Defective DNA double-strand break repair underlies enhanced tumorigenesis and chromosomal instability in p27-deficient mice with growth factor-induced oligodendrogliomas

The DNA Double-Strand Break Repair in Glioma: Molecular Players and Therapeutic Strategies

Gliomas are the most frequent type of tumor in the central nervous system, which exhibit properties that make their treatment difficult, such as cellular infiltration, heterogeneity, and the presence of stem-like cells responsible for tumor recurrence. The response of this type of tumor to chemoradiotherapy is poor, possibly due to a higher repair activity of the genetic material, among other causes. The DNA double-strand breaks are an important type of lesion to the genetic material, which have the potential to trigger processes of cell death or cause gene aberrations that could promote tumorigenesis. This review describes how the different cellular elements regulate the formation of DNA double-strand breaks and their repair in gliomas, discussing the therapeutic potential of the induction of this type of lesion and the suppression of its repair as a control mechanism of brain tumorigenesis.

Cyclin-dependent kinase inhibitors (CDKIs) and the DNA damage response: The link between signaling pathways and cancer

At the cellular level, DNA repair mechanisms are crucial in maintaining both genomic integrity and stability. DNA damage appears to be a central culprit in tumor onset and progression. Cyclin-dependent kinases (CDKs) and their regulatory partners coordinate the cell cycle progression. Aberrant CDK activity has been linked to a variety of cancers through deregulation of cell-cycle control. Besides DNA damaging agents and chromosome instability (CIN), disruptions in the levels of cell cycle regulators including cyclin-dependent kinase inhibitors (CDKIs) would result in unscheduled proliferation and cell division. The INK4 and Cip/Kip (CDK interacting protein/kinase inhibitor protein) family of CDKI proteins are involved in cell cycle regulation, transcription regulation, apoptosis, and cell migration. A thorough understanding of how these CDKIs regulate the DNA damage response through multiple signaling pathways may provide an opportunity to design efficient treatment strategies to inhibit carcinogenesis.

The T197A Knock-in Model of Cdkn1b Gene to Study the Effects of p27 Restoration In Vivo

The CDK inhibitor, p27^kip1, encoded by the Cdkn1b gene can negatively modulate cell proliferation. The control of p27 activity during the cell cycle is regulated at multiple levels, including transcription, translation, and protein stability. The last residue of p27 (threonine 198 in human, threonine 197 in mouse) is involved in the control of protein stability. We have generated a murine knock-in model (Cdkn1b ^T197A) in which threonine 197 is replaced by alanine, which renders p27 protein highly unstable due to a high rate of proteasomal degradation. Expectedly, Cdkn1b ^T197A/T197A mice present with increased body size and weight, organomegaly, and multiple organ hyperplasia, similar to what is observed in Cdkn1b ^KO/KO mice. We investigated the effects exerted by the restoration of normal levels of p27 protein in the tissue of Cdkn1b ^T197A/T197A mice. We found that proteasome inhibition with bortezomib rescues the hyperplasia induced by the lack of p27 expression in Cdkn1b ^T197A/T197A but not in Cdkn1b ^KO/KO mice. However, BAY 11-7082, a proteasome inhibitor that stabilizes IκB but not p27, fails to rescue hyperplasia in Cdkn1b ^T197A/T197A mice. Bortezomib increases p27 half-life and reduces the proliferation in MEFs derived from Cdkn1b ^T197A/T197A but not from Cdkn1b ^WT/WT mice, whereas BAY 11-7082 had no effect on the protein levels of p27 and on the proliferation rate of Cdkn1b ^T197A/T197A MEFs.The results presented here demonstrate that Cdkn1b ^T197A/T197A mice represent an attractive in vivo model to investigate whether the targeting of p27 degradation machinery might prove beneficial in the treatment of a variety of human proliferative disorders caused by increased turnover of p27 protein.

p27Kip1 regulates the microtubule bundling activity of PRC1

Cytokinesis begins in anaphase with the formation of the central spindle. PRC1 is a microtubule associated protein that plays an essential role in central spindle formation by crosslinking antiparallel microtubules. We have identified PRC1 as a novel binding partner for p27^Kip1 (p27). p27 is a cyclin-CDK inhibitor that causes cell cycle arrest in G1. However, p27 has also been involved in the regulation of G2/M progression and cytokinesis, as well as of other cellular processes, including actin and microtubule cytoskeleton dynamics. We found that p27 interferes with the ability of PRC1 to bind to microtubules, without affecting PRC1 dimerization or its capacity to interact with other partners such as KIF4. In this way, p27 inhibited microtubule bundling by PRC1 in vitro and prevented the extensive microtubule bundling phenotype caused by PRC1 overexpression in cells in culture. Finally, co-expression of p27 or a p27 mutant that does not bind cyclin-CDKs inhibited multinucleation induced by PRC1 overexpression. Together, our results suggest that p27 may participate in the regulation of mitotic progression in a CDK-independent manner by modulating PRC1 activity.

