Biochemical and molecular basis of Glanzmann's thrombasthenia.

Glanzmann's thrombasthenia is a rare autosomal recessive bleeding disorder characterized by a quantitative deficiency or a functional abnormality of the major platelet membrane integrin receptor: the glycoprotein (GP) IIb/IIIa complex. The GPIIb/IIIa complex functions as a platelet receptor for fibrinogen, von Willebrand factor, fibronectin and vitronectin; therefore it plays an important role in platelet adhesion and aggregation. Thrombasthenic platelets are severely deficient in GPIIb/IIIa content or function, and fail to aggregate and form the hemostatic plug at the site of vessel injury. On the other hand, heterozygous subjects (having about half the number of normal GPIIb/IIIa complexes) do not show bleeding problems. It has been demonstrated that a molecular defect affecting one of the two GP coding genes is sufficient to determine a contemporary deficit of both GPIIb and GPIIIa, and hence the thrombasthenic phenotype. Up to now, few molecular abnormalities giving rise to Glanzmann's thrombasthenia have been characterized. Large rearrangements within the GPIIb or GPIIIa coding genes appear to be unusual, whereas small modifications in the nucleotide sequence of the coding regions occur with higher frequency.