Molecular biology and genetics of the Rh blood group system

A novel frameshift mutation in RHAG leads to Rhnull phenotype in a Chinese individual

BACKGROUND

We recently encountered a Rhnull phenotype proband within one family in the Chinese population. Rhnull is a rare autosomal recessive disorder characterized by the absence of the Rh antigens on the erythrocyte membrane, resulting in chronic hemolytic anemia. This study described the serological and molecular analysis of a Chinese Rhnull proband and his immediate family.

METHODS

Red blood cells antigen phenotyping and antibody screening/identification were conducted. RHD, RHCE, and RHAG were analyzed using genomic DNA by polymerase chain reaction and sequence analysis.

RESULTS

Serologic tests showed a D-C-E-c-e- phenotype in the proband associated with the suspicion of anti-Rh29 (titer 16). Molecular analyses showed a new mutation (c.406dupA) in exon 3 of RHAG. This duplication introduced a reading frameshift (p.Thr136AsnfsTer21). The RHAG mutation was found in the homozygous state for the proband and heterozygous state for his parents.

CONCLUSION

We identified a novel RHAG mutation resulting in the Rhnull phenotype of the regulator type. Inheritance of the novel allele was shown by family study.

BGvar: A comprehensive resource for blood group immunogenetics

BACKGROUND

Blood groups form the basis of effective and safe blood transfusion. There are about 43 well-recognised human blood group systems presently known. Blood groups are molecularly determined by the presence of specific antigens on the red blood cells and are genetically determined and inherited following Mendelian principles. The lack of a comprehensive, relevant, manually compiled and genome-ready dataset of red cell antigens limited the widespread application of genomic technologies to characterise and interpret the blood group complement of an individual from genomic datasets.

MATERIALS AND METHODS

A range of public datasets was used to systematically annotate the variation compendium for its functionality and allele frequencies across global populations. Details on phenotype or relevant clinical importance were collated from reported literature evidence.

RESULTS

We have compiled the Blood Group Associated Genomic Variant Resource (BGvar), a manually curated online resource comprising all known human blood group related allelic variants including a total of 1700 International Society of Blood Transfusion approved alleles and 1706 alleles predicted and curated from literature reports. This repository includes 1682 single nucleotide variations (SNVs), 310 Insertions, Deletions (InDels) and Duplications (Copy Number Variations) and about 1360 combination mutations corresponding to 43 human blood group systems and 2 transcription factors. This compendium also encompasses gene fusion and rearrangement events occurring in human blood group genes.

CONCLUSION

To the best of our knowledge, BGvar is a comprehensive and a user-friendly resource with most relevant collation of blood group alleles in humans. BGvar is accessible online at URL: http://clingen.igib.res.in/bgvar/.

Practical Solutions for Problems in Red Blood Cell Compatibility Testing

Hematologists often encounter transfusion problems, one of which is crossmatch incompatibility. In many countries, transfusion medicine is not a recognized specialty, there are no reference immunohematology laboratories, and most blood banks can only perform “type and screen” and crossmatch analyses. Therefore, hematologists should have basic knowledge about blood banking procedures and how to use them. This review aims to provide hematologists who do not have access to advanced blood bank laboratories some practical tips for handling problems in pretransfusion testing.

Molecular characterization of a rare Rh phenotype Dc-from the Indian subcontinent

Unusual Rh phenotypes such as Rh_null, D-- and Dc- etc. are rarely encountered in routine blood bank testing. The Rh_null phenotype is characterized by the absence of all Rh antigens, D-- phenotype does not express any RhCcEe antigens whereas Dc- phenotype individual lacks expression of antithetical E /e antigens. These individuals may produce multiple Rh antibodies against missing antigens. An old woman (B RhD positive) from Bangladesh with end-stage renal disease developed severe anaemia. Cross-matching with ABO and RhD compatible blood units showed +3 agglutination reaction. Detailed immunohaematological investigations showed a lack of C, E and e antigens, thus identifying the rare Rh variant as Dc-. Antibodies against C and e antigens were also detected in the patient's serum. PCR-SSP confirmed the absence of the molecular region defining the C, E and e antigens. Copy number analysis by QMPSF revealed the homozygous state of (RHCE-D(4-9)-CE) allele at the RHCE gene locus. This is the first report of the rare Dc- variant individual from the Indian subcontinent.

The Necessity of Clinical Rh Phenotypic Serological Detection and Homotypic Infusion in Patients with Repeated Blood Transfusion

Background

This study analyzed the distribution of Rh serological phenotype in people living in Hangzhou, China, and assessed the necessity of its routine clinical detection and homotypic infusion.

Material/Methods

Blood donors and patients who might need blood transfusion were enrolled into the study, and ABO and 5 major Rh serological antigens (C, c, D, E, and e) were routinely detected. The consistent ABO and Rh serological phenotype blood was transfused between the blood donors and recipients. Irregular antibodies were screened and identified in patients before the blood transfusion. Then, the transfusion adverse effects were monitored and compared with the previous data in the hospital.

Results

The phenotypic frequencies of Rh blood groups were D>C>E>c>e. The CCDee was the most common phenotype and CcdEe was the least common. The detection rate of unexpected antibodies gradually increased, while the unexpected antibodies slowly decreased in the Rh system. There was a correlation between the isotypic infusion of 5 Rh antigens and the detection rate of antibodies in the Rh system (R=0.845). The adverse effects of blood transfusion declined from 19.95% in 2011 with just homotypic ABO infusion to 3.098% in 2019 with the transfusion of homotypic ABO and the 5 major Rh serological antigens.

Conclusions

The consistency of the transfusion with ABO and 5 significant Rh serological antigens could prevent and decrease the high frequency production of isoantibodies, which is of vital importance in reducing the incidence rate of adverse effects in patients receiving transfusions.

