Cell3™ Target: Tumor Exome

For best results, we recommend that you extract cfDNA and quantify just before you start your library prep. However, sometimes there is a need to extract and store cfDNA at an earlier time. In this case we advise you to store extracted cfDNA in a low-bind tube at -20˚C and quantify your samples after thawing when you are ready to start the library preparation.

We don’t recommend mixing Cell3™ Target enrichment probes with libraries prepared from other library prep kits because, we cannot guarantee the results. However, if you choose to do this you should be aware that some compatibility issues may arise. For example, the blockers in our kits are designed for our library prep kits and are for hybridisation-based kits with 8nt or lower indexing. Therefore, depending on the kit you used, you may see a high percentage of off target reads.

Our elution buffer consists of 10 mM Tris-HCl, pH 8.5, 0.1 mM EDTA; therefore, this low level of EDTA should not affect most downstream applications, including NGS library prep. It  is perfectly fine to use water in place of the supplied buffer to elute your cfDNA. If doing so, please ensure that the pH is >6.0.  However, if you are storing your samples the buffer should be used. In addition, it is advisable to only extract cfDNA at the point where you plan to proceed with the library preparation.

We have successfully run the protocol using cyclers that have a set max volume of 50 or 100ul.

A heated lid that has a max of 99˚C should work fine. However, we would not recommend adjusting the denaturation temperature to 95 ˚C in the library amplification step and recommend keeping this at 98 ˚C.

We recommend the hybridisation to run between 4 and 16 hours. However, we regularly run the hybridization overnight. For example, start the reaction at 4/5pm and then perform probe capture at 9 or 10am the following day. We have tested longer hybridisation times only up to 24 hours. If you want to break in between, you can store the dried down library pool with COT-1 DNA and universal blockers overnight at 4˚C if you want to delay starting the hybridisation reaction.

Section 2C Captured library amplification, of the Cell3 Target protocol gives general guidelines on the suggested number of post capture PCR cycles. For very small targets the number of cycles would need to be increased. For example, 0.004Mb may need an additional 2 cycles (total 18) to ensure there is enough library for sequencing. Therefore, please note that for very small targets you may need to optimise the number of post capture PCR cycles.

The correct type of Dyna Beads is critical to the success of the capture. Only M-270 Streptavidin beads are suitable and will give the correct post capture yield. Any other beads are likely to be a different diameter and concentration and have shown to give a significant decrease in final yield.

As you have plenty of DNA for your WGS, and therefore not using the UMIs, we recommend performing a PCR free method to eliminate PCR bias. To do this you would need a minimum of 100ng of DNA to input and if your FFPE DNA has very low DIN scores (e.g. less than 3) you would need to increase your DNA input by 5-10 fold (see pg. 11 in Cell3 Target protocol).

A.    The following are the steps for running a PCR free method for Cell3 Target library prep:

1. Fragmentation/End repair/A-tailing (use 1 ul for tapestation to verify size)
2. Ligation of adapters
3. Bead clean-up
4. Quantitation by qPCR (not Qubit and TapeStation)

When conducting a PCR free library prep, the QC at the end (following adapter ligation) will not give accurate Qubit readings (they will appear lower than what they should be) and the Tapestation will not show fragments in the right range. This is due to the ligation of Y-shaped adapters. The best way to QC is using qPCR. If you want to confirm the average fragment size, you could use 1 ul of the fragmentation reaction (post incubation) and run that on the tapestation. The average fragment length + adapters (144bp) will give the final library average fragment length.

Quantity of probe set is 18 ul for a 4x reaction kit for the cancer 50 panel. This allows you to prepare 4 x hybridisation pools with one probe set.

Library pooling guidelines for all custom and off the shelf products are listed on pg. 27 of the protocol. For the Cancer 50 panel up to 16 libraries can be pooled into 1 hybridisation reaction and you use 4 ul of probe set per reaction. For the exome panel, up to 8 libraries can be pooled. By all means, you can pool less than this number. However, you will need to ensure that the concentration of the combined pool reaches a total of 1000ng.

A. In all cases it is important to accurately determine the DNA concentration in your sample, especially when using <100 ng of DNA as input. To do this we recommend using a fluorometric method (such as the Qubit assay, Invitrogen).  Additionally, you may want to perform different QC steps on your input material according to your material type.

a) FFPE derived DNA may be compromised due to formalin fixation so we recommend performing a quality check by running your samples on a genomic screen tape (TapeStation) to determine the DIN (DNA integrity) score. This allows you to optimise the DNA input adding more material with lower DIN scores.

b) DNA derived from plasma may be contaminated with gDNA and the extent of this can be determined using a High Sensitivity D1000 Screen Tape on the TapeStation. Contamination with gDNA can arise during the processing of plasma due to haemolysis of white blood cells. A peak at >1500 bp is evidence of sample contamination with gDNA. You may also want to run this check to determine that the TapeStation profile reveals fragments of the expected size range (see Fig. 2 above).