Clinical significances of p27 in digestive tract cancers: a comprehensive analysis on immunohistochemistry staining, published literatures, microarray and RNA-seq data

In the present study, we conducted a comprehensive analysis on the clinical roles of p27 protein and p27 gene in digestive tract cancers (DTCs). First, we performed immunohistochemistry staining and found that p27 protein was down-regulated in DTCs. Then we collected 62 publications and calculated the combined hazard ratios (HRs), odds ratios (ORs) and 95% confidence intervals (95% CIs) to clarify the relationships of p27 protein expression with prognoses and clinicopathological parameters. The overall HRs indicated that the down-regulated p27 protein was an independent prognostic biomarker for overall survival (HR: 1.58, 95% CI: 1.38-1.81, P < 0.0001) but not for disease-free survival and cancer-specific survival. The combined ORs indicated that a low expression of p27 protein was positively related to lymph node metastasis (OR: 2.15, 95% CI: 1.57-2.96, P < 0.0001), distant metastasis (OR: 2.02, 95% CI: 1.12-3.63, P = 0.019) and pathology grading (OR: 2.14, 95% CI: 1.75-2.62, P < 0.0001). Additionally, 60 DTCs-related microarray and RNA-seq datasets were obtained to investigate the expression level and clinical value of p27 gene in DTCs patients. We found that the expression level of p27 gene in DTCs was similar to that in normal controls. And no significant associations of p27 gene expression with prognoses and clinicopathological factors were observed. In conclusion, according to our results, it was p27 protein, but not p27 gene, that can function as an effective biomarker to predict the clinical outcome in patients with DTCs. The down-regulation of p27 protein in DTCs may not result from the altered expression of p27 gene.

Citron Kinase Deficiency Leads to Chromosomal Instability and TP53-Sensitive Microcephaly

Mutations in citron (CIT), leading to loss or inactivation of the citron kinase protein (CITK), cause primary microcephaly in humans and rodents, associated with cytokinesis failure and apoptosis in neural progenitors. We show that CITK loss induces DNA damage accumulation and chromosomal instability in both mammals and Drosophila. CITK-deficient cells display "spontaneous" DNA damage, increased sensitivity to ionizing radiation, and defective recovery from radiation-induced DNA lesions. In CITK-deficient cells, DNA double-strand breaks increase independently of cytokinesis failure. Recruitment of RAD51 to DNA damage foci is compromised by CITK loss, and CITK physically interacts with RAD51, suggesting an involvement of CITK in homologous recombination. Consistent with this scenario, in doubly CitK and Trp53 mutant mice, neural progenitor cell death is dramatically reduced; moreover, clinical and neuroanatomical phenotypes are remarkably improved. Our results underscore a crucial role of CIT in the maintenance of genomic integrity during brain development.

Wharton's Jelly Mesenchymal Stem Cells Protect the Immature Brain in Rats and Modulate Cell Fate

The development of a mammalian brain is a complex and long-lasting process. Not surprisingly, preterm birth is the leading cause of death in newborns and children. Advances in perinatal care reduced mortality, but morbidity still represents a major burden. New therapeutic approaches are thus desperately needed. Given that mesenchymal stem/stromal cells (MSCs) emerged as a promising candidate for cell therapy, we transplanted MSCs derived from the Wharton's Jelly (WJ-MSCs) to reduce the burden of immature brain injury in a murine animal model. WJ-MSCs transplantation resulted in protective activity characterized by reduced myelin loss and astroglial activation. WJ-MSCs improved locomotor behavior as well. To address the underlying mechanisms, we tested the key regulators of responses to DNA-damaging agents, such as cyclic AMP-dependent protein kinase/calcium-dependent protein kinase (PKA/PKC), cyclin-dependent kinase (CDK), ataxia-telangiectasia-mutated/ATM- and Rad3-related (ATM/ATR) substrates, protein kinase B (Akt), and 14-3-3 binding protein partners. We characterized WJ-MSCs using a specific profiler polymerase chain reaction array. We provide evidence that WJ-MSCs target pivotal regulators of the cell fate such as CDK/14-3-3/Akt signaling. We identified leukemia inhibitory factor as a potential candidate of WJ-MSCs' induced modifications as well. We hypothesize that WJ-MSCs may exert adaptive responses depending on the type of injury they are facing, making them prominent candidates for cell therapy in perinatal injuries.

