PTPN11 Mutations in JMML Confer Dasatinib Sensitivity in a TNK2-Dependent Manner

Introduction: Juvenile myelomonocytic leukemia (JMML) is a rare disease characterized by an overproduction of granulocytes and monocytes, with cells displaying hypersensitivity to stimulation by the cytokine GM-CSF in culture. The only curative approach for JMML is allogeneic hematopoietic stem cell transplant, and disease recurrence is the leading cause of death in these patients, occurring within two years in 50% of patients. New therapeutic strategies are needed to improve treatment and survival.

Methods: In order to better identify therapeutic strategies, our lab has developed ex vivo functional assays with small-molecule inhibitors and RNAi for the profiling of actionable targets in primary patient samples. In these assays, mononuclear cells from patient bone marrow or blood are cultured in the presence of small molecule inhibitor or RNAi libraries and cell viability is measured by colorimetric assay. To investigate targets identified in these assays, gene constructs are transfected into HEK293 T17 cells for immunoblots or transduced into mouse bone marrow cells for methylcellulose-based colony formation assays.

Results: Mononuclear white blood cells from a patient with recurrent JMML were found to be sensitive to the multi-family kinase inhibitor dasatinib as well as siRNA targeting the dasatinib target tyrosine kinase non-receptor 2 (TNK2, also known as ACK1), suggesting an over-reliance on TNK2 for cell viability. TNK2 was recently demonstrated by our lab and others to regulate signaling downstream of signaling pathways including EGFR, ER, Src, and CSF3R. Based on the ex vivo functional data for this JMML specimen, the patient was placed on daily dasatinib and achieved marked improvement, which bridged the patient to an additional bone marrow transplant, resulting in disease-free survival for another year.

This patient had a mutation in PTPN11 (also known as SHP2), which encodes a protein tyrosine phosphatase that is mutated in approximately 35% of JMML patients. PTPN11 is an auto-inhibited phosphatase that dephosphorylates targets in many signaling pathways including RAS-MAPK, with an overall positive effect on pathway activity and cellular proliferation. In JMML, mutations in residues of the N-terminal SH2 domain of PTPN11 result in a constitutively active form of the protein, which increases proliferation and cell survival and is linked to dysregulation of RAS/MAPK.

Results of in vitro experiments demonstrate, for the first time, an interaction of TNK2 with mutant PTPN11. Co-expression of mutant-PTPN11 and TNK2 results in increased phospho-PTPN11 at two sites associated with activation of PTPN11 (Y542 and Y580), and co-immunoprecipitation experiments indicate direct interaction of TNK2 with PTPN11. These phosphorylation events coincide with significantly increased MEK/ERK signaling in the context of TNK2/mutant PTPN11 co-expression relative to expression of either gene alone. Expression of a TNK2 mutant harboring a loss-of-function mutation at a key activating residue (TNK2 Y284F) reduces PTPN11/MAPK activation, whereas expression of a gain-of-function, drug-resistant TNK2 gatekeeper mutant increases PTPN11/MAPK activation when compared to wild-type TNK2. Inhibition of TNK2 with either dasatinib or the TNK2-specific inhibitor AIM100 also reduces PTPN11/MAPK activation. Interestingly, phospho-TNK2 levels are significantly reduced in the presence of mutant-PTPN11, suggesting a feedback mechanism that has yet to be fully elucidated.

Functional experiments suggest that mutant-PTPN11 and TNK2 work in a synergistic fashion to increase cell transformation and cytokine hypersensitivity. Mouse bone marrow cells transduced with both mutant PTPN11 and TNK2 have significantly more colonies in methylcellulose colony formation assays than with mutant- PTPN11 alone or with wild-type PTPN11 and TNK2. Treatment of cells in colony formation assays with either dasatinib or AIM100 inhibits colony formation, suggesting a dependence on TNK2 kinase activity for cell transformation.

Conclusions: Taken together, the preliminary data suggest that PTPN11 mutations may be a functional marker for dasatinib sensitivity, and that the underlying mechanism for this sensitivity may be through the inhibition of TNK2 activity by dasatinib. Our aim is further validate the role of PTPN11 mutations as a marker for efficacy of dasatinib or other TNK2 inhibitors.