FPTT is a low-incidence Rh antigen associated with a "new" partial Rh D phenotype, DFR

A Review of the Literature Organized Into a New Database: RHeference

Hundreds of articles containing heterogeneous data describe D variants or add to the knowledge of known alleles. Data can be difficult to find despite existing online blood group resources and genetic and literature databases. We have developed a modern, elaborate database for D variants, thanks to an extensive literature search with meticulous curation of 387 peer-reviewed articles and 80 abstracts from major conferences and other sources. RHeference contains entries for 710 RHD alleles, 11 RHCE alleles, 30 phenotype descriptions (preventing data loss from historical sources), 35 partly characterized alleles, 3 haplotypes, and 16 miscellaneous entries. The entries include molecular, phenotypic, serological, alloimmunization, haplotype, geographical, and other data, detailed for each source. The main characteristics are summarized for each entry. The sources for all information are included and easily accessible through doi and PMID links. Overall, the database contains more than 10,000 individual pieces of data. We have set up the database architecture based on our previous expertise on database setup and biocuration for other topics, using modern technologies such as the Django framework, BioPython, Bootstrap, and Jquery. This architecture allows an easy access to data and enables simple and complex queries: combining multiple mutations, keywords, or any of the characteristics included in the database. RHeference provides a complement to existing resources and will continue to grow as our knowledge expands and new articles are published. The database url is http://www.rheference.org/.

D variants at the RhD vestibule in the weak D type 4 and Eurasian D clusters

BACKGROUND

One branch of the RHD phylogenetic tree is represented by the weak D type 4 cluster of alleles with F223V as the primordial amino acid substitution. F223V as well as a large number of further substitutions causing D variants are located at the extracellular RhD protein vestibule, which represents the entrance to the transmembraneous channel of the RhD protein.

STUDY DESIGN AND METHODS

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

RESULTS

The RHD alleles DOL-1 and DOL-2 had the two amino acid substitutions M170T (509T>C) and F223V (667T>G) in common. DOL-2 harbored the additional substitution L378V (1132C>G). Both alleles were observed in Africans and are probably evolutionary related. DMI carried M170I (510G>A), which differed from the DOL-typical substitution. DFW and DFL harbored the substitutions H166P (497A>C) and Y165C (494A>G). The antigen densities of DOL-1, DFL, and DFW were only moderately reduced.

CONCLUSION

DOL-1 and DOL-2 belong to the weak D type 4 cluster of RHD alleles. Together with DMI, DFL, and DFW they represent D variants with amino acid substitutions located at extracellular loops 3 or 4 lining the RhD protein vestibule. These substitutions were of minor influence on antigen density while adjacent substitutions in the transmembraneous section caused weak D antigen expression. All these D variants were partial D and alloanti-D immunizations have been observed in DOL-1, DMI, and DFL carriers. The substitution at position 170 causes partial D although located deep in the vestibule.

Molecular basis of the LOCR (Rh55) antigen

BACKGROUND

In 1994 during the investigation of a case of hemolytic disease of the newborn, a new low-incidence red cell (RBC) antigen, LOCR, was described. Although the presence of LOCR was associated with altered expression of Rh antigens, its formal assignment to the Rh blood group system did not occur until haplotype and linkage analysis conducted in 2003 provided the necessary proof. The current study was undertaken in an attempt to define the underlying RH mutation in LOCR+ individuals.

STUDY DESIGN AND METHODS

Genomic DNA from five unrelated LOCR+ individuals and three Rh-matched control individuals was amplified by polymerase chain reaction with intronic primers flanking all 10 exons of RH. Amplified products were separated on 1 percent agarose gels and isolated for DNA sequence analysis in both the forward and the reverse directions.

RESULTS

DNA sequence analysis of the three LOCR+ D- individuals revealed a single heterozygous 286G>A nucleotide substitution resulting in a predicted Gly>Ser substitution at amino acid 96. DNA sequence analysis from the two LOCR+ D+ individuals revealed the identical mutation, as well as all of the changes associated with the common RHD gene.

CONCLUSIONS

Based on our results, a Gly96Ser substitution in the Rhce polypeptide defines the low-incidence RBC antigen known as LOCR. This same amino acid change has previously been shown to be involved in the Rh:-26 phenotype, which suggests that LOCR and Rh26 are antithetical. Serologic investigations with various Rh:-26 cells and serum samples, however, reveal that only some c+ Rh:-26 phenotypes are LOCR+.

