Genotyping approach for non-invasive foetal RHD detection in an admixed population

BACKGROUND

Non-invasive foetal RHD genotyping can predict haemolytic disease of the foetus and the newborn in pregnancies with anti-D alloantibodies and also avoid antenatal anti-D prophylaxis in pregnant women carrying an RHD negative foetus. Considering that the Argentine genetic background is the result of generations of intermixing between several ethnic groups, we evaluated the diagnostic performance of a non-invasive foetal RHD determination strategy to guide targeted antenatal RhD immunoprophylaxis. This algorithm is based on the analysis of four regions of the RHD gene in cell-free foetal DNA in maternal plasma and maternal and paternal RHD genotyping.

MATERIALS AND METHODS

DNA from 298 serologically D negative pregnant women between 19-28 weeks gestation were RHD genotyped. Foetal RHD status was determined by real-time PCR in 296 maternal plasma samples. In particular cases, RHDΨ and RHD-CE-D^s alleles were investigated in paternal DNA. Umbilical cord blood was collected at birth, and serological and molecular studies were performed.

RESULTS

Of the 298 maternal samples, 288 were D-/RHD- and 10 D-/RHD+ (2 RHD*DAR; 5 RHD-CE-D^s; 3 RHDΨ). Plasma from RHD*DAR carriers was not analysed. Real-time PCR showed 210 RHD+ and 78 RHD- foetuses and 8 inconclusive results. In this latter group, paternal molecular studies were useful to report a RHD negative status in 5 foetuses while only 3 remained inconclusive. All the results, except one false positive due to a silent allele (RHD[581insG]), agreed with the neonatal typing performed in cord blood.

DISCUSSION

The protocol used for non-invasive prenatal RHD genotyping proved to be suitable to determine foetal RHD status in our admixed population. The knowledge of the genetic background of the population under study and maternal and paternal molecular analysis can reduce the number of inconclusive results when investigating foetal RHD status.

Distribution of the FYBES and RHCE*ce(733C>G) alleles in an Argentinean population: implications for transfusion medicine

BACKGROUND

The understanding of the molecular bases of blood groups makes possible the identification of red cell antigens and antibodies using molecular approaches, especially when haemagglutination is of limited value. The practical application of DNA typing requires the analysis of the polymorphism and allele distribution of the blood group genes under study since genetic variability was observed among different ethnic groups. Urban populations of Argentina are assumed to have a white Caucasian European genetic component. However, historical and biological data account for the influence of other ethnic groups. In this work we analyse FY and RH blood group alleles attributed to Africans and that could have clinical implications in the immune destruction of erythrocytes.

METHODS

We studied 103 white trios (father, mother and child, 309 samples) from the city of Rosario by allele specific PCRs and serological methods. The data obtained were analysed with the appropriate statistical test considering only fathers and mothers (n = 206).

RESULTS

We found the presence of the FY*BES and RHCE*ce(733C>G) alleles and an elevated frequency (0.0583) for the Dce haplotype. The number of individuals with a concomitant occurrence of both alleles was significantly higher than that expected by chance. We found that 4.68% of the present gene pool is composed by alleles primarily associated with African ancestry and about 10% of the individuals carried at least one RH or FY allele that is predominantly observed among African populations. Thirteen percent of Fy(b-) subjects were FY*A/FY*BES.

CONCLUSION

Taken together, the results suggest that admixture events between African slaves and European immigrants at the beginning of the 20th century made the physical characteristics of black Africans to be invisible nowadays. Considering that it was a recent historical event, the FY*BES and RHCE*ce(733C>G) alleles did not have time to become widespread but remain concentrated within families. These findings have considerable impact for typing and transfusion strategy in our population, increasing the pool of compatible units for Fy(b-) individuals requiring chronic transfusion. Possible difficulties in transfusion therapy and in genotyping could be anticipated and appropriately improved strategies devised, allowing a better management of the alloimmunization in the blood bank.

The Dc(G48)e(s) haplotype is frequent among the Dce haplotypes within a white population

BACKGROUND

The absence of hybrid Rhesus boxes denotes an RHD homozygous status and helps to detect the presence of Dce haplotypes instead of dce. RHCE exon 1 C48, characteristic of RHC alleles, and RHCE exon 5 G733, responsible for VS antigenicity, have been noted in many RHce alleles but it was not clearly established whether they occurred in the same allele and/or cosegregate together with RHD.

STUDY DESIGN AND METHODS

Samples from 148 white trios (father, mother, and child) were studied. Rh phenotype was performed by hemagglutination. Hybrid Rhesus box, RHCE exon 1 G48C, RHCE exon 5 C733G, and RHC intron 2 polymorphisms were analyzed by polymerase chain reaction. Haplotypes were determined considering serologic, molecular, and segregation data.

RESULTS

RHCE exon 1 C48 and RHCE exon 5 G733 were present in RHce alleles that cosegregated with RHD forming Dce haplotypes. Both transversions were not frequently found in the same RHce allele. Of the 33 Dce haplotypes, 16 (48.5%) had a C at position 48 [Dc(C48)e], 11 (33.3%) had a G at position 48 with a G at position 733 [Dc(G48)e(s)], 5 (15.2%) had a G at position 48 [Dc(G48)e], and 1 (3.0%) had a C at position 48 with a G at position 733 [Dc(C48)e(s)].