Combinatory Evaluation of Transcriptome and Metabolome Profiles of Low Temperature-induced Resistant Ascites Syndrome in Broiler Chickens

To select metabolic biomarkers and differentially expressed genes (DEGs) associated with resistant-ascites syndrome (resistant-AS), we used innovative techniques such as metabolomics and transcriptomics to comparatively examine resistant-AS chickens and AS controls. Metabolomic evaluation of chicken serum using ultra-performance liquid chromatography-quadruple time-of-flight high-sensitivity mass spectrometry (UPLC-QTOF/HSMS) showed significantly altered lysoPC(18:1), PE(18:3/16:0), PC(20:1/18:3), DG(24:1/22:6/0:0), PS(18:2/18:0), PI(16:0/16:0), PS(18:0/18:1), PS(14:1/14:0), dihydroxyacetone, ursodeoxycholic acid, tryptophan, L-valine, cycloserine, hypoxanthine, and 4-O-Methylmelleolide concentrations on day 21 and LysoPC(18:0), LysoPE(20:1/0:0), LysoPC(16:0), LysoPE(16:0/0:0), hypoxanthine, dihydroxyacetone, 4-O-Methylmelleolide, LysoPC(18:2), and PC(14:1/22:1) concentrations on day 35, between the susceptible and resistant groups. Compared to the susceptible group, transcriptomic analysis of liver samples using RNA-seq revealed 413 DEGs on day 21 and 214 DEGs on day 35 in the resistant group. Additional evaluations using gene ontology (GO) indicate that significant enrichment occurred in the oxygen transportation, defensive reactions, and protein modifications of the decreased DEGs as well as in the cell morphological formation, neural development, and transforming growth factor (TGF)-beta signalling of the increased DEGs on day 21. Oxygen transportation was also significantly enriched for downregulated DEGs on day 35. The combinatory evaluation of the metabolome and the transcriptome suggests the possible involvement of glycerophospholipid metabolism in the development of resistant-AS in broilers.

Variability of cholesterol accessibility in human red blood cells measured using a bacterial cholesterol-binding toxin

Cholesterol partitions into accessible and sequestered pools in cell membranes. Here, we describe a new assay using fluorescently-tagged anthrolysin O, a cholesterol-binding bacterial toxin, to measure accessible cholesterol in human red blood cells (RBCs). Accessible cholesterol levels were stable within individuals, but varied >10 fold among individuals. Significant variation was observed among ethnic groups (Blacks>Hispanics>Whites). Variation in accessibility of RBC cholesterol was unrelated to the cholesterol content of RBCs or plasma, but was associated with the phospholipid composition of the RBC membranes and with plasma triglyceride levels. Pronase treatment of RBCs only modestly altered cholesterol accessibility. Individuals on hemodialysis, who have an unexplained increase in atherosclerotic risk, had significantly higher RBC cholesterol accessibility. Our data indicate that RBC accessible cholesterol is a stable phenotype with significant inter-individual variability. Factors both intrinsic and extrinsic to the RBC contribute to variation in its accessibility. This assay provides a new tool to assess cholesterol homeostasis among tissues in humans.

DOI:http://dx.doi.org/10.7554/eLife.23355.001

Flow Cytometry in Detection of Fetal Red Blood Cells and Maternal F Cells to Identify Fetomaternal Hemorrhage

Accurate detection and quantitation of fetomaternal hemorrhage (FMH) is critical to the obstetric management of rhesus D alloimmunization in Rh-negative pregnant women. The flow cytometry is based on the detection of fetal red blood cells using a monoclonal anti-HbF antibody, and is the method most indicated for this estimation. The objective of this study was to quantify fetal red blood cell levels of pregnant women using flow cytometry. We analyzed 101 peripheral blood samples from Rh-negative and Rh-positive women, whose mean age was 24 years (20-32 years), after vaginal delivery or cesarean section. Our study showed that 53% of pregnant women had fetal red blood cells levels <2.0 mL, 31% between 2.0-3.9 mL, 16% between 4.0-15.0 mL, and 1% >15.0 mL. Accurate quantitation of fetal red blood cells is necessary to determine the appropriate dose of anti-D (RHD) immunoglobulin to be administered to pregnant or postpartum women.

Does aberrant membrane transport contribute to poor outcome in adult acute myeloid leukemia?

Acute myeloid leukemia in adults is a highly heterogeneous disease. Gene expression profiling performed using unsupervised algorithms can be used to distinguish specific groups of patients within a large patient cohort. The identified gene expression signatures can offer insights into underlying physiological mechanisms of disease pathogenesis. Here, the analysis of several related gene expression clusters associated with poor outcome, worst overall survival and highest rates of resistant disease and obtained from the patients at the time of diagnosis or from previously untreated individuals is presented. Surprisingly, these gene clusters appear to be enriched for genes corresponding to proteins involved in transport across membranes (transporters, carriers and channels). Several ideas describing the possible relationship of membrane transport activity and leukemic cell biology, including the "Warburg effect," the specific role of chloride ion transport, direct "import" of metabolic energy through uptake of creatine phosphate, and modification of the bone marrow niche microenvironment are discussed.

BGMUT Database of Allelic Variants of Genes Encoding Human Blood Group Antigens

The Blood group antigen Gene MUTation (BGMUT) database documents variations in genes of human blood group systems. In March 2014, the database, accessible at www.ncbi.nlm.nih.gov/gv/mhc/xslcgi.cgi?cmd=bgmut, listed 1,545 alleles of 44 genes of 34 blood group systems. Besides allelic information, the BGMUT resource also presents comprehensive and current information on blood group systems. This review describes the database and notes its utility for the transfusion medicine and human genetics communities.

