Loss of the tumor suppressor p15Ink4b enhances myeloid progenitor formation from common myeloid progenitors

Cyclin-dependent kinase inhibitors in malignant hematopoiesis

The cell-cycle is a tightly orchestrated process where sequential steps guarantee cellular growth linked to a correct DNA replication. The entire cell division is controlled by cyclin-dependent kinases (CDKs). CDK activation is balanced by the activating cyclins and CDK inhibitors whose correct expression, accumulation and degradation schedule the time-flow through the cell cycle phases. Dysregulation of the cell cycle regulatory proteins causes the loss of a controlled cell division and is inevitably linked to neoplastic transformation. Due to their function as cell-cycle brakes, CDK inhibitors are considered as tumor suppressors. The CDK inhibitors p16^INK4a and p15^INK4b are among the most frequently altered genes in cancer, including hematopoietic malignancies. Aberrant cell cycle regulation in hematopoietic stem cells (HSCs) bears severe consequences on hematopoiesis and provokes hematological disorders with a broad array of symptoms. In this review, we focus on the importance and prevalence of deregulated CDK inhibitors in hematological malignancies.

CDKL1 promotes tumor proliferation and invasion in colorectal cancer

BACKGROUND

CDKL1 is a member of the cell division cycle 2 (CDC2)-related serine threonine protein kinase family and is overexpressed in malignant tumors such as melanoma, breast cancer, and gastric cancer.

OBJECTIVE

This study aimed to evaluate whether CDKL1 can serve as a potential molecular target for colorectal cancer therapy.

MATERIALS AND METHODS

Expression of CDKL1 in colorectal cancer tissues and cell lines was measured by immunohistochemistry and Western blot, respectively. To investigate the role of CDKL1 in colorectal cancer, CDKL1-small hairpin RNA-expressing lentivirus was constructed and infected into HCT116 and Caco2 cells. The effects of RNA interference (RNAi)-mediated CDKL1 downregulation on cell proliferation and invasion were assessed by CCK-8, colony formation, transwell, and tumorigenicity assays in nude mice. The effects of CDKL1 downregulation on cell cycle and apoptosis were analyzed by flow cytometry. Furthermore, microarray method and data analysis elucidated the molecular mechanisms underlying the phenomenon.

RESULTS

CDKL1 protein was overexpressed in colorectal cancer tissues compared with paired normal tissues. Knockdown of CDKL1 in HCT116 and Caco2 significantly inhibited cell growth, colony formation ability, tumor invasion, and G1-S phase transition of the cell cycle. The knockdown of CDKL1 stimulated the upregulation of p15 and retinoblastoma protein.

CONCLUSION

CDKL1 plays a vital role in tumor proliferation and invasion in colorectal cancer in vitro and in vivo and, thus, may be considered as a valuable target for therapeutic intervention.

Calcium and calcineurin-NFAT signaling regulate granulocyte-monocyte progenitor cell cycle via Flt3-L

Maintenance of myeloid progenitor cells is controlled by complex regulatory mechanisms and is orchestrated by multiple different transcription factors. Here, we report that the activation of the transcription factor nuclear factor of activated T cells (NFAT) by calcium-sensing protein calcineurin inhibits the proliferation of myeloid granulocyte–monocyte progenitors (GMPs). Myeloid progenitor subtypes exhibit variable sensitivity to induced Ca²⁺ entry and consequently display differential engagement of the calcineurin-NFAT pathway. This study shows that inhibition of the calcineurin-NFAT pathway enhances the proliferation of GMPs both in vitro and in vivo and demonstrates that calcineurin-NFAT signaling in GMPs is initiated by Flt3-L. Inhibition of the calcineurin-NFAT pathway modified expression of the cell cycle regulation genes Cdk4, Cdk6, and Cdkn1a (p21), thus enabling rapid cell cycle progression specifically in GMPs. NFAT inhibitor drugs are extensively used in the clinic to restrict the pathological activation of lymphoid cells, and our data reveal for the first time that these therapies also exert potent effects on maintenance of the myeloid cell compartment through specific regulation of GMP proliferation. Stem Cells2014;32:3232–3244

