Biochemistry and molecular genetics of Rh antigens

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.

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-.

Molecular biology and genetics of the Rh blood group system

The Rh (Rhesus) blood group system is the most complex of the known human blood group polymorphisms. The expression of its antigens is controlled by a two-component genetic system consisting of RH and RHAG loci, which encode Rh30 polypeptides and Rh50 glycoprotein, respectively. Over the past decade, there has been a rapid advance in knowledge of the biochemistry, molecular biology, and genetics of the Rh genes and proteins. The primary structures of D and CcEe antigens have become well understood and the molecular genetic basis of a vast array of phenotype polymorphisms has been delineated. The identification of various molecular defects associated with Rh deficiency syndrome clarifies the nature of the amorph, suppressor, and modifier genes. The observed mutation spectrum defines a basic set of components essential for Rh complex assembly in the erythrocyte membrane. The resulting molecular information, combined with new experimental tools, is helping to dissect the fine structure of Rh antigens in terms of epitope mapping. The discovery of novel Rh homologs in primitive organisms and in nonerythroid tissues opens new avenues of research beyond the scope of erythrocytes and Rh antigens. This review provides an update on the Rh family in antigen expression, phenotype diversity, and disease association.

Rhmod syndrome: a family study of the translation-initiator mutation in the Rh50 glycoprotein gene

Rhmod syndrome is a rare genetic disorder thought to result from mutations at a "modifier" but not at the suppressor underlying the regulator type of Rhnull disease. We studied this disorder in a Jewish family with a consanguineous background and analyzed RH and RHAG, the two loci that control Rh-antigen expression and Rh-complex assembly. Despite the presence of a d (D-negative) haplotype, no other gross alteration was found at RH, and cDNA sequencing showed a normal structure for D, Ce, and ce Rh transcripts in family members. However, analysis of RHAG transcript, which encodes Rh50 glycoprotein, identified a single G-->T transversion in the initiation codon, causing a missense amino acid change (ATG[Met]-->ATT[Ile]). This point mutation also occurred in the genomic region spanning exon 1 of RHAG, and its genotypic status in the mother and two children was confirmed by analysis of single-strand conformation polymorphism. Although blood typing showed a very weak expression of Rh antigens, immunoblotting barely detected the Rh proteins in the Rhmod membrane. In vitro transcription-coupled translation assays showed that the initiator mutants of Rhmod-but not those of the wild type-could be translated from ATG codons downstream. Our findings point to incomplete penetrance of the Rhmod mutation, in the form of "leaky" translation, leading to some posttranslational defects affecting the structure, interaction, and processing of Rh50 glycoprotein.

Sequence analysis of cDNA derived from reticulocyte mRNAs coding for Rh polypeptides and demonstration of E/e and C/c polymorphisms

RNA derived from enriched reticulocytes of Rh-phenotyped donors was isolated, reversely transcribed into cDNA and amplified with Rh-specific primers by polymerase chain reaction. Nucleotide sequence analysis of the entire coding region of the Rh cDNAs was carried out. Four types of cDNAs were identified, tentatively designated as RhSCI, RhSCII, RhSCIII and RhSCIV. Comparison of RhSCII with RhSCI (identical to the previously reported RhIXb/30A cDNA), showed single base pair difference. Since RhSCI and RhSCII were found to be related to the presence of E or e antigen, respectively, the P226A amino acid polymorphism appears to be the genetic basis of the E/e polymorphism. RhSCIII was demonstrated to be a transcript derived from the RhD gene, with 35 amino acid substitutions as compared to RhSCI. RhSCIV was found to be present only in RhC-positive individuals, indicating that RhSCIV encodes a polypeptide carrying the C antigen. Six nucleotide changes, resulting in four amino acid substitutions W16C, L60I, N68S and P103S, were observed between RhSCII and RhSCIV, probably representing the C/c polymorphism.

Blood group antigens on human erythrocytes-distribution, structure and possible functions

Human erythrocyte blood group antigens can be broadly divided into carbohydrates and proteins. The carbohydrate-dependent antigens (e.g., ABH, Lewis, Ii, P1, P-related, T and Tn) are covalently attached to proteins and/or sphingolipids, which are also widely distributed in body fluids, normal tissues and tumors. Blood group gene-specific glycosyltransferase regulate the synthesis of these antigens. Protein-dependent blood group antigens (e.g., MNSs, Gerbich, Rh, Kell, Duffy and Cromer-related) are carried on proteins, glycoproteins and proteins with glycosylphosphatidylinositol anchor. The functions of these molecules on human erythrocytes remain unknown; some of them may be involved in maintaining the erythrocyte shape. This review describes the distribution, structures and probable biological functions of some of these antigens in normal and pathological conditions.