The rhesus blood group system: insights from recent advances in molecular biology

Determination of fetal rhesus d status by maternal plasma DNA analysis

In this study, we assessed the feasibility of fetal RhD genotyping by analysis of cell-free fetal DNA(cffDNA) extracted from plasma samples of Rhesus (Rh) D-negative pregnant women by using real-time polymerase chain reaction (PCR). Fetal genotyping was performed on 30 RhD-negative women between 9 and 39 weeks of gestation who were referred to us for invasive testing [amniocentesis/chorionic villi sampling (CVS)]. The fetal RHD genotype was determined based on real-time PCR method. Exons 7 and 10 of the RHD and SRY genes were targeted.

Among the pregnant women, 12 were carrying male and 17 were carrying female fetuses. Out of 29 pregnant women, 21 had RhD-positive and nine had RhD-negative fetuses. One sample (case 12, whose blood group was found to be AB Rh [+]) was excluded due to controversial results from repeated serological analyses. All prenatal results were in concordance with postnatal RhD status and fetal sex without false- positive or -negative results. Performing real-time PCR on cffDNA showed accurate, efficient and reliable results, allowing rapid and high throughput non invasive determination of fetal sex and RhD status in clinical samples.

The use of cell-free fetal nucleic acids in maternal blood for non-invasive prenatal diagnosis

BACKGROUND

Cell-free fetal nucleic acids (cffNA) can be detected in the maternal circulation during pregnancy, potentially offering an excellent method for early non-invasive prenatal diagnosis (NIPD) of the genetic status of a fetus. Using molecular techniques, fetal DNA and RNA can be detected from 5 weeks gestation and are rapidly cleared from the circulation following birth.

METHODS

We searched PubMed systematically using keywords free fetal DNA and NIPD. Reference lists from relevant papers were also searched to ensure comprehensive coverage of the area.

RESULTS

Cell-free fetal DNA comprises only 3-6% of the total circulating cell-free DNA, therefore diagnoses are primarily limited to those caused by paternally inherited sequences as well as conditions that can be inferred by the unique gene expression patterns in the fetus and placenta. Broadly, the potential applications of this technology fall into two categories: first, high genetic risk families with inheritable monogenic diseases, including sex determination in cases at risk of X-linked diseases and detection of specific paternally inherited single gene disorders; and second, routine antenatal care offered to all pregnant women, including prenatal screening/diagnosis for aneuploidy, particularly Down syndrome (DS), and diagnosis of Rhesus factor status in RhD negative women. Already sex determination and Rhesus factor diagnosis are nearing translation into clinical practice for high-risk individuals.

CONCLUSIONS

The analysis of cffNA may allow NIPD for a variety of genetic conditions and may in future form part of national antenatal screening programmes for DS and other common genetic disorders.

Large scale blood group genotyping

Determination of predicted blood group phenotype by determination of genotype has been performed since the 1990s. This evolved due to the rapid accrual of information surrounding the molecular basis of blood group antigen expression, which started in 1990 with ABO and RH systems and has now resulted in the molecular description of 28 of the 29 blood groups. Blood group genotyping is currently performed mostly for fetal blood group incompatibility and for assessment of multi-transfused patients. Both of these clinical scenarios are either dangerous or technically difficult, respectively to define serologically. With the simultaneous development of mass scale genotyping platforms it has now permitted the application of them to blood group genotype determination. In this paper, I describe some recently published work that has demonstrated that mass scale genotyping approaches are feasible. These approaches may lead to more effective management of blood stocks and patient cross-matching by reducing the dependence on serology during the time critical pre-transfusion phase. It is most probable that large scale studies, perhaps involving many European Union and North American based blood suppliers, may drive the introduction of this technology and convince red cell serologists that this approach may allow their work to be more focussed.

Noninvasive determination of fetal RHD status by examination of cell-free DNA in maternal plasma

BACKGROUND

Cell-free fetal DNA in maternal plasma opens the way for routine risk-free diagnosis of fetal D status of D- mothers. The focus was on accuracy of RHD typing and confirmation of fetal DNA in maternal plasma while RHD was not detected.

STUDY DESIGN AND METHODS

Plasma DNA was extracted (by manual and/or automatic method) from 255 D- pregnant women and amplified in exons 7 and 10 and intron 4 of RHD gene with real-time polymerase chain reaction. The presence of fetal DNA was confirmed by testing SRY and, when negative, by one of 11 different polymorphisms found in the father but not in the mother. The results were compared with the D status of the newborns.

