Molecular Basis of the Rare Gene Complex, D(C)-, Which Encodes Four Low Prevalence Antigens In the Rh Blood Group System.

Over 40 years ago, the investigation of a case of fatal HDN in the third child of Madame Nou, a native of Ivory Coast, revealed that Madame Nou's RBCs had an unusual phenotype in the Rh blood group system denoted DIVa(C)-/DIVa(C)-. Initially, her RBCs were shown to express a partial D, a weak form of C, and Go^a (RH30) [Salmon, et al., Rev Franc Transf 1969;12:239]. Later her RBCs were shown to also express RH33, Riv (RH45), and FPTT (RH50) [Bizot, et al., Transfusion 1988;28:342; Delehanty, et al., Transfusion 1983;23:410, abstract]. R₀^Har and CeVA phenotypes are encoded by hybrid RHCE-D(5)-CE alleles (respectively, c+ and C+) and the RBCs express RH33 and FPTT antigens but not Go^a or Riv [Noizat-Pirenne, et al. Transfusion 2002;42:627]. RHD*DIVa.2 encodes a partial D and the Go^a antigen and frequently travels with RHCE*ce(1025T) (RHCE*ceTI) (Vege, et al., Transfusion 2007;47:159A). The purpose of this study was to determine the molecular basis associated with the rare DIVa(C)- complex.

Blood samples were obtained from three donors previously identified as having the DIVa(C)- haplotype. Molecular analyses were performed by standard methods and included AS-PCR, PCR-RFLP, genomic sequencing of specific exons, and cloning and direct sequencing of cDNA.

At the RHD locus all donors were heterozygous for RHD and RHD*DIVa.2 and at the RHCE locus all had a compound hybrid allele, which contains exons 2 and 3 from RHD*DIVa.2 (based on RHD*186G/T, RHD*410C/T, RHD*455A/C), and exon 5 from RHD. The altered RHCE is presumed to be in cis to RHD*DIVa.2. In all three probands RHCE*48 in exon 1 is G/C; presumably the G belonging to the in trans RHCE and the nt48C to the hybrid allele, and this assumption favors exon 1 of the hybrid being from RHCE. Thus, the RHCE allele is likely RHCE*CE-DIVa.2(2,3)-CE-D(5)-CE. The in trans allele in Proband 1 is RH*cE, in Proband 2 it is RHCE*ce 254C, 733G, and in Proband 3 it is RHCE*ce.

The compound hybrid provides an explanation for the expression of the four low prevalence antigens on RBCs with the DIVa(C)- phenotype. RHD*DIVa.2 encodes the Go^a antigen. The flanking of RHD exon 5 by RHCE exons in the compound hybrid likely results in RH33 and FPTT antigen expression because R₀^Har and CeVA RBCs express these two antigens. It is possible that the junction of RHD exon 3 to RHCE exon 4 is involved in the expression of Riv. The weak C expression could be a consequence of exons 2 and 3 from RHD*DIVa.2 in the compound hybrid because exon 2 of the wild type RHD is identical in sequence to exon 2 of RHCE*C. The three probands in our study had RHCE nt1025C/C (wild type) and thus, are not RHCE*ce(1025T). This is the first report of RHD*DIVa.2 being involved in a hybrid gene at the RHCE locus. Such a hybrid is not unprecedented in that RHD*DIIIa is involved in the RHD*DIIIa-CE(4-7)-D hybrid [(C)ce^S type 1 in the r’^S haplotype] As only one example of anti-Riv has been described, our findings provide a tool by which to predict the expression of Riv.

No relevant conflicts of interest to declare.