An Ile-568 to Asn Polymorphism Prevents Normal Trafficking and Function of the Human P2X7 Receptor

The P2X7 receptor is a ligand-gated channel that is highly expressed on mononuclear cells and that mediates ATP-induced apoptosis of these cells. Wide variations in the function of the P2X7 receptor have been observed, in part because of a loss-of-function polymorphism that changes Glu-496 to Ala without affecting the surface expression of the receptor on lymphocytes. In this study a second polymorphism (Ile-568 to Asn) has been found in heterozygous dosage in three of 85 normal subjects and in three of 45 patients with chronic lymphocytic leukemia. P2X7 function was measured by ATP-induced fluxes of Rb+, Ba2+, and ethidium+ into various lymphocyte subsets and was decreased to values of ∼257 of normal. The expression of the P2X7 receptor on lymphocytes was approximately half that of normal values as measured by the binding of fluorescein-conjugated monoclonal antibody. Transfection experiments showed that P2X7 carrying the Ile-568 to Asn mutation was non-functional because of the failure of cell surface expression. The differentiation of monocytes to macrophages with interferon-γ up-regulated P2X7 function in cells heterozygous for the Ile-568 to Asn mutation to a value around 507 of normal. These data identify a second loss-of-function polymorphism within the P2X7 receptor and show that Ile-568 is critical to the trafficking domain, which we have shown to lie between residues 551 and 581.