Haemolytic disease of the fetus and newborn

Determination of Fetal RHD Genotype Including the RHD Pseudogene in Maternal Plasma

OBJECTIVE

To examine the accuracy of fetal RHD genotype and RHD pseudogene determination in a multiethnical population.

METHODS

Prospective study involving D-negative pregnant women. Cell-free DNA was extracted from 1 ml of maternal plasma by an automated system (MagNA Pure Compact, Roche) and real-time PCR was performed in triplicate targeting the RHD gene exons 5 and 7. Inconclusive samples underwent RHD pseudogene testing by real-time PCR analysis employing novel primers and probe.

RESULTS

A positive result was observed in 128/185 (69.2%) samples and negative in 50 (27.0%). Umbilical cord blood phenotype confirmed all cases with a positive or negative PCR result. Seven (3.8%) cases were found inconclusive (exon 7 amplification only) and RHD pseudogene testing with both conventional and real-time PCR demonstrated a positive result in five of them, while two samples were also RHD pseudogene negative.

CONCLUSION

Real-time PCR targeting RHD exons 5 and 7 simultaneously in maternal plasma is an accurate method for the diagnosis of fetal D genotype in our population. The RHD pseudogene real-time PCR assay is feasible and is particularly useful in populations with a high prevalence of this allele.

Delayed cord clamping in red blood cell alloimmunization: safe, effective, and free?

Hemolytic disease of the newborn (HDN), an alloimmune disorder due to maternal and fetal blood type incompatibility, is associated with fetal and neonatal complications related to red blood cell (RBC) hemolysis. After delivery, without placental clearance, neonatal hyperbilirubinemia may develop from ongoing maternal antibody-mediated RBC hemolysis. In cases refractory to intensive phototherapy treatment, exchange transfusions (ET) may be performed to prevent central nervous system damage by reducing circulating bilirubin levels and to replace antibody-coated red blood cells with antigen-negative RBCs. The risks and costs of treating HDN are significant, but appear to be decreased by delayed umbilical cord clamping at birth, a strategy that promotes placental transfusion to the newborn. Compared to immediate cord clamping (ICC), safe and beneficial short-term outcomes have been demonstrated in preterm and term neonates receiving delayed cord clamping (DCC), a practice that may potentially be effective in cases RBC alloimmunization.

Frequency and characterization of known and novel RHD variant alleles in 37 782 Dutch D-negative pregnant women

To guide anti-D prophylaxis, Dutch D- pregnant women are offered a quantitative fetal-RHD-genotyping assay to determine the RHD status of their fetus. This allowed us to determine the frequency of different maternal RHD variants in 37 782 serologically D- pregnant women. A variant allele is present in at least 0·96% of Dutch D- pregnant women The D- serology could be confirmed after further serological testing in only 54% of these women, which emphasizes the potential relevance of genotyping of blood donors. 43 different RHD variant alleles were detected, including 15 novel alleles (11 null-, 2 partial D- and 2 DEL-alleles). Of those novel null alleles, one allele contained a single missense mutation (RHD*443C>G) and one allele had a single amino acid deletion (RHD*424_426del). The D- phenotype was confirmed by transduction of human D- erythroblasts, consolidating that, for the first time, a single amino acid change or deletion causes the D- phenotype. Transduction also confirmed the phenotypes for the two new variant DEL-alleles (RHD*721A>C and RHD*884T>C) and the novel partial RHD*492C>A allele. Notably, in three additional cases the DEL phenotype was observed but sequencing of the coding sequence, flanking introns and promoter region revealed an apparently wild-type RHD allele without mutations.

Using red blood cell genomics in transfusion medicine

Blood types (blood group antigens) are heritable polymorphic antigenic molecules on the surface of blood cells. These were amongst the first human Mendelian traits identified, and the genetic basis of nearly all of the hundreds of blood types is known. Clinical laboratory methods have proven useful to identify selected blood group gene variants, and use of genetic blood type information is becoming widespread. However, the breadth and complexity of clinically relevant blood group genetic variation poses challenges. With recent advances in next-generation sequencing technologies, a more comprehensive DNA sequence-based genetic blood typing approach is now feasible. This chapter introduces the practitioner to high-resolution genetic blood typing beginning with an overview of the genetics of blood group antigens, the clinical problem of allosensitization, current blood type testing methods, and then discussion of next-generation sequencing and its application to the problem of genetic blood typing.