Molecular biology of the Rh blood group system

The Amt/MEP/Rh Family: Structure of AmtB and the Mechanism of Ammonia Gas Conduction

The atomic structures of the first members of the Amt/MEP/Rh family show that they are 11-crossing membrane proteins that form trimers in the membrane. Each monomer supports a hydrophobic channel that conducts NH3but not any water or ions. The reprotonation of NH3on the receiving side raises the pH on that side in the absence of metabolism of NH3, and there is no transfer of protons through the protein.

Rh proteins: Key structural and functional components of the red cell membrane

Rh (Rhesus) proteins (D, CcEe) are expressed in red cells (RBC) in association with other membrane proteins (RhAG, LW, CD47 and GPB). By interacting with the spectrin-based skeleton through protein 4.2 and ankyrin, the Rh complex contributes to the maintenance of the mechanical properties of the erythrocyte membrane. The RH system is one of the most immunogenic and polymorphic human blood group system. Molecular basis of most Rh phenotypes, including the Rh(null) phenotype associated with hemolytic anemia, have been determined. The demonstration that the RHD-positive locus is composed of the RHD and RHCE genes, whereas the RHD gene is deleted in most RhD-negative individuals, allowed fetal RhD genotyping by non-invasive PCR assays for antenatal diagnosis of pregnancy at risk for Rh hemolytic disease of the newborn. In mammals, the Rh protein family includes two non-erythroid members, RhBG and RhCG, mainly expressed in liver and kidney, two organs specialized in ammonia genesis and excretion. Functional analyses in heterologous systems revealed that RhAG, RhBG and RhCG can mediate ammonium (NH(3) and/or NH(4)(+)) transport across the cell membrane and might represent mammalian specific ammonium transporters. Furthermore, recent studies performed in human and murine red blood cells (RBC) indicate that RhAG facilitates CH(3)NH(2)/NH(3) movement across the membrane and represents a potential example of gas channel. The crystallographic structure of the bacterial ammonia channel AmtB and functional studies showing that AmtB conducts NH(3) into reconstituted vesicles is fully consistent with these latter studies. In RBCs, RhAG may transport NH(3) to detoxifying organs like kidney and liver and with non-erythroid tissues orthologs may contribute to regulation of the acid-base balance.

Utilisation en routine clinique du génotypage fœtal RHD sur plasma maternel : bilan de deux ans d’activité

OBJECTIVES

To evaluate the predictive value of RHD fetal genotype in maternal plasma of Rh D negative mothers after 10 weeks of gestation in a clinical use.

MATERIAL AND METHOD

Prospective, comparative study between fetal RHD genotyping in maternal plasma, with amplification of exons 4,5,10 of the RHD gene, by real-time multiplex PCR, and Rh D serology at birth, in 218 pregnancy and their 223 babies, between November 2002 and 2004.

RESULTS

Combining the amplification of three exons, the concordance rate of fetal Rh D genotyping in maternal plasma and baby phenotyping at delivery was 100%. Four women whose the babies were Rh D negative were positive for RHD exon 10 during pregnancy. This positivity was, in three cases, correlated with the presence of RHDpsi pseudogene and in last case, with a haplotype Cdes (r's). RHD genotyping was performed for five twin pregnancies.

CONCLUSION

Multiplex PCR using maternal plasma provides perfect prenatal prediction of fetal RHD gene. These results confirm that this non invasive procedure is the preferred method for assessing Rh D fetal status in Rh negative mothers. Using this method for two years in routine practice has led us to modify our management scheme for sensitized Rh D-negative pregnant women.

High frequency of partial DIIIa and DAR alleles found in sickle cell disease patients suggests increased risk of alloimmunization to RhD

We have set out to determine the frequency of DIIIa and DAR alleles among sickle cell disease (SCD) patients. These D variants permit the unexpected development of antibodies to RhD among individuals who are otherwise classified as RhD+. DNA samples from 130 SCD patients were tested for 455A>C (specific for DIIIa), 602C>G, 667T>G (common for both DIIIa and DAR) and 1025T>C (specific for DAR) by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and sequence analysis. The PCR-RFLP showed that 12 (9.2%) of the SCD patients were carrying DIIIa and DAR alleles. Genomic DNA analysis performed by sequence showed that three samples were heterozygous DIIIa (2.3%), seven heterozygous DAR (4.6%) and two (1.5%) samples carried a partial D with four mutations: 455A>C (heterozygous), 602C>G and 667T>G (homozygous) and 1025T>C (heterozygous), indicating compound heterozygosity for one DIIIa allele and one DAR allele. The predicted phenotypes of eight (6.2%) SCD patients were DIIIa, DAR and DIIIa/DAR. Three patients were anti-D immunized (DAR, n = 1; DIIIa/DAR, n = 2). These findings suggest that SCD patients who are candidates for chronic transfusion may benefit from genotyping for DIIIa and DAR to prevent alloimmunization.

Genetic basis of blood group diversity

In the last 18 years the genes that encode all but one of the 29 blood group systems present on red blood cells (RBCs) have been identified. This body of knowledge has permitted the application of molecular techniques to characterize the common blood group antigens and to elucidate the background for some of the variant phenotypes. Just as the RBC was used as a model for the biochemical characterization of cell membranes, so the genes encoding blood groups provide a readily accessible model for the study of gene expression and diversity. The application of genotyping techniques to identify fetuses at risk of haemolytic disease of the newborn is now the standard of care, and the expansion of nucleic acid testing platforms to include both disease testing and blood typing in the blood centre is on the horizon. This review summarizes the molecular basis of blood groups and illustrates the mechanisms that generate diversity through specific examples.

