Evaluation of Single Nucleotide Variants in Intron 1 of the ABO Gene as Diagnostic Markers for the A1 Blood Group

Introduction

The molecular diagnosis of the A1 blood group is based on the exclusion of ABO gene variants causing blood groups A2, B, or O. A specific genetic marker for the A1 blood group is still missing. Recently, long-read ABO sequencing revealed four sequence variations in intron 1 as promising markers for the ABO*A1 allele. Here, we evaluated the diagnostic values of the 4 variants in blood donors with regular and weak A phenotypes and genotypes.

Methods

ABO phenotype data (A, B, AB, or O) were taken from the blood donor files. The ABO genotypes (low resolution) were known from a previous study and included the variants c.261delG, c.802G>A, c.803G>C, and c.1061delC. ABO variant alleles (ABO*AW.06,*AW.08,*AW.09,*AW.13, *AW.30, and *A3.02) were identified in weak A donors by sequencing the ABO exons before. For genotyping of the ABO intron 1 variants rs532436, rs1554760445, rs507666, and rs2519093, we applied TaqMan assays with endpoint fluorescence detection according to a standard protocol. Genotypes of the variants were compared with the ABO phenotype and genotype. Evaluation of diagnostic performance included sensitivity, specificity, positive (PPV), and negative predictive value (NPV).

Results

In 1,330 blood donors with regular ABO phenotypes and genotypes, the intron 1 variants were significantly associated with the proposed A1 blood group. In 15 donors, we found discrepancies to the genotype of at least one of the 4 variants. For the diagnosis of the ABO*A1 allele, the variants showed 98.79-99.48% sensitivity, 99.66-99.81% specificity, 98.80-99.31% PPV, and 99.66-99.86% NPV. Regarding the A phenotype, the diagnostic values were 99.02-99.41% sensitivity, 99.63-99.76% specificity, 99.41-99.61% PPV, and 99.39-99.63% NPV. The *A1 marker allele of all intron 1 variants was also associated with the *AW.06, *AW.13, and *AW.30 variants. Samples with *AW.08, *AW.09, and *A3.02 variants lacked this association.

Conclusion

The ABO intron 1 variants revealed significant association with the ABO*A1 allele and the A phenotype. However, the intron 1 genotype does not exclude variant alleles causing weak A phenotypes. With the introduction of reliable tag, single nucleotide variants for the A1, A2, B, and O blood groups and the genotyping instead of phenotyping of the ABO blood group are getting more feasible on a routine basis.

Increased plasma level of soluble P-selectin in non-hospitalized COVID-19 convalescent donors

Background

The coronavirus disease-2019 (COVID-19) is a systemic disease with severe implications on the vascular and coagulation system. A procoagulant platelet phenotype has been reported at least in the acute disease phase. Soluble P-selectin (sP-sel) in the plasma is a surrogate biomarker of platelet activation. Increased plasma levels of sP-sel have been reported in hospitalized COVID-19 patients associated with disease severity. Here, we evaluated in a longitudinal study the sP-sel plasma concentration in blood donors who previously suffered from moderate COVID-19.

Methods

154 COVID-19 convalescent and 111 non-infected control donors were recruited for plasma donation and for participation in the CORE research trial. First donation (T1) was performed 43–378 days after COVID-19 diagnosis. From most of the donors the second (T2) plasma donation including blood sampling was obtained after a time period of 21–74 days and the third (T3) donation after additional 22–78 days. Baseline characteristics including COVID-19 symptoms of the donors were recorded based on a questionnaire. Platelet function was measured at T1 by flow cytometry and light transmission aggregometry in a representative subgroup of 25 COVID-19 convalescent and 28 control donors. The sP-sel plasma concentration was determined in a total of 704 samples by using a commercial ELISA.

Results

In vitro platelet function was comparable in COVID-19 convalescent and control donors at T1. Plasma samples from COVID-19 convalescent donors revealed a significantly higher sP-sel level compared to controls at T1 (1.05 ± 0.42 ng/mL vs. 0.81 ± 0.30 ng/mL; p < 0.0001) and T2 (0.96 ± 0.39 ng/mL vs. 0.83 ± 0.38 ng/mL; p = 0.0098). At T3 the sP-sel plasma level was comparable in both study groups. Most of the COVID-19 convalescent donors showed a continuous decrease of sP-sel from T1 to T3.

Conclusion

Increased sP-sel plasma concentration as a marker for platelet or endothelial activation could be demonstrated even weeks after moderate COVID-19, whereas, in vitro platelet function was comparable with non-infected controls. We conclude that COVID-19 and additional individual factors could lead to an increase of the sP-sel plasma level.

Introduction of Noninvasive Prenatal Testing for Blood Group and Platelet Antigens from Cell-Free Plasma DNA Using Digital PCR

Background

Noninvasive prenatal testing (NIPT) for fetal antigens is a common standard for targeted immune prophylaxis in RhD-mediated hemolytic disease of the fetus and newborn, and is most frequently done by quantitative PCR (qPCR). A similar approach is considered for other blood group and human platelet alloantigens (HPA). Because of a higher sensitivity compared to qPCR for rare molecule detection, we established and validated digital PCR (dPCR) assays for the detection of RHD exons 3, 5 and 7, KEL1, HPA-1a, and HPA-5b from cell-free DNA (cfDNA) in plasma. The dPCR assays for the Y-chromosomal marker amelogenin and autosomal SNPs were implemented as controls for the proof of fetal DNA.

