-Caveat Lector-
Library: Assessment of the allergenicity of genetically modified foods
Steve L. Taylor Department of Food Science & Technology, University of
Nebraska, Lincoln, NE 68583-0919, USA
ABSTRACT Food allergies are caused by abnormal immunological responses to
substances in foods, usually naturally-occurring proteins. Allergic
reactions can be manifested by symptoms ranging from mild cutaneous or
gastrointestinal problems to life-threatening anaphylactic shock reactions.
Virtually all food allergens are proteins, but only a small fraction of the
many proteins found in foods are allergenic. Since genetic modification
results in the introduction of new proteins into the transgenic species,
the potential allergenicity of the newly introduced protein should be a
major component of the safety assessment process. An assessment of the
allergenicity of the newly introduced proteins can be accomplished by
evaluating the source of the gene, the sequence homology of the newly
introduced protein to known allergens, the immunochemical reactivity of the
newly introduced protein with immunoglobulin E (IgE) antibodies from the
blood serum of individuals with known allergies to the source from which
the genetic material was obtained, and the physicochemical properties of
the newly introduced protein. Such an approach was used recently to assess
the possible allergenicity of a transgenic soybean with an inserted gene
from Brazil nuts that expressed a high-methionine protein. Brazil nuts are
known to be allergenic, and it was demonstrated that the high-methionine
protein was indeed a major allergen from Brazil nuts. As a result of this
assessment, commercial interest in this transgenic soybean variety was
abandoned.
INTRODUCTION The term, food allergy, is probably overused by many
consumers and even some physicians to describe any abnormal, undesirable
reaction to a food (Anderson, 1996). However, it is important for many
reasons to distinguish between the many different types of illnesses that
fall within the overall category of food sensitivities, also known as
individualistic adverse reactions to foods (Taylor, 1987). As noted below,
the true food allergies involve abnormal immunological responses to
substances, usually naturally occurring proteins, in foods (Mekori, 1996).
Since the true food allergies involve abnormal responses to proteins, this
type of food sensitivity is of concern to biotechnologists developing new
transgenic foods. Also, with the true food allergies, the tolerance level
for exposure to the offending food protein is extremely low, though
variable from one individual to another (Taylor and Lehrer, 1996). True
food allergies must be distinguished from food intolerances, those types of
food sensitivity which do not involve abnormal responses of the immune
system (Taylor, 1987; Anderson, 1996). Several forms of food intolerances
exist including anaphylactoid reactions, metabolic food disorders, and
idiosyncratic reactions (Taylor, 1987). Food intolerances are not typically
caused by food proteins, but instead are caused by a variety of small
molecular weight components of foods. Good examples include lactose
intolerance (Lifshitz, 1988), a metabolic food disorder linked to a
deficiency of b -galactosidase in the small intestine, and sulphite-induced
asthma (Taylor et al., 1997), an idiosyncratic illness involving an unknown
mechanism which is triggered in sensitive individuals by the ingestion of
sulphites, a common category of food additives. With food intolerances,
sensitive individuals can often tolerate the ingestion of some quantity of
the offending food (Lemke and Taylor, 1994). Since food intolerances do not
typically involve proteins, these types of food sensitivities will not be
discussed further in this article. However, it should be emphasised again
that many consumers and even some physicians do not make the distinction
between true food allergies and food intolerances. Since some types of food
intolerance, such as lactose intolerance, are fairly prevalent in the
population, failure to distinguish between these illnesses can lead to a
very false impression of the prevalence of "food allergies" in the
population.
TRUE FOOD ALLERGIES True food allergies can involve several types of
abnormal immunological responses to food proteins (Sampson and Burks,
1996). The most common type of food allergies are mediated by
immunoglobulin E (IgE). IgE-mediated reactions are known as immediate
hypersensitivity reactions because symptoms occur within minutes to a few
hours after the ingestion of the offending food. IgE-mediated reactions can
occur to pollens, mould spores, animal dander, insect venoms and other
environmental stimuli as well as foods. IgE-mediated allergic diseases
affect perhaps 10-25% of the population in developed countries (Mekori,
1996), although food allergies only represent a small fraction of all
allergic disease. The other major type of true food allergy is the delayed
hypersensitivity reaction which involves sensitised tissue-bound
lymphocytes rather than antibodies (Sampson, 1990a). In delayed
hypersensitivity reactions, the onset of symptoms occurs more than 8 hours
after ingestion of the offending food. The role of foods in delayed
hypersensitivity reactions remains uncertain (Burks and Sampson, 1993).
