Diversification of the celiac disease [alpha]-gliadin complex in wheat: a 33-mer peptide with six overlapping epitopes, evolved following polyploidization

Summary The gluten proteins from wheat, barley and rye are responsible both for celiac disease (CD) and for non-celiac gluten sensitivity, two pathologies affecting up to 6-8% of the human population worldwide. The wheat [alpha]-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-[alpha]3 epitope which partially overlaps with the 33-mer. Next-generation sequencing (NGS) and Sanger sequencing of [alpha]-gliadin genes from diploid and polyploid wheat provided six types of [alpha]-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 [alpha]-gliadins. Variants of the DQ2.5-glia-[alpha]3 epitope were associated with specific types of [alpha]-gliadins. Remarkably, only type 1 [alpha]-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 [alpha]-gliadins found only in Aegilops tauschii, the D-genome donor of hexaploid wheat. Type 3 [alpha]-gliadins seem to be the ancestral type as they are found in most of the [alpha]-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 [alpha]-gliadins containing only immunogenic peptides. Significance Statement The [alpha]-gliadins contain the most-immunogenic epitopes responsible of gluten intolerances. By NGS technology a comprehensive study of [alpha]-gliadins containing these immunogenic epitopes in diploid and polyploid wheats showed six types of [alpha]-gliadins. However, only Type 1, but not the others, contains all the immunogenic peptides and epitopes. These findings are important for preventing/reducing the incidence of CD and wheat breeding as we identify five types of [alpha]-gliadins that do not contain epitopes for the 33-mer.