Inhibition of the DOT1L/MPL/JAK2 Signaling Axis Selectively Targets TET2 Mutation-Driven Clonal Hematopoiesis

Clonal Hematopoiesis of Indeterminate Potential (CHIP) refers to the clonal expansion of hematopoietic stem and progenitor cells (HSPCs) carrying mutations associated with hematologic malignancies in individuals without evidence of a blood disorder. CHIP carriers are at a 10-fold higher risk of developing blood cancers relative to non-carriers. The time interval from acquisition of CHIP to overt neoplasia can span many years. This finding suggests a potential window of opportunity to intervene to prevent or delay malignant transformation; however, this goal is not yet possible due to the lack of known effective interventions. One of the most commonly mutated genes in CHIP is TET2, which encodes an epigenetic modifier involved in DNA demethylation. Tet2 knockout mouse models have an expansion of HSPCs and are at increased risk of development of myeloid malignancies, thus establishing TET2 mutation as a bona fide driver of pre-leukemia. To identify drugs that selectively target Tet2-deficient HSPCs, we generated isogenic murine cell lines by overexpressing HOXB4 in Sca-1 + HSPCs derived from a Tet2 knockout (Tet2-/-) mouse and a Tet2 wildtype (Tet2 +/+) littermate. HOXB4 overexpression has previously been shown to expand and immortalize HSPCs indefinitely without causing leukemogenesis. Using this isogenic cell line system termed HPC HOXB4, we conducted a drug screen of 37 small-molecule inhibitors of epigenetic signaling to find compounds that selectively reduced the proportion of Tet2-/- to Tet2+/+ HPC HOXB4 cells in competition assays. Through this screen, we discovered that DOT1L inhibitors, including SGC0946 and pinometostat, preferentially reduced the competitive advantage of Tet2 -/- HPC HOXB4 cells. Importantly, SGC0946 treatment also reduced the competitive advantage of unmodified Tet2 -/- murine HSPCs over wildtype cells in short-term competition assays. Mechanistic studies revealed that DOT1L inhibition induced higher levels of apoptosis in Tet2 -/- than in Tet2+/+ HPC HOXB4 cells and promoted the differentiation of Tet2-/- HPC HOXB4 cells. Genetic knockdown of Dot1l expression using shRNAs phenocopied the effects of pharmacologic DOT1L inhibition. DOT1L is a histone methyltransferase that catalyzes histone H3K79 methylation. Unexpectedly, we found that global H3K79me2 and H3K79me3 levels were higher in Tet2 -/- than in Tet2+/+ HPC HOXB4 cells. SGC0946 treatment effectively removed the H3K79me2/3 mark in Tet2 -/- cells. Next, we conducted RNA-seq analysis to gain insights into the transcriptomic changes in Tet2 -/- HPC HOXB4 after DOT1L inhibition. We found that expression of Mpl, which encodes the thrombopoietin receptor (TPOR), was over 20-fold higher in Tet2 -/- HPC HOXB4 than in wildtype cells at baseline and was strongly decreased after SGC0946 treatment. In contrast, SGC0946 had no effect on Mpl expression in Tet2 +/+ HPC HOXB4 cells. Mpl expression was also elevated in unmodified Tet2 -/- HSPCs compared with Tet2+/+ cells. Given that TPO signaling promotes HSC self-renewal and proliferation, we hypothesized that the effects of Dot1L inhibition are mediated through suppression of Mpl. Consistent with our hypothesis, enforced expression of Mpl in Tet2 -/- HPC HOXB4 cells completely rescued the effects of DOT1L inhibition, and downregulation of Mpl expression using shRNAs phenocopied the effects of DOT1L inhibition on Tet2 -/- HPC HOXB4 cells. Based on the findings above, we hypothesized that inhibition of TPOR signaling would selectively target Tet2 -/- over Tet2+/+ HSPCs. Given that Janus Kinase 2 (JAK2) is required to transduce signals downstream of the TPOR, we studied the effects of two potent JAK2 inhibitors, fedratinib and AZD1480, along with a JAK1/2 inhibitor, ruxolitinib, on Tet2 -/- and Tet2+/+ HPC HOXB4 cells in competition assays. In line with our hypothesis, all three compounds reduced the competitive advantage of Tet2 -/- HPC HOXB4 cells. Ruxolitinib also reduced the competitive advantage of unmodified Tet2 -/- HPSCs cells over wildtype cells in a dose-dependent manner. In summary, our data demonstrate that 1) the TPOR signaling pathway is upregulated in TET2-mutated HSPCs through epigenetic dysregulation by DOT1L, and 2) inhibition of DOT1L or the MPL/JAK2 signaling axis selectively targets TET2-mutated HSPCs. Our findings have important implications in the development of pharmacologic interventions against TET2-mutation driven CHIP. Disclosures Chan: AbbVie: Research Funding; BMS: Research Funding.