RESULTS

After exclusion of 25 cases (10%) because of material shortage, in 230 cases (90%) available for complete study, the predictive value of the procedure of fetal RHD testing (RHD genotyping plus confirmation of fetal DNA) was 99.6 percent. SRY detection confirmed fetal DNA presence in maternal plasma in all boys, whereas the detection of various polymorphisms in all girls but one.

CONCLUSIONS

Fetal RHD genotyping from maternal plasma may be used with confidence, although additional polymorphisms for confirmation of fetal DNA should be included for 100 percent predictive value (instead of 99.6%).

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.

Rare RHCE phenotypes in black individuals of Afro-Caribbean origin: identification and transfusion safety

The molecular backgrounds of variants encountered in Afro-Caribbean black individuals and associated with the production of clinically significant antibodies against high-incidence antigens (anti-RH18, anti-RH34) and against Rhe epitopes were determined. We showed that RH:-18 phenotypes are produced by 3 distinct RHCE alleles: ceEK carrying 48G>C (exon 1), 712A>G, 787A>G, 800T>A (exon 5); ceBI carrying 48G>C (exon 1), 712A>G (exon 5), 818C>T (exon 6), 1132C>G (exon 8); and the already known ceAR allele carrying 48G>C (exon 1), 712A>G, 733C>G, 787A>G, 800T>A (exon 5), and 916A>G (exon 6). The RH:-34 phenotype is produced by the (C)ce(s) haplotype described previously and composed of a hybrid D-CE(3-8)-D gene with 4 extra mutations next to a ce(s) allele (733C>G; exon 5) with an extra mutation in exon 7 (1006G>T). Partial Rhe with risk of immunization against lacking epitopes can be produced by the new ce(s) allele carrying an extra mutation in exon 3 (340C>T) and by the ceMO allele described previously. A population of sickle cell disease patients was screened to estimate the incidence of these rare alleles, with the conclusion that a procedure is required to detect the associated phenotypes in black donors to ensure transfusion safety for patients. We also described a new variant [ce(s)(748)] and variants carrying different altered alleles in nonimmunized patients and for whom the risk of immunization is discussed.

On the mechanism of tolerance to the Rh D antigen mediated by passive anti-D (Rh D prophylaxis)

Anti-D prophylaxis is the most successful clinical application of antibody-mediated immune suppression. Passive IgG anti-D is given to Rh D-negative women to prevent immunisation to foetal Rh D-positive red blood cells (RBC) and subsequent haemolytic disease of the newborn. Despite its widespread use and efficacy, the mechanism of action of this therapy is unproven. The known facts about the antigen, antibody response, dose of anti-D, RBC clearance and effects of the passive anti-D on subsequent primary and secondary immune responses are discussed in relation to recent information on ways by which immune responses may be suppressed. Most Rh D antigen sites on RBC are not bound by passive anti-D, and thus epitope masking (which may occur in experimental murine models using xenogeneic RBC) is not the reason why anti-D responses are prevented by administration of prophylactic anti-D. It is hypothesised that although clearance and destruction of the antigenic RBC may be a contributing factor in preventing immunisation, down-regulation of antigen-specific B cells through co-ligation of B cell receptors and inhibitory IgG Fc receptors must also occur.

Molecular background of D(C)(e) haplotypes within the white population

BACKGROUND

D(C)(e) and D(C)e haplotypes may be encountered in the white population. Few data are available on the molecular backgrounds responsible for depressed expression of C and e.

STUDY DESIGN AND METHODS

Individuals of white origin carrying a D(C)(e) genotype resulting in depressed expression of C or both C and e were subdivided into two categories based on the RBC reactivity with the human sera Mol and Hor, which contain antibodies against low-frequency antigens of the Rh (RH) system and other non-Rh low-frequency antigens. Neither Hor+, Mol+ nor Hor+, Mol- RBCs expressed the V (RH10), VS (RH20), and/or Rh32 (RH32) low-frequency antigens. These results suggested that Hor+, Mol+ variants expressed Rh33 (RH33 or Har) and FPTT (RH50), whereas Hor+, Mol- variants might express an undefined low-frequency antigen. Further serologic and molecular analyses were performed.