Tumor suppressor ASXL1 is essential for the activation of INK4B expression in response to oncogene activity and anti-proliferative signals

ASXL1 mutations are frequently found in hematological tumors, and loss of Asxl1 promotes myeloid transformation in mice. Here we present data supporting a role for an ASXL1-BAP1 complex in the deubiquitylation of mono-ubiquitylated lysine 119 on Histone H2A (H2AK119ub1) in vivo. The Polycomb group proteins control the expression of the INK4B-ARF-INK4A locus during normal development, in part through catalyzing mono-ubiquitylation of H2AK119. Since the activation of the locus INK4B-ARF-INK4A plays a fail-safe mechanism protecting against tumorigenesis, we investigated whether ASXL1-dependent H2A deubiquitylation plays a role in its activation. Interestingly, we found that ASXL1 is specifically required for the increased expression of p15(INK4B) in response to both oncogenic signaling and extrinsic anti-proliferative signals. Since we found that ASXL1 and BAP1 both are enriched at the INK4B locus, our results suggest that activation of the INK4B locus requires ASXL1/BAP1-mediated deubiquitylation of H2AK119ub1. Consistently, our results show that ASXL1 mutations are associated with lower expression levels of p15(INK4B) and a proliferative advantage of hematopoietic progenitors in primary bone marrow cells, and that depletion of ASXL1 in multiple cell lines results in resistance to growth inhibitory signals. Taken together, this study links ASXL1-mediated H2A deubiquitylation and transcriptional activation of INK4B expression to its tumor suppressor functions.

Brief report: Loss of p15Ink4b accelerates development of myeloid neoplasms in Nup98-HoxD13 transgenic mice

Homeostasis of hematopoietic stem and progenitor cells is a tightly regulated process. The disturbance of the balance in the hematopoietic progenitor pool can result in favorable conditions for development of diseases such as myelodysplastic syndromes and leukemia. It has been shown recently that mice lacking p15Ink4b have skewed differentiation of common myeloid progenitors toward the myeloid lineage at the expense of erythroid progenitors. The lack of p15INK4B expression in human leukemic blasts has been linked to poor prognosis and increased risk of myelodysplastic syndromes transformation to acute myeloid leukemia. However, the role of p15Ink4b in disease development is just beginning to be elucidated. This study examines the collaboration of the loss of p15Ink4b with Nup98-HoxD13 translocation in the development of hematological malignancies in a mouse model. Here, we report that loss of p15Ink4b collaborates with Nup98-HoxD13 transgene in the development of predominantly myeloid neoplasms, namely acute myeloid leukemia, myeloproliferative disease, and myelodysplastic syndromes. This mouse model could be a very valuable tool for studying p15Ink4b function in tumorigenesis as well as preclinical drug testing.

A combination of STI571 and BCR-ABL1 siRNA with overexpressed p15INK4B induced enhanced proliferation inhibition and apoptosis in chronic myeloid leukemia

p15INK4B, a cyclin-dependent kinase inhibitor, has been recognized as a tumor suppressor. Loss of or methylation of the p15INK4B gene in chronic myeloid leukemia (CML) cells enhances myeloid progenitor formation from common myeloid progenitors. Therefore, we examined the effects of overexpressed p15INK4B on proliferation and apoptosis of CML cells. Overexpression of p15INK4B inhibited the growth of K562 cells by downregulation of cyclin-dependent kinase 4 (CDK4) and cyclin D1 expression. Overexpression of p15INK4B also induced apoptosis of K562 cells by upregulating Bax expression and downregulating Bcl-2 expression. Overexpression of p15INK4B together with STI571 (imatinib) or BCR-ABL1 small interfering RNA (siRNA) also enhanced growth inhibition and apoptosis induction of K562 cells. The enhanced effect was also mediated by reduction of cyclin D1 and CDK4 and regulation of Bax and Bcl-2. In conclusion, our study may provide new insights into the role of p15INK4B in CML and a potential therapeutic target for overcoming tyrosine kinase inhibitor resistance in CML.

