Microcephaly Gene Mcph1 Deficiency Induces p19ARF-Dependent Cell Cycle Arrest and Senescence

MCPH1 has been identified as the causal gene for primary microcephaly type 1, a neurodevelopmental disorder characterized by reduced brain size and delayed growth. As a multifunction protein, MCPH1 has been reported to repress the expression of TERT and interact with transcriptional regulator E2F1. However, it remains unclear whether MCPH1 regulates brain development through its transcriptional regulation function. This study showed that the knockout of Mcph1 in mice leads to delayed growth as early as the embryo stage E11.5. Transcriptome analysis (RNA-seq) revealed that the deletion of Mcph1 resulted in changes in the expression levels of a limited number of genes. Although the expression of some of E2F1 targets, such as Satb2 and Cdkn1c, was affected, the differentially expressed genes (DEGs) were not significantly enriched as E2F1 target genes. Further investigations showed that primary and immortalized Mcph1 knockout mouse embryonic fibroblasts (MEFs) exhibited cell cycle arrest and cellular senescence phenotype. Interestingly, the upregulation of p19ARF was detected in Mcph1 knockout MEFs, and silencing p19Arf restored the cell cycle and growth arrest to wild-type levels. Our findings suggested it is unlikely that MCPH1 regulates neurodevelopment through E2F1-mediated transcriptional regulation, and p19ARF-dependent cell cycle arrest and cellular senescence may contribute to the developmental abnormalities observed in primary microcephaly.

CDKN2A deletions are associated with poor outcomes in 101 adults with T-cell acute lymphoblastic leukemia

The identification of genetic risk subgroups of T-cell acute lymphoblastic leukemia (T-ALL) may provide evidence for risk stratification and individualized treatment. We investigated the characteristics and prognostic value of tumor suppressor gene CDKN2A deletions in 101 patients with T-ALL. The CDKN2A deletion was present in 23% (23/101) of T-ALL by fluorescence in situ hybridization (FISH). The most common type of CDKN2A deletion was homozygous deletion (70%, 16/23). A lower frequency of CDKN2A deletion was found in patients with early T-cell precursor (ETP) ALL than in patients with non-ETP-ALL (10.4% vs 34.0%; P = .008). Deletion of CDKN2A was significantly associated with younger age (P = .001), higher white blood cell (WBC) count (P < .001) and higher lactate dehydrogenase (LDH) level (P = .002). Patients with CDKN2A deletion had lower 2-year overall survival (OS) and event-free survival (EFS) rates than patients without CDKN2A deletion (2-year OS: 18.6% ± 8.9% vs 47.4% ± 6.2%, P = .032; EFS: 16.4 ± 8.3 vs 38.6 ± 5.9%, P = .022). In multivariable analysis, CDKN2A deletion was an independent adverse prognostic factor for OS (P = .016). In conclusion, adult T-ALL patients with CDKN2A deletion had a poor prognosis, and these patients might benefit from intensive chemotherapy or allogeneic hematopoietic stem-cell transplantation.

Loss of Both CDKN2A and CDKN2B Allows for Centrosome Overduplication in Melanoma

Centrosomes duplicate only once in coordination with the DNA replication cycle and have an important role in segregating genetic material. In contrast, most cancer cells have centrosome aberrations, including supernumerary centrosomes, and this correlates with aneuploidy and genetic instability. The tumor suppressors p16 (CDKN2A) and p15 (CDKN2B) (encoded by the familial melanoma CDKN2 locus) inhibit CDK4/6 activity and have important roles in cellular senescence. p16 is also associated with suppressing centrosomal aberrations in breast cancer; however, the role of p15 in centrosome amplification is unknown. Here, we investigated the relationship between p15 and p16 expression, centrosome number abnormalities, and melanoma progression in cell lines derived from various stages of melanoma progression. We found that normal human melanocyte lines did not exhibit centrosome number abnormalities, whereas those from later stages of melanoma did. Additionally, under conditions of S-phase block, p15 and p16 status determined whether centrosome overduplication would occur. Indeed, removal of p15 from p16-negative cell lines derived from various stages of melanoma progression changed cells that previously would not overduplicate their centrosomes into cells that did. Although this study used cell lines in vitro, it suggests that, during clinical melanoma progression, sequential loss of p15 and p16 provides conditions for centrosome duplication to become deregulated with consequences for genome instability.

