Candidate gene acting as a suppressor of the RH locus in most cases of Rh-deficiency

Biological ammonium transporters: evolution and diversification

Although ammonium is the preferred nitrogen source for microbes and plants, in animal cells it is a toxic product of nitrogen metabolism that needs to be excreted. Thus, ammonium movement across biological membranes, whether for uptake or excretion, is a fundamental and ubiquitous biological process catalysed by the superfamily of the Amt/Mep/Rh transporters. A remarkable feature of the Amt/Mep/Rh family is that they are ubiquitous and, despite sharing low amino acid sequence identity, are highly structurally conserved. Despite sharing a common structure, these proteins have become involved in a diverse range of physiological process spanning all domains of life, with reports describing their involvement in diverse biological processes being published regularly. In this context, we exhaustively present their range of biological roles across the domains of life and after explore current hypotheses concerning their evolution to help to understand how and why the conserved structure fulfils diverse physiological functions.

A novel nonsense variant in RHAG underlies a Nordic Rhnull phenotype

BACKGROUND AND OBJECTIVES

The extremely rare Rhnull phenotype is characterized by the absence of all Rh antigens on erythrocytes. It is divided into the regulator and amorph types based on the underlying genetic background. The more common regulator type depends on critical variants silencing RHAG, which encodes RhAG glycoprotein, necessary for RhD/RhCE expression. Rhnull cells have altered expression of glycophorin B and LW glycoprotein.

MATERIALS AND METHODS

Four unrelated Rhnull individuals were investigated. Serological testing was performed according to standard blood bank practice. RHD/RHCE and S/s allele-specific Polymerase chain reaction (PCR) genotyping was done on genomic DNA using in-house PCR assays. RHAG, and in some cases also RHD/RHCE, were sequenced. Initial s phenotyping results triggered additional serological investigation.

RESULTS

Anti-Rh29 was identified in all four individuals. Extended typing with anti-S and anti-s showed that the three samples predicted to type as s+ failed to react with 2 of 5 anti-s. Sequence analysis of all 10 RHAG exons and the immediate intron/exon boundaries revealed a single nucleotide variant in the 3'-end of intron 6, c.946 -2a>g in all samples. RHD/RHCE showed no alterations.

CONCLUSION

A novel Nordic Rhnull allele was identified. In addition, it was shown that s+ Rhnull red blood cells are not only U- but also have qualitative changes in their s antigen expression.

Lack of the human choline transporter-like protein SLC44A2 causes hearing impairment and a rare red blood phenotype

Blood phenotypes are defined by the presence or absence of specific blood group antigens at the red blood cell (RBC) surface, due to genetic polymorphisms among individuals. The recent development of genomic and proteomic approaches enabled the characterization of several enigmatic antigens. The choline transporter-like protein CTL2 encoded by the SLC44A2 gene plays an important role in platelet aggregation and neutrophil activation. By investigating alloantibodies to a high-prevalence antigen of unknown specificity, found in patients with a rare blood type, we showed that SLC44A2 is also expressed in RBCs and carries a new blood group system. Furthermore, we identified three siblings homozygous for a large deletion in SLC44A2, resulting in complete SLC44A2 deficiency. Interestingly, the first-ever reported SLC44A2-deficient individuals suffer from progressive hearing impairment, recurrent arterial aneurysms, and epilepsy. Furthermore, SLC44A2_null individuals showed no significant platelet aggregation changes and do not suffer from any apparent hematological disorders. Overall, our findings confirm the function of SLC44A2 in hearing preservation and provide new insights into the possible role of this protein in maintaining cerebrovascular homeostasis.

The use of pluripotent stem cells to generate diagnostic tools for transfusion medicine

Red blood cell (RBC) transfusion is one of the most common medical treatments, with more than 10 million units transfused per year in the United States alone. Alloimmunization to foreign Rh proteins (RhD and RhCE) on donor RBCs remains a challenge for transfusion effectiveness and safety. Alloantibody production disproportionately affects patients with sickle cell disease who frequently receive blood transfusions and exhibit high genetic diversity in the Rh blood group system. With hundreds of RH variants now known, precise identification of Rh antibody targets is hampered by the lack of appropriate reagent RBCs with uncommon Rh antigen phenotypes. Using a combination of human-induced pluripotent stem cell (iPSC) reprogramming and gene editing, we designed a renewable source of cells with unique Rh profiles to facilitate the identification of complex Rh antibodies. We engineered a very rare Rh null iPSC line lacking both RHD and RHCE. By targeting the AAVS1 safe harbor locus in this Rh null background, any combination of RHD or RHCE complementary DNAs could be reintroduced to generate RBCs that express specific Rh antigens such as RhD alone (designated D--), Goa+, or DAK+. The RBCs derived from these iPSCs (iRBCs) are compatible with standard laboratory assays used worldwide and can determine the precise specificity of Rh antibodies in patient plasma. Rh-engineered iRBCs can provide a readily accessible diagnostic tool and guide future efforts to produce an alternative source of rare RBCs for alloimmunized patients.

Missense RHD single nucleotide variants induce weakened D antigen expression by altering splicing and/or protein expression

BACKGROUND

Although D variant phenotype is known to be due to genetic defects, including rare missense single nucleotide variants (SNVs), within the RHD gene, few studies have addressed the molecular and cellular mechanisms driving this altered expression. We and others showed previously that splicing is commonly disrupted by SNVs in constitutive splice sites and their vicinity. We thus sought to investigate whether rare missense SNVs located in "deep" exonic regions could also impair this mechanism.

