A Familial Platelet Function Disorder Associated with Abnormal Signaling Through the Glycoprotein VI Pathway

Abstract number: P0320

Dunkley¹ S, Arthur² J, Evans¹ S, Shen Y, Andrews² R

¹¹University of New South Wales and Prince of Wales Hospital, Sydney, Australia ¹¹University of New South Wales and Prince of Wales Hospital, Sydney, Australia ²²Monash University, Melbourne, Australia

We report a familial case of abnormal platelet aggregation consistent with an abnormality of signaling through the platelet glycoprotein (GP) VI collagen receptor/pathway. The patient is a 58 year old lady with a history of immune disorders including small vessel vasculitis, Sjogren's disease and an associated MALT lymphoma. There was a history from childhood of excessive bleeding, with recurrent epistaxis requiring transfusion, menorrhagia, and a life threatening haemorrhage following trauma. The bleeding phenotype has improved with age. A mild bleeding tendency is described in her mother, daughter and several sisters. Coagulation and von Willebrand factor was normal. The platelet count was mildly reduced at 123 × 10⁹/L with mild macrothrombocythemia and a MPV of 12.5fL. Platelet antibodies are negative by flow cytometry and MAIPA. The bleeding time was prolonged, >12 minutes, but PFA-100 analysis normal. There was abnormal platelet aggregation with collagen at concentrations up to 4 µg/mL. Aggregation with convulxin or the GPVI-specific agonist XL-CRP, were also abnormal. Platelet aggregation to epinephrine was mildly abnormal but normal with ADP, TRAP and ristocetin. Identical abnormalities of platelet aggregation were also present in her mother, daughter and niece. However, despite defective collagen-dependent aggregation, the collagen receptors, GPVI and the alpha2beta1, were expressed at normal levels on the platelet surface as assessed by flow cytometry, as was the FcRgamma chain (required for GPVI expression) in permeabilized platelets. These results were confirmed by western blotting. Sequencing of the coding regions of the GPVI gene showed no abnormalities. Dense granules appeared normal by mepacrine labeling and serotonin uptake/release. A genetic defect in a signaling protein involved with the GPVI pathway is hypothesized in this family. Further investigation of GPVI-dependent tyrosine phosphorylation may help suggest a candidate protein.