Loss of ARF/INK4A Promotes Liver Progenitor Cell Transformation Toward Tumorigenicity Supporting Their Role in Hepatocarcinogenesis

Liver progenitor cells (LPCs) contribute to liver regeneration during chronic damage and are implicated as cells of origin for liver cancers including hepatocellular carcinoma (HCC). The CDKN2A locus, which encodes the tumor suppressors alternate reading frame protein (ARF) and INK4A, was identified as one of the most frequently altered genes in HCC. This study demonstrates that inactivation of CDKN2A enhances tumorigenic transformation of LPCs. The level of ARF and INK4A expression was determined in a panel of transformed and nontransformed wild-type LPC lines. Moreover, the transforming potential of LPCs with inactivated CDKN2A was shown to be enhanced in LPCs derived from Arf^-/- and CDKN2A^fl/fl mice and in wild-type LPCs following CRISPR-Cas9 suppression of CDKN2A. ARF and INK4A abundance is consistently reduced or ablated following LPC transformation. Arf^-/- and CDKN2A^-/- LPCs displayed hallmarks of transformation such as anchorage-independent and more rapid growth than control LPC lines with unaltered CDKN2A. Transformation was not immediate, suggesting that the loss of CDKN2A alone is insufficient. Further analysis revealed decreased p21 expression as well as reduced epithelial markers and increased mesenchymal markers, indicative of epithelial-to-mesenchymal transition, following inactivation of the CDKN2A gene were required for tumorigenic transformation. Loss of ARF and INK4A enhances the propensity of LPCs to undergo a tumorigenic transformation. As LPCs represent a cancer stem cell candidate, identifying CDKN2A as a driver of LPC transformation highlights ARF and INK4A as viable prognostic markers and therapeutic targets for HCC.

Sexually dimorphic leanness and hypermobility in p16Ink4a/CDKN2A-deficient mice coincides with phenotypic changes in the cerebellum

p16Ink4a/CDKN2A is a tumor suppressor that critically regulates the cell cycle. Indeed, p16Ink4a deficiency promotes tumor formation in various tissues. We now report that p16Ink4a deficiency in female mice, but not male mice, induces leanness especially in old age, as indicated by lower body weight and smaller white adipose tissue, although other major organs are unaffected. Unexpectedly, the integrity, number, and sizes of adipocytes in white adipose tissue were unaffected, as was macrophage infiltration. Hence, hypermobility appeared to be accountable for the phenotype, since food consumption was not altered. Histological analysis of the cerebellum and deep cerebellar nuclei, a vital sensorimotor control center, revealed increased proliferation of neuronal cells and improved cerebellum integrity. Expression of estrogen receptor β (ERβ) and PCNA also increased in deep cerebellar nuclei, implying crosstalk between p16Ink4a and ERβ. Furthermore, p16Ink4a deficiency expands LC3B+ cells and GFAP+ astrocytes in response to estrogen. Collectively, the data suggest that loss of p16INK4a induces sexually dimorphic leanness in female mice, which appears to be due to protection against cerebellar senescence by promoting neuronal proliferation and homeostasis via ERβ.

A natural WNT signaling variant potently synergizes with Cdkn2ab loss in skin carcinogenesis

Cdkn2ab knockout mice, generated from 129P2 ES cells develop skin carcinomas. Here we show that the incidence of these carcinomas drops gradually in the course of backcrossing to the FVB/N background. Microsatellite analyses indicate that this cancer phenotype is linked to a 20 Mb region of 129P2 chromosome 15 harboring the Wnt7b gene, which is preferentially expressed from the 129P2 allele in skin carcinomas and derived cell lines. ChIPseq analysis shows enrichment of H3K27-Ac, a mark for active enhancers, in the 5' region of the Wnt7b 129P2 gene. The Wnt7b 129P2 allele appears sufficient to cause in vitro transformation of Cdkn2ab-deficient cell lines primarily through CDK6 activation. These results point to a critical role of the Cdkn2ab locus in keeping the oncogenic potential of physiological levels of WNT signaling in check and illustrate that GWAS-based searches for cancer predisposing allelic variants can be enhanced by including defined somatically acquired lesions as an additional input.

