Outliers in RhD membrane integration are explained by variant RH haplotypes

RHD-negative red cells may be avoided for patients with ambiguous serologic typing for the RHD antigen

BACKGROUND AND OBJECTIVES

Serologic typing with monoclonal anti-D is mandatory for RHD antigen determination before transfusion, but due to aberrant (weak or partial) variants of RHD, results may be ambiguous and molecular RHD-typing is required. Before that, RHD-negative (RHD -) red blood cells concentrates (RBCs) shall be transfused to avoid anti-D formation, which probably leads to wastage of RHD - RBCs. STUDY DESIGN AND METHODS: All patients with ambiguous results in serologic RHD-typing and molecular RHD-typing were assessed retrospectively. The proportions of patients at risk for anti-D formation and the proportion of RHD - RBCs transfused unnecessarily were evaluated for the following transfusion strategies: (1) RHD-positive (RHD + )RBCs for all patients, (2) RHD + RBCs for patients with at least 2+ reaction with anti-D, (3) RHD + RBCs for patients with C and/or E in their RHCE-phenotype, (4) RHD + RBCs for patients with C and/or E and at least 2+ reaction, and (5) RHD - RBCs for all patients.

RESULTS

A total of 112 patients were included. Most had weak D type 1-3 and a minority had other, rare RHD variants. The risk of anti-D formation was 4.5%, 2.9%, 1.8%, 1.0%, and 0% for strategies 1-5, respectively. The proportion of RHD - RBCs transfused unnecessarily was 0%, 49.5%, 0.9%, 50.5%, and 95.5%.

CONCLUSION

Transfusing patients with a C and/or E in their RHCE-phenotype with RHD + RBCs resulted in a very low risk of immunization while avoiding wastage of RHD - RBCs. Therefore, this strategy should be used for some patients with ambiguous results in serologic RHD-typing and pending results of molecular RHD-typing.

Rh flow cytometry: An updated methodology for D antigen density applied to weak D types 164 and 165

BACKGROUND

An original methodology for determining the D antigen density on red cells was published in 2000 and has been applied in many publications since. This flow cytometry-based assay remained largely unrevised utilizing monoclonal anti-Ds that are not readily available anymore. We updated the methodology to quantify erythrocyte D antigen sites using microspheres and monoclonal anti-Ds that are commercially available today.

METHODS

The absolute D antigen density of a frozen standard CcDEe cell, drawn in 2003, a fresh blood donation from the same individual, drawn in 2022, and an internal control CcDEe cell, was quantified by flow cytometry using fluorescence-labeled microspheres. The internal control CcDEe cell was used in conjunction with 9 commercial anti-Ds to determine D antigen densities of 7 normal D, 4 partial D, and 11 weak D type samples, including 2 novel alleles.

RESULTS

The reproducibility of the updated assay was evaluated with red cells of published D antigen densities. The current results matched the known ones closely. The new weak D types 164 and 165 carried 4500 and 1505 D antigens/red cell, respectively. The absolute D antigen density decreased from 27,231 to 26,037 in an individual over 19 years.

DISCUSSION

The updated assay gave highly reproducible results for the D antigen densities of Rh phenotypes. Readily available anti-Ds allowed for the determination of the D antigen densities of 7 weak D types. The assay is suitable to evaluate the effects of distinct amino acid substitutions on the RhD phenotype.

A practical and effective strategy in East Asia to prevent anti-D alloimmunization in patients by C/c phenotyping of serologic RhD-negative blood donors

