Genomic organization of peanut allergen gene, Ara h 3

Prospects for developing allergen-depleted food crops

In addition to the challenge of meeting global demand for food production, there are increasing concerns about food safety and the need to protect consumer health from the negative effects of foodborne allergies. Certain bio-molecules (usually proteins) present in food can act as allergens that trigger unusual immunological reactions, with potentially life-threatening consequences. The relentless working lifestyles of the modern era often incorporate poor eating habits that include readymade prepackaged and processed foods, which contain additives such as peanuts, tree nuts, wheat, and soy-based products, rather than traditional home cooking. Of the predominant allergenic foods (soybean, wheat, fish, peanut, shellfish, tree nuts, eggs, and milk), peanuts (Arachis hypogaea) are the best characterized source of allergens, followed by tree nuts (Juglans regia, Prunus amygdalus, Corylus avellana, Carya illinoinensis, Anacardium occidentale, Pistacia vera, Bertholletia excels), wheat (Triticum aestivum), soybeans (Glycine max), and kidney beans (Phaseolus vulgaris). The prevalence of food allergies has risen significantly in recent years including chance of accidental exposure to such foods. In contrast, the standards of detection, diagnosis, and cure have not kept pace and unfortunately are often suboptimal. In this review, we mainly focus on the prevalence of allergies associated with peanut, tree nuts, wheat, soybean, and kidney bean, highlighting their physiological properties and functions as well as considering research directions for tailoring allergen gene expression. In particular, we discuss how recent advances in molecular breeding, genetic engineering, and genome editing can be used to develop potential low allergen food crops that protect consumer health.

Critical review on proteotypic peptide marker tracing for six allergenic ingredients in incurred foods by mass spectrometry

Peptide marker identification is one of the most important steps in the development of a mass spectrometry (MS) based method for allergen detection, since the robustness and sensitivity of the overall analytical method will strictly depend on the reliability of the proteotypic peptides tracing for each allergen. The European legislation in place issues the mandatory labelling of fourteen allergenic ingredients whenever used in different food formulations. Among these, six allergenic ingredients, namely milk, egg, peanut, soybean, hazelnut and almond, can be prioritized in light of their higher occurrence in food recalls for undeclared presence with serious risk decision. In this work, we described the results of a comprehensive evaluation of the current literature on MS-based allergen detection aiming at collecting all available information about proteins and peptide markers validated in independent studies for the six allergenic ingredients of interest. The main features of the targeted proteins were commented reviewing all details available about known isoforms and sequence homology particularly in plant-derived allergens. Several critical aspects affecting peptide markers reliability were discussed and according to this evaluation a final short-list of candidate markers was compiled likely to be standardized and implemented in MS methods for allergen analysis.

Peanut allergens

Peanut allergens have the potential to negatively impact on the health and quality of life of millions of consumers worldwide. The seeds of the peanut plant Arachis hypogaea contain an array of allergens that are able to induce the production of specific IgE antibodies in predisposed individuals. A lot of effort has been focused on obtaining the sequences and structures of these allergens due to the high health risk they represent. At present, 16 proteins present in peanuts are officially recognized as allergens. Research has also focused on their in-depth immunological characterization as well as on the design of modified hypoallergenic derivatives for potential use in clinical studies and the formulation of strategies for immunotherapy. Detailed research protocols are available for the purification of natural allergens as well as their recombinant production in bacterial, yeast, insect, and algal cells. Purified allergen molecules are now routinely used in diagnostic multiplex protein arrays for the detection of the presence of allergen-specific IgE. This review gives an overview on the wealth of knowledge that is available on individual peanut allergens.

Comparative and evolutionary analysis of major peanut allergen gene families

Peanut (Arachis hypogaea L.) causes one of the most serious food allergies. Peanut seed proteins, Arah1, Arah2, and Arah3, are considered to be among the most important peanut allergens. To gain insights into genome organization and evolution of allergen-encoding genes, approximately 617 kb from the genome of cultivated peanut and 215 kb from a wild relative were sequenced including three Arah1, one Arah2, eight Arah3, and two Arah6 gene family members. To assign polarity to differences between homoeologous regions in peanut, we used as outgroups the single orthologous regions in Medicago, Lotus, common bean, chickpea, and pigeonpea, which diverged from peanut about 50 Ma and have not undergone subsequent polyploidy. These regions were also compared with orthologs in many additional dicot plant species to help clarify the timing of evolutionary events. The lack of conservation of allergenic epitopes between species, and the fact that many different proteins can be allergenic, makes the identification of allergens across species by comparative studies difficult. The peanut allergen genes are interspersed with low-copy genes and transposable elements. Phylogenetic analyses revealed lineage-specific expansion and loss of low-copy genes between species and homoeologs. Arah1 syntenic regions are conserved in soybean, pigeonpea, tomato, grape, Lotus, and Arabidopsis, whereas Arah3 syntenic regions show genome rearrangements. We infer that tandem and segmental duplications led to the establishment of the Arah3 gene family. Our analysis indicates differences in conserved motifs in allergen proteins and in the promoter regions of the allergen-encoding genes. Phylogenetic analysis and genomic organization studies provide new insights into the evolution of the major peanut allergen-encoding genes.