Determination of fetal rhesus d status by maternal plasma DNA analysis

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

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

Mice expressing RHAG and RHD human blood group genes

Anti-RhD prophylaxis of haemolytic disease of the fetus and newborn (HDFN) is highly effective, but as the suppressive mechanism remains uncertain, a mouse model would be of interest. Here we have generated transgenic mice expressing human RhAG and RhD erythrocyte membrane proteins in the presence and, for human RhAG, in the absence, of mouse Rhag. Human RhAG associates with mouse Rh but not mouse Rhag on red blood cells. In Rhag knockout mice transgenic for human RHAG, the mouse Rh protein is “rescued” (re-expressed), and co-immunoprecipitates with human RhAG, indicating the presence of hetero-complexes which associate mouse and human proteins. RhD antigen was expressed from a human RHD gene on a BAC or from RHD cDNA under control of β-globin regulatory elements. RhD was never observed alone, strongly indicative that its expression absolutely depends on the presence of transgenic human RhAG. This first expression of RhD in mice is an important step in the creation of a mouse model of RhD allo-immunisation and HDFN, in conjunction with the Rh-Rhag knockout mice we have developed previously.

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.

Patterns of human genetic variation inferred from comparative analysis of allelic mutations in blood group antigen genes

Comparative analysis of allelic variation of a gene sheds light on the pattern and process of its diversification at the population level. Gene families for which a large number of allelic forms have been verified by sequencing provide a useful resource for such studies. In this regard, human blood group-encoding genes are unique in that differences of cell surface traits among individuals and populations can be readily detected by serological screening, and correlation between the variant cell surface phenotype and the genotype is, in most cases, unequivocal. Here, we perform a comprehensive analysis of allelic forms, compiled in the Blood Group Antigen Gene Mutation database, of ABO, RHD/CE, GYPA/B/E and FUT1/2 gene families that encode the ABO, RH, MNS, and H/h blood group system antigens, respectively. These genes are excellent illustrative examples showing distinct mutational patterns among the alleles, and leading to speculation on how their origin may have been driven by recurrent but different molecular mechanisms. We illustrate how alignment of alleles of a gene may provide an additional insight into the DNA variation process and its pathways, and how this approach may serve to catalog alleles of a gene, simplifying the task and content of mutation databases.

DIIIa and DIII Type 5 are encoded by the same allele and are associated with altered RHCE*ce alleles: clinical implications

BACKGROUND

The partial D phenotype DIIIa was originally reported to be associated with 455A>C in Exon 3, 602C>G in Exon 4, and 667T>G in Exon 5. Other alleles with these changes were subsequently identified and designated DIII Types 5, 6, and 7, as they had additional alterations. The observation that DNA samples associated with the DIIIa phenotype had more changes than those originally reported motivated us to reanalyze the DIIIa probands (BP and DJ) from the original study. We also studied additional DIIIa samples to clarify the RHD background and establish the associated RHCE.

STUDY DESIGN AND METHODS

Hemagglutination testing was performed by standard methods. RHD and RHCE were analyzed by combinations of polymerase chain reaction-restriction fragment length polymorphism, exon-specific sequencing, cloning, or direct sequencing of Rh-cDNAs.

RESULTS

The RHD alleles from BP, DJ, and 58 additional DIIIa samples had the three reported nucleotide changes as well as 186G>T, 410C>T, and 819G>A. The DIIIa allele was associated with several altered RHCE*ce-alleles, the prominent one being ceS (48C, 733G, 1006T).

CONCLUSION

The DIIIa phenotype is associated with six RHD changes, five of which encode amino acid changes, and partial DIIIa and DIII Type 5 are encoded by the same RHD allele. In all samples, RHD*DIIIa was inherited with altered RHCE*ce. Patients with partial DIIIa are at risk for production of alloanti-D, but they are also at risk for alloanti-e, -c, or antibodies to high-prevalence Rh antigens if there is no conventional RHCE*ce in trans. Among 39 patients studied, 16 had alloanti-D and 27 had alloanti-e or anti-hrB.

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.

Generation and characterisation of Rhd and Rhag null mice

Mouse Rhd* and Rhag* genes were targeted using insertional vectors; the resulting knockout mice, and double-knockout descendants, were analysed. Rhag glycoprotein deficiency entailed defective assembly of the erythroid Rh complex with complete loss of Rh and intercellular adhesion molecule 4 (ICAM-4), but not CD47, expression. Absence of the Rh protein induced a loss of ICAM-4, and only a moderate reduction of Rhag expression. Double knockout phenotype was similar to that of Rhag targeted mice. Rhd and Rhag deficient mice exhibited neither the equivalent of human Rh(null) haemolytic anaemia nor any clinical or cellular abnormalities. Rhd-/- and Rhag-/- erythrocytes showed decreased basal adhesion to an endothelial cell line resulting from defective ICAM-4 membrane expression. There was no difference in recovery from phenylhydrazine-induced haematopoietic stress for double knockout mice as compared to controls, suggesting that ICAM-4 might be dispensable during stress erythropoiesis. Ammonia and methylammonia transport in erythrocytes was severely impaired in Rhag-/- but only slightly in Rhd-/- animals that significantly expressed Rhag, supporting the view that RhAG and Rhag, but not Rh, may act as ammonium transporters in human and mouse erythrocytes. These knockout mice should prove useful for further dissecting the physiological roles of Rh and Rhag proteins in the red cell membrane.