Serologic RhD-negative red cells can cause anti-D alloimmunization if they carry the Asian-type DEL or other DEL variants. RHD genotyping is a viable countermeasure if available, but inexpensive alternatives are worthy of consideration. RhD-negative blood donors in Japan were studied by anti-D adsorption-elution and RHD genotyping. We collated published case reports of RhD-negative red cell transfusions associated with inexplicable anti-D immunization. Of 2754 serologic RhD-negative donors, 378 were genotyped D/d. Anti-D adsorption-elution revealed 63.5% (240 of 378) to be DEL, of whom 96.7% (232 of 240) had the 1227G > A variant, diagnostic for the Asian-type DEL. All 240 donors also carried at least one C antigen; none had a cc phenotype. The chance of transfusing DEL red cells to genuinely RhD-negative Asian patients (based on a three-unit transfusion) ranges from 16.7% in Korea to 69.4% in Taiwan, versus 0.6% in Germany. Among 22 RhD-negative recipients of serologic RhD-negative red cells, who produced new or increased anti-D antibody titers, all 17 from East Asia were transfused with red cells with a C-positive phenotype or known to be Asian-type DEL or both. Serologic RhD-negative East Asians with a cc phenotype can be red cell donors for RhD-negative recipients, especially those of childbearing potential.

DEL in China: the D antigen among serologic RhD-negative individuals

BACKGROUND

Providing RhD-negative red cell transfusions is a challenge in East Asia, represented by China, Korea, and Japan, where the frequency of RhD-negative is the lowest in the world.

FINDINGS

Among 56 ethnic groups in China, the RhD-negative frequency in Han, the prevalent ethnicity, is 0.5% or less, similar to most other ethnic groups. The Uyghur ethnic group has the highest reported RhD-negative frequency of up to 4.7%, as compared to 13.9% in the US. However, an estimated 7.15 million RhD-negative people live in China. The RhD-negative phenotype typically results from a loss of the entire RHD gene, causing the lack of the RhD protein and D antigen. The DEL phenotype carries a low amount of the D antigen and types as RhD-negative in routine serology. The DEL prevalence in RhD-negative individuals averages 23.3% in the Han, 17% in the Hui and 2.4% in the Uyghur ethnicities. The Asian type DEL, also known as RHD*DEL1 and RHD:c.1227G > A allele, is by far the most prevalent among the 13 DEL alleles observed in China.

CONCLUSION

The purpose of this review is to summarize the data on DEL and to provide a basis for practical strategy decisions in managing patients and donors with DEL alleles in East Asia using molecular assays.

High prevalence of weak D type 42 in a large-scale RHD genotyping program in the province of Quebec (Canada)

BACKGROUND

The determination of the RhD phenotype is crucial to avoid alloimmunization, especially in childbearing women. Following the 2015 recommendation from the Work Group on RHD Genotyping, a large-scale RHD genotyping program was implemented in the province of Quebec (Canada) and offered to women ≤45 years old with a serological weak D or discordant results. Since weak D type 42 was previously shown to be prevalent among French Canadians, genotyping for that variant was also performed. Our aim was to report the prevalence of the weak D alleles in the province of Quebec.

STUDY DESIGN AND METHODS

A retrospective study of 2105 women with serological weak D referred to Hema-Quebec's immunohematology reference laboratory (IRL) between June 2016 and May 2020 was conducted. Results from the serological tests performed by the referring hospital were compiled and RHD were genotyped.

RESULTS

Most patients presented at least one serological result ≤2+ before being referred to Hema-Quebec. Weak D type 42 was the most prevalent variant, representing 17.5% (368/2105) of all individuals tested. Only 15.3% (323/2105) of patients were weak D type 1, 3.3% (69/2105) were type 2, and 8.6% (180/2105) were type 3. Weak D type 42 is highly expressed in regions with low immigration rate and known for their founder effect.

CONCLUSION

Our RHD genotyping program allowed for a better management of weak D. The province of Quebec presents a unique RHD genotype distribution. We confirmed that weak D type 42 is associated with a founder effect found in Caucasian French Canadians.

Frequency and characterization of RHD variant alleles in a population of blood donors from southeastern Brazil: Comparison with other populations

BACKGROUND

The correct determination of D antigen could help to avoid alloimmunization in pregnant women and patients receiving blood transfusions. However, there are limitations in the identification of D variants as the partial and weak D phenotypes make the determination of D antigen a great challenge in the transfusion routine.'

STUDY DESIGN AND METHODS

The molecular characterization of D variants was performed on blood donors from southeastern Brazil with atypical D typing. Furthermore, the serological profile of all RHD variant alleles identified was analyzed using different Anti-D clones. The prevalence of RHD alleles and genotypes found was compared with those described in other countries and in other regions from Brazil.