However, coeliac disease, also known as gluten-sensitive enteropathy, is a
well-established form of delayed hypersensitivity reaction that will be
discussed.
IgE-mediated food allergies The best estimates are that IgE-mediated food
allergies afflict approximately 1-2% of the population (Taylor et al.,
1989; Anderson, 1996 ). Infants are more commonly affected than adults; the
prevalence among infants under the age of three may be as high as 5-8%
(Bock, 1987; Sampson, 1990b). Although all humans have IgE antibodies, only
certain individuals will form IgE that specifically recognises a protein in
their environment, including food proteins. Children born to allergic
parents are more likely than others to develop allergic sensitisation
including food allergies (Mekori, 1996). Children tend to outgrow food
allergies, especially allergies to cows' milk, eggs, and soybeans (Bock,
1982; Bishop et al., 1990). Thus, the prevalence of food allergies in
adults is considerably lower than among infants. The mechanisms involved in
the development of increased tolerance are not entirely understood, but
probably involve the development of oral tolerance through activation of
suppressor T cells, the maturation of the intestinal barrier, and the
development of blocking secretory IgA antibodies in the gut lumen (Burks
and Sampson, 1993; Eigenmann and Sampson, 1994). Eight foods or food
groups account for perhaps 90% or more of all serious allergic reactions to
food (FAO, 1995; Hefle et al., 1996). These foods are cows' milk, eggs,
fish, crustacea (shrimps, crab, lobster), peanuts, soybeans, tree nuts
(almonds, walnuts, cashews, etc.) and wheat. Allergic reactions to fresh
fruits and vegetables, the so-called oral allergy syndrome, are also rather
common, but the allergens involved in fresh fruits and vegetables tend to
be labile to heat (cooking or processing) and digestion (acid and enzymes)
and the symptoms are rather mild being confined primarily to itching and
hives in the oropharyngeal area (Parker et al., 1990). In addition to the
eight foods mentioned above, 160 other foods or food-related substances
have been documented to cause allergic reactions in one or more individuals
(Hefle et al., 1996). These include most major grain, oilseed, and
vegetable crops as well as processed foods such as beer and chocolate.
Figure 1. Mechanism of IgE-mediated food allergy reaction. The mechanism
involved in IgE-mediated reactions is depicted in Figure 1. During the
sensitisation process, exposure to the allergen induces B lymphocytes to
form allergen-specific IgE antibodies probably with the aid of helper T
lymphocytes (Eigenmann and Sampson, 1994; Taylor et al., 1989). The IgE
antibodies attach to specialised cells, mast cells which are present in
most tissues and basophils which are present in blood. Upon subsequent
exposure to the allergen, the allergen cross-links two IgE molecules on the
surface of the mast cell or basophil membrane. This induces these cells to
release a host of mediators of allergic reactions including histamine,
leukotrienes, prostaglandins, and many others (Winbery and Lieberman,
1995). As a result, exposure to a small amount of allergen can induce the
release of massive quantities of allergic mediators. Thus, the tolerance
level for the offending allergens is quite low in many cases. With food
allergies, the tolerance level is not precisely known and is likely to be
variable from one individual to another (Taylor and Lehrer, 1996). However,
ingestion of milligram quantities of the allergenic food are likely to be
necessary to provoke allergic symptoms (Taylor and Lehrer, 1996). The
mediators released by mast cells and basophils can induce a wide range of
symptoms affecting many organs of the body. Table 1 provides a list of the
most common manifestations of food allergies (Lemke and Taylor, 1994).
