Multiple isoforms excluding normal RhD mRNA detected in Rh blood group Del phenotype with RHD 1227A allele

Frequency of the 'Asia type' DEL with weak D phenotype in chinese

BACKGROUND

DEL individuals account for 9-30% of serological RhD negative population in east Asia and majority of them carrying the RHD*DEL1 allele are referred to as 'Asia type' DEL individuals. There is a lack of data on the molecular basis for 'Asia type' DELs with weak RhD phenotype. Therefore, the aim of this study is to unveil 'Asia type' DELs by elucidating the genetic background and analyzing the serological results.

METHODS

With a microplate typing protocol, RhD characterization was performed in samples from one million blood donors collected at Chengdu blood center during the period from 2019 to 2022. RhD confirmatory test was performed by direct antiglobulin test and indirect antiglobulin test with five anti-D reagents to detect RhD variants. Molecular characterization of samples categorized as RhD variants was studied by using direct genomic DNA sequencing and RHD zygosity analysis, followed by adsorption and elution tests conduced for samples carrying RHD*DEL1 allele to confirm the presence of RhD antigens on the red cells.

RESULTS

We reported here 21 RhD variant samples were detected by micro-column gel agglutination assay with IgG anti-D antibodies. Moreover, the agglutination reaction was stronger with IgG anti-D reagents in micro-column gel card than with IgM/IgG blended anti-D antibodies. Each of the 21 samples carried the RHD*DEL1 allele, which indicated that they were 'Asia type' DEL. Of the 21 'Asia type' DEL samples, 9 samples were detected to be RHD+/ RHD + homozygotes, whereas the other 12 samples were RHD+/RHD- hemizygotes. Among the samples phenotyped for RhCE, seven were CCee and four were Ccee.

CONCLUSIONS

In this study, DEL samples carrying RHD*DEL1 showed weak RhD phenotype with some anti-D reagents in RhD confirmatory test, which suggest that a serology strategy using several anti-D reagents may be helpful to detect this 'Asia type' DEL. Further studies are needed to elucidate whether the 'Asia type' DELs with weak RhD phenotype have stronger antigenicity and could cause serious transfusion reaction.

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

BACKGROUND

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

STUDY DESIGN AND METHODS

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

RESULTS

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

CONCLUSION

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

Genotyping of RHD c.1227G>A allele by melting curve analysis

BACKGROUND

DEL is the weakest known D-positive phenotype and is detectable only by adsorption and elution tests. RHD c.1227G>A is an important marker for DEL phenotype in East Asians. The aim of this study was to develop a method for RHD c.1227G>A genotyping by single-tube PCR with melting curve analysis.

METHODS

Two GC-rich tails of different lengths were attached to the 5'-end of allele-specific primers for RHD 1227G and 1227A alleles, such that RHD c.1227G>A could be distinguished by the melting temperature. A total of 145 D-negative Chinese Han blood donors were genotyped for RHD c.1227G>A by melting curve analysis, conventional polymerase chain reaction with sequence-specific primers (PCR-SSP), and sequencing.

RESULTS

In 143 subjects (143/145, 98.6%), PCR-SSP and melting curve analysis produced consistent results with RHD exon 9 sequencings. Two samples were genotyped as RHD 1227G/A by PCR-SSP, but as RHD 1227A/A or A/- by melting curve analysis. These two samples were confirmed to be RHD 1227A/A or A/-. Based on RHD exon 9 sequencing, the accuracy, sensitivity, specificity, positive predictive value, and negative predictive value of the melting curve analysis for detecting both RHD 1227A and 1227G were all 100%. In contrast, the accuracy, specificity and positive predictive value of PCR-SSP for RHD 1227G detection were 98.62%, 98.21% and 94.29%, respectively, which were lower than those observed with the melting curve analysis.

CONCLUSION

Melting curve analysis for RHD c.1227G>A genotyping is a simple, rapid, and reliable method, superior to conventional PCR-SSP.

