MOLECULAR-BASIS OF THE ALTERED ANTIGENIC EXPRESSION OF RHD IN WEAK D(D-U) AND RHC E IN R(N) PHENOTYPES/

The RH blood group locus is composed of two sequence-related genes, RH D and RHCE, encoding the D, Cc, and Ee antigens in common ph-positive phenotypes. In this report. we have analyzed the molecular basis of Rh antigens expression in weak D (D-u) and R(N) donors, in whom there is a severe reduction of the D and C/e antigens, respectively. Genomic a nd transcript analysis of three unrelated low-grade weak D (D-u) varia nts indicated that the very low expression of the D antigen is not the result of rearrangement or mutation in the coding sequence of the RHD gene. Accordingly. weak D (D-u) erythrocytes should carry a normal Rh D polypeptide, which is in agreement with the observation that these v ariants never produce anti-D antibodies. Comparative polymerase chain reaction analysis showed a lower steady-state level of RhD transcripts in weak D (D-u) reticulocytes, as compared with normal RhD-positive c ontrols, thus providing direct evidence that the difference between th e D antigen of D-positive and weak D (D-u) red blood cells is quantita tive only. Conversely, analysis of the molecular genetic basis of the R(N) phenotype indicated that the severely decreased expression of the RhC and Rhe antigens in three variants is associated with a qualitati ve alteration identified as a segmental DNA exchange between the RHCE and RHD genes. These genomic rearrangements, which resulted in hybrid RhCe-D-Ce proteins expressing the low frequency Rh32 but not the high incidence Rh46 antigens, involved either exon 4 alone or both exons 3 and 4. These findings show that an identical phenotypical alteration o f ph antigens (reduced expression) may result either from a quantitati ve or a qualitative alteration of the RH genes expression. (C) 1996 by The American Society of Hematology.