p27Kip1 Is Required to Mediate a G1 Cell Cycle Arrest Downstream of ATM following Genotoxic Stress

The DNA damage response (DDR) is a coordinated signaling network that ensures the maintenance of genome stability under DNA damaging stress. In response to DNA lesions, activation of the DDR leads to the establishment of cell cycle checkpoints that delay cell-cycle progression and allow repair of the defects. The tumor suppressor p27^Kip1 is a cyclin-CDK inhibitor that plays an important role in regulating quiescence in a variety of tissues. Several studies have suggested that p27^Kip1 also plays a role in the maintenance of genomic integrity. Here we demonstrate that p27^Kip1 is essential for the establishment of a G1 checkpoint arrest after DNA damage. We also uncovered that ATM phosphorylates p27^Kip1 on a previously uncharacterized residue (Ser-140), which leads to its stabilization after induction of DNA double-strand breaks. Inhibition of this stabilization by replacing endogenous p27^Kip1 with a Ser-140 phospho-mutant (S140A) significantly sensitized cells to IR treatments. Our findings reveal a novel role for p27^Kip1 in the DNA damage response pathway and suggest that part of its tumor suppressing functions relies in its ability to mediate a G1 arrest after the induction of DNA double strand breaks.

Novel piperazine core compound induces death in human liver cancer cells: possible pharmacological properties

The current study evaluates the cytotoxic mechanism of a novel piperazine derivate designated as PCC against human liver cancer cells. In this context, human liver cancer cell lines, SNU-475 and 243, human monocyte/macrophage cell line, CRL-9855, and human B lymphocyte cell line, CCL-156, were used to determine the IC₅₀ of PCC using the standard MTT assay. PCC displayed a strong suppressive effect on SNU-475 and SNU-423 cells with an IC₅₀ value of 6.98 ± 0.11 μg/ml and 7.76 ± 0.45 μg/ml respectively, after 24 h of treatment. Significant dipping in the mitochondrial membrane potential and elevation in the released of cytochrome c from the mitochondria indicated the induction of the intrinsic apoptosis pathway by PCC. Activation of this pathway was further evidenced by significant activation of caspase 3/7 and 9. PCC was also shown to activate the extrinsic pathways of apoptosis via activation of caspase-8 which is linked to the suppression of NF-ƙB translocation to the nucleus. Cell cycle arrest in the G1 phase was confirmed by flow cytometry and up-regulation of glutathione reductase expression was quantified by qPCR. This study suggests that PCC is a simultaneous inducer of intrinsic and extrinsic pathways of apoptosis in liver cancer cell lines.

Mechanism of action of novel piperazine containing a toxicant against human liver cancer cells

The purpose of this study was to assess the cytotoxic potential of a novel piperazine derivative (PCC) against human liver cancer cells. SNU-475 and 423 human liver cancer cell lines were used to determine the IC50 of PCC using the standard MTT assay. PCC displayed a strong suppressive effect on liver cancer cells with an IC50 value of 6.98 ± 0.11 µM and 7.76 ± 0.45 µM against SNU-475 and SNU-423 respectively after 24 h of treatment. Significant dipping in the mitochondrial membrane potential and elevation in the released of cytochrome c from the mitochondria indicated the induction of the intrinsic apoptosis pathway by PCC. Activation of this pathway was further evidenced by significant activation of caspase 3/7 and 9. PCC was also shown to activate the extrinsic pathways of apoptosis via activation of caspase-8 which is linked to the suppression of NF-κB translocation to the nucleus. Cell cycle arrest in the G1 phase was confirmed by flow cytometry and up-regulation of glutathione reductase expression was quantified by qPCR. Results of this study suggest that PCC is a potent anti-cancer agent inducing both intrinsic and extrinsic pathways of apoptosis in liver cancer cell lines.