STUDY DESIGN AND METHODS

Forty-six missense SNVs reported within exons 6 and 7 were first selected from the Human RhesusBase. Their respective effect on splicing was assessed by using an in vitro assay. An RhD-negative cell model was further generated by using the CRISPR-Cas9 approach. RhD-mutated proteins were overexpressed in the newly created model, and cell membrane expression of the D antigen was measured by flow cytometry.

RESULTS

Minigene splicing assay showed that 14 of 46 (30.4%) missense SNVs alter splicing. Very interestingly, further investigation of two missense SNVs, which both affect codon 338 and confer a weak D phenotype, showed various mechanisms: c.1012C>G (p.Leu338Val) disrupts splicing only, while c.1013T>C (p.Leu338Pro) alters only the protein structure, in agreement with in silico prediction tools and 3D protein structure visualization.

CONCLUSION

Our functional data set suggests that missense SNVs damage quantitatively D antigen expression by, at least, two different mechanisms (splicing alteration and protein destabilization) that may act independently. These data thereby contribute to extend the current knowledge of the molecular mechanisms governing weakened D expression.

Insights into anti-D formation in carriers of RhD variants through studies of 3D intraprotein interactions

BACKGROUND

Many RhD variants associated with anti-D formation (partial D) in carriers exposed to the conventional D antigen carry mutations affecting extracellular loop residues. Surprisingly, some carry mutations affecting transmembrane or intracellular domains, positions not thought likely to have a major impact on D epitopes.

STUDY DESIGN AND METHODS

A wild-type Rh trimer (RhD₁ RhAG₂ ) was modeled by comparative modeling with the human RhCG structure. Taking trimer conformation, residue accessibility, and position relative to the lipid bilayer into account, we redefine the domains of the RhD protein. We generated models for RhD variants carrying one or two amino acid substitutions associated with anti-D formation in published articles (25 variants) or abstracts (12 variants) and for RHD*weak D type 38. We determined the extracellular substitutions and compared the interactions of the variants with those of the standard RhD.

RESULTS

The findings of the three-dimensional (3D) analysis were correlated with anti-D formation for 76% of RhD variants: 15 substitutions associated with anti-D formation concerned extracellular residues, and structural differences in intraprotein interactions relative to standard RhD were observed in the others. We discuss the mechanisms by which D epitopes may be modified in variants in which the extracellular residues are identical to those of standard RhD and provide arguments for the benignity of p.T379M (RHD*DAU0) and p.G278D (RHD*weak D type 38) in transfusion medicine.

CONCLUSION

The study of RhD intraprotein interactions and the precise redefinition of residue accessibility provide insight into the mechanisms through which RhD point mutations may lead to anti-D formation in carriers.

Drosophila ammonium transporter Rh50 is required for integrity of larval muscles and neuromuscular system

Rhesus glycoproteins (Rh50) have been shown to be ammonia transporters in many species from bacteria to human. They are involved in various physiological processes including acid excretion and pH regulation. Rh50 proteins can also provide a structural link between the cytoskeleton and the plasma membranes that maintain cellular integrity. Although ammonia plays essential roles in the nervous system, in particular at glutamatergic synapses, a potential role for Rh50 proteins at synapses has not yet been investigated. To better understand the function of these proteins in vivo, we studied the unique Rh50 gene of Drosophila melanogaster, which encodes two isoforms, Rh50A and Rh50BC. We found that Drosophila Rh50A is expressed in larval muscles and enriched in the postsynaptic regions of the glutamatergic neuromuscular junctions. Rh50 inactivation by RNA interference selectively in muscle cells caused muscular atrophy in larval stages and pupal lethality. Interestingly, Rh50-deficiency in muscles specifically increased glutamate receptor subunit IIA (GluRIIA) level and the frequency of spontaneous excitatory postsynaptic potentials. Our work therefore highlights a new role for Rh50 proteins in the maintenance of Drosophila muscle architecture and synaptic physiology, which could be conserved in other species.

Enhancement of red blood cell transfusion compatibility using CRISPR-mediated erythroblast gene editing

Regular blood transfusion is the cornerstone of care for patients with red blood cell (RBC) disorders such as thalassaemia or sickle‐cell disease. With repeated transfusion, alloimmunisation often occurs due to incompatibility at the level of minor blood group antigens. We use CRISPR‐mediated genome editing of an immortalised human erythroblast cell line (BEL‐A) to generate multiple enucleation competent cell lines deficient in individual blood groups. Edits are combined to generate a single cell line deficient in multiple antigens responsible for the most common transfusion incompatibilities: ABO (Bombay phenotype), Rh (Rh_null), Kell (K₀), Duffy (Fy_null), GPB (S−s−U−). These cells can be differentiated to generate deformable reticulocytes, illustrating the capacity for coexistence of multiple rare blood group antigen null phenotypes. This study provides the first proof‐of‐principle demonstration of combinatorial CRISPR‐mediated blood group gene editing to generate customisable or multi‐compatible RBCs for diagnostic reagents or recipients with complicated matching requirements.

The Shape Shifting Story of Reticulocyte Maturation

The final steps of erythropoiesis involve unique cellular processes including enucleation and reorganization of membrane proteins and the cytoskeleton to produce biconcave erythrocytes. Surprisingly this process is still poorly understood. In vitro erythropoiesis protocols currently produce reticulocytes rather than biconcave erythrocytes. In addition, immortalized lines and iPSC-derived erythroid cell suffer from low enucleation and suboptimal final maturation potential. In light of the increasing prospect to use in vitro produced erythrocytes as (personalized) transfusion products or as therapeutic delivery agents, the mechanisms driving this last step of erythropoiesis are in dire need of resolving. Here we review the elusive last steps of reticulocyte maturation with an emphasis on protein sorting during the defining steps of reticulocyte formation during enucleation and maturation.

