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Single-event transgene-product levels predict levels in genetically modified breeding stacks Satyalinga Srinivas Gampala, Brandon J. Fast, Kimberly A Richey, Zhifang Gao, Ryan Christopher Hill, Bryant Wulfkuhle, Guomin Shan, Greg A Bradfisch, and Rod A. Herman J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03098 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 22, 2017
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Single-event transgene-product levels predict levels in genetically modified breeding stacks
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Satyalinga Srinivas Gampala*, Brandon J. Fast, Kimberly A. Richey, Zhifang Gao, Ryan Hill,
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Bryant Wulfkuhle, Guomin Shan, Greg A. Bradfisch, Rod A. Herman
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Dow AgroSciences, Indianapolis, Indiana, USA
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*Corresponding Author:
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Satyalinga Srinivas Gampala
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Dow AgroSciences, Building 312, 9330 Zionsville Road, Indianapolis, IN 46268 USA
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Telephone: 1-317-337-3830
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Email:
[email protected] 11
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Abstract:
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The concentration of transgene products (proteins and double-stranded RNA) in genetically
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modified (GM) crop tissues is measured to support food, feed, and environmental risk
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assessments. Measurement of transgene-product concentrations in breeding stacks of previously
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assessed and approved GM events is required by many regulatory authorities to evaluate
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unexpected transgene interactions that might affect expression. Research was conducted to
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determine how well concentrations of transgene products in single GM events predict levels in
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breeding stacks composed of these events. The concentrations of transgene products were
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compared between GM maize, soybean, and cotton breeding stacks (MON-87427 × MON-89034
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× DAS-Ø15Ø7-1 × MON-87411 × DAS-59122-7 × DAS-40278-9 corn; DAS-81419-2 × DAS-
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44406-6 soybean; DAS-21023-5 x DAS-24236-5 x SYN-IR102-7 × MON-88913-8 × DAS-
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81910-7) and their component single events (MON-87427, MON-89034, DAS-Ø15Ø7-1, MON-
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87411, DAS-59122-7, DAS-40278-9 corn; DAS-81419-2, DAS-44406-6 soybean; DAS-21023-
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5, DAS-24236-5, SYN-IR102-7, MON-88913-8, DAS-81910-7). Comparisons were made
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within a crop and transgene product across plant tissue types, and were also made across
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transgene products in each breeding stack for grain/seed. Scatter plots were generated
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comparing expression in the stacks to their component events, and the percent of variability
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accounted for by the line of identity (y = x) was calculated (coefficient of identity, i2). Results
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support transgene concentrations in single events predicting similar concentrations in breeding
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stacks containing the single events. Therefore, food, feed, and environmental risk assessments
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based on concentrations of transgene products in single GM events are generally applicable to
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breeding stacks composed of these events.
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Keywords:
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Transgene expression; Genetically Modified (GM) breeding stacks; Genetically Modified
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Organisms (GMO), food, feed, and environmental risk assessment; safety assessment, coefficient
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of identity
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Introduction:
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As part of the safety assessment for genetically modified (GM) crops, the transgene products
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(proteins or double-stranded RNA) are assessed for safety. Risk is a product of hazard (toxicity)
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and exposure. There have been no hazards identified for commercialized transgene products in
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the context of food and feed safety1, 2, and existing transgene products have been issued an
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exemption from the requirement for a tolerance in pesticidal GM crops regulated by the US
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Environmental Protection Agency (EPA). Therefore exposure measurements should not be
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necessary for food and feed safety assessments of these products.
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Insecticidal transgene products may present a potential hazard to a subset of non-target insects,
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typically taxonomically related to target pests. Thus, an exposure assessment for these transgene
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products is required as part of the assessment of the environmental risk of pesticidal transgene
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products to beneficial (or otherwise valued) non-target organisms. For pesticidal transgene
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products where sensitive non-target organisms might be exposed to crop tissues, it is important
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to characterize the concentrations of transgene products in the tissues to which exposure is
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expected.
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In practice, the concentrations of all transgene products in commercialized GM crops have been
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characterized in multiple plant tissues at various crop stages irrespective of any identified hazard.
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These experiments are commonly referred to as expression studies. Furthermore, most
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regulatory authorities require expression studies to be completed for GM breeding stacks (single
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GM events bred together using traditional breeding) to bridge the environmental risk assessments
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of single GM transformation events to these breeding stacks.3 When breeding stacks express
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transgene products at levels similar to those expressed in single GM events, the environmental
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risk assessments done for the single events apply to the breeding stack.
