Paired end reads with different lengths

[copacetik]

Hello All,

I am very new to bioinformatics. I am a wet-lab biologist trying to teach myself about RNA-seq.

Using the sra toolkit, I looked at an RNA-seq study on the GEO database. I downloaded the data as fastq files using "fastq-dump --split-files thedata"

I ended up with thedata_1.fastq and thedata_2.fastq, when I ran these through fastqc, the 1 file had a sequence length of 75, while the 2 file had a sequence length of 25.

Is this a mistake I made? I couldn't find any previous topics that covered this. I assumed using --split-files would show me if it was paired end reads, and if so they should be the same size.

If not, is the data still usable? Since I am just trying to teach myself how to work with this data, it would not be a big deal to abandon it, but that would also not exactly fulfill the goal.

Thanks for any help/advice

[dpryan]

That would seem unusual, but then again sometimes people upload weird stuff to GEO. What was the accession number? With that info I or someone else could just double check for you.

If that turns out to be the correct data then it should still be usable. You can still map reads like that and, even if not, you could always just ditch read #2 (though 25bp should suffice).

[Michael.Ante]

AFAIK, Solid reads came in a 50+25 read pair. I think they extended it later (75+35 or so).
Just have a look, if the reads are coded in base-space (ACGT) or in color-space (0123).
So if you found ABI Solid reads, you can use them, but beware of the color-space coding. TopHat1 could handle those, maybe TopHat2 by now; you need to build the index on color-space and align the color-space reads.

[copacetik]

Thank you for the help, the accession number is SRP061544 and I looked at two samples, SRR2125888 and SRR2125889 and both gave me that same 75+25 situation.

It gave the sequencer as a HiSeq 2500.

[dpryan]

Well, you did everything correctly and got what they uploaded. The only problem is that what they uploaded is questionable. A HiSeq 2500 produces paired-end reads of the same length, so it won't produce this dataset. My guess is that they screwed up creating the SRA file and that they actually have 2x50bp reads rather than 2x75. You'll probably be able to tell if this is the case when you align the data, since if I'm correct the alignment metrics will be very weird (i.e., a low alignment rate with lots of soft-clipping of bases 51-75 or read #1).

[GenoMax]

SRA listing does indicate this as an asymmetric submission (75bp+25bp). Perhaps there are some clues in the associated publication (if any).

[HESmith]

Originally Posted by dpryan

A HiSeq 2500 produces paired-end reads of the same length, so it won't produce this dataset.

Actually, the instrument can be programmed for paired ends of different read lengths. For example, one user engineered his barcode on the wrong end of his amplicon library, so we ran 50+10 cycles to allow demultiplexing.

[dpryan]

Originally Posted by HESmith

Actually, the instrument can be programmed for paired ends of different read lengths. For example, one user engineered his barcode on the wrong end of his amplicon library, so we ran 50+10 cycles to allow demultiplexing.

Good point, hopefully whomever uploaded the data can shed some light on things (assuming there's no publication yet).

[copacetik]

Alright, so at least it was not an error on my part. There is no associated publication yet and since I'm only using it for training purposes I probably won't contact the lab. Just a couple followup questions regarding this scenario:
1. Many of the preprocessing tutorials I've read suggest removing reads below a certain length, should I still attempt that? If so, what would be an appropriate length? Most of them suggest removing anything bellow ~35 which of course would not work here. The initial QC shows the data has a lot of adapter reads and skewed GC content, etc, so it seems like this would be appropriate.
2. GenoMax, you mentioned the SRA listing pointing this out, where can I find that? I did not see it anywhere on the site.
3. It seems to me, although maybe I'm misunderstanding this, that since using paired end reads relies on matching the pairs it will only be as good (on average) as the shorter read, is that right? Would it make sense to just use the forward reads and ignore the 25bp reads?

Thanks again for the help.

[GenoMax]

Originally Posted by copacetik

Alright, so at least it was not an error on my part. There is no associated publication yet and since I'm only using it for training purposes I probably won't contact the lab. Just a couple followup questions regarding this scenario:
1. Many of the preprocessing tutorials I've read suggest removing reads below a certain length, should I still attempt that? If so, what would be an appropriate length? Most of them suggest removing anything bellow ~35 which of course would not work here. The initial QC shows the data has a lot of adapter reads and skewed GC content, etc, so it seems like this would be appropriate.
2. GenoMax, you mentioned the SRA listing pointing this out, where can I find that? I did not see it anywhere on the site.
3. It seems to me, although maybe I'm misunderstanding this, that since using paired end reads relies on matching the pairs it will only be as good (on average) as the shorter read, is that right? Would it make sense to just use the forward reads and ignore the 25bp reads?

Thanks again for the help.

I am attaching a screenshot of SRA run browser that shows the length of the two reads graphically.

If this dataset is not correctly uploaded (i.e. it is actually 50x50 but uploaded/parsed as a 75x25) you will start finding spurious/no alignments (as Devon mentioned above).

There should be plenty other datasets to select from that look more normal for training.