WGS analysis of ALL sample TARGET-10-PALETF-03A-01D
While analyzing this WGS library for subclonal mutations, we discovered that this sample has been contaminated (most likely during library construction) with a significant amount of DNA from another patient in the study (TARGET-10-PALJDL-03A-01D). We determined that the contaminating DNA is likely from the malignant cells of the other patient as sequences corresponding to polymorphisms and somatic mutations in the contaminating patient’s genome can be detected.  Based on the mean level of read support for variants unique to the contaminating patient’s genome, we estimate the amount of contamination to be 20-30%. Despite this contamination, we had sufficient power to detect clonal somatic mutations in the genome of this patient as described in Roberts et al, 2012. Genomic DNA for construction of whole genome shotgun sequencing (WGSS) libraries was prepared from the same biopsy material using the Qiagen AllPrep DNA/RNA Mini Kit (Qiagen, Valencia, CA, USA). DNA quality was assessed by spectrophotometry (260/280 and 260/230) and gel electrophoresis before library construction. Depending on the availability of DNA, between 2 and 10ug was used in WGSS library construction. Briefly, DNA was sheared for 10 min using a Sonic Dismembrator 550 with a power setting of "7" in pulses of 30 seconds interspersed with 30 seconds of cooling (Cup Horn, Fisher Scientific, Ottawa, Ontario, Canada), and analyzed on 8% PAGE gels. The 200-300bp DNA fraction was excised and eluted from the gel slice overnight at 4 degrees Celsius in 300 ul of elution buffer (5:1, LoTE buffer (3 mM Tris-HCl, pH 7.5, 0.2 mM EDTA)-7.5 M ammonium acetate), and was purified using a Spin-X Filter Tube (Fisher Scientific), and by ethanol precipitation. WGSS libraries were prepared using a modified paired-end protocol supplied by Illumina Inc. (Illumina, Hayward, USA). This involved DNA end-repair and formation of 3' A overhangs using Klenow fragment (3' to 5' exo minus) and ligation to Illumina PE adapters (with 5' overhangs). Adapter-ligated products were purified on Qiaquick spin columns (Qiagen, Valencia, CA, USA) and PCR-amplified using Phusion DNA polymerase (NEB, Ipswich, MA, USA) and 10 cycles with the PE primer 1.0 and 2.0 (Illumina). PCR products of the desired size range were purified from adapter ligation artifacts using 8% PAGE gels. DNA quality was assessed and quantified using an Agilent DNA 1000 series II assay (Agilent, Santa Clara CA, USA) and Nanodrop 7500 spectrophotometer (Nanodrop, Wilmington, DE, USA) and DNA was subsequently diluted to 10nM. The final concentration was confirmed using a Quant-iT dsDNA HS assay kit and Qubit fluorometer (Invitrogen, Carlsbad, CA, USA).