Structure, Evolution and Functional Inference on the CDP Gene Family in European Hazelnut

Food allergy is an IgE-mediated response of the immune system of sensitive subjects to specific proteins. The recent WHO/FAO consultation included hazelnut (Corylus avellana L.) in the list of global priority allergenic ingredients. One of the most common hazelnut allergen groups encompasses the Cupin_1 domain-containing proteins (CDPs). To date, three CDP allergens (Cor a 9, Cor a 11, and Cor a 16) have been deposited into the official WHO/IUIS database. Recently, using a whole-genome approach, the Cor a loci were mapped in four Corylus species. However, the task of identifying novel CDP allergen candidates directly within the genome has not yet been addressed. The recently available C. avellana cv. 'Tombul' genome sequence allowed the identification of 114 hazelnut CDP-encoding genes. The maximum likelihood analysis allowed us to reconstruct the evolutionary relationships underlying the diversification and expansion of the CDP superfamily. Several duplication events associated with lineage-specific evolution were discovered. Interestingly, conserved epitopes and amino acid motifs that likely play a role in allergenic function were also identified. A total of 42 cis-acting regulatory elements were shared in the promoter regions of known and new putative allergens. Using the well-known CDP allergens as an internal benchmark, we have identified and molecularly analyzed new CDP allergen candidates. To provide a first transcriptional characterization, Cor a 9, Cor a 11, and Cor a 16 were analyzed by real-time PCR. The expression analysis showed generally low relative transcript levels of Cor a genes in immature reproductive tissues compared with young leaves. Interestingly, mass spectrometry analysis confirmed the accumulation of known allergens and several predicted CDP allergen candidates in commercial hazelnut flour. The data reported here may help to further address food safety questions and could be conveniently used in breeding research aimed at developing hypoallergenic cultivars.