Partial D, weak D types, and novel RHD alleles among 33,864 multiethnic patients: implications for anti-D alloimmunization and prevention

BACKGROUND

The D antigen includes category D, partial D, and weak D types, which are important because anti-D alloimmunization can occur in some but not all persons that express a variant RHD allele. At present, there is little prospective information on the prevalence of D variants among obstetric patients and potential transfusion recipients.

STUDY DESIGN AND METHODS

The RHD alleles were prospectively examined in a large patient population identified on the basis of a difference in anti-D reactivity between two reagents.

RESULTS

Fifty-five discrepancies (0.96% of D-) were noted among 33,864 ethnically diverse patients over 18 months, of which 54 represented mutated RHD alleles. Seven obstetric patients were assigned D- status based on serology; only 1 patient had a partial RHD allele. Ten of 25 (36%) obstetric patients and 4 of 6 (67%) female potential transfusion recipients of childbearing age or younger were assigned D+ status, and they expressed a D variant known to permit anti-D alloimmunization. In total 20 RHD alleles were identified including category, DVa or DVa-like alleles (n = 7), DAR (n = 8), and four novel RHD alleles including two new DAU alleles.

CONCLUSION

Given the complexity of D antigen expression, it is concluded that some clinically important D variants identified by standard serologic analysis phenotype as D+ and are potentially at risk for the development of anti-D.

DNA microsatellite and linkage analysis supports the inclusion of LOCR in the Rh blood group system

BACKGROUND

In 1994, a new low-incidence RBC antigen called LOCR was described. It was established that RBCs expressing LOCR had altered expression of Rh antigens (c or e). Unfortunately, because of an insufficient number of informative families, it was not possible to formally assign LOCR to the Rh blood group system by serology alone.

STUDY DESIGN AND METHODS

Genomic DNA from 19 family members segregating for LOCR was analyzed for repeat polymorphisms of the chromosome 1p microsatellite markers D1S1612, D1S1597, D1S552, D1S247, and D1S2134.

RESULTS

No evidence of recombination (in either paternal or maternal meioses) between LOCR and D1S1597, D1S552, or D1S247 was observed. Peak lods for combined paternal and maternal meioses were 2.41 for either LOCR:D1S552 or LOCR:D1S247. Lods for linkage between LOCR and D1S1597 peaked at 1.81 for maternal meioses alone.

CONCLUSIONS

With serologic methods, a peak lod of 2.107 was determined previously between LOCR and RH. In this study, DNA analysis of the only informative family (with seven children) not segregating for RH yielded a peak lod of 1.81 between LOCR and D1S1597-D1S552-D1S247. By combining the results generated by each approach (lods of 3.917), evidence has been provided that supports the placement of LOCR in the Rh blood group system.

RHC and RHc genotyping in different ethnic groups

BACKGROUND

RH genotyping assays are mainly based on research in whites. These assays may not be reliable in a multiracial society because of the genetic variation in RH among ethnic groups.

STUDY DESIGN AND METHODS

Five groups from different ethnic backgrounds were serologically typed for C and c and were genotyped on nucleotide C48 and intron 2 for RHC and RHc on nucleotides C178 and C307.

RESULTS

RHc genotyping with both methods proved to be reliable. RHC genotyping on C48 is not reliable because of a 48G>C mutation in the RHce allele (false-positive prediction of C). This mutation was found in every ethnic group and does not affect c or e expression. RHC genotyping on intron 2 is unreliable because of r's (Cdes) alleles (a false-negative prediction of C). This allele was found in whites and blacks from Curaçao and South Africa. Reactions of r's cells with anti-C are weaker, but no negative reactions with various MoAbs were found. A new method (RHC/c/hex3-intron 4/exon 7 multiplex PCRs) was developed based on intron 2 and r's hybrid exon 3 characteristics (RHC) and C307 (RHc).

CONCLUSIONS

Reliable RHC and RHc genotyping is possible in different ethnic groups with the RHC/c/hex3-intron 4/exon 7 multiplex PCR approach.

