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

Minor histocompatibility antigen HA-1 and HPA-5 polymorphisms in HLA-identical related bone marrow transplantation

The minor histocompatibility antigens (mHags), HA-1 and HPA-5, are immunogenic alloantigens shown to be responsible for graft-versus-host disease (GVHD) in HLA-identical bone marrow transplantation. Both antigens have two known alleles each, resulting in a single amino acid polymorphism. The HA-1H allele encodes histidine, whereas the HA-1R allele encodes arginine. The HPA-5b (Br(a)) allele encodes lysine, whereas the HPA-5a (Br(b)) encodes glutamic acid. In this study, 49 bone marrow transplant recipients and their genetically related HLA-identical donors were evaluated for the presence of HA-1, whereas 39 recipients, different from the abovementioned ones, and their HLA-identical siblings were analyzed for the presence of HPA-5. The frequencies of the two alleles of HA-1 in the recipient population were HA-1R = 0.663 and HA-1H = 0.336. In the donor population, the respective frequencies were 0.704 and 0.296. Seven donors (14.5%) were mismatched with the recipients for HA-1H. In contrast, the frequencies of the two alleles of HPA-5 in the recipient population were HPA-5a = 0.859 and HPA-5b = 0.141; whereas, among donors, they were 0.820 and 0.180, respectively. Five donors (12.8%) were found to be mismatched with their recipients for HPA-5. These results provide insight into the polymorphism of mH antigens based on the study of their frequencies in bone marrow transplant recipients and their genetically HLA-identical siblings, an endeavor that is essential to investigate the presence of HA-1 and HPA-5 mHags.

HPA-5b (Bra ) neonatal alloimmune thrombocytopenia in Quebec: incidence and clinical outcome in 31 cases

BACKGROUND

Clinical detection of neonatal allo-immune thrombocytopenia (NAITP) is often less than the incidence predicted in prospective studies. The aims of this study were to calculate the observed and expected incidence of NAITP in Quebec, Canada, and to evaluate the clinical outcome of infants with anti-HPA-5b NAITP.

STUDY DESIGN AND METHODS

Records from 1998 to 2002 of the only PLT serology laboratory for the province of Quebec were reviewed, as were hospital charts of all HPA-5 cases. The number of expected NAITP cases was estimated with known alloantigen gene frequencies.

RESULTS

Ninety cases of NAITP were identified. The clinical detection rate was 1 in 4100, with 9 to 30 percent of expected HPA-1a and 11 to 23 percent of expected HPA-5b NAITP cases detected. Seventy-eight percent of cases of HPA-5b NAITP were asymptomatic. Sixty-three percent of all deliveries but 94 percent of HPA-5b cases occurred in hospitals performing cord blood PLT counts.

CONCLUSION

The NAITP detection rate was 25 to 50 percent of the rate reported in prospective trials. The less severe clinical presentation of NAITP owing to HPA-5b was confirmed. The frequent routine determination of cord blood PLT counts may explain increased detection of asymptomatic NAITP involving anti-HPA-5b.

Genotyping for platelet-specific antigens: techniques for the detection of single nucleotide polymorphisms

Accurate typing of patients for platelet-specific (human platelet) antigens (HPA) is required in several different clinical situations, and blood services need to maintain panels of HPA-typed apheresis platelet donors and whole-blood donors to support HPA alloimmunized patients. Six clinically relevant HPA alloantigen systems have been described and, in addition, a significant number of HPA alloantigens with a highly skewed allele frequency or of very low immunogenicity have been reported. Certain well-characterized biallelic systems such as Gov have not as yet been included in the HPA nomenclature but are included in this review. Biochemical studies have identified the platelet membrane proteins on which the HPA antigens are localized. Cloning of the genes encoding these proteins and the realization that there is adequate mRNA in fresh platelets has led to identification of the molecular basis of HPA antigens over the last decade. All but one of the biallelic platelet-specific alloantigen systems are based on a single nucleotide polymorphism in the DNA sequence, corresponding to a single amino acid substitution in the encoded primary protein sequence. The discovery of the genetic basis of the alloantigens has allowed the development of polymerase chain reaction-based techniques for HPA genotyping using genomic DNA. The genetic basis of the HPA alloantigens, the most commonly used genome typing techniques and their pitfalls, and future developments, are discussed in this review.

The number of platelet glycoprotein Ia molecules is associated with the genetically linked 807 C/T and HPA-5 polymorphisms

BACKGROUND

The neutral 807 C/T (Phe224) polymorphism (807 C/T polymorphism) of the glycoprotein (GP)Ia gene has been recently associated with the number of GPIa molecules on the platelet surface. The association of the number of GPIa molecules with other GPIa polymorphisms, such as HPA-5 (Glu/Lys505) (HPA-5 polymorphism), involved in alloimmune thrombocytopenias is unknown.

STUDY DESIGN AND METHODS

The association of the HPA-5 polymorphism with the number of GPIa molecules on the platelet surface in 159 white blood donors was investigated. The genetic linkage between the HPA-5 and the 807 C/T polymorphisms in 316 individuals was also determined.

RESULTS

Both the 807 C/T and HPA-5 polymorphisms correlate with the number of GPIa molecules on the platelet surface. The 807 T and HPA-5b alleles are associated with increased numbers of GPIa molecules on the platelet surface. Moreover, the HPA-5b allele is genetically linked to 15.8 percent of the 807 C alleles. Therefore, the number of GPIa molecules on the platelet surface is dependent on both GPIa polymorphisms as follows: 807 T/T, HPA-5 a/a > 807 C/T, HPA-5 a/b > 807 C/T, HPA-5 a/a > 807 C/C, HPA-5 a/b > 807 C/C, HPA-5 a/a.

CONCLUSION

Two GPIa polymorphisms (807 C/T and HPA-5) responsible for the variability in the numbers of GPIa/IIa molecules on the platelet surface in whites have been identified. Despite the genetic linkage between the two polymorphisms, their influence on the number of GPIa molecules on the platelet surface may occur through different mechanisms.