Summary
We studied a 66-year-old man with a myeloproliferative disorder who presented with
a prolonged bleeding time and marked thrombocytosis (platelet count, 3,890 × 109/1). There was no past history of a bleeding disorder. The patient had normal coagulation
data. His platelets completely lacked collagen-induced platelet aggregation and adhesion,
but showed normal responses to other agonists. All family members tested showed normal
platelet aggregation with collagen.
Analysis of 125I surface-labeled platelets by two-dimensional SDS gel electrophoresis disclosed absence
of the spot corresponding to platelet membrane GPIa (α2) but no other significant deficiencies of major platelet glycoproteins i.e., GPIb,
IIb-IIIa, and IV. Immunoisolation studies of the patient’s platelets indicated that
neither anti-GPIa nor anti-GPIIa (β1) monoclonal antibody (mAb) isolated any surface membrane proteins corresponding to
GPIa. GPVI, a putative collagen receptor, was immunoisolated from the platelets. Indirect
immunofluorescence study using flow cytometry confirmed that the patient’s platelets
were totally deficient in surface expression of the GPIa-IIa complex (α2β1, integrin). In contrast, phytohemoagglutinin-activated T-lymphocytes from the patient
expressed normal concentrations of this complex.
The data suggest that our patient had an acquired deficiency of the platelet GPIa-IIa
complex, due to a myeloproliferative disorder, which might account for the absence
of responsiveness of his platelet to collagen.