Transfusion in the age of molecular diagnostics

DNA-based tests are increasingly being used to predict a blood group phenotype to improve transfusion medicine. This is possible because genes encoding 29 of the 30 blood group systems have been cloned and sequenced, and the molecular bases associated with most antigens have been determined. RBCs carrying a particular antigen, if introduced into the circulation of an individual who lacks that antigen (through transfusion or pregnancy), can elicit an immune response. It is the antibody from such an immune response that causes problems in clinical practice and the reason why antigen-negative blood is required for safe transfusion. The classical method of testing for blood group antigens and antibodies is hemagglutination; however, it has certain limitations, some of which can be overcome by testing DNA. Such testing allows conservation of antibodies for confirmation by hemagglutination of predicted antigen-negativity. High-throughput platforms provide a means to test relatively large numbers of donors, thereby opening the door to change the way antigen-negative blood is provided to patients and to prevent immunization. This review summarizes how molecular approaches, in conjunction with conventional hemagglutination, can be applied in transfusion medicine.

Applications and Experience with PCR-Based Assays to Predict Blood Group Antigens

DNA-based tests are increasingly being used to predict a blood group phenotype. This is possible because genes encoding 29 of the 30 blood group systems have been cloned and sequenced, and the molecular bases associated with most antigens have been determined. RBCs carrying a particular antigen, if introduced into the circulation of an individual who lacks that antigen, can elicit an immune response. It is the antibody from such an immune response that causes problems in clinical practice and the reason why antigen-negative blood is required for safe transfusion. The classical method of testing for blood group antigens and antibodies is hemagglutination; however, it has certain limitations, some of which can be overcome by testing DNA. Such testing allows conservation of antibodies for confirmation by hemagglutination of predicted antigen-negativity. High-throughput platforms provide a means to test relatively large numbers of donors, thereby opening the door to change the way antigen-negative blood is provided to patients. This chapter discusses how molecular approaches can be applied in transfusion medicine, and summarizes experiences of using laboratory developed tests and DNA arrays at the New York Blood Center.

Two novel Jk(null) alleles derived from 222C>A in Exon 5 and 896G>A in Exon 9 of the JK gene

BACKGROUND

Polynesian Jk(null) is well known for its mutation as Intron 5 g>a at the 3' splice acceptor site. After sequencing analysis, however, it was noticed that only three of eight samples with the Jknull phenotype carried typical homozygous Polynesian Jk(null) mutation. Five others were noted to be unreported heterozygous Polynesian Jk(null) mutation. An investigation was then conducted to characterize the underlying mechanism leading to this particular Jk(null) genotype.

STUDY DESIGN AND METHODS

Genomic DNA covering 5'-untranslated region exons and intervening introns of the JK gene was amplified by polymerase chain reaction, and the fragments were directly sequenced. The sequencing results were compared with those published in literature and related biologic Web sites.

RESULTS

In all five samples with a heterozygous Polynesian Jk(null) mutation, additional mutations were identified. Two samples carried missense mutations: 222C>A (Asn74Lys) in Exon 5 and 499A>G (Met167Val) in Exon 7. Three others had missense mutation 896G>A (Gly299Glu) in Exon 9. These substituted amino acids were located either near or at transmembrane domains, respectively. In addition, two polymorphic nucleotides at positions -103 (a>g) and -119(c>a) from the 3' end of Intron 1 were also Polynesian mutation-related.

CONCLUSIONS

In contrast to the typical homozygous Polynesian Jk(null) mutation, two novel heterozygous Jk(null) alleles were noted to be associated with the Jknull phenotype. One carried missense mutation 222C>A in Exon 5, and the other had 896G>A missense mutation in Exon 9. These findings may have implications in designing a molecular screening assay for people with the Jknull phenotype.

Hereditary spherocytosis

Hereditary spherocytosis is a common inherited disorder that is characterised by anaemia, jaundice, and splenomegaly. It is reported worldwide and is the most common inherited anaemia in individuals of northern European ancestry. Clinical severity is variable with most patients having a well-compensated haemolytic anaemia. Some individuals are asymptomatic, whereas others have severe haemolytic anaemia requiring erythrocyte transfusion. The primary lesion in hereditary spherocytosis is loss of membrane surface area, leading to reduced deformability due to defects in the membrane proteins ankyrin, band 3, beta spectrin, alpha spectrin, or protein 4.2. Many isolated mutations have been identified in the genes encoding these membrane proteins; common hereditary spherocytosis-associated mutations have not been identified. Abnormal spherocytes are trapped and destroyed in the spleen and this is the main cause of haemolysis in this disorder. Common complications are cholelithiasis, haemolytic episodes, and aplastic crises. Splenectomy is curative but should be undertaken only after careful assessment of the risks and benefits.

Erythroid urea transporter deficiency due to novel JKnull alleles

BACKGROUND

The Kidd blood group antigens Jka and Jkb are encoded by the red blood cell (RBC) urea transporter gene. Homozygosity for silent JK alleles results in the rare Jk(a-b-) phenotype. To date, seven JKnull alleles have been identified, and of these, two are more frequent in the Polynesians and Finns. This study reports the identification of other JKnull alleles in Jk(a-b-) individuals of different ethnic or geographic origins.

STUDY DESIGN AND METHODS

Nine Jk(a-b-) samples and a sample from a Jk(a-b+) mother of a Jk(a+b-) baby were investigated. Polymerase chain reaction amplification and sequence analysis of the JK gene was performed. Western blotting and urea lysis were used to confirm Jk(a-b-) RBCs.

RESULTS

Four novel alleles were identified: two different nonsense mutations, 202C>T (Gln68Stop) and 723delA (Ile262Stop) were identified on otherwise consensus JK*1 and JK*2 alleles, respectively. A missense mutation, 956C>T (Thr319Met), was identified in a JK*1 allele from an African-American and a JK*2 allele in two people of subcontinental Indian descent. Immunoblotting and urea lysis confirmed absence of JK glycoprotein in RBC membranes from a sample carrying the 956C>T mutation. Other previously described JKnull mutations were found in samples of origins other than in which they were first identified.

CONCLUSION

The molecular bases of the Jk(a-b-) phenotype are diverse and this is the first report of JKnull alleles in individuals of African and subcontinental Indian descent. Although rare, these alleles should be taken into consideration when planning genotyping strategies for blood donors and patients.