Concise review: erythroid versus myeloid lineage commitment: regulating the master regulators

Developmental processes, like blood formation, are orchestrated by transcriptional networks. Those transcriptional networks are highly responsive to various environmental stimuli and affect common precursors resulting in increased production of cells of the erythroid lineage or myeloid lineage (granulocytes, neutrophils, and macrophages). A significant body of knowledge has accumulated describing transcription factors that drive differentiation of these two major cellular pathways, in particular the antagonistic master regulators such as GATA-1 and PU.1. However, little is known about factors that work upstream of master regulators to enhance differentiation toward one lineage. These functions become especially important under various stress conditions like sudden loss of red blood cells or pathogen infection. This review describes recent studies that begin to provide evidence for such factors. An increased understanding of factors regulating cellular commitment will advance our understanding of the etiology of diseases like anemia, cancer, and possibly other blood related disorders.

RPRD1B promotes tumor growth by accelerating the cell cycle in endometrial cancer

RPRD1B, the regulation of nuclear pre-mRNA domain containing 1B gene, functions as a cell cycle manipulator and has been found overexpressed in a small panel of endometrial cancer types. In the present study, we investigated the roles of RPRD1B in endometrial cancer using various in vitro and in vivo experiments. According to our results, RPRD1B mRNA was significantly upregulated in endometrial cancer tissues (P=0.0012). RPRD1B overexpression was correlated with tumor stage (P=0.0004), histology type (P=0.0146) and depth of myometrial invasion (P=0.024). In vitro, RPRD1B promoted cellular proliferation (P=0.032 for MTT assay and P=0.018 for colony formation assay), and accelerated the cell cycle (P=0.007) by upregulating cyclin D1, CDK4 and CDK6, while knockdown of RPRD1B suppressed cellular proliferation (P=0.02 for MTT assay and P=0.031 for colony formation assay), and led to G1 phase arrest (P=0.025) through downregulating cyclin D1, CDK4 and CDK6. Consistently, in the nude mice model, RPRD1B overexpression significantly accelerated the tumor xenograft growth (P=0.0012), accompanied by elevated Ki-67 and cyclin D1. In addition, we demonstrated that downregulating RPRD1B could sensitize Ishikawa cells to Raloxifene (P=0.01). In summary, we demonstrated that RPRD1B was frequently overexpressed in human endometrial cancer. Both in vitro and in vivo, over-abundant RPRD1B could promote tumor growth and accelerate cellular cell cycle. In addition, knockdown of RPRD1B also increased cell sensitivity to Raloxifene, making RPRD1B a potent therapeutic target for endometrial cancer, particularly in patients with resistance to the selective ER modulators.

The myelodysplastic syndrome as a prototypical epigenetic disease

The myelodysplastic syndrome (MDS) is a clonal disorder characterized by increased stem cell proliferation coupled with aberrant differentiation resulting in a high rate of apoptosis and eventual symptoms related to bone marrow failure. Cellular differentiation is an epigenetic process that requires specific and highly ordered DNA methylation and histone modification programs. Aberrant differentiation in MDS can often be traced to abnormal DNA methylation (both gains and losses of DNA methylation genome wide and at specific loci) as well as mutations in genes that regulate epigenetic programs (TET2 and DNMT3a, both involved in DNA methylation control; EZH2 and ASXL1, both involved in histone methylation control). The epigenetic nature of MDS may explain in part the serendipitous observation that it is the disease most responsive to DNA methylation inhibitors; other epigenetic-acting drugs are being explored in MDS as well. Progression in MDS is characterized by further acquisition of epigenetic defects as well as mutations in growth-controlling genes that seem to tip the proliferation/apoptosis balance and result in the development of acute myelogenous leukemia. Although MDS is clinically and physiologically heterogeneous, a case can be made that subsets of the disease can be largely explained by disordered stem cell epigenetics.