RESULTS

Atypical serologic D typing occurred in 0.79 % of blood donors. The majority of RHD variant alleles (88 %) were first characterized by multiplex PCR and PCR-SSP as RHD*weak partial 4 (47 %), followed by RHD*weak D type 3 (29.9 %), RHD*weak D type 2 (3.9 %) and RHD*weak D type 1 (3.1 %). Genomic DNA sequencing characterized the RHD*weak partial 4 variants found in RHD*DAR1.2 (weak 4.2.2) (22 %), RHD*DAR3 (weak 4.0.1) (2.4 %), RHD*DAR3.1 (weak 4.0) (22 %) and RHD*DAR4 (weak 4.1) (0.8 %). RHD variant alleles associated with partial D, such as, RHD*DAU-4 (1.6 %), RHD*DAU-5 (2.4 %), RHD*DAU-6 (1.6 %), RHD* DIII type 8 (1.6 %), RHD*DVII (3.9 %) and RHD* DMH (0.8 %) were also observed.

CONCLUSION

The prevalence of RHD variant alleles observed in this cohort differ from those found in other populations, including Brazilians from other regions. RHD allele distribution in specific regions should be considered for implementation of algorithms and genotyping strategies aiming at a more effective and safe transfusion.

Correlation among automated scores of agglutination, antigen density by flow cytometry and genetics of D variants

BACKGROUND

Laboratory testing to identify the molecular basis of serologic weak D phenotypes is recommended to determine whether a pregnant woman or potential transfusion recipient should be managed as RhD-positive or RhD-negative. The variation in D antigen expression on RBCs, different potencies of anti-D typing reagents, lack of standardized test methods, and the subjectivity of interpreting agglutination reactions complicate the detection of D variants. We evaluated the correlation of agglutination scores by an automated immunoassay analyzer with D antigen densities determined by flow cytometry, and D variant types identified by molecular analysis.

MATERIALS AND METHODS

We selected 273 blood donor samples with agglutination scores of less than 92 (4+), measured by an automated analyzer (NEO®, Immucor, Norcross, GA, USA). D antigen densities were measured by flow cytometry for 89 samples. Samples were classified as molecularly-determined weak D or partial D variants by multiplex PCR, PCR RFLP and DNA sequencing.

RESULTS

All samples with a D antigen density ≥15% had an agglutination score >80 (4+). Agglutination scores for weak D types varied from 10 to 90. Agglutination scores for partial D antigens were graded with scores varying from 60 to 99. D antigen densities varied from 0.55% to 10.67% for weak Ds and 4.1% to 30.5% for partial Ds.

DISCUSSION

Our results showed that score values follow a pattern among D variants that could be related to antigen density and to the RhD variant classification.

A novel double-variant RHAG allele leads to Rhmod phenotype

AIMS/OBJECTIVES

We aimed to analyse the molecular backgrounds and red blood cell (RBC) antigen expression of a male blood donor with Rh_mod phenotype and his family members.

BACKGROUND

Rh deficiency phenotypes are rarely found worldwide and are characterised by the lack of Rh antigen expression on RBCs. During routine screening, we found a blood donor who seemingly lacked Rh antigens. Therefore, we recruited the donor and his family for further investigation.

METHODS

RBC serotyping and antibody screening/identification were performed for each sample. A routine blood examination was also conducted. RHD, RHCE and RHAG were sequenced at the genomic DNA or RNA level. Eleven antigens or proteins associated with Rh complex were tested using flow cytometry analysis.

RESULTS

The proband and one of his brothers showed extremely weak D antigen and Rh expression levels but did not manifest anaemia. Most of the expressed RBC antigens of the two Rh-deficient individuals were similar to the previously reported cases but with some exceptions. Molecular analyses demonstrated homozygous expression of a novel RHAG allele, namely, c.[572G>A;707A>C], both in the proband and one of his brothers.

