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ASSESSMENT OF TOXICITY AND ALLERGENICITY IN TRANSGENIC CROPS
Several laboratiories have used recombinant DNA technology in plant breeding to improved compositional, processing and agronomic characteristics of plants. These plants have been extensively field tested, have gained regulatory approvals and are currently being marketed in a number of countries around the world. These products have demonstrated numerous benefits including a reliable means to control the targeted insect pests which maintaining beneficial insect populations, reduced use of chemical insecticides, reduced level of mycotoxins in maize kernels, increased yields and reduced labor and mechanical inputs. This report will briefly summarize the approach used to assure the safety of foods derived from transgenic crops.
The approach used to assure the safety of transgenic crops is based on the guidance provided by international organisations like the World Health Organisation (WHO), the United Nations Food and Agricultural Organisation (FAO), the International Life Sciences Institute (ILSI) and the Organisation for Economic Cooperation and Development (OECD). All these organisations have accepted the concept of substantial equivalence as cornerstone of the food safety assessment and depending on the extent of change in terms of composition, a tiered approach to safety assessment is recommended. Basically three categories of transgenic crops can be considered (FAO/WHO 1996): (a) transgenic crops which have the same composition as the parent crop, (b) transgenic crops which have the same composition as the parent crop with the exceptions of a well-defined trait, and (c) transgenic crops whose composition differs from the parent crop.
Key areas for assessing the safety of transgenic plants include: molecular characterisation of the genetic modification, agronomic characterisation, nutritional assessment (key nutrients), toxicological assessment (anti-nutrients or toxicants) and an assessment of the gene expression product. The overall goal of this assessment is to determine whether the transgenic plant is substantially equivalent to the conventional plant which has a history of safe use. If the new product is considered to be as safe as the non-modified counterpart. Furthermore, if the transgenic product and counterpart are substantially equivalent in all respects but the introduced traits, then the further assessment focuses on demonstrating the safety of those traits.
Molecular characterisation is a key part of the safety assessment and is necessary to understand which part of the plasmid has been transferred to the host plant and how it is inserted in the plant genome. Data generally required include information on the source of the gene, the gene transfer system used to insert the gene into the plant genome, the number of inserts, the number of copies of inserted genes, and the integrity and stability of the genetic insert.
The agronomic traits from a good starting point for evaluating substantial equivalence of transgenic plants to their conventional counterpart. These traits are normally examined as an integral part of the development program leading to a new food plant variety. For example, in the case of potatoes the traits commonly examined are yield, tuber size and distribution, dry matter content and disease resistance. Agronomic properties may vary depending on local environmental conditions. Therefore, it is critical to compare the agronomic traits of the transgenic crops with the non-modified crops in the same geographical regions as it will be grown commercially. The most elaborate part of the determination of substantial equivalence is the compositional comparison (nutrients and anti nutrients) of the transgenic crop to the parental variety. Chemical analysis can be used to determine substantial equivalence of the whole plant, specific parts of the plant (e.g. roots, leaves, seeds etc.) or products that have been derived by processing the plant (e.g. oil, sugar etc.). As with the agronomic properties, environmental conditions have a significant influence on composition, therefore, the data from a transgenic plant are compared to a parental variety grown under identical soil and climatologic conditions.
When a transgenic food crop has been shown to be substantially equivalent to conventional crop with the exception of the introduced trait(s) which may impart one or more characteristics such as pest resistance, selectively to preferred herbicides or modification of the ripening process, the safety assessment focuses on the introduced trait and the protein expression product of the cloned gene. The biological function/specificity, mode of action of the protein and the history of known consumption determine the kind and extent of assessment undertaken. For example, if the protein is an enzyme, the potential effects of the enzyme on metabolic pathways and levels of endogenous metabolites based on its mode of action and specificity are assessed.
Another aspects of the safety testing of the newly introduced protein is the determination of its amino acid sequence and comparison to known sequences to determine if the protein has sequence homology to food proteins, toxins or allergens. The inherent digestibility of the protein in simulated gastric and intestinal preparations and stability to heat, provide useful information for assessing the potential toxicity and appropriate raw agricultural product plant or a specific processed food is determined to assess the extent of human exposure. The last step in the safety assessment of the introduced protein is an evaluation of acute toxicity in the mouse.
If the newly introduced protein is functionally/structurally related to proteins than are known toxins or antinutrients or is derived from food with history of allergy or is structurally similar to known protein allergens or is not degraded by digestive enzymes or the biological function of the protein has not been characterized and there are no structurally related proteins identified in protein data bases then, additional testing may be undertaken after a case-by-case assessment. Toxicological and nutritional endpoints are evaluated in rat feeding studies to determine a “no-effect level” for antinutrient effects and compared to potential human exposures to determine if an adequate safety margin exists. To establish wholesomeness of the feed, studies on livestock and poultry have been considered. Consideration of additional animal feeding studies may also be necessary if foods derived from transgenic crops are not substantially equivalent to those derived from conventional varieties.
Where there are indications for potential allergenicity, a tiered testing scheme has been developed for additional evaluation by ILSI/IFBC (Metcalfe et al., 1996). The immunogenic potential of the introduced protein is tested in one of the various test tube tests such as the RAST or RAST-inhibition assay or the enzyme-linked immunosorbent assay (ELISA). When test tube tests are negative or equivocal, skin prick tests are carried out. If non positive response are detected in the prick test, then double-blind placebo controlled food challenges with patients known to be allergic to the food can be carried out under controlled clinical conditions. The testing scheme to detect allergens summarized above has worked effectively as demonstrated in the product development case of the Brazil nut 2S storage protein. This protein was introduced into soybean to increase the sulphur containing amino acid content and thereby improve its nutritional value for use in animal feeds. Since there was a small number of individuals who were allergic to Brazil nuts, it was decided to test sera from these individuals to see if they contained IgE that would cross react with the Brazil nut 2S storage protein, sera from 8 out of 9 Brazil nut allergic individuals reacted with this storage protein. Based on the introduced Brazil nut storage protein was discontinued. This variety of soybean was never commercialized.
In conclusion, biotechnology provides plants breeders the opportunity to develop new varieties of food crops efficiently and with greater potential benefit than has been so far possible with conventional breeding practices. Consideration of the overall data developed as per the scheme discussed above provide assurance that his technology will generate food “as safe as” that produced by traditional breeding programs.
FAO/WHO.1996. Joint FAO/WHO Expert Consultation on Biotechnology and Food Safety. Review of existing safety evaluation strategies and guidelines. September 30- October 4, Rome
Metcalfe, D.D., R.L. Fuchs, R. Townsend, H.A. Sampson, S.L. Taylor and J.R. Fordham. 1996. Allergenicity of foods produced by genetic modification. Critical Reviews in Food Science and Nutrition 36: 165-186