Gly m 6
Summary
Gly m 6 is one of the main soybean (Glycine max) storage proteins, associated majorly with severe allergic reactions to soy. It is a legumin-like protein (glycinin) belonging to the 11S globulin protein group. This glycinin protein has five subunits namely, G1-G5. Each subunit of Gly m 6 comprises of a 40 kDa acidic unit and 20 kDa basic unit connected by a disulfide bond. Both the acidic and basic subunits are found to be potentially allergenic. Patients sensitized to Gly m 6 may present a combination of skin, gastrointestinal, respiratory, or cardiovascular symptoms and sometimes even anaphylaxis. Gly m 6 is cross-reactive with allergens of other legume allergens like Ara h 2 and Ara h 3 of peanut. Gly m 6 could also serve as diagnostic marker for identifying soy-allergic subjects who are at high risk for severe clinical symptoms. Further, it can better identify soy allergy compared to IgE towards soy extract.
Epidemiology
Worldwide distribution
Soybean is one of the “big eight” foods that are considered to be accountable for 90% of all food allergies. It forms a major allergen in the United States (US) and Canada and is responsible for the most important food allergies in the US, Europe, and Japan. Soy allergy can trigger anaphylaxis in severe cases, which may be life-threatening.
Gly m 6, besides Gly m 5, is the principal soya protein. The presence of these soy allergens possibly suggests severe allergic reactions and can lead to immediate-type soybean allergy in children along with Gly m 5.
In a study conducted on 91 Japanese children with soybean sensitization, Gly m 6 sensitization was found to be between 90-93% in the symptomatic patients (40 out of 91) depending on its sensitization to different subunits while it was 90.2% in the non-symptomatic patients (51 out of 91). The patients with a positive oral food challenge (OFC) were considered symptomatic while those with negative OFC were considered non-symptomatic.
Usually, soybean allergy is observed in young children and by 10 years of age, they are most often tolerant and thus, soybean allergy is rarely observed in adults. However, a study was conducted on 10 soy-allergic adults (4 from Switzerland and 6 from the US). IgE binding to Gly m 6 was observed in 80% (8 out of 10) allergic adults. However, another study on 30 European subjects (25 adults and 5 children) found Gly m 6-sIgE in only 37% (11 out of 30) of soybean-allergic patients including 3 children.
Clinical Relevance
Disease Severity
Gly m 6, together with Gly m 5 is one of the major allergens of soybean leading to food allergy and is mainly responsible for severe systemic reactions in allergic individuals. The severe symptoms were manifested as a combination of skin, gastrointestinal, respiratory, or cardiovascular symptoms and sometimes even anaphylaxis. A European study on 30 soybean-allergic participants (25 adults and 5 children) found 40% (12 out of 30) suffering from mild symptoms, 37% (11 out of 30) having moderate symptoms, and 23% (7 out of 30) having severe symptoms. Gly m 5 or Gly m 6 sensitization was found in 86% (6 out of 7) of patients with severe allergic reactions like anaphylaxis while the same was observed in only 33% (4 out of 12) of patients with mild symptoms. Furthermore, it was found that 60% (3 out of 5) of children demonstrated IgE-binding to Gly m 6. Thus, postulating them to be major allergens of soybean in children.
Some studies have shown that sIgE levels to Gly m 6 are found to be significantly higher in symptomatic soy allergic patients as compared to non-symptomatic individuals. This was reported in a study conducted on 74 soybean-sensitized Japanese individuals. Out of 74 individuals, 33 individuals were found symptomatic (severe symptoms in 14 and mild symptoms in 19) while 41 were non-symptomatic based on OFC. IgE reactivity to Gly m 6 was observed in 58% of symptomatic patients and 39% of non-symptomatic individuals. Gly m 5-sIgE levels were significantly higher (p<0.05) in symptomatic patients as compared to non-symptomatic soy-sensitized individuals. Similarly, in another study in Japan conducted on 55 soy-sensitized children, higher levels of sIgE to Gly m 6 was observed in symptomatic (19 out of 55) as compared to non-symptomatic children (36 out of 55). However, it was not found to be statistically significant.