Mechanism of Action of the Novel Nickel(II) Complex in Simultaneous Reactivation of the Apoptotic Signaling Networks Against Human Colon Cancer Cells

The aim of this study was to evaluate the cytotoxic potential of a novel nickel(II) complex (NTC) against WiDr and HT-29 human colon cancer cells by determining the IC50 using the standard MTT assay. The NTC displayed a strong suppressive effect on colon cancer cells with an IC50 value of 6.07 ± 0.22 μM and 6.26 ± 0.13 μM against WiDr and HT-29 respectively, after 24 h of treatment. Substantial reduction in the mitochondrial membrane potential and increase in the release of cytochrome c from the mitochondria directed the induction of the intrinsic apoptosis pathway by the NTC. Activation of this pathway was further evidenced by significant activation of caspase 3/7 and 9. The NTC was also shown to activate the extrinsic pathway of apoptosis via activation of caspase-8 which is linked to the suppression of NF-κB translocation to the nucleus. Cell cycle arrest in the G1 phase was confirmed by flow cytometry and up-regulation of glutathione reductase expression was quantified by qPCR. Results of the current work indicates that NTC possess a potent cancer cell abolishing activity by simultaneous induction of intrinsic and extrinsic pathways of apoptosis in colon cancer cell lines.

Apoptotic effect of novel Schiff based CdCl₂(C₁₄H₂₁N₃O₂) complex is mediated via activation of the mitochondrial pathway in colon cancer cells

The development of metal-based agents has had a tremendous role in the present progress in cancer chemotherapy. One well-known example of metal-based agents is Schiff based metal complexes, which hold great promise for cancer therapy. Based on the potential of Schiff based complexes for the induction of apoptosis, this study aimed to examine the cytotoxic and apoptotic activity of a CdCl2(C14H21N3O2) complex on HT-29 cells. The complex exerted a potent suppressive effect on HT-29 cells with an IC50 value of 2.57 ± 0.39 after 72 h of treatment. The collapse of the mitochondrial membrane potential and the elevated release of cytochrome c from the mitochondria to the cytosol indicate the involvement of the intrinsic pathway in the induction of apoptosis. The role of the mitochondria-dependent apoptotic pathway was further proved by the significant activation of the initiator caspase-9 and the executioner caspases-3 and -7. In addition, the activation of caspase-8, which is associated with the suppression of NF-κB translocation to the nucleus, also revealed the involvement of the extrinsic pathway in the induced apoptosis. The results suggest that the CdCl2(C14H21N3O2) complex is able to induce the apoptosis of colon cancer cells and is a potential candidate for future cancer studies.

Suppression of C6 gliomas via application of rat hyperplasia gene

BACKGROUND

Among all neurological tumors, tumor incidence of the neuroepithelial tissue is the highest, where 50% are gliomas. Treatment for gliomas has traditionally included surgery and adjuvant therapy. With advancements in medicine, gene therapy has entered the clinical setting, in which control of tumor growth, tumor volume and decrease of supply of blood to the tumor have been observed. Rat hyperplasia suppressor gene (rHSG) has been proven to inhibit the injury-mediated proliferation of vascular smooth muscle cells.

METHODS

A recombinant adenovirus, Adv-rHSG-GFP, was constructed and characterized by in vitro and in vivo studies. The function of rHSG on cell proliferation was determined in vitro by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) exclusion assay and plate clone formation, while a C6/Sprague Dawley rat glioma model was established to observe the effect of rHSG in vivo.

RESULTS

Overexpression of rHSG displayed a strong effect on suppressing C6 cells proliferation in vitro and growth of glioma in vivo, which suggests the use of rHSG as a possible treatment strategy for glioma. p21Cip1, p27Kip1 and proliferating cell nuclear antigen were found to be involved in the tumor suppression mechanism of rHSG.

CONCLUSIONS

rHSG can markedly inhibit of the growth of rat glioma cells. The suppression mechanism of rHSG may be related to cell cycle regulation, which shows that rHSG is a potential therapeutic target of glioma tumor. This preclinical study supports a further in-depth study on the effect of rHSG on cell proliferation, migration and change in the extracellular matrix component of glioma cells.

