Background: RhD and RhCE are structurally related to ammonium transporter proteins, yet their physiologic function remains unclear. Recent three-dimensional homology modeling with Escherichia coli AmtB as a template defined a putative transmembraneous channel. Three RhD variants with amino acid substitutions located at the extracellular channel aperture are described.
Study design and methods: Blood samples were selected because of serologic abnormalities. RHD, RHCE, and LW nucleotide sequences were determined from genomic DNA. D epitope patterns were established with monoclonal anti-D panels. Three-dimensional Rh structures were calculated by alignment to AmtB.
Results: The RHD allele DCS-1 was found to carry the two amino acid substitutions F223V (667T > G) and A226P (676G > C) caused by missense mutations in RHD exon 5. This study compared DCS-1 with the D variants DFV (F223V) and DCS-2 (A226P), harboring solely one or the other of the two substitutions. All three D variants were associated with a cDE haplotype. The antigen densities were approximately 3,000 D antigens per red blood cell for DCS-1, 800 for DCS-2, and 9,300 for DFV. DCS-1 and DCS-2 were partial D, because they lacked distinct epitopes. DFV presented as an almost normal D phenotype; the sample contained allo-anti-LW(a). The D(w) antigen was absent from DCS-1, DFV, and DAU-4, but expressed by DAU-5.
Conclusion: DCS-1, DCS-2, and DFV carry amino acid substitutions at the extracellular vestibule, visualized by 3-dimensional modeling. Proline at position 226 greatly influenced the D antigen density and may reduce the RhD membrane integration. Although the F223V substitution is regarded as the initial event in the evolution of the weak D Type 4 cluster, the current DFV allele probably evolved independently, as evident from different RHCE haplotypes.