CONCLUSIONS

The results show four molecular backgrounds for the Dce haplotype and reflect the contribution of African alleles to the genetic pool of the population under study. The molecular characterization of Dce and its frequency distribution may develop a better understanding of the phylogeny of Rh haplotypes.

Effect of membrane-bound IgG and desialysation in the interaction of monocytes with senescent erythrocytes

Determination of the erythrocyte lifespan is a complex process affected by many cellular parameters. In the present study we measured and characterised the red blood cell (RBC) membrane proteins, mainly band 3, and quantified membrane-bound IgG in senescent RBC (SeRBC) and young RBC (YRBC). We also investigated, through a functional assay, the interaction between SeRBC and peripheral blood monocytes. We applied this erythrophagocytosis assay to study the phagocytosis of desialysed RBC. The results obtained showed no changes in the protein content between SeRBC and YRBC and no differences when examining membrane proteins by SDS-PAGE. Then, considering that the accumulation of autologous IgG on RBC membrane provides a direct mechanism for the removal of SeRBC, we measured the IgG content of intact RBC using an enzyme-linked anti-immunoglobulin test finding that the number of IgG molecules bound to SeRBC was significantly higher than that observed for YRBC. The increase observed in the percentage of erythrophagocytosis with SeRBC and sensitised RBC (SRBC) confirmed the involvement of autologous IgG in the selective removal of erythrocytes. We also observed a higher percentage of monocytes with phagocytosed and adherent RBC (AM) obtained with neuraminidase-treated RBC than those obtained with YRBC. This finding suggests that a decrease in sialic acid content of SeRBC may be involved in physiological erythrophagocytosis.

Use of the erythrophagocytosis assay for predicting the clinical consequences of immune blood cell destruction

The erythrophagocytosis assay is a useful parameter to evaluate the immune erythrocyte destruction occurring in vivo. The aim of this work was to use this assay in: a) the diagnostic and therapeutic assessment of autoimmune haemolytic anaemia (AIHA), b) the selection of blood for immunized patients requiring transfusion and c) the prediction of the severity of haemolytic disease of the newborn (HDN). This assay was also used to study the physiological removal of senescent erythrocytes from the circulation. The erythrophagocytosis assay was carried out incubating different erythrocyte suspensions and peripheral blood monocytes. A total of 200 monocytes were analysed to determine the percentage of active monocytes (% AM). We have demonstrated the usefulness of the erythrophagocytosis assay in the diagnosis of AIHA, mainly in patients with a negative direct antiglobulin test. This assay is also more adequate than classic immunohaematologic tests to obtain a better evaluation of the patients' response to treatment. The erythrophagocytosis assay was performed in immunized patients requiring transfusion and allowed us to select the least incompatible units. This method showed a better correlation with the newborns' clinical outcome than serological tests in cases of HDN. This functional assay also indicated an increased rate of erythrophagocytosis of senescent erythrocytes compared with young erythrocytes.

A Dc- phenotype encoded by an RHCE-D(5-7/8)-CE hybrid allele

BACKGROUND AND OBJECTIVES

The Rh system is genetically controlled by the homologous RHD and RHCE genes that encode the RhD and RhCcEe polypeptides, respectively. Deletions, point mutations and rearrangements between both genes are responsible for the great polymorphism of this system. The aim of this work was to analyse the genetic basis of a Dc- phenotype.

MATERIALS AND METHODS

DNA samples from the Dc- propositus and family members were obtained from peripheral blood. RHCE intron 4-exon 5 and RH exons 4, 5, 6 and 7 were analysed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Exon 9 was studied by PCR-sequence-specific primers (SSP). The RH locus was further analysed by using a PCR designed for a hybrid allele.

RESULTS

No RHCE-specific fragments were found when analysing exons 5, 6 and 7 of the RH locus from the propositus' DNA, while exons 4 and 9 of both RH genes were present.

CONCLUSIONS

The results obtained indicated that the Dc- phenotype is encoded by a novel RHCE-D(5-7/8)-CE hybrid allele.

Clinical aspects of Rh genotyping

The Rh antigens are encoded by the RHD and RHCE genes. In RhD negative individuals the RHD gene is absent or grossly deleted. Routinely, Rh typing is performed by haemagglutination. However, there are some clinical situations in which serological techniques are not suitable for determining the red blood cell phenotype accurately. Most anti-D sera may not agglutinate erythrocytes possessing a reduced expression of the D antigen. In these cases, DNA-based analyses may be better than serological typing to infer the appropriate phenotype. Agglutination methods are also of limited use for determining the red blood cell phenotype of a foetus at risk of haemolytic disease of the newborn. Molecular RHD typing using amniocytes or DNA obtained from maternal plasma may obviate the need of amniocenteses during pregnancy when the foetus is RhD negative, thus providing an important tool in managing possible sensitization by foetal erythrocytes. Classical haemagglutination has limitation in patients with autoimmune haemolytic anaemia. Erythrocytes coated with IgG cannot be accurately typed for red blood cell antigens, particularly when directly agglutinating antibodies are not available or IgG removal by chemical treatment is insufficient. Molecular genotyping is very important for determination of the true blood group antigens of these patients. RHD genotyping with a specificity and sensitivity comparable to serologic methods is of practical importance to overcome the limitations of serology and, in addition, to improve the currently possible resolution.