Allergic individuals do not often display all of these symptoms. However,
the nature of the symptoms involved in food allergies is quite variable
among individuals. The most serious manifestations of food allergies are
asthma, laryngeal oedema, and anaphylactic shock. All of these symptoms are
life-threatening. Deaths have been documented from inadvertent ingestion of
allergenic foods by food-allergic individuals (Yunginger et al., 1988;
Sampson et al., 1992). Comparatively few food-allergic individuals are
likely to suffer life-threatening reactions from exposure to the allergenic
food. Mild symptoms, such as the aforementioned oral allergy syndrome, are
much more common manifestations. IgE-mediated food allergies are
associated with certain proteins or allergens in the offending foods. Bush
and Hefle (1996) have described the known allergens from a variety of
commonly and less commonly allergenic foods. Virtually all food allergens
are proteins. However, only a very small percentage of the many thousands
of proteins in foods are allergens. Food allergens are proteins capable of
eliciting the formation of allergen-specific IgE antibodies and
cross-linking IgE molecules on the surface of the mast cell or basophil
membrane to trigger the release of histamine and other mediators of
allergic disease. Many allergenic foods contain multiple allergens. Foods
can contain both major and minor allergens (Bush and Hefle, 1996). Major
allergens are generally defined as proteins for which 50% or more of
allergic patients have specific IgE (King et al., 1994). A nomenclature
system has been developed for allergens from food and non-food sources
(King et al., 1994). This allergen nomenclature system uses the first three
letters of the genus, followed by the first letter of the species, and then
an Arabic number. The numbers are assigned to allergens from specific
sources according to the order of their identification. For example,
peanuts contain several known allergens including Ara h 1 and Ara h 2
(Burks et al., 1991; 1992). The amino acid sequences of many food allergens
are known (Metcalfe et al., 1996). The amino acid sequences of the epitopes
or IgE-binding regions of these proteins are also known in some cases (Bush
and Hefle, 1996). Many food allergens, especially those in the common
allergenic foods, are present as major protein components, typically
ranging between 1.0 and 80% of total protein (Metcalfe et al., 1996). Food
allergens tend to be stable to digestion and processing (Taylor and Lehrer,
1996). The ability of food allergens to reach and cross the intestinal
mucosal barrier in immunologically intact form is probably a prerequisite
to allergenicity (Metcalfe et al., 1996). Astwood et al. (1996)
demonstrated that food allergens were quite stable to digestion in a
simulated gastrointestinal digestive model. Food allergens also tend to
resist heat denaturation (Taylor and Lehrer, 1996). This property allows
the allergens to survive heat processing and cooking. IgE-mediated food
allergies are usually treated with specific avoidance diets (Taylor et al.,
1986). Since the tolerance level for the offending food is very low, great
care must be taken in constructing safe and effective avoidance diets.
Coeliac disease Coeliac disease is an abnormal immunological response to
the gluten protein fractions of wheat, rye, barley and triticale that is
mediated by the T lymphocytes in the small intestine (Ferguson, 1996). The
inflammatory reaction destroys the absorptive function of the small
intestinal mucosa. As a result, coeliac disease is a malabsorption syndrome
characterised by body wasting, anaemia, diarrhoea, and bone pain along with
other symptoms. Specific proteins in the gliadin fraction of wheat have
been implicated in coeliac disease (Wieser, 1996). Coeliac disease is also
treated with avoidance diets.
ALLERGENICITY ASSESSMENT Recently, the International Food Biotechnology
Council in conjunction with Allergy and Immunology Institute of the
International Life Sciences Institute developed a decision-tree approach
for the assessment of the allergenicity of genetically modified foods
(Metcalfe et al., 1996). This decision-tree approach is depicted in Figure
2. This strategy focuses on the source of the gene, the sequence homology
of the newly introduced protein to known allergens, the immunochemical
reactivity of the newly introduced protein with IgE from the blood serum of
individuals with known allergies to the source of the transferred genetic
material and the physicochemical properties of the newly introduced protein
(Metcalfe et al., 1996; Taylor, 1997).
Figure 2. Assessment of the allergenic potential of foods derived from
genetically modified food crops. Reprinted with permission from: Critical
Reviews in Food Science and Nutrition, Volume 36, Supplement, S165-S186,
December, 1996. The sources of genetic material can be classified as
commonly allergenic (the eight foods or food groups mentioned earlier),
less commonly allergenic (the 160 foods identified by Hefle et al., 1996),
or unknown allergenic potential. As noted in Figure 2, the approaches to
allergenicity assessment vary according to the nature of the source of the
transferred genetic material. If the genetic material is obtained from a
known allergenic source, either commonly or uncommonly allergenic, and the
encoded protein is expressed in the edible portion of the genetically
modified organism, then data should be generated to determine if the gene
codes for an allergen. In such situations, the first step in the process is
an assessment of the immunoreactivity of the newly introduced protein with
IgE antibodies from the sera of individuals allergic to the donor organism.
The blood serum can be tested for reactivity with the newly introduced
protein or extracts of the transgenic food using immunoassays (Yunginger
and Adolphson, 1992; Taylor and Lehrer, 1996). If the genetic material is
obtained from a commonly allergenic source, 14 test sera should be used
(Metcalfe et al., 1996). With 14 sera, a negative result in the immunoassay
provides >99.9% assurance that a major allergen from the donor organism has
not been transferred and >95% assurance that a minor allergen affecting at
least 20% of the sensitive population has not been transferred. If the
genetic material is obtained from a less commonly allergenic source, five
test sera should be used recognising that it will be much more difficult to
identify individuals with these allergies (Metcalfe et al., 1996). With
five sera, a negative result in the immunoassay provides >95% assurance
that a major allergen from the donor organism has not been transferred.
