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Rh Blood Group System

RH
Gene locus - RHCE, RHD
Alleles

Introduction

Whereas previously the Rh blood group system included the structurally related antigens RhCE, RhD and RhAG, recently, RhAG was given a separate blood group system status (see entry "Rg-associated glycoprotein (RhAg)").

Antigens of the Rh blood group system are products of RHD and RHCE (collectively referred to as RH30 or RHCED), two tightly linked and highly homologous genes residing on chromosome 1p36.1. RhD carries the D antigen, the most potent blood group immunogen. The D epitope is not expressed in a relatively large segment of the population (i.e., Rh-negative phenotype), as a result of RHD gene deletion or other gene alterations.

RHCE exists in four allelic forms and each allele determines the expression of two antigens in Ce, ce, cE or CE combination (RHCE is the collective name of the four alleles). RHD and RHCE genes, each, contain 10 exons and span ~75 kb DNA sequence.

The antigen of the RHAG blood group system is Rh50 glycoprotein, the product of another single copy gene RHAG (also refered to as RH50). Complex formation with RhD and/or RhCE is essential for the presentation of the Rh antigenic activity. RHAG is similarily organized into 10 exons and shares 36% sequence identity with RHCE/D but it is located at a separate locus, on chromosome 6p12.3.

While alterations in the RHAG locus are relatively rare, the RHCE/D locus harbors a large repertoire of allelic diversity at the level of population. It was proposed that the open reading frames of RHCE and RHD occur in opposite orientation - their 3'ends facing each other and being separated by about a 30 kb region that contains the SMP1 gene; whether this occurs in all populations still needs to be demonstrated; in addition, two 9kb highly homologous sequences, named "Rhesus Boxes", are located at the 5' and 3' ends of RHD. It is proposed that the deletion of RHD , frequently observed in the population, occurs through unequal homologous recombination confined to these homologous regions (Wagner and Flegel, Blood, 95:3662, 2000, and in Blood, 99, 2272, 2002).

Products of both RHCED and RHAG are integral membrane proteins showing a similar 12-transmembrane helix topology; Rh 30 polypeptides are palmitolyated and their epitopes are defined by five specific amino acids located on the extracellular loops. Rh 50 is N-glycosylated at a single site, Asn37.

The existence of two non-erythroid homologues of RHAG, named RHBG and RHCG, was demonstrated; they show a high level of homology but their patterns of expression are not identical (Liu et al., J. Biol. Chem., 275, 25641, 2000; Liu et al., J. Biol. Chem., 276, 1424, 2001; Huang and Liu, Blood Cells, Molecules, and Diseases, 27, 90, 2001). In addition, novel RH homologues have been identified in a large number of species, from lower vertebrates to invertebrates and unicellular organisms, including several prokaryotes (Huang and Peng, PNAS USA, 2005, 102, 15512-15517).

The genes

RHD and RHCE reside on chromosome 1 at position 1p36.1 - the open reading frames of the two genes may occur in opposite orientation - their 3'ends facing each other and being separated by about a 30kb region that contains the SMP1 gene (Wagner and Flegel, Blood, 95:3662, 2000).

Function of proteins

Role in erythrocyte membrane integrity and a transport function; RHAG,RHBG and RHCG may function as ammonium transporters (Huang et al., J. Biol. Chem., 276, 1424, 2001 and 275, 25641, 2000; Marini et al., Nature Genet. 26, 341, 2000, Westhoff et al., J. Biol. Chem., 277, 12499, 2002). In addition, a study in green algae showed that these Rh proteins may function as a gas channel for CO2 (Soupeneset al., PNAS USA, 99, 7769, 2002).Recent phylogenetic, physiological, developmental and crystallographic studies on various RH homologues strongly suggest that Rh proteins may be CO2 transporters, but their role in ammonium transport is still an open question (Peng and Huang Transf Clin Biol 2006 13 85-94; Kustu and Inwood Transf Clin Biol 2006 13 103-110; Endeward et al. FASEB J 2008 22 64-73; Li et al. PNAS USA 2007 104 19279-19284; Lupo et al. PNAS USA 2007 104 19303-19308).

Tissue distribution

Expression of RHCE, RHD and RHAG is confined to erythroid tissues; products of the non-erythroid homologues are expressed in kidney, liver, skin, testis and brain.

Disease association

Rh incompatible transfusion may result in death of patients. Rh incompatibility is still the leading cause of hemolytic disease of the newborn (HDN) and may involve some forms of graft-versus-host (GVH) disease in organ transplantation. Absence or severely reduced expression of all Rh30 polypeptides and/or their associated Rh antigens is referred to as Rhnull or Rhmod (also known as Rh deficiency syndrome). This autosomal recessive disorder manifests a varying degree of compensated hemolytic anemia and spherostomatocytosis. The responsible mutations are located at either the RHCED (antigen) locus proper or at the suppressor locus RHAG.

Serological aspects

In a recent article W.A. Flegel ( Transfusion and Apheresis Science 2011 44 81-91) provides an excellent description of the molecular basis proposed for a number of relatively common RHD variant phenotypes that express decreased amounts of the D antigen. In particular, "weak D" and "partial D" may present confusing results and may even escape detection by serological testing. In those cases molecular analysis may be helpful. In addition, it may indicate two categories of "partial D" that can be correlated with either amino acid changes in the extracellular loops or by hybrid alleles; in contrast, DNA of "weak D" phenotypes often shows amino acid substitutions within transmembrane or intracellular regions.Problems with Integration into the red cell membrane may be characteristic of the "DEL" phenotype. In all cases, folding or conformational changes may be responsible for the pattern of expression of these variant forms.

