CHARACTERIZATION OF THE HYBRID RHD GENE LEADING TO THE PARTIAL-D CATEGORY-IIIC PHENOTYPE

Background: A D-positive white woman was found to have produced alloan ti-D leading to hemolytic disease of the newborn in her third D-positi ve child. The maternal D was identified as the partial D category IIIc antigen (D-IIIc). The molecular basis of this phenotype was studied. Study Design and Methods: The proposita and her relatives were phenoty ped for Rh system antigens with standard reagents. D-IIIc typing of D- positive red cells was done with serum that contained anti-D from the proposita. Southern blot analysis and RHD-specific polymerase chain re actions were performed with genomic DNA. Rh transcripts were cloned an d sequenced. Results: Six relatives of the proposita were found to exp ress the D-IIIc phenotype, which traveled with Ce. The D-IIIc phenotyp e was inherited in a Mendelian fashion. Southern blot analysis showed an identical digestion pattern in D-IIIc individuals and in DD control s. Three different Rh transcripts were found. Two Rh transcripts were derived from RHCE (RHce and RHCe). The RHD-derived Rh transcript was t he same as that of the published RHD sequence, apart from exon 3, whic h appeared to be exon 3 of RHCE. At the genomic level, RHD exon 3 was missing in all individuals expressig D-IIIc. Conclusion: This study sh ows the characteristics of a new hybrid D-CE-D allele encoding D-IIIc. It may be concluded that exon 3 of RHD is not involved in the formati on of any of the D epitopes known at present, but rather encodes a new D epitope or D epitopes, as yet undefined by monoclonal anti-D reagen ts.