An improved DNA-based identification of fetuses at risk for HPA-1a (PlA1) neonatal alloimmune thrombocytopenia

Analysis of platelet membrane glycoprotein polymorphisms in Glanzmann thrombasthenia showed the French gypsy mutation in the alphaIIb gene to be strongly linked to the HPA-1b polymorphism in beta3

We have tested the DNA of a large series of Glanzmann thrombasthenia patients for polymorphisms in platelet membrane glycoproteins. To our surprise, we noted a high prevalence of the HPA-1b allele of beta3, the minority allele in a normal population. This proved to be due to the presence of nine patients homozygous for the so-called French gypsy mutation (IVS15[ + 1]G-->A) in alphaIIb. Seven of these patients were homozygous for the HPA-1b alloantigen and the other two heterozygous HPA-1a/1b. As the alphaIIb and beta3 genes are both on chromosome 17, it is highly probable that the French gypsy mutation first arose on a chromosome encoding HPA-1b. For other adhesion receptors, no major differences were seen in the distribution of the A1, A2 and A3 alleles in the alpha2 gene, or in the Kozak or HPA-2 polymorphisms of GPIbalpha, suggesting that none of these alleles result in increased survival in Glanzmann thrombasthenia.

The Natural History of Fetomaternal Alloimmunization to the Platelet-Specific Antigen HPA-1a (PlA1, Zwa) as Determined by Antenatal Screening

Immunization against the human platelet antigen (HPA)-1 alloantigen is the most common cause of severe fetal and neonatal thrombocytopenia. Fetal therapy has substantial risks and its indications need better definition. Of 24,417 consecutive pregnant women, 618 (2.5%) were HPA-1a negative of whom 385 entered an observational study. All were HLA-DRB3*0101 genotyped and screened for anti–HPA-1a. Their partners and neonates were HPA-1 genotyped and the latter were assessed by cord blood platelet counts and cerebral ultrasound scans. Anti–HPA-1a was detected in 46 of 387 pregnancies (12.0%; 95% CI 8.7%-15.2%). All but one were HLA-DRB3*0101 positive (odds ratio 140; 95% CI 19-1035;P< .00001). One baby died in utero, and of 26 HPA-1a–positive babies born to women with persistent antenatal antibodies, 9 were severely thrombocytopenic (8 with a count <10 × 109/L, 1 with a large porencephalic cyst), 10 were mildly thrombocytopenic, whereas 7 had normal platelet counts. Severe thrombocytopenia was significantly associated with a third trimester anti–HPA-1a titer ≥ 1:32 (P = .004), but was not observed in babies of women with either transient or postnatal-only antibodies. HPA-1a alloimmunization complicates 1 in 350 unselected pregnancies, resulting in severe thrombocytopenia in 1:1,200. HPA-1a and HLA-DRB3*0101 typing combined with anti–HPA-1a titration allows selection of the majority of pregnancies at risk of severe thrombocytopenia.

The Natural History of Fetomaternal Alloimmunization to the Platelet-Specific Antigen HPA-1a (PlA1, Zwa) as Determined by Antenatal Screening

Immunization against the human platelet antigen (HPA)-1 alloantigen is the most common cause of severe fetal and neonatal thrombocytopenia. Fetal therapy has substantial risks and its indications need better definition. Of 24,417 consecutive pregnant women, 618 (2.5%) were HPA-1a negative of whom 385 entered an observational study. All were HLA-DRB3*0101 genotyped and screened for anti–HPA-1a. Their partners and neonates were HPA-1 genotyped and the latter were assessed by cord blood platelet counts and cerebral ultrasound scans. Anti–HPA-1a was detected in 46 of 387 pregnancies (12.0%; 95% CI 8.7%-15.2%). All but one were HLA-DRB3*0101 positive (odds ratio 140; 95% CI 19-1035;P< .00001). One baby died in utero, and of 26 HPA-1a–positive babies born to women with persistent antenatal antibodies, 9 were severely thrombocytopenic (8 with a count <10 × 109/L, 1 with a large porencephalic cyst), 10 were mildly thrombocytopenic, whereas 7 had normal platelet counts. Severe thrombocytopenia was significantly associated with a third trimester anti–HPA-1a titer ≥ 1:32 (P = .004), but was not observed in babies of women with either transient or postnatal-only antibodies. HPA-1a alloimmunization complicates 1 in 350 unselected pregnancies, resulting in severe thrombocytopenia in 1:1,200. HPA-1a and HLA-DRB3*0101 typing combined with anti–HPA-1a titration allows selection of the majority of pregnancies at risk of severe thrombocytopenia.

Prenatal determination of human platelet antigen type 4 by DNA amplification of amniotic fluid cells

To predict a fetus at risk for neonatal alloimmune thrombocytopenia (NATP) caused by human platelet antigen (HPA)-4 incompatibility, we applied a sequence-specific polymerase chain reaction (PCR-SSP). We were able to determine the HPA-4 genotype of three infants at risk using amniotic fluid cells without the need for fetal blood sampling. The HPA-4 genotypes of amniotic fluid cells determined in this way were completely concordant with the genotype and phenotype of infants' venous blood samples obtained after delivery. Therefore, this technique is also convenient to a fetus at risk in the antenatal management of NATP induced by HPA-4 incompatibility.