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Regulatory authorities also consider expression in food/feed safety assessments of GM breeding
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stacks despite the lack of hazard associated with the transgene products. Furthermore, some
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geographies regulate the efficacy and/or durability of pesticidal GM crops (which can be
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dependent on transgene expression levels), but this evaluation is related to product performance
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rather than safety.4 It is noteworthy that, to ensure market success, the phenotype and trait
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performance of new crop varieties and hybrids are routinely evaluated by plant breeders
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independent of government regulation or whether the trait originated from GM or traditional
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breeding.
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Since transgenes in GM events are introgressed into many different commercialized varieties or
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hybrids with many different genotypes, the requirement to reassess expression in GM breeding
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stacks appears to be based on the hypothesis that transgenes are more likely to interact with each
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other than with the many new endogenous genotypes into which they are introgressed. To our
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knowledge, there is no scientific evidence to support this hypothesis. The requirement to
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reassess transgene product levels in breeding stacks also does not take into consideration whether
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a hazard for these products has been identified. Furthermore, consideration is often not taken as
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to whether the margins of exposure previously afforded by empirical toxicology tests are
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insufficient to protect against harm if expression in the stack was to increase within biologically-
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plausible limits.
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Here we report empirical transgene-expression data for breeding stacks compared with their
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individual component events across multiple crops and tissue types. We plot the expression level
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of each transgene product in each tissue/crop-stage for the single events against the stack, and 5 ACS Paragon Plus Environment
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quantify the predictive capability for the single-event expression to predict stack expression by
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calculating the percent of variation (coefficient of identity) captured by the line of identity (y =
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x).5, 6 Because of the importance of grain/seed in trade and the food and feed safety assessment,
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we also plot the grain/seed expression levels for each transgene product for the single events
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against the breeding stacks across transgene products, and calculate the coefficient of identity (I2)
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for these relationships in each crop/stack. These results help visualize and quantify the
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predictability of expression in the breeding stacks based on expression in the single events, and
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evaluate the usefulness of transgene expression data for breeding stacks in their safety
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assessment.
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Materials and Methods:
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GM breeding stacks:
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The largest Dow AgroSciences (Indianapolis, IN) maize, soybean, and cotton GM breeding
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stacks were chosen for investigating the transgene product expression levels compared with
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expression in their component single events. These breeding stacks included MON-87427 ×
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MON-89034 × DAS-Ø15Ø7-1 × MON-87411 × DAS-59122-7 × DAS-40278-9 maize
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expressing CP4 EPSPS, Cry1A.105, Cry2Ab2, Cry1F, PAT, Cry3Bb1, Cry34Ab1, Cry35Ab1,
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and AAD-1 proteins, and DvSnf7 double stranded RNA; DAS-81419-2 × DAS-44406-6 soybean
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expressing Cry1Ac, Cry1F, PAT, AAD-12, and 2mEPSPS proteins; and DAS-21023-5 x DAS-
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24236-5 x SYN-IR102-7 × MON-88913-8 × DAS-81910-7 cotton expressing the Cry1Ac,
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Cry1F, PAT, Vip3Aa19, APH4, CP4 EPSPS, and AAD-12 proteins (Table 1). DAS-21023-5
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and DAS-24236-5 cotton events were never commercialized as separate events, and accordingly
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expression data were not generated for the single events in these studies; therefore the two-event
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breeding stack of these events was treated as a single event for the purposes of this analysis. The
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Bacillus thuringiensis crystal (Cry) and vegetative (Vip) proteins and the DvSnf7 double
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stranded RNA confer insect resistance, and the CP4 EPSPS, PAT, AAD-1, AAD-12, and
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2mEPSPS proteins confer herbicide tolerance (Table 1).
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Field trials:
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Field trials were conducted at multiple locations with four replicate blocks at each location
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arranged in a randomized complete block design. MON-87427 × MON-89034 × DAS-Ø15Ø7-1
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× MON-87411 × DAS-59122-7 × DAS-40278-9 maize was grown during 2015 at eight sites
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(Kimballton, IA; Cresco, IA; Richland, IA; Stewardson, IL; Pickard, IN; Kirksville, MO; York,
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NE; Germansville, PA); DAS-81419-2 × DAS-44406-6 soybean was grown during 2012 at nine
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sites (Richland, IA; Atlantic, IA; Carlyle, IL; Wyoming, IL; Sheridan, IN; Kirksville, MO; Fisk,
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MO; York, NE; Germansville, PA); and DAS-21023-5 × DAS-24236-5 × SYN-IR102-7 ×
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MON-88913-8 x DAS-81910-7 cotton was grown during 2013 at five sites (Tallassee, AL;
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Sycamore, GA; Greenville, MS; Cedar Grove, NC; East Bernard, TX). All plots within a study
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location were treated with the same maintenance chemicals and agronomic practices. Plant
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tissue types and the crop stage at collection are indicated in figure captions.