Deletion of the p16INK4a tumor suppressor and expression of the androgen receptor induce sarcomatoid carcinomas with signet ring cells in the mouse prostate

The tumor suppressor p16^Ink4a, encoded by the INK4a gene, is an inhibitor of cyclin D-dependent kinases 4 and 6, CDK4 and CDK6. This inhibition prevents the phosphorylation of the retinoblastoma protein (pRb), resulting in cellular senescence through inhibition of E2F-mediated transcription of S phase genes required for cell proliferation. The p16^Ink4a plays an important role in tumor suppression, whereby its deletion, mutation, or epigenetic silencing is a frequently observed genetic alteration in prostate cancer. To assess its roles and related molecular mechanisms in prostate cancer initiation and progression, we generated a mouse model with conditional deletion of p16^Ink4a in prostatic luminal epithelium. The mice underwent oncogenic transformation and developed prostatic intraepithelial neoplasia (PIN) from eight months of age, but failed to develop prostatic tumors. Given the prevalence of aberrant androgen signaling pathways in prostate cancer initiation and progression, we then generated R26hAR^L/wt:p16^L/L: PB-Cre4 compound mice, in which conditional expression of the human AR transgene and deletion of p16^Ink4a co-occur in prostatic luminal epithelial cells. While R26hAR^L/wt:PB-Cre4 mice showed no visible pathological changes, R26hAR^L/wt:p16^L/L: PB-Cre4 compound mice displayed an early onset of high-grade PIN (HGPIN), prostatic carcinoma, and metastatic lesions. Strikingly, we observed tumors resembling human sarcomatoid carcinoma with intermixed focal regions of signet ring cell carcinoma (SRCC) in the prostates of the compound mice. Further characterization of these tumors showed they were of luminal epithelial cell origin, and featured characteristics of epithelial to mesenchymal transition (EMT) with enhanced proliferative and invasive capabilities. Our results not only implicate a biological role for AR expression and p16^Ink4a deletion in the pathogenesis of prostatic SRCC, but also provide a new and unique genetically engineered mouse (GEM) model for investigating the molecular mechanisms for SRCC development.

The tumour suppressor CDKN2A/p16INK4a regulates adipogenesis and bone marrow-dependent development of perivascular adipose tissue

The genomic CDKN2A/B locus, encoding p16^INK4a among others, is linked to an increased risk for cardiovascular disease and type 2 diabetes. Obesity is a risk factor for both cardiovascular disease and type 2 diabetes. p16^INK4a is a cell cycle regulator and tumour suppressor. Whether it plays a role in adipose tissue formation is unknown. p16^INK4a knock-down in 3T3/L1 preadipocytes or p16^INK4a deficiency in mouse embryonic fibroblasts enhanced adipogenesis, suggesting a role for p16^INK4a in adipose tissue formation. p16^INK4a-deficient mice developed more epicardial adipose tissue in response to the adipogenic peroxisome proliferator activated receptor gamma agonist rosiglitazone. Additionally, adipose tissue around the aorta from p16^INK4a-deficient mice displayed enhanced rosiglitazone-induced gene expression of adipogenic markers and stem cell antigen, a marker of bone marrow-derived precursor cells. Mice transplanted with p16^INK4a-deficient bone marrow had more epicardial adipose tissue compared to controls when fed a high-fat diet. In humans, p16^INK4a gene expression was enriched in epicardial adipose tissue compared to other adipose tissue depots. Moreover, epicardial adipose tissue from obese humans displayed increased expression of stem cell antigen compared to lean controls, supporting a bone marrow origin of epicardial adipose tissue. These results show that p16^INK4a modulates epicardial adipose tissue development, providing a potential mechanistic link between the genetic association of the CDKN2A/B locus and cardiovascular disease risk.

