A Glu-496 to Ala polymorphism leads to loss of function of the human P2X(7) receptor

P2X(7) receptor is a ligand-gated cation-selective channel that mediates AT P-induced apoptosis of cells of the immune system. We and others have shown that P2X(7) is nonfunctional both in lymphocytes and monocytes from some s ubjects. To study a possible genetic basis we sequenced DNA coding for the carboxyl-terminal tail of P2X(7). In 9 of 45 normal subjects a heterozygous nucleotide substitution (1513A-->C) was found, whereas 1 subject carried t he homozygous substitution that codes for glutamic acid to alanine at amino acid position 496. Surface expression of P2X(7) on lymphocytes was not aff ected by this E496A polymorphism, demonstrated both by confocal microscopy and immunofluorescent staining. Monocytes and lymphocytes from the E496A ho mozygote subject expressed nonfunctional receptor, whereas heterozygotes sh owed P2X(7) function that was half that of germline P2X(7). Results of tran sfection experiments showed that the mutant P2X(7) receptor was nonfunction al when expressed at low receptor density but regained function at a high r eceptor density. This density dependence of mutant P2X(7) function was also seen on differentiation of fresh monocytes to macrophages with interferon- gamma, which up-regulated mutant P2X(7) and partially restored its function . P2X(7)-mediated apoptosis of lymphocytes was impaired in homozygous mutan t P2X(7) compared with germline (8.6 versus 35.2%). The data suggest that t he glutamic acid at position 496 is required for optimal assembly of the P2 X(7) receptor.