PLATELETS WITH 10-PERCENT OF THE NORMAL AMOUNT OF GLYCOPROTEIN-VI HAVE AN IMPAIRED RESPONSE TO COLLAGEN THAT RESULTS IN A MILD BLEEDING TENDENCY

Platelet glycoprotein VI (GPVI), a 62 kD membrane protein, has been id entified as one of the platelet receptors for collagen, since GPVI-def icient platelets exhibit abnormal responses to collagen and an abnorma l bleeding tendency. We report a female patient with a mild bleeding h istory whose platelets expressed 10% GPVI of normal platelets. Shape c hange, aggregation and ATP release of the patient's platelets were com pletely absent in response to 1-5 mu g/ml collagen but present normall y in response to ADP and Ca2+ ionophore A23187. Adhesion of the patien t's platelets to coated collagen was mildly affected (40-60% of normal platelets) in spite of only 10% expression of GPVI. Flow cytometrical studies revealed that the patient's platelets expressed normal amount s of the GPIa/IIa complex. These results suggest that platelet GPVI is less involved in adhesion to collagen than shape change and aggregati on induced by collagen.