Inhibition of signaling in MPN patients with a selective JAK2 inhibitor. (A) Representative plots of inhibition of STAT5 signaling in CML (left panel) and JAK2 V617F–positive PV (middle left panel) patient CD34⁻ cells and the HEL cell line (middle right panel) after exposure to imatinib (CML) or TG101209 (PV and HEL) for 4 hours (filled histogram). DMSO-treated controls are middle histograms in the overlay (solid line). The relative value, percent inhibition was calculated as described in “Methods.” As is demonstrated in the bar chart (right panel), HEL showed marked inhibition of STAT5 signaling. MPNs showed only a modest level of inhibition that was independent of the JAK2 mutant genotype and similar to the level of inhibition of phospho-STAT5 in CML cells treated with imatinib. However, CD34⁻ samples from controls were also inhibited to a similar degree. (B) A similar comparison in CD34⁺ cells. Representative histograms of a CML patient (left panel), a JAK2 V617F–positive PV patient (middle left panel), and a control (middle right panel) are shown. As is demonstrated in the bar chart in the right panel, the level of inhibition is modest but is independent of JAK2 genotype and is similar to the level obtained in CD34⁺ cells from CML patients treated with imatinib. However, once again, at least comparable levels of inhibition are seen in normal CD34⁺ samples.