Allergenic molecules
The World Health Organization (WHO) and International Union of Immunological Societies’ Allergen Nomenclature Subcommittee (IUIS) have registered four allergenic components, i.e., Sin a 1 (2S albumin), Sin a 2 (11S globulin), Sin a 3 (LTP) and Sin a 4 (Profilin) for white/yellow mustard (S. alba), while only one allergen has been identified from brown/Indian/oriental mustard (B. juncea): Bra j 1 (2S albumin) (4, 22).
| Allergen |
Isoallergen |
Biochemical name |
Molecular weight (kDa) |
Allergenicity |
|---|
| Sinapis alba (White/Yellow mustard) |
| Sin a 1 |
Sin a 1.0101 |
2S albumin |
14 |
Major and most prevalent allergen.
100% of mustard-sensitive patients showed IgE binding to Sin a 1 on immunoblot (n=11) (4, 22).
78.1% patients tested positive for SPT (n=32), while 73.5% patient’s sera showed positive IgE on ELISA (n=34) (23).
|
| Sin a 2 |
Sin a 2.0101 |
11S legumin like protein |
51 |
Major allergen.
44% of mustard-allergic patients were reported positive in ELISA (n=39) (4, 22).
59.3% patients tested positive for SPT (n=32), while 47% patient’s sera showed positive IgE on ELISA (n=34) (23).
|
| Sin a 3 |
Sin a 3.0101 |
Non-specific LTP type I |
12.3 |
Minor allergen.
41.1% of patient’s sera had positive IgE to recombinant (r) Sin a 3 on ELISA (n=34) (23).
|
| Sin a 4 |
Sin a 4.0101 |
Profilin |
13-14 |
Minor allergen.
23.5% of patient’s sera had positive IgE to recombinant (r) Sin a 4 on ELISA (n=34) (23).
|
| Brassica juncea (Indian or oriental mustard) |
| Bra j 1 |
Bra j 1.0101 |
2S albumin seed storage protein |
14 |
Closely related to Sin a 1.
64% mustard-sensitive patients showed IgE binding to synthetic peptide of large chain of Bra j 1 (n=11) (4, 22).
|
| Allergen |
Isoallergen |
Biochemical name |
Molecular weight (kDa) |
Allergenicity |
|---|
kDa: kilodaltons, IgE: Immunoglobulin E, SPT: Skin prick test; LTP: Lipid transfer protein; ELISA: Enzyme-linked immunosorbent assay.
From S. alba variety, eight isoforms of Sin a 1 have been identified via deoxyribonucleic acid cloning and sequencing techniques (Sin a 1.0101, Sin a 1.0102, Sin a 1.0103, Sin a 1.0104. Sin a 1.0105, Sin a 1.0106, Sin a 1.0107 and Sin a 1.0108) (24), while from the B. juncea variety, a new IgE-binding mustard protein has been identified, as oleosin (oil-body protein) and enolase enzyme. However, biological and structural characterization of this newly recognized allergen is still pending (5).
Biomarkers of severity
In a study conducted on 34 mustard allergic patients, Sin a 1 allergen was recognized as a true potential marker of sensitization to mustard, while Sin a 2 was found to be associated with severe symptoms, after ingestion of mustard (4, 23).
It has been shown that steps such as thermal processing/enzymatic digestion are not enough for suppressing the allergenicity of mustard seed allergens. However, extrusion process (preconditioning [35℃] and flaking, steam heating [80-125℃ for 15-20 min], mechanical pressing, solvent extraction and finally de-solventising [103-107℃]) can suppress the allergenicity by the way of removing the potential allergens from mustard oil (4, 25).
Cross-reactivity
Cross-reactivity between aeroallergens, like pollens and mustard, is widely reported (18). Association between mustard and mugwort (Artemisia vulgaris) pollen was found to be significant (p<0.001) in studies conducted on patients with mustard hypersensitivity (13, 18).
The mugwort-mustard allergy is a term coined to describe this association of mugwort pollinosis, with food allergy from the Brassicaceae family members like white and Indian mustard, and others such as cabbage, broccoli and cauliflower. The cross-reactive allergen components, responsible, are not yet confirmed, but possible ones include LTPs (Art v 3, Sin a 3), profilins (Art v 4, Sin a 4) and Art v 60 kDa (26, 27).
Mustard as a hidden allergen is found to have potential cross-reactivity with allergens of nuts, legumes, corn and Rosaceae fruits, especially peach (4, 28, 29). In a prospective study conducted on 38 Spanish mustard-allergic patients, all were found sensitized to other members of the Brassicaceae family (cabbage, cauliflower and broccoli), while 97.4% were sensitized to nuts (excluding almond and peanut), 94.7% to legumes, 78.9% to corn and 89.5% to Rosaceae fruits, suggesting cross-reactivity among them (4, 18). Additionally, cross-sensitization was observed to be significant (Pearson correlation=0.006) among mustard and peach, in an observational cohort study involving 35 peach-allergic patients (30).
A review suggested that no clinical cross-reactivity at either inhalation or food allergy level exists between rapeseed and mustard, although cross-sensitization may be present (28). However, Sin a 1 showed marked cross-reactivity with rapeseed allergen Bn III at IgE antibody level. Furthermore, 2S albumin from sunflower seed was found to be cross-reactive with 2S albumin of mustard (4).
Sensitization to foods from the Rosaceae family and pollens may be attributed to Sin a 3 and Sin a 4 (23). A study revealed that 11S globulins from tree nuts, such as pistachio, almond, hazelnut and walnut shared IgG epitopes with Sin a 2 and reduced its IgE binding ability. A similar effect was not observed with peanut (31). Further, no cross-reactivity was observed between Sin a 1 and proteins of the Brassicaceae family, with similar genetic homology (28).
Sequence similarity of mustard LTP Sin a 3 is reported to be 65% with LTP from cabbage (Bra o 3) and 50-55% with LTP from peach (Pru p 3), apple (Mal d 3), cherry (Pru a v 3) and hazelnut (Cor a 8), as well as with pathogenesis-related PR-10 protein (Fra a 3). Moreover, the sequence similarity of mustard profilin Sin a 4 has been shown to be around 80% with profilin from melon (Cuc m 2) and peach (Pru p 4), and also some similarity with profilins from chenopod (Che a 2) and birch (Bet v 2) (4).