A Schiff base-derived copper (II) complex is a potent inducer of apoptosis in colon cancer cells by activating the intrinsic pathway

Metal-based drugs with extensive clinical applications hold great promise for the development of cancer chemotherapeutic agents. In the last few decades, Schiff bases and their complexes have become well known for their extensive biological potential. In the present study, we examined the antiproliferative effect of a copper (II) complex on HT-29 colon cancer cells. The Cu(BrHAP)2 Schiff base compound demonstrated a potent antiproliferative effect in HT-29 cells, with an IC50 value of 2.87  μg/ml after 72 h of treatment. HT-29 cells treated with Cu (II) complexes underwent apoptosis death, as exhibited by a progressive elevation in the proportion of the G1 cell population. At a concentration of 6.25  μg/ml, the Cu(BrHAP)2 compound caused significant elevation in ROS production following perturbation of mitochondrial membrane potential and cytochrome c release, as assessed by the measurement of fluorescence intensity in stained cells. Furthermore, the activation of caspases 3/7 and 9 was part of the Cu (II) complex-induced apoptosis, which confirmed the involvement of mitochondrial-mediated apoptosis. Meanwhile, there was no significant activation of caspase-8. Taken together, these results imply that the Cu(BrHAP)2 compound is a potential candidate for further in vivo and clinical colon cancer studies to develop novel chemotherapeutic agents derived from metal-based agents.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

TRIM3, a tumor suppressor linked to regulation of p21(Waf1/Cip1.)

The TRIM family of genes is largely studied because of their roles in development, differentiation and host cell antiviral defenses; however, roles in cancer biology are emerging. Loss of heterozygosity of the TRIM3 locus in ∼20% of human glioblastomas raised the possibility that this NHL-domain containing member of the TRIM gene family might be a mammalian tumor suppressor. Consistent with this, reducing TRIM3 expression increased the incidence of and accelerated the development of platelet-derived growth factor -induced glioma in mice. Furthermore, TRIM3 can bind to the cdk inhibitor p21(WAF1/CIP1). Thus, we conclude that TRIM3 is a tumor suppressor mapping to chromosome 11p15.5 and that it might block tumor growth by sequestering p21 and preventing it from facilitating the accumulation of cyclin D1-cdk4.

Tyrosine phosphorylation of the p21 cyclin-dependent kinase inhibitor facilitates the development of proneural glioma

Phosphorylation of Tyr-88/Tyr-89 in the 3(10) helix of p27 reduces its cyclin-dependent kinase (CDK) inhibitory activity. This modification does not affect the interaction of p27 with cyclin-CDK complexes but does interfere with van der Waals and hydrogen bond contacts between p27 and amino acids in the catalytic cleft of the CDK. Thus, it had been suggested that phosphorylation of this site could switch the tumor-suppressive CDK inhibitory activity to an oncogenic activity. Here, we examined this hypothesis in the RCAS-PDGF-HA/nestin-TvA proneural glioma mouse model, in which p21 facilitates accumulation of nuclear cyclin D1-CDK4 and promotes tumor development. In these tumor cells, approximately one-third of the p21 is phosphorylated at Tyr-76 in the 3(10) helix. Mutation of this residue to glutamate reduced inhibitory activity in vitro. Mutation of this residue to phenylalanine reduced the tumor-promoting activity of p21 in the animal model, whereas glutamate or alanine substitution allowed tumor formation. Consequently, we conclude that tyrosine phosphorylation contributes to the conversion of CDK inhibitors from tumor-suppressive roles to oncogenic roles.

Oncogene interactions are required for glioma development and progression as revealed by a tissue specific transgenic mouse model

The aggressive and invasive nature of brain tumors has hampered progress in the design and implementation of efficacious therapies. The recent success of targeted therapies in other tumor types makes this an attractive area for research yet complicating matters is the ability of brain tumors to circumvent the targeted pathways to develop drug resistance. Effective therapies will likely need to target more than one signaling pathway or target multiple nodes within a given pathway. Key to identifying these targets is the elucidation of the driver and passenger molecules within these pathways. Animal models provide a useful tool with many advantages in the study of these pathways. These models provide a means to dissect the critical components of tumorigenesis, as well as serve as agents for preclinical testing. This review focuses on the use of the RCAS/tv-a mouse model of brain tumors and describes their unique ability to provide insight into the role of Oncogene cooperation in tumor development and progression.

