Differential Sensitivity of Normal and CML-Derived CD34+ Cells to Inhibition of SHP-2 Gene Expression by RNA Interference (RNAi).

RNA interference (RNAi) has rapidly evolved into an efficient tool for functional genomics in a variety of organisms. Stable expression of shRNA (short hairpin RNA) driven by pol III promoters upon retro- or lentiviral gene transfer can induce long-term gene silencing in mammalian cells, including human hematopoietic cells. We recently demonstrated that lentivirus mediated anti bcr-abl RNAi can specifically silence bcr-abl gene expression, inhibit oncogene driven cell proliferation, and eradicate leukemic cells depending on the dose of lentivirus-mediated shRNA expression (Scherr et al. Gene Therapy 2004). Since effective depletion requires a threshold of lentiviral integrations into target cell genomes, the risk of insertional mutagenesis may limit the therapeutic value of this approach. We therefore applied lentivirus-mediated RNAi for functional genomics in purified primary normal and CD34+ cells from chronic phase CML patients harvested at initial diagnosis. Several SHP-2 shRNAs were generated according to established rules and were functionally evaluated using a bicistronic reporter system as described earlier. Effective shRNA expression cassettes were subsequently cloned into lentiviral plasmids encoding RFP to track lentiviral transduction. Transduction of K562, U937, NB-4 and TF-1 cells with lentiviral supernatants results in a reduction of SHP-2 mRNA and protein by more than 90 %. Interestingly, anti-SHP-2 shRNA induced almost complete depletion of RFP+ cells in all four cell lines, demonstrating that SHP-2 expression is essential for proliferation and survival in these cells. We next transduced normal and CML-derived CD34+ cells with a puritiy of > 95% with control and anti-SHP-2 lentiviruses, and stimulated methylcellulose cultures of the cells with high (GM-CSF: 20 ng/ml; IL-3: 10 ng/ml) or low (GM-CSF: 0.2 ng/ml; IL-3: 0.1 ng/ml) cytokine concentrations. This assay relies on the fact that colony formation of CML-CFU is mediated by both cytokine receptor and bcr-abl signaling. Therefore differential numbers of transduced, i.e. RFP+ colonies under different cytokine stimulations reflect the role of the RNAi-target in normal or malignant CFU. Whereas anti-SHP-2 RNAi did not reduce the proliferation of normal transduced CFU (n=5), proliferation of CFU from CML patients was specifically reduced between 50 to 85 % under low cytokine concentration (n= 9). These data suggest that primary normal cells are more resistant to inhibition of SHP-2 gene expression than leukemic cell lines and CD34+ cells from CML patients and identify SHP-2 as a potential target for anti bcr-abl therapy.