A new Ser472Asn (Cab2(a+)) polymorphism localized within the αIIb "thigh" domain is involved in neonatal thrombocytopenia.
BACKGROUND
A new platelet antigen, Cab2(a+), was identified in a case of severe neonatal alloimmune thrombocytopenia (<8 × 10(9)/L) in twins.


METHODS
Coding sequences of αIIb and β3 genes from parents were amplified and sequenced. CHO cell lines expressing wild-type or mutated forms of the complex were established to study the role of the mutation in alloimmunization and in αIIbβ3 functions.


RESULTS
The father and twins were heterozygous for a single αIIb c.1508G>A mutation leading to a Ser472Asn substitution. Immunologic assays with transfected CHO cells revealed the Asn472 form of αIIbβ3 responsible for the Cab2(a+) epitope but not an Ala472 form. Using these cells lines we demonstrated that both Ser472Asn and Ser472Ala substitutions produced limited structural alteration as revealed by the reactivity of a panel of anti-αIIbβ3 monoclonal antibodies (MoAbs). Activated Asn472 and Ala472 forms of αIIbβ3 supported 1) binding of soluble fibrinogen and of the ligand mimetic MoAb PAC-1, 2) ligand-induced binding site epitopes exposure (MoAbs AP-5 and D3GP3), and 3) cell aggregation. Adhesion onto adsorbed fibrinogen was conserved and was specifically inhibited by MoAb AP-2 or peptide RGDS. Finally outside-in signaling was not affected.


CONCLUSIONS
We have characterized a new low-frequency alloantigen (<1%) resulting from the Ser472Asn substitution in αIIb and shown this polymorphism to have a limited effect, if any, on the αIIbβ3 complex functions.
Jallu
Vincent
V
Platelet Immunology Laboratory, INTS, Paris, France.
Dusseaux
Mathilde
M
Kaplan
Cecile
C
D000328:Adult; D019943:Amino Acid Substitution; D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D004200:Diseases in Twins / Q000235:genetics*; D000939:Epitopes / Q000276:immunology; D005260:Female; D005340:Fibrinogen / Q000378:metabolism; D006579:Heterozygote; D006801:Humans; D007112:Immunity, Maternally-Acquired; D007231:Infant, Newborn; D039661:Integrin beta3 / Q000737:chemistry; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology; D010973:Platelet Adhesiveness / Q000187:drug effects; D040201:Platelet Membrane Glycoprotein IIb / Q000737:chemistry / Q000235:genetics* / Q000276:immunology; D020641:Polymorphism, Single Nucleotide; D025941:Protein Interaction Mapping; D013329:Structure-Activity Relationship; D054098:Thrombocytopenia, Neonatal Alloimmune / Q000235:genetics*; D014162:Transfection
D000911:Antibodies, Monoclonal; D000939:Epitopes; D039661:Integrin beta3; D009842:Oligopeptides; D040201:Platelet Membrane Glycoprotein IIb; D005340:Fibrinogen; C046617:arginyl-glycyl-aspartyl-serine