PDGF and PDGF receptors in glioma

The family of platelet-derived growth factors (PDGFs) plays a number of critical roles in normal embryonic development, cellular differentiation, and response to tissue damage. Not surprisingly, as it is a multi-faceted regulatory system, numerous pathological conditions are associated with aberrant activity of the PDGFs and their receptors. As we and others have shown, human gliomas, especially glioblastoma, express all PDGF ligands and both the two cell surface receptors, PDGFR-α and -β. The cellular distribution of these proteins in tumors indicates that glial tumor cells are stimulated via PDGF/PDGFR-α autocrine and paracrine loops, while tumor vessels are stimulated via the PDGFR-β. Here we summarize the initial discoveries on the role of PDGF and PDGF receptors in gliomas and provide a brief overview of what is known in this field.

p27kip1 protein levels reflect a nexus of oncogenic signaling during cell transformation

SV40 small t-antigen (ST) collaborates with SV40 large T-antigen (LT) and activated rasv12 to promote transformation in a variety of immortalized human cells. A number of oncogenes or the disruption of the general serine-threonine phosphatase protein phosphatase 2A (PP2A) can replace ST in this paradigm. However, the relationship between these oncogenes and PP2A activity is not clear. To address this, we queried the connectivity of these molecules in silico. We found that p27 was connected to each of those oncogenes that could substitute for ST. We further determined that p27 loss can substitute for the expression of ST during transformation of both rodent and human cells. Conversely, knock-in cells expressing the degradation-resistant S10A and T187A mutants of p27 were resistant to the transforming activities of ST. This suggests that p27 is an important target of the tumor-suppressive effects of PP2A and likely an important target of the multitude of cellular oncoproteins that emulate the transforming function of ST.

p27(Kip1) controls cytokinesis via the regulation of citron kinase activation

p27(Kip1) (p27) acts as a tumor suppressor by inhibiting cyclin-cyclin-dependent kinase (cyclin-CDK) activity. However, mice expressing a form of p27 that is unable to bind or inhibit cyclin-CDK complexes (p27(CK-)) have increased incidence of tumor development as compared with wild-type and p27(-/-) mice, revealing an oncogenic role for p27. Here, we identified a phenotype of multinucleation and polyploidy in p27(CK-) mice not present in p27(-/-) animals, suggesting a role for p27 in G2/M that is independent of cyclin-CDK regulation. Further analysis revealed that p27(CK-) expression caused a cytokinesis and abscission defect in mouse embryonic fibroblasts. We identified the Rho effector citron kinase (citron-K) as a p27-interacting protein in vitro and in vivo and found that p27 and citron-K colocalized at the contractile ring and mid-body during telophase and cytokinesis. Moreover, overexpression of the minimal p27-binding domain of citron-K was sufficient to rescue the phenotype caused by p27(CK-). Conversely, expression of a mutant p27(CK-) unable to bind citron-K did not induce multinucleation. Finally, by binding to citron-K, p27 prevented the interaction of citron-K with its activator RhoA. Taken together, these data suggest a role for p27 during cytokinesis via the regulation of citron-K activity.

Molecular systems biology of Sic1 in yeast cell cycle regulation through multiscale modeling

Cell cycle control is highly regulated to guarantee the precise timing of events essential for cell growth, i.e., DNA replication onset and cell division. Failure of this control plays a role in cancer and molecules called cyclin-dependent kinase (Cdk) inhibitors (Ckis) exploit a critical function in cell cycle timing. Here we present a multiscale modeling where experimental and computational studies have been employed to investigate structure, function and temporal dynamics of the Cki Sic1 that regulates cell cycle progression in Saccharomyces cerevisiae. Structural analyses reveal molecular details of the interaction between Sic1 and Cdk/cyclin complexes, and biochemical investigation reveals Sic1 function in analogy to its human counterpart p27(Kip1), whose deregulation leads to failure in timing of kinase activation and, therefore, to cancer. Following these findings, a bottom-up systems biology approach has been developed to characterize modular networks addressing Sic1 regulatory function. Through complementary experimentation and modeling, we suggest a mechanism that underlies Sic1 function in controlling temporal waves of cyclins to ensure correct timing of the phase-specific Cdk activities.