Whole-exome sequencing for RH genotyping and alloimmunization risk in children with sickle cell anemia

RH genes are highly polymorphic and encode the most complex of the 35 human blood group systems. This genetic diversity contributes to Rh alloimmunization in patients with sickle cell anemia (SCA) and is not avoided by serologic Rh-matched red cell transfusions. Standard serologic testing does not distinguish variant Rh antigens. Single nucleotide polymorphism (SNP)-based DNA arrays detect many RHD and RHCE variants, but the number of alleles tested is limited. We explored a next-generation sequencing (NGS) approach using whole-exome sequencing (WES) in 27 Rh alloimmunized and 27 matched non-alloimmunized patients with SCA who received chronic red cell transfusions and were enrolled in a multicenter study. We demonstrate that WES provides a comprehensive RH genotype, identifies SNPs not interrogated by DNA array, and accurately determines RHD zygosity. Among this multicenter cohort, we demonstrate an association between an altered RH genotype and Rh alloimmunization: 52% of Rh immunized vs 19% of non-immunized patients expressed variant Rh without co-expression of the conventional protein. Our findings suggest that RH allele variation in patients with SCA is clinically relevant, and NGS technology can offer a comprehensive alternative to targeted SNP-based testing. This is particularly relevant as NGS data becomes more widely available and could provide the means for reducing Rh alloimmunization in children with SCA.

Evolution of the RH gene family in vertebrates revealed by brown hagfish (Eptatretus atami) genome sequences

In vertebrates, there are four major genes in the RH (Rhesus) gene family, RH, RHAG, RHBG, and RHCG. These genes are thought to have been formed by the two rounds of whole-genome duplication (2R-WGD) in the common ancestor of all vertebrates. In our previous work, where we analyzed details of the gene duplications process of this gene family, three nucleotide sequences belonging to this family were identified in Far Eastern brook lamprey (Lethenteron reissneri), and the phylogenetic positions of the genes were determined. Lampreys, along with hagfishes, are cyclostomata (jawless fishes), which is a sister group of gnathostomata (jawed vertebrates). Although those results suggested that one gene was orthologous to the gnathostome RHCG genes, we did not identify clear orthologues for other genes. In this study, therefore, we identified three novel cDNA sequences that belong to the RH gene family using de novo transcriptome analysis of another cyclostome: the brown hagfish (Eptatretus atami). We also determined the nucleotide sequences for the RHBG and RHCG genes in a red stingray (Dasyatis akajei), which belongs to the cartilaginous fishes. The phylogenetic tree showed that two brown hagfish genes, which were probably duplicated in the cyclostome lineage, formed a cluster with the gnathostome RHAG genes, whereas another brown hagfish gene formed a cluster with the gnathostome RHCG genes. We estimated that the RH genes had a higher evolutionary rate than the RHAG, RHBG, and RHCG genes. Interestingly, in the RHBG genes, only the bird lineage showed a higher rate of nonsynonymous substitutions. It is likely that this higher rate was caused by a state of relaxed functional constraints rather than positive selection nor by pseudogenization.

Clinical and laboratory update on the DEL variant

Serological assays for the RhD blood group are based on detection of the RhD antigen on human red blood cells using a specific anti-D antibody. The weak expression of the RhD antigen in the DEL variant hinders the sensitivity of conventional serological assays. Evidence of anti-D immunization in patients with D-negativity who have received DEL-variant blood units has been reported in various populations. This observation has prompted the need for genetic epidemiological and clinical data on the DEL variant in the development of DEL molecular diagnostic testing. This review highlights the molecular features of the DEL variant, the clinical consequences of DEL-blood transfusion, and current approaches for detection of the DEL-variant for donor screening and transfusion.

The human Kell blood group binds the erythroid 4.1R protein: new insights into the 4.1R-dependent red cell membrane complex

Protein 4.1R plays an important role in maintaining the mechanical properties of the erythrocyte membrane. We analysed the expression of Kell blood group protein in erythrocytes from a patient with hereditary elliptocytosis associated with complete 4.1R deficiency (4.1(-) HE). Flow cytometry and Western blot analyses revealed a severe reduction of Kell. In vitro pull down and co-immunoprecipitation experiments from erythrocyte membranes showed a direct interaction between Kell and 4.1R. Using different recombinant domains of 4.1R and the cytoplasmic domain of Kell, we demonstrated that the R(46) R motif in the juxta-membrane region of Kell binds to lobe B of the 4.1R FERM domain. We also observed that 4.1R deficiency is associated with a reduction of XK and DARC (also termed ACKR1) proteins, the absence of the glycosylated form of the urea transporter B and a slight decrease of band 3. The functional alteration of the 4.1(-) HE erythrocyte membranes was also determined by measuring various transport activities. We documented a slower rate of HCO3 (-) /Cl(-) exchange, but normal water and ammonia transport across erythrocyte membrane in the absence of 4.1. These findings provide novel insights into the structural organization of blood group antigen proteins into the 4.1R complex of the human red cell membrane.