TET2 Mutation and High miR-22 Expression as Biomarkers to Predict Clinical Outcome in Myelodysplastic Syndrome Patients Treated with Hypomethylating Therapy

Tet methylcytosine dioxygenase 2 (TET2) is one of the most frequently mutated genes in myelodysplastic syndrome (MDS). TET2 is known to involve a demethylation process, and the loss of TET2 is thought to cause DNA hypermethylation. Loss of TET2 function is known to be caused by genetic mutations and miRNA, such as miR-22. We analyzed 41 MDS patients receiving hypomethylating therapy (HMT) to assess whether TET2 mutation status and miR-22 expression status were associated with their clinical characteristics and treatment outcomes. Responsiveness to HMT was not affected by both TET2 mutation (odds ratio (OR) 0.900, p = 0.909) and high miR-22 expression (OR 1.548, p = 0.631). There was a tendency for TET2 mutation to be associated with lower-risk disease based on IPSS (Gamma = −0.674, p = 0.073), lower leukemic transformation (OR 0.170, p = 0.040) and longer survival (Hazard ratio 0.354, p = 0.059). Although high miR-22 expression also showed a similar tendency, this tendency was weaker than that of TET2 mutation. In summary, the loss of TET2 function, including both TET2 mutation and high miR-22 expression, was not a good biomarker for predicting the response to HMT but may be associated with lower-risk disease based on IPSS, lower leukemic transformation and longer survival.

Pedigree Investigation, Clinical Characteristics and Prognosis Analysis of Hematological Disease Patients with Germline TET2 Mutation

Background: More and more germline gene mutations have been discovered in hematological malignances with the development of next gene sequencing (NGS). Tet methylcytosine dioxygenase 2 (TET2) is one of the most common mutation genes in hematological neoplasms. We aimed to analyzed whether germline TET2 mutation has a family aggregation or is a tumor predisposition gene. Further we compared its impact with somatic TET2 mutation in hematological diseases.Methods: A total cohort of 103 hematological patients with TET2 mutation were included from December 2016 to December 2019. Data were extracted from hematology department of West China Hospital of Sichuan University. Bone marrow (BM) or peripheral blood (PB) as somatic DNA origin to be detected by next-generation sequencing (NGS), and nails and hairs as germline DNA origin to be detected by Sanger sequencing, respectively. Further, we compared the clinical characteristics between the patients with germline and somatic TET2 mutation.Results: 103 patients were included, including 33 (32.03%) patients with germline TET2 mutation and 70 (67.97%) patients with somatic TET2 mutation. Variant allele frequency (VAF) of germline TET2 mutation was more stable in our study, ranging from 40% to 55% and mutation sites were more concentrated. Patients with germline TET2 mutation were younger with median age 48 (range, 16-82) (P=0.0078). Further, patients with germline TET2 mutation were mainly myelodysplastic syndromes (MDS) (n=13, 39.4%), while patients with somatic TET2 mutation were acute myeloid leukemia (AML) (n=28, 40.0%) (P=0.0003). Germline TET2 mutation affected the distribution of peripheral blood cell count and the proportion in bone marrow (P<0.05). Germline TET2 mutation was a poor prognosis factor in MDS patients via univariate analysis (HR=5.3, 95%CI: 0.89-32.2, P=0.0209), but not in multivariate analysis by Cox regression model (P=0.062).Conclusions: Some family members were asymptomatic carriers, which indicated germline TET2 mutation might have a family aggregation. More importantly, TET2 gene may be a predisposition gene of hematological malignant when the other gene mutations as the second hit. The VAF of germline TET2 mutation is more stable. At the same time, the germline TET2 mutation may be an adverse factor for the MDS patients.

