Do count-based differential expression methods perform poorly when genes are expressed in only one condition?
© Zhou and Robinson. 2015
Received: 11 December 2014
Accepted: 15 September 2015
Published: 8 October 2015
A response to ‘Comprehensive evaluation of differential gene expression analysis methods for RNA-seq data’ by Rapaport F, Khanin R, Liang Y, Pirun M, Krek A, Zumbo P, Mason CE, Socci ND and Betel D in Genome Biology, 2013, 14:R95
Statistical methods for determining transcriptional changes between (replicated) groups of cell populations using RNA sequencing (RNA-seq) data are now quite mature. Several themes that emerged from the past decade of modeling microarray data apply analogously to RNA-seq data: parameter moderation is critical, multiple testing corrections are necessary and flexible frameworks (e.g., linear models) to account for the effect of covariates are essential. For RNA-seq data, popular packages such as edgeR, DESeq and DESeq2 [1–3] perform detailed modeling of the dispersion–mean relationship, with variations on fitting a dispersion by mean trend and moderating estimates toward the trend. Likewise, careful modeling of the mean–variance relationship of transformed data has been proven effective, essentially ‘unlocking’ the world of heteroskedastic linear regression .
A recent report in Genome Biology from Rapaport and co-authors claimed that some methods, namely PoissonSeq  and limma , ‘have improved modeling of genes expressed in one condition’, where they showed a striking difference in the ability to separate differential expression (DE) . From a methodological perspective, this result caught our interest and prompted us to understand how aspects of the all-zero-in-one-condition manifest undesirable properties in count-based models. Briefly, (i) we found a coding error in the calculation of edgeR’s signal-to-noise (S/N) metric and (ii) our re-analysis suggests that count-based methods perform as well or better than other methods, counter to the original conclusion.
The Rapaport manuscript is an excellent model of modern bioinformatics research, in terms of making processed data and code available that reproduce figures from their manuscript. In many cases, the small details can be important and this open-source model facilitates quick access in understanding precisely what settings were used. We fully support this model and by default, also make our code available. In this correspondence, we investigate the genesis of differences in method performance that Rapaport and co-authors observed and provide our view of how performance results can be sensitive to decisions made.
Genes expressed in only one condition
We first briefly summarize the analysis that Rapaport and colleagues reported, with respect to the all-zero-in-one-condition case.
Using gene-level read counts, they isolated genes that exhibit zero-counts across all replicates of a single condition; in general, the number of such genes is related to the depth of sequencing dedicated to each sample, with deeper sequencing resulting in fewer such cases. The dataset in question, comparing GM12892 cells to H1-hESC cells , with three and four replicates, respectively, had typical read depths for such experiments (16–39 million mapped reads). They used the following pipeline: (i) from the count table, generate DE P values for several methods; (ii) calculate S/N using ‘normalized’ data; (iii) plot negative log P value versus S/N, where they expect a monotonic positive dependency (correlation); and, (iv) generate receiver operating characteristic (ROC) curves with thresholds on the S/N to illustrate the ability to separate low S/N (<3) from high S/N (>3).
They highlighted that count-based methods such as DESeq and edgeR, which infer changes in expression via the negative binomial (NB) model, do not perform very well in this case. It is worth noting that this is a non-standard use of ROC curves: here, all genes are strictly DE, but they vary in their magnitude of change. So, the ROC curve represents the ability to separate low S/N from high S/N. Rapaport and colleagues postulated that the NB model reduces to Poisson (dispersion ≈ 0) and lacks the ability to handle the ‘wide variations’ in gene counts among replicate libraries. Our aim with this report is to understand the origins of this result, whether it is a shortcoming of the dispersion estimation strategy or in the inference machinery, since parameter estimates are on the boundary of the parameter space.
Signal-to-noise has some potential limitations
We became interested in the suitability and robustness of the S/N metric itself, since it forms the basis for the ‘truth’ in Rapaport’s ROC result. In theory, the S/N of the non-zero observations should accurately reflect the significance of model-based P values for the expressed-in-one-condition versus zero differences. In practice, however, there are some potential difficulties: the sample sizes are small and therefore, the S/N itself is subject to considerable estimation uncertainty; it is well known that for count data the variance is intimately tied to the mean, so it is not clear whether S/N should be calculated on a linear scale. In addition, a notable aspect of the Rapaport ROC comparison is that while the same S/N cutoff (= 3) is used across all methods, different sets of true and false DE labels are used; this makes the curves difficult to compare, since both the truth and score change by method. We explore these issues here.
Top ten genes originally calculated using S/N (for edgeR-normalized data; first ten rows) and top ten genes calculated using FDR for DE (edgeR P values; second ten rows). The table includes the counts-per-million table (A = GM12892 and B = H1-hESC), S/N and estimated false discovery rate (FDR) for edgeR and limma-voom for the ENCODE dataset comparing three replicates of GM12892 to four replicates of H1-hESC
Count-based methods perform well on zero-in-one-condition simulation
Given recent efforts in simulating RNA-seq count tables [9–11], we tried to create a representative simulation for the zero-in-one-condition situation. The simulation was designed as follows: (i) generate a dataset with no DE and (ii) randomly select genes across the spectrum of expression levels and set counts for one condition (chosen at random) equal to zero to represent ‘true’ DE genes. As previously, we sampled NB mean and dispersion estimates from the joint distribution of estimates using a large dataset (here, from ) and filtered out extreme dispersion values. Altogether, 30,000 features were generated in a 5 versus 5 two-group comparison and zero-counts were introduced to 5 % of the features. To reflect that zeros occur somewhat more often at lower expression across various datasets (see Additional file 2: Fig. S4), we increased the frequency of zero-counts at low expression strength.
As developers and users of bioinformatics strategies, we are particularly interested in the metrics and methods that differentiate performance between the available tools. In this paper, we claim that count-based methods perform well when genes are only expressed in one condition, in contrast to an earlier report. We showed that a code error and the chosen scale of S/N resulted in the earlier conclusion that count-based methods suffer performance in this situation. By calculating the S/N on a different scale and using the same set of labels across methods, a reversal of method performance was observed. This highlights a sensitivity to decisions made in constructing the benchmark.
Using a customized simulation that introduces zero-counts in one experimental condition, we demonstrated that the performance of the count-based method is actually on a par with or better than other methods. We also debunked the postulation that poor performance is related to dispersion estimation in count models.
In the process of seeking the origins of this statistical performance difference, we discovered another potentially interesting phenomenon that may affect the interpretation of results. Looking at Table 1 and Additional file 1, it is evident that the scale of P values is drastically different between edgeR and voom. Although this observation appears rather unrelated to the ability to separate true from false DE genes, it is an indication that the scale of observations modeled affects the magnitude of statistical evidence derived. Not surprisingly, method performance is ultimately dependent on the scales, parameters and datasets used for the evaluation.
R code and data that can be used to reproduce the figures in the main manuscript and in the supplement are available online .
XZ is supported by an Swiss National Science Foundation project grant (143883). MDR acknowledges funding from the European Commission through the 7th Framework Collaborative Project RADIANT (grant agreement number 305626). We acknowledge data processing help from Malgorzata Nowicka at the outset of the project and to her and Charity Law for helpful feedback on an earlier version of the manuscript.
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