While this degree of confidence is less than with the proteins obtained
from commonly allergenic source, the size of the affected population and
thus the risk to consumers is also much less. A positive result in the in
vitro immunoassays would constitute sufficient proof of the allergenicity
of the transgenic food. In the case of negative or equivocal results in
the in vitro immunoassays with transgenic foods containing genetic material
from commonly allergenic foods, the transgenic food should be tested
further using in vivo skin-prick tests with an appropriate number of
allergic test subjects (Bock et al., 1977; Taylor and Lehrer, 1996). A
positive result in the skin-prick tests signifies that the transgenic food
extract is able to elicit histamine release from skin mast cells in vivo.
Ultimately, the decision-tree approach suggests that the safety of the
transgenic product be confirmed using double-blind, placebo-controlled food
challenges with allergic individuals (Bock et al., 1988; Taylor and Lehrer,
1996). A positive result in either of these in vivo tests would be
sufficient evidence of the allergenicity of the transgenic food. While a
similar approach might be logical for transgenic foods containing genetic
material from less commonly allergenic sources, it is unreasonable to
expect that a sufficient number of subjects with the allergy in question
could be assembled at any site for the in vivo testing. However, the risk
is very small in such cases when negative results are obtained in the
immunoassay using five or more test sera. If fewer than five test sera are
used, the decision-tree approach (Figure 2) advocates an assessment of the
stability of the newly introduced protein(s) to digestion and processing.
The most difficult assessment involves genetic material obtained from
sources of unknown allergenic potential such as viruses, bacteria, insects,
non-food plants, etc. The likelihood that the proteins derived from such
sources of DNA will be allergens is not very high, since most proteins in
nature are not allergens (Taylor, 1997). Additionally, many of these
proteins will be expressed in the transgenic food at very low levels, while
allergic sensitisation is more likely to occur to the major proteins that
exist in foods (Taylor, 1997). The first step in the evaluation of such
transgenic foods is a comparison of the amino acid sequence of the
introduced protein with the amino acid sequences of known allergens (Figure
2). The amino acid sequences of 198 major allergens including food
allergens are known (Metcalfe et al., 1996). In this decision-tree
strategy, a significant sequence similarity would require a match of at
least eight contiguous identical amino acids (Metcalfe et al., 1996). In
general, the minimal peptide length for binding to T cells, which would be
required for allergen sensitisation, appears to be eight or nine amino
acids (Rothbart and Gefter, 1991; Pernis, 1992). The peptide length
required for an IgE-binding epitope would be even longer (Rothbart and
Gefter, 1991). Thus, a match based upon the identity of eight contiguous,
identical amino acids seems to be a conservative approach. Since this
approach assesses the entire protein sequence, it is not based upon the
identity of amino acid sequences just to known epitopes of known allergens
(Taylor, 1997). Because insufficient information is available on the amino
acid sequences of allergen epitopes especially for food allergens, the more
appropriate approach is to look for matches along the entire length of the
polypeptide (Taylor, 1997). Of course, such an approach will probably yield
some frivolous matches that have nothing to do with IgE binding. Also, this
approach is clearly limited in that it cannot identify discontinuous or
conformational epitopes which depend upon the tertiary structure of the
protein (Metcalfe et al., 1996). However, the stability of food allergens
to heat argues for greater significance of linear, continuous epitopes for
these particular allergens. If a sequence homology match to a known
allergen is identified for the introduced protein, then this protein should
be assessed for immunoreactivity using sera from individuals sensitive to
that particular allergen. The approach becomes similar to the one used when
the genetic material is obtained from known allergenic sources (Figure 2).
If no sequence homology to known allergens is identified, then the
stability of the protein to digestion and processing is assessed (Figure
2). Simulated gastric and intestinal digestive models of mammalian
digestion have been used to assess the digestive stability of known food
allergens and proteins introduced into foods through genetic modification
(Astwood et al., 1996). Stability to processing must be approached on a
case-by-case basis knowing the typical processes that might be applied to
that particular food. This decision-tree approach has already been applied
in the assessment of the allergenicity of transgenic food products. A
transgenic soybean that contained a high-methionine storage protein derived
from Brazil nuts was developed for its potential as an improved animal
feed. An allergenicity assessment was performed because the Brazil nut is a
commonly allergenic food. Immunoassays using blood serum from individuals
with documented Brazil nut allergy revealed that the transferred storage
protein from Brazil nuts was probably an allergen (Nordlee et al., 1996).