Characteristics of mammalian Rh glycoproteins (SLC42 transporters) and their role in acid-base transport

The mammalian Rh glycoproteins belong to the solute transporter family SLC42 and include RhAG, present in red blood cells, and two non-erythroid members RhBG and RhCG that are expressed in various tissues, including kidney, liver, skin and the GI tract. The Rh proteins in the red blood cell form an "Rh complex" made up of one D-subunit, one CE-subunit and two RhAG subunits. The Rh complex has a well-known antigenic effect but also contributes to the stability of the red cell membrane. RhBG and RhCG are related to the NH4(+) transporters of the yeast and bacteria but their exact function is yet to be determined. This review describes the expression and molecular properties of these membrane proteins and their potential role as NH3/NH4(+) and CO2 transporters. The likelihood that these proteins transport gases such as CO2 or NH3 is novel and significant. The review also describes the physiological importance of these proteins and their relevance to human disease.

Refined views of multi-protein complexes in the erythrocyte membrane

The erythrocyte membrane has been extensively studied, both as a model membrane system and to investigate its role in gas exchange and transport. Much is now known about the protein components of the membrane, how they are organised into large multi-protein complexes and how they interact with each other within these complexes. Many links between the membrane and the cytoskeleton have also been delineated and have been demonstrated to be crucial for maintaining the deformability and integrity of the erythrocyte. In this study we have refined previous, highly speculative molecular models of these complexes by including the available data pertaining to known protein-protein interactions. While the refined models remain highly speculative, they provide an evolving framework for visualisation of these important cellular structures at the atomic level.

Human RhAG ammonia channel is impaired by the Phe65Ser mutation in overhydrated stomatocytic red cells

In red cells, Rh-associated glycoprotein (RhAG) acts as an ammonia channel, as demonstrated by stopped-flow analysis of ghost intracellular pH (pH(i)) changes. Recently, overhydrated hereditary stomatocytosis (OHSt), a rare dominantly inherited hemolytic anemia, was found to be associated with a mutation (Phe65Ser or Ile61Arg) in RHAG. Ghosts from the erythrocytes of four of the OHSt patients with a Phe65Ser mutation were resealed with a pH-sensitive probe and submitted to ammonium gradients. Alkalinization rate constants, reflecting NH(3) transport through the channel and NH(3) diffusion unmediated by RhAG, were deduced from time courses of fluorescence changes. After subtraction of the constant value found for Rh(null) lacking RhAG, we observed that alkalinization rate constant values decreased ∼50% in OHSt compared with those of controls. Similar RhAG expression levels were found in control and OHSt. Since half of the expressed RhAG in OHSt most probably corresponds to the mutated form of RhAG, as expected from the OHSt heterozygous status, this dramatic decrease can be therefore related to the loss of function of the Phe65Ser-mutated RhAG monomer.

Critical band 3 multiprotein complex interactions establish early during human erythropoiesis

Band 3, the major anion transport protein of human erythrocytes, forms the core of a multiprotein complex in the erythrocyte membrane. Here we studied the spatiotemporal mechanisms of band 3 multiprotein complex assembly during erythropoiesis. Significant pools of intracellular band 3 and Rh-associated glycoprotein (RhAG) were found in the basophilic erythroblast. These intracellular pools decreased in the polychromatic erythroblast, whereas surface expression increased and were lowest in the orthochromatic erythroblast and reticulocytes. Protease treatment of intact cells to remove extracellular epitopes recognized by antibodies to band 3 and RhAG was used to study surface delivery kinetics and intracellular complex composition from the proerythroblast stage to the enucleated reticulocyte. Newly synthesized band 3 and protein 4.2 interact initially in the early stages of the secretory pathway and are found associated at the plasma membrane from the basophilic stage of erythropoiesis. Although we could successfully coimmunoprecipitate Rh with RhAG from plasma membrane pools at a similar stage, no intracellular interaction between these proteins was detectable. Knockdown of RhAG during early erythropoiesis was accompanied by a concomitant drop in membrane expression of Rh polypeptides. These data are consistent with assembly of major components of the band 3 macrocomplex at an early stage during erythropoiesis.

The Rh protein family: gene evolution, membrane biology, and disease association

The Rh (Rhesus) genes encode a family of conserved proteins that share a structural fold of 12 transmembrane helices with members of the major facilitator superfamily. Interest in this family has arisen from the discovery of Rh factor's involvement in hemolytic disease in the fetus and newborn, and of its homologs widely expressed in epithelial tissues. The Rh factor and Rh-associated glycoprotein (RhAG), with epithelial cousins RhBG and RhCG, form four subgroups conferring upon vertebrates a genealogical commonality. The past decade has heralded significant advances in understanding the phylogenetics, allelic diversity, crystal structure, and biological function of Rh proteins. This review describes recent progress on this family and the molecular insights gleaned from its gene evolution, membrane biology, and disease association. The focus is on its long evolutionary history and surprising structural conservation from prokaryotes to humans, pointing to the importance of its functional role, related to but distinct from ammonium transport proteins.

A new blood group system, RHAG: three antigens resulting from amino acid substitutions in the Rh-associated glycoprotein

BACKGROUND AND OBJECTIVES

Rh-associated glycoprotein (RhAG) is closely associated with the Rh proteins in the red cell membrane. Two high frequency antigens (Duclos and DSLK) and one low frequency antigen (Ol(a)) have serological characteristics suggestive of expression on RhAG.

MATERIALS AND METHODS

RHAG was sequenced from the DNA of one Duclos-negative, one DSLK-negative, and two Ol(a+) individuals. Recombinant protein was expressed in HEK 293 cells. Protein models with RhAG subunits were constructed.