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Analytical Methods:
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Proteins were quantified using ELISA methods validated under EPA Good Laboratory Practice
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(GLP) standards. CP4 EPSPS, Cry2Ab2, Cry34Ab1, AAD-1 and PAT in maize, PAT in
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soybean and CP4 EPSPS, AAD-12, and PAT in cotton were quantified using validated Dow
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AgroSciences methods with kits purchased from Acadia, LLC (Portland, ME) or EnviroLogix
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(Portland, ME). Cry1F and Cry35Ab1 in maize, Cry1Ac and Cry1F in soybean, and Cry1Ac,
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Cry1F, and Vip3A in cotton were also quantified using validated Dow AgroSciences methods
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with kits purchased from Acadia, LLC (Portland, ME) or Romer Labs (Newark, DE).
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Cry1A.105 was quantified using a validated method following a protocol provided by Monsanto
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(St. Louis, MO). Cry3Bb1 and APH4 were quantified using a validated method with ELISA kits
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produced at Dow AgroSciences or Acadia, LLC (Portland, ME). AAD-12 and 2mEPSPS in
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soybean were quantified using a validated method with ELISA kits produced at Acadia
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(Portland, ME) or at Dow AgroSciences. The double stranded RNA transgene product, DvSnf7,
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was quantified in using the QuantiGene Plex 2.0 RNA assay platform from Affymetrix (Santa
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Clara, CA) as previously described.7 All ELISA methods were fully validated under GLP (Good
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Laboratory Practices) according to ICH guidelines which include evaluation of recovery, limit of
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detection, and precision.
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Data Analysis:
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Mean expression levels (expressed as ng/mg dry-weight of tissue for proteins and ng/g dry-
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weight for DvSnf7) within each single event for each transgene product were plotted against the
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relevant breeding stack across crop tissue types. For grain/seed, mean expression levels within
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each single event were plotted against the relevant breeding stack across transgene products.
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Where the same transgene product is present in multiple single events found in the breeding
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stack (i.e. CP4 EPSPS protein in maize, and PAT protein in maize, soybean, and cotton),
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expression levels were summed to represent total expression in the component single events.
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The amount of variation in the data accounted for by the line of identity (I2) was calculated in a
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manner analogous to how the coefficient of determination (R2) is calculated for a regression line5
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. The I2 is calculated as follows: 8 ACS Paragon Plus Environment
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It is noteworthy that the I2 will never exceed the R2 for any given dataset because, unlike a
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regression line that is fit to the data, the line of identity is fixed (y = x). Analyses were
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conducted in both the natural and base-10 logarithmic (Log10) scales. Analyses in the Log10
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scale weights lower expressing proteins more heavily than in the natural scale.
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Results and Discussion:
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With the exception of APH4, which was undetectable in most cotton tissues, the other transgenes
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products expressed at detectable levels in sampled plant tissues. The plots of single-event
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transgene expression levels against stack expression levels, across crop tissue types, show
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limited scatter around the line of identity (Figures 1-4). The variation observed between the
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single and stack transgene expression levels across crops and crop tissue types from single-year
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multi-site field trials is well accounted for by the line of identity for all transgene products across
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all breeding stacks (I2 > 0.86; I2 > 0.93 in Log10 scale) with the exception of Cry1Ac expression
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in soybean (I2 = 0.14; I2 = 0.85 in Log10 scale; Figure 3) and marginally for Cry1F in cotton (I2
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= 0.71; I2 = 0.84 in Log10 scale; Figure 2).
