The RhD− Trait in a White Patient With the RhCCee Phenotype Attributed to a Four-Nucleotide Deletion in theRHD Gene

RHD gene deletion occurred in the Rhesus box

The Rh blood group antigens derive from 2 genes,RHD and RHCE, that are located at chromosomal position 1p34.1-1p36 (chromosome 1, short arm, region 3, band 4, subband 1, through band 6). In whites, a cde haplotype with a deletion of the whole RHD gene occurs with a frequency of approximately 40%. The relative position of the 2 RH genes and the location of the RHD deletion was previously unknown. A model has been developed for the RH locus using RHD- and RHCE-related nucleotide sequences deposited in nucleotide sequence databases along with polymerase chain reaction (PCR) and nucleotide sequencing. The open reading frames of bothRH genes had opposite orientations. The 3′ ends of the genes faced each other and were separated by about 30 000 base pair (bp) that contained the SMP1 gene. The RHD gene was flanked by 2 DNA segments, dubbed Rhesus boxes, with a length of approximately 9000 bp, 98.6% homology, and identical orientation. The Rhesus box contained the RHD deletion occurring within a stretch of 1463 bp of identity. PCR with sequence-specific priming (PCR-SSP) and PCR with restriction fragment length polymorphism (PCR-RFLP) were used for specific detection of the RHDdeletion. The molecular structure of the RH gene locus explains the mechanisms for generating RHD/RHCE hybrid alleles and the RHD deletion. Specific detection of theRHD− genotype is now possible.

The presence of an RHD pseudogene containing a 37 base pair duplication and a nonsense mutation in Africans with the Rh D-negative blood group phenotype

Antigens of the Rh blood group system are encoded by 2 homologous genes, RHD and RHCE, that produce 2 red cell membrane proteins. The D-negative phenotype is considered to result, almost invariably, from homozygosity for a complete deletion ofRHD. The basis of all PCR tests for predicting fetal D phenotype from DNA obtained from amniocytes or maternal plasma is detection of the presence of RHD. These tests are used in order to ascertain the risk of hemolytic disease of the newborn. We have identified an RHD pseudogene (RHD ψ) in Rh D-negative Africans. RHDψ contains a 37 base pair (bp) insert in exon 4, which may introduce a stop codon at position 210. The insert is a sequence duplication across the boundary of intron 3 and exon 4.RHDψ contains another stop codon in exon 6. The frequency ofRHDψ in black South Africans is approximately 0.0714. Of 82 D-negative black Africans, 66% hadRHDψ, 15% had the RHD-CE-D hybrid gene associated with the VS+ V– phenotype, and only 18% completely lackedRHD. RHDψ is present in about 24% of D-negative African Americans and 17% of D-negative South Africans of mixed race. No RHD transcript could be detected in D-negative individuals with RHDψ, probably as a result of nonsense-mediated mRNA decay. Existing PCR-based methods for predicting D phenotype from DNA are not suitable for testing Africans or any population containing a substantial proportion of people with African ethnicity. Consequently, we have developed a new test that detects the 37 bp insert in exon 4 of RHDψ. (Blood. 2000; 95:12-18)

Rh phenotype prediction by DNA typing and its application to practice

The complexity of the RHD and RHCE genes, which is the greatest of all blood group systems, confounds analysis at the molecular level. RH DNA typing was introduced in 1993 and has been applied to prenatal testing. PCR-SSP analysis covering multiple polymorphisms was recently introduced for the screening and initial characterization of partial D. Our objective is to summarize the accrued knowledge relevant to the approaches to Rh phenotype prediction by DNA typing, their possible applications beyond research laboratories and their limitations. The procedures, results and problems encountered are highly detailed. It is recommended that DNA typing comprises an analysis of more than one polymorphism. We discuss future directions and propose a piecemeal approach to improve reliability and cost-efficiency of blood group genotyping that may eventually replace the prevalent serology-based techniques even for many routine tasks. Transfusion medicine is in the unique position of being able to utilize the most extensive phenotype databases available to check and develop genotyping strategies.