A DEL phenotype attributed to RHD Exon 9 sequence deletion: slipped-strand mispairing and blood group polymorphisms

BACKGROUND

The RhD blood group antigen is extremely polymorphic and the DEL phenotype represents one such class of polymorphisms. The DEL phenotype prevalent in East Asian populations arises from a synonymous substitution defined as RHD*1227A. However, initially, based on genomic and cDNA studies, the genetic basis for a DEL phenotype in Taiwan was attributed to a deletion of RHD Exon 9 that was never verified at the genomic level by any other independent group. Here we investigate the genetic basis for a Caucasian donor with a DEL partial D phenotype and compare the genomic findings to those initial molecular studies.

STUDY DESIGN AND METHODS

The 3'-region of the RHD gene was amplified by long-range polymerase chain reaction (PCR) for massively parallel sequencing. Primers were designed to encompass a deletion, flanking Exon 9, by standard PCR for Sanger sequencing. Targeted sequencing of exons and flanking introns was also performed.

RESULTS

Genomic DNA exhibited a 1012-bp deletion spanning from Intron 8, across Exon 9 into Intron 9. The deletion breakpoints occurred between two 25-bp repeat motifs flanking Exon 9 such that one repeat sequence remained.

CONCLUSION

Deletion mutations bordered by repeat sequences are a hallmark of slipped-strand mispairing (SSM) event. We propose this genetic mechanism generated the germline deletion in the Caucasian donor. Extensive studies show that the RHD*1227A is the most prevalent DEL allele in East Asian populations and may have confounded the initial molecular studies. Review of the literature revealed that the SSM model explains some of the extreme polymorphisms observed in the clinically significant RhD blood group antigen.

The synonymous nucleotide substitution RHD 1056C>G alters mRNA splicing associated with serologically weak D phenotype

BACKGROUND

D antigen is one of the most clinically significant blood group antigens. Variation of the RHD gene can cause weak D or partial D phenotypes. While most variations are missense substitutions with amino acid changes, those without are called "silent" or "synonymous" substitutions. Synonymous substitutions often have little effect on the protein, not altering the phenotype. However, effect on splicing can affect end-product protein. We report a new synonymous variation, RHD 1056C>G, that resulted in weak D phenotype, and predicted its effect with various in silico methods.

METHODS

Serologic testing of the D antigen with full sequencing of the RHD gene was done. Human Splice Finder was used to predict the effect of this variation, and validation of this method was done with all known RHD variations reported in the literature.

RESULTS

RHD 1056C>G was predicted to cause the formation of an exonic splicing silencer (ESS) site. The creation of new ESS site potentially inhibits the splicing event, resulting alteration of splicing. This is similar to remodeling of splice acceptor or donor site, as this kind of deep exonic variation could affect the D antigen's quality or quantity. This is in concordance with serologic results, which showed only delayed weak agglutination to anti-D reagents.

CONCLUSIONS

The analytic methods we applied showed good correlation with the actual phenotype, along with concordant results when analyzing other known variants reported in the literature. We conclude that RHD 1056C>G results in serologic weak D phenotype.

Prospective Evaluation of a Transfusion Policy of RhD-Positive Red Blood Cells into DEL Patients in China

BACKGROUND

The D antigen is highly immunogenic, requiring only a small quantity of transfused red blood cells (RBCs) to cause alloimmunization in D- immunocompetent recipients. DEL was reported arousing alloimmunization to true Rh- patients. Molecular studies of the RHD gene have revealed that DEL individuals retain a grossly intact RHD gene or have a portion of RHD in their genomes. Avoiding immunization with clinically important antibodies is a primary objective in transfusion medicine.

METHODS

In order to determine whether pregnant DEL women carrying an RhD+ fetus are at risk of anti-D alloimmunization, 808 Rh- pregnant women with a history of gestations or parturitions who regularly visited hospitals for their prenatal anti-D screening and postpartum care from January 2011 to December 2012 were investigated. Samples were analyzed for DEL by PCR with specific primers, PCR-sequence-specific primers (PCR-SSP), reverse transcription-PCR (RT-PCR), PCRrestriction fragment length polymorphism (PCR-RFLP), and by gene sequencing to characterize different alleles.

