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

A case of haemolytic disease of the fetus and newborn attributed to a novel antigen in the RHAG blood group system

BACKGROUND AND OBJECTIVES

A newborn presented with jaundice in Thailand. The cord red cells tested positive by direct antiglobulin test (DAT) for an unknown maternal red cell antibody. Initial blood group sequencing suggested that the infant carried a novel variant RHAG c.140T>C, responsible for a low-prevalence antigen in the RHAG blood group system (ISBT 030). We report here on testing of samples from the infant's parents and older sibling to define a new antigen in the RHAG system.

MATERIALS AND METHODS

Massive parallel sequencing (MPS) using a custom-designed panel was performed on all four family members. Extended serological testing was also performed to determine whether family members with the same variant as the infant showed reactivity with the antibody in the maternal plasma.

RESULTS

We identified a novel single nucleotide variant (SNV) (RHAG c.140T>C, p.[Phe47Ser]) in samples from three of the four family members tested (the infant, the older sibling and the father). The variant was not detected in the mother's sample. Maternal plasma showed positive agglutination with all family members tested; however, when tested with routine panel cells, no reactivity was observed.

CONCLUSION

This case study showed that the presence of the novel variant (RHAG c.140T>C), encoding a p.(Phe47Ser) change in the RhAG glycoprotein, was the apparent cause of incompatibility between maternal plasma and that of red cells from the proband, father and older sibling of the proband. We propose this variant to be a new low-prevalence antigen in the RHAG blood group system.

Asian-type DEL (RHD*DEL1) with an allo-anti-D: A paradoxical observation in a healthy multiparous woman

BACKGROUND

The DEL phenotype is the D variant expressing the least amounts of D antigen per red cell. Asian-type DEL (RHD:c:1227G > A) is the most prevalent DEL in East Asia without any anti-D alloimmunization reported before. We investigated the first observation of an anti-D in any DEL phenotype, reported in the Japanese language at a 1987 conference, only 3 years after the discovery of DEL.

METHODS

We contacted the proband 35 years after the initial report. Standard hemagglutination, adsorption/elution, and flow cytometry tests were performed, as was nucleotide sequencing for the RHD, RHCE, and HLA class I and class II genes.

RESULTS

The healthy multiparous Japanese woman, a regular blood donor, still had the anti-D of titer 8 representing an alloantibody by standard serologic methods. Unexpectedly, she carried an Asian-type DEL without any additional RHD gene variation. All 12 HLA alleles identified were known in the Japanese population. Interestingly, one of her HLA-DRB1 and a variant of her HLA-DQB1 alleles had previously been associated with anti-D immunization.

CONCLUSION

We described an allo-anti-D, maintained for more than three decades, in an Asian-type DEL. The combination of two implicated HLA alleles were rare and could have contributed to the anti-D immunization. Continued monitoring of anti-D immunization events in patients with DEL is warranted, and we discuss possible mechanisms for further study. As only this single observation has been recognized in the last 35 years, the current recommendation is affirmed: Individuals with Asian-type DEL should be treated as Rh D-positive for transfusion and Rh immune prophylaxis purposes.

Identification of novel mutations in BCKDHB and DBT genes in Vietnamese patients with maple sirup urine disease

Background

Maple sirup urine disease (MSUD) is an autosomal recessive inherited metabolic disorder. The disease‐causing mutations can affect the BCKDHA, BCKDHB, and DBT genes encoding for the E1α, E1β, and E2 subunits of the multienzyme branched‐chain α‐keto acid dehydrogenase (BCKDH) complex. In the present study, novel pathogenic variants in BCKDHB and DBT genes were identified in three Vietnamese families with MSUD.

Methods

Three newborn patients from three unrelated Vietnamese families were diagnosed with MSUD at the Metabolic Clinic, National Hospital of Pediatrics. Blood samples of 11 relatives from two generations of the three families diagnosed with MSUD were analyzed using exome and Sanger sequencing analyses.

Results

Novel pathogenic variants in BCKDHB (c.1103C>T, c.989A>G, and c.704G>A), and DBT (c.263_265delAAG) genes were identified in three pediatric patients with MSUD.