Isolated ten-eleven translocation 2 positive in triple negative essential thrombocythemia: Case report and literature review

Essential thrombocythemia is one of the famous diseases under the category of myeloproliferative disorder. It is an end result of a genetic mutation of one or more of the most frequent oncogenes such as Janos kinase 2 (JAK2), MPL proto-oncogene, thrombopoietin receptor (MPL), and calreticulin (CALR). However, negative genetic markers, so-called (triple negative disease), can happen in the presence of other uncommon types of mutation. TET2 (ten-eleven translocation 2) positive as isolated genetic marker in triple negative essential thrombocythemia is uncommon genetic presentation. For that, we are reporting a 22-year-old lady who presented with a feature of dyspepsia and accidentally found to have persistently high platelet count, even after treating her mild iron deficiency anemia with no other secondary causes. Further investigations and bone marrow biopsy supported the diagnosis of isolated TET2 positive in triple negative essential thrombocythemia. We treated her conservatively with good hydration and low dose of aspirin. In conclusion, isolated TET2 positive in triple negative essential thrombocythemia at presentation is uncommon with no clear management or risk stratification guideline. However, it is hypothesized that TET2 mutation precedes JAK2; therefore, the detection of isolated TET2 in a triple negative essential thrombocythemia case should be closely followed for clonal evolution in long term. Further study and guidelines required in this area.

Recurrent and Refractory Hemophagocytic Lymphohistiocytosis in an Elderly. Role of Immune Aberration Due to Myeloid Gene Mutation

Hemophagocytic lymphohistiocytosis (HLH) is a potentially fatal hyper-inflammatory disease induced by aberrant immune activation and subsequent proliferation of macrophages, histiocytes and T-helper cells. In this abstract we present a case of HLH, which relapsed twice despite ongoing treatment, and we hypothesize on possible causes and mechanisms. A 77 year old female presented to our hospital with ongoing fevers and worsening cytopenia. Blood counts from three years before the current presentation showed Hb 120g/L, WBC 4.0 x 10^9/L, Neutrophil count 1.8 x 10^9/L, Lymphocytes count 1.8 x 10^9/L and Platelet count 104 x 10^9/L. A bone marrow examination at that time revealed a normocellular marrow with 28% lymphocytes of which70% were CD 4+, CD 3+, CD5+ and CD 7-. Molecular studies confirmed T cell receptor (TCR) gamma gene rearrangement. The karyotype on the bone marrow was normal. In the absence of clinical symptoms, the patient was regularly followed up without specific therapy. During the current admission, however, the patient was febrile, had progressive pancytopenia and biochemistry suggestive of HLH (Fig 1). She was extensively evaluated which ruled out infective and malignant causes. A bone marrow aspirate and biopsy was performed and treatment initiated as per HLH-94 protocol. The bone marrow examination showed marked features of haemophagocytosis on a normocellular background. A small clone of T-lymphocytes was again noted with similar features as in the first biopsy. In addition, a prominent population (10%) of promonocytes was apparent with an uncertain significance. Karyotype was normal. Next Generation Sequencing showed TET2 frame shift mutation at low variant allele frequency (5%). Patient responded well to treatment. While on tapering dose of steroids, the disease flared up (Fig 1) and the patient was restarted on high dose steroids with etoposide. After a quick initial response, while still on active treatment, she again relapsed within 3 weeks, coupled with sepsis and acute myocardial ischemia, followed by sudden death. We were unable to identify a cause for HLH. There were 10% promonocytes in bone marrow and evidence of aberrant T-cells on flow cytometry. Although there was no obvious evidence of dysplasia on microscopy, the flow cytometry showed up- regulation of CD 64 and CD 14, down regulation of CD13 and 11b, and CD 34 expression in granulocytes possible indicating dysplasia as per the Wells criteria. (Wells et al., Blood 2003; 102(1):393) The Next generation sequencing showed TET2 mutation as mentioned above. Mutations in TET2 have been found to have overrepresented in chronic myelomonocytic leukemia in as much as 50% of patients and around 20-35% of patients with myelodysplastic syndrome (MDS). These patients with TET2 mutation have been found to have altered methylation. Recently TET2 has been implicated in immune regulation with evidence of abnormal CD 4 T cell proliferation (present in our patient) and disruption of T cell homeostasis. In addition, patients with TET2 mutation associated myelodysplastic syndrome are known to have auto-immune manifestations (Yimei Feng et al., Frontiers in Oncology, 2019 (9):1). Alyssa H et al have shown that TET2 mutant in patients with MDS (Myelodysplasia) leads to alteration of immune environment in the macrophage differentiation (Alyssa et al., Experimental Hematology, 2017:55; 56). Whether these immune aberration caused recurrent florid relapse of hemophagocytosis in our patient within a span of 2-3 months remained unclear and it could be considered in future research. Even though the occurrence of hemophagocytic syndrome has been described in acute leukemias, the association of the same with myeloid gene aberrations with or without overt myelodysplastic features is unknown. However since hemophagocytosis itself is not common, a careful evaluation to look for uncommon associations which may be a triggering factor may pave the way for identifying their possible role in the pathogenesis. And therapeutic options such as hypo methylating agents can evolve when such associations are confirmed in future studies. Figure 1 Disclosures No relevant conflicts of interest to declare.