RESULTS

The original Duclos-negative patient was homozygous for RHAG 316C>G, encoding Gln106Glu. HEK 293 cells expressing Gln106Glu mutant RhAG did not react with anti-Duclos. An individual with DSLK-negative red cells was homozygous for 490A>C, encoding Lys164Gln. Two Ol(a+) members of the original Norwegian family were heterozygous for 680C>T, encoding Ser227Leu. A Japanese donor with Rh(mod) phenotype had Ol(a+) red cells and was homozygous for 680C>T.

CONCLUSION

The three red cell antigens encoded by RHAG form the RHAG blood group system: Duclos is RHAG1 (030001); Ol(a) is RHAG2 (030002); and DSLK is provisionally RHAG3 (030003).

The molecular genetics of blood group polymorphism

Over 300 blood group specificities on red cells have been identified, many of which are polymorphic. The molecular mechanisms responsible for these polymorphisms are diverse, though many simply represent single nucleotide polymorphisms (SNPs). Other mechanisms include the following: gene deletion; single nucleotide deletion and sequence duplication, which introduce reading-frame shifts; nonsense mutation; intergenic recombination between closely linked genes, giving rise to hybrid genes and hybrid proteins; and a SNP in the promoter region of a blood group gene. Examples of these various genetic mechanisms are taken from the ABO, Rh, Kell, and Duffy blood group systems. Null phenotypes, in which no antigens of a blood group system are expressed, are not generally polymorphic, but provide good examples of the effect of inactivating mutations on blood group expression. As natural human 'knock-outs', null phenotypes provide useful clues to the functions of blood group antigens. Knowledge of the molecular backgrounds of blood group polymorphisms provides a means to predict blood group phenotypes from genomic DNA. This has two main applications in transfusion medicine: determination of foetal blood groups to assess whether the foetus is at risk from haemolytic disease and ascertainment of blood group phenotypes in multiply transfused, transfusion-dependent patients, where serological tests are precluded by the presence of donor red cells. Other applications are being developed for the future.

Nitrogen excretion in developing zebrafish (Danio rerio): a role for Rh proteins and urea transporters

Injection of antisense oligonucleotide morpholinos to elicit selective gene knockdown of ammonia (Rhag, Rhbg, and Rhcg1) or urea transporters (UT) was used as a tool to assess the relative importance of each transporter to nitrogen excretion in developing zebrafish (Danio rerio). Knockdown of UT caused urea excretion to decrease by approximately 90%, whereas each of the Rh protein knockdowns resulted in an approximately 50% reduction in ammonia excretion. Contrary to what has been hypothesized previously for adult fish, each of the Rh proteins appeared to have a similar effect on total ammonia excretion, and thus all are required to facilitate normal ammonia excretion in the zebrafish larva. As demonstrated in other teleosts, zebrafish embryos utilized urea to a much greater extent than adults and were effectively ureotelic until hatching. At that point, ammonia excretion rapidly increased and appeared to be triggered by a large increase in the mRNA expression of Rhag, Rhbg, and Rhcg1. Unlike the situation in the adult pufferfish (35), the various transporters are not specifically localized to the gills of the developing zebrafish, but each protein has a unique expression pattern along the skin, gills, and yolk sac. This disparate pattern of expression would appear to preclude interaction between the Rh proteins in zebrafish embryos. However, this may be a developmental feature of the delayed maturation of the gills, because as the embryos matured, expression of the transporters in and around the gills increased.

Functions of red cell surface proteins

The external membrane of the red cell contains numerous proteins that either cross the lipid bilayer one or more times or are anchored to it through a lipid tail. Many of these proteins express blood group activity. The functions of some of these proteins are known; in others their function can only be surmised from the protein structure or from limited experimental evidence. They are loosely divided into four categories based on their functions: membrane transporters; adhesion molecules and receptors; enzymes; and structural proteins that link the membrane with the membrane skeleton. Some of the proteins carry out more than one of these functions. Some proteins may complete their major functions during erythropoiesis or may only be important under adverse physiological conditions. Furthermore, some might be evolutionary relics and may no longer have significant functions. Polymorphisms or rare changes in red cell surface proteins are often responsible for blood groups. The biological significance of these polymorphisms or the selective pressures responsible for their stability within populations are mostly not known, although exploitation of the proteins by pathogenic micro-organisms has probably played a major role.

Evolution and functional characterization of the RH50 gene from the ammonia-oxidizing bacterium Nitrosomonas europaea

The family of ammonia and ammonium channel proteins comprises the Amt proteins, which are present in all three domains of life with the notable exception of vertebrates, and the homologous Rh proteins (Rh50 and Rh30) that have been described thus far only in eukaryotes. The existence of an RH50 gene in bacteria was first revealed by the genome sequencing of the ammonia-oxidizing bacterium Nitrosomonas europaea. Here we have used a phylogenetic approach to study the evolution of the N. europaea RH50 gene, and we show that this gene, probably as a component of an integron cassette, has been transferred to the N. europaea genome by horizontal gene transfer. In addition, by functionally characterizing the Rh50(Ne) protein and the corresponding knockout mutant, we determined that NeRh50 can mediate ammonium uptake. The RH50(Ne) gene may thus have replaced functionally the AMT gene, which is missing in the genome of N. europaea and may be regarded as a case of nonorthologous gene displacement.