Dendritic cells in myelodysplastic syndromes: from pathogenesis to immunotherapy

Myelodysplastic syndromes (MDS) are clonal disorders of the hematopoietic stem cell characterized by ineffective hematopoiesis leading to peripheral cytopenias. Different processes are involved in its pathogenesis, such as (epi)genetic alterations and immunological dysfunctions. The nature of immune dysregulation is markedly different between various MDS risk groups. In low-risk MDS, the immune system is in a proinflammatory state, whereas in high-risk disease, immunosuppressive features facilitate expansion of the dysplastic clone and can eventually lead to disease progression to acute myeloid leukemia. Various cell types contribute to dysregulation of immune responses in MDS. Dendritic cells (DCs) are important regulators of immunity. However, the role of DCs in MDS has yet to be elucidated. It has been suggested that impaired DC function can hamper adequate immune responses. This review focuses on the involvement of DCs in immune dysregulation in low- and high-risk MDS and the implications for DC-targeted therapies.

p15Ink4b Functions in determining hematopoietic cell fates: implications for its role as a tumor suppressor

The p15Ink4b gene is frequently hypermethylated in myeloid neoplasia and has been demonstrated to be a tumor suppressor. Since it is a member of the INK4b family of cyclin-dependent kinase inhibitors, it was initially presumed that its loss in leukemic blasts caused a dysregulation of the cell cycle. However, animal model experiments over the last several years have produced a very different picture of how p15Ink4b functions in hematopoietic cells and how its loss contributes to myelodysplastic syndrome and myeloid leukemia. It is clear now, that in early hematopoietic progenitors, p15Ink4b functions outside of its canonical role as a cell cycle inhibitor. Its functions are involved in signal transduction and influence the development of erythroid, monocytic and dendritic cells.

The role of tumor suppressor p15Ink4b in the regulation of hematopoietic progenitor cell fate

Epigenetic silencing of the tumor suppressor gene p15Ink4b (CDKN2B) is a frequent event in blood disorders like acute myeloid leukemia and myelodysplastic syndromes. The molecular function of p15Ink4b in hematopoietic differentiation still remains to be elucidated. Our previous study demonstrated that loss of p15Ink4b in mice results in skewing of the differentiation pattern of the common myeloid progenitor towards the myeloid lineage. Here, we investigated a function of p15Ink4b tumor suppressor gene in driving erythroid lineage commitment in hematopoietic progenitors. It was found that p15Ink4b is expressed more highly in committed megakaryocyte–erythroid progenitors than granulocyte–macrophage progenitors. More importantly, mice lacking p15Ink4b have lower numbers of primitive red cell progenitors and a severely impaired response to 5-fluorouracil- and phenylhydrazine-induced hematopoietic stress. Introduction of p15Ink4b into multipotential progenitors produced changes at the molecular level, including activation of mitogen-activated protein kinase\extracellular signal-regulated kinase (MEK/ERK) signaling, increase GATA-1, erythropoietin receptor (EpoR) and decrease Pu1, GATA-2 expression. These changes rendered cells more permissive to erythroid commitment and less permissive to myeloid commitment, as demonstrated by an increase in early burst-forming unit-erythroid formation with concomitant decrease in myeloid colonies. Our results indicate that p15Ink4b functions in hematopoiesis, by maintaining proper lineage commitment of progenitors and assisting in rapid red blood cells replenishment following stress.

Engineering mouse models with myelodysplastic syndrome human candidate genes; how relevant are they?

Myelodysplastic syndromes represent particularly challenging hematologic malignancies that arise from a large spectrum of genetic events resulting in a disease characterized by a range of different presentations and outcomes. Despite efforts to classify and identify the key genetic events, little improvement has been made in therapies that will increase patient survival. Animal models represent powerful tools to model and study human diseases and are useful pre-clinical platforms. In addition to enforced expression of candidate oncogenes, gene inactivation has allowed the consequences of the genetic effects of human myelodysplastic syndrome to be studied in mice. This review aims to examine the animal models expressing myelodysplastic syndrome-associated genes that are currently available and to highlight the most appropriate model to phenocopy myelodysplastic syndrome disease and its risk of transformation to acute myelogenous leukemia.

Role of key regulators of the cell cycle in maintenance of hematopoietic stem cells

BACKGROUND

Hematopoietic stem cells (HSCs) are characterized by pluripotentiality and self-renewal ability. To maintain a supply of mature blood cells and to avoid HSC exhaustion during the life span of an organism, most HSCs remain quiescent, with only a limited number entering the cell cycle.

SCOPE OF REVIEW

The molecular mechanisms by which quiescence is maintained in HSCs are addressed, with recent genetic studies having provided important insight into the relation between the cell cycle activity and stemness of HSCs.