Clonal Architecture Analysis of TET2 Identified Distinct Origins in Myelodysplastic Syndromes

Next-generation sequencing (NGS) has deepened our understanding of myelodysplastic syndromes (MDS). Genetic mutations of TET2 were common in MDS and were also detected in asymptomatic elder persons, which were defined as aged related clonal haematopoiesis (ARCH) and considered as a myelodysplastic syndrome precursor state. In this study, we analyzed the mutation profiles of TET2 in 770 newly-diagnosed MDS subjects. 73 mutations were found in 67 of 770(8.7%) subjects. 14.9% were frame shifts, 29.9% were nonsense, 44.8% were missense and 10.7% harbored 2 TET2 mutations. TET2MT subjects were older than TET2WT subjects (P=0.024) and the variant allele frequency (VAF) of TET2 was positively correlated with age (r=0.318, P=0.009). Clonal architecture was determined using a copy number-adjusted VAF difference between 2 mutation events in 49 TET2MT subjects with 2 or more mutations. 19 (38.3%) were TET2ancestral and 30(61.2%) were TET2subclonal. TET2ancestral subjects were significantly older than TET2WT subjects (P=0.013) while no difference was found between TET2subclonal subjects and TET2WT subjects (P=0.509). The frequency of cytosine-to-thymine (C→T) transition was significantly higher in TET2ancestralsubjects compared with TET2subclonal subjects (P=0.029), which was considered to be a somatic mutational signature of aging, indicating that MDS driven by TET2ancestral was likely derived from TET2MT ARCH. The most common upstream mutations in TET2subclonal subjects were U2AF1 (16.7%) and ASXL1 (13.3%), and the most common downstream mutations in TET2ancestral subjects were ASXL1 (21.1%) and RUNX1 (15.8%). TET2 mutations rarely existed alone in TET2ancestral subjects (14.3%) and were always accompanied by another mutations like U2AF1, SF3B1 and ASXL1, suggesting that the acquisition of another mutation led to the progress of ARCH to MDS in TET2ancestral subjects. TET2MT had no impact on survivals in overall cohort (P=0.218) while predicted poorer survivals in IPSS-R lower risk group (P=0.004). Additional ASXL1, U2AF1 and RUNX1 mutations in TET2MT subjects indicated poorer prognosis compared with TET2WT subjects (P=0.005; P=0.04; P&lt;0.001) .There was no significant difference in OS of TET2MT subjects with different clonal architecture (P=0.76). Subjects with TET2 VAF≥30% had poorer survivals compared with TET2WT subjects (P=0.057). In conclusion, TET2MT MDS subjects with different clonal architectures may have different origins. TET2ancestral subjects may be derived from ARCH and progressed to MDS after secondary hits. The effects of TET2 mutation on the prognosis depended on the accompanying mutations and clonal burdens. Disclosures No relevant conflicts of interest to declare.