Activated platelets of patients with paroxysmal nocturnal hemoglobinuria express cellular prion protein.

Cellular prion protein (PrPc) is a glycosylphosphatidylinositol (GPI)-anchored membrane glycoprotein that contains a putative membrane-spanning section. Patients with paroxysmal nocturnal hemoglobinuria (PNH) lack GPI proteins on the surface of somatically mutated hematopoietic stem cell and its progeny. Platelet expression of PrPc was studied in 8 PNH patients. Resting PNH (CD55(-)) platelets were devoid of surface PrPc, but activation of platelets resulted in the surface expression of PrPc. Expressed PrPc was detected by 2 monoclonal antibodies (mAbs) against the N-terminal part of the molecule but not by mAb 6H4, which binds at the C-terminus beyond the membrane-spanning section. However, 6H4 detected PrPc on Western blots of PNH platelets, demonstrating that the lack of 6H4 binding was not caused by PrPc truncation. Our results indicate that in the absence of GPI anchor, PrPc can be expressed intracellularly and up-regulated on the platelet membrane, likely in a transmembrane form with the C-terminal part of the molecule inserted into the cytoplasm.