In This Issue

See article, p. 125.

Although Ftl3 inhibitor (Flt3i)–targeting drugs offer clinical ben-efits in Flt3-mutant acute myeloid leukemia (AML), some patients are unresponsive (primary resistance) and others progress after initial response (secon-dary resistance). Alotaibi, Yilmaz, and colleagues characterize the genetic spectrum of resistance to type 1 and 2 Flt3i in primary and secondary cases. By targeted next-generation sequencing panel spanning over 20 genes commonly affected in AML, they compare the dynamics of genetic lesion occurrence and allele frequencies in baseline and relapse samples of 67 AML patients who progressed after achieving a complete response to a Flt3i in combination with chemotherapy. In 26% of the patients, the original Flt3 mutation was no longer detectable at relapse, whereas in the majority of the remaining cases, it persisted at near-pretreatment levels. Flt3-D835 mutation emerged in 30% of patients at relapse on type 2 Flt3i and correlated with 2-fold shorter overall survival (OS). Mutations in the Ras/MAPK pathway are most common in patients relapsed on type 1 Flt3i. Ras/MAPK mutations emerging at the time of relapse correlate with 2.5-fold shorter OS. In a cohort of 106 primary resistance patients, lack of response to type 1 Flt3i is more likely in patients with high variant allele frequency of RAS mutations. The findings offer a rationale for therapeutic combinations targeting Flt3 along with its likely genetic escape pathways.

See article, p. 135.

Discoveries of mechanisms and treatments for human myelodysplastic syndrome (MDS) are hampered by the lack of patient-derived mouse models, as mouse host engraftment with primary MDS cells is only transient and requires large amounts of sample. In this work, Mian and colleagues find that MDS patient cells rely on human-derived stromal factors much more than normal hematopoietic stem cells (HSC). Scaffolds seeded with human mesenchymal stem cells (hMSC) and as little as 25,000 patient mononuclear bone marrow cells implanted in irradiated NSG mice achieve on average 10% or higher long-term multilineage engraftment and repopulation of secondary recipients. Human hematopoiesis in these mice preserves the patient's mutation frequencies, myeloid skewing, and dysplastic morphology. Healthy donor–derived hMSCs support even higher engraftment than hMDS-derived stroma. Intravital imaging of the scaffold shows it is vascularized and perfused. MDS cells can migrate out of it and repopulate another scaffold if it is preseeded with hMSC cells, but they do not settle in the mouse bone marrow. This is in contrast to healthy human donor HSCs, which colonize mouse bone marrow as well as other scaffolds. In summary, the study highlights greater dependence of MDS progenitors on healthy, species-matched MSCs as compared to normal HSCs and presents a valuable tool to interrogate MDS in vivo.

See article, p. 146.

The TET family of dioxygenases function as indirect erasers of DNA methylation. Loss-of-function TET2 mutations are one of the most common frequent lesions in myeloid neoplasms. Current pharmacologic approaches targeting TET2 mutations centered on restoring the activity of TET2 are often ineffective. IDH1/2-mutant cells accumulate 2HG, a potent competitive inhibitor of αKG-dependent enzymes including TET, which suggests that IDH and TET mutations could be synthetically lethal, due to minimum basal TET activity requirement for the survival and proliferation of leukemia progenitor and stem cells. Guan and colleagues test the hypothesis that inhibiting all TET activity in TET-mutant leukemia cells could be detrimental to their survival. Mutations of TET2 and IDH1/2 are found mutually exclusive in MDS and AML cohorts, and IDH1/2-mutant cases have higher TET2 expression, indicating that 2HG derived from IDH1/2 mutation could inhibit the residual TET activity and cause synergistic lethality in TET2-mutant cases. When TET2-mutant cell lines are transduced with IDH1^R132C, their growth is inhibited both in vitro and in vivo. The authors then design a small-molecule 2HG-mimetic, TETi76, which selectively inhibits all three TETs but not other related demethylases in a cell-free system. In cell culture models, TETi76 reduces TET activity, mimics TET2 deficiency, and preferentially restricts the growth of TET dioxygenase–deficient neoplastic cells by promoting apoptotic cell death. In mice with TET2-mutated human leukemia xenografts, TETi76 selectively restricts the clonal evolution and tumor growth of TET2-mutant cancer cells. Taken together, these results provide a therapeutic strategy to target TET2-mutant myeloid neoplasms and related disorders.

See article, p. 162.

The MYC gene is frequently amplified and mutated in AML, which contributes to leukemogenesis. It remains poorly understood how MYC drives the development of AML. TFEB is a transcription factor and master regulator of autophagy and lysosome biogenesis. Since MYC is an oncogene in AML and given that autophagy limits cell proliferation and survival in AML, Yun and colleagues investigate the interplay between MYC and TFEB in AML. They report that TFEB expression inversely correlates with MYC in cancer cell lines, bone marrow blasts from AML patients, and tumor samples from three independent AML cohorts. Gain- and loss-of-function studies show that MYC directly suppresses the gene expression of TFEB, its downstream target genes, and the autophagy-lysosome responses both in vitro and in vivo. TFEB functions as tumor suppressor in AML, since ectopic expression of TFEB in tumor cells improves survival in mouse xenografts. In mice and humans, MYC and TFEB expression is inversely regulated during monocytic and granulocytic differentiation, and TFEB promotes myeloid and AML cell differentiation and apoptosis. Mechanistically, RNA-seq, bisulfite-seq, and oxidation bisulfite-seq analyses reveal that TFEB directly upregulates the gene expression of IDH1/2 to drive the expression of key genes involved in myeloid differentiation and cell death. Lastly, a combination of DNMT inhibitor with TFEB-activating agent GSK-621 decreases AML cell survival and provokes AML regression. These results indicate that MYC controls myeloid differentiation through the TFEB–IDH1/2 epigenetic axis, which can be therapeutically targeted in AML.

In This Issue is written by Blood Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details.