Interestingly, in contrast to the above studies, a study in Utrecht, The Netherlands found significantly high sIgE for Gly m 6 in the soy-allergic patients with mild symptoms as compared to patients with severe symptoms (p=0.02). The study was conducted on 35 soy-allergic adults with 15 patients presenting mild symptoms while 20 patients presenting severe symptoms to soy food challenge.
Furthermore, Gly m 6, along with Gly m 5, is known to be involved in baker’s asthma. A study on 135 soy flake processing workers (SPWs) in the age range of 19–66 years in Tennessee, US, found that IgE binding for soybean was evident in all the SPWs, especially for Gly m 5 and Gly m 6. The study highlighted that exposure to Gly m 5 and Gly m 6 at the workplace, may result in respiratory morbidity in the soy-sensitized individuals by causing occupational sensitization.
Gly m 6, together with Gly m 5 has recently been reported to be the causative allergens for soybean related food-dependant exercise-induced anaphylaxis (FDEIA) in 2 children (9–10-year-old boys). It was observed after the consumption of a soy-containing food (tofu) that contains more storage proteins (Gly m 5 and Gly m 6) as compared to other soy products.
Cross-reactive molecules
Clinical cross-reactivity has been reported among patients allergic to peanuts, soybean, and other legumes. A study on 35 soy-allergic adults in the Netherlands found co-existing peanut allergy in 60% (21 out of 35) of the individuals. The sIgE to Gly m 6 was found to be significantly correlated (p<0.001) with Ara h 2-sIgE.
Further, a similar co-existing pattern of clinical peanut and soy allergy was observed in a study in Gothenburg, Sweden conducted on 47 severe peanut-allergic adults, 22 peanut-sensitized adults, and 22 healthy controls. Sensitization to soy-IgE was observed in 72.3% (34 out of 47) of severe peanut-allergic adults and 50% (11 out of 22) of peanut-sensitized adults. Further, it was observed that IgE reactivities to Gly m 6 or Gly m 5 were correlated with IgE, SPT and BAT to soy, suggesting it to be a clinically relevant soy allergy in peanut-allergic patients. In a study on 10 soy and/or peanut-allergic patients, sIgE reactivity to Gly m 6 was observed in all of them, however, it was found to be very weak in 5 patients. Its corresponding 11S globulin in peanut (Ara h 3) was observed in 8 patients.
This pattern of cross-reactivity was confirmed by a retrospective cohort study in 1057 food-allergic patients in Vienna. The study found that “peanut-soybean” is an important cluster, majorly due to cross-reactivity of peanut allergen Ara h 3 with soybean allergens, Gly m 6 together with Gly m 5. Furthermore, peanut and soy allergy was found to be co-existing in 2 cases of FDEIA, which was attributed due to sensitivity to seed storage proteins of soy (Gly m 5 and Gly m 6).
Diagnostics
Disease Severity
Sensitization to Gly m 6-sIgE can serve as an indicator of developing severe allergic reactions to soy in children and can better identify soy allergy compared to IgE towards soy extract. Further, IgE reactivity to Gly m 6 and Gly m 5 could help to diagnose soy allergic patients who are at risk of developing severe reactions.
Exposure
The main route of exposure is through ingestion.
Cross-Reactivity
Nearly 50% amino acid sequence similarity has been observed between Gly m 6 and other legume proteins. Ara h 3 has been recognized as a homolog of Gly m 5 and found to share few IgE epitopes with the acidic chain of G1 glycinin. Immuno-cross-reactivity between peanut and soybean can be attributed to similarities between their allergens like Ara h 3 and Gly m 6 based on amino acid sequence, IgE epitope mapping, and molecular modeling.
References
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