The structure and function of the Rh antigen complex

The Rh system is one of the most important and complex blood group systems because of the large number of antigens and the serious complications for the fetus of a woman sensitized by transfusion or pregnancy. Major advances in our understanding of the Rh system have occurred with the cloning of the genes and with functional evidence that the Rh blood group proteins belong to an ancient family of membrane proteins involved in ammonia transport. The arrangement and configuration of the genes at the RH locus promotes genetic exchange, generating new antigens. Importantly, RH genetic testing can now be applied to clinical transfusion medicine and prenatal practice. This includes testing for RHD zygosity, confirmation or resolution of D antigen status, and detection of altered RHD and RHCE genes in individuals at risk for producing antibodies to high-incidence Rh antigens, particularly sickle cell disease (SCD) patients. The Rh proteins form a core complex that is critical to the structure of the erythrocyte membrane, and they may play a physiologic role in the sequestration of blood ammonia. The Rh family of proteins now includes non-erythroid homologs present in many other tissues, and comparative genomics reveal Rh homologs in all domains of life.

The challenge of understanding ammonium homeostasis and the role of the Rh glycoproteins

Rh glycoproteins belong to the superfamily of ammonium transporters, but until recent functional studies their functional role was unknown. This review focuses on the functional results obtained in our laboratory after the heterologous expression of RhAG (the erythroid Rh glycoprotein) and RhCG (an epithelial Rh glycoprotein). RhAG and RhCG were expressed in two different expression systems (HeLa cells and Xenopus laevis oocytes) that differed in their endogenous membrane permeabilities for NH3 and NH4+. To check if RhAG and RhCG are ammonium transporters, we measured intracellular pH changes in cells exposed to an ammonium-containing solution, and analyzed the ammonium-induced NH3 and NH4+ transmembrane fluxes in control versus transfected cells. We observed that RhAG and RhCG expression induced an enhancement of the ammonium-induced initial alkalinization (related to NH3 influx into the cell) and secondary acidification (related to NH4+ influx into the cell). Moreover, sub-millimolar ammonium concentrations induced inward currents in voltage-clamped RhAG- and in RhCG-expressing oocytes. Taken together, these results show not only that RhAG and RhCG are ammonium transporters, but also that they are promoting the transmembrane transport of NH3 and of NH4+. Data from our laboratory and from other groups raise several questions that are discussed.

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.

Modelling the human rhesus proteins: implications for structure and function

The mammalian rhesus (Rh) proteins that carry the Rh blood group antigens of red blood cells are related to the ammonium channel (Amt) proteins found in both pro- and eukaryotes. However, despite their clinical importance the structure of the Rh antigens is presently unknown. We have constructed homology models of the human Rh proteins, RhD and RhAG using the structure of the Escherichia coli ammonia channel AmtB as a template, together with secondary structure predictions and the extensive available biochemical data for the Rh proteins. These models suggest that RhAG and the homologous non-erythrocyte Rhesus glycoproteins, RhBG and RhCG, have a very similar channel architecture to AmtB. By comparison, RhD and RhCE have a different arrangement of residues, indicating that if they function as ammonia channels at all, they must do so by a different mechanism. The E. coli AmtB protein is a homotrimer and our models provoke a reassessment of the widely accepted tetrameric model of the organisation of the erythrocyte Rh complex. A critical analysis of previously published data, together with sequencing yield data, lead us to suggest that the erythrocyte Rh complex could indeed also be trimeric.

Molecular aspects of Rh antigens

Since the cloning of two Rh related cDNAs, several attempts to clarify the molecular basis of Rh system have been performed. The RHD gene is deleted in most Caucasians who lack the D antigen, whereas some Japanese RhD(-) individuals appear to have an intact RHD gene. The expression study in erythroleukemic cells revealed that c and E antigens were expressed on a single polypeptide encoded by a full length cE transcript and RhD transcript give rise to D and G antigens. Recent finishing of human genome euchromatin sequencing confirmed the genomic organization of RH locus in which RHD and CE face each other at their 3' tails, and the gene SMP1 is interspersed between them. Two 9 kb transposon-like DNA segments, called 'rhesus boxes' were identified in both upstream and downstream of the RHD gene. Murine genome sequence of rh locus indicate not only the ancient duplication of RH gene by the mechanism of retro-transposition before hominoids branched off but also the unequal crossing over at the 'rhesus boxes' resulting in an RHD deletion. The macrocomplex formed by Rh, RhAG and band3 is thought contributing for the physiological functions, anchoring lipid bilayer to spectrin-based framework and membrane transportation of CO2 and HCO3-.

A comprehensive analysis of DEL types: partial DEL individuals are prone to anti-D alloimmunization

BACKGROUND

The D antigen of the polymorphic Rh blood group system is of particular clinical importance regarding transfusion- and pregnancy-induced alloimmunization. Different RhD variants with specific clinical implications have been characterized. The least expressed D variants collectively called DEL are serologically detectable only by adsorption-elution techniques, with so far only poorly defined antigenic properties.

STUDY DESIGN AND METHODS

A comprehensive immunohematologic analysis of five of the six currently known DEL genotypes was performed. DEL phenotypes associated with the RHD(M295I), RHD(IVS3+1g>a), RHD(K409K), RHD(X418L), or RHD(IVS5-38del4) allele were characterized with extended serology and flow cytometry.

RESULTS

Epitope mapping with adsorption-elution revealed a prominent D epitope loss in the RHD(IVS3+1g>a)-associated DEL phenotype, whereas in the other four DEL types no signs of qualitative D antigen alteration were detected. The observation of alloanti-D in two RHD(IVS3+1g>a) cases confirmed the partial nature of this DEL phenotype. The RHD(M295I) phenotype exhibited the highest D antigen expression among all investigated DEL types, as determined by a semiquantitative adsorption-elution approach and flow cytometry.