MAJOR CONCLUSIONS

The cell cycle is tightly regulated in HSCs by complex factors. Key regulators of the cell cycle in other cell types-including cyclins, cyclin-dependent kinases (CDKs), the retinoblastoma protein family, the transcription factor E2F, and CDK inhibitors-also contribute to such regulation in HSCs. Most, but not all, of these regulators are necessary for maintenance of HSCs, with abnormal activation or suppression of the cell cycle resulting in HSC exhaustion. The cell cycle in HSCs is also regulated by external factors such as cytokines produced by niche cells as well as by the ubiquitin-proteasome pathway.

GENERAL SIGNIFICANCE

Studies of the cell cycle in HSCs may shed light on the pathogenesis of hematopoietic disorders, serve as a basis for the development of new therapeutic strategies for such disorders, prove useful for the expansion of HSCs in vitro as a possible replacement for blood transfusion, and provide insight into stem cell biology in general. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

The tumor suppressor p15Ink4b regulates the differentiation and maturation of conventional dendritic cells

The tumor suppressor p15Ink4b is frequently inactivated by methylation in acute myeloid leukemia and premalignant myeloid disorders. Dendritic cells (DCs) as potent APCs play critical regulatory roles in antileukemic immune responses. In the present study, we investigated whether p15Ink4b can function as modulator of DC development. The expression of p15Ink4b is induced strongly during differentiation and activation of DCs, and its loss resulted in significant quantitative and qualitative impairments of conventional DC (cDC) development. Accordingly, ex vivo-generated BM-derived DCs from p15Ink4b-knockout mice express significantly decreased levels of the antigen-presenting (MHC II) and costimulatory (CD80 and CD86) molecules and have impaired immunostimulatory functions, such as antigen uptake and T-cell stimulation. Reexpression of p15Ink4b in progenitors restored these defects, and confirmed a positive role for p15Ink4b during cDC differentiation and maturation. Furthermore, we have shown herein that p15Ink4b expression increases phosphorylation of Erk1/Erk2 kinases, which leads to an elevated activity of the PU.1 transcription factor. In conclusion, our results establish p15Ink4b as an important modulator of cDC development and implicate a novel function for this tumor suppressor in the regulation of adaptive immune responses.

p57 is required for quiescence and maintenance of adult hematopoietic stem cells

Quiescence is required for the maintenance of hematopoietic stem cells (HSCs). Members of the Cip/Kip family of cyclin-dependent kinase (CDK) inhibitors (p21, p27, p57) have been implicated in HSC quiescence, but loss of p21 or p27 in mice affects HSC quiescence or functionality only under conditions of stress. Although p57 is the most abundant family member in quiescent HSCs, its role has remained uncharacterized. Here we show a severe defect in the self-renewal capacity of p57-deficient HSCs and a reduction of the proportion of the cells in G(0) phase. Additional ablation of p21 in a p57-null background resulted in a further decrease in the colony-forming activity of HSCs. Moreover, the HSC abnormalities of p57-deficient mice were corrected by knocking in the p27 gene at the p57 locus. Our results therefore suggest that, among Cip/Kip family CDK inhibitors, p57 plays a predominant role in the quiescence and maintenance of adult HSCs.

Regulation of murine hematopoietic stem cell quiescence by Dmtf1

The cell-cycle status of hematopoietic stem cells (HSCs) is tightly regulated, most likely to balance maintenance of stem-cell status through quiescence and expansion/differentiation of the hematopoietic system. Tumor-suppressor genes (TSGs), with their cell cycle-regulatory functions, play important roles in HSC regulation. The cyclin-D binding myb-like transcription factor 1 (Dmtf1) was recently recognized as a TSG involved in human cancers by repressing oncogenic Ras/Raf signaling. However, the role of Dmtf1 in the hematopoietic system is entirely unknown. In the present study, we demonstrate that Dmtf1 regulates HSC function under both steady-state and stress conditions. Dmtf1(-/-) mice showed increased blood cell counts in multiple parameters, and their progenitor cells had increased proliferation and accelerated cell-cycle progression. In addition, long-term HSCs from Dmtf1(-/-) mice had a higher self-renewal capacity that was clearly demonstrated in secondary recipients in serial transplantation studies. Dmtf1(-/-) BM cells showed hyper proliferation after 5-fluorouracil-induced myeloablation. Steady-state expression and Induction of CDKN1a (p21) and Arf were impaired in HSCs from Dmtf1(-/-) mice. The function of Dmtf1 was mediated by both Arf-dependent and Arf-independent pathways. Our results implicate Dmtf1 in the regulation of HSC function through novel cell cycle-regulatory mechanisms.