RESULTS

Molecular analysis of Hor+, Mol+ variants revealed a hybrid gene structure RHCe-D(5)-Ce, in which exon 5 of RHCE (RHCe allele) was replaced by exon 5 of RHD (the so-called RHCeVA allele). The presence of exon 5RHD resulted in several amino acid alterations predicted in the external loop 4 of the CeVA polypeptide. Molecular analysis of Hor+, Mol- variants revealed the presence of a new RHCe allele characterized by a single point mutation C340T within exon 3 (the so-called RHCeMA allele), resulting in a R114W substitution predicted on the external loop 2 of the CeMA polypeptide. A serologic study showed a different pattern of reactivity with C and e MoAbs.

CONCLUSION

Two types of mutations resulted in amino acid substitutions predicted in external loops 4 and 2, respectively, which altered both the C and e reactivity, and indicated conformation changes or defective interaction between nonadjacent loops of the Ce polypeptide. Serologic analysis showed that together with Hor and Mol sera testing, the use of different C and e MoAbs could help to identify these variants within the white population.

Section 1B: Rh flow cytometry. Coordinator's report. Rhesus index and antigen density: an analysis of the reproducibility of flow cytometric determination

Fifty-seven IgG monoclonal anti-D antibodies were evaluated in the Rh flow cytometry section, in which 12 laboratories participated. Staining protocols and a fluorescein (FITC)-conjugated Fab fragment goat anti-human IgG (H + L) as a secondary antibody were recommended but not mandatory. A CcDEe red blood cell (RBC) sample that was shown to be homozygous for RHD by molecular methods was supplied and used as internal 'standard RBC' throughout all experiments. An RBC panel comprising two partial D and four weak D types was supplied as well. The use of standard RBC reduced the variability of the data among the laboratories and allowed the conversion of fluorescence data into epitope densities, which were compounded in an antigen density (antigen D per RBC). The highest antigen density was determined for DVI type III, followed by DVII and weak D type 3; the lowest antigen density were determined for weak D type 1 and type 2. Nine of the 12 participating laboratories discriminated three groups of aberrant RhD that had similar Rhesus indices (RI): D category VI with RI = 0; weak D type 2 and type 3 with an high RI; and D category VII and weak D type 1 with an intermediate RI. The antigen densities and the Rhesus indices obtained correlated well among the laboratories of this Workshop section despite different staining protocols, secondary antibodies and instrumentation.

Molecular biology of the Rh blood group system

Rh molecular biology has made many advances since the first Rh cDNA was cloned in 1990. This review summarizes the current knowledge concerning the molecular basis of Rh antigenicity, D-epitope expression, and the structures of the Rh genes and proteins. Although many recent reviews have appeared regarding these subjects, advances in Rh protein function that have been published within the last 12 months have had a fundamental impact on the future direction of Rh research. In November 2000, an article described the role of Rh proteins in ammonium transport, which has remained undescribed in vertebrates, except for non-specific transport via K+ channels. The recent identification of nonerythroid Rh proteins, their expression in diverse tissues, and notably polarized epithelial and endothelial cells will be of broad functional significance and will greatly increase our understanding of the role of Rh in ammonium transport and the biology of ammonium metabolism as a whole. The advances in Rh molecular genetics have enabled the development of diagnostic tests in the clinic. At present, this is largely confined to the prenatal diagnosis of fetal blood group status in alloimmunized pregnancies, but could be extended to the noninvasive prenatal testing of all D-negative pregnant women and eventually, perhaps, to all patient and donor blood.

Two new alleles of the RHCE gene in Black individuals: the RHce allele ceMO and the RHcE allele cEMI

Six unrelated individuals of Afro-Caribbean origin, whose red cells have a marked reduction of the Rhe antigen expression, have been identified. All exhibited the same serological profile with anti-e monoclonal antibodies and lacked expression of the high frequency e-related antigen hrS. Transcripts and genomic analysis showed that these phenotypes resulted from the presence of two new RHCE alleles, ceMO and cEMI. The ceMO allele corresponded to a RHce gene carrying a G667T mutation (exon 5) and was detected at the homozygous state in sample 1 and at the heterozygous state in samples 2-6. The G667T mutation resulted in a Val223Phe substitution on the Rhce polypeptide, in close proximity to Ala226 (e-antigen polymorphism), which might account for the altered expression of e. The ceMO allele is also associated with the lack of expression of the hrS antigen. The absence of the hrS antigen expression may have implications in transfusion as hrS-negative individuals may develop clinically significant antibodies. The cEMI allele corresponded to a silent RHE allele carrying a nine nucleotide deletion within exon 3 and was detected at the heterozygous state in sample 2. This deletion resulted in a shortened polypeptide of 414 residues (instead of 417) that was absent (or severely reduced) at the red cell surface, as the E antigen was undetectable using serology and Western blot analysis with anti-E reagents. In DNA-based polymerase chain reaction genotyping for RHE determination, the cEMI allele provided a false positive result as the cells carrying this allele are serologically phenotyped as E-negative. The incidence of this allele in the Black population is unknown but, as shown already for D genotyping, one must exercise caution when genotyping is performed to detect the e/E polymorphism.

