Synthetic lethality in leukemic stem cells in juvenile myelomonocytic leukemia

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Project Summary Juvenile myelomonocytic leukemia (JMML), a clonal hematological malignancy of early childhood, has limited therapeutic options. Relapse remains the main cause of treatment failure, most likely due to the persistence of leukemic stem cells (LSCs), a small population of self-renewing precursor cells that give rise to the bulk of tumor cells. This reservoir of tumor cells is responsible for long-term maintenance of leukemia growth and is also a major source of drug resistance. Therefore, a novel treatment approach focused on the unique characteristics and vulnerabilities of LSCs is needed in order to target this root cause of JMML. However, it remains a critical challenge how these leukemic precursor cells may be eradicated. JMML is caused by various genetic mutations in the signaling proteins involved in the Ras pathway, among which protein tyrosine phosphatase Ptpn11 (Shp2), a positive regulator of the Ras pathway, is most frequently mutated. These mutations cause greatly increased catalytic activity of Shp2, and JMML patients with Ptpn11 mutations have the worst prognosis in all subtypes of JMML. We previously created a line of conditional knock-in mice with the Ptpn11E76K mutation, the most common and active Ptpn11 mutation found in JMML. Induction of the Ptpn11E76K/+ mutation in these mice (Ptpn11E76K/+/ Mx1-Cre+) resulted in JMML-like myeloproliferative neoplasm (MPN) with full penetrance, suggesting a causative role of this mutation in the pathogenesis of JMML. In an effort to understand the biological properties of leukemic precursor cells in JMML, we recently examined mutated hematopoietic stem cells in this mouse model (referred to as LSCs) and found that Stat5 was hyper-activated. More importantly, Ptpn11E76K/+ LSCs appeared to rely on Stat5 for maintenance and self-renewal because deletion of Stat5 in Ptpn11E76K/+/Mx1-Cre+/Stat5fl/fl double mutant mice resulted in massive LSC cell death while neither Stat5 knockout nor Ptpn11E76K/+ knock-in alone mice had such phenotypes in stem cells. Consequently, the LSC pool in Ptpn11E76K/+/Mx1-Cre+/Stat5fl/fl double mutant mice was markedly decreased. Further mechanistic investigation revealed that this was because tyrosine (Tyr705) phosphorylation of Stat3, one of the Shp2 substrate proteins, was decreased by 3 to 5-fold in Ptpn11E76K/+ LSCs likely due to enhanced dephosphorylation by the hyperactive Shp2 E76K mutant. Thus, diminished Stat3 activity represents an Achilles’ heel of Ptpn11E76K/+ LSCs, making them vulnerable to Stat5 depletion-induced cell death. The synthetic lethality induced by loss of Stat5 in Ptpn11-activated JMML stem cells raises an intriguing possibility, i.e., pharmacological inhibition of Stat5 might be effective in eradicating LSCs in this subtype of JMML. We plan to test this hypothesis and accomplish the objective of this project by pursuing the following two aims. 1) To define the role of Stat5 in maintenance of LSCs in Ptpn11-associated JMML. 2) To test the therapeutic effects of the Stat5 inhibitor pimozide, a clinically used antipsychotic drug, in Ptpn11 (Shp2)-activated JMML.