Gly m 4
Summary
Gly m 4 acts as the major soy allergen for patients allergic to birch pollen, accounting for pollen-related soybean allergy in adults. It is a pan-allergen, belonging to group 10 of pathogenesis-related proteins (PR-10) with a molecular weight of 17 kDa. Gly m 4 concentration varies widely among the different soybean products, depending strongly on the total quantity of soybean in the product, the extent of food processing, and heating. The differences in Gly m 4 content of different soy-containing products may account for the differences in symptom severity (mild to severe). Patients sensitized to Gly m 4 may present oral allergy syndrome (facial edema, itchy eyes, angioedema, nasal congestion, nasopharyngitis, painful swallowing or other buccal symptoms), gastrointestinal, or respiratory symptoms, and even anaphylaxis upon consumption of soy-containing products. The sensitization to Gly m 4 in birch-pollen allergic patients poses as a risk factor for the development of oral allergy syndrome (OAS) of severe type as well as systemic reactions to soy. Gly m 4 is cross-reactive with allergens of other legumes like Bet v 1 (Betulaceae pollen) as well as with other PR-10 proteins such as Aln g 1 (alder pollen), Ara h 8 (peanut), Cor a 1.01 (hazel pollen), Cor a 1.04 (hazelnut), Mal d 1 (apple), Pru p 1 (peach). Importantly, Gly m 4 has been recognized as a diagnostic marker for for severe allergic reactions to soy in especially birch pollen allergic patients.
Epidemiology
Worldwide distribution
Soybean is one of the “big eight” foods, 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 4, a pan-allergen and Bet v 1- homolog, accords for pollen-related soybean allergy in adults, majorly accounting for immediate-type soybean allergy in birch pollen allergic patients after consumption of food products containing soy. Nevertheless, the relevance of Gly m 4 as a clinically significant allergen is restricted to areas with birch pollen allergy.
In a study conducted on 91 Japanese children with soybean sensitization, Gly m 4 sensitization (related to pollen-food allergy) was found to be 17.5% (7 out of 40) in the symptomatic patients while it was 21.6% (11 out of 91) in the non-symptomatic patients. The patients with a positive oral food challenge (OFC) were considered as symptomatic (40 out of 91) while those with negative OFC were considered non-symptomatic (51 out of 91).
A study on 30 European subjects (25 adults and 5 children) found Gly m 4-sIgE in 70% (21 out of 30) of soybean-allergic patients. Furthermore, in another study in Austria (birch pollen environment) on 20 patients with soy allergy (10-69 years), 95% (19 out of 20) patients had specific immunoglobulin (sIgE) to Gly m 4. Also, another study in Utrecht, the Netherlands on 35 soy allergic adults found Gly m 4-sIgE in 74% of individuals. While a study conducted in the Netherlands on 305 patients sensitized to one or more PR-10 proteins found Gly m 4 sensitization to be 38% (115 out of 305).
Clinical Relevance
Disease severity
Gly m 4 is found to be associated with pollen-food allergy syndrome (PFAS) and is mainly accountable for severe OAS as well as moderate to severe generalized reactions in allergic individuals. The severe symptoms include oral allergy syndrome (facial edema, itchy eyes, angioedema, nasal congestion, nasopharyngitis, painful swallowing or other buccal symptoms), gastrointestinal, or respiratory symptoms, and even anaphylaxis. The differences in Gly m 4 content of different soy-containing products may account for the differences in symptom severity (mild to severe).Gly m 4 sensitivity is frequently found in birch pollen allergic patients. However, the sensitization to this soy allergen possibly suggests severe allergic reactions, unlike other Bet v 1 homolog allergens.
Severe allergic reactions may develop following ingestion of mildly processed soy, such as soy drinks, in patients primarily sensitized to birch pollen. This was observed in a study on four Swedish children (10–14-year-old) allergic to birch pollen. All the four children manifested severe allergic reactions (ranging from allergic rhino-conjunctivitis to OAS and gastrointestinal symptoms) after consumption of soymilk during peak pollen season for birch.
Further, it has been observed that higher Gly m 4-sIgE is associated with soy-milk allergy and not with other soy products. This may be due to different ways of processing the soy products. A study in Utrecht, The Netherlands found significantly high sIgE for Gly m 4 in the soy-allergic patients who manifested anaphylactic reactions to soy milk. 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.
In contrast, 92% of soybean allergic patients suffering from mild symptoms were found to be sensitized to Gly m 4 in a European study on 30 soybean-allergic participants (25 adults and 5 children). This study could not establish a strong association of Gly m 4-sIgE to the development of severe symptoms as compared to Gly m 5 or Gly m 6. However, they suggest that Gly m 4 could induce severe reactions in patients with high levels of IgE to Bet v 1.
Cross-reactive molecules
Gly m 4 is a common cross-reactive protein in Europe due to frequent sensitization to birch. Thus, pollen-related allergy to soybean in adolescents and adults may be attributed to the IgE cross-reactivity between Bet v 1 (Betulaceae pollen) and its homolog, Gly m 4. Usually, birch pollen allergic patients may develop a secondary allergy to soy, as a result of the cross-reactivity between birch pollen and soy. Further, this clinical cross-reactivity may manifest as severe symptoms.
