f441 Jug r 1

Summary

Jug r 1 is heat resistant 2S albumin storage protein, like allergens in pecan nut, which is also digestion stable. Sensitization to 2S albumins is through the gastrointestinal tract. High stability and structural resistance due to cysteine residues make it sustainable in the gastrointestinal tract's harsh acidic environment. Hence, they can cross the gut mucosal barrier, can present to the immune system, and trigger allergic responses. sIgE to Jug r 1 is strongly associated with primary walnut and pecan nut allergy and systemic symptoms risk. Jug r 1 sensitization has a high prevalence in children from Northern Italy, showing that 71.4 % of cases were positive for Jug r 1, leading to anaphylaxis. A study found that the concentration of Jug r 1-sIgE was higher than other components from walnut. Jug r 1 is small and occurs in large amounts in many edible trees which act as seed storage proteins. The specificity of recombinant protein (r Jug r 1) to IgE are possible markers to detect severity and persistence. Another study with pediatric subjects found anaphylaxis, angioedema, and urticarial to be major symptoms. IgE for walnut Jug r 1 protein was found in 96.9 % of cases. IgE binding in storage proteins of Juglans species is weak and not able to cross-react with Jug r 1 or Jug r 2. Peanut allergens like Ara h 1, 2 and 3 are homologous to Jug r 1, r 3, and r 4 from walnut, respectively.

Epidemiology

Worldwide distribution

Euro Prevall project presented data which suggests that 3 % of the adult European population were sensitized to walnut. In  Iceland, Spain and Switzerland, the prevalence was found to be 0.1%, 8% and 6%, respectively (1).

Jug r 1, a 2S albumin, storage protein is the major component allergen from walnut (Juglans regia). Ballmer-Weber et al (2019) studied the prevalence of sensitization to Jug r 1 in children. The study observed out of 88 patients sensitized to Jug r 1, 60 of them had the allergy onset at the age of less than 14 (2).

A study was conducted in Korea on young children with walnut allergy. Walnut currently ranks as the third most common food that induces anaphylaxis in Korean children. In this study, 32 children cases were analyzed, out of which 96.9 % cases were found to be sensitized to Jug r 1 (3). In 2014, a study conducted on 45 children in Northern Italy showed that 71.4 % of cases were positive for Jug r 1, leading to anaphylaxis (4).

A Korean study administered an oral food challenge to 108 children (median age of 6–7 years). It was found that the concentration of Jug r 1-sIgE was higher than other components (3, 5). A study by Sato, Yamamoto et al, conducted in Japan, suggested that specific IgE measurement for Jug r 1 is essential in the diagnosis of walnut allergy in children and youth because of its enhanced clinical specificity in comparison to IgE for walnut extract (6). 

Environmental Characteristics

Source and tissue

Walnut has different allergic storage proteins. One of them is a 2S albumin (Jug r 1). Jug r 1 is an important component for analysis as it belongs to the main distributed family of inherently allergenic 2S albumins. Jug r 1 is small in an allergen, small in size-found in significant quantity in various edible trees. It also acts as seed storage proteins (7). Natural Jug r 1 and recombinant Jug r 1 have different amino acid lengths but exhibit similar IgE-binding reactivity (8). Recombinant Jug r 1 is usually sourced from E.coli. (7). 

Clinical Relevance

Specific molecules

Measurement of sIgE for Jug r 1 protein enhances its specificity for the diagnosis of walnut allergy (6).  The application of recombinant allergens protein molecule and their synthetic sequential epitopes contribute to the detection of sensitization profiles. The specificity of recombinant protein to IgE are possible markers to detect persistence and severity. Hence the use of new technology like microarrays is detected as an exceptional means to enhance allergy diagnostic techniques (9).

A study by Rasi Varaee et al. 2020, focused on Jug r 1 protein produced its recombinant form to apply for in vitro diagnosis of Persian walnut allergy. In this study, cloning of the 2S albumin of J. Regia (Jug r 1) is carried out and revealed that its DNA sequence consisted of 663 base pairs, which encodes 142 amino acids (8).

Cross-reactive molecules

IgE binding for seed storage proteins in Juglans species is weak and may show an inability for cross-reactivity with Jug r 1 or Jug r 2. Immunoblot inhibition is used to determine if in-vitro Jug r 1 and Jug r 2  are cross-reactive with various other species of walnut from the same family. Extensive cross-reactivity has been observed between the different walnut species. It has been reported that J. Regia species is unable to inhibit binding towards recombinant Jug r 1 (the 2S albumin) completely (10). Rosenfeld, Shreffler et al reported that walnut proteins, Jug r 1, Jug r 3, and Jug r 4 shows homology with peanut allergenic proteins Ara h 1, Ara h 2 and Ara h 3, respectively. The sequence identified as sequential IgE binding epitopes of Ara h 1, Ara h 2, and Ara h 3 has no IgE binding equivalents in walnut allergens (11). 

