EGFR inhibitors induced apoptosis of both acute and chronic myeloid leukemia stem/progenitor cells.
To test the effects of EGFRi on leukemic stem/progenitor cells, we isolated CD34+ leukemic stem/progenitor cells from AML patients by FACS and then treated CD34+ AML cells in culture with AZD9291(osimertinib), a third generation covalent EGFR inhibitors. The percentage of apoptosis was measured using Annexin V/PI assay. As shown in Fig. 1a, treatment with osimertinib from 0.5 µΜ dose-dependently induced apoptosis of CD34+ leukemic stem/progenitor cells. Similar effects were also observed from other covalent EGFR inhibitors including afatinib and rociletinib (Fig. 1b)11, 25. Surprisingly, the EGFR inhibitors also displayed remarkable apoptosis-inducing effects on primary CD34+ cells sorted from CML patients (Fig. 1c), which population is believed to be responsible for relapse of CML due to insensitivity to BCR-ABL inhibitor imatinib6, demonstrating that EGFR inhibitors kill stem/progenitor cells of myeloid leukemia.
We next evaluate the effect of EGFR inhibitors on CD34− leukemia cells purified from AML and CML patients. As depicted in Fig. 2a, exposure of CD34− cells from AML and CML individuals to different EGFR inhibitors did not induce significant cell death compared to its CD34+ counterparts, suggesting a specificity of EGFR-inhibitors towards CD34+ cells. Moreover, incubation of the bone marrow mononuclear cells (BMMCs) from AML samples with osimertinib demonstrated that CD34+ cells within the bulk population were more susceptible than CD34− population (Fig. 2b).
Osimertinib Induced Loss Of Y329 Phosphorylation Of Cd34
To reveal the mechanism of EGFRi, we first looked for the expression of EGFR on CD34+ leukemia stem/progenitors cells from AML and CML patients. Taking the EGFR+ lung carcinoma cell line A549 as a positive control, we conducted western blot, immunofluorescent staining and flow cytometry analysis using antibody against EGFR or Y1068-phosphorylated EGFR, an active form of EGFR26. In accordance with previous observations14, 18, 20, EGFR protein in CD34+ leukemia stem/progenitors cells were barely detectable (Supplementary Fig. 1).
Next, we investigated EGFR-independent mechanisms for the inhibitory effects of EGFRi in CD34+ leukemia stem/progenitor cells. We carried out quantitative proteomic analyses of phospho-tyrosine(Y) peptides to explore intracellular signaling events in response to osimertinib. CD34+ cells isolated from 3 AML individuals were pooled together and treated with osimertinib followed by analysis of tyrosine-phosphorylated sites targeted by osimertinib using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). Previously reported large-scale phosphoproteomic studies indicated that off-targets of EGFR inhibitors are preferentially intracellular non-receptor tyrosine kinases including Syk and Lyn12, 18, 27, 28. As expected, the group of proteins annotated as tyrosine protein kinases was significantly overrepresented as being under-phosphorylated in the phosphoproteomes of osimertinib-treated cells (Fig. 3a, Supplementary Table 1). Notably, phosphorylation of a significant portion of membrane proteins was inhibited upon osimertinib exposure, which was further supported by STRING analysis (Fig. 3a and 3b, Supplementary Table 2). Meanwhile, motif analysis revealed the osimertinib targeted “-E-X-X-pY” motif, which is enriched (fold increase 4.66 vs 3.88) in downregulated phospho-tyrosine sites compared to all regulated sites (right panel, Fig. 3b). Among the 23 proteins containing the “-E-X-X-pY” motif (Supplementary Table 3), there are 7 protein tyrosine kinases and unexpectedly, one membrane molecule CD34, the marker of stem/progenitor cells that lacks tyrosine kinase activity(right panel, Fig. 3b)29, 30. A remarkable loss of Y329 phosphorylation of CD34 after osimertinib exposure was observed by phosphoproteomic analysis (Fig. 3a and Supplementary Fig. 2). Next we validated the downregulation of Y329 phosphorylation of CD34 (Fig. 3d), but not changes in the protein level of CD34 (Fig. 3c) by osimertinib treatment using parallel reaction monitoring-based targeted MS (PRM-MS) in 5 AML patient samples as well as CD34-expressing KG-1 and Kasumi-1 leukemia cell lines (Fig. 3c-e, Supplementary Fig. 2–8). This finding was recapitulated in an independent set of AML specimens using a custom-designed antibody against the phosphorylated Y329 residue of CD34(CD34-pY329) (Supplementary Fig. 9). Taken together, these data demonstrate that treatment with osimertinib inhibits phosphorylation of Y329 in CD34.
