Division 1.5.1 Assessment of allergenicity of the newly expressed protein

Allergenicity is not an intrinsic, fully predictable property of a given protein but is a biological activity requiring an interaction with individuals with a pre-disposed genetic background. Allergenicity therefore depends upon the genetic diversity and variability in atopic humans. Frequency, severity and specificity of allergic reactions also depend upon geographic and environmental factors. Given this lack of complete predictability, it is necessary to consider several aspects in the assessment of allergenicity to obtain a cumulative body of evidence which minimises any uncertainty with regard to the protein(s) in question.

When studying the structural characteristics and the biological and physicochemical properties of a newly expressed protein, it is essential that the tested protein is equivalent with respect to structure and activity to the newly expressed protein in the genetically modified plant. Studies carried out using purified target proteins prepared by expression in organisms such as Escherichia coli shall be acceptable provided that the properties of the microbial substitute protein are identical to those of the protein expressed in the plant, thus taking into account all post-translational modifications that specifically occur in the plant.

The applicant shall verify whether the source of the transgene is allergenic. When the introduced genetic material is obtained from wheat, rye, barley, oats or related cereal grains, the applicant shall also assess the newly expressed proteins for a possible role in the elicitation of gluten-sensitive enteropathy or other enteropathies which are not IgE-mediated. Where transformation events have been stacked, the applicant shall provide an assessment of any potential for increased allergenicity to humans and animals on a case-by-case approach. These potential effects may arise from additive, synergistic or antagonistic effects of the gene products.

The applicant shall follow an integrated, case-by-case approach, that is to say, a weight-of-evidence approach, in the assessment of possible allergenicity of newly expressed proteins. This approach shall include:

1. (a)

   Amino acid sequence homology comparison between the newly expressed protein and known allergens

   In every case, a search for sequence homologies and/or structural similarities between the expressed protein and known allergens shall be performed to identify potential IgE cross-reactivity between the newly expressed protein and known allergens. The applicant shall ensure that the quality and the comprehensiveness of the databases are state of the art. The alignment-based criterion involving 35 % sequence identity to a known allergen over a window of at least 80 amino acids is considered a minimal requirement. All sequence alignment parameters used in the analysis shall be provided including calculation of percent identity (PID). The calculation of PID shall be performed on a window of 80 amino acids with gaps so that inserted gaps are treated as mismatches. In some cases, for assessing short peptidic fragments such as ORFs, a search for sequences of contiguous identical or chemically similar amino acid residue can be conducted. However, this search shall not be carried out routinely for the identification of potential linear IgE binding epitopes because of its poor sensitivity or specificity.

2. (b)

   Specific serum screening

   When there is an indication of sequence homology or structure similarities, an important procedure for assessing the potential that exposure to the newly expressed proteins might elicit an allergic reaction in individuals already sensitised to cross-reactive proteins, is based on in vitro tests that measure the capacity of specific IgE from serum of allergic patients to bind the test protein(s). There is inter-individual variability in the specificity and affinity of the human IgE response. In particular, the specificity of the IgE antibodies to the different allergens present in a given food/source and/or to the different epitopes present on a given protein may vary amongst allergic individuals. In order to optimise the sensitivity of the test, individual sera from well-characterised allergic individuals shall be used. The applicant shall perform specific serum screening in the following cases:

   1. (i)

      the source of the introduced gene is considered allergenic, even if no sequence homology of the newly expressed protein to a known allergen is demonstrated; or

   2. (ii)

      the source is not known to be allergenic, but there are indications of a relationship between the newly expressed protein and a known allergen, based on sequence homology or structure similarity.

   Specific serum screening shall be undertaken with individual sera from individuals with a proven and well-characterised allergy to the source or to the potentially cross-reacting allergen using relevant immunochemical tests. IgE-binding assays (such as Radio or Enzyme Allergosorbent Assay (RAST or EAST), Enzyme Linked Immunosorbent Assay (ELISA) and electrophoresis followed by immunoblotting with specific IgE-containing sera) are adequate methods.

3. (c)

   Pepsin resistance and in vitro digestibility tests

   Stability to digestion by proteolytic enzymes has long been considered a characteristic of allergenic proteins. Although it has been established that no absolute correlation exists, resistance of proteins to pepsin digestion is an additional criterion to be considered in the weight-of-evidence approach for the assessment of allergenicity. The pepsin resistance test is generally performed under quite standardised conditions, at low pH values and high pepsin:protein ratios. It is recognised that the pepsin resistance test does not reflect the physiological conditions of the digestion. The digestibility of the newly expressed proteins in specific segments of the population, such as infants and individuals with impaired digestive functions, may be assessed using in vitro digestibility tests using different conditions. Also, since the protein encoded by the newly introduced genes will be present in the product as a complex matrix, the impact of the possible interaction between the protein and other components of the matrix, as well as the effects of the processing, shall be taken into account in additional in vitro digestibility tests. Depending on the outcome of the in vitro digestibility test, a comparison of the intact, the heat-denatured and the pepsin-digested proteins for IgE binding shall be assessed since an altered digestibility may impact on the allergenicity of the newly expressed protein.

4. (d)

   Additional tests

   Although additional tests including in vitro cell based assays or in vivo tests on animal models have not been validated to date for regulatory purposes, they may provide useful additional information, for example, on the potential of the newly expressed protein for de novo sensitisation.