CONCLUSION

In conclusion, evidence is provided that different DEL genotypes code either for partial or complete D antigen expression and that this finding is clinically relevant.

The application of a new quantitative assay for the monitoring of integrin-associated protein CD47 on red blood cells during storage and comparison with the expression of CD47 and phosphatidylserine with flow cytometry

BACKGROUND

After the introduction of universal leukoreduction, the role of factors other than white blood cells in red cell (RBC) storage lesion is attracting increasing attention. These include changes in the levels of CD47 and phosphatidylserine (PS) markers on RBCs during storage. The aim of this study was to monitor these changes with both flow cytometry (FACS) and a newly developed quantitative enzyme-linked immunosorbent assay (ELISA).

STUDY DESIGN AND METHODS

A new quantitative ELISA (monoclonal antibody immobilization of RBC antigens [MAIRA]) was developed. The assay yielded consistent linear curves that enabled the measurement of CD47 expression on RBCs. In addition, FACS was used to measure both CD47 expression and PS on RBCs (n = 3 units) during storage (Days 4, 10, 24, and 31).

RESULTS

A significant reduction in CD47 expression was observed both by MAIRA assay and by FACS by Days 24 and 31 (p < 0.01), and the correlation between the two assays was significant (p < 0.01). In addition, a significant increase in PS was observed by the same storage days with FACS (p < 0.01).

CONCLUSION

The MAIRA assay appears to be suitable for the quantitative measurement of RBC markers during storage. Significant changes in CD47 and PS levels were observed during storage, which may have detrimental immunomodulatory and hemostatic effects on the transfused RBCs.

The molecular genetics of blood group polymorphism

Nearly 300 blood group specificities on red cells are known, many of which are polymorphic. The molecular mechanisms responsible for these polymorphisms are diverse, though the majority represent single nucleotide polymorphisms (SNPs) encoding amino acid substitutions. Other mechanisms include the following: gene deletion; single nucleotide deletion and sequence duplication, which introduce reading-frame shifts; nonsense mutation; intergenic recombination between closely-linked genes, giving rise to hybrid genes and hybrid proteins; and a SNP in the promoter region of a blood group gene. Examples of these genetic mechanisms are taken from the ABO, Rh, Kell, and Duffy blood group systems. Null phenotypes, in which no antigens of a blood group system are expressed, are not generally polymorphic, but provide good examples of the effect of inactivating mutations on blood group expression. As natural human 'knock-outs' they provide useful clues to the functions of blood group antigens. Knowledge of the molecular bases to blood group polymorphisms provides a means to predict blood group phenotype from genomic DNA with a high degree of accuracy. This currently has two main applications in transfusion medicine: for determining fetal blood groups to assess whether the fetus is at risk from haemolytic disease; and to determine blood group phenotypes in multiply transfused, transfusion-dependent patients, where serological tests are precluded by the presence of donor red cells. Other applications are being developed for the future.

Expression of the human erythroid Rh glycoprotein (RhAG) enhances both NH3 and NH4+ transport in HeLa cells

The erythroid Rh-associated glycoprotein (RhAG) is strictly required for the expression of the Rh blood group antigens carried by Rh (D,CE) proteins. A biological function for RhAG in ammonium transport has been suggested by its ability to improve survival of an ammonium-uptake-deficient yeast. We investigated the function of RhAG by studying the entry of NH3/NH4+ in HeLa cells transiently expressing the green fluorescent protein (GFP)-RhAG fusion protein and using a fluorescent proton probe to measure intracellular pH (pHi). Under experimental conditions that reduce the intrinsic Na/H exchanger activity, exposure of control cells to a 10 mM NH4Cl- containing solution induces the classic pHi response profile of cells having a high permeability to NH3 (PNH3) but relatively low permeability to NH4+ (PNH4). In contrast, under the same conditions, the pHi profile of cells expressing RhAG clearly indicated an increased PNH4, as evidenced by secondary reacidification during NH4Cl exposure and a pHi undershoot below the initial resting value upon its removal. Measurements of pHi during methylammonium exposure showed that RhAG expression enhances the influx of both the unprotonated and ionic forms of methylammonium. Using a mathematical model to adjust passive permeabilities for a fit to the pHi profiles, we found that RhAG expression resulted in a threefold increase of PNH4 and a twofold increase of PNH3. Our results are the first evidence that the human erythroid RhAG increases the transport of both NH3 and NH4+.

Protein 4.2 is critical to CD47-membrane skeleton attachment in human red cells

The reduction in expression of the integral membrane protein CD47 in human red blood cells (RBCs) deficient in protein 4.2 suggests that protein 4.2 may mediate a linkage of CD47 to the membrane skeleton. We compared the fractions of membrane skeleton-attached CD47, Rh-associated glycoprotein (RhAG), Rh, and band 3 in normal and protein 4.2-deficient cells using fluorescence-imaged microdeformation. We found that CD47 attachment decreases from 55% in normal cells to 25% to 35% in 4.2-deficient cells. RhAG, which has been shown to have no significant variation in expression among the cells studied, shows a significant decrease in membrane skeleton attachment in 4.2-deficient cells from 60% to 40%. Both Rh and band 3, which have also been shown to have no change in expression, show a smaller decrease from 75% attached in normal RBCs to 55% attached in 4.2-deficient cells. In normal cells, Rh phenotype influences CD47 expression but not the level of membrane skeleton attachment of CD47. In contrast, the results indicate that protein 4.2 strongly influences CD47 levels as well as the extent of membrane skeleton attachment in the RBC, whereas protein 4.2 affects membrane skeletal attachment of RhAG, Rh, and band 3 to a lesser extent.