Epigenetic changes in the myelodysplastic syndrome

Epigenetic mechanisms, such as DNA methylation and histone modifications, drive stable, clonally propagated changes in gene expression and can therefore serve as molecular mediators of pathway dysfunction in neoplasia. Myelodysplastic syndrome (MDS) is characterized by frequent epigenetic abnormalities, including the hypermethylation of genes that control proliferation, adhesion, and other characteristic features of this leukemia. Aberrant DNA hypermethylation is associated with a poor prognosis in MDS that can be accounted for by more rapid progression to acute myeloid leukemia. In turn, treatment with drugs that modify epigenetic pathways (DNA methylation and histone deacetylation inhibitors) induces durable remissions and prolongs life in MDS, offering some hope and direction in the future management of this deadly disease.

Myeloid-specific inactivation of p15Ink4b results in monocytosis and predisposition to myeloid leukemia

Inactivation of p15INK4b, an inhibitor of cyclin-dependent kinases, through DNA methylation is one of the most common epigenetic abnormalities in myeloid leukemia. Although this suggests a key role for this protein in myeloid disease suppression, experimental evidence to support this has not been reported. To address whether this event is critical for premalignant myeloid disorders and leukemia development, mice were generated that have loss of p15Ink4b specifically in myeloid cells. The p15Ink4b(fl/fl)-LysMcre mice develop nonreactive monocytosis in the peripheral blood accompanied by increased numbers of myeloid and monocytic cells in the bone marrow resembling the myeloproliferative form of chronic myelomonocytic leukemia. Spontaneous progression from chronic disease to acute leukemia was not observed. Nevertheless, MOL4070LTR retrovirus integrations provided cooperative genetic mutations resulting in a high frequency of myeloid leukemia in knockout mice. Two common retrovirus insertion sites near c-myb and Sox4 genes were identified, and their transcript up-regulated in leukemia, suggesting a collaborative role of their protein products with p15Ink4b-deficiency in promoting malignant disease. This new animal model demonstrates experimentally that p15Ink4b is a tumor suppressor for myeloid leukemia, and its loss may play an active role in the establishment of preleukemic conditions.

Hematopoietic stem cell function requires 12/15-lipoxygenase-dependent fatty acid metabolism

Fatty acid metabolism governs multiple intracellular signaling pathways in many cell types, but its role in hematopoietic stem cells (HSCs) is largely unknown. Herein, we establish a critical role for 12/15-lipoxygenase (12/15-LOX)-mediated unsaturated fatty acid metabolism in HSC function. HSCs from 12/15-LOX-deficient mice are severely compromised in their capacity to reconstitute the hematopoietic compartment in competitive and serial reconstitution assays. Furthermore, we demonstrate that 12/15-LOX is required for the maintenance of long-term HSC quiescence and number. The defect in HSCs is cell-autonomous and associated with a selective reduction in 12/15-LOX-mediated generation of bioactive lipid mediators and reactive oxygen species and with a decrease in canonical Wnt signaling as measured by nuclear beta-catenin staining. These results have implications for development, aging, and transformation of the hematopoietic compartment.

Signatures of polycomb repression and reduced H3K4 trimethylation are associated with p15INK4b DNA methylation in AML