Prevention and Therapy

Experimental trials

In a single-center, prospective cohort study by Elizur et al 2018, oral allergen immunotherapy (OAI) for walnut can induce desensitization to walnut and cross-desensitization to pecan and hazelnut in cases with tree nut co-allergies. The daily dose in small amounts of the allergenic components can maintain desensitization in the body. The reported result concluded that, out of 55 patients, 49 cases (89%) were completely desensitized, and three were partially desensitized. Eighteen cases were under the control group with no dose of allergenic components were found to be allergenic for walnut (12). 

Molecular Aspects

Biochemistry

Jug r 1, the proteins of the 2S albumin superfamily, like prolamins, are small globular, water-soluble proteins. The molecular weight is 12-15 kDa and high content of asparagine, glutamine, cysteine, and arginine residues. Jug r 1 is 2S albumin protein wherein S stands for sedimentation coefficient (8). Jug r 1 has two subunits, a large and a smaller subunit bonded through disulfide bridges (13).

Walnut’s major allergen, Jug r 1, can trigger severe allergic reactions through epitopes. Five digestion-resistant peptides that are AA19-33, AA117-137, AA96-106, AA40-45, and AA54-74 were identified by HPLC method (14). 

Isoforms, epitopes, antibodies

Jug r 1 can elicit food allergy due to its epitope like structure. Six epitopes that have been predicted were AA28-35, AA42-49, AA55-62, AA65-73, AA97-104, and AA109-121 (14). A study observed that the reduced and non-reduced forms of 2S albumin from English walnut could bind with sera IgE of subjects allergic to walnut. This investigation showed that epitopes could be either three-dimensional motifs (conformational epitopes) or linear sequences of amino acids (15). Cloning of the 2S albumin of J. Regia (Jug r 1) revealed that its DNA sequence consisted of 663 base pairs and encoded 142 amino acids. A common IgE binding epitope has been described in the large chain of 2S albumins especially for Jug r 1 (8).

Cross-reactivity

Rosenfeld, Shreffler et al determined the importance of sequence similarity in peanuts and walnuts for clinically relevant co-reactivity (11). Allergy to specific mixtures of nuts is due to cross-reactivity between closely related epitopes. Such epitopes are more common in the phylogenetically associated source. Therefore, the recombinant form of allergenic protein can provide a suitable strategy for the detection and management of allergy (8).

Cross-reactivity of 2S albumins from various plant sources, such as black walnut, Brazil nut, pecan, mustard, Corylus, and sesame, are frequently noticed. Hence the similarity between 2S albumin sequences of walnut and other plants could be distinguished using recombinant 2S albumin. Sensitization to recombinant Jug r 1 can improve prophylaxis and treatment aspects (8).

Lee, Jeong et al conducted a study in 2019 in which fifteen patients (46.9%) were co-sensitized to more than one storage protein. The protein with the highest positivity was the soy storage protein Gly m 6 (n = 8, 25.0%), followed by hazel (Cor a 9, n = 7) and peanut (Ara h 2, n = 6). Only two patients (37 months and 66 months old) were co-sensitized to PR-10, and both had a history of pollen food syndrome. Cross-reactive carbohydrates (CCD) were negative in all patients. Meanwhile, six patients in the tolerant group (85.7%) were co-sensitized to more than one storage protein (3).

Jug r 1 is similar to 2S albumin proteins from cottonseed, brazil nut, mustard, and castor bean (13). Recombinant 2S albumin of walnut is sourced from a bacterial host and applicable for in vitro studies (8). Cross-reactivity between tree nuts and peanut were found to be homogeneous (11). 

Diagnostic Relevance

Disease Severity

Jug r 1 is more specific and has a greater positive predictive value than other walnut food allergenic components. Jug r 1 is the major component considered for diagnosing allergy to walnut (16). In a study, the recombinant Jug r 1 epitope mapping confirmed that a linear 12 amino acid sequence (a large subunit of 2S albumin) is responsible for strong IgE binding in walnut-sensitive patients (8).

Exposure

Sensitization to 2S albumins is considered to occur through the gastrointestinal tract. High stability and structural resistance due to cysteine residues make it sustainable in the gastrointestinal tract's harsh acidic environment. Hence they can cross the gut mucosal barrier, present to the immune system, and trigger an allergic response (8). 

Compiled By

Author: Turacoz Healthcare Solutions

Reviewer: Dr. Magnus Borres

Last reviewed: November 2020