RAD51 can inhibit PDGF-B-induced gliomagenesis and genomic instability

Faithful replication and DNA repair are vital for maintenance of genome integrity. RAD51 is a central protein in homologous recombination repair and during replication, when it protects and restarts stalled replication forks. Aberrant RAD51 expression occurs in glioma, and high expression has been shown to correlate with prolonged survival. Furthermore, genes involved in DNA damage response (DDR) are mutated or deleted in human glioblastomas, corroborating the importance of proper DNA repair to suppress gliomagenesis. We have analyzed DDR and genomic instability in PDGF-B-induced gliomas and investigated the role of RAD51 in glioma development. We show that PDGF-B-induced gliomas display genomic instability and that co-expression of RAD51 can suppress PDGF-B-induced tumorigenesis and prolong survival. Expression of RAD51 inhibited proliferation and genomic instability of tumor cells independent of Arf status. Our results demonstrate that the RAD51 pathway can prevent glioma initiation and maintain genome integrity of induced tumors, suggesting reactivation of the RAD51 pathway as a potential therapeutic avenue.

Cyclin D1 and cdk4 mediate development of neurologically destructive oligodendroglioma

Although the molecular changes that characterize gliomas have been studied, the pathogenesis of tumor development remains unclear. p21 contributes to gliomagenesis by stabilizing cyclin D1-cdk4 kinase complexes, suggesting that cyclin D1 and cdk4 may also be required for glial tumor development. In this study, we used a mouse model to attempt to confirm this hypothesis, finding that cyclin D1 and cdk4 played active roles in not only the tumor but also the tumor microenvironment. Loss of cdk4 blocked tumor development, but loss of cyclin D1 did not prevent gliomas from developing. Instead, loss of cyclin D1 impeded progression to higher stages of malignancy. Enforcing expression of cyclin D1 was insufficient to correct the progression defect observed in cyclin D1-deficient animals. In contrast, restoration of cdk4 in the cdk4-deficient animals restored cell proliferation and tumor formation, although at lower tumor grades. Notably, the failure of tumors in the cyclin D1- and cdk4-deficient animals to progress to higher grades was correlated with a failure to fully activate microglia in the tumor microenvironment. Moreover, when platelet-derived growth factor-transformed glial cells were engrafted orthotopically into the mice, the tumors that formed progressed to high grades in wild-type mice but not cyclin D1-deficient animals. Together, our findings establish that the cyclin D1-cdk4 axis is not only critical in glial tumor cells but also in stromal-derived cells in the surrounding tumor microenvironment that are vital to sustain tumor outgrowth.

CDKN1B, encoding the cyclin-dependent kinase inhibitor 1B (p27), is located in the minimally deleted region of 12p abnormalities in myeloid malignancies and its low expression is a favorable prognostic marker in acute myeloid leukemia

BACKGROUND

Alterations of the short arm of chromosome 12 (12p) occur in various hematologic malignancies and ETV6 and CDKN1B, which are located on 12p, have been implicated as leukemogenic genes of interest.

DESIGN AND METHODS

We selected seven patients with myeloid malignancies and small 12p deletions detected by fluorescence in situ hybridization encompassing only the region centromeric of ETV6 and further evaluated them by single nucleotide polymorphism microarrays.

RESULTS

The minimally deleted region contained only nine genes. These genes were subsequently analyzed by microarray expression profiling in an independent cohort of 781 patients, most, but not all, of whom had different hematologic malignancies CREBL2, MANSC1, and CDKN1B were expressed in more than 25% of cases, while the other six genes were expressed in only a minority of cases. As CDKN1B is a cell cycle regulator and functions as a tumor suppressor gene, this gene was selected for further expression studies in 286 patients with acute myeloid leukemia. When comparing patients with low CDKN1B expression (expression level<1,160; 1st quartile) with those with intermediate or high expression (2nd-4th quartiles), certain mutations were observed more frequently in the former: RUNX1-RUNX1T1 (11/83, 13.3% versus 5/203; 2.5%; P=0.001), PML-RARA rearrangements (11/83, 13.3% versus 4/203, 2.0%; P<0.001), 11q23/MLL rearrangements (6/83, 7.2% versus 4/203, 2.0%; P=0.038), and FLT3-TKD mutations (7/63, 11.1% versus 6/167, 3.6%; P=0.047). The median overall survival of patients with low CDKN1B expression was longer than that of patients with intermediate/high expression (not reached versus 14.9 months; P=0.005). Likewise, patients with low CDKN1B expression had a longer event-free survival than those with intermediate/high expression (31.0 versus 9.7 months; P=0.013).

CONCLUSIONS

CDKN1B is an interesting candidate gene as a potential biomarker for prognostication in acute myeloid leukemia.