <- which(is.na(meth.sva), arr.ind=T)
    if (nrow(idx) > 0)
        meth.sva[idx] <- meth.mean[idx[,1]]
    sva(meth.sva, mod=mod, mod0=mod0)$sv
}, simplify=F)

Test the associations with gestational age

Testing associations takes a few seconds for each dataset.

library(limma)
stats <- sapply(accessions, function(acc) {
    mod <- model.matrix(~ ., data.frame(ga=variable[[acc]],
                                        covariates[[acc]]))
    fit <- lmFit(methylation[[acc]], mod)
    fit <- eBayes(fit)
    data.frame(estimate=fit$coefficients[,"ga"],
               se=sqrt(fit$s2.post) * fit$stdev.unscaled[,"ga"],
               p.value=fit$p.value[,"ga"])
},simplify=F)

Now we check the agreement between the datasets.

kable(sapply(stats, function(a) sapply(stats, function(b) {
    sites <- intersect(rownames(a), rownames(b))
    sites <- sites[order(a[sites,"p.value"])[1:100]]
    cor(a[sites,"estimate"], b[sites,"estimate"])
})))

A previous study reported thousands of associations with gestational age:

Bohlin J, et al. Prediction of gestational age based on genome-wide differentially methylated regions. Genome Biol. 2016;17(1):207.

The effect estimates are strongly associated.

bohlin <- read.csv("bohlin.csv",stringsAsFactors=F,row.names=1)
kable(sapply(stats, function(stats) {
    cor(bohlin$estimate, stats[rownames(bohlin),"estimate"], use="p")
}))

Annotate summary statistics with genomic locations

If your data was generated using one of the Illumina BeadChips, then it is possible to annotate your EWAS summary statistics using the appropriate Bioconductor annotation package.

For the Illumina 450K microaray, we use the IlluminaHumanMethylation450kanno.ilmn12.hg19 package.

if (!require(IlluminaHumanMethylation450kanno.ilmn12.hg19, quietly=T)) {
    install.packages("BiocManager")
    BiocManager::install("IlluminaHumanMethylation450kanno.ilmn12.hg19")
}
library(IlluminaHumanMethylation450kanno.ilmn12.hg19)

If we were using the more recent Illumina MethylationEPIC BeadChip, then we would load the IlluminaHumanMethylationEPICanno.ilm10b2.hg19 package instead.

We construct an annotation data frame as follows:

data(list="IlluminaHumanMethylation450kanno.ilmn12.hg19")
data(Locations)
data(Other)
annotation <- cbind(as.data.frame(Locations), as.data.frame(Other))

We then add the annotation to the stats object (Warning: we assume that the row names of the stats object are the Illumina CpG site identifiers, e.g. cg05775921).

stats <- sapply(stats, function(stats) {
    annotation <- annotation[match(rownames(stats), rownames(annotation)),]
    cbind(stats, annotation)
}, simplify=F)

Construct 'pre' objects for DMR meta-analysis

Takes about 2 minutes per dataset to calculate CpG site correlations.

library(dmrff)
pre <- sapply(accessions, function(acc) {
    cat(date(), "pre", acc, "\n")
    with(stats[[acc]], dmrff.pre(estimate, se, methylation[[acc]], chr, pos))
}, simplify=F)

Perform DMR meta-analysis

There are hundreds of DMRs for this phenotype, so the final meta-analysis takes about 15 minutes with a single processor.

options(mc.cores=20) ## if you have 20 processors available
meta <- dmrff.meta(pre)

The output contains a data frame of the meta-analysed regions and the EWAS meta-analysis on which it was based.

kable(meta$dmrs[1:2,])

kable(meta$ewas[1:2,])

Here we add the CpG sites and gene names to the EWAS sites:

idx <- match(with(meta$ewas, paste(chr,pos)),
             with(annotation, paste(chr,pos)))
meta$ewas$cpg <- rownames(annotation)[idx]
meta$ewas$gene <- annotation$UCSC_RefGene_Name[idx]

Meta-analyzed DMRs

dmrs <- meta$dmrs[which(meta$dmrs$p.adjust < 0.05 & meta$dmrs$n >= 2), ]

We've identified r nrow(dmrs) DMRs with Bonferroni adjusted p < 0.05.

For convenience, we'll just look at DMRs on chromosome 6 with at least 5 CpG sites.

dmrs6 <- meta$dmrs[which(meta$dmrs$p.adjust < 0.05
                         & meta$dmrs$n >= 5
                         & meta$dmrs$chr == "chr6"), ]
dmrs6 <- dmrs6[order(dmrs6$start),]
kable(dmrs6[,c("chr","start","end","n","estimate","se","p.value","p.adjust")])

Below we use the dmrff.sites function to show the CpG sites in each DMR and their meta-analysed summary statistics.

sites <- dmrff.sites(dmrs6, meta$ewas$chr, meta$ewas$pos)
sites <- cbind(sites[,c("region","chr","pos")],
               meta$ewas[sites$site,c("cpg", "estimate","se","p.value")])
kable(sites,row.names=F)

best6 <- dmrs6[which.min(dmrs6$p.value),]
dmrff.plot(
    best6$chr, best6$start, best6$end,
    meta$ewas$estimate, meta$ewas$se, meta$ewas$chr, meta$ewas$pos) 

We can use the same function to show the summary statistics for these sites from one of the original studies.

acc <- "GSE79056"
sites <- dmrff.sites(dmrs6, stats[[acc]]$chr, stats[[acc]]$pos)
sites <- cbind(sites[,c("region","chr","pos")],
               stats[[acc]][sites$site,c("estimate","se","p.value")])
kable(sites,row.names=F)

best6 <- dmrs6[which.min(dmrs6$p.value),]
dmrff.plot(
    best6$chr, best6$start, best6$end,
    stats[[acc]]$estimate, stats[[acc]]$se, stats[[acc]]$chr, stats[[acc]]$pos) 

Going back to full set of DMRs, we can ask which regions are novel, i.e. contain no CpG sites identified by Bohlin et al.

## consider only autosomal regions
dmrs <- dmrs[which(dmrs$chr %in% paste0("chr", 1:22)),]
## collect CpG sites in the regions
sites <- dmrff.sites(dmrs, meta$ewas$chr, meta$ewas$pos)
sites <- cbind(sites, meta$ewas[sites$site,c("cpg","estimate","se","p.value")])
## identify sites with associations according to Bohlin et al.
sites$bohlin <- sites$cpg %in% rownames(bohlin)
## novel regions do not contain any of the Bohlin sites
dmrs$novel <- !(1:nrow(dmrs) %in% sites$region[sites$bohlin])

Of r nrow(dmrs) DMRs identified, r round(100*sum(dmrs$novel)/nrow(dmrs),2)% are novel.

They appear on the following chromosomes:

freq.novel <- table(dmrs$chr[dmrs$novel])
freq <- table(dmrs$chr)
prop <- 100*freq.novel/freq
prop <- as.data.frame(prop)
colnames(prop) <- c("chr","pct")
prop$novel <- freq.novel
prop$total <- freq
prop <- prop[order(prop$pct),]
kable(prop,row.names=F,digits=2)

perishky/dmrff documentation built on July 19, 2024, 5:16 a.m.