Conclusions

We have identified novel pathogenic variants in the MSUD‐related genes in the pedigree of the three patient's families. Our findings expand the mutational spectrum of MSUD and provide the scientific basis for genetic counseling for the patient's families.

A RHAG point mutation selectively disrupts Rh antigen expression

OBJECTIVES

The aim of this study was to characterise a novel mutation in the gene encoding RhAG in order to elucidate a molecular mechanism for Rh antigen expression and spherocytosis.

BACKGROUND

Rhesus-associated glycoprotein (RhAG) is critical for maintaining the structure and stability of erythrocytes. Single missense mutations in the gene encoding RhAG are sufficient to induce spherocytosis and deficiencies in Rh complex formation. We report a novel missense mutation that incompletely disrupts Rh antigen expression and selectively knocks out RhD antigen expression.

METHODS

Blood samples were taken from a 38-year-old male, his brother, his wife and his daughter in Xi'an, China. To detect the proband's RhAG and D antigen expression, the RBC were stained with anti-D and anti-RhAG and analysed by flow cytometry. Red blood cell morphology was detected with atomic force microscopy (AFM). Genomic DNA was isolated from whole blood samples, and the RHD, RHCE and RHAG alleles were sequenced and analysed. The mutation was mapped onto a predicted crystal structure of RhAG by the I-TASSER server and visualised using PyMOL.

RESULTS

Morphological testing by AFM found clear evidence of spherocytosis in the proband's erythrocytes. RHAG gene sequencing identified the mutation at sequence 236G > A, resulting in a serine to asparagine substitution at residue 79 (S79N). Family survey indicated that inheriting this allele is necessary and sufficient to cause the condition. Mapping the mutation onto a predicted crystal structure of RhAG revealed the proximity of the mutation to the critical structural elements of the protein.

CONCLUSIONS

A novel RHAG mutation significantly lowers RhAG antigen expression and antigen-mediated agglutination intensity.

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).

Autosomal recessive ichthyosis with hypotrichosis syndrome: further delineation of the phenotype

Autosomal recessive ichthyosis with hypotrichosis (ARIH) syndrome, which is characterized by congenital ichthyosis, abnormal hair and corneal involvement, has recently been shown in one consanguineous Israeli Arab family to be caused by a mutation in the ST14 gene, which encodes serine protease matriptase. No other families have so far been described since the original report. In this current report we describe a female patient from a second family with ARIH syndrome who carries a homozygous novel mutation, p.M1I. The patient has congenital ichthyosis, light brown, curly, sparse hair, improving with age, and sparse body hair, eyebrows and eyelashes. She does not suffer from photophobia, but has blepharitis. The phenotype of this patient closely resembles that of the affected individuals in the previously reported family, although she does not have tooth abnormalities and the ichthyosis is milder.

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.

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.

Evolutionarily conserved non-AUG translation initiation in NAT1/p97/DAP5 (EIF4G2)

Only a few cases of exclusive translation initiation at non-AUG codons have been reported. We recently demonstrated that mammalian NAT1 mRNA, encoded by EIF4G2, uses GUG as its only translation initiation codon. In this study, we identified NAT1 orthologs from chicken, Xenopus, and zebrafish and found that in all species, the GUG codon also serves as the initiation codon. In all species, the GUG codon fulfilled the reported requirements for non-AUG initiation: an optimal Kozak motif and a downstream hairpin structure. Site-directed mutagenesis showed that nucleotides at positions -3 and +4 are critical for the GUG-mediated translation initiation in vitro. We found that NAT1 orthologs in Drosophila melanogaster and Halocynthia roretzi also use non-AUG start codons, demonstrating evolutionary conservation of the noncanonical translation initiation.

Four novel mutations in the thiazide-sensitive Na-Cl co-transporter gene in Japanese patients with Gitelman's syndrome

BACKGROUND

Gitelman's syndrome (GS) is an autosomal recessive disorder resulting from inactivating mutations in the thiazide-sensitive Na-Cl co-transporter (NCCT) gene. To date, almost 90 mutations have been identified. It is possible that there is a population-specific distribution of mutations. In this study, we analysed mutations in the NCCT gene of seven Japanese patients with GS.