CONCLUSIONS

To our knowledge, we identified the second double-variant RHAG allele and the first one related to Rh_mod phenotype. The novel allele was also confirmed to be heritable by family analyses.

Transfusion strategy for weak D Type 4.0 based on RHD alleles and RH haplotypes in Tunisia

BACKGROUND

With more than 460 RHD alleles, this gene is the most complex and polymorphic among all blood group systems. The Tunisian population has the largest known prevalence of weak D Type 4.0 alleles, occurring in one of 105 RH haplotypes. We aimed to establish a rationale for the transfusion strategy of weak D Type 4.0 in Tunisia.

STUDY DESIGN AND METHODS

Donors were randomly screened for the serologic weak D phenotype. The RHD coding sequence and parts of the introns were sequenced. To establish the RH haplotype, the RHCE gene was tested for characteristic single-nucleotide positions.

RESULTS

We determined all RHD alleles and the RH haplotypes coding for the serologic weak D phenotype among 13,431 Tunisian donations. A serologic weak D phenotype was found in 67 individuals (0.50%). Among them, 60 carried a weak D Type 4 allele: 53 weak D Type 4.0, six weak D Type 4.2.2 (DAR), and one weak D Type 4.1. An additional four donors had one variant allele each: DVII, weak D Type 1, weak D Type 3, and weak D type 100, while three donors showed a normal RHD sequence. The weak D Type 4.0 was most often linked to RHCE*ceVS.04.01, weak D Type 4.2.2 to RHCE*ceAR, and weak D Type 4.1 to RHCE*ceVS.02, while the other RHD alleles were linked to one of the common RHCE alleles.

CONCLUSIONS

Among the weak D phenotypes in Tunisia, no novel RHD allele was found and almost 90% were caused by alleles of the weak D Type 4 cluster, of which 88% represented the weak D Type 4.0 allele. Based on established RH haplotypes for variant RHD and RHCE alleles and the lack of adverse clinical reports, we recommend D+ transfusions for patients with weak D Type 4.0 in Tunisia.

D category IV: a group of clinically relevant and phylogenetically diverse partial D

BACKGROUND

The D typing strategies in several European countries protect carriers of D category VI (DVI) from anti-D immunization but not carriers of other partial D. Besides DVI, one of the clinically most important partial D is D category IV (DIV). A detailed description and direct comparison of the different DIV types was missing.

STUDY DESIGN AND METHODS

RHD nucleotide sequences were determined from genomic DNA. D epitope patterns were established with commercial monoclonal anti-D panels.

RESULTS

DIV comprises several variants of the D antigen with distinct serology, molecular structures, evolutionary origins, and ethnic prevalences. The DIV phenotype is determined by 350H shared by all, but not limited to, DIV variants which are further divided into DIVa and DIVb. The DIVa phenotype is expressed by DIV Type 1.0 harboring 350H and the dispersed amino acids 62F, 137V, and 152T. The DIVb phenotype is expressed by DIV Type 3 to Type 5 representing RHD-CE-D hybrids. Four of the six postulated DIV variants were encountered among 23 DIV samples analyzed. Of 12 DIV carriers with anti-D, 10 were female and seven likely immunized by pregnancy. Two DIV-related alleles are newly described: DWN, which differs from DIV Type 4 by 350D and epitope pattern. DNT carries 152T, known to cause a large D antigen density.

CONCLUSION

DIV alleles arose from at least two independent evolutionary events. DIV Type 1.0 with DIVa phenotype belongs to the oldest extant human RHD alleles. DIV Type 2 to Type 5 with DIVb phenotype arose from more recent gene conversions. Anti-D immunization, especially dreaded in pregnancies, will be avoided not only in carriers of DVI but also in carriers of other D variants like DIV, if our proposed D typing strategy is adopted.