Generation of an immortalized mouse embryonic palatal mesenchyme cell line

Palatogenesis is a complex morphogenetic process, disruptions in which result in highly prevalent birth defects in humans. In recent decades, the use of model systems such as genetically-modified mice, mouse palatal organ cultures and primary mouse embryonic palatal mesenchyme (MEPM) cultures has provided significant insight into the molecular and cellular defects underlying cleft palate. However, drawbacks in each of these systems have prevented high-throughput, large-scale studies of palatogenesis in vitro. Here, we report the generation of an immortalized MEPM cell line that maintains the morphology, migration ability, transcript expression and responsiveness to exogenous growth factors of primary MEPM cells, with increased proliferative potential over primary cultures. The immortalization method described in this study will facilitate the generation of palatal mesenchyme cells with an unlimited capacity for expansion from a single genetically-modified mouse embryo and enable mechanistic studies of palatogenesis that have not been possible using primary culture.

Enhanced Megakaryopoiesis and Platelet Activity in Hypercholesterolemic, B6-Ldlr-/-, Cdkn2a-Deficient Mice

BACKGROUND

Genome-wide association studies for coronary artery disease/myocardial infarction revealed a 58 kb risk locus on 9p21.3. Refined genetic analyses revealed unique haplotype blocks conferring susceptibility to atherosclerosis per se versus risk for acute complications in the presence of underlying coronary artery disease. The cell proliferation inhibitor locus, CDKN2A, maps just upstream of the myocardial infarction risk block, is at least partly regulated by the noncoding RNA, ANRIL, overlapping the risk block, and has been associated with platelet counts in humans. Thus, we tested the hypothesis that CDKN2A deficiency predisposes to increased platelet production, leading to increased platelet activation in the setting of hypercholesterolemia.

METHODS AND RESULTS

Platelet production and activation were measured in B6-Ldlr(-/-)Cdkn2a(+/-) mice and a congenic strain carrying the region of homology with the human 9p21.3/CDKN2A locus. The strains exhibit decreased expression of CDKN2A (both p16(INK4a) and p19(ARF)) but not CDKN2B (p15(INK4b)). Compared with B6-Ldlr(-/-) controls, both Cdkn2a-deficient strains exhibited increased platelet counts and bone marrow megakaryopoiesis. The platelet overproduction phenotype was reversed by treatment with cyclin-dependent kinase 4/6 inhibitor, PD0332991/palbociclib, that mimics the endogenous effect of p16(INK4a). Western diet feeding resulted in increased platelet activation, increased thrombin/antithrombin complex, and decreased bleeding times in Cdkn2a-deficient mice compared with controls.

CONCLUSIONS

Together, the data suggest that one or more Cdkn2a transcripts modulate platelet production and activity in the setting of hypercholesterolemia, amenable to pharmaceutical intervention. Enhanced platelet production and activation may predispose to arterial thrombosis, suggesting an explanation, at least in part, for the association of 9p21.3 and myocardial infarction.

Cooperativity of oncogenic K-ras and downregulated p16/INK4A in human pancreatic tumorigenesis

Activation of K-ras and inactivation of p16 are the most frequently identified genetic alterations in human pancreatic epithelial adenocarcinoma (PDAC). Mouse models engineered with mutant K-ras and deleted p16 recapitulate key pathological features of PDAC. However, a human cell culture transformation model that recapitulates the human pancreatic molecular carcinogenesis is lacking. In this study, we investigated the role of p16 in hTERT-immortalized human pancreatic epithelial nestin-expressing (HPNE) cells expressing mutant K-ras (K-ras^G12V). We found that expression of p16 was induced by oncogenic K-ras in these HPNE cells and that silencing of this induced p16 expression resulted in tumorigenic transformation and development of metastatic PDAC in an orthotopic xenograft mouse model. Our results revealed that PI3K/Akt, ERK1/2 pathways and TGFα signaling were activated by K-ras and involved in the malignant transformation of human pancreatic cells. Also, p38/MAPK pathway was involved in p16 up-regulation. Thus, our findings establish an experimental cell-based model for dissecting signaling pathways in the development of human PDAC. This model provides an important tool for studying the molecular basis of PDAC development and gaining insight into signaling mechanisms and potential new therapeutic targets for altered oncogenic signaling pathways in PDAC.