Weakened expression of 'e' owing to concomitant occurrence of Cys16 and Val245 (VS antigen)

BACKGROUND AND OBJECTIVES

The 48 G>C transversion in exon 1 of the RHCE gene leads to Trp16Cys, usually present in the conventional RHCE Ce, while Trp16 is associated with RHCE ce. The presence of Cys16 in RHCE ce is associated with the R(0) (Dce) haplotype in Africans, leading to a weak 'e' antigen expression on red blood cells (RBCs). VS is a common red cell antigen in individuals of African descent and results from a single point mutation in exon 5 of the RHCE (733C>G), leading to Leu245Val substitution; VS positivity is also associated with weak expression of 'e'. This study investigated the association of Cys16 and/or VS with the RHCE ce alleles in a cohort of sickle cell disease (SCD) patients phenotyped as R(0)r or R(0)R(0) and rr.

MATERIALS AND METHODS

DNA samples from 58 SCD patients were tested for the 48 G>C transversion, encoding Cys16, by allele-specific polymerase chain reaction (PCR). We also amplified exon 5 of the RHCE by PCR and subjected the amplified product to restriction fragment length polymorphism analysis, using BfaI, in order to determine the VS status. Further cDNA analysis was performed on three samples to verify whether the mutations were located on the same or on different alleles.

RESULTS

Fifty-six of the 58 SCD patients studied (97%) were heterozygous for 48G/48C (Cys16). Of these, 18 (32%) were also heterozygous for 733C/G (245Val). All of these 18 samples showed weak 'e' expression on RBCs when tested with at least one monoclonal antibody to e antigen. cDNA sequencing of three of 18 patient samples showed that the genes encoding Cys16 and Val245 (VS) were on different alleles.

CONCLUSIONS

We found a high incidence of Cys16 associated with the RHCE ce in our SCD cohort. A high percentage of these patients were also found to be heterozygous for VS. cDNA analysis showed that, in at least three samples, the two mutations were on different alleles, with consequent weakening of expression of the e antigen on RBCs.

The evolution and formation of RH genes

The Rh system clinically is one of the important blood groups. The major Rh antigens, which are constituted by over 40 types, are RhD, RhC/c, and RhE/e. Furthermore, Rh blood group system is characterized by the existence of many variants. It was considered that Rh blood group system was encoded on two genes termed the RHCE and RHD, which are composed of ten exons, respectively. It is inferred that the RHD gene encodes the RhD antigen and that the RHCE gene encodes the Rh C/c and RhE/e antigens. There are RHce, RHCe, RHcE and RHCE alleles as polymorphisms of RHCE gene. In 2000, the entire nucleotide sequences in all introns of both the RHD and RHCE genes were determined. Due to the new findings on RH genes, it is thought that multiple recombination (and/or gene conversion), nucleotide substitutions, small nucleotide gaps, replication slippage of microsatellite, large nucleotide gaps (due to Alu sequence) and the high level of the homology (%) between both RH genes are the important factors in the formation and evolution of both RH genes and Rh variants. Based on the advance of human genome project, the new interpretations on the evolution and formation of RH genes and Rh variants will be performed. Human Rh family (superfamily) and its counterparts in primates, mammals, fish, amphibians, bacteria, lower eukaryotes, archaea and plants have been identified. A lot of findings have been accumulated in their evolution and function. As gene conversions or recombination events confuse the phylogenetic tree of human RH genes and their counterparts, careful attention is necessary for researchers to calculate the time of gene duplication and to discuss the evolution of Rh family and its counterparts.Rh genotyping methods will never be perfect and both the clinicians and researchers have to recognize the limitation of Rh genotyping, especially RhD genotyping, because new Rh variants must have formed continually. In applying the Rh genotyping to clinical medicine, especially transfusion medicine, it is necessary to compare and examine the serological (phenotypic) data in Rh blood group system with caution.

Prediction of fetal D status from maternal plasma: introduction of a new noninvasive fetal RHD genotyping service

BACKGROUND

Invasive procedures to obtain fetal DNA for prenatal blood grouping present a risk to the fetus. During pregnancy, cell-free fetal DNA is present in maternal blood. The detection of RHD sequences in maternal plasma has been used to predict fetal D status, based on the assumption that RHD is absent in D- genomes.

STUDY DESIGN AND METHODS

Real-time PCR assays were designed to distinguish RHD from RHDpsi (possessed by the majority of D- black Africans). Plasma-derived DNA from 137 D- women was subjected to real-time PCR to detect fetal RHD and Y chromosome-associated SRY sequences. The accuracy of RHD genotyping from maternal plasma was investigated by comparing results with those obtained by conventional RHD genotyping from fetal tissue or serologic tests on the infant's RBCs. The quantity of fetal DNA in maternal plasma was investigated in 94 pregnancies.

RESULTS

Fetal D status was predicted with 100-percent accuracy from maternal plasma. The number of copies of fetal DNA in maternal plasma was found to increase with gestation.

CONCLUSION

Combination of the sensitivity of real-time PCR with an improved RHD typing assay to distinguish RHD from RHDpsi enables highly accurate prediction of fetal D status from maternal plasma. This has resulted in the implementation of a clinical noninvasive fetal RHD genotyping service.