Droplet digital PCR (ddPCR) has been used for absolute quantification of genetically engineered (GE) events. Absolute quantification of GE events by duplex ddPCR requires the use of appropriate primers and probes for target and reference gene sequences in order to accurately determine the amount of GE materials. Single copy reference genes are generally preferred for absolute quantification of GE events by ddPCR. Study has not been conducted on a comparison of reference genes for absolute quantification of GE canola events by ddPCR. The suitability of four endogenous reference sequences (HMG-I/Y, FatA(A), CruA and Ccf) for absolute quantification of GE canola events by ddPCR was investigated. The effect of DNA extraction methods and DNA quality on the assessment of reference gene copy numbers was also investigated. ddPCR results were affected by the use of single vs. two copy reference genes. The single copy, FatA(A), reference gene was found to be stable and suitable for absolute quantification of GE canola events by ddPCR. For the copy numbers measured, the HMG-I/Y reference gene was less consistent than FatA(A) reference gene. The expected ddPCR values were underestimated when CruA and Ccf (two copy endogenous Cruciferin sequences) were used because of high number of copies. It is important to make an adjustment if two copy reference genes are used for ddPCR in order to obtain accurate results. On the other hand, real-time quantitative PCR results were not affected by the use of single vs. two copy reference genes.
Keywords: Canola; Ccf, Cruciferin; CruA, Cruciferin A; DMF, DNeasy® mericon Food kit; DNA extraction; Digital PCR; FID, Fast ID Genomic DNA extraction kit; FatA(A), Acyl-ACP thioesterase; GE, genetically engineered; GMO; GMO, genetically modified organism; GMQ2, GM Quicker II DNA extraction kit; HMG-I/Y, high-mobility group protein; NSF, NucleoSpin Food kit; PCR, polymerase chain reaction; Reference genes; SNP, single nucleotide polymorphism; dPCR, digital PCR; ddPCR, droplet digital PCR.