Managing Allergenic Proteins
As a preliminary evaluation of the safety of transgenic plants, the verification of substantial equivalence with the genetically unmodified counterpart is now widely employed (Kuiper et al., 2001). Modern, transcriptomic, proteomic, and metabolomic profiling techniques can be a vital part of such testing. Although substantial equivalence measurements are not safety assessments in themselves, they can reveal biochemical differences that can then be subjected to more rigorous toxicological and immunologic testing.
A potential consequence of the genetic modification of crop plants is introduction or creation of allergens. This could occur in several possible ways, including introduction of unknown allergens with the transgenic protein itself, modification by the host transgenic plant of the immunogenic properties of the transgenic protein, modification of the immunogenicity of endogenous proteins in the transgenic plant, and dissemination of an allergen through pollen that induces respiratory sensitization (Moneret-Vautrin, 2002). Such risks need to be evaluated prior to widespread use of a transgenic crop plant. Unfortunately, the possibility of allergen induction can be exaggerated to the general public and used to fuel the idea that genetic modification is an unpredictable and irresponsible science. It is true that the allergenicity of proteins, such as BNA, may not be widely known before their introduction into a crop plant. However, the scientific community quickly becomes aware of such potential problems (Nordlee et al., 1996) and acts appropriately. For example, the transgenic soybean plants expressing BNA were never commercially developed. As we gain a better understanding of the identity and epitopic composition of common allergenic proteins, their selective modification or elimination becomes feasible, and this could lead to the development of hypoallergenic versions.
Soybean consumption is a problem for some people and animals as it contains several dominant allergenic proteins: Gly m Bd 68K, Gly m Bd 28K, and Gly m Bd 30K (P34) (Ogawa et al., 2000). The widespread use of soybean in the human foods and animal feeds makes it an obvious target for genetic engineering to remove or reduce these allergens. Gly m Bd 68K and Gly m Bd 28K are seed storage proteins, and some reduction of their levels has been achieved through the development of mutant lines (Ogawa et al., 2000). However, such a strategy has not been successful with P34, which is an albumin and a member of the papain family of cysteine proteases (Ogawa et al., 2000). Although this protein is a minor seed constituent, it is the most dominant soybean allergen (Yaklich et al., 1999). While considered an albumin, P34 partitions into oil body membranes during processing, as well as with the globulin fraction (Kalinski et al., 1992). Consequently, it is almost impossible to completely remove this protein from soybean isolates. Furthermore, its ubiquitous presence in cultivated and wild soybean varieties suggests that it will not be possible to reduce its level through conventional breeding (Yaklich et al., 1999). However, through the sense expression of a Gly m Bd 30K cDNA, transgenic lines have been developed in which the endogenous Gly m Bd 30K gene is completely silenced (Herman et al., 2003). A function for this protein has not been demonstrated but no overt phenotypic change was observed in the gene-silenced plants. These transgenic soybeans are currently being further evaluated in field trials (Herman et al., 2003).
Rice induces allergic reactions in some people and this is a growing problem in some countries, like Japan (Watanabe, 1993). One of the major rice allergens was identified as a 16-kDa albumin (Matsuda et al., 1988; Urisu et al., 1991). This protein is encoded by a multigene family composed of at least ten members (Tada et al., 2003), each of which has allergenic properties (Matsuda et al., 1991). An antisense strategy was used to reduce the abundance of the 16-kDa albumin as well as other gene family members (Tada et al., 1996). An 80% reduction in abundance of the 16-kDa rice allergen was achieved (Tada et al., 1996), and the reduction in protein levels of other family members was proportional to their degree of nucleotide sequence identity with the transgene. Highly homologous proteins were markedly lowered, and proteins with less identity were hardly reduced at all (Tada et al., 2003). Highly immunogenic proteins need only be present in minute quantities in order to elicit an immune response. Thus, these results suggest that the antisense strategy may not be suitable for complete removal of allergenic proteins, especially if they are encoded by divergent multigene families. In this regard, gene silencing strategies (Waterhouse et al., 1998) that require smaller regions of DNA sequence identity may prove to be more suitable.
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