RESULTS

Among the 808 Rh- pregnant women of our sample, 178 (22.0%) were typed as DEL; 168 DEL samples were confirmed to have the RHD (1,227 G>A) allele, 8 DEL samples were characterized by one base mutation of the RHD (3G >A) allele, and the remaining two DEL samples were determined to carry RHD-CE(4-9)-D or RHD-CE(2-5)-D. The observation of allo-anti-D in two prominent D epitope loss cases confirmed the partial nature of these DEL phenotypes.

CONCLUSIONS

In conclusion, evidence is provided that different DEL genotypes code either for partial or complete D antigen expression. It is suggested that the use of RhD+ RBCs in complete D antigen DEL patients does not induce adverse reaction.

Analysis of density and epitopes of D antigen on the surface of erythrocytes from DEL phenotypic individuals carrying the RHD1227A allele

BACKGROUND

The characteristics of the D antigen are important as they influence the immunogenicity of D variant cells. Several studies on antigenic sites have been reported in normal D positive, weak D and partial D cases, including a comprehensive analysis of DEL types in Caucasians. The aim of this study was to assess D antigen density and epitopes on the erythrocyte surface of Asian type DEL phenotypic individuals carrying the RHD1227A allele in the Chinese population.

MATERIALS AND METHODS

A total of 154 DEL phenotypic individuals carrying the RHD1227A allele were identified through adsorption and elution tests and polymerase chain reaction analysis with sequence-specific primers in the Chinese population. D antigen density on the erythrocyte surface of these individuals was detected using a flow cytometric method. An erythrocyte sample with known D antigen density was used as a standard. Blood samples from D-negative and D-positive individuals were used as controls. In addition, D antigen epitopes on the erythrocyte surface of DEL individuals carrying the RHD1227A allele were investigated with 18 monoclonal anti-D antibodies specific for different D antigen epitopes.

RESULTS

The means of the median fluorescence intensity of D antigen on the erythrocyte membrane surface of D-negative, D-positive and DEL individuals were 2.14±0.25, 193.61±11.43 and 2.45±0.82, respectively. The DEL samples were estimated to have approximately 22 D antigens per cell. The samples from all 154 DEL individuals reacted positively with 18 monoclonal anti-D antibodies specific for different D antigen epitopes.

DISCUSSION

In this study, D antigen density on the erythrocyte surface of DEL individuals carrying the RHD1227A allele was extremely low, there being only very few antigenic molecules per cell, but the D antigen epitopes were grossly complete.

RHD PCR of D-Negative Blood Donors

SUMMARY

RHD PCR of blood donors may be used to reveal weak D, partial D, DEL and chimeric D+/D- donors among presumed D-negative blood donors. Units donated by such donors pose a definite yet low risk for anti-D immunization of transfusion recipients. The frequency of DEL donors among D-negative donors is 1:350 to 1:2,000 in Europe and up to 1:5 in Asian countries. Different strategies for RHD PCR of blood donors have been used. Probably, the most cost-efficient implementation is replacement of sensitive D antigen testing with the indirect antiglobulin test by RHD PCR in pools which might even reduce total testing cost.

An investigation of secondary anti-D immunisation among phenotypically RhD-negative individuals in the Chinese population

BACKGROUND

Despite the introduction of anti-D prophylaxis into clinical practice, RhD alloimmunisation remains a problem, particularly in the context of transfusions and pregnancy-induced alloimmunisation. The incidence of RhD alloimmunisation among phenotypically RhD-negative individuals is unknown in most countries. We investigated RhD alloimmmunisation in RhD-negative pregnant women and transfusion recipients in south-east China in order to optimise the prevention of this phenomenon.

METHODS

We analysed the RhD alloimmunisation status of RhD-negative pregnant women and transfusion recipients in south-east China. The RhD blood types of the study population were identified by standard serological methods. The D antigen was further tested with the indirect antiglobulin test to exclude or confirm weak D or partial D types. RhC, c, E and e antigens were typed in all subjects. If anti-D antibody screening was positive, the specificity and titre of the antibody were determined. The Del phenotype was investigated by the polymerase chain reaction sequence-specific primer method.

