In mdeber/BRGenomics: Tools for the Efficient Analysis of High-Resolution Genomics Data

Using DESeq2 for Pairwise Differential Expression

Rationale

DESeq2's default treatment of data relies on the assumption that genewise estimates of dispersion are largely unchanged across samples. While this assumption holds for a typical RNA-seq data, it can be violated if there are samples within the DESeqDataSet object for which there are meaningful signal changes across a majority of regions of interest.

The BRGenomics functions getDESeqDataSet() and getDESeqResults() are simple and flexible wrappers for making pairwise comparisons between individual samples, without relying on the assumption of globally-similar dispersion estimates. In particular, getDESeqResults() follows the logic that the presence of a dataset $X$ within the DESeqDataSet object will not affect the comparison of datasets $Y$ and $Z$.

While the intuition above is appealing, users should note that if the globally-similar dispersions assumption does hold, then DESeq2's default behavior should be more sensitive in its estimates of genewise dispersion. In this case, users can still take advantage of the convenience of the BRGenomics function getDESeqDataSet(), but they should subsequently call DESeq2::DESeq() and DESeq2::results() directly.

If the globally-similar dispersions assumption is violated, but something beyond simple pairwise comparisons is desired (e.g. group comparisons or additional model terms), we note that, with some prying, DESeq2 can be used without "blind dispersion estimation" (see the DESeq2 manual).

Formatting Data for DESeq2

Just like the functions that generate batch-normalized spike-in normalization factors, the DESeq-oriented functions require that the names of the input datasets end in "rep1", "rep2", etc.

Let's first make a toy list of multiple datasets to compare:

library(BRGenomics)
data("PROseq")

ps_list <- lapply(1:6, function(i) PROseq[seq(i, length(PROseq), 6)])
names(ps_list) <- c("A_rep1", "A_rep2", 
                    "B_rep1", "B_rep2",
                    "C_rep1", "C_rep2")

ps_list[1:2]
names(ps_list)

As you can see, the names all end in "repX", where X indicates the replicate. Replicates will be grouped by anything that follows "rep". If the sample names do not conform to this standard, the sample_names argument can be used to rename the samples within the call to getDESeqDataSet().

data("txs_dm6_chr4")

dds <- getDESeqDataSet(ps_list, txs_dm6_chr4,
                       gene_names = txs_dm6_chr4$gene_id,
                       ncores = 1)
dds

Notice that the dim attribute of the DESeqDataSet object is c(111, 6). There are 6 samples, but length(txs_dm6_chr4) is not 111. This is because we provided gene names to getDESeqDataSet(), which were non-unique. The feature being exploited here is for use with discontinuous gene regions, not for multiple overlapping transcript isoforms.

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By default, getDESeqDataSet() will combine counts across all ranges belonging to a gene, but if they overlap, they will be counted twice. For addressing issues related to overlaps, see the reduceByGene() and intersectByGene() functions.

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We could have added normalization factors, which DESeq2 calls "size factors", in the call to getDESeqDataSet(), or we can supply them to getDESeqResults() below. (Supplying them twice will overwrite the previous size factors).

Important note on normalization factors: DESeq2 "size factors" are the inverse of BRGenomics normalization factors. So if you calculate normalization factors with NF <- getSpikeInNFs(...), set sizeFactors <- 1/NF.

Getting DESeq2 Results

Generating DESeq2 results is simple:

getDESeqResults(dds, contrast.numer = "B", contrast.denom = "A",
                quiet = TRUE, ncores = 1)

We can also make multiple pairwise-comparisons by ignoring the contrast.numer and contrast.denom arguments, and instead using the comparisons argument. The resulting list of results is named according to the comparisons:

comparison_list <- list(c("B", "A"), 
                        c("C", "A"),
                        c("C", "B"))
dsres <- getDESeqResults(dds, comparisons = comparison_list,
                         quiet = TRUE, ncores = 1)
names(dsres)
dsres$C_vs_B

mdeber/BRGenomics documentation built on March 4, 2024, 9:46 a.m.