MinHash signatures as ways to find samples, and collaborators?

As I wrote last week my latest enthusiasm is MinHash sketches, applied (for the moment) to RNAseq data sets. Briefly, these are small "signatures" of data sets that can be used to compare data sets quickly. In the previous blog post, I talked a bit about their effectiveness and showed that (at least in my hands, and on a small data set of ~200 samples) I could use them to cluster RNAseq data sets by species.

What I didn't highlight in that blog post is that they could potentially be used to find samples of interest as well as (maybe) collaborators.

Finding samples of interest

The "samples of interest" idea is pretty clear - supposed we had a collection of signatures from all the RNAseq in the the Sequence Read Archive? Then we could search the entire SRA for data sets that were "close" to ours, and then just use those to do transcriptome studies. It's not yet clear how well this might work for finding RNAseq data sets with similar expression patterns, but if you're working with non-model species, then it might be a good way to pick out all the data sets that you should use to generate a de novo assembly.

More generally, as we get more and more data, finding relevant samples may get harder and harder. This kind of approach lets you search on sequence content, not annotations or metadata, which may be incomplete or inaccurate for all sorts of reasons.

In support of this general idea, I have defined a provisional file format (in YAML) that can be used to transport around these signatures. It's rather minimal and fairly human readable - we would need to augment it with additional metadata fields for any serious use in databases(but see below for more discussion on that). Each record (and there can currently only be one record per signature file) can contain multiple different sketches, corresponding to different k-mer sizes used in generating the sketch. (For different sized sketches with the same k-mers, you just store the biggest one, because we're using bottom sketches so the bigger sketches properly include the smaller sketches.)

If you want to play with some signatures, you can -- here's an executable binder with some examples of generating distance matrices between signatures, and plotting them. Note that by far the most time is spent in loading the signatures - the comparisons are super quick, and in any case could be sped up a lot by moving them from pure Python over to C.

I've got a pile of all echinoderm SRA signatures already built, for those who are interested in looking at a collection -- look here.

Finding collaborators

Searching public databases is all well and good, and is a pretty cool application to enable with a few dozen lines of code. But I'm also interested in enabling the search of pre-publication data and doing matchmaking between potential collaborators. How could this work?

Well, the interesting thing about these signatures is that they are irreversible signatures with a one-sided error (a match means something; no match means very little). This means that you can't learn much of anything about the original sample from the signature unless you have a matching sample, and even then all you know is the species and maybe something about the tissue/stage being sequenced.

In turn, this means that it might be possible to convince people to publicly post signatures of pre-publication mRNAseq data sets.

Why would they do this??

An underappreciated challenge in the non-model organism world is that building reference transcriptomes requires a lot of samples. Sure, you can go sequence just the tissues you're interested in, but you have to sequence deeply and broadly in order to generate good enough data to produce a good reference transcriptome so that you can interpret your own mRNAseq. In part because of this (as well as many other reasons), people are slow to publish on their mRNAseq - and, generally, data isn't made available pre-publication.

What if you could go fishing for collaborators on building a reference transcriptome? Very few people are excited about just publishing a transcriptome (with some reason, when you see papers that publish 300), but those are really valuable building blocks for the field as a whole.

So, suppose you had some RNAseq, and you wanted to find other people with RNAseq from the same organism, and there was this service where you could post your RNAseq signature and get notified when similar signatures were posted? You wouldn't need to do anything more than supply an e-mail address along with your signature, and if you're worried about leaking information about who you are, it's easy enough to make new e-mail addresses.

I dunno. Seems interesting. Could work. Right?

One fun point is that this could be a distributed service. The signatures are small enough (~1-2 kb) that you can post them on places like github, and then have aggregators that collect them. The only "centralized" service involved would be in searching all of them, and that's pretty lightweight in practice.

Another fun point is that we already have a good way to communicate RNAseq for the limited purpose of transcrpiptome assembly -- diginorm. Abundance-normalized RNAseq is useless for doing expression analysis, and if you normalize a bunch of samples together you can't even figure out what the original tissue was. So, if you're worried about other people having access to your expression levels, you can simply normalize the data all together before handing it over.

Further thoughts

As I said in the first post, this was all nucleated by reading the mash and MetaPalette papers. In my review for MetaPalette, I suggested that they look at mash to see if MinHash signatures could be used to dramatically reduce their database size, and now that I actually understand MinHash a bit more, I think the answer is clearly yes.

Which leads to another question - the Mash folk are clearly planning to use MinHash & mash to search assembled genomes, with a side helping of unassembled short and long reads. If we can all agree on an interchange format or three, why couldn't we just start generating public signatures of all the things, mRNAseq and genomic and metagenomic all? I see many, many uses, all somewhat dimly... (Lest anyone think I believe this to be a novel observation, clearly the Mash folk are well ahead of me here -- they undersold it in their paper, so I didn't notice until I re-read it with this in mind, but it's there :).

Anyway, it seems like a great idea and we should totally do it. Who's in? What are the use cases? What do we need to do? Where is it going to break?


p.s. Thanks to Luiz Irber for some helpful discussion about YAML formats!

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