Targeting of cancer stem cells is believed to be essential for curative therapy of cancers, but supporting evidence is limited. Few selective target genes in cancer stem cells have been identified. Here we identify the arachidonate 5-lipoxygenase (5-LO) gene (Alox5) as a critical regulator for leukemia stem cells (LSCs) in BCR-ABL-induced chronic myeloid leukemia (CML). In the absence of Alox5, BCR-ABL failed to induce CML in mice. This Alox5 deficiency caused impairment of the function of LSCs but not normal hematopoietic stem cells (HSCs) through affecting differentiation, cell division, and survival of long-term LSCs (LT-LSCs), consequently causing a depletion of LSCs and a failure of CML development. Treatment of CML mice with a 5-LO inhibitor also impaired the function of LSCs similarly by affecting LT-LSCs, and prolonged survival. These results demonstrate that a specific target gene can be found in cancer stem cells and its inhibition can completely inhibit the function of these stem cells.

SUMMARY Fanconi Anemia (FA) is an inherited DNA repair disorder characterized by progressive bone marrow failure (BMF) from hematopoietic stem and progenitor cell (HSPC) attrition. A greater understanding of the pathogenesis of BMF could improve the therapeutic options for FA patients. Using a genome-wide shRNA screen in human FA fibroblasts, we identify transforming growth factor-β (TGF-β) pathway-mediated growth suppression as a cause of BMF in FA. Blocking the TGF-β pathway improves the survival of FA cells and rescues the proliferative and functional defects of HSPCs derived from FA mice and FA patients. Inhibition of TGF-β signaling in FA HSPCs results in elevated homologous recombination (HR) repair with a concomitant decrease in non-homologous end-joining (NHEJ), accounting for the improvement in cellular growth. Together, our results suggest that elevated TGF-β signaling contributes to the BMF in FA by impairing HSPC function, and may be a potential therapeutic target for the treatment of FA.

N 6 -methyladenosine (m 6 A) is a commonly present modification of mammalian mRNAs and plays key roles in various cellular processes. m 6 A modifiers catalyze this reversible modification. However, the underlying mechanisms by which these m 6 A modifiers are regulated remain elusive. Here we show that expression of m 6 A demethylase ALKBH5 is regulated by chromatin state alteration during leukemogenesis of human acute myeloid leukemia (AML), and ALKBH5 is required for maintaining leukemia stem cell (LSC) function but is dispensable for normal hematopoiesis. Mechanistically, KDM4C regulates ALKBH5 expression via increasing chromatin accessibility of ALKBH5 locus, by reducing H3K9me3 levels and promoting recruitment of MYB and Pol II. Moreover, ALKBH5 affects mRNA stability of receptor tyrosine kinase AXL in an m 6 Adependent way. Thus, our findings link chromatin state dynamics with expression regulation of m 6 A modifiers and uncover a selective and critical role of ALKBH5 in AML that might act as a therapeutic target of specific targeting LSCs.

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IntroductionThe human Philadelphia chromosome (Ph) arises from a reciprocal translocation between chromosome 9 and 22, resulting in the formation of chimeric BCR-ABL oncogene. BCR-ABL encodes a constitutively activated, oncogenic tyrosine kinase. 1 Ph ϩ leukemia induced by BCR-ABL includes chronic myeloid leukemia (CML) and B-cell acute lymphoid leukemia (B-ALL). The BCR-ABL kinase inhibitor imatinib mesylate induces a complete hematologic and cytogenetic response in the majority of chronic-phase CML patients 2 but is unable to completely eradicate BCR-ABLexpressing leukemic cells, 3,4 suggesting that leukemia stem cells are not eliminated. Over time, patients frequently become drugresistant and develop progressive disease despite continued treatment. [5][6][7] Moreover, B-ALL is less sensitive to imatinib, suggesting that inhibition of BCR-ABL kinase activity is not enough to suppress B-ALL development. New therapeutic strategies need to be developed for Ph ϩ leukemia.Tumors progress to more advanced stages after acquiring additional genetic alterations, and inactivation of tumor suppressor genes are common in human cancers. Phosphatase and tensin homolog (PTEN) 8 is often deleted or inactivated in many tumor types, including glioblastoma, 9 endometrial carcinoma, 10 and lymphoid malignancies. 11 PTEN is a phosphatase that dephosphorylates phosphatidylinositol-3-trisphosphate. 12,13 Phosphatidylinositol-3-trisphosphate is a direct product of phosphoinositide 3-kinase (PI3K) activity and plays a critical role in the regulation of cell survival and growth by activating the Ser/Thr protein kinase PDK1 and its downstream target Akt. 14,15 Activated Akt mediates several well-described PI3K responses that include cell survival and growth, cellular metabolism, angiogenesis, and cell migration.Mice with a complete null mutation of Pten develop early embryonic lethality at E9.5. [16][17][18] Pten-heterozygous mice die within 1 year after birth, and survivors develop a broad range of tumors, including mammary, thyroid, endometrial, and prostate cancers, [16][17][18] as well as autoimmunity related to Fas-mediated response. 19 Mice with the tissuespecific deletion of Pten using the Cre-loxP system have become available for studying physiologic functions of Pten in adult tissues and organs. 20,21 For example, mice with Pten deletion in adult hematopoietic cells develop and die of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL). 22 Akt1 is a major downstream signaling molecule of PTEN and is activated after PTEN is mutated in human cancers. Investigators in a recent study 23 showed that the deficiency of Akt1 is sufficient to suppress the development of several types of tumors in Ptenheterozygous mice, including prostate cancer, endometrial carcinoma, thyroid neoplasia, intestinal polyps, and lymphoid hyperplasia. Moreover, rapamycin, which directly inhibits the Akt downstream molecule mammalian target of rapamycin (mTOR), effectively inhibits survival and proliferation of AML cells from PTEN fl/fl ;Mx-1...