Response to Comment on "PP2A inhibition sensitizes cancer stem cells to ABL tyrosine kinase inhibitors in BCR-ABL+ human leukemia".

LB100 sensitizes resistant chronic phase CML stem and progenitor cells to TKIs and spares healthy bone marrow cells.

PP2A inhibition sensitizes cancer stem cells to ABL tyrosine kinase inhibitors in BCR-ABL+ human leukemia.

Overcoming drug resistance and targeting leukemic stem cells (LSCs) remain major challenges in curing BCR-ABL+ human leukemia. Using an advanced drug/proliferation screen, we have uncovered a prosurvival role for protein phosphatase 2A (PP2A) in tyrosine kinase inhibitor (TKI)-insensitive leukemic cells, regulated by an Abelson helper integration site-1-mediated PP2A-ß-catenin-BCR-ABL-JAK2 protein complex. Genetic and pharmacological inhibition of PP2A impairs survival of TKI nonresponder cells and sensitizes them to TKIs in vitro, inducing a dramatic loss of several key proteins, including ß-catenin. We also demonstrate that the clinically validated PP2A inhibitors LB100 and LB102, in combination with TKIs, selectively eliminate treatment-naïve TKI-insensitive stem and progenitor cells, while sparing healthy counterparts. In addition, PP2A inhibitors and TKIs act synergistically to inhibit the growth of TKI-insensitive cells, as assessed by combination index analysis. The combination eliminates infiltrated BCR-ABL+ blast cells and drug-insensitive LSCs and confers a survival advantage in preclinical xenotransplant models. Thus, dual PP2A and BCR-ABL inhibition may be a valuable therapeutic strategy to synergistically target drug-insensitive LSCs that maintain minimal residual disease in patients.

Targeting primitive chronic myeloid leukemia cells by effective inhibition of a new AHI-1-BCR-ABL-JAK2 complex.

BACKGROUND: Imatinib mesylate (IM) induces clinical remission of chronic myeloid leukemia (CML). The Abelson helper integration site 1 (AHI-1) oncoprotein interacts with BCR-ABL and Janus kinase 2 (JAK2) to mediate IM response of primitive CML cells, but the effect of the interaction complex on the response to ABL and JAK2 inhibitors is unknown. METHODS: The AHI-1-BCR-ABL-JAK2 interaction complex was analyzed by mutational analysis and coimmunoprecipitation. Roles of the complex in regulation of response or resistance to ABL and JAK2 inhibitors were investigated in BCR-ABL (+) cells and primary CML stem/progenitor cells and in immunodeficient NSG mice. All statistical tests were two-sided. RESULTS: The WD40-repeat domain of AHI-1 interacts with BCR-ABL, whereas the N-terminal region interacts with JAK2; loss of these interactions statistically significantly increased the IM sensitivity of CML cells. Disrupting this complex with a combination of IM and an orally bioavailable selective JAK2 inhibitor (TG101209 [TG]) statistically significantly induced death of AHI-1-overexpressing and IM-resistant cells in vitro and enhanced survival of leukemic mice, compared with single agents (combination vs TG alone: 63 vs 53 days, ratio = 0.84, 95% confidence interval [CI] = 0.6 to 1.1, P = .004; vs IM: 57 days, ratio = 0.9, 95% CI = 0.61 to 1.2, P = .003). Combination treatment also statistically significantly enhanced apoptosis of CD34(+) leukemic stem/progenitor cells and eliminated their long-term leukemia-initiating activity in NSG mice. Importantly, this approach was effective against treatment-naive CML stem cells from patients who subsequently proved to be resistant to IM therapy. CONCLUSIONS: Simultaneously targeting BCR-ABL and JAK2 activities in CML stem/progenitor cells may improve outcomes in patients destined to develop IM resistance.