In a first with RNA-Seq technology, scientists at Stanford University have broken through the single-cell barrier. In a recently published paper, A.R. Wu et al., Quantitative assessment of single-cell RNA-sequencing methods, Nature Methods, 20 (2013), report the results of sequencing RNA from zero human cells. How was this accomplished? The gist of it is that an empty tube was filled with spike-in, and then submitted for RNA-Seq… The details are as follows: In order to assess the quality of single-cell RNA-Seq, Wu et al. performed numerous single-cell RNA-Seq experiments on HCT116 cells, as summarized in the figure below (Figure 1a from their paper).
Figure 1a from the Wu et al. paper showing the experimental design.
I was interested in this study because for the regularized pooling project I’m working on with Nicolas Bray (see recent post), it would be useful to demonstrate improvements in quantification accuracy by joint analysis of single-cell RNA-Seq. I asked Nick to look at the Wu et al. data when it came out two weeks ago, and as a first step he aligned the reads to the human transcriptome. Strangely, he found very low alignment rates, and in some cases literally almost no reads aligned at all. At first we thought there was some trimming issue, so we went to look at the Cufflinks output of the authors. The figure below, made by Nick, shows the percent spike-in (assessed by examining the abundance of ERCC-*) for each of the SMARTer based 96 samples:
The worst sample is C70 (GSM1241223) for which only 252 human transcripts have non-zero abundance. It is 99.828339% spike-in! The fact that the results of RNA-Seq on an empty test tube were published is in and of itself just a minor (?) embarrassment; more interesting is the range of quality obtained as measured by the amount of spike-in sequenced– a plot that we have made above and that seems crucial to the paper, but that was not produced by the authors. In fact, what the authors do show is slightly suspect: reproduced below is their Figure S2 from the Supplement:
Why would the authors show correlations for just four randomly picked samples? Why not show results for all of the data? We dug a bit deeper into this, and noticed that 93/96 of the FPKM file names look like [GEO accession]_CXX_ILXXXX. But the remaining three look like GSM1241223_C70_NTC_tube_ctrl_IL3196.sorted.genes.fpkm_tracking.txt.gz (which is the apparently empty tube), GSM1241245_C92_cell_tube_ctrl_IL3198.sorted.genes.fpkm_tracking.txt.gz and GSM1241195_C42_100ng_RNA_ctrl_IL3198.sorted.genes.fpkm_tracking.txt.gz. Therefore, these were presumably intended controls, but they were not published as such. There is the separate issue, that aside from the controls, the experiment in general appears to have some failure rate that is not clearly presented. This is evident in the following plot which Nick made, showing the average log-correlation of each experiment with the others after removing zeroes (the bottom one is C09 and the runner up is C70):
This figure is showing the honest truth of the paper. It is what it is; everyone I’ve talkedto that has actually performed single-cell RNA-Seq tells me that it is difficult and there is a non-trivial failure rate, on top of variable quality across cells. In fact, there is subtle evidence of failure in other papers. In the single-cell RNA-Seq technology race, the paper preceding Wu et al was A.K. Shalek et al., Single-cell transcriptomics reveals bimodality in expression and splicing in immune cells, Nature (2013). In Shalek et al., the authors describe 18 single-cell experiments. Specifically, they claim to have constructed DNA libraries “from 18 single BMDCs (S1–S18), three replicate populations of 10,000 cells, and two negative controls (empty wells), and sequenced each to an average depth of 27 million read pairs.” However a close inspection of the GEO reveals the following IDs and descriptors:
|GSM1012777||Single cell S1|
|GSM1012778||Single cell S2|
|GSM1012779||Single cell S3|
|GSM1012780||Single cell S4|
|GSM1012781||Single cell S5|
|GSM1012782||Single cell S6|
|GSM1012783||Single cell S7|
|GSM1012784||Single cell S8|
|GSM1012785||Single cell S9|
|GSM1012786||Single cell S10|
|GSM1012787||Single cell S11|
|GSM1012788||Single cell S13|
|GSM1012789||Single cell S14|
|GSM1012790||Single cell S15|
|GSM1012791||Single cell S16|
|GSM1012792||Single cell S22|
|GSM1012793||Single cell S23|
|GSM1012794||Single cell S24|
While there are 18 consecutive IDs, the cell labels range from 1–24. Where are the 6 missing cells? I can’t be sure, but they were probably failures. Update: the authors of the Shalek et al. paper explained to me after seeing the post that two of the missing labels were negative controls, and 3 were population replicates (the names of these were altered in GEO). which was indeed a failure (S12); it gave no signal on the BioAnalyzer and was therefore not sequenced. I was told that the authors are working on fixing the GEO sample names to clarify the reason for missing labels of samples. As such, it turns out the experiment was extremely successful with a success rate of 18/19.
Returning to Wu et al., they should be commended for releasing all their data (to their credit they also release the R code they used for analysis). The problem with the paper is that instead of reporting the failures and discarding them before analysis, they instead use all of the data when performing comparisons between single-cell and bulk RNA-Seq. This is is evident in some of the strange techniques they end up using. For example, the method for generating the crucial Figure 4a is described as:
“(a) Correlation between the merged
single cells (“ensemble”) and the bulk RNA-seq measurement of gene
expression. The ensemble was created by computationally pooling all
the raw reads obtained from the 96 single-cell transcriptomes
generated using the C1 system and then sampling 30 million reads
randomly. The bulk and ensemble libraries were depth matched before
alignment was performed. For each gene, the log2-transformed median
FPKM values from the ensemble and bulk were plotted against each
I’m guessing that the odd idea of sampling and then taking the median is precisely to throw out outliers coming from the control tubes. Yes, the data were tortured, and yes, the FPKMs confessed. The paper has some other issues that suggest it was not carefully reviewed by the authors (let alone the reviewers). In the Methods I found the statement “FPKM values used for analyses were generated by TopHat”. I, of all people, can attest to the fact that it is Cufflinks, not TopHat, that estimates (not generates!) FPKM values. Thankfully, in the GEO entries Cufflinks is correctly cited together with the version used.
In summary, in the last two high profile publications on single-cell RNA-Seq, there were failures in the experiment and they were not reported clearly by the authors. Neither committed an egregious offense, but I wish they had fully reported the number of experiments attempted and the number that succeeded. That seems to me to be important data in papers describing new technology. I believe that fear of rejection from the journal, or fear of embarrassment of the state of single-cell RNA-Seq is what drove Wu et al. to spin the results positively. All part of the fear of failure, that seems to hold back a lot of science. But single-cell RNA-Seq has a bright future and these papers would both be better if they were more open about failure. The only thing we have to fear is fear itself.