RhAG protein of the Rhesus complex is a CO2channel in the human red cell membrane

We have determined CO2 permeabilities, P(CO2), of red cells of normal human blood and of blood deficient in various blood group proteins by a previously described mass spectrometric technique. While P(CO2) of normal red cells is approximately 0.15 cm/s, we find in red blood cells (RBCs) lacking the Rh protein complex (Rh(null)) a significantly reduced P(CO2) of 0.07 cm/s +/-0.02 cm/s (P<0.02). This value is similar to the value we have reported previously for RBCs lacking aquaporin-1 protein (AQP-1(null)), suggesting that each of the Rh and AQP-1 proteins is responsible for approximately 1/2 of the normal CO2 permeability of the RBC membrane. Four other blood group deficiencies tested lack diverse membrane proteins but exhibit normal CO2 permeability. The CO2 pathway constituted by Rh proteins was inhibitable at pH(e)= 7.4 by NH4Cl with an I50 of approximately 10 mM corresponding to an I50 for NH3 of approximately 0.3 mM. The pathway independent of Rh proteins, presumably that constituted by AQP-1, was not inhibitable by NH4Cl/NH3. However, both pathways were strongly inhibited by DIDS, which accounts for the marked inhibitory effect of DIDS on normal P(CO2), while in contrast another AE1 inhibitor, DiBAC, does not inhibit P(CO2), although it markedly reduces P(HCO3-). We conclude that Rh protein, presumably the Rh-associated glycoprotein RhAG, possesses a gas channel that allows passage of CO2 in addition to NH3.

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.

Red cell antigens: Structure and function

Landsteiner and his colleagues demonstrated that human beings could be classified into four groups depending on the presence of one (A) or another (B) or both (AB) or none (O) of the antigens on their red cells. The number of the blood group antigens up to 1984 was 410. In the next 20 years, there were 16 systems with 144 antigens and quite a collection of antigens waiting to be assigned to systems, pending the discovery of new information about their relationship to the established systems. The importance of most blood group antigens had been recognized by immunological complications of blood transfusion or pregnancies; their molecular structure and function however remained undefined for many decades. Recent advances in molecular genetics and cellular biochemistry resulted in an abundance of new information in this field of research. In this review, we try to give some examples of advances made in the field of 'structure and function of the red cell surface molecules.'

CeRh1 (rhr-1) is a dominant Rhesus gene essential for embryonic development and hypodermal function in Caenorhabditis elegans

Rhesus (Rh) proteins share a conserved 12-transmembrane topology and specify a family of putative CO(2) channels found in diverse species from microbes to human, but their functional essentiality and physiological importance in metazoans is unknown. To address this key issue and analyze Rh-engaged physiologic processes, we sought to explore model organisms with fewer Rh genes yet are tractable to genetic manipulations. In this article, we describe the identification in nematodes of two Rh homologues that are highly conserved and similar to human Rh glycoproteins, and we focus on their characterization in Caenorhabditis elegans. RNA analysis revealed that CeRh1 is abundantly expressed in all developmental stages, with highest levels in adults, whereas CeRh2 shows a differential and much lower expression pattern. In transient expression in human cells, both CeRh1 and CeRh2-GFP fusion proteins were routed to the plasma membrane. Transgenic analysis with GFP or LacZ-fusion reporters showed that CeRh1 is mainly expressed in hypodermal tissue, although it is also in other cell types. Mutagenesis analysis using deletion constructs mapped a minimal promoter region driving CeRh1 gene expression. Although CeRh2 was dispensable, RNA interference with CeRh1 caused a lethal phenotype mainly affecting late stages of C. elegans embryonic development, which could be rescued by the CbRh1 homologue from the worm Caenorhabditis briggsae. Taken together, our data provide direct evidence for the essentiality of the CeRh1 gene in C. elegans, establishing a useful animal model for investigating CO(2) channel function by cross-species complementation.

Rh proteins vs Amt proteins: an organismal and phylogenetic perspective on CO2 and NH3 gas channels

Rh (Rhesus) proteins are homologues of ammonium transport (Amt) proteins. Physiological and structural evidence shows that Amt proteins are gas channels for NH(3), but the substrate of Rh proteins, be it CO2 as shown in green alga, or NH3/NH4+ as shown in mammalian cells, remains disputed. We assembled a large dataset generated of Rh and Amt to explore how Rh originated from and evolved independently of Amt relatives. Analysis of this rich data implies that Rh was split from Amt first to emerge in archaeal species. The Rh ancestor underwent divergence and duplication along speciation, leading to neofunctionalization and subfunctionalization of the Rh family. The characteristic organismal distribution of Rh vs. Amt reflects their early separation and subsequent independent evolution: they coexist in microbes and invertebrates but do not in fungi, vascular plants or vertebrates. Rh gene-duplication was prominent in vertebrates: while epithelial RhBG/RhCG displayed strong purifying selection, erythroid Rh30 and RhAG experienced different episodes of positive selection in each of which adaptive evolution occurred at certain time points and in a few codon sites. Mammalian Rh30 and RhAG were subject to particularly strong positive selection in some codon sites in the lineage from rodents to human. The grounds of this adaptive evolution may be driven by the necessity to increase the surface/volume ratio of biconcave erythrocytes for facilitative gas diffusion. Altogether, these results are consistent with Rh proteins not being the orthologue of Amt proteins but having gained the function for CO2/HCO3- transport, with important roles in systemic pH regulation.

