Scott C. Kogan

RNAi screen identifies Brd4 as a therapeutic target in acute myeloid leukaemia

Epigenetic pathways can regulate gene expression by controlling and interpreting chromatin modifications. Cancer cells are characterized by altered epigenetic landscapes, and commonly exploit the chromatin regulatory machinery to enforce oncogenic gene expression programs. Although chromatin alterations are, in principle, reversible and often amenable to drug intervention, the promise of targeting such pathways therapeutically has been limited by an incomplete understanding of cancer-specific dependencies on epigenetic regulators. Here we describe a non-biased approach to probe epigenetic vulnerabilities in acute myeloid leukaemia (AML), an aggressive haematopoietic malignancy that is often associated with aberrant chromatin states. By screening a custom library of small hairpin RNAs (shRNAs) targeting known chromatin regulators in a genetically defined AML mouse model, we identify the protein bromodomain-containing 4 (Brd4) as being critically required for disease maintenance. Suppression of Brd4 using shRNAs or the small-molecule inhibitor JQ1 led to robust antileukaemic effects in vitro and in vivo, accompanied by terminal myeloid differentiation and elimination of leukaemia stem cells. Similar sensitivities were observed in a variety of human AML cell lines and primary patient samples, revealing that JQ1 has broad activity in diverse AML subtypes. The effects of Brd4 suppression are, at least in part, due to its role in sustaining Myc expression to promote aberrant self-renewal, which implicates JQ1 as a pharmacological means to suppress MYC in cancer. Our results establish small-molecule inhibition of Brd4 as a promising therapeutic strategy in AML and, potentially, other cancers, and highlight the utility of RNA interference (RNAi) screening for revealing epigenetic vulnerabilities that can be exploited for direct pharmacological intervention.

Survival signalling by Akt and eIF4E in oncogenesis and cancer therapy

Evading apoptosis is considered to be a hallmark of cancer, because mutations in apoptotic regulators invariably accompany tumorigenesis. Many chemotherapeutic agents induce apoptosis, and so disruption of apoptosis during tumour evolution can promote drug resistance. For example, Akt is an apoptotic regulator that is activated in many cancers and may promote drug resistance in vitro. Nevertheless, how Akt disables apoptosis and its contribution to clinical drug resistance are unclear. Using a murine lymphoma model, we show that Akt promotes tumorigenesis and drug resistance by disrupting apoptosis, and that disruption of Akt signalling using the mTOR inhibitor rapamycin reverses chemoresistance in lymphomas expressing Akt, but not in those with other apoptotic defects. eIF4E, a translational regulator that acts downstream of Akt and mTOR, recapitulates Akts action in tumorigenesis and drug resistance, but is unable to confer sensitivity to rapamycin and chemotherapy. These results establish Akt signalling through mTOR and eIF4E as an important mechanism of oncogenesis and drug resistance in vivo, and reveal how targeting apoptotic programmes can restore drug sensitivity in a genotype-dependent manner.

An improved method for prenatal diagnosis of genetic diseases by analysis of amplified DNA sequences: application to hemophilia A

Abstract We report the development of a rapid nonradioactive technique for the genetic prediction of human disease and its diagnostic application to hemophilia A. This method is based on enzymatic amplification of short segments of human genes associated with inherited disorders. A novel feature of the procedure is the use of a heat-stable DNA polymerase, which allows the repeated rounds of DNA synthesis to proceed at 63°C. The high sequence specificity of the amplification reaction at this elevated temperature permits restriction-site polymorphisms, contained in the amplified samples, to be analyzed by visual inspection of their digestion products on polyacrylamide gels. By means of this method, we have performed carrier detection and prenatal diagnosis of hemophilia in two families with use of the factor VIII intragenic polymorphisms identified by the restriction enzymes Bc/I and XbaI. Predictions can be made directly from chorionic villi, without previous DNA extraction, and fetal sex can be determined...