Peanut allergens: an overview

Peanut is recognized as a potent food allergen producing one of the most frequent food allergies. This fact has originated the publication of an elevated number of scientific reports dealing with peanut allergens and, especially, the prevalence of peanut allergy. For this reason, the information available on peanut allergens is increasing and the debate about peanut allergy is always renewed. This article reviews the information currently available on peanut allergens and on the techniques used for their chemical characterization. Moreover, a general overview on the current biotechnological approaches used to reduce or eliminate peanut allergens is also provided.

Epigenetic regulation of peanut allergen gene Ara h 3 in developing embryos

Peanut (Arachis hypogaea) allergy is one of the most serious food allergies. Peanut seed protein, Ara h 3, is considered to be one of the most important peanut allergens. Little is known about the temporal and spatial regulation mechanism of Ara h 3 during seed development. In this study, chromatin structure of the Ara h 3 promoter was analyzed to examine its transcriptional regulation. Analysis of transgenic plants of Arabidopsis thaliana expressing Arah3: GUS showed that the Ara h 3 promoter could efficiently direct the seed-specific expression of the GUS reporter gene. Chromatin immunoprecipitation revealed that nucleosomes were depleted at the core promoter of the Ara h 3 upon full activation in the late stage of embryo maturation, which was accompanied by a dramatic decrease of histone acetylation. However, in the early stage of embryo maturation, histone H3 hyperacetylation at the core promoter of Ara h 3 was detected. A decrease of histone H3-K9 dimethylation levels at core promoter of Ara h 3 was also observed with concomitant repression of Ara h 3 in the vegetative tissues. Our results suggest that the histone modification status of Ara h 3 undergoes targeted changes including the increase of histone H3 acetylation and decrease of histone H3-K9 dimethylation in early maturation embryos. In addition, the loss of histone H3 from the proximal promoter of Ara h 3 is associated with its high expression during late embryo maturation.

Crystal structure of Ara h 3, a major allergen in peanut

The prevalence of food allergy has increased dramatically in recent years. Tremendous research progress has been made in understanding the pathophysiological mechanisms of allergy and in identifying and characterizing food allergens. Peanut is a major food allergen source and Ara h 3 is a major peanut allergen. Using overlapping short peptides, several linear IgE-binding epitopes in Ara h 3 have been defined before. However, the structure of Ara h 3 of the native allergen is not clear and information on conformational epitopes is lacking. Structural characterization of allergens is required for understanding the allergenicity of food allergens and for the development of immunotherapeutic agents. Previously, we have reported the crystallization of Ara h 3 purified from raw peanut. Here we report the crystal structure of Ara h 3 at 1.73A resolution. Mapping of the previously defined linear epitopes on the crystal structure of Ara h 3 indicated that linear epitopes with more solvent exposure were those indicated by the literature to react with more patient sera. The structure of Ara h 3 may be used to assess the importance of conformational epitopes in further investigations.

Purification and characterization of natural Ara h 8, the Bet v 1 homologous allergen from peanut, provides a novel isoform

The peanut allergen Ara h 8 is an important allergen for birch pollen allergic patients because of the cross-reactivity to the homologous Bet v 1. As the existence of Ara h 8 has been shown at the cDNA level so far (AY328088) and the allergen has indirectly been detected as natural protein, it was the aim of our study to identify natural Ara h 8 in peanut extract and to develop a purification strategy. This was achieved using a unique combination of purification steps, including optimized extraction conditions, size exclusion and ion exchange chromatography and treatment of the interfering contaminants with iodoacetic acid. A characterization of the protein by microsequencing showed discrepancies to the deduced amino acid sequence of AY328088. For this reason, we cloned and expressed a new Ara h 8 isoform from cDNA (EU046325). This IgE-reactive protein corresponds to the results of microsequencing, ESI-FTICR-MS and trypsin fingerprinting analysis of the authentic and purified nAra h 8. Apart from the ultimate use of recombinant allergens for diagnostic procedures, there is also a scientific need for the natural counterpart, as it represents an excellent reference point by which to compare protein characteristics and to standardize diagnostic and therapeutic allergens.

A genetic engineering strategy to eliminate peanut allergy

Peanut allergy is an IgE-mediated hypersensitivity reaction with an increasing prevalence worldwide. Despite its seriousness, to date, there is no cure. Genetic engineering strategies can provide a solution. The post-transcriptional gene silencing (PTGS) model can be used effectively to knock out the production of allergenic proteins in peanut by specific degradation of the endogenous target messenger RNA (mRNA). Ara h 2, the most potent peanut allergenic protein, was selected as a model to demonstrate the feasibility of this concept. Transgenic peanut plants were produced via microprojectile-mediated transformation of peanut embryos using a plasmid construct, which contains a fragment of the coding region of Ara h 2 linked to an enhanced CaMV 35S constitutive promoter. Molecular analyses, including polymerase chain reaction and Southern blots, confirmed the presence of the stable integration of the Ara h 2 transgene into the peanut genome. Northern hybridization showed the expression of the Ara h 2 transgene in all vegetative tissues of the mature transgenic peanut plants, indicating the stable expression of the truncated Ara h 2 transgene throughout the development of the plants. It is, therefore, reasonable to expect that the truncated Ara h 2 transgene transcripts will be synthesized in the seeds and will trigger the specific degradation of endogenous Ara h 2 mRNA. The next step will be to grow the transgenic peanut plants to full maturity for seed production and to determine the level of allergen Ara h 2.