TRANSCRIPT ANALYSIS OF D-CATEGORY PHENOTYPES PREDICTS HYBRID RH D-CE-D PROTEINS ASSOCIATED WITH ALTERATION OF D-EPITOPES

The RH blood group locus from RhD-positive donors is composed of two c losely 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 e xpression of some of these D epitopes at the surface of variant red bl ood cells defines the D category phenotypes. In this report, we have a nalyzed the Rh transcripts from reticulocytes of different D category phenotypes (D-IVa, D-IVb, D-Va, and DFR). Although Southern blot analy sis did not show obvious deletions within the RHD gene, sequence analy sis of the RhD transcripts indicated that, in all cases studied, the l ack of D epitopes is associated with substitutions, in the deduced pol ypeptides, of amino acids specific of the RhD protein by those encoded at the equivalent position by the RHCE gene. These results strongly s uggested that the D category phenotypes resulted from segmental DNA re placement between RHD-specific fragments and their equivalents in the RHCE gene. The regions involved in the D-IVa, D-IVb, D-Va, and DFR phe notypes were shown to encompass all or part of the exons 3 and 7, exon s 7 to 9, exon 5, and exon 4, respectively. All protein variants encod ed by these rearranged RH genes represent new CE-D-CE hybrid molecules that retain only some of the nine D epitopes. Because segmental DNA r eplacements have been previously identified in other Rh variant genome s, we postulate that such genomic rearrangements between different reg ions of the RHCE and RHD genes should be one of the most frequent even ts involved in the extreme polymorphism of the RH blood group system. (C) 1995 by The American Society of Hematology.