Methods

Validation was performed on dilution series of mixed plasma samples from volunteer donors with known genotypes. After preamplification of the target loci, two-color (FAM and VIC) TaqMan^TM probe chemistry and chip-based dPCR were applied. The assays for RHD included GAPDH as an internal control. For the diallelic markers KEL1/2, HPA-1a/b, HPA-5a/b, and AMEL-X/Y and 3 autosomal SNPs, the probes enabled allelic discrimination in the two fluorescence channels. The dPCR protocol for NIPT was applied to plasma samples from pregnant women.

Results

The RHD exon 5 assay allowed the detection of a 0.05% RHD target in an RhD-negative background, whereas the exon 7 assay required at least a 0.25% target. The exon 3 assay showed the highest background and required at least a 2.5% RHD target for reliable detection. The dPCR assays for the diallelic markers revealed similar sensitivity and enabled the detection of at least a 0.5% target allele. The HPA-1a assay was the most sensitive and allowed target detection in plasma mixtures containing only 0.05% HPA-1a. The plasma samples from 13 pregnant women at different gestational ages showed unambiguous positive and negative results for the analyzed targets.

Conclusion

Analysis of cfDNA from maternal plasma using dPCR is suitable for the detection of fetal alleles. Because of the high sensitivity of the assays, the NIPT protocol for RhD, KEL1, and HPA can also be applied to earlier stages of pregnancy.

Evaluation of Diagnostic Tests by Evanescence Biosensor Technology for Rapid Phenotyping of the Human Platelet Alloantigens 1a and 5b

Background

The human platelet alloantigens (HPA) HPA-1a and HPA-5b are located on glycoproteins on the platelet surface and are the most relevant to cause neonatal alloimmune thrombocytopenia (NAIT). The antigens are defined by single nucleotide polymorphisms (SNPs) in the glycoprotein genes, and the antigen status can be determined by genotyping the SNPs. However, genotyping is time-consuming and costly depending on the method and sample throughput. Here, we tested the reliability of the evanescence wave based fluorescence (EVA) biosensor technology for the rapid phenotyping of the HPA-1a and HPA-5b antigens on blood donor samples in two laboratories.

Methods

HPA-1a and HPA-5b phenotyping was performed on EDTA blood samples from 336 blood donors (Lyon: 216 donors; Mannheim: 120 donors) using EVA typing assays and the biosensor system (Davos Diagnostics, Davos, Switzerland). For genotyping, validated PCR-SSP and TaqMan-PCR methods were used.

Results

HPA-1a phenotyping was positive for all samples with HPA-1aa (n = 244; EVA value 807 ± 167 U/s) and HPA-1ab (n = 82; 542 ± 110 U/s) genotypes. All samples (n = 10) with negative EVA values (<10 U/s) had the HPA-1bb genotype. HPA-5b phenotyping was negative for all HPA-5aa genotypes (n = 267) and positive for the HPA-5ab (n = 66; 83 ± 22 U/s) and HPA-5bb (n = 3; 118 ± 25 U/s) genotypes. EVA values from heterozygotes were significantly lower compared to HPA-1a or HPA-5b homozygotes. A strong correlation of the EVA values with the platelet count in the blood samples was observed.

Conclusion

EVA is a reliable method for rapid phenotyping of the clinically relevant HPA-1a and HPA-5b platelet antigens. All phenotyping results were 100% concordant with the HPA-1 or HPA-5 genotype. The test can be performed from only 10 µl of fresh or frozen blood samples within less than 15 min time-to-result.

Towards a Regional Registry of Extended Typed Blood Donors: Molecular Typing for Blood Group, Platelet and Granulocyte Antigens

BACKGROUND

The provision of compatible blood products to patients is the most essential task of transfusion medicine. Besides ABO and Rh, a number of additional blood group antigens often have to be considered for the blood supply of immunized or chronically transfused patients. It also applies for platelet antigens (HPA) and neutrophil antigens (HNA) for patients receiving platelet or granulocyte concentrates. Here, we describe the molecular screening for a number of blood group, HPA, and HNA alleles. Based on the screening results we are building up a regional blood donor registry to provide extended matched blood products on demand.

METHODS

We developed and validated TaqMan™ PCR and PCR-SSP methods for genetic markers defining 37 clinically relevant blood group antigens (beyond ABO and Rh), 10 HPA, and 11 HNA. Furthermore, we describe a feasible method for fast molecular screening of the HNA-2null phenotype. All data were statistically evaluated regarding genotype distribution. Allele frequencies were compared to ExAC data from non-Finnish Europeans.