Control of the senescence-associated secretory phenotype by NF-κB promotes senescence and enhances chemosensitivity

Cellular senescence acts as a potent barrier to tumorigenesis and contributes to the anti-tumor activity of certain chemotherapeutic agents. Senescent cells undergo a stable cell cycle arrest controlled by RB and p53 and, in addition, display a senescence-associated secretory phenotype (SASP) involving the production of factors that reinforce the senescence arrest, alter the microenvironment, and trigger immune surveillance of the senescent cells. Through a proteomics analysis of senescent chromatin, we identified the nuclear factor-κB (NF-κB) subunit p65 as a major transcription factor that accumulates on chromatin of senescent cells. We found that NF-κB acts as a master regulator of the SASP, influencing the expression of more genes than RB and p53 combined. In cultured fibroblasts, NF-κB suppression causes escape from immune recognition by natural killer (NK) cells and cooperates with p53 inactivation to bypass senescence. In a mouse lymphoma model, NF-κB inhibition bypasses treatment-induced senescence, producing drug resistance, early relapse, and reduced survival. Our results demonstrate that NF-κB controls both cell-autonomous and non-cell-autonomous aspects of the senescence program and identify a tumor-suppressive function of NF-κB that contributes to the outcome of cancer therapy.

New Evidence Supporting Megakaryocyte-Erythrocyte Potential of Flk2/Flt3+ Multipotent Hematopoietic Progenitors

A model of hematopoietic development wherein multipotentiality is conserved until segregation of myeloid and lymphoid potential has recently been challenged, proposing that megakaryocyte/erythrocyte (MegE) potential is lost in Flk2/Flt3-expressing early progenitors. Here, we used sensitive in vivo approaches to quantitatively and kinetically assess the MegE potential of hematopoietic stem cells and various Flk2(+) early progenitors and compared it with the MegE potential of downstream committed myeloid and lymphoid progenitors and with their ability to give rise to mature myelomonocytic and lymphoid cells. We demonstrate that Flk2(+) early progenitors retain MegE potential in vivo both at the population and clonal levels. These results indicate that Flk2 expression by early progenitors is not at the expense of full multipotency and support the current model of hematopoietic development with segregation of myeloid and lymphoid lineages from multipotent progenitors.

mTORC1 promotes survival through translational control of Mcl-1

Activation of the phosphatidylinositol 3-kinase (PI3K)/AKT signaling pathway is a frequent occurrence in human cancers and a major promoter of chemotherapeutic resistance. Inhibition of one downstream target in this pathway, mTORC1, has shown potential to improve chemosensitivity. However, the mechanisms and genetic modifications that confer sensitivity to mTORC1 inhibitors remain unclear. Here, we demonstrate that loss of TSC2 in the Eμ-myc murine lymphoma model leads to mTORC1 activation and accelerated oncogenesis caused by a defective apoptotic program despite compromised AKT phosphorylation. Tumors from Tsc2+/−Eμ-Myc mice underwent rapid apoptosis upon blockade of mTORC1 by rapamycin. We identified myeloid cell leukemia sequence 1 (Mcl-1), a bcl-2 like family member, as a translationally regulated genetic determinant of mTORC1-dependent survival. Our results indicate that the extent by which rapamycin can modulate expression of Mcl-1 is an important feature of the rapamycin response.

Mouse models of human AML accurately predict chemotherapy response

The genetic heterogeneity of cancer influences the trajectory of tumor progression and may underlie clinical variation in therapy response. To model such heterogeneity, we produced genetically and pathologically accurate mouse models of common forms of human acute myeloid leukemia (AML) and developed methods to mimic standard induction chemotherapy and efficiently monitor therapy response. We see that murine AMLs harboring two common human AML genotypes show remarkably diverse responses to conventional therapy that mirror clinical experience. Specifically, murine leukemias expressing the AML1/ETO fusion oncoprotein, associated with a favorable prognosis in patients, show a dramatic response to induction chemotherapy owing to robust activation of the p53 tumor suppressor network. Conversely, murine leukemias expressing MLL fusion proteins, associated with a dismal prognosis in patients, are drug-resistant due to an attenuated p53 response. Our studies highlight the importance of genetic information in guiding the treatment of human AML, functionally establish the p53 network as a central determinant of chemotherapy response in AML, and demonstrate that genetically engineered mouse models of human cancer can accurately predict therapy response in patients.