Genotyping of the platelet-specific alloantigen HPA-5 (Br(a)/Br(b)) using polymerase chain reaction with sequencespecific primers (PCR-SSP)

A DNA-based one-stage technique, polymerase chain reaction with sequence-specific primers (PCR-SSP) was developed for genotyping of the platelet specific alloantigen HPA-5 (Bra/Brb). Sequence-specific primers, matching the wild type and the point mutation responsible for the HPA-5 (Bra/Brb) phenotype, were constructed. Conjointly a fragment of the gene coding for glycoprotein (GP) IIIa was amplified as an internal control of the enzyme reaction. Using these HPA-5 (Bra/Brb) sequence-specific primers the correct fragment of the GPIa gene was amplified, as evidenced by the PCR product size, the restriction map and by the nucleotide sequence. This assay was applied on 187 Swedish blood donors; 157 individuals were found to have a homozygous HPA-5a (Bra/Brb) genotype and 30 individuals a heterozygous HPA-5a,b (Bra/Brb) genotype. None of the donors was found to display a homozygous HPA-5b (Bra/Brb) genotype. Thus, the (HPA-5b) Bra antigen frequency in this population will be approximately 16.0% with a gene frequency of 8.0%. It is concluded that this assay is an attractive technique for genotyping of the HPA-5 (Bra/Brb) alloantigens on genomic DNA. The technique can replace serological alloantigen typing, especially in cases where platelets and rare human alloantisera are not available.

A rapid genotype assay for Pla haplotypes

The method described here results in simple visual analysis of 1, 2, or 3 ethidium bromide-stained bands corresponding to the PlA1, PlA2 or PlA1/A2 phenotypes, respectively. The amplification primers, designed to yield a 711bp product which encompasses exon 2 and substantial intronic sequence both 5' and 3' of the exon enable exquisite resolution of the restriction enzyme digestion products which define the alleles. In addition to prediction of fetal risk of neonatal alloimmune thrombocytopenia using this method, an ethnically heterogeneous pool of platelet donors was screened for the platelet antigen PlA (HPA-1) and gave unequivocal results which should be useful for platelet matching.

Molecular genotyping of fetal platelet antigens with uncultured amniocytes

OBJECTIVE

Amino acid substitutions in platelet membrane glycoproteins result in alloantigens implicated in neonatal alloimmune thrombocytopenia. We report the use of the reverse dot blot technique to genotype the five major fetal platelet alloantigens from amniotic fluid cells.

STUDY DESIGN

We evaluated a patient with Bakb platelet antibodies who had a previous pregnancy complicated by fetal intracranial hemorrhage. The father was heterozygous Baka/Bakb, giving the pregnancy a 50% risk for platelet incompatibility between mother and fetus. Amniotic fluid was obtained at 16 weeks. Deoxyribonuleic acid was extracted from uncultured amniocytes and amplified with polymerase chain reaction. These products were hybridized to filters containing oligonucleotides specific for each of the 10 different platelet antigen alleles. Reactivity was detected with a chromogenic substrate.

RESULTS

The reverse dot blot genotyping of uncultured amniocytes revealed the fetus to be Baka/Baka, thus not at risk for neonatal alloimmune thrombocytopenia.

CONCLUSION

Precise knowledge of fetal platelet type by amniocentesis could obviate the need for fetal blood sampling and significantly alter prenatal management of neonatal alloimmune thrombocytopenia.

Non-radioactive SSCP for genotyping human platelet alloantigens

Human platelet alloantigen systems are responsible for neonatal and post-transfusional thrombocytopenias. The determination of the different allotypes can be performed using immunological or DNA-based methods. The most used DNA-based procedure requires the digestion by specific restriction enzymes of PCR products containing the genetic determinants of these alloantigens. We now report a rapid method of genotyping which does not use restriction enzymes and is less prone to misinterpretation. This is non-radioactive PCR-SSCP (single strand conformation polymorphism), which we illustrate for two different HPA systems, one on GPIIIa (HPA-1) and the other on GPIIb (HPA-3).

The platelet-specific alloantigen PlA1 (HPA-1a): a comparison of flow cytometric immunophenotyping and genotyping using polymerase chain reaction and restriction fragment length polymorphism in a Swedish blood donor population

BACKGROUND

There is an increasing interest in the development of rapid and reliable techniques for platelet alloantigen typing.

STUDY DESIGN AND METHODS

By use of standardized flow cytometry and a specific human alloantiserum, 236 Swedish blood donors were immunophenotyped for the platelet-specific alloantigen, PlA1 (HPA-1a).

RESULTS

Ten individuals (4.2%) had low fluorescence intensities and were considered PlA1-negative (HPA-1a-negative); all of them also demonstrated a PlA2/PlA2 (HPA-1b/1b) genotype in a polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) assay of the underlying DNA polymorphism. The remaining population had clear positive fluorescence and was regarded as PlA1-positive (HPA-1a-positive). The fluorescence distribution histogram among PlA1-positive (HPA-1a-positive) individuals was dome-shaped, and those individuals who were homozygous for PlA1 (HPA-1a) could not be distinguished from those who were heterozygous. This finding was further substantiated by PCR-RFLP analysis of the PlA1/PlA2 (HPA-1a/1b) genotype; a heterozygous genotype was found among those having a medium fluorescence intensity as well as among those having a strong fluorescence intensity.

CONCLUSION

Flow cytometry is a valuable tool for large-scale detection of PlA1 (HPA-1a). However, flow cytometry based on only one antiserum cannot distinguish between homozygous and heterozygous carriers of PlA1 (HPA-1a). For zygosity testing and when platelets are difficult to obtain, the PCR-RFLP technique is the assay of choice.