Using SDS-polyacrylamide gel electrophoresis and immunoblotting with the
sera of Brazil nut-allergic individuals, it was demonstrated that the
high-methionine protein from Brazil nuts was indeed the major allergen from
Brazil nuts (previously unidentified; now identified as Ber e 1) as it was
recognised by the sera of eight of the nine patients (Nordlee et al.,
1996). This protein was also identified in the transgenic soybeans. The
potential allergenicity of the transgenic soybeans was further confirmed by
positive skin-prick tests on three Brazil nut-allergic individuals (Nordlee
et al., 1996). As a result, commercial interest in this transgenic soybean
variety was abandoned. Monsanto has also used this decision-tree approach
to evaluate the potential allergenicity of their glyphosate-tolerant
soybeans (Harrison et al., 1996). The enzyme transferred into these
transgenic soybeans has no significant sequence homology to known
allergens. This enzyme is also rapidly digested in simulated mammalian
digestion systems (Harrison et al., 1996).
CONCLUSION A logical decision-tree strategy exists for the assessment of
the potential allergenicity of genetically modified organisms. Thus far,
this approach has been tested on only a small number of transgenic foods.
However, this strategy was quite successful in preventing the introduction
of a transgenic soybean into the marketplace that contained the major
allergen from Brazil nuts.
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Nickson, T.E.; Mitsky, T.A.; Taylor, M.L.; Fuchs, R.L.; Padgette, S.R.
(1996) The expressed protein in glyphosate-tolerant soybean,
5-enolpyruvylshikimate-3-phosphate synthase from Agrobacterium sp. Strain
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D.G.; Platts-Mills, T.A.; Thomas, W. (1994) Allergen nomenclature.
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allergenic potential of foods derived from genetically engineered crop
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(1996) Identification of a Brazil nut allergen in transgenic soybeans. New
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peptides and MHC proteins. Annual Reviews of Immunology 9, 527-565.
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and near-fatal anaphylactic reactions to foods in children and adolescents.
New England Journal of Medicine 327, 380-384. Taylor, S.L. (1987) Allergic
and sensitivity reactions to food components. In: Nutritional Toxicology,
2nd edition. Orlando, FL, USA; Academic Press. pp. 173-198. Taylor, S.L.
(1997) Food from genetically modified organisms and potential for food
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S.L.; Bush, R. K.; Busse, W.W. (1986) Avoidance diets - how selective
should we be? New England Regional Allergy Proceedings 7, 527-532. Taylor,
S.L.; Lehrer, S.B. (1996) Principles and characteristics of food allergens.
Critical Reviews in Food Science and Nutrition 36, S91-S118. Taylor, S.L.;
Nordlee, J.A.; Rupnow, J.H. (1989) Food allergies and sensitivities. In:
Food Toxicology - A Perspective on the Relative Risks. New York, USA;
Marcel Dekker. pp. 255-295. Taylor, S.L.; Bush, R.K., Nordlee, J.A. (1997)
Sulfites. In: Food Allergies - Adverse Reactions to Foods and Food
Additives, 2nd edition. Boston, MA, USA; Blackwell Scientific Publishers.
pp. 339-357. Weiser, H. (1996) Relation between gliadin structure and
coeliac toxicity. Acta Paediatrica S412, 3-9. Winbery, S.L.; Lieberman,
P.L. (1995) Anaphylaxis. Immunology and Allergy Clinics of North America
15, 447-475. Yunginger, J.W.; Adolphson, C.R. (1992) Standardization of
allergens. In: Manual of Clinical Immunology, 4th edition. Washington,
D.C., USA; American Society of Microbiology. pp. 678-684. Yunginger, J.W.;
Sweeney, K.G.; Sturner, W.Q., Giannandrea, L.A., Tiegland, J.D., Bray, M.,
Benson, P.A.; York, J.A., Biedrzycki, L., Squillace, D.L.; Helm, R.M.
(1988) Fatal food induced anaphylaxis. Journal of the American Medical
Association 260, 1450-1452. Table 1. Common manifestations of IgE-mediated
food allergies
Type of symptoms Manifestation
Gastrointestinal symptoms Nausea
Vomiting
Diarrhoea
Abdominal pain
Cutaneous symptoms Urticaria
Eczema or dermatitis
Angioedema
Pruritis
Respiratory symptoms Rhinitis
Asthma
Other symptoms Laryngeal oedema
Anaphylactic shock
Hypotension
Oral allergy syndrome
Copyright CRC Press, Boca Raton, Florida, USA. � 1996.
Library: Assessment of the allergenicity of genetically modified foods
http://binas.unido.org/binas/Library/cabi/taylor.html
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