Evolutionary genetics of the human Rh blood group system

The evolutionary history of variation in the human Rh blood group system, determined by variants in the RHD and RHCE genes, has long been an unresolved puzzle in human genetics. Prior to medical treatments and interventions developed in the last century, the D-positive (RhD positive) children of D-negative (RhD negative) women were at risk for hemolytic disease of the newborn, if the mother produced anti-D antibodies following sensitization to the blood of a previous D-positive child. Given the deleterious fitness consequences of this disease, the appreciable frequencies in European populations of the responsible RHD gene deletion variant (for example, 0.43 in our study) seem surprising. In this study, we used new molecular and genomic data generated from four HapMap population samples to test the idea that positive selection for an as-of-yet unknown fitness benefit of the RHD deletion may have offset the otherwise negative fitness effects of hemolytic disease of the newborn. We found no evidence that positive natural selection affected the frequency of the RHD deletion. Thus, the initial rise to intermediate frequency of the RHD deletion in European populations may simply be explained by genetic drift/founder effect, or by an older or more complex sweep that we are insufficiently powered to detect. However, our simulations recapitulate previous findings that selection on the RHD deletion is frequency dependent and weak or absent near 0.5. Therefore, once such a frequency was achieved, it could have been maintained by a relatively small amount of genetic drift. We unexpectedly observed evidence for positive selection on the C allele of RHCE in non-African populations (on chromosomes with intact copies of the RHD gene) in the form of an unusually high F( ST ) value and the high frequency of a single haplotype carrying the C allele. RhCE function is not well understood, but the C/c antigenic variant is clinically relevant and can result in hemolytic disease of the newborn, albeit much less commonly and severely than that related to the D-negative blood type. Therefore, the potential fitness benefits of the RHCE C allele are currently unknown but merit further exploration.

A novel laboratory technique demonstrating the influences of RHD zygosity and the RhCcEe phenotype on erythrocyte D antigen expression

D antigen is the most immunogenic and clinically relevant antigen within the complex Rh blood group system. Variability of D antigen expression was first described decades ago but has rarely been investigated quantitatively, particularly in the context of RHD zygosity along with RhCcEe serological phenotype. With IRB approval, 107 deidentified blood samples were analyzed. Rh phenotypes were determined serologically by saline technique using monoclonal antibodies against D, C, c, E, and e antigens. RHD zygosity was determined using both PCR-restriction fragment length polymorphisms and quantitative real-time PCR techniques. A novel and robust method was developed for quantitation of erythrocyte D antigen sites using calibrated microspheres and flow cytometry, allowing correlation of D antigen density with RHD zygosity and expression of Rh CcEe antigens. Subjects homozygous for RHD expressed nearly twice the number of D antigen sites compared with RHD hemizygotes (33,560 ± 8,222 for DD versus 17,720 ± 4,471 for Dd, P < 0.0001). Expression of c or E antigens was associated with significantly increased erythrocyte D antigen expression, whereas presence of C or e antigens reduced expression. These data and this novel quantitation method will be important for future studies investigating the clinical relevance of D antigen variability.

Molecular genetics and clinical applications for RH

Rhesus is the clinically most important protein-based blood group system. It represents the largest number of antigens and the most complex genetics of the 30 known blood group systems. The RHD and RHCE genes are strongly homologous. Some genetic complexity is explained by their close chromosomal proximity and unusual orientation, with their tail ends facing each other. The antigens are expressed by the RhD and the RhCE proteins. Rhesus exemplifies the correlation of genotype and phenotype, facilitating the understanding of general genetic mechanisms. For clinical purposes, genetic diagnostics of Rhesus antigens will improve the cost-effective development of transfusion medicine.

Overcoming methodical limits of standard RHD genotyping by next-generation sequencing

BACKGROUND AND OBJECTIVES

Molecular variations of the RHD gene may result in the reduced expression of the D antigen and altered Rh phenotypes. In many occasions, they cannot be typed reliably by standard serological methods. Sequence-based typing is the gold standard to determine rare and unknown RHD genotypes. For this pilot study, sequence-based typing by standard Sanger sequencing was compared to a newly established next-generation sequencing approach based on pyrosequencing.

MATERIALS AND METHODS

Twenty-six DNA samples were selected after primary serological testing exhibiting a weak reaction in Rh phenotype. Parallel sequence analysis of the complete coding sequence including adjacent intronic sequences allowed a comparison of the methodical potency in mutation detection of Sanger with next-generation sequencing.