EGFR covalent inhibitors directly bind with CD34 at cysteine 199
We next investigated how EGFR inhibitors associate with CD34. One common feature of leukemia stem/progenitors cells from AML and CML patient samples is the expression of CD34. Moreover, EGFRi do not kill CD34− leukemia cells. We proposed that CD34 might be a non-tyrosine kinase target of EGFRi. To test this hypothesis, we incubated purified recombinant CD34 with osimertinib or afatinib, two covalent EGFRi, followed by MS analysis. We got 44% sequence coverage of CD34 protein and identified all the 6 cysteines within the cysteine-rich domain in CD34 sequence. Notably, the m/z ratio of the C199-containing peptide TSSCAEFKK (Fig. 4a) was measured as 1,056.49 in the absence of osimertinib and 1,498.74 in the presence of osimertinib. For afatinib binding assay, the m/z ratio of the same C199-containing peptide was measured as 1,484.63 and 1,056.49 with/without afatinib (Fig. 4b). The calculated mass shift was consistent with the addition of one molecule of osimertinib or afatinib respectively. Clearly, EGFR covalent inhibitors bind the C199 residue of CD34 in vitro.
To clarify the role of CD34 in mediating the apoptotic effect induced by osimertinib, we depleted CD34 in primary CD34+ AML cells, KG-1 and Kasumi-1 cell with the CRISPR-Cas9 system. As shown in Fig. 4c, knockout of CD34 caused primary leukemia cells less sensitive to osimertinib, which could be restored by re-expression of CD34WT but not CD34C199S (Fig. 4d), indicating that the binding to C199 of CD34 is essential in mediating the effect of osimertinib.
Osimertinib inhibits phosphorylation of STAT3 through impairing the interaction between CD34 and Src
As leukemia cell maintenance and growth depends upon kinase-mediated signal cascade, we next investigated the kinase signals that are associated with modification of CD34 in response to osimertinib. We treated primary CD34+ cells with osimertinib and probed several major signaling pathways. Constitutive signal transducer and activator of transcription 3(STAT3) activation were observed in CD34+ AML patient samples (Fig. 4e) as well as KG-1 and Kasumi-1 leukemia cell lines (Fig. 4g), but not in purified CD34− primary leukemia cells (Fig. 4f). Osimertinib significantly suppressed STAT3 activation without altering the protein level of STAT3 (Fig. 4e). Phosphorylation of ERK (p44/42) and AKT was not altered upon osimertinib treatment (Fig. 4e and 4g). Moreover, osimertinib treatment did not induce significant downregulation of STAT3 in CD34C199S-expressing KG-1 cells compared to its WT counterpart (Fig. 4h), indicating that STAT3 is the major kinase that is responsible for apoptosis-inducing effect of osimertinib. In support of this hypothesis, STAT3 inhibitor stattic31 induced synthetic lethality with osimertinib (Fig. 4i and 4j).
STAT3 can be activated by growth factor receptors or cytokine receptors, usually via kinases such as janus-activated kinase (JAK) or proto-oncogene tyrosine-protein kinase Src, leading to increased cell survival32, 33. To clarify the intracellular signals between CD34 and activated STAT3, we pooled CD34+ cells from 3 AML individuals, treated with/without osimertinib and conducted endogenous immunoprecipitation–mass spectrometry (IP–MS) assays using an antibody to CD34. STAT3 was not observed in the CD34-immunoprecipitate, suggesting absence of direct interaction between CD34 and STAT3 (Supplementary Table 4). Among the CD34-interacting proteins, Src was identified to be downregulated upon osimertinib treatment (Fig. 5a, Supplementary Table 4) and was further validated in KG-1 and Kasumi-1 cells as well as primary CD34+ cells using antibody against CD34 (Fig. 5b-d). The association between Src and CD34 was further confirmed through IP assay using an antibody to Src (Fig. 5b and 5c) and immunofluorescent staining (Fig. 5e and 5f). More importantly, the endogenous interaction between CD34 and Src was impaired upon osimertinib treatment (Fig. 5g), providing a molecular basis for osimertinib-induced downregulation of STAT3 phosphorylation. Of note, Crk-like protein(CRKL), a previously reported CD34-interacting adaptor protein, was not immunoprecipitated by anti-CD34 antibody in our system (data not shown)34, 35. Collectively, we proposed that the binding of osimertinib to CD34 impaired its interaction with Src, leading to impaired activation of STAT3 that is necessary for the maintenance of CD34+ leukemia cells. Supporting this finding, phosphorylation of Src was apparently suppressed by osimertinib (Fig. 4e-g) and Src inhibitor saracatinib36 synergistically induced apoptosis with osimertinib in primary CD34+ cells (Fig. 5h).
