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Thursday, April 16, 2009

Multi-match reads in ChIP-Seq

I had an interesting comment left on my blog today, which is worth taking a few minutes to write a response to:
"Hi Anthony, I just discovered your blog and it looks very interesting to me!
Since this article on Eland is now more than one year old, I was wondering
if the description at point 3 about multi matching locations is still
applicable to the Eland program in the Illumina pipeline 1.3. More in general,
would you trust the multi matching locations extracted from the multi_eland
output files to perform a repeat enrichment analysis over an experiment of
ChIP-seq? If no, why? Thank you in advance for your attention."

The first question asks about multi-matching locations - and if the point in question (point 3) applies to the Illumina Pipeline 1.3. Since point 3 was just that the older pipeline didn't provide the locations of the multi-matche reads, I suppose this no longer really applies: I understand the new version of Eland does provide multi-match alignment information, as do other aligners such as Bowtie. However, I should also mention that since I adopted Maq as my preferred aligner, I haven't used Eland much - so it's hard for me to give an informed opinion on the quality of the matches. I simply don't know if they're any good, and I won't belabour that point. I have used Bowtie specifically because it was able to do mutli-matches, but we didn't use it for ChIP-Seq, and the multi-matches had other uses in that experiment.

So, the more interesting question is whether I'd use multi-match reads in a ChIP-Seq analysis. And, off hand, my answer has to be no. But let me explain my reasoning, and the conditions in which I would change that answer.

First, lets assume that we have Single End Tags, so the multi-match information is not resolvable. That means anytime we have a read that maps to more than one location, we have the possibility that we can either map it to it's source - or we're mapping it incorrectly. A 50% change of "getting it right." The greater the number of multi-match locations, the smaller the chance we're actually finding it's correct origin. So, at best we've got a 50-50 chance that we're not adversely affecting the outcome of the experiment. That's not great.

In contrast, there are things we could do to make them usable. The most widely used method from FindPeaks is the weighted fragment distribution type. Thus, we could expand the principle to weight the fragments according to the number of sites. That would be... bearable. But would it significantly add to the quality of the alignment?

I'm still going to say no. Fragments we see in ChIP-Seq experiments tend to fall within 200-300bp of the regions in which the transcription factor (or other sites) bind. Thus, even if we were concerned that a particular transcription factor binds primarily to the similar motif regions at two sites, there should be more than enough (unique) sequence around that site (which is usually <30-40bp in length) to which you'll still see fragments aligning. That should compensate for the loss of the multi-match fragments.

Even more importantly, as read lengths increase, the amount of non-unique sequence decreases rapidly, making the shrinking number of multi-match reads less important.

The same argument can be extended for paired end tags: Just as read lengths improve and reduce the number of multi-match sites, more of the multi-match reads will be resolved by pairing them with a second read, which is unlikely to be within the same repeat region, thus reducing the number of reads that become unresolvable multi-matches. Proportionally, one would then expect that leaving out these reads become a smaller and smaller segment of the population, and would have to worry less and less about their contribution.

So, then, when would I want them?

Well, on the odd chance you're working with very short reads, you can pull off the weighting properly, and you have single end tags - and the multi-match reads make up a significant proportion of the reads, then it's worth exploring.

You'd need to start asking the tough questions: did the aligner simply find that a small k-mer of the read aligned to multiple locations (and was then unable to resolve the tie by extension the way some Eland aligners work)? Does the aligner use quality scores to identify mis-alignments? How reliable are the alignments (what's their error rate)? What was your sample, and how divergent is it from reference ? (e.g., cancer samples have a high variation rate, and so encourage many false alignments, making the alignments less reliable.)

Overall, I really don't see too many cases where you're going to gain a lot by digging in the multi-match files. That's not too say that you won't find anything good in there - you probably would, if you knew where to look, but the noise to signal ratio is going to be pretty poor - just by definition of the fact that they're mutli-match reads alone. You'll just have to ask if it's worth your time.

For the moment, I don't think my time (even at grad student wages) is worth it. It's just not low hanging fruit, when it comes to ChIP-Seq.

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Friday, March 20, 2009

Universal format converter for aligned reads

Last night, I was working on FindPeaks when I realized what an interesting treasure trove of libraries I was really sitting on. I have readers and writers for many of the most common aligned read formats, and I have several programs that do useful functions. So, that raise the distinctly interesting point that all of them should be applied together in one shot... and so I did exactly that.

I now have an interesting set of utilities that can be used to convert from one file format to another: bed, gff, eland, extended eland, MAQ .map (read only), mapview, bowtie.... and several other more obscure formats.

For the moment, the "conversion utility" forces the output to bed file format (since that's the file type with the least information, and I don't have to worry about unexpected file information loss), which can then be viewed with the UCSC browser, or interpreted by FindPeaks to generate wig files. (BED files are really the lowest common denominator of aligned information.) But why stop there?

Why not add a very simple functionality that lets one format be converted to the other? Actually, there's no good reason not to, but it does involve some heavy caveats. Conversion from one format type to another is relatively trivial until you hit the quality strings. since these aren't being scaled or altered, you could end up with some rather bizzare conversions unless they're handled cleanly. Unfortunately, doing this scaling is such a moving target that it's just not possible to keep up with that and do all the other devlopment work I have on my plate. (I think I'll be asking for a co-op student for the summer to help out.)

Anyhow, I'll be including this nifty utility in my new tags. Hopefully people will find the upgraded conversion utility to be helpful to them. (=

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Monday, April 21, 2008

Eland file Format

I haven't written much over the past couple of days. I have a few things piled up that all need doing urgently... and it never fails, that's when you get sick. I spent today in bed, fighting off a cold, sore throat and fever. Wonderful combination.

Anyhow, since I'm starting to feel better, I thought I'd write a few lines before going to bed, and wanted to mention that I've finally seen a file produced by the new Eland. It's a little different, and the documentation provided (ahem, that I was able to obtain from a colleague who uses the pipeline) was pretty scarce.

In fact, much of what you see in the file is pretty obvious, with the same general concept as the previous Eland files, except this has a few caveats:

1) the library name and 4-coordinate position of the sequence are all separated by tabs in one of the files I saw, but concatenated with a separating ":" in another. I'm not sure which is the real format, but there are at least 2 formats for line identification.

2) There's a string that seems to encode the base quality scores from the prb files, but it's in a format for which I can't find any information.

3) there's a new format for mismatches within the alignment. Instead of telling you the location of the mismatch, you now get a summary of the alignment itself. If Eland could do insertions, it would work well for those too. From the document, it tells you the number of aligned bases, with letters interspersed to show the mismatch. (e.g. if you had a 32 base alignment, with a mismatch A at character 10, you'd get the string "9A22".) I also understand that upper and lower case mismatches mean something different, though I haven't probed the format too much.

So, in the discussion of formats, I understand there's some community effort around using a so-called "short read format" or SRF format. It's been adopted by Helicos, GEO, as well as several other groups.

Maybe it's time I start converting Eland formats to this as well. Wouldn't it be nice if we only had to work with one format? (If only Microsoft understood that too! - ps, don't let the community name fool you, it's well known Microsoft sponsored that site.)

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