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A single tissue type (leaf at V5) drove the variability (low I2 value) seen between the soybean
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breeding stack (8.7 ng Cry1Ac/mg) and the single event (25.4 ng Cry1Ac/mg) (Figure 4);
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however, the stack sprayed with the trait-relevant herbicides (2,4-D + glyphosate + glufosinate)
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expressed similar levels (20.7 ng Cry1Ac/mg) as the single event (25.4 ng Cry1Ac/mg) in the
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same tissue and study. Furthermore, the expression in a later-stage leaf tissue (V10-12) was
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similar for the single event (23.7 ng Cry1Ac/mg) and the stack (22.6 ng Cry1Ac/mg). This
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suggests that the lower expression in the stack for a single plant tissue for one GM protein in a
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single crop is likely an anomaly due to the large number of transgene products measured across
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several plant tissues in three different crops (115 comparisons). Regardless, the magnitude of
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reduction in Cry1Ac level observed for the stack in this single early stage leaf tissue does not
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raise any safety concerns.
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The marginally lower capacity of the single cotton event to predict the stack for Cry1F (I2 =
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0.71) compared with other transgene products across the three crops (I2 > 0.86) was driven by
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slightly lower expression in the breeding stack compared with the single event across plant
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tissues (Figure 3). As with the Cry1Ac level in V5 leaf, the magnitude of reduction in Cry1F
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level observed for the stack in these leaf tissue does not raise any safety concerns.
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The results expressed in the Log10 scale weight lower-expressing tissues more heavily than in
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the natural scale and thus balance the contribution of the different tissues more evenly than in the
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natural scale. It is noteworthy that the variation observed between single and stack transgene
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expression levels across crops and crop tissue types is well accounted for by the line of identity
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for all transgene products across all breeding stacks in the Log10 scale (I2 > 0.83).
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Considering the weight of evidence from these results, expression of transgene products in single
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events predicts expression in stacks very well. Since crop-tissue expression levels are used to
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estimate exposure in environmental risk assessments, these data strongly support the
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applicability of environmental risk assessments of single events to breeding stacks where hazard
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has not increased (no biologically significant synergism among pesticidal transgene products in
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the breeding stack).
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Plots of single-event transgene expression levels against stack expression levels in grain/seed,
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across all transgene products, also show limited scatter around the line of identity (Figure 5).
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The variation observed between the single and stack transgene expression levels in grain/seed
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across transgene product types was well accounted for by the line of identity (I2 > 0.96 across all
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breeding stacks in both the natural and Log10 scales). While transgene products in currently
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commercialized crops have shown no hazard in the context of a food and feed risk assessment, if
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a transgene product were to demonstrate some level of hazard in a future GM crop, then
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expression levels in grain could be used to estimate worst-case exposure in humans and
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livestock. It is noteworthy that transgene products in current commercial GM commodity crops
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are all inactivated and/or degraded by the processing of the GM grain/seed to produce food, and
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thus exposure is dramatically lower than predicted from raw grain/seed (8). The results of this
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analysis show that the transgene expression levels in grain/seed for single GM events predict the
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levels in GM breeding stacks very well across different transgene products within the same
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breeding stack. Thus, food and feed safety assessments for single events should apply to
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breeding stacks containing these same GM events.
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The results of this investigation indicate that transgene expression levels of single GM events
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predict expression levels in GM breeding stacks very well across crop tissues and among
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transgene products within a crop tissue (grain/seed) for the three major GM crops investigated
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(maize, soybean, and cotton). The analysis reported here avoids some of the pitfalls of
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conducting pairwise statistical comparisons between expression levels in single GM events and
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stacks using difference tests (ANOVA). These weaknesses include: 1) false positives due to
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many individual comparisons (multiplicity), 2) differences in transgene expression due to slight
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genotype differences between single events and stacks (common when traits from different
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developers are combined in stacks due to different initial transformant germplasm) 9, 3 detection
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of statistically significant differences that are very small in magnitude and have no biological
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relevance in the context of a risk assessment. The profiling plots used here put the magnitude of
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observed expression levels into the context of differences among crop tissues and among
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transgene products.
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Transgene expression can help inform risk assessments, and the results reported here provide
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strong indication that stacking individual GM events through traditional breeding has a
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negligible effect on transgene expression. This finding is consistent with both the expectation
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and observation that transgenes are no more likely to interact with each other than with unrelated
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endogenous genes.5, 10-13 While genotype and environment certainly can affect the expression of
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endogenous proteins, and thus expected to affect transgenes in a similar manner, previous
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investigations have demonstrated that environmental factors typically have the greatest impact
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on the production of endogenous proteins, as recently exemplified by proteinaceous soybean
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allergens.14, 15 Since transgenes have been observed to behave the same as endogenous genes 16,
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plant breeders evaluate GM trait performance in each new crop variety or hybrid under diverse
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environmental conditions (as they do for non-GM traits) before their commercial release to
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ensure consistency across the geographies where they will be grown. In the context of
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agricultural production and risk assessment, the measurement of transgene expression for single
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GM events under diverse environmental conditions provides a robust evaluation of potential
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exposure to a GM event alone or in breeding stacks.