RESULTS

Up to now more than 2,000 non-selected regular blood donors in south-west Germany have been screened for blood group, HPA, and HNA alleles. The screening results were confirmed by serology and PCR-SSP methods for selected numbers of samples. The allele frequencies were similar to non-finnish Europeans in the ExAC database except for the alleles encoding the S, HPA-3b and HNA-4b antigens, with significantly lower prevalence in our cohort, as well as the LU14 and the HNA-3b antigens, with a higher frequency compared to the ExAC data. We identified 71 donors with rare blood groups such as Lu(a+b-), Kp(a+b-), Fy(a-b-) and Vel-, and 169 donors with less prevalent HPA or HNA types.

CONCLUSION

Molecular screening for blood group alleles by using TaqMan™ PCR is an effective and reliable high-throughput method for establishing a rare donor registry.

The Impact of Using Genotyped Reagent Red Blood Cells in Antibody Identification

Background

The detection and identification of antibodies to red blood cell (RBC) antigens is one of the most important and challenging issues in transfusion medicine. Up to date there are 354 RBC antigens recognized by the International Society of Blood Transfusion (ISBT). The reagent RBCs used in commercial antibody screening and identification panels however are usually serologically typed for up to 40 clinically important antigens. Thus the identification of many antibody specificities remains impossible when using reagent RBCs with only limited information about their antigens. To improve the pre-transfusion diagnostics, we developed antibody identification panels with reagent RBCs serologically typed for 26 antigens and additionally genotyped for 30 blood group alleles.

Methods

The reagent RBCs in the panels were characterized serologically for the clinically most significant 'standard' antigens. The reagent RBC donors were additionally genotyped by using in-house PCR-SSP methods. The antibody identification was performed in the indirect antiglobulin test using untreated and papain-treated RBCs in the gel technique. Antibodies identified due to the genotype information were confirmed by serology using appropriate reference RBCs.

Results

Within a time period of 3 years and 8 months, 16,878 blood samples from 8,467 patients were tested in our reference laboratory. In total, 21 different antibodies from 10 different blood group systems could be identified in 126 patients (1.5%) due to the genotype information obtained for the reagent RBCs. Antibodies to antigens from the Knops system (53 patients; 42%, 8 patients with anti-Kn^b) and to Cartwright antigens (31 patients; 25%) were the most frequent.

Conclusion

The use of genotyped reagent RBCs in antibody identification panels extends the range of detectable antibody specificities, accelerates the antibody identification, and improves the pre-transfusion diagnostics.

Molecular Screening for Vel- Blood Donors in Southwestern Germany

BACKGROUND

The SMIM1 protein carries the Vel blood group antigen, and homozygosity for a 17 bp deletion in the coding region of the SMIM1 gene represents the molecular basis of the Vel- blood group phenotype. We developed PCR-based methods for typing the SMIM1 17 bp (64-80del) gene deletion and performed a molecular screening for the Vel- blood type in German blood donors.

METHODS

For SMIM1 genotyping, TaqMan-PCR and PCR-SSP methods were developed and validated using reference samples. Both methods were used for screening of donors with blood group O from southwestern Germany. Heterozygotes and homozygotes for the SMIM1 64-80del allele were serologically typed for the Vel blood group antigen. In addition, the rs1175550 SNP in SMIM1 was typed and correlated to the results of the phenotyping.

RESULTS

Both genotyping methods, TaqMan-PCR and PCR-SSP, represent reliable methods for the detection of the SMIM1 64-80del allele. Screening of 10,598 blood group O donors revealed 5 individuals homozygous for the deletional allele. They were confirmed Vel- by serological typing. Heterozygotes for the 64-80del allele showed different antigen expressions ranging from very weak to regular positive.

CONCLUSION

Molecular screening of blood donors for the Vel- blood type is feasible and avoids the limitations of serological typing which might show false-negative results with heterozygous individuals. The identification of Vel- blood donors significantly contributes to the adequate blood supply of patients with anti-Vel.

A single-tube real-time PCR assay for Mycoplasma detection as a routine quality control of cell therapeutics

BACKGROUND

Contamination of cell culture and biological material by mollicute species is an important safety issue and requires testing. We have developed a singletube real-time polymerase chain reaction (PCR) assay for rapid detection of Mollicutes species stipulated by the European Pharmacopeia.

METHODS

Primers and TaqMan probes (FAM-labeled) were deduced from 16S rDNA sequence alignment of 18 mollicutes species. A synthetic internal control (IC) DNA and an IC-specific TaqMan probe (VIC-labeled) were included. The analytical sensitivity of the assay was determined on DNA dilutions from 12 mollicute strains. Specificity was proven by the use of DNA from other bacteria.

RESULTS

Analytical sensitivities of the PCR assay were in the range of 405-2,431 genomes/ml for 11 of the 12 tested mollicute DNA samples. The lowest sensitivity was found for Ureaplasma urealyticum (19,239 genomes/ml). Negative results for DNA samples from 3 different ubiquitous bacteria demonstrated the specificity of the PCR assay for Mollicutes. Direct testing of cell culture supernatants spiked with Mycoplasma orale revealed similar sensitivity compared to isolated DNA.

CONCLUSIONS

Our single-tube real-time PCR assay with internal reaction control enables rapid and specific detection of mollicute contaminants. The test protocol is suitable for routine quality control of cell therapeutics.