Generation of mice with inactivated Rh or Rhag genes

Mice carrying inactivated Rh and Rhag genes were generated by insertional targeting. KO animals exhibited normal growth, development and fertility and both types were indistinguishable at a gross phenotypic level from their wild type littermates. Preliminary analysis revealed that red cells from Rh-/- mice lack Rh protein and have a moderate decrease of Rhag protein, whereas those from Rhag-/- mice have a total absence of Rhag and Rh proteins. Studies are in progress to delineate the antigenic, biochemical and functional abnormalities of red cells from these animals as well as the impact on hematological parameters and erythropoiesis.

Hydrophobic cluster analysis and modeling of the human Rh protein three-dimensional structures

Rh (Rhesus) is a major blood group system in man, which is clinically significant in transfusion medicine. Rh antigens are carried by an oligomer of two major erythroid specific polypeptides, the Rh (D and CcEe) proteins and the RhAG glycoprotein, that shared a common predicted structure with 12 transmembrane a-helices (M0 to M11). Non erythroid homologues of these proteins have been identified (RhBG and RhCG), notably in diverse organs specialized in ammonia production and excretion, such as kidney, liver and intestine. Phylogenetic studies and experimental evidence have shown that these proteins belong to the Amt/Mep/Rh protein superfamily of ammonium/methylammonium permease, but another view suggests that Rh proteins might function as CO2 gas channels. Until recently no information on the structure of these proteins were available. However, in the last two years, new insight has been gained into the structural features of Rh proteins (through the determination of the crystal structures of bacterial AmtB and archeaebacterial Amt-1. Here, models of the subunit and oligomeric architecture of human Rh proteins are proposed, based on a refined alignment with and crystal structure of the bacterial ammonia transporter AmtB, a member of the Amt/Mep/Rh superfamily. This alignment was performed considering invariant structural features, which were revealed through Hydrophobic Cluster Analysis, and led to propose alternative predictions for the less conserved regions, particularly in the N-terminal sequences. The Rh models, on which an additional Rh-specific, N-terminal helix M0 was tentatively positioned, were further assessed through the consideration of biochemical and immunochemical data, as well as of stereochemical and topological constraints. These models highlighted some Rh specific features that have not yet been reported. Among these, are the prediction of some critical residues, which may play a role in the channel function, but also in the stability of the subunit structure and oligomeric assembly. These results provide a basis to further understand the structure/function relationships of Rh proteins, and the alterations occurring in variant phenotypes.

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.

Red cell membrane CO2 permeability in normal human blood and in blood deficient in various blood groups, and effect of DIDS

The red cell membrane has an exceptionally high permeability for CO2, PCO2 approximately 0.15 cm/s, which is two to three orders of magnitude greater than that of some epithelial membranes and similarly greater than the permeability of the red cell membrane for HCO3-. As shown previously, this high PCO2 can be drastically inhibited by 10 microM 4,4'-diisothiocyanato-2,2'-stilbenedisulfonate (DIDS), indicating that membrane proteins may be involved in this high gas permeability. Here, we have studied the possible contribution of several blood group proteins to CO2 permeation across the red cell membrane by comparing PCO2 of red cells deficient in specific blood group proteins with that of normal red cells. While PCO2 of normal red cells is approximately 0.15 cm/s and that of Fy(null) and Jk(null) red cells is similar, PCO2's of Colton null (deficient in aquaporin-1) and Rh(null) cells (deficient in Rh/RhAG) are both reduced to about 0.07 cm/s, i.e. to about one half. In addition, the inhibitory effect of DIDS is about half as great in Rh(null) and in Colton null red cells as it is in normal red cells. We conclude that aquaporin-1 and Rh/RhAG proteins contribute substantially to the high permeability of the human red cell membrane for CO2. Together these proteins are responsible for 50% or more of the CO2 permeability of red cell membranes. The CO2 pathways of both proteins can be partly inhibited by DIDS, which is why this compound very effectively reduces membrane CO2 permeability.

Biological gas channels for NH3 and CO2: evidence that Rh (Rhesus) proteins are CO2 channels

Physiological evidence from our laboratory indicates that Amt/Mep proteins are gas channels for NH3, the first biological gas channels to be described. This view has now been confirmed by structural evidence and is displacing the previous belief that Amt/Mep proteins were active transporters for the NH4+ ion. Still disputed is the physiological substrate for Rh proteins, the only known homologues of Amt/Mep proteins. Many think they are mammalian ammonium (NH4+ or NH3) transporters. Following Monod's famous dictum, "Anything found to be true of E. coli must also be true of elephants" [Perspect. Biol. Med. 47(1) (2004) 47], we explored the substrate for Rh proteins in the unicellular green alga Chlamydomonas reinhardtii. C. reinhardtii is one of the simplest organisms to have Rh proteins and it also has Amt proteins. Physiological studies in this microbe indicate that the substrate for Rh proteins is CO2 and confirm that the substrate for Amt proteins is NH3. Both are readily hydrated gases. Knowing that transport of CO2 is the ancestral function of Rh proteins supports the inference from hematological research that a newly evolving role of the human Rh30 proteins, RhCcEe and RhD, is to help maintain the flexible, flattened shape of the red cell.

Role of RhAG and AQP1 in NH3 and CO2 gas transport in red cell ghosts: a stopped-flow analysis

