Silencing of genes required for glycosylphosphatidylinositol anchor biosynthesis in Burkitt lymphoma

Objective To investigate the mechanism of glycosylphosphatidylinositol (GPI) anchor deficiency in Burkitt lymphoma cell lines. Methods We identified a large GPI anchor protein deficient population in three different Burkitt lymphoma cell lines through proaerolysin treatment of the cells and flow cytometry analysis using a proaerolysin variant (FLAER). The mechanism of GPI anchor protein deficiency was studied by DNA gene sequencing, a cell-free assay to investigate the GPI anchor biosynthetic pathway, microarray analysis, and quantitative real-time polymerase chain reaction. Results Burkitt lymphoma cell lines harbor large populations of FLAERneg cells, which are resistant to proaerolysin. In all three cell lines, silencing of a gene involved in an early step in GPI-anchor biosynthesis was responsible for the lack of GPI-anchored proteins on the cell surface. Quantitative polymerase chain reaction and microarray analysis demonstrate that the level of mRNA for PIGL and PIGY is lower in the FLAERneg Ramos cells and that mRNA levels of PIGY are reduced in the Akata and Daudi cells. Hypermethylation of these genes was associated with the low levels of mRNA and treatment of the cells with 5-aza-2′ deoxycytidine restored cell surface GPI-anchored proteins to the FLAERneg cells. Conclusion GPI-anchored protein deficiency in Burkitt lymphoma cells is not due to a genetic mutation (e.g., PIGA ); rather, the lack of GPI-anchored proteins results from transcriptional silencing of PIGL and PIGY.