A Leu262Pro mutation in the integrin beta(3) subunit results in an alpha(IIb)-beta(3) complex that binds fibrin but not fibrinogen

Platelet retraction of a fibrin clot is mediated by the platelet fibrinogen receptor, alpha(IIb)beta(3). In certain forms of the inherited platelet disorder, Glanzmann thrombasthenia (GT), mutant alpha(IIb)beta(3) may interact normally with fibrin yet fail to support fibrinogen-dependent aggregation. We describe a patient (LD) with such a form of GT. Platelets from LD supported normal clot retraction but failed to bind fibrinogen. Platelet analysis using flow cytometry and immunoblotting showed reduced but clearly detectable alpha(IIb)beta(3), findings consistent with type II GT. Genotyping of LD revealed 2 novel beta(3) mutations: a deletion of nucleotides 867 to 868, resulting in a premature stop codon at amino acid residue 267, and a T883C missense mutation, resulting in a leucine (Leu) 262-to-proline (Pro) substitution. Leu262 is highly conserved among beta integrin subunits and lies within an intrachain loop implicated in subunit association. Leu262Probeta(3) cotransfected with wild-type alpha(IIb) into COS-7 cells showed delayed intracellular maturation and reduced surface expression of easily dissociable complexes. In human embryonic kidney 293 cells, Leu262Probeta(3) formed a complex with endogenous a(v) and retracted fibrin clots similarly to wild-type beta(3). The same cells, however, were unable to bind immobilized fibrinogen. The molecular requirements for alpha(IIb)beta(3) to interact with fibrin compared with fibrinogen, therefore, appear to differ. The region surrounding beta(3) Leu262 may maintain beta(3) in a fibrinogen-binding, competent form, but it appears not to be required for receptor interactions with fibrin.