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 1A: Rh serology. Coordinator's report

One hundred forty-two Rh-specific monoclonal antibodies (Mabs) were evaluated by serology in 27 laboratories. Evaluators were asked to test each Mab at three dilutions in specified serological techniques against normal positive and normal negative phenotype cells, and any Rh variant cells that they had available. Raw data was submitted to the coordinator for overall analysis. Results were analysed by expressing the sum of reaction grades for each Mab with each variant cell as a percentage of the sum of reaction grades of that Mab with normal phenotype cells. Anti-D Mabs were sorted into 23 groups which had the same pattern of reactions with different partial D phenotype cells. Eighteen of these corresponded to previously defined patterns; five were new patterns. Combined with data from the previous workshop, this means that 30 different reaction patterns have been defined. A new nomenclature is introduced for numbering the epitopes. Reactions with new variants DNB, DNU and DAR indicated some further subsplits of these patterns. Reactions with Category Va cells indicated that there were five different types of Va cells that could be distinguished serologically with monoclonal antibodies. No patterns of reactivity corresponding to the epitope groups could be observed with the different types of weak D tested. Anti-E Mabs were sorted into 14 groups, and the E variant cells into seven groups.

Weak D alleles express distinct phenotypes

The weak D phenotype is caused by many different RHD alleles encoding aberrant RhD proteins, raising the possibility of distinct serologic phenotypes and of anti-D immunizations in weak D. We reported 6 new RHD alleles, D category III type IV, DIM, and the weak D types 4.1, 4.2.1, 4.2.2, and 17. The immunohematologic features of 18 weak D types were examined by agglutination and flow cytometry with more than 50 monoclonal anti-D. The agglutination patterns of the partial D phenotypes DIM, DIII type IV, and DIVtype III correlated well with the D epitope models, those of the weak D types showed no correlation. In flow cytometry, the weak D types displayed type-specific antigen densities between 70 and 4000 RhD antigens per cell and qualitatively distinct D antigens. A Rhesus D similarity index was devised to characterize the extent of qualitative changes in aberrant D antigens and discriminated normal D from all tested partial D, including D category III. In some rare weak D types, the extent of the alterations was comparable to that found in partial Ds that were prone to anti-D immunization. Four of 6 case reports with anti-D in weak D represented auto-anti-D. We concluded that, in contrast to previous assumptions, most weak D types, including prevalent ones, carry altered D antigens. These observations are suggestive of a clinically relevant potential for anti-D immunizations in some, but not in the prevalent weak D types, and were used to derive an improved transfusion strategy in weak D patients.

Weak D alleles express distinct phenotypes

The weak D phenotype is caused by many different RHD alleles encoding aberrant RhD proteins, raising the possibility of distinct serologic phenotypes and of anti-D immunizations in weak D. We reported 6 new RHD alleles, D category III type IV, DIM, and the weak D types 4.1, 4.2.1, 4.2.2, and 17. The immunohematologic features of 18 weak D types were examined by agglutination and flow cytometry with more than 50 monoclonal anti-D. The agglutination patterns of the partial D phenotypes DIM, DIII type IV, and DIVtype III correlated well with the D epitope models, those of the weak D types showed no correlation. In flow cytometry, the weak D types displayed type-specific antigen densities between 70 and 4000 RhD antigens per cell and qualitatively distinct D antigens. A Rhesus D similarity index was devised to characterize the extent of qualitative changes in aberrant D antigens and discriminated normal D from all tested partial D, including D category III. In some rare weak D types, the extent of the alterations was comparable to that found in partial Ds that were prone to anti-D immunization. Four of 6 case reports with anti-D in weak D represented auto-anti-D. We concluded that, in contrast to previous assumptions, most weak D types, including prevalent ones, carry altered D antigens. These observations are suggestive of a clinically relevant potential for anti-D immunizations in some, but not in the prevalent weak D types, and were used to derive an improved transfusion strategy in weak D patients.

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.

The serological profile and molecular basis of a new partial D phenotype, DHR

BACKGROUND AND OBJECTIVES

The Rh D antigen comprises a mosaic of at least 30 epitopes expressed on a 30-kD non-glycosylated Rh D polypeptide. The equivalent Rh CeEe polypeptide expressing the Rh C/c and E/e antigens differs in only 36 of the 417 amino acid residues. Partial D individuals have been described who fail to express a number of D epitopes.

MATERIALS AND METHODS

Serologic methods were applied with monoclonal anti-D to map epitopes on the red cells of a proposita aberrant D typing. Polymerase chain reaction (PCR) and DNA sequencing were also done.

RESULTS

DNA sequence analysis derived by RT-PCR using total RNA isolated from peripheral blood of this person suggests two mechanisms for the genetic basis of this variants: one here gene conversion events result in the replacement of RHD gene exons with the equivalent RHCE exons; the second where point mutation in the RHD gene generates an amino acid substitution in the Rh D protein.

