Abstract: 【Objective】Transgenic maize NK603 approved for import is an important target of genetically modified organism (GMO) regulation in China. The implementation of GMO regulation requires reference materials (RMs) and standardized detection methods. Establishment of a duplex droplet digital PCR (ddPCR) would provide accurate measurement technology for quantification of NK603 event and development of NK603 RMs. 【Method】 A standard plasmid molecule pUC57-NK603 was constructed by DNA synthetic technique; the primer/probe set of NK603 event was combined with different maize reference genes one by one to select reference gene PCR assay with identical amplification ability to NK603 event-specific PCR assay; main reaction parameters, such as annealing temperature and primer/probe concentration，were optimized in the course of establishing duplex ddPCR; the standard plasmid solution was serially diluted to investigate the limit of detection (LOD), limit of quantification (LOQ) and dynamic range of the duplex ddPCR assay; blinded samples with mass fractions of 100%, 10% and 6% were prepared by mixing NK603 powder with non-GM counterpart, to evaluate the accuracy of quantitative results of the duplex ddPCR. 【Result】 The reference gene zSSIIb was determined to combine with NK603 event to establish the NK603/zSSIIb duplex ddPCR method with the standard plasmid molecule pUC57-NK603 as a quality control after analyzing the fluorescence amplitude of positive droplets, separation between positive and negative droplets, raindrop number, and consistency between measured copy number ratio and expected copy number ratio of NK603 event to reference gene. The primer/probe concentration was optimized to be 400 nmol·L^-1/200 nmol·L^-1 for both NK603 event and zSSIIb gene, and the annealing temperature was determined to be 60°C. The LOD of NK603/zSSIIb duplex ddPCR was estimated to be 2 copies of DNA template, the LOQ was 48 copies of DNA template, both NK603 assay and zSSIIb assay showed good linearity between measured values and theoretical values over the dynamic range from 10 to 60000 copies of DNA template. The NK603/zSSIIb duplex ddPCR achieved accurate quantitative results of NK603 content in blinded maize samples with less than 25% of coefficient of variation; the quantitative results of ddPCR were not significantly different from those of real-time quantitative PCR (qPCR), moreover, the duplex ddPCR showed an advantage over qPCR in term of precision. 【Conclusion】 The selection of reference genes affects the accuracy of quantitative results by ddPCR. Establishment of ddPCR methods should use samples with accurate GMO content as quality controls to evaluate the applicability of reference genes. The NK603/zSSIIb duplex ddPCR method was successfully established using the synthetic standard plasmid molecule pUC57-NK603 as a quality control. NK603 certified reference materials (CRMs) have been successfully developed in China by applying the established NK603/zSSIIb duplex ddPCR assay to characterize the property values.

Key words: transgenic maize NK603, duplex droplet digital PCR, reference plasmid molecule, reference gene, quantification