RESULTS

Sanger sequencing revealed 39 RHD polymorphisms in 21 of 26 samples in the RHD coding region, while pyrosequencing detected all but two alterations resulting in a concordance rate of 94·9% and clearly revealed a heterozygous compound mutation in one sample with RHDψ and Weak D type 4 alleles. The resolution of cis/trans linkage of polymorphisms and exact characterization of a 37 bp duplication was achieved by next-generation sequencing.

CONCLUSION

Our data suggest that next-generation sequencing offers a new development for high-throughput and clonal sequencing for molecular RHD genotyping. However, further attempts in the methodical set-up have to be undertaken prior to validation and introduction as a routine service.

Investigation of whether the acute hemolysis associated with Rh(o)(D) immune globulin intravenous (human) administration for treatment of immune thrombocytopenic purpura is consistent with the acute hemolytic transfusion reaction model

BACKGROUND

Immune thrombocytopenic purpura and secondary thrombocytopenia patients treated with Rh(o)(D) immune globulin intravenous (human; anti-D IGIV) have experienced acute hemolysis, which is inconsistent with the typical presentation of extravascular hemolysis -- the presumed mechanism of action of anti-D IGIV. Although the mechanism of anti-D-IGIV-associated acute hemolysis has not been established, the onset, signs/symptoms, and complications appear consistent with the intravascular hemolysis of acute hemolytic transfusion reactions (AHTRs). In transfusion medicine, the red blood cell (RBC) antigen-antibody incompatibility(-ies) that precipitate AHTRs can be detected in vitro with compatibility testing. Under the premise that anti-D-IGIV-associated acute hemolysis results from RBC antigen-antibody-mediated complement activation, this study evaluated whether the incompatibility(-ies) could be detected in vitro with a hemolysin assay, which would support the AHTR model as the hemolytic mechanism.

STUDY DESIGN AND METHODS

Seven anti-D IGIV lots were tested to determine the RBC antibody identities in those lots, including four lots that had been implicated in acute hemolytic episodes. Hemolysin assays were performed that tested each of 73 RBC specimens against each lot, including the RBCs of one patient who had experienced acute hemolysis after anti-D IGIV administration.

RESULTS

Only two anti-D IGIV lots contained RBC antibodies beyond those expected. No hemolysis endpoint was observed in any of the hemolysin assays.

CONCLUSION

Although the findings did not support the AHTR model, the results are reported to contribute knowledge about the mechanism of anti-D-IGIV-associated acute hemolysis and to prompt continued investigation into cause(s), prediction, and prevention of this potentially serious adverse event.

RHCE alleles detected after weak and/or discrepant results in automated Rh blood grouping of blood donors in Northern Germany

BACKGROUND

More than 170 weak or partial RHD alleles are currently known. A similar heterogeneity of RHCE alleles may be anticipated, but a large-scale systematic analysis of the molecular bases of altered C, c, E, and e antigenicity in European blood donors was lacking.

STUDY DESIGN AND METHODS

Between November 2004 and October 2006, samples collected from 567,105 blood donors in the northwest of Germany were surveyed for weakened and/or discrepant serologic reaction patterns of the C, c, E, or e antigens in automated testing. Samples from 187 donors with systematic typing problems were further investigated by manual typing and in 122 donors by DNA typing. The polymorphisms determining C, c, E, and e, as well as three repeatedly found substitutions, M167K, G96S, and L115R, were tested by PCR-SSP. Further analysis consisted of sequencing of the exons of RHCE. In addition, 13 referred samples were analyzed.

RESULTS

RHcE(M167K) known as E variant I was the most frequent allele, found in 70 of 122 analyzed donors. Among 13 referred samples, C typing problems predominated. Overall, 34 different underlying alleles were detected, 23 of which were new. Molecular causes included single-amino-acid substitutions, gene conversions, multiple dispersed amino acid substitutions, protein extensions, and in-frame amino acid deletions.

