Platelet ABCA1 Expression Is Normal in Scott Syndrome Dogs.

Abstract

Scott syndrome is a rare platelet function defect characterized by a failure of Ca2+-stimulated phosphatidylserine (PS) externalization and lack of platelet prothrombinase activity. In 2005 Albrecht et al. (Blood 106:542) described low levels of ATP-binding transporter A1 (ABCA1) mRNA in lymphocytes and heterozygosity for a novel ABCA1 missense mutation in a patient with Scott Syndrome. The proband's expression of platelet ABCA1, however, was not measured. Platelet dysfunction has been described as a feature of Tangier disease, a monogenic disorder caused by ABCA1 mutations (Nofer et al, JBC 279:34032, 2004). The disease phenotype results from altered lipid trafficking, with a virtual absence of plasma HDL cholesterol and accumulation of lipid in cells of the RE system. In the current study, we systematically evaluated platelets from Scott syndrome dogs to find evidence of ABCA1 deficiency. Platelets from control dogs and affected German shepherd dogs (GSD) were compared with regard to cholesterol content, ABCA1 protein expression, and reactivity to collagen, convulxin (CVX), and thrombin. All GSD had normal plasma cholesterol and triglyceride levels, with no significant difference in mean platelet cholesterol content from that of control dogs (control cholesterol = 13.4 ug/108 platelets, GSD cholesterol = 12.6 ug/108 platelets; n= 6; p = 0.726). Western blot analysis of platelet extracts from control dogs and GSD using a polyclonal antibody to human ABCA1 revealed comparable levels of a 220 kD protein band, the expected size for human ABCA1. We found no significant differences in maximal platelet aggregation in response to 12 ug/mL collagen stimulation [control dog mean response = 82.9% (n=10), GSD = 75.0 % (n= 7); p = 0.133)]. CVX concentration-response aggregation curves obtained for washed platelets were similar for control dogs (n=4) and GSD (n=3) in the range of 1 to 100 nM CVX, with a plateau of maximal aggregation at 10 nM CVX. In flow cytometry analyses, we found no difference in mean platelet mepacrine uptake between control dogs and GSD (mean positive platelets = 64%, n= 3 control, n=5 GSD). There were no significant differences in mean CD62P expression post-thrombin (0.5 U/mL) or CVX (2 nM) stimulation [control CD62P positive platelets post-thrombin = 57%, post-CVX = 47.5% (n=7); GSD CD62P positive platelets post-thrombin = 49%, post-CVX = 56% (n=4)]. Although ABCA1 has been proposed to play a role in PS transport, we found no evidence of ABCA1 deficiency and none of the characteristic platelet abnormalities associated with Tangier disease in canine Scott syndrome, a model of impaired transmembrane PS movement.Comparison of Platelet PhenotypesANALYSISTANGIER DISEASESCOTT SYNDROME GSDIonophore induced PS expressionPresentAbsentCollagen or CVX induced aggregationDecreasedNormalCVX or thrombin induced alpha granule releaseDecreasedNormalPlatelet ABCA1 expressionAbsentPresent