METHODS

Peripheral blood mononuclear cells were isolated from patients with GS, their family members and healthy control subjects. A mutation analysis of the NCCT gene was performed completely by direct automated sequencing of polymerase chain reaction-amplified DNA products. In patients with a deletion or splice site mutation, we undertook cDNA sequence analysis.

RESULTS

We identified nine mutations. Five of them [c.185C>T (Thr60Met), c.1712C>T (Ala569Val), c.1930C>T (Arg642Cys), c.2552T>A (Leu849His) and c.1932delC] have been reported in Japanese patients, but not in GS patients from other ethnic groups. The remaining four mutations [c.7A>T (Met1Leu), c.1181_1186+20del26, c.1811_1812delAT and IVS16+1G>A] were novel. In cDNA derived from a patient with c.1181_1186+20del26, a deletion of exon 9 and a frameshift at the start of exon 10 were observed. In cDNA derived from patients with IVS16+1G>A, an additional 96 bp insertion between exons 16 and 17 was observed. Six out of seven patients were compound heterozygotes, and the remaining one carried a single heterozygous mutation.

CONCLUSIONS

We found four novel mutations in the NCCT gene in seven Japanese patients with GS. Moreover, our study suggests that the distribution of mutations in the NCCT gene in Japanese GS patients potentially differs from that in other populations.

Pushing the limits of the scanning mechanism for initiation of translation

Selection of the translational initiation site in most eukaryotic mRNAs appears to occur via a scanning mechanism which predicts that proximity to the 5' end plays a dominant role in identifying the start codon. This "position effect" is seen in cases where a mutation creates an AUG codon upstream from the normal start site and translation shifts to the upstream site. The position effect is evident also in cases where a silent internal AUG codon is activated upon being relocated closer to the 5' end. Two mechanisms for escaping the first-AUG rule--reinitiation and context-dependent leaky scanning--enable downstream AUG codons to be accessed in some mRNAs. Although these mechanisms are not new, many new examples of their use have emerged. Via these escape pathways, the scanning mechanism operates even in extreme cases, such as a plant virus mRNA in which translation initiates from three start sites over a distance of 900 nt. This depends on careful structural arrangements, however, which are rarely present in cellular mRNAs. Understanding the rules for initiation of translation enables understanding of human diseases in which the expression of a critical gene is reduced by mutations that add upstream AUG codons or change the context around the AUG(START) codon. The opposite problem occurs in the case of hereditary thrombocythemia: translational efficiency is increased by mutations that remove or restructure a small upstream open reading frame in thrombopoietin mRNA, and the resulting overproduction of the cytokine causes the disease. This and other examples support the idea that 5' leader sequences are sometimes structured deliberately in a way that constrains scanning in order to prevent harmful overproduction of potent regulatory proteins. The accumulated evidence reveals how the scanning mechanism dictates the pattern of transcription--forcing production of monocistronic mRNAs--and the pattern of translation of eukaryotic cellular and viral genes.

RhCG is downregulated in oesophageal squamous cell carcinomas, but expressed in multiple squamous epithelia

To better understand the molecular events underlying the development of oesophageal cancer, we have isolated the genes dysregulated in primary oesophageal cancer tissues using a modified differential display polymerase chain reaction (DD-PCR). In the present study, a gene designated C15orf6 was identified. The C15orf6 gene, encompassing 25 kb, is composed of 11 exons with a mRNA of 1948 bp. Database searching showed that C15orf6 was 100% homologous to the Rh type C-glycoprotein (RhCG) with the same open reading frame, but 16 bp longer than RhCG at the 5'-end. The gene was highly expressed in human oesophagus, cervix, oral cavity, skin and kidney, but undetectable in the other 14 adult normal tissues examined. Northern blot, RT-PCR and western blot analysis showed that RhCG/C15orf6 was frequently lost or dramatically reduced in primary oesophageal cancer tissues (30/34) compared with the corresponding normal oesophageal mucosa. Three oesophageal-cancer cell lines tested lacked RhCG/C15orf6 expression. Immunohistochemistry revealed that in normal oesophageal tissues, RhCG/C15orf6 was mainly expressed in the plasma membrane of the epithelial cells. In addition, Rh-associated glycoprotein (RhAG) expression was also commonly silenced in both oesophageal cancer cell lines (2/3) and primary oesophageal cancer tissues (11/13). To our knowledge, this is the first time that RhAG expression has been seen in oesophageal epithelium and extends the functional role of the RhAG protein beyond the erythrocyte. These data suggest that inactivation of RhCG/C15orf6 and RhAG occurs frequently during the development of human oesophageal cancer.

