Summary

We have recently studied a case series of 46 unrelated patientswith inherited thrombocytopenias and identified 18 cases thatdid not fit any known platelet disorder.In two unrelated families,a mild thrombocytopenia with normal platelet size was transmittedin an autosomal dominant fashion. Bleeding time wasprolonged in 5 investigated patients. In all of them, flow cytometryand SDS-PAGE of platelet glycoproteins (GP) showed areduced content of GPIa,a subunit of the GPIa-IIa complex (alsoknown as integrin a 2 ß 1 ) that is a major collagen receptor on platelets.All other membrane GPs were within the normal range.GPIa deficiency was associated with severely reduced in vitro platelet adhesion to molecules known to interact selectively withGPIa. In vitro platelet aggregation was normal in all subjects, exceptfor a suboptimal platelet response to fibrillar collagen intwo patients.A mild defect of a -granules was observed in all affectedsubjects. No mutation was identified in the genes encodingfor GPIa or GPIIa. Since no other similar cases have beenreported in the literature, we suggest that an autosomal dominantthrombocytopenia associated with GPIa deficiency anda -granule defect represents a new form of inherited thrombocytopenia.