Structural and functional diversity of blood group antigens

Biochemical and molecular genetic studies have revealed that blood group antigens are present on cell surface molecules of wide structural diversity, including carbohydrate epitopes on glycoproteins and/or glycolipids, and peptide antigens on proteins inserted within the membrane via single or multi-pass transmembrane domains, or via glycosylphosphatidylinositol linkages. These studies have also shown that some blood group antigens are carried by complexes consisting of several membrane components which may be lacking or severely deficient in rare blood group 'null' phenotypes. In addition, although all blood group antigens are serologically detectable on red blood cells (RBCs), most of them are also expressed in non-erythroid tissues, raising further questions on their physiological function under normal and pathological conditions. In addition to their structural diversity, blood group antigens also possess wide functional diversity, and can be schematically subdivided into five classes: i) transporters and channels; ii) receptors for ligands, viruses, bacteria and parasites; iii) adhesion molecules; iv) enzymes; and v) structural proteins. The purpose of this review is to summarize recent findings on these molecules, and in particular to illustrate the existing structure-function relationships.

New Insights into the Rh Superfamily of Genes and Proteins in Erythroid Cells and Nonerythroid Tissues

The past decade has seen extensive studies of the erythrocyte Rh30 polypeptides and Rh-associated glycoprotein, which specify the clinically important Rh blood group system. Here we consider recent advances on these and other Rh homologues in the context of gene organization, molecular evolution, tissue-specific expression, protein structure, and potential biological functions. The Rh family is now known to contain a large number of homologues that form a unique branch in the eucarya life domain. The ancient origin and broad distribution imply central roles for the various Rh proteins in maintaining normal cellular and organismal homeostatic conditions. Rh homologues occur in the form of multiple chromosomal loci in mice and humans, but as single-copy genes in unicellular organisms (e.g., green alga and slime mold). While primitive Rh genes vary largely in exon/intron design, the mammalian Rh homologues bear a similar genomic organization. Sequence comparisons have revealed the signatures and a consensus 12-transmembrane fold characteristic of the Rh family. Phylogenetic analysis has placed all Rh homologues as a related cluster that intercepts ammonium transporter (Amt) clusters, indicating an intimate evolutionary and structural relationship between the Rh and Amt families. The biochemical identification and epithelial expression of RhBG and RhCG orthologues in mammalian kidney, liver, skin, testis, and brain suggest that they serve as transporters likely participating in ammonia homeostasis. Further inquires into the structure, function, biosynthesis, and interaction of Rh proteins will shed new light on ammonia homeostasis in a wide range of human physiological and pathological states.

RhD haemolytic disease of the fetus and the newborn

When an RhD negative mother is exposed to the RhD positive red cells (usually as transplacental haemorrhage), she develops allo-anti-D which crosses the placenta and then results in the destruction of fetal red cells. Clinical manifestations of RhD haemolytic disease (HDN) range from asymptomatic mild anaemia to hydrops fetalis or stillbirth associated with severe anaemia and jaundice. HDN was a significant cause of fetal mortality and morbidity until the introduction of amniocentesis, intrauterine transfusion, controlled early delivery and exchange transfusion in the management of severely alloimmunised women and their fetuses. The objective of monitoring alloimmunised women is to identify fetal anaemia and prevent the development of life-threatening hydrops. Evaluation involves assessing the history of previous pregnancies; serial estimation of maternal anti-D levels; serial ultrasound measurements; serial amniocentesis; fetal blood sampling, and intrauterine transfusion when indicated. Diagnostic genotyping by DNA-based methods can identify at-risk RhD positive fetuses early in gestation. Identification of transplacental haemorrhage (TPH) as the stimulus for anti-D antibody production led to the development of anti-D immunoglobulin prophylaxis for at-risk RhD negative women who are not already alloimmunised. Prevention includes administration of anti-D immunoglobulin for any event associated with TPH during pregnancy, and at delivery of an RhD positive infant. Prophylactic routine administration of anti-D immunoglobulin at 28 (and 34) weeks gestation, in addition to the above, has reduced alloimmunisation to <1% of RhD negative women carrying an RhD positive fetus.