Hematopoietic stem cell quiescence is maintained by compound contributions of the retinoblastoma gene family.

Individual members of the retinoblastoma (Rb) tumor suppressor gene family serve critical roles in the control of cellular proliferation and differentiation, but the extent of their contributions is masked by redundant and compensatory mechanisms. Here we employed a conditional knockout strategy to simultaneously inactivate all three members, Rb, p107, and p130, in adult hematopoietic stem cells (HSCs). Rb family triple knockout (TKO) mice develop a cell-intrinsic myeloproliferation that originates from hyperproliferative early hematopoietic progenitors and is accompanied by increased apoptosis in lymphoid progenitor populations. Loss of quiescence in the TKO HSC pool is associated with an expansion of these mutant stem cells but also with an enhanced mobilization and an impaired reconstitution potential upon transplantation. The presence of a single p107 allele is sufficient to largely rescue these defects. Thus, Rb family members collectively maintain HSC quiescence and the balance between lymphoid and myeloid cell fates in the hematopoietic system.

p53 loss promotes acute myeloid leukemia by enabling aberrant self-renewal

The p53 tumor suppressor limits proliferation in response to cellular stress through several mechanisms. Here, we test whether the recently described ability of p53 to limit stem cell self-renewal suppresses tumorigenesis in acute myeloid leukemia (AML), an aggressive cancer in which p53 mutations are associated with drug resistance and adverse outcome. Our approach combined mosaic mouse models, Cre-lox technology, and in vivo RNAi to disable p53 and simultaneously activate endogenous Kras(G12D)-a common AML lesion that promotes proliferation but not self-renewal. We show that p53 inactivation strongly cooperates with oncogenic Kras(G12D) to induce aggressive AML, while both lesions on their own induce T-cell malignancies with long latency. This synergy is based on a pivotal role of p53 in limiting aberrant self-renewal of myeloid progenitor cells, such that loss of p53 counters the deleterious effects of oncogenic Kras on these cells and enables them to self-renew indefinitely. Consequently, myeloid progenitor cells expressing oncogenic Kras and lacking p53 become leukemia-initiating cells, resembling cancer stem cells capable of maintaining AML in vivo. Our results establish an efficient new strategy for interrogating oncogene cooperation, and provide strong evidence that the ability of p53 to limit aberrant self-renewal contributes to its tumor suppressor activity.

MLL3 Is a Haploinsufficient 7q Tumor Suppressor in Acute Myeloid Leukemia

Recurring deletions of chromosome 7 and 7q [-7/del(7q)] occur in myelodysplastic syndromes and acute myeloid leukemia (AML) and are associated with poor prognosis. However, the identity of functionally relevant tumor suppressors on 7q remains unclear. Using RNAi and CRISPR/Cas9 approaches, we show that an ∼50% reduction in gene dosage of the mixed lineage leukemia 3 (MLL3) gene, located on 7q36.1, cooperates with other events occurring in -7/del(7q) AMLs to promote leukemogenesis. Mll3 suppression impairs the differentiation of HSPC. Interestingly, Mll3-suppressed leukemias, like human -7/del(7q) AMLs, are refractory to conventional chemotherapy but sensitive to the BET inhibitor JQ1. Thus, our mouse model functionally validates MLL3 as a haploinsufficient 7q tumor suppressor and suggests a therapeutic option for this aggressive disease.