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This 2014 PNAS paper by S. Lin et al (Lin et al., PNAS, 2014) that compares transcription of tissues between species has a flawed experimental design, where species is almost perfectly confounded with machine / lane on which the sequencing was done. Y. Golad and O. Mizrahi-Man have published a manuscript describing the confounding and the results of removing it. This was possible because the original authors supplied the information about which publically available files were used in the original analysis. The data from this experiment is probably only suitable as an example of what not to do in high-throughput biology experimental design, and that there may be similarities in human and mouse transcriptional programs.
We discussed these papers in the Systems Biology and Omics Integration University of Kentucky Journal Club on June 1, 2015. I lead that discussion.
In 2014, two papers were published by members of the ENCODE project purporting that tissue gene expression clustered more by species than by tissue. In the first (ENCODE Consortium, Nature, 2014, doi:10.1038/nature13992 2014), a large number of experiments were combined and compared. Figure 2a shows a PCA plot of expression in across 10 tissues in human and mouse.
Interestingly enough, if you collapse PC1 (definitely species), then the tissues start to look quite similar. Which does not seem that unexpected, there are species specific differences, but the tissues are doing something similar in each species.
This was not the end of the story, however. A subsequent publication (S Lin et al., PNAS, 2014, doi: 10.1073/pnas.1413624111) went further in doing fresh sequencing of 13 tissues in both species, still showing bigger differences between species than between tissues.
On April 28, Y. Gilad sent out this tweet:
The left figure shows the original data, clustering by species, and on the right, reprocessed data, clustering by tissue. Now, I have to admit when he posted this I was kind of ticked off. Where was the blog-post or manuscript showing what exactly was done? If you look at the comments to Yoav on that tweet, it seems others were wondering the same thing. Thankfully, on May 19, the manuscript hit F1000Research (Gilad Y and Mizrahi-Man O. A reanalysis of mouse ENCODE comparative gene expression data [v1; ref status: approved with reservations 1, http://f1000r.es/5ez] F1000Research 2015, 4:121 (doi: 10.12688/f1000research.6536.1)).
Yoav asked for the list of data files that were used in the PNAS paper, and then examined the read id line to extract the experimental design. This experimental design is captured in Figure 1:
Do you notice a problem with this design?
. . . . .
Hopefully you noticed that species (one of the main effects to investigate) is not randomly or even semi-randomly distributed across sequencers and / or lanes, but is almost perfectly confounded with sequencer / lane. It doesn't matter if one technician handled all the samples, this is potentially a large batch effect / confounding variable.
And Yoav shows that ignoring the batch effect produces data much like that reported in the Lin et al PNAS pub, while removing the batch effect using ComBat results in the tissues clustering together.
I presented these papers and the discussion around them at our weekly Systems Biology and Omics Integration (SBOI) Journal Club on June 1, 2015. There were a couple of concerns with the overall study:
Finally, I think the best use of the 2014 PNAS pub and this dataset is an example of how not to design a biological experiment.
As I looked at the comment section of the Gilad & Man article yesterday (June 1, 2015), I noticed that there were direct replies from S. Lin in a couple of places. In particular is a comment that Lin et al did a second set of sequencing with a new design, and reanalysed the data. Links are provided to two figures, a table of the new design and a new 3D PCA plot:
The new sequencing design seems much more reasonable, and the PCA plot has many characteristics of the original one from the original comparative analysis by Mouse ENCODE (see above), in that yes, there are species specific differences, but there also appears to be a way to collapse along PC2 and PC3 where the tissues will line up with each other, which I kind of would expect.