CDK4/6 and IGF1 receptor inhibitors synergize to suppress the growth of p16INK4A-deficient pancreatic cancers

Loss-of-function mutations in p16(INK4A) (CDKN2A) occur in approximately 80% of sporadic pancreatic ductal adenocarcinoma (PDAC), contributing to its early progression. Although this loss activates the cell-cycle-dependent kinases CDK4/6, which have been considered as drug targets for many years, p16(INK4A)-deficient PDAC cells are inherently resistant to CDK4/6 inhibitors. This study searched for targeted therapies that might synergize with CDK4/6 inhibition in this setting. We report that the IGF1R/IR inhibitor BMS-754807 cooperated with the CDK4/6 inhibitor PD-0332991 to strongly block proliferation of p16(INK4A)-deficient PDAC cells in vitro and in vivo. Sensitivity to this drug combination correlated with reduced activity of the master cell growth regulator mTORC1. Accordingly, replacing the IGF1R/IR inhibitor with the rapalog inhibitor temsirolimus broadened the sensitivity of PDAC cells to CDK4/6 inhibition. Our results establish targeted therapy combinations with robust cytostatic activity in p16(INK4A)-deficient PDAC cells and possible implications for improving treatment of a broad spectrum of human cancers characterized by p16(INK4A) loss.

PD-0332991, a CDK4/6 inhibitor, significantly prolongs survival in a genetically engineered mouse model of brainstem glioma

Diffuse intrinsic pontine glioma (DIPG) is an incurable tumor that arises in the brainstem of children. To date there is not a single approved drug to effectively treat these tumors and thus novel therapies are desperately needed. Recent studies suggest that a significant fraction of these tumors contain alterations in cell cycle regulatory genes including amplification of the D-type cyclins and CDK4/6, and less commonly, loss of Ink4a-ARF leading to aberrant cell proliferation. In this study, we evaluated the therapeutic approach of targeting the cyclin-CDK-Retinoblastoma (Rb) pathway in a genetically engineered PDGF-B-driven brainstem glioma (BSG) mouse model. We found that PD-0332991 (PD), a CDK4/6 inhibitor, induces cell-cycle arrest in our PDGF-B; Ink4a-ARF deficient model both in vitro and in vivo. By contrast, the PDGF-B; p53 deficient model was mostly resistant to treatment with PD. We noted that a 7-day treatment course with PD significantly prolonged survival by 12% in the PDGF-B; Ink4a-ARF deficient BSG model. Furthermore, a single dose of 10 Gy radiation therapy (RT) followed by 7 days of treatment with PD increased the survival by 19% in comparison to RT alone. These findings provide the rationale for evaluating PD in children with Ink4a-ARF deficient gliomas.

Bmi1 is required for hepatic progenitor cell expansion and liver tumor development

Bmi1 is a polycomb group transcriptional repressor and it has been implicated in regulating self-renewal and proliferation of many types of stem or progenitor cells. In addition, Bmi1 has been shown to function as an oncogene in multiple tumor types. In this study, we investigated the functional significance of Bmi1 in regulating hepatic oval cells, the major type of bipotential progenitor cells in adult liver, as well as the role of Bmi1 during hepatocarcinogenesis using Bmi1 knockout mice. We found that loss of Bmi1 significantly restricted chemically induced oval cell expansion in the mouse liver. Concomitant deletion of Ink4a/Arf in Bmi1 deficient mice completely rescued the oval cell expansion phenotype. Furthermore, ablation of Bmi1 delayed hepatocarcinogenesis induced by AKT and Ras co-expression. This antineoplastic effect was accompanied by the loss of hepatic oval cell marker expression in the liver tumor samples. In summary, our data demonstrated that Bmi1 is required for hepatic oval cell expansion via deregulating the Ink4a/Arf locus in mice. Our study also provides the evidence, for the first time, that Bmi1 expression is required for liver cancer development in vivo, thus representing a promising target for innovative treatments against human liver cancer.