Effect of Osimertinib on CD34 + cells in PDX preclinical models
To assess the clinical relevance of these findings, the in vivo effects of osimertinib on CD34+ leukemia stem/progenitor cells were further evaluated by patient-derived xenograft (PDX) AML and CML models. We sorted CD34+ cells from AML or CML patients and exposed them to osimertinib for 48 hours before transplantation into sub-lethally irradiated NSG mice (Fig. 6a). Human CD45+ cells were detectable by flow cytometry in bone marrow 16 weeks post-transplantation, representing successful engraftment of human leukemia stem cells. Percentage of CML CD34+ cells were largely reduced in osimertinib-treated group (Fig. 6c). Osimertinib clearly reduced engraftment of CD34+ cells as indicated by decreased numbers of CD45+ and CD33+ cells in the bone marrow of both AML and CML PDX models (Fig. 6b-c).
Osimertinib Therapy In Cd34-high AML Patients
Based on our analyses that CD34+ leukemia stem/progenitors cells can be targeted by covalent EGFR inhibitors, we reasoned that the controversial outcomes of clinical trials might be due to difference of the expression of CD34+ across clinical samples such that administration of EGFR inhibitors might only be of benefit to the subsets of AML with high percentages of CD3437. We next purified the CD34+ cells and measured the level of CD34 expression on leukemia cells using quantitative proteomics (Fig. 7a). We observed that CD34 expression on AML CD34+ cells were higher than CD34+ cells from healthy donors. The group of patients(n = 15) in the upper quartile was designated as “CD34-high” population among all the AML(n = 59). Furthermore, osimertinib treatment on normal CD34+ cells displayed less toxicity compared to its AML counterpart (Fig. 6d), suggesting a therapeutic window. Next, after obtaining informed consent, two CD34-high patients who have failed to respond to any available therapy was initiated on 80 mg once daily of osimertinib.
Patient 1, a 47-year-old woman was diagnosed with AML-M0 with a peripheral white blood cell (WBC) count of 223×109/L, and morphologically 90% of which were myoblasts. The percentage of CD34+ blasts was 80.3% (Fig. 7b). After diagnosed with AML-M0, induction chemotherapy with HA(homoharringtonine and cytosine arabinoside)was begun, but on day 21 of chemotherapy interval bone marrow showed 97% blasts indicating refractory disease. Her disease was refractory to a second induction on a clinical trial (ChiCTR1800019049) using FLAG (fludarabine, cytarabine and filgrastim) combined with decitabine, and then a third line treatment with CLAG (cladribine, cytarabine and G-CSF) with 34% blasts in the bone marrow on day 28. Finally, she rejected any further chemotherapy for the intolerable myotoxicity, and requested oral drugs to expect an accidental prolongation for her life. Considering the status of multiple drug resistance and the potential efficacy for AML from the case reports, she was voluntarily started on osimertinib (off-label, non-protocol) 80 mg once daily (day 0) after evaluating the response of her CD34+ BMMC to osimertinib in vitro (Fig. 1a, AML-#4). Her WBC count dropped sharply from 24.9×109/L to 7.1×109/L at day 2 and remained below 5×109/L till day 17(Fig. 7b). A bone marrow biopsy demonstrated 5% of blast in the bone marrow on day 14(Fig. 7d).
Patient 2, a 69-year-old man was diagnosed with AML-M2a, when he presented with dyspnea, dizziness, and a WBC count of 68.3×109/L with 50.1% of marrow blasts CD34+. He developed pneumonia of left upper lobe. Given his poor performance status, he rejected any chemotherapy and voluntarily started on osimertinib 80 mg once daily (day 0) and the WBC count dropped from 24.9×109/L to 3.1×109/L on day 3 and remained below 5×109/L till day 9 (Fig. 7c and 7e).
In parallel, the level of Y329-phosphorylated CD34(pCD34-Y329) decreased significantly after administration of osimertinib in both patients (Fig. 7f). Collectively, these data suggested an opportunity to evaluate osimertinib’s usage for treating AML patients from the subgroup with high percentage of CD34 and warrants clinical investigation in the treatment of refractory/resistant AML.