To clarify the potential role Rh/RhAG and AQP1 proteins in erythrocyte gas transport, NH3 and CO2 transport was measured in erythrocyte ghost membrane vesicles from rare human variants (Rh(null), CO(null),) and knockout mice (homozygous AQP1-/-, Rh-/- and Rhag-/-) exhibiting well-characterized protein defects. Transport was measured from intracellular pH (pHi) changes in a stopped-flow fluorimeter. NH3 transport was measured in chloride-free conditions with ghosts exposed to 20 mM inwardly directed gradients of gluconate salts of ammonium, hydrazine and methylammonium at 15 degrees C. Alkalinization rates of control samples were 6.5+/-0.3, 4.03+/-0.17, 0.95+/-0.08 s(-1) for each solute, respectively, but were significantly reduced for Rh(null) and CO(null) samples that are deficient in RhAG and AQP1 proteins, respectively. Alkalinization rates of Rh(null) ghosts were about 60%, 83% and 94% lower than that in control ghosts, respectively, for each solute. In CO(null) ghosts, the lack of AQP1 resulted in about 30% reduction of the alkalinization rates as compared to controls, but the transport selectivity of RhAG for the three solutes was preserved. Similar observations were made with ghosts from KO mice Rhag-/- and AQP1-/-. These results confirm the major contribution of RhAG/Rhag in the NH3 conductance of erythrocytes and suggest that the reduction of transport rates in the absence of AQP1 would be better explained by a direct or indirect effect on RhAG/Rhag-mediated transport. When ghosts were preloaded with carbonic anhydrase and exposed to a 25 mM CO2/HCO3- gradient at 6 degrees C, an extremely rapid kinetics of acidification corresponding to CO2 influx was observed. The rate constants were not significantly different between controls and human variants (125+/-6 s(-1)), or between wild-type and KO mice, suggesting no major role of RhAG or AQP1 in CO2 transport, at least in our experimental conditions.

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-.

Rhnull syndrome: identification of a novel mutation in RHce

BACKGROUND

The deficiency of Rh proteins on red blood cells (RBCs) from individuals of the Rh(null) amorph type are the result of homozygosity for a silent RHCE in cis with a deleted RHD. A novel mutation in RHce was identified in two Caucasian Brazilian girls with the amorph type of Rh(null) who were born to parents who were first cousins.

STUDY DESIGN AND METHODS

RBCs from the Rh(null) sisters and from family members were analyzed by serology and flow cytometry with specific antibodies. Genomic DNA and transcripts were tested by polymerase chain reaction and sequence analysis.

RESULTS

Rh(null) RBCs were nonreactive with anti-Rh and anti-LW. Molecular analyses showed a deletion of RHD and of one nucleotide (960/963; GGGG-->GGG) in exon 7 of the RHce. This deletion introduced a frameshift after Gly321, a new C-terminal sequence, and a premature stop codon, resulting in a shorter predicted protein with 357 amino acids.

CONCLUSION

The detection of a unique RHce transcript indicated that the two sisters were homozygous, whereas the other family members were heterozygous for the mutation. A novel mutation resulting in the amorph Rh(null) with loss of Rh antigen expression is described.

Evolutionary conservation and diversification of Rh family genes and proteins

Rhesus (Rh) proteins were first identified in human erythroid cells and recently in other tissues. Like ammonia transporter (Amt) proteins, their only homologues, Rh proteins have the 12 transmembrane-spanning segments characteristic of transporters. Many think Rh and Amt proteins transport the same substrate, NH(3)/NH(4)(+), whereas others think that Rh proteins transport CO(2) and Amt proteins NH(3). In the latter view, Rh and Amt are different biological gas channels. To reconstruct the phylogeny of the Rh family and study its coexistence with and relationship to Amt in depth, we analyzed 111 Rh genes and 260 Amt genes. Although Rh and Amt are found together in organisms as diverse as unicellular eukaryotes and sea squirts, Rh genes apparently arose later, because they are rare in prokaryotes. However, Rh genes are prominent in vertebrates, in which Amt genes disappear. In organisms with both types of genes, Rh had apparently diverged away from Amt rapidly and then evolved slowly over a long period. Functionally divergent amino acid sites are clustered in transmembrane segments and around the gas-conducting lumen recently identified in Escherichia coli AmtB, in agreement with Rh proteins having new substrate specificity. Despite gene duplications and mutations, the Rh paralogous groups all have apparently been subject to strong purifying selection indicating functional conservation. Genes encoding the classical Rh proteins in mammalian red cells show higher nucleotide substitution rates at nonsynonymous codon positions than other Rh genes, a finding that suggests a possible role for these proteins in red cell morphogenetic evolution.

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

BACKGROUND

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

STUDY DESIGN AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Molecular characterization of D- Korean persons: development of a diagnostic strategy

BACKGROUND

D- frequencies show wide racial differences: in particular, in Korean persons it is approximately 1/100th of that in Caucasians (0.15% vs. 15%). The molecular mechanisms of D- may be unique to races, and thus specific molecular diagnostic approaches are necessary for individual races. Such marked racial differences have been attributed to the founder effect. It was hypothesized that D- frequencies may be affected by genomic variations of Rhesus boxes, which are instruments of unequal crossing-over.

STUDY DESIGN AND METHODS

First, the molecular basis of D- in Korean donors was characterized. A total of 264 D- persons were analyzed by performing RHD exon polymerase chain reaction (PCR), Rhesus box hybrid PCR-PstI, RHD sequencing, and a novel RHD-CE-D hybrid PCR. Second, with sequencing analysis, Rhesus boxes in Korean and Caucasian persons were compared.

RESULTS

Of 264 D- Korean individuals, 74 percent completely lacked RHD, 9 percent had RHD-CE-D hybrid, and 17 percent had point mutations. Three genetic causes, RHD deletion, RHD-CE(2-9)-D2, and 1227G>A, explained 99.4 percent of D- alleles in Korean persons. Three novel mutations were also found. The identity region of 10 Rhesus boxes in Korean persons was in complete concordance with that in Caucasian persons.

CONCLUSION

In this study, a molecular diagnostic strategy was established and the genetic causes of almost all D- Korean persons was able to be diagnosed with a simple decision tree. The study provides a good example of the molecular diagnosis of D- persons, especially in low-frequency areas. Our hypothesis that D- frequencies are affected by genomic variations in Rhesus boxes was excluded. Rather, racial differences may be influenced by the founder effect.