CONCLUSIONS

We report here a new partial D, DHR, where a single point mutation (G to A at nucleotide 686) in exon 5 of the RHD gene results in a conservative amino acid substitution (Arg229Lys), in the predicted Rh D protein. This residue is localised on the fourth predicted exofacial loop of the Rh D polypeptide as determined by hydropathy analysis. This substitution results in the lack of epD 1, 2, 12 and 20 (30 epitope model) and indicates the involvement of loop 4, and in particular the requirement of Arg229, in the expression of these epitopes.

Selection of monoclonal antibodies for the identification of D variants: ability to detect weak D and to split epD2, epD5 and epD6/7

Red cells from known D variant donors were tested with 41 monoclonal anti-D reagents, 26 IgG and 15 IgM, with the view to selecting a panel to aid the identification of unusual D types. These antibodies gave reaction patterns which allowed the identification of most of the known D category cells, recognizing epD2, epD5, epD6/7, epD8 and epD9, but were unable to distinguish category III from normal D-positive cells. Reactivity with HMi, HMii, DFR, DBT and RoHar cells split epD2, epD5 and epD6/7 into two, three and eight groups, respectively. A panel comprising 15 monoclonal anti-D, 11 IgG and four IgM, was selected as representative of the antibodies tested. Reactivity of monoclonal anti-D was dependent on antibody concentration and antibody avidity. An antibody concentration of at least 12 micrograms/ml was required for optimum reactivity of the two monoclonal antibodies tested. A simple calculation of division of the titre by the antibody concentration provided a relatively simple means of establishing the reactivity performance of the antibody and correlated well with ability to detect weak D (Du) cells. A characteristic variable reduction in reaction strength with all the IgG anti-D was observed with weak D cells. The IgM antibodies, except the high avidity RUM-1, T3D2T6, D9A4 and BS226, performed poorly in detecting weak D. The majority of the IgM antibodies tested reacted with RoHarr cells, while only one IgG antibody was positive.

The Rh antigen D: partial D antigens and associated low incidence antigens

The expression of the Rh antigen D varies quantitatively and qualitatively (partial D); published information and 15 years' work studying D variants are discussed in this review. D epitopes correspond to the reaction patterns of monoclonal anti-D with partial D antigens. Partial D antigens can be reported in terms of their D epitopes but the epitope profile of cells with a quantitative variant of D (weak D) is difficult to determine reliably by haemagglutination tests. Nine partial D antigens, categories II-VII, DFR and two not previously reported, are identified by their epitope profiles and by association with low incidence antigens. Monoclonal anti-D recognize 16 D epitopes and more epitopes are anticipated. The specificities of polyclonal anti-D made by people with partial D antigens are considered in terms of possible D epitope specificities: recognized epitope specificities, or combination thereof, were not able to account for all observed reaction patterns of anti-D made by immunized individuals with partial D phenotypes. An attempt is made to understand partial D antigens and their associated low incidence antigens in terms of the molecular genetic information available.

Tentative model for the mapping of D epitopes on the RhD polypeptide

The partial D phenotypes correspond to D-positive individuals that may develop anti-D antibodies following immunization by transfusion or pregnancy, since they lack some of the D epitopes that compose the D antigen. When these red cells are tested with a panel of human monoclonal anti-D, different patterns of reactivity are observed and at least nine distinct epitopes termed epD1 to epD9 can be identified. Molecular analysis of partial D variants have shown that the loss of some D epitopes is associated either with intergenic recombination events between the D and CE genes generating hybrid gene structures D-CE-D or CE-D-CE, or with point mutations of the D gene. Based on these findings, a tentative model that correlates critical amino acid positions and D epitope expression on the D protein was proposed. Although recent studies suggest that the D antigen may be composed of as many as 30 epitopes, the relatively simple model presented here may be useful to serologists as a preliminary approach to understanding the basis of D antigenic variation in terms of structure-activity relationship.

A structural model for 30 Rh D epitopes based on serological and DNA sequence data from partial D phenotypes

Both cDNA RHD sequences and reactivity with monoclonal anti-D have been reported in a number of partial D phenotypes, where parts (some epitopes) of the normal D antigen are missing, and anti-D of restricted specificity may be made in response to challenge with normal D positive blood. This paper analyses these reports together and proposes a model for the structure which comprise the epitopes of the Rh D antigen. Some epitopes are proposed to be comprised of continuous peptide sequence within one extracellular loop, whereas others require interactions between two or the extracellular peptide loops.

