New mutations on platelet GPIIb in Sub-Saharan African populations revealed by genotyping discrepancies

The αIIb p.Leu841Met (Cab3(a+) ) polymorphism results in a new human platelet alloantigen involved in neonatal alloimmune thrombocytopenia

BACKGROUND

Fetal-neonatal alloimmune thrombocytopenia (FNAIT) diagnosis relies on maternofetal incompatibility and alloantibody identification. Genotyping for rare platelet (PLT) polymorphisms allowed the identification of three families with suspected or confirmed maternofetal incompatibility for the αIIb-c.2614C>A mutation (Halle et al., Transfusion 2008;48:14-15).

STUDY DESIGN AND METHODS

A polymerase chain reaction-sequence-specific primers amplification assay was designed to genotype the αIIb-c.2614C>A mutation. HEK293 cells expressing αIIb-Leu841 or αIIb-Met841 αIIbβ3 forms were used to probe the reactivity of maternal sera from these families and to study the effects of the substitution on αIIbβ3 expression and functions.

RESULTS

Tested by flow cytometry (FCM), one serum sample specifically reacted with αIIb-Met841 but not with αIIb-Leu841 αIIbβ3. This specificity revealed the αIIb-Leu841 polymorphism as a new alloantigen named Cab3(a+) . Cross-match testing using FCM also showed the Cab3(a+) antigen to be expressed at the PLT surface. As for anti-human PLT alloantigen (HPA)-3a (or -3b) and anti-HPA-9bw, detection of anti-Cab3(a+) alloantibodies appeared difficult and required whole PLT assays when classical monoclonal antibody-specific immobilization of PLT antigen test failed. In our FNAIT set, the immune response to Cab3(a+) maternofetal incompatibility could induce severe thrombocytopenias and life-threatening hemorrhages. The p.Leu841Met substitution has limited effects, if any, on local αIIb structure, preserving both αIIbβ3 expression and functions.

CONCLUSION

The Cab3(a+) polymorphism is a new rare alloantigen (allelic frequency <1%) carried by αIIb that might result in severe life-threatening thrombocytopenias. In Sub-Saharan African populations, higher Cab3(a+) gene frequencies (up to 8.2%; Halle et al., Transfusion 2008;48:14-15) and homozygous people are observed.

New K103 β3 allele identified in a context of severe neonatal thrombocytopenia

BACKGROUND

A new β3 allele was identified in a severe case of neonatal alloimmune thrombocytopenia (<7 × 10(9) /L).

STUDY DESIGN AND METHODS

Diagnosis was done by use of monoclonal antibody-specific immobilization of platelet (PLT) antigen for serologic analyses and polymerase chain reaction (PCR)-sequence-specific primers (SSP) and PCR-restriction fragment length polymorphism (RFLP) for genotyping. Direct sequencing of PCR product was done and mutant αIIbβ3 expressed in HEK-293 cells.

RESULTS

Serologic analysis revealed in the maternal serum an anti-human PLT alloantigen (HPA)-1a alloantibody associated to an anti-α2β1. Anti-HPA-1a alloimmunization diagnosis was confirmed by genotyping showing maternofetal incompatibility. However, investigation of rare HPA polymorphisms revealed discrepant HPA-16b assignation between PCR-RFLP and PCR-SSP. Sequencing revealed a new c.385C>A mutation in the β3 coding sequence resulting in a false assignation of the HPA-16b allele by PCR-RFLP. This mutation leads to a Q103K substitution in mature β3. The K103-β3 form of the complex was expressed in HEK-293 cells but did not react with the maternal serum.

CONCLUSION

We have characterized a new rare allele (frequency < 1%) of β3 that yields false HPA-16b genotyping in PCR-RFLP. This new case of false typing assignation emphasizes the necessity to use two genotyping techniques in diagnosis. This particularly applies for rare HPA polymorphisms when PLT phenotyping cannot be used.

[Genotyping applied to platelet immunology: when? How? Limits]

Platelet alloantigens named Human Platelet Antigens (HPA) are involved in immune conflicts such as post-transfusion purpura, platelet transfusion refractoriness and neonatal alloimmune thrombocytopenia. Biological diagnosis relies on: (1) detection of alloantibodies; (2) identification of the alloantigen involved in the immune conflict. Since the development of methods based on molecular biology, platelet genotyping is preferred to phenotyping. Today, most of the Platelet Immunology Units use PCR-RFLP or PCR-SSP, and few use real-time PCR. An increasing amount of commercial kits based on new technologies is now available, for example microarrays, fluorescent or coloured microbeads, or a combination of both technologies. However, an increasing number of polymorphisms have been discovered that are responsible for erroneous platelet genotypings. Consequently, it would be of interest to develop alternative technologies based on antigen/antibody interaction instead of DNA.