DNA hypermethylation of the p15INK4b tumor suppressor gene is commonly observed in acute myeloid leukemia (AML). Repressive histone modifications and their associated binding proteins have been implicated in the regulation of DNA methylation and the transcriptional repression of genes with DNA methylation. We have used high-density chromatin immunoprecipitation-on-chip to determine the histone modifications that normally regulate p15INK4b expression in AML cells and how these marks are altered in cells that have p15INK4b DNA methylation. In AML patient blasts without p15INK4b DNA methylation, a bivalent pattern of active (H3K4me3) and repressive (H3K27me3) modifications exist at the p15INK4b promoter. AML patient blasts with p15INK4b DNA methylation lose H3K4me3 at p15INK4b and become exclusively marked by H3K27me3. H3K27me3, as well as EZH2, extends throughout p14ARF and p16INK4a, indicating that polycomb repression of p15INK4b is a common feature in all AML blasts irrespective of the DNA methylation status of the gene. Reactivation of p15INK4b expression in AML cell lines and patient blasts using 5-aza-2'-deoxycytidine (decitabine) and trichostatin A increased H3K4me3 and maintained H3K27me3 enrichment at p15INK4b. These data indicate that AML cells with p15INK4b DNA methylation have an altered histone methylation pattern compared with unmethylated samples and that these changes are reversible by epigenetic drugs.

Cell cycle, CDKs and cancer: a changing paradigm

Tumour-associated cell cycle defects are often mediated by alterations in cyclin-dependent kinase (CDK) activity. Misregulated CDKs induce unscheduled proliferation as well as genomic and chromosomal instability. According to current models, mammalian CDKs are essential for driving each cell cycle phase, so therapeutic strategies that block CDK activity are unlikely to selectively target tumour cells. However, recent genetic evidence has revealed that, whereas CDK1 is required for the cell cycle, interphase CDKs are only essential for proliferation of specialized cells. Emerging evidence suggests that tumour cells may also require specific interphase CDKs for proliferation. Thus, selective CDK inhibition may provide therapeutic benefit against certain human neoplasias.

The H3K36 demethylase Jhdm1b/Kdm2b regulates cell proliferation and senescence through p15(Ink4b)

The Ink4a/Arf/Ink4b locus plays a critical role in both cellular senescence and tumorigenesis. Jhdm1b/Kdm2b/Fbxl10, the mammalian paralogue of the histone demethylase Jhdm1a/Kdm2a/Fbxl11, has been implicated in cell cycle regulation and tumorigenesis. In this report, we demonstrate that Jhdm1b is an H3K36 demethylase. Knockdown of Jhdm1b in primary MEFs inhibits cell proliferation and induces cellular senescence in a pRb and p53 pathway-dependent manner. Importantly, the effect of Jhdm1b on cell proliferation and cellular senescence is mediated through de-repression of p15^Ink4b as loss of p15^Ink4b function rescues cell proliferation defects in Jhdm1b knockdown cells. Chromatin immunoprecipitation on ectopically expressed Jhdm1b demonstrates that Jhdm1b targets the p15^Ink4b locus and regulates its expression in an enzymatic activity-dependent manner. Alteration of Jhdm1b level affects Ras-induced neoplastic transformation. Collectively, our results indicate that Jhdm1b is an H3K36 demethylase that regulates cell proliferation and senescence through p15^Ink4b.

Roles of p15Ink4b and p16Ink4a in myeloid differentiation and RUNX1-ETO-associated acute myeloid leukemia

Inactivation of p15(Ink4b) expression by promoter hypermethylation occurs in up to 80% of acute myeloid leukemia (AML) cases and is particularly common in the FAB-M2 subtype of AML, which is characterized by the presence of the RUNX1-ETO translocation in 40% of cases. To establish whether the loss of p15(Ink4b) contributes to AML progression in association with RUNX1-ETO, we have expressed the RUNX1-ETO fusion protein from a retroviral vector in hematopoietic progenitor cells isolated from wild-type, p15(Ink4b) or p16(Ink4a) knockout bone marrow. Analysis of lethally irradiated recipient mice reconstituted with RUNX1-ETO-expressing cells showed that neither p15(Ink4b) or p16(Ink4a) loss significantly accelerated disease progression over the time period of one year post-transplantation. Loss of p15(Ink4b) alone resulted in increased myeloid progenitor cell frequencies in bone marrow by 10-month post-transplant and a 19-fold increase in the frequency of Lin(-)c-Kit(+)Sca-1(+) (LKS) cells that was not associated with expansion of long-term reconstituting HSC. These results strongly suggest that p15(Ink4b) loss must be accompanied by additional oncogenic changes for RUNX1-ETO-associated AML to develop.