NADPH oxidase NOX4 mediates stellate cell activation and hepatocyte cell death during liver fibrosis development

A role for the NADPH oxidases NOX1 and NOX2 in liver fibrosis has been proposed, but the implication of NOX4 is poorly understood yet. The aim of this work was to study the functional role of NOX4 in different cell populations implicated in liver fibrosis: hepatic stellate cells (HSC), myofibroblats (MFBs) and hepatocytes. Two different mice models that develop spontaneous fibrosis (Mdr2(-/-)/p19(ARF-/-), Stat3(Δhc)/Mdr2(-/-)) and a model of experimental induced fibrosis (CCl(4)) were used. In addition, gene expression in biopsies from chronic hepatitis C virus (HCV) patients or non-fibrotic liver samples was analyzed. Results have indicated that NOX4 expression was increased in the livers of all animal models, concomitantly with fibrosis development and TGF-β pathway activation. In vitro TGF-β-treated HSC increased NOX4 expression correlating with transdifferentiation to MFBs. Knockdown experiments revealed that NOX4 downstream TGF-β is necessary for HSC activation as well as for the maintenance of the MFB phenotype. NOX4 was not necessary for TGF-β-induced epithelial-mesenchymal transition (EMT), but was required for TGF-β-induced apoptosis in hepatocytes. Finally, NOX4 expression was elevated in patients with hepatitis C virus (HCV)-derived fibrosis, increasing along the fibrosis degree. In summary, fibrosis progression both in vitro and in vivo (animal models and patients) is accompanied by increased NOX4 expression, which mediates acquisition and maintenance of the MFB phenotype, as well as TGF-β-induced death of hepatocytes.

Induction of histiocytic sarcoma in mouse skeletal muscle

Myeloid sarcomas are extramedullary accumulations of immature myeloid cells that may present with or without evidence of pathologic involvement of the bone marrow or peripheral blood, and often coincide with or precede a diagnosis of acute myeloid leukemia (AML). A dearth of experimental models has hampered the study of myeloid sarcomas and led us to establish a new system in which tumor induction can be evaluated in an easily accessible non-hematopoietic tissue compartment. Using ex-vivo transduction of oncogenic Kras(G12V) into p16/p19^−/− bone marrow cells, we generated transplantable leukemia-initiating cells that rapidly induced tumor formation in the skeletal muscle of immunocompromised NOD.SCID mice. In this model, murine histiocytic sarcomas, equivalent to human myeloid sarcomas, emerged at the injection site 30–50 days after cell implantation and consisted of tightly packed monotypic cells that were CD48+, CD47+ and Mac1+, with low or absent expression of other hematopoietic lineage markers. Tumor cells also infiltrated the bone marrow, spleen and other non-hematopoietic organs of tumor-bearing animals, leading to systemic illness (leukemia) within two weeks of tumor detection. P16/p19^−/−; Kras(G12V) myeloid sarcomas were multi-clonal, with dominant clones selected during secondary transplantation. The systemic leukemic phenotypes exhibited by histiocytic sarcoma-bearing mice were nearly identical to those of animals in which leukemia was introduced by intravenous transplantation of the same donor cells. Moreover, murine histiocytic sarcoma could be similarly induced by intramuscular injection of MLL-AF9 leukemia cells. This study establishes a novel, transplantable model of murine histiocytic/myeloid sarcoma that recapitulates the natural progression of these malignancies to systemic disease and indicates a cell autonomous leukemogenic mechanism.