Rh serology--coordinator's report

168 Rh specific monoclonal antibodies (Mabs) were evaluated in 43 laboratories. Red cells from 63 Rh variant phenotype donations were circulated in panels to evaluators. Circulation was organised to ensure adequate duplicate testing. Evaluators were asked to test each Mab at 3 dilutions in two specified techniques against normal phenotype cells, panel cells received and any other Rh variant cells they had available. Results were analysed by expressing the sum of reaction grades for each Mab with each variant cell as a percentage of the sum of reaction grades of that Mab with normal phenotype cells. Reactions with enzyme-treated cells were found to be highly variable between laboratories, and further analysis of these results was not attempted. Anti-D Mabs were sorted into groups which had the same pattern of reactions with different phenotype cells. 24 such patterns were identified, 17 corresponding to previously reported patterns and 7 new ones.

Monoclonal anti-D specificity and Rh D structure: criteria for selection of monoclonal anti-D reagents for routine typing of patients and donors

The Rh blood group system is the next most important to the ABO system in terms of its clinical significance in blood transfusion. It is vital to the safe, efficient practice of transfusion medicine that Rh D phenotyping tests are selected, executed and interpreted correctly. However, the Rh D blood group antigen has been shown to be subject to many phenotypic variations, and different reagents and typing techniques vary in their ability to detect these variants. The range of D-positive phenotypes are reviewed in terms of their reactivity with monoclonal antibody reagents and their clinical significance. In view of the available evidence, it is suggested that patient typing can be safely achieved by the duplicate use of one high-avidity or two very similar IgM monoclonal anti-D reagents that detect most variants except category DVI in simple tube or microplate saline tests. Antiglobulin testing for weak D should not be carried out on patient samples. Donor typing can be safely achieved by the use of the same monoclonal, used in parallel with a polyclonal anti-D reagent that detects DVI on sensitive automated equipment.

Defining the Rh blood group antigens. Biochemistry and molecular genetics

The Rh blood group antigens (D, Cc and Ee series) are carried by a family of non glycosylated hydrophobic transmembrane proteins of 30-32 kDa which are missing from the red cells of rare Rhnull individuals that express several membrane defects. The structure of these proteins has been deduced from cDNA cloning and studies have shown that the Rh proteins are erythroid specific and share no sequence homology with any known protein. The RhD and non-D proteins exhibit 92% sequence identity and their predicted membrane topology is similar as most of the molecules appear to reside between the leaflets of the phospholipid bilayer with only short hydrophilic loops connecting the twelve putative transmembrane helices. The RHD and RHCE genes encoding the Rh proteins (D and Cc/Ee, respectively) are organized in tandem on chromosome 1p34-p36 and most likely derived by duplication of a common ancestral gene. This concept is supported by the identification of RH-like genes in non human primates. The human RH locus is best described as a two-gene model in which all RhD-positive and most RhD-negative haplotypes are composed of two (RHD and RHCE) or only one (RHCE) structural genes, respectively. The RHD gene encodes the D protein and the RHCE gene encodes the C/c and E/e proteins presumably by alternative splicing of a pre messenger RNA. The correlation between the blood group D epitopes and the amino acid polymorphism of the Rh proteins is not yet established, but amino acid polymorphisms at positions 103 and 226 determine the molecular basis for the C/c (Ser-->Pro) and E/e (Pro-->Ala) specificities, respectively. Most variants analyzed so far are caused by gene conversion which appears as the principal mechanism responsible for polymorphism and gene diversity in the RH system. However, gene deletions have also been found in some occasions. To date, all Rhnull phenotypes investigated most likely result from transcriptional regulatory mechanisms that are not yet understood. Rhnull individuals suffer a clinical syndrome of varying severity and their red cells are characterized by morphological and functional abnormalities of cation transport and phospholipid asymmetry. In addition, several membrane components including the Rh proteins and other glycoproteins recently characterized (Rh50 glycoprotein, CD47, glycophorin B, Duffy, LW) are absent or severely decreased on these cells. These findings suggest that the Rh proteins are assembled into a multimeric complex with these glycoproteins and further studies should clarify the role in biosynthesis and the potential function of each component in this complex.