SummaryBackground Mantle-cell lymphoma is an aggressive B-cell lymphoma with a poor prognosis. Both ibrutinib and temsirolimus have shown single-agent activity in patients with relapsed or refractory mantle-cell lymphoma. We undertook a phase 3 study to assess the effi cacy and safety of ibrutinib versus temsirolimus in relapsed or refractory mantle-cell lymphoma.

High expression of BMI1 in acute myeloid leukemia (AML) cells is associated with an unfavorable prognosis. Therefore, the effects of down-modulation of BMI1 in normal and leukemic CD34 ؉ AML cells were studied using a lentiviral RNA interference approach. We demonstrate that downmodulation of BMI1 in cord blood CD34 ؉ cells impaired long-term expansion and progenitor-forming capacity, both in cytokine-driven liquid cultures as well as in bone marrow stromal cocultures. In addition, long-term culture-initiating cell frequencies were dramatically decreased upon knockdown of BMI1, indicating an impaired maintenance of stem and progenitor cells. The reduced progenitor and stem cell frequencies were associated with increased expression of p14ARF and p16INK4A and enhanced apoptosis, which coincided with increased levels of intracellular reactive oxygen species and reduced FOXO3A expression. In AML CD34 ؉ cells, down-modulation of BMI1 impaired long-term expansion, whereby selfrenewal capacity was lost, as determined by the loss of replating capacity of the cultures. These phenotypes were also associated with increased expression levels of p14ARF and p16INK4A. Together our data indicate that BMI1 expression is required for maintenance and selfrenewal of normal and leukemic stem and progenitor cells, and that expression of IntroductionBMI1 is a member of the Polycomb group (PcG) genes, which are transcriptional repressors that play essential roles in the maintenance of appropriate gene expression during development. [1][2][3][4][5] Two distinct multiprotein PcG complexes have been identified, the Polycomb repression complex (PRC) 1 and PRC2. 1 The PRC2 complex is involved in initiation of silencing and contains histone deacetylases and methyltransferases that can methylate H3 lysine 9 and 27. 6 Deletion of PRC2 genes in mice results in embryonic lethality, emphasizing their importance in development. [7][8][9] PRC1 is implicated in stable maintenance of gene repression and recognizes the methylation marks set by PRC2. 10,11 Mice mutant for most PRC1 genes survive until birth as a result of partial functional redundancy provided by their homologues, but developmental defects do arise thereafter, as is, for example, the case in the hematopoietic compartment after deletion of BMI1. [12][13][14] Targeted deletion of Bmi1 has shown that although the numbers of fetal liver-derived hematopoietic stem cells (HSCs) are normal in these mice, their proliferative and self-renewal capacity is severely impaired. 15,16 In adult Bmi1-deficient mice, the HSCs are less frequent and display an impaired competitive repopulation capacity. 16,17 Gain-of-function studies demonstrated enhanced selfrenewal of murine HSCs and with a shift in balance toward more symmetric stem cell divisions. 17 We have demonstrated that constitutive expression of BMI1 in human cord blood (CB) cells results in prolonged maintenance of the stem cell pool and enhances self-renewal of human stem and progenitor cells. 18 BMI1 is a potent negative regulator of the IN...

The polycomb group (PcG) gene BMI1 has been identified as one of the key epigenetic regulators of cell fates during different stages of development in multiple murine tissues. In a clinically relevant model, we demonstrate that enforced expression of BMI1 in cord blood CD34 ؉ cells results in long-term maintenance and self-renewal of human hematopoietic stem and progenitor cells. Longterm culture-initiating cell frequencies were increased upon stable expression of BMI1 and these cells engrafted more efficiently in NOD-SCID mice. Week 5 cobblestone area-forming cells (CAFCs) were replated to give rise to secondary CAFCs. Serial transplantation studies in NOD-SCID mice revealed that secondary engraftment was only achieved with cells overexpressing BMI1. Importantly, BMI1-transduced cells proliferated in stromafree cytokine-dependent cultures for more than 20 weeks, while a stable population of approximately 1% to 5% of CD34 ؉ cells was preserved that retained colonyforming capacity. Whereas control cells lost most of their NOD-SCID engraftment potential after 10 days of ex vivo culturing in absence of stroma, NOD-SCID multilineage engraftment was retained by overexpression of BMI1. Thus, our data indicate that self-renewal of human hematopoietic stem cells is enhanced by BMI1, and we classify BMI1 as an intrinsic regulator of human stem/progenitor cell self-renewal. IntroductionCellular memory induced by chromatin modifications can be maintained through subsequent cell divisions by the opposing effects of transcriptional activators of Trithorax (Trx) proteins and repressors of the polycomb proteins (PcG). 1 Two functionally different PcG complexes have been identified. Polycomb repressive complex (PRC) 2, also termed as "initiation" complex, consists of several subunits, including EED, SU(Z)12, and EZH2, functions as a histone methyltransferase that specifically trimethylates histone 3 on lysine residue 27 (H3K27) resulting in compaction of the chromatin and subsequent gene silencing. 2 PRC1 which is referred to as "maintenance" complex consists of various subunits, including BMI1, RING1A/B, CBX, MEL18, and MPH/ RAE28. 2 This complex does not possess methyltransferase activity itself, and the only observed enzymatic activity is ubiquitinylation of lysine 119 of H2A via its RING subdomains. 3 However, it has recently been demonstrated that DNA methyltransferase (Dnmt1) is required for proper assembly of PRC1 complex and that it can associate with BMI1 via Dmap1 and thus regulate DNA and histone methylation. 4,5 These observations suggest that not only PRC2, but also PRC1, might affect the epigenetic state of cells via methylation in a more direct fashion.Polycomb target genes were mapped in genome-wide screens in human embryonic fibroblasts, 6 murine embryonic stem cells, 7,8 and Drosophila melanogaster. 9,10 In embryonic fibroblasts, PRC1, PRC2, and H3K27-3me co-occupied genes that tend to be involved in embryonic development and cell fate decisions. 6 In ES cells, SU(Z)12, EED, and H3K27-3me co-occupied genes th...