Abstract
Glycosylphosphatidylinositol is an important means of anchoring many cell surface proteins. Glycosylphosphatidylinositol anchored proteins (GPI-AP) are distributed on all hematopoietic lineages, but are absent on hematopoietic cells from patients with paroxysmal nocturnal hemoglobinuria (PNH). The absence of GPI-AP in PNH is due to mutations in PIG-A, whose product is necessary for the 1st step in GPI biosynthesis. A small percentage of GPI-AP deficient cells can be found in cell lines and that these GPI-APlo/neg cells do not harbor PIG-A mutations. The significance and mechanism of the GPI-AP deficiency in these cells are unclear. We found that 25% of the Burkitts’ lymphoma cell line, Ramos, is GPI-APlo/neg after staining with FLAER and that these GPI-APlo/neg cells do not harbor PIG-A mutations. After 2 days cultured in regular medium, the GPI-APlo/neg Ramos cells reverted to a mix of GPI-APlo/neg and GPI-APpos cells demonstrating that the GPI-APlo/neg cells appear to be precursors to the GPI-APpos cells. An in vitro assay for early steps in GPI anchor biosynthesis using UDP-[3H]GlcNAc found that the GPI-APlo/neg cells generated reduced amounts of the 1st and 2nd GPI intermediates, GlcNAc-PI and GlcN-PI. RT-PCR of the 24 known genes involved in GPI anchor biosynthesis revealed silencing of PIGL and to a lesser extent, PIGY. Methylation specific PCR demonstrated that PIGL was hypermethylated in the FLAERlo/neg Ramos cells. Furthermore, culturing the FLAERlo/neg Ramos cells in 5-Aza-2′ deoxycytidine greatly increased the percentage of cells displaying surface GPI-AP, suggesting that demethylating PIGL and perhaps PIGY may restore surface expression of GPI-AP. We hypothesized that primitive HSC may also enriched for GPI-APlo/neg cells. Thus, we isolated small lineage depleted Fr25Lin− cells from C57Bl6/NCR (Ly 5.2) mice as described1 and found that 30% were GPI-APlo/neg. Fr25lin−GPI-APlo/neg cells were highly enriched for HSC/progenitor cells using hematopoietic colony forming assays. We next transplanted lethally irradiated female recipients with either 100 Fr25Lin−FLAERlo/neg or Fr25Lin−FLAERpos, respectively. 3/4 animals transplanted with Fr25lin−FLAERlo/neg cells survived 17 weeks and revealed 81%, 85% and 90% engraftment; 2/4 animals transplanted with 100 Fr25lin−FLAER pos cells survived 17 weeks with 17% and 48% engraftment. Similar to the Ramos cells, RT-PCR analysis of the Fr25lin−GPI-APlo/neg cells revealed silencing of pigl and to a lesser extent, pigy. In this study, we found that reduced surface expression of GPI-AP is a common feature of cancer cell lines and primitive hematopoietic progenitor cells. In contrast to the fixed GPI-AP deficiency in HSC from PNH patients, the absence of GPI-AP in certain cancer cell lines and normal HSC is not fixed; the progeny of these cells acquire GPI-AP upon cellular differentiation. Furthermore, our data suggest that silencing of the genes required for early steps in GPI anchor biosynthesis, most notably PIG-L and PIG-Y, are responsible for the paucity of GPI-AP on the HSC/progenitor cells.
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