About the alleles

Gene recombinations between the RHD and RHCE alleles, as well as other mutations at the RHCE/D locus are responsible for the origin of a large number of rare alleles whose expression is apparent from serological studies. The opposite orientation of the two genes would suggest that gene recombinations occur predominantly through gene conversion rather than crossings-over; however this still is an open question. In the RHCE/D locus, gene conversion has been implicated in both large- and small-scale transfers of genetic material from donor to the recipient; they are defined as macroconversions or microconversion events, respectively. Unequal homologous recombination occurring within these "Rhesus Boxes" flanking the RHD gene may be responsible for its deletion (Wagner and Flegel, Blood 95:3662, 2000). Other molecular mechanisms include missense changes, nonsense mutations and small in-frame and out-of-frame deletions.

In the list of alleles, designation of the hybrid alleles is based on their hybrid structures when entire exons or their portions are transferred (exons are shown in parenthesis); designation of all other alleles, including those where microconversions occur, incorporates the nature of the alteration at the protein level (Antorakis et al., Human Mutation, 11:1, 1998; also, frsh=frame shift); the generic "CE" will be used to designate the RHCE alleles when their exact allele specificity is not known.

The most common phenotypic alteration includes the absence of expression of the D antigen (RhD-negative, gene frequency in Caucasian population is 45%). This may be due to deletion of the entire gene, gene rearrangements, or mutations, as well as deletions or insertions resulting in frameshifts. In all these instances the D epitope is absent or unavailable.
The incidence of variants of RHCE and RHD alleles in the population still lacks complete documentation; whereas in some cases, less than 0.1% of the population tested show the variant phenotype and the occurrence of some phenotypes has been documented in single families only, the incidence of other variants may be much higher. A recent report documents the prevalence of a RHD pseudogene in a large segment of RhD-negative African populations (Singleton et al., Blood, 95, 12, 2000). In most cases the sites of recombinations are known but the breakpoints, which most often occur within introns, have not been defined or are ambiguous because of a high sequence identity. Complementary or additional information on the RHD alleles can be obtained on the Rhesus Site.

Rare Alleles of Rh Deficiency Syndrome - Because the Rh antigens are expressed as a complex of RHCED and RHAG products, alterations in either locus can result in Rh deficiency syndrome. Rh null refers to the complete absence of Rh antigens and is classified as amorph or regulator type. The amorph type defines mutations in the RHCED antigen locus proper, whereas the regulator type defines mutations in the RHAG locus that supresses the expression of Rh antigens. The Rh mod phenotype reflects either incomplete penetrance of RHAG mutations or other, as yet, unknown mutations. To date, three types of mutations have been shown to cause the Rh null phenotype: missense changes, small exonic deletions or splice site mutations. Missense mutations result in single amino acid replacements, whereas the other two types eventually lead to frameshift and premature chain terminations. In the amorph type of Rh null, a homozygous mutation in the RHCE gene generally occurs on the genetic background of RHD gene deletion. In the regulator type Rh null, the location of mutations appears to be clustered in the RHAG gene. In one subject, the Rh mod phenotype has been shown to result from a defective translation initiation, together with alternative use of downstream ATG codons in the Rh50 mutant.
In the list of alleles, RHD sequence with acc. no. X63097.1 is taken as reference for RHD alleles, and DQ322275 for RHCE alleles.

Compilation of RHCE/D alleles in BGMUT. (Excel; March 2011)

When searching for a particular allele, use "alias" if DNA alteration is known or, if you wish to search by phenotype or the designation used by the author, use "name" (see "Details").
Please note that a number of alleles suffer from a nomenclature problem; in particular,the DAR (DARE,DARA) series of alleles are scattered because authors use different names for their designation;thus some may appear under "DAR" or"weak D type..."or others. When searching, try both the "name" and the "alias" terms but best is to search,if known, under a typical nucleotide change, or the first author or PMID.

Other database IDs and links

NCBI genes
6006 for RHCE
6007 for RHD
Uniprot ID
P18577 for RHCE
Q02161 for RHD
Genbank proteins
NP_065231 for RHCE
NP_057208 for RHD
Gene nomenclature database ID
10008 for RHCE
10009 for RHD
Genbank nucleic acids
NM_020485.4 for RHCE
NM_016124 for RHD
NCBI homologenes for homologs and orthologs
32310 for RHCE
NCBI dbSNP for single nucleotide polymorphisms
6006 for RHCE
6007 for RHD
OMIM ID - at Online Mendelian Inheritance in Man
111700 for RHCE
111680 for RHD

Literature

New PubMed entries with the terms Rh and blood from the last 30 days.
NCBI Book Sections with the terms Rh and blood.

Other

Information is also available on the orthologs of Rh system genes, the Rh box alleles, and the non-erythroid Rh homologs.

Contributors

Cheng-Han Huang, Laboratory of Biochemistry and Molecular Genetics, Lindsley F. Kimball Research Institute, New York Blood Center, New York, N.Y.

Contributors for specific alleles are listed with the alleles.

Links


Updated 2014-08-15 15:34:47.760
 
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