Diversification of the celiac disease α‐gliadin complex in wheat: a 33‐mer peptide with six overlapping epitopes, evolved following polyploidization

Abstract

SUMMARYThe gluten proteins from wheat, barley and rye are responsible both for celiac disease (CD) and for nonceliac gluten sensitivity, two pathologies affecting up to 6-8% of the human population worldwide. The wheat a-gliadin proteins contain three major CD immunogenic peptides: p31-43, which induces the innate immune response; the 33-mer, formed by six overlapping copies of three highly stimulatory epitopes; and an additional DQ2.5-glia-a3 epitope which partially overlaps with the 33-mer. Next-generation sequencing (NGS) and Sanger sequencing of a-gliadin genes from diploid and polyploid wheat provided six types of a-gliadins (named 1-6) with strong differences in their frequencies in diploid and polyploid wheat, and in the presence and abundance of these CD immunogenic peptides. Immunogenic variants of the p31-43 peptide were found in most of the a-gliadins. Variants of the DQ2.5-glia-a3 epitope were associated with specific types of a-gliadins. Remarkably, only type 1 a-gliadins contained 33-mer epitopes. Moreover, the full immunodominant 33-mer fragment was only present in hexaploid wheat at low abundance, probably as the result of allohexaploidization events from subtype 1.2 a-gliadins found only in Aegilops tauschii, the D-genome donor of hexaploid wheat. Type 3 a-gliadins seem to be the ancestral type as they are found in most of the a-gliadin-expressing Triticeae species. These findings are important for reducing the incidence of CD by the breeding/selection of wheat varieties with low stimulatory capacity of T cells. Moreover, advanced genome-editing techniques (TALENs, CRISPR) will be easier to implement on the small group of a-gliadins containing only immunogenic peptides.

Molecular diversity of α-gliadin expressed genes in genetically contrasted spelt (Triticum aestivum ssp. spelta) accessions and comparison with bread wheat (T. aestivum ssp. aestivum) and related diploid Triticum and Aegilops species

“…Among these, 446 showed a full open reading frame (full ORF). The 18 remaining sequences (3.9 %) had at least one premature stop codon (PSC) and were designated as pseudogenes, in line with previous publications (Noma et al 2015; Van Herpen et al 2006; Ozuna et al 2015). …”

“…Working on genomic DNA, Anderson and Greene (1997) showed that about half of the α-gliadin genes from bread wheat displayed at least one PSC. More recently, Ozuna et al (2015) found pseudogene proportions of 39, 76 and 63 % in the genomes of diploid, tetraploid and hexaploid wheat species, respectively. Polyploidization might have contributed to this increase in PSC occurrence because the genetic redundancy created by polyploidy can change the dynamics of coding sequence evolution and lead to the accumulation of PSC in duplicated genes (Akhunov et al 2013; Mighell et al 2000).…”

“…The 33-mer, involving six overlapping copies of DQ2.5-glia-α1 and DQ2.5-glia-α2, is the most immunogenic fragment of α-gliadin sequences (Molberg et al 2005; Shan et al 2002). Alpha-gliadins with these epitope duplications are D-genome specific and the 33-mer fragment is generally found at a low frequency in bread wheat α-gliadins (Molberg et al 2005; Ozuna et al 2015). In this study, we noticed the same trend, with only 13.8 % of the D-genome spelt α-gliadins containing the full 33-mer fragment.…”

Properties of Gluten Intolerance: Gluten Structure, Evolution, Pathogenicity and Detoxification Capabilities

“…Based on the differences in epitopes comprising 33-mer peptide, α-gliadins can be divided into six types. Only Type 1 encompasses proteins including 33-mer peptide (from hexaploid wheat), whereas other types do not [30]. …”

“…Using RNA-amplicon sequencing (NGS) technology it was shown that α-gliadins can be separated into six types and only one type contains all the immunogenic peptides and epitopes, whereas the other five types do not contain all the epitopes disabling 33-mer peptide formation [30]. Thus, distinct types of α-gliadins differ mainly in the number of repeat blocks consisting in interspersed motifs PFPPQQ and PYPQPQ.…”