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.

The Rh blood group system: a review

The Rh blood group system is one of the most polymorphic and immunogenic systems known in humans. In the past decade, intense investigation has yielded considerable knowledge of the molecular background of this system. The genes encoding 2 distinct Rh proteins that carry C or c together with either E or e antigens, and the D antigen, have been cloned, and the molecular bases of many of the antigens and of the phenotypes have been determined. A related protein, the Rh glycoprotein is essential for assembly of the Rh protein complex in the erythrocyte membrane and for expression of Rh antigens. The purpose of this review is to provide an overview of several aspects of the Rh blood group system, including the confusing terminology, progress in molecular understanding, and how this developing knowledge can be used in the clinical setting. Extensive documentation is provided to enable the interested reader to obtain further information.

Structure of the SLC7A7 gene and mutational analysis of patients affected by lysinuric protein intolerance

Lysinuric protein intolerance (LPI) is a rare autosomal recessive defect of cationic amino acid transport caused by mutations in the SLC7A7 gene. We report the genomic structure of the gene and the results of the mutational analysis in Italian, Tunisian, and Japanese patients. The SLC7A7 gene consists of 10 exons; sequences of all of the exon-intron boundaries are reported here. All of the mutant alleles were characterized and eight novel mutations were detected, including two missense mutations, 242A-->C (M1L) and 1399C-->A (S386R); a nonsense mutation 967G-->A (W242X); two splice mutations IVS3 +1G-->A and IVS6 +1G-->T; a single-base insertion, 786insT; and two 4-bp deletions, 455delCTCT and 1425delTTCT. In addition, a previously reported mutation, 1625insATCA, was found in one patient. It is noteworthy that 242A-->C causes the change of Met1 to Leu, a rare mutational event previously found in a few inherited conditions. We failed to establish a genotype/phenotype correlation. In fact, both intrafamilial and interfamilial phenotypic variability were observed in homozygotes for the same mutation. The DNA-based tests are now easily accessible for molecular diagnosis, genetic counseling, and prenatal diagnosis of LPI.

RH blood group system and molecular basis of Rh-deficiency

Rhesus (Rh) antigens are defined by a complex association of membrane polypeptides that are missing or severely deficient from the red cells of rare Rhnull individuals who suffer a clinical syndrome of varying severity characterized by abnormalities of the red cell shape, cation transport and membrane phospholipid organization. The Rhnull phenotype is an inherited condition that may arise from homozygosity either for a 'suppressor' gene unrelated to the RH locus ('regulator type') or for a silent allele at the RH locus itself ('amorph type'). A current model suggests that the proteins of the Rh complex (Rh, RhAG, CD47, LW, GPB) are assembled by non-covalent bonds and that it is not assembled or transported to the cell surface when one subunit is missing. Rh and RhAG proteins belong to the same protein family and are quantitatively the major components that form the core of the complex, which is firmly linked to the membrane skeleton. Molecular analysis of Rhnull individuals has revealed that abnormalities occur only at the RHAG and RH loci, without alteration of the genes encoding the accessory chains. Mutations of the RHAG gene, but not of RH, occur in all Rhnull individuals of the regulator type (including Rhmod) investigated so far (13 cases), strongly suggesting that RHAG mutants act as 'suppressors' and not as transcriptional regulators of the RH genes and that variable expression of the RHAG alleles may account for the Rhmod phenotypes (exhibiting weak expression of Rh antigens). Conversely, mutations of the RHCE gene, but not of RHAG, occur in two unrelated Rhnull individuals of the amorph type, supporting the view that RH mutants result from a 'silent' allele at the RH locus. These findings strongly support the Rh complex model since when either the Rh or RhAG protein is missing, the assembly and/or transport of the Rh complex is defective. Transcriptional as well as post-transcriptional mechanisms may account for the molecular abnormalities, but experimental evidence based on expression models is required to test these hypotheses, in the hope that they may help to clarify the biological role of the Rh proteins in the red cell membrane.