A study reported facial, oropharyngeal, and/or systemic symptoms in 20 patients in less than 30 minutes of consuming a soy product. The birch pollen allergy was co-existent in 80% (16 out of 20) patients while 85% (17 out of 20) patients demonstrated sIgE to Gly m 4. Moreover, allergic symptoms were most commonly observed during peak pollen season for trees. Also, recognition of Gly m 4 in patients with high IgE levels to Bet v 1 was associated with the development of severe symptoms.
Clinical cross-reactivity has also been reported among patients allergic to peanuts, soybean, and other legumes. It may be due to cross-reactivity between Gly m 4 and Bet v 1 homolog, Ara h 8 in peanuts. A co-existing pattern of clinical birch, peanut and soy allergy was observed in a study in Gothenburg, Sweden conducted on 47 severe peanut-allergic adults. Sensitization to soy-IgE was observed in 72.3% (34 out of 47) of these adults. It was observed that IgE reactivities to Gly m 4 were correlated with sIgE to Ara h 8 as well as to birch.
This pattern of co-sensitization was further confirmed in a study conducted on 305 patients sensitized to one or more PR-10 proteins. Gly m 4 sensitization was found in 38% of patients. The co-sensitization prevalence of Bet v 1, Cor a 1.01 (hazel pollen), Cor a 1.04 (hazelnut), Mal d 1 (apple), Aln g 1 (alder pollen), Pru p 1 (peach), Ara h 8 (peanut), Api g 1 (celery) and Act d 8 (kiwi) was found to be 100%, 100%, 100%, 100%, 100%, 97%, 94%, 50% and 41%, respectively.
Furthermore, a retrospective cohort study in 1057 food-allergic patients in Vienna found cross-reactivity among soybean, peanut, hazelnut, apple, and peach allergens which was termed as “soybean-nuts-fruits” cross-reactive cluster. It was found in 132 patients, majorly due to cross-reactivity between Ara h 8 (peanut) and Cor a 1.0101 and Cor a 1.0401 (hazelnut), Gly m 4 (soybean), Mal d 1 (apple), and Pru p 1 (peach).
Diagnostics
Disease Severity
Sensitization to Gly m 4 can serve as a marker for severe allergic reactions to soy in especially birch pollen allergic patients.
Interestingly, in a study conducted on 21 pollen-related soybean allergic patients and 93 pollen sensitized controls in Japan, sensitization to Gly m 4 in soybean allergic adult patients were found to be of more relevance than Gly m 5 or Gly m 6 (considered to be more relevant in children). Further, Gly m 4-sIgE levels could differentiate between pollen-related soybean allergy and pollen sensitization (p<0.001).
However, Gly m 4 sensitization may not be useful for determining the clinical expression of soybean allergy in sensitized individuals. This was observed 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 4 was observed to be almost similar in symptomatic patients (21%) and non-symptomatic individuals (20%).
Exposure
The main route of exposure is through ingestion.
Cross-Reactivity
Gly m 4 is homologous to Bet v 1, sharing 53% sequence identity to Bet v 1. Extensive IgE cross-reactivity has been reported between Bet v 1 and Gly m 4. It was demonstrated by a study on 4 soy-allergic patients where IgE to Gly m 4 in all the 4 patients was inhibited by Bet v 1. Further, sIgE to Bet v 1 was found to be significantly correlated (p<0.001) with Gly m 4-sIgE in a study on 35 soy-allergic and 11 soy-tolerant adults in the Netherlands. The high degree of sequence similarity and highly similar tertiary structure accounts for the cross-reactivity that occurs between the amino acid sequences of Gly m 4 and Bet v 1.
A study on 57 birch pollen allergic patients identified the major epitope of Bet v 1 to be Bet v 1142-156. This major epitope was considered to play a role in cross-reactivity and was recognized by Gly m 4 in 44.4% of the 11 T cell clones. Similarly, another study on 203 Chinese patients with a clinical history of allergic rhinitis/asthma found a high correlation between specific IgE for Bet v 1 and Gly m 4 (p<0.0001).
Gly m 4 has been extensively described to cross-react with other PR-10 proteins in food. A study identified amino acid sequence homology between Gly m 4 and other PR-10 proteins. It was found to be 44% for Act d 8 (kiwi), 35.4% for Api g 1 (celery), 70.2% for Ara h 8, 56.5% for Cor a 1.04 (hazelnut), 50.6% for Mal d 1, 52.5% for Pru p 1, 47.5% for Aln g 1 and 67.1% for Bet v 1 respectively. Further, it was also reported that IgE-binding to Gly m 4 was inhibited by Pru av 1 in 2 out of 4 soy-allergic patients.
Immuno-cross-reactivity between peanut and soybean may be partly attributed to similarities between their allergens, Ara h 8 and Gly m 4, in amino acid sequence, IgE epitope mapping, and molecular modeling.
Further, the sequence similarity of 74%-84 % has been found between Gly m 4 and its homologous PR-10 proteins of yellow lupine, a flowering plant belonging to the legume family Fabaceae.
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