CONCLUSION

In addition to RHcE(M167K), a large number of different alleles are underlying CcEe typing problems. Molecular mechanisms parallel those found in RHD. Elucidation of the molecular bases of variant antigens is important to improve serologic and molecular typing methods.

Blood group genotyping in Germany

BACKGROUND

Molecular methods for blood group genotyping became available more than 10 years ago as one major aspect of immunogenetics. Since then, the clinical applications have been expanded and refined. Their implementation varies considerably among different health-care systems, notably between North America and Europe.

STUDY DESIGN

This summary is based on studies published mostly during the last 3 years and on workshop reports from the German and Swiss transfusion societies. It represents an edited transcript of the author's presentation given at the Workshop on Molecular Methods in Immunohematology organized by the Food and Drug Administration (FDA) in Bethesda on September 25, 2006.

RESULTS

Current applications of blood group genotyping in Germany, Switzerland, and Austria are detailed: weak D testing in patients and pregnant women; blood group genotyping in perinatal care, in patients who received a transfusion, and in patients with immunohematologic problems; RHD genotyping in donors for DEL and D(+/-) chimera; and RHD zygosity testing.

CONCLUSION

Since around 2000, molecular tests for blood groups have been widely offered as a routine service. Many samples are shipped to reference laboratories in the German-speaking countries with the specific request for such testing. The advent of Conformité Européenne (CE)-labeled test kits renders it technically and legally possible, within the specifications of the CE-certification process for in vitro diagnostic devices in the European Union, to replace several blood group serology tasks by genotyping.

How I manage donors and patients with a weak D phenotype

PURPOSE OF REVIEW

Since the adoption of molecular blood-group typing, the considerable heterogeneity of the serologic entities weak D and DEL at the molecular level has come to light. I offer an approach to the management of donors and patients expressing D antigen weakly and carrying any of the various molecular types of weak D and DEL.

RECENT FINDINGS

More than 50 distinct weak D alleles have been described. An internet-based survey of anti-D immunizations occurring in D-positive transfusion recipients reveals that no allo-anti-D has been observed in patients carrying prevalent weak D types. Allo-immunizations are documented for weak D types 4.2 (also known as DAR), 11 and 15. Anti-D immunizations have been reported in D-negative persons transfused with weak D and DEL red blood cells.

SUMMARY

Patients carrying any of the prevalent weak D types 1, 2, 3 or 4.1 are not prone to allo-anti-D immunization and may safely be transfused with D-positive red blood cells. Pregnant women with these weak D types need not receive RhIg. We should pay attention to weak D- or DEL-positive blood units that are labelled D-negative. The clinical benefit of removing DEL blood units from our supply of D-negative red blood cell units should be determined.

Molecular genetics of RH and its clinical application

BACKGROUND

The RH genes RHD and RHCE encode two proteins that represent the clinically most important blood group system defined by the sequences of red cell membrane proteins. In the last five years the field has been moving from defining the underlying molecular genetics to applying the molecular genetics in clinical practice.

MATERIALS AND METHODS

The state of the current knowledge is briefly summarized using recent reviews and original work since 2000.

RESULTS

The RHD and RHCE genes are strongly homologous and located closely adjacent at the human chromosomal position 1p36.11. Part of the genetic complexity is explained by the clustered orientation of both genes with their tail ends facing each other. The SMP1 gene is located interspersed between both RH genes. Using additional genetic features of the RH gene locus, RHCE was shown to represent the ancestral RH position, while RHD is the duplicated gene. More than 150 alleles have been defined for RHD alone. They were classified based on antigenic and clinical properties into phenotypes like partial D, weak D and DEL. Among the D negative phenotype a large variety of non-functional alleles were found. The frequencies of these distinct alleles vary widely among human populations, which has consequences for clinical practice.

CONCLUSION

Rhesus is a model system for the correlation of genotype and phenotype, facilitating the understanding of underlying genetic mechanisms in clustered genes. With regard to clinical practice, the genetic diagnostics of blood group antigens will advance the cost-effective development of transfusion medicine.