Loss of p16(Ink4a) function rescues cellular senescence induced by telomere dysfunction

p16^Ink4a is a tumor suppressor and a marker for cellular senescence. Previous studies have shown that p16^Ink4a plays an important role in the response to DNA damage signals caused by telomere dysfunction. In this study, we crossed Wrn^−/− and p16^Ink4a−/− mice to knock out the p16^Ink4a function in a Wrn null background. Growth curves showed that loss of p16^Ink4a could rescue the growth barriers that are observed in Wrn^−/− mouse embryonic fibroblasts (MEFs). By challenging the MEFs with the global genotoxin doxorubicin, we showed that loss of p16^Ink4a did not dramatically affect the global DNA damage response of Wrn^−/− MEFs induced by doxorubicin. However, in response to telomere dysfunction initiated by the telomere damaging protein TRF2^ΔBΔM, loss of p16^Ink4a could partially overcome the DNA damage response by disabling p16^Ink4a up-regulation and reducing the accumulation of γ-H2AX that is observed in Wrn^−/− MEFs. Furthermore, in response to TRF2^ΔBΔM overexpression, Wrn^−/− MEFs senesced within several passages. In contrast, p16^Ink4a−/− and p16^Ink4a−/−Wrn^−/− MEFs could continuously grow and lose expression of the exogenous TRF2^ΔBΔM in their late passages. In summary, our data suggest that in the context of telomere dysfunction, loss of p16^Ink4a function could prevent cells from senescence. These results shed light on the anti-aging strategy through regulation of p16^Ink4a expression.

Bone marrow p16INK4a-deficiency does not modulate obesity, glucose homeostasis or atherosclerosis development

Objective

A genomic region near the CDKN2A locus, encoding p16^INK4a, has been associated to type 2 diabetes and atherosclerotic vascular disease, conditions in which inflammation plays an important role. Recently, we found that deficiency of p16^INK4a results in decreased inflammatory signaling in murine macrophages and that p16^INK4a influences the phenotype of human adipose tissue macrophages. Therefore, we investigated the influence of immune cell p16^INK4a on glucose tolerance and atherosclerosis in mice.

Methods and Results

Bone marrow p16^INK4a-deficiency in C57Bl6 mice did not influence high fat diet-induced obesity nor plasma glucose and lipid levels. Glucose tolerance tests showed no alterations in high fat diet-induced glucose intolerance. While bone marrow p16^INK4a-deficiency did not affect the gene expression profile of adipose tissue, hepatic expression of the alternative markers Chi3l3, Mgl2 and IL10 was increased and the induction of pro-inflammatory Nos2 was restrained on the high fat diet. Bone marrow p16^INK4a-deficiency in low density lipoprotein receptor-deficient mice did not affect western diet-induced atherosclerotic plaque size or morphology. In line, plasma lipid levels remained unaffected and p16^INK4a-deficient macrophages displayed equal cholesterol uptake and efflux compared to wild type macrophages.

Conclusion

Bone marrow p16^INK4a-deficiency does not affect plasma lipids, obesity, glucose tolerance or atherosclerosis in mice.

Chronic activation of wild-type epidermal growth factor receptor and loss of Cdkn2a cause mouse glioblastoma formation

Glioblastoma multiforme (GBM) is characterized by overexpression of epidermal growth factor receptor (EGFR) and loss of the tumor suppressors Ink4a/Arf. Efforts at modeling GBM using wild-type EGFR in mice have proven unsuccessful. Here, we present a unique mouse model of wild-type EGFR-driven gliomagenesis. We used a combination of somatic conditional overexpression and ligand-mediated chronic activation of EGFR in cooperation with Ink4a/Arf loss in the central nervous system of adult mice to generate tumors with the histopathologic and molecular characteristics of human GBMs. Sustained, ligand-mediated activation of EGFR was necessary for gliomagenesis, functionally substantiating the clinical observation that EGFR-positive GBMs from patients express EGFR ligands. To gain a better understanding of the clinically disappointing EGFR-targeted therapies for GBM, we investigated the molecular responses to EGFR tyrosine kinase inhibitor (TKI) treatment in this model. Gefitinib treatment of primary GBM cells resulted in a robust apoptotic response, partially conveyed by mitogen-activated protein kinase (MAPK) signaling attenuation and accompanied by BIM(EL) expression. In human GBMs, loss-of-function mutations in the tumor suppressor PTEN are a common occurrence. Elimination of PTEN expression in GBM cells posttumor formation did not confer resistance to TKI treatment, showing that PTEN status in our model is not predictive. Together, these findings offer important mechanistic insights into the genetic determinants of EGFR gliomagenesis and sensitivity to TKIs and provide a robust discovery platform to better understand the molecular events that are associated with predictive markers of TKI therapy.