Large-scale pre-diagnosis study of fetal RHD genotyping by PCR on plasma DNA from RhD-negative pregnant women

BACKGROUND

The routine prenatal determination of fetal RhD blood group would be very useful in the management of pregnancies in RhD-negative women, as up to 40% of these pregnancies bear a RhD-negative fetus. The fetal DNA present in maternal plasma offers an opportunity for risk-free prenatal diagnosis.

AIM

This study focused on the feasibility and accuracy of large-scale RhD fetal diagnosis in non-immunized and anti-D immunized RhD-negative women.

METHODS

Plasma DNA was extracted from 893 RhD-negative pregnant women and amplified in exons 7 and 10 of the RHD gene using conventional and real-time PCR. The results were then compared with the RHD fetal genotype determined on amniotic cells and/or the RhD phenotype of the red blood cells of the infants at birth.

RESULTS

After exclusion of 42 samples from women exhibiting a nonfunctional or rearranged RHD gene, fetal RhD status was predicted with a 99.5% accuracy. A strategy is also proposed to avoid the small number of false-positive and -negative results.

CONCLUSION

Fetal RHD genotyping from maternal plasma DNA in different clinical situations may be used with confidence.

Impact of Rh(D) Blood Group System on Graft Function and Survival in Live-Donor Kidney Transplantation: A Single-Institution Experience

One thousand five hundred consecutive live-donor renal transplants were performed in a single institution. Among these patients, 1,372 patients (group I) received Rh-identical allografts and 128 (group II) received Rh-nonidentical allografts. The two groups were homogenous apart from the prevalence of the Rh nonidentity among unrelated donor-recipient pairs. The rate of acute and chronic rejection was comparable in both groups (P = 0.33 for acute rejection and P = 0.66 for chronic rejection). The mean serum creatinine at 5 years was 1.8 +/- 1 mg/dL for group I and 1.7 +/- 0.9 mg/dL for group II (P = 0.5). The 1-, 5-, and 10-year graft survival rates were 94%, 78%, and 54% for group I and 95%, 82%, and 57% for group II. We found that the Rh(D) blood group system is not likely to be a clinically relevant histocompatibility barrier to live-donor renal transplantation.

Human Rhesus-associated glycoprotein mediates facilitated transport of NH(3) into red blood cells

Rhesus (Rh) antigens are carried by a membrane complex that includes Rh proteins (D and CcEe), Rh-associated glycoproteins (RhAG), and accessory chains (LW and CD47) associated by noncovalent bonds. In heterologous expression systems, RhAG and its kidney orthologs function as ammonium transporters. In red blood cells (RBCs), it is generally accepted that NH(3) permeates by membrane lipid diffusion. We have revisited these issues by studying RBC and ghosts from human and mouse genetic variants with defects of proteins that comprise the Rh complex. In both normal and mutant cells, stopped-flow analyses of intracellular pH changes in the presence of inwardly directed methylammonium (CH(3)NH(+)(3)+CH(3)NH(2)) or ammonium (NH(+)(4)+NH(3)) gradients showed a rapid alkalinization phase. Cells from human and mouse variants exhibited a decrease in their kinetic rate constants that was strictly correlated to the degree of reduction of their RhAG/Rhag expression level. Rate constants were not affected by a reduction of Rh, CD47, or LW. CH(3)NH(2)/NH(3) transport was characterized by (i) a sensitivity to mercurials that is reversible by 2-mercaptoethanol and (ii) a reduction of alkalinization rate constants after bromelain digestion, which cleaves RhAG. The results show that RhAG facilitates CH(3)NH(2)/NH(3) movement across the RBC membrane and represents a potential example of a gas channel in mammalian cells. In RBCs, RhAG may transport NH(3) to detoxifying organs, like kidney and liver, and together with nonerythroid tissue orthologs may contribute to the regulation of the systemic acid-base balance.

Red blood cell blood group antigens: structure and function

Red blood cell (RBC) blood group antigens are polymorphic, inherited, carbohydrate or protein structures located on the extracellular surface of the RBC membrane. They contribute to the architecture of the RBC membrane, and their individual function(s) are being slowly revealed. The biological qualities assigned to these RBC membrane structures are based on observed physiological alteration in RBCs that lack the component, by documenting similarities in its protein sequence (predicted from the nucleotide sequence of the gene) to proteins of known function and by extrapolation to identified functional homologues in other cells. The varied roles of RBC antigens include membrane structural integrity, the transport of molecules through the membrane, as receptors for extracellular ligands, adhesion molecules, enzymes, complement components and regulators, and in glycocalyx formation.

RH locus contraction in a novel Dc-/D-- genotype resulting from separate genetic recombination events

BACKGROUND

The rare phenotypes Dc- and D-- lack the expression of E/e and CcEe antigens, respectively; their cotransmission in a single family has not been reported.

STUDY DESIGN AND METHODS

Six members of a Chinese family with two exhibiting the Dc- phenotype were studied using standard serologic methods. Rh genotypes were analyzed by Southern blot, and RH loci, by exon PCR. Rh transcripts were characterized by gene-specific RT-PCR and sequencing.

RESULTS

Although Rh typing detected two members as Dc- homozygotes, RFLP analysis and exon PCR showed them to be Dc- heterozygotes with a partial deletion of RHCE. cDNA sequencing showed the expression in the family of normal RHD and RHCe as well as hybrid transcripts, RHD(1-9)/RHCE(10) and RHCE(1-3)/RHD(4-10). Thus, the Dc- members had the genotype of Dc-/ D-- and expressed both hybrid genes that were inherited from their parents, respectively.

DISCUSSION

This is the first demonstration in a family that the Dc- and D-- complexes neither are linked with a normal RHD or RHCE gene. The segregation of these two different hybrid genes with single break points suggests their independent genetic origin and provides molecular insights into the dynamic nature of genomic rearrangements leading to RH locus contraction.