Red cell antigens as functional molecules and obstacles to transfusion

Blood group antigens (BGAs) can act as functional molecules but also can evoke autoantibodies and alloantibodies, causing autoimmune hemolytic anemia, hemolytic disease of the newborn and hemolytic transfusion reactions. In Section I, Dr. Marilyn Telen discusses physiologic and pathologic functions of RBC BGA-bearing molecules. She reviews some associations of BGAs with RBC membrane integrity and hemolytic anemia; association of BGAs with enzymatic and transport functions; and adhesion molecules expressed by RBCs, especially with reference to their pathophysiological role in sickle cell disease. In Section II, Dr. Lawrence Petz discusses the problems of providing blood for patients who have RBC autoantibodies. He provides an algorithm for excluding the presence of "hidden" alloantibodies, when all units appear to be incompatible due to the autoantibody. He emphasizes that clinicians should be aware of these approaches and not accept "the least incompatible unit." In Section III, Dr. George Garratty describes two processes, in development, that produce RBCs that result in RBCs that can be described as "universal" donor or "stealth" RBCs. The first process involves changing group A, B, or AB RBCs into group O RBCs by removing the immunospecific sugars responsible for A and B specificity by using specific enzymes. The second process involves covering all BGAs on the RBC surface using polyethylene glycol (PEG). Results of in vitro and in vivo studies on these modified RBCs are discussed.

Rh-RhAG/ankyrin-R, a new interaction site between the membrane bilayer and the red cell skeleton, is impaired by Rh(null)-associated mutation

Several studies suggest that the Rh complex represents a major interaction site between the membrane lipid bilayer and the red cell skeleton, but little is known about the molecular basis of this interaction. We report here that ankyrin-R is capable of interacting directly with the C-terminal cytoplasmic domain of Rh and RhAG polypeptides. We first show that the primary defect of ankyrin-R in normoblastosis (nb/nb) spherocytosis mutant mice is associated with a sharp reduction of RhAG and Rh polypeptides. Secondly, our flow cytometric analysis of the Triton X-100 extractability of recombinant fusion proteins expressed in erythroleukemic cell lines suggests that the C-terminal cytoplasmic domains of Rh and RhAG are sufficient to mediate interaction with the erythroid membrane skeleton. Using the yeast two-hybrid system, we demonstrate a direct interaction between the cytoplasmic tails of Rh and RhAG and the second repeat domain (D2) of ankyrin-R. This finding is supported by the demonstration that the substitution of Asp-399 in the cytoplasmic tail of RhAG, a mutation associated with the deficiency of the Rh complex in one Rhnull patient, totally impaired interaction with domain D2 of ankyrin-R. These results identify the Rh/RhAG-ankyrin complex as a new interaction site between the red cell membrane and the spectrin-based skeleton, the disruption of which might result in the stomato-spherocytosis typical of Rhnull red cells.

The Rh complex exports ammonium from human red blood cells

The Rh blood group system represents a major immunodominant protein complex on red blood cells (RBC). Recently, the Rh homologues RhAG and RhCG were shown to promote ammonium ion transport in yeast. In this study, we showed that also in RBC the human Rh complex functions as an exporter of ammonium ions. We measured ammonium import during the incubation of RBC in a solution containing a radiolabelled analogue of NH4Cl (14C-methyl-NH3Cl). Rhnull cells of the regulator type (expressing no Rh complex proteins) accumulated significantly higher levels (P = 0.05) of radiolabelled methyl-ammonium ions than normal RBC, at room temperature. Rhnull cells of the amorph type (expressing limited amounts of Rh complex proteins) accumulated an intermediate amount of methyl-ammonium ions. To show that decreased ammonium export contributes to its accumulation, the release of intracellular methyl-ammonium from the cells was measured over time. In 30 s, normal RBC released 87% of the intracellular methyl-ammonium ions, whereas Rhnull cells of the regulator type released only 46%. We conclude that the Rh complex is involved in the export of ammonium from RBC.

Fractional attachment of CD47 (IAP) to the erythrocyte cytoskeleton and visual colocalization with Rh protein complexes

Interactions of CD47 and RhAG and the Rh proteins are visualized between one another and with the cytoskeleton of intact erythrocytes. In a first study, CD47 is labeled with a phycoerythrin (PhE)- tagged antibody, which generates discrete spots that reflect induced clusters of CD47. Rh and RhAG colocalize with each other and to these induced clusters, whereas Band 3 and glycophorin C remain more homogeneously dispersed on the cell periphery. In a second study, red cells are aspirated into a micropipette, and immunofluorescent maps of the surface gradients that develop for CD47 and RhAG determine cytoskeletal connectivity. CD47 and RhAG gradients on normal red cells prove to be nearly identical and also appear intermediate to those found for the fluid bilayer and network-linked glycophorin C. Similar gradients are obtained for CD47 on Rh(null) cells, suggesting that linkage of CD47 to the spectrin-actin skeleton is independent of Rh or RhAG and is not affected by CD47's reduced surface expression on these cells. The results show that CD47 colocalizes with Rh and RhAG but is fractionally attached to the red cell membrane skeleton independent of these and other major integral membrane proteins involved in cytoskeletal attachment. The results imply a homogeneous base distribution of CD47, restrained by cytoskeleton linkages, plus a smaller fraction of CD47, which is able to diffuse in the membrane.

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.