Editing BioMicroCenter:News2013 (section)

=== APRIL 20, 2013 ===

We have noticed a number of technical issues with some Illumina runs. We want to share with you to make sure you are aware of some changes and newly identified technical issues with the platform and what we are doing to correct them where we can. All of these changes are from the Illumina side and none were especially well documented (some not at all). These issues are unlikely to be limited to the BMC, so samples from elsewhere on campus or around the country may also have these issues. Please read this as it may have some impact on your analyses.  <BR><BR>

Just to begin, all of these changes are subtle and not obvious in most cases directly from the sequencers. It was the rare cases that had dramatic effects that caused us to notice them. If you decide you need to have samples rerun, we will work with you to try to get Illumina to replace the reagents and to get the samples rerun. Unfortunately, there is no way we can possibly do bulk reruns of several months’ worth of studies.  <BR><BR>

The most concerning issue is a dropout of GC rich regions in clustering. This has been an on-again off-again issue with Illumina that we have addressed over a year ago by improvements in amplification cycling conditions and enzyme selection. Some time, several months ago (we do not have a precise window), Illumina appears to have changed the chemistry of one of their clustering components and that caused a major change in performance on GC rich areas. This can be seen as an absence of reads from very GC rich areas but, because these areas are rare in most genomes, they cannot be seen on the flowcell wide metrics. This issue is found on current HiSeq and MiSeqV2 kits but not on MiSeqV1 kits nor, we suspect, on the GAII. We have been able to address this problem by adding a brief boiling step during NaOH denaturation of the samples and have implemented this as SOP starting about two weeks ago. This drop out of regions can cause significant issues for several studies – most notably ChIP analyses – when you are comparing data from different chemistries.  <BR><BR>

A second concern is one that has been reported in the community but we have not identified on our machines – yet – where samples from a run are being observed in the following run as minor contaminants. This issue is limited to the MiSeq and HiSeq2500 (we do not have the latter) where the tubes that add sample to the flowcell are not changed. This contamination is reported to be <1% and so would not show up on our quality metrics. However, if your MiSeq analyses are being based on finding a few reads in a large pool of discarded data or you are doing a number of sequential runs, you may wish to validate your data more carefully using an alternative technique such as qPCR or sanger sequencing. There is currently no technical fix to this problem.  <BR><BR>

A third issue has been around for a while though we had not appreciated the implications. Illumina’s newer versions of basecalling software have become less capable of handling uniform sequence (all A’s for example). In earlier versions, only 5 basepairs of variability were needed and intensities could be determined by the control lane we run on all HiSeq flowcells. Now, it appears that nt 1-25 all must have representation of all 4 bases at all positions, even with a control lane. This has always been an issue on the MISeq and we have solved it by spiking in 30%PhiX in the lane (as opposed to our normal 0.1% spike in). Similar solutions can be used on the HiSeq. Given this change, we are re-evaluating whether there is value in using the 8th lane as a control. The latest version of MiSeq software (only a couple days old) supposedly allows us to lower the fraction to 5%, but how successful this is remains to be seen. Base rearrangement with the GAII allows the GAII to avoid this issue.  <BR><BR>

Finally, it appears that custom priming on the MiSeq is not the same as custom priming on the HiSeq and GAII. It can still be done, but the Tm requirement is much higher. Primers that work on the HiSeq may fail on the MiSeq. As long as your Tm matches or exceeds the Tm used for Illumina primers, the MiSeq should work, but the MiSeq’s different chemistry (formamide instead of heat denaturation) is less forgiving.  <BR><BR>

In summary, we have a number of technical challenges that may (or may not) effect you and we want to make sure you have all the information we can give you. I want to thank the researchers and labs that have been very patient as we have struggled running their samples which led us to identify these problems. If you believe these issues have effected your data, please do not hesitate to contact me and we can discuss how to move forward.  <BR><BR>