Transcript analysis of D category phenotypes predicts hybrid Rh D-CE-D proteins associated with alteration of D epitopes

Abstract

The RH blood group locus from RhD-positive donors is composed of two closely related genes, RHCE and RHD, encoding the Cc/Ee and D antigens, respectively. The major Rh antigen, D, is serologically defined as a mosaic of at least nine determinants (epD1 to epD9), and the lack of expression of some of these D epitopes at the surface of variant red blood cells defines the D category phenotypes. In this report, we have analyzed the Rh transcripts from reticulocytes of different D category phenotypes (DIVa, DIVb, DVa, and DFR). Although Southern blot analysis did not sow obvious deletions within the RHD gene, sequence analysis of the RhD transcripts indicated that, in all cases studied, the lack of D epitopes is associated with substitutions, in the deduced polypeptides, of amino acids specific of the RhD protein by those encoded at the equivalent position by the RHCE gene. These results strongly suggested that the D category phenotypes resulted from segmental DNA replacement between RHD-specific fragments and their equivalents in the RHCE gene. The regions involved in the DIVa, DIVb, DVa, and DFR phenotypes were shown to encompass all or part of the exons 3 and 7, exons 7 to 9, exon 5, and exon 4, respectively. All protein variants encoded by these rearranged RH genes represent new CE-D-CE hybrid molecules that retain only some of the nine D epitopes. Because segmental DNA replacements have been previously identified in other Rh variant genomes, we postulate that such genomic rearrangements between different regions of the RHCE and RHD genes should be one of the most frequent events involved in the extreme polymorphism of the RH blood group system.