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Notes:
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All authors are employed by Dow AgroSciences LLC, a wholly owned subsidiary of Dow
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Chemical Company, which develops transgenic crops.
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Acknowledgement:
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We thank Nick Storer and Chandrashekar Aradhya for reviewing the manuscript and providing
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input.
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Figure captions:
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Expression in single events is plotted on X-axis and expression in breeding stacks is plotted on
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Y-axis. Results were also expressed in the Log10 scale, and coefficient of identity (I2) was
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shown as insert within each figure panel.
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Figure 1. Maize single-event versus breeding-stack expression (ng/mg dw) of AAD-1, CP4
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EPSPS, Cry1A.105, Cry1F, Cry2Ab2, and Cry34Ab1 across tissue types showing line of
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identity. Tissue types are represented in the plots by the following symbols:
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= leaf V9,
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Figure 2. Maize single-event versus breeding-stack expression (ng/mg dw) of Cry35Ab1,
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Cry3Bb1, DvSnf7 RNA (ng/gm), and PAT across tissue types showing line of identity. Tissue
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types are represented in the plots by the following symbols:
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leaf R1,
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Figure 3. Cotton single-event versus breeding-stack expression (ng/mg dw) of transgene
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products (AAD-12, CP4 EPSPS, Cry1Ac, Cry1F, PAT and Vip3Aa19) across tissue types
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showing line of identity. Tissue types are represented in the plots by the following symbols:
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leaf BBCH 14,
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seed BBCH 99.
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Figure 4. Soybean single-event versus breeding-stack expression (ng/mg dw) of transgene
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products (2mEPSPS, AAD-12, Cry1Ac, Cry1F, PAT) across tissue types showing line of
316
identity. Tissue types are represented in the plots by the following symbols:
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leaf V10-12,
= leaf R1,
= root,
= root R1,
= forage, and
= leaf BBCH 60,
= forage R3,
= forage R5, and
= leaf V2-V4,
= grain R6.
= leaf V2-V4,
= leaf V9,
=
= grain.
= leaf BBCH 80,
= root R3, and
= root BBCH 86, and
= grain R8.
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=
= cotton
= leaf V5,
=
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Figure 5. Maize, soybean, and cotton single-event versus breeding-stack grain/seed expression
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(ng/mg dw) of transgene products across transgene products within each stack. Transgene
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products are represented by the following symbols: :
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Cry1A.105,
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DvSnf7 RNA, and
= Cry1F,
= Cry2Ab2,
= AAD-1,
= Cry34Ab1,
= CP4 EPSPS,
= Cry35Ab1, .
= PAT.
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=
= Cry3Bb1,
=
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Table 1: GM events and corresponding transgenic gene products.
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GM Event
Transgenic Gene Products
Trait (s)
DAS-Ø15Ø7-1 maize
Cry1F, PAT
Insect resistance, Herbicide tolerance
DAS-59122-7 maize
Cry34Ab1, Cry35Ab1, PAT
Insect resistance, Herbicide tolerance
DAS-4Ø278-9 maize
AAD-1
Herbicide tolerance
MON-89Ø34 maize
Cry1A.105, Cry2Ab2
Insect resistance
MON-87427 maize
CP4 EPSPS
Herbicide tolerance
MON-87411 maize
Cry3Bb1, DVsnf7, CP4
Insect resistance, Herbicide
EPSPS
tolerance
Cry1F, Cry1Ac, PAT
Insect resistance, Herbicide
DAS-81419-2 soybean
tolerance DAS-44406-6 soybean
AAD-12, 2mEPSPS, PAT
Herbicide tolerance
DAS-21023-5 cotton
Cry1Ac, PAT
Insect resistance, Herbicide tolerance
DAS-24236-5 cotton
Cry1F, PAT
Insect resistance, Herbicide tolerance
SYN-IR102-7 cotton
Vip3Aa19, APH4
Insect resistance
MON-88913-8 cotton
CP4 EPSPS
Herbicide tolerance
DAS-81910-7 cotton
AAD-12, PAT
Herbicide tolerance
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Figure 1
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Figure 3
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Figure 5
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