RESULTS

An anti-D antibody was found in 61 of 416 RhD-negative pregnant women (14.66%), and in 11 of 227 RhD-negative transfusion recipients (4.85%). None of the 72 RhD-negative pregnant women or transfusion recipients with anti-D had the Del phenotype. Anti-D antibodies were not detected among Del phenotype individuals and Del phenotypes were not found in anti-D antibody producing individuals.

DISCUSSION

Our study suggests that the risk of alloimmunity-induced neonatal haemolysis increases in true RhD-negative multipara. Perinatal protection would be necessary in these patients, while antenatal anti-D testing and Rh immune globulin prophylaxis would be unnecessary for RhDel pregnant women. Pregnant women and transfusion recipients with the Del type seldom produce anti-D antibody. RhD-negative recipients are not at risk of alloimmunisation after transfusion with Del red blood cells.

Molecular basis of blood group expression

Antigen diversity arises from changes at the gene level that range from single nucleotide polymorphisms (SNPs) to intra- and inter-genic exchanges, inversions, insertions, and deletions. Nucleotide changes often result in amino acid difference from the wild-type gene product and with those changes new blood group antigens arise. Alternatively, there is loss of expression altogether, which is deemed the 'null' phenotype. Near complete knowledge of the genetic changes underlying the expression of blood group antigens will lead to the reality that red cell genotyping as a test-of-record. The importance of molecular testing in immunohematology necessitates appropriate training and competency programs to ensure that the highly skilled staff has the appropriate knowledge background. This review summarizes the core mechanisms for gene expression and provides a compilation of the molecular basis for blood group expression.

Aberrant RNA splicing in RHD 7-9 exons of DEL individuals in Taiwan: a mechanism study

BACKGROUND

The Rh blood D group provides a clinically important model of aberrant splicing with skipped exons. Approximately 30% of serologically D-negative Chinese individuals have an intact RHD gene (DEL phenotype) and induce allo-immunization in transfusions. The RHD1227GNA polymorphism occurs in >95% DEL phenotype of Asian descent. The effects of RHD 1227A and a novel allele on exon 9 splicing were examined.

RESULTS

Amplified DEL RNA products revealed that 3 transcripts involved skipping of exons 8-9, exon 9, or exon 9 with an inserted 170-bp cryptic exon located between exons 7 and 8. A novel, single nucleotide polymorphism was identified in the 7th intron, (IVS7) 923C>T, and present in all DEL patients. The odds ratio of RHD1227G>A allele with DEL phenotype was 2711. Splicing analysis of transcripts from minigenes containing the 1227GNA allele, but not the (IVS7) 923C>T allele, demonstrated aberrant exon 9 skipping.

CONCLUSIONS

A combined haplotype of 1227G>A and IVS7 923C>T alleles was apparent in >95% DEL Chinese individuals. RHD1227A mutation significantly increased aberrant mRNA splicing, producing a hybrid RHD mRNA lacking exon 9. These results provide a molecular basis of the DEL phenotype in the Chinese population.

Sequencing analysis of RHD intron 7 and 9

Whole length of RHD introns 7 and 9 of one normal Rh D-positive individual and 2 DEL samples, carrying RHD1227A allele, were sequenced and aligned. Thirty-three and 27 nucleotide variants were totally observed in intron 7 and intron 9, respectively (EMBL/GenBank/DDBJ EU372940 approximately 2). Among them, 8 variants in intron 7 and 7 in intron 9 were observed commonly in all 3 samples, whereas 2 variants in intron 7 and one in intron 9 were only found in 2 DEL samples, but not in the normal D-positive. The variants observed in intron 7 in DEL cannot explain enough for that DEL mRNA has one segment of 170 base pairs sequence from intron 7. But the nucleotides AG-GT at both sides of the segment may be related to this molecular even as AG-GT may cut intron 7 with its normal splicing site (GT-AG) into two "new introns" although the mechanisms are complicated in fact. We also have not found any suspicious splicing-affecting variants in intron 9 of DEL allele. However, this may make out further that the reason of whole exon 9 spliced out in DEL mRNAs may be no more than the 1227A>G mutation in DEL allele.