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IntroductionNormal and leukemic hematopoietic cells and stem cells reside in the bone marrow in specialized areas ("niches") that provide the structural and physiologic conditions for their growth and survival. 1 Subpopulations of leukemic cells can be sequestered in niches and thereby evade chemotherapy-induced death. 2 We and others have reported that stromal cells protect acute myeloid leukemia (AML) and chronic lymphocytic leukemia cells from the apoptosis induced by chemotherapy. [3][4][5][6] While the mechanisms of stroma-mediated protection are pleiotropic and involve a complex interplay of stroma-produced cytokines, chemokines, and adhesion molecules, the stroma-secreted chemokine stromal-derived factor 1␣ (SDF-1␣) and its cognate receptor CXCR4 have recently emerged as critical mediators of stromal/leukemic cell interactions. 7,8 SDF-1␣ and CXCR4 primarily regulate the migration, homing, and mobilization of hematopoietic cells. 9,10 Binding of SDF-1␣ to CXCR4 causes CXCR4 to be incorporated into lipid rafts 11 and increases its phosphorylation. 12 The latter leads to prolonged activation of the extracellular signaling-regulated kinase (ERK) and phosphoinositol 3-kinase (PI3K) pathways, 13 which are key signaling pathways that promote leukemia cells survival. 14,15 Both surface and intracellular 16 CXCR4 levels were found to be elevated in a subset of AML cases. Further, CXCR4 has been shown to mediate the homing and engraftment of AML cells to the bone marrow of nonobese diabetes (NOD)/severe combined immunodeficiency (SCID) mice. 17,18 Finally, CXCR4 was recently reported to be expressed at higher levels in cases of AML associated with an internal tandem duplication (ITD) type of mutation of the gene that encodes fetal liver tyrosine . 19 This is one of the most frequent mutations in AML, which confers poor response to chemotherapy and only transient response to FLT3 inhibitors. 20,21 Our recent studies, in addition, indicated that CXCR4 expression is associated with poor prognosis in patients with diploid AML regardless of FLT3 mutation status. 22,23 Altogether, these findings suggest that disruption of these interactions by SDF-1␣/CXCR4 antagonists represents a novel strategy for targeting leukemia/bone marrow microenvironment interactions. We have reported that inhibition of CXCR4 by specific synthetic peptides (ie, RCP168) interferes with stromal/ leukemic cell interactions and increases the sensitivity of leukemic cells to chemotherapy. 24 In this study, we used AMD3465 (Anormed and Genzyme, Cambridge, MA), a second-generation smallmolecule reversible inhibitor of SDF-1␣/CXCR4 with a half maximal inhibitory concentration (IC 50 An Inside Blood analysis of this article appears at the front of this issue.The online version of this article contains a data supplement.The publication costs of this article were defrayed in part by page charge payment. Therefore, and solely to indicate this fact, this article is hereby marked ''advertisement'' in accordance with 18 USC section 1734. For personal use on...

XRCC1 is a molecular scaffold protein that assembles multi-protein complexes involved in DNA single-strand break repair1,2. Here, we show that biallelic mutations in human XRCC1 are associated with ocular motor apraxia, axonal neuropathy, and progressive cerebellar ataxia. XRCC1-mutant patient cells exhibit not only reduced rates of single-strand break repair but also elevated levels of protein ADP-ribosylation; a phenotype recapitulated in a related syndrome caused by mutations in the XRCC1 partner protein PNKP3-5 and implicating hyperactivation of poly (ADP-ribose) polymerase/s as a cause of cerebellar ataxia. Indeed, remarkably, genetic deletion of Parp1 rescued normal cerebellar ADP-ribose levels and reduced the loss of cerebellar neurons and ataxia in Xrcc1-defective mice, identifying a molecular mechanism by which endogenous single-strand breaks trigger neuropathology. Collectively, these data establish the importance of XRCC1 protein complexes for normal neurological function and identify PARP1 as a therapeutic target in DNA strand break repair-defective disease.

The clinical challenge posed by p53 abnormalities in hematological malignancies requires therapeutic strategies other than standard genotoxic chemotherapies. ONC201 is a first-in-class small molecule that activates p53-independent apoptosis, has a benign safety profile, and is in early clinical trials. We found that ONC201 caused p53-independent apoptosis and cell cycle arrest in cell lines and in mantle cell lymphoma (MCL) and acute myeloid leukemia (AML) samples from patients; these included samples from patients with genetic abnormalities associated with poor prognosis or cells that had developed resistance to the nongenotoxic agents ibrutinib and bortezomib. Moreover, ONC201 caused apoptosis in stem and progenitor AML cells and abrogated the engraftment of leukemic stem cells in mice while sparing normal bone marrow cells. ONC201 caused changes in gene expression similar to those caused by the unfolded protein response (UPR) and integrated stress responses (ISRs), which increase the translation of the transcription factor ATF4 through an increase in the phosphorylation of the translation initiation factor eIF2α. However, unlike the UPR and ISR, the increase in ATF4 abundance in ONC201-treated hematopoietic cells promoted apoptosis and did not depend on increased phosphorylation of eIF2α. ONC201 also inhibited mammalian target of rapamycin complex 1 (mTORC1) signaling, likely through ATF4-mediated induction of the mTORC1 inhibitor DDIT4. Overexpression of BCL-2 protected against ONC201-induced apoptosis, and the combination of ONC201 and the BCL-2 antagonist ABT-199 synergistically increased apoptosis. Thus, our results suggest that by inducing an atypical ISR and p53-independent apoptosis, ONC201 has clinical potential in hematological malignancies.