Lysine-specific demethylase 2B (KDM2B)-let-7-enhancer of zester homolog 2 (EZH2) pathway regulates cell cycle progression and senescence in primary cells

Sustained expression of the histone demethylase, KDM2B (Ndy1/FBXL10/JHDM1B), bypasses cellular senescence in primary mouse embryonic fibroblasts (MEFs). Here, we show that KDM2B is a conserved regulator of lifespan in multiple primary cell types and defines a program in which this chromatin-modifying enzyme counteracts the senescence-associated down-regulation of the EZH2 histone methyltransferase. Senescence in MEFs epigenetically silences KDM2B and induces the tumor suppressor miRNAs let-7b and miR-101, which target EZH2. Forced expression of KDM2B promotes immortalization by silencing these miRNAs through locus-specific histone H3 K36me2 demethylation, leading to EZH2 up-regulation. Overexpression of let-7b down-regulates EZH2, induces premature senescence, and counteracts immortalization of MEFs driven by KDM2B. The KDM2B-let-7-EZH2 pathway also contributes to the proliferation of immortal Ink4a/Arf null fibroblasts suggesting that, beyond its anti-senescence role in primary cells, this histone-modifying enzyme functions more broadly in the regulation of cellular proliferation.

RNA helicase DDX5 is a p53-independent target of ARF that participates in ribosome biogenesis

The p19ARF tumor suppressor limits ribosome biogenesis and responds to hyperproliferative signals to activate the p53 checkpoint response. Although its activation of p53 has been well characterized, the role of ARF in restraining nucleolar ribosome production is poorly understood. Here we report the use of a mass spectroscopic analysis to identify protein changes within the nucleoli of Arf-deficient mouse cells. Through this approach, we discovered that ARF limited the nucleolar localization of the RNA helicase DDX5, which promotes the synthesis and maturation of rRNA, ultimately increasing ribosome output and proliferation. ARF inhibited the interaction between DDX5 and nucleophosmin (NPM), preventing association of DDX5 with the rDNA promoter and nuclear pre-ribosomes. In addition, Arf-deficient cells transformed by oncogenic RasV12 were addicted to DDX5, because reduction of DDX5 was sufficient to impair RasV12-driven colony formation in soft agar and tumor growth in mice. Taken together, our findings indicate that DDX5 is a key p53-independent target of the ARF tumor suppressor and is a novel non-oncogene participant in ribosome biogenesis.

The in vivo role of the RP-Mdm2-p53 pathway in signaling oncogenic stress induced by pRb inactivation and Ras overexpression

The Mdm2-p53 tumor suppression pathway plays a vital role in regulating cellular homeostasis by integrating a variety of stressors and eliciting effects on cell growth and proliferation. Recent studies have demonstrated an in vivo signaling pathway mediated by ribosomal protein (RP)-Mdm2 interaction that responds to ribosome biogenesis stress and evokes a protective p53 reaction. It has been shown that mice harboring a Cys-to-Phe mutation in the zinc finger of Mdm2 that specifically disrupts RP L11-Mdm2 binding are prone to accelerated lymphomagenesis in an oncogenic c-Myc driven mouse model of Burkitt's lymphoma. Because most oncogenes when upregulated simultaneously promote both cellular growth and proliferation, it therefore stands to reason that the RP-Mdm2-p53 pathway might also be essential in response to oncogenes other than c-Myc. Using genetically engineered mice, we now show that disruption of the RP-Mdm2-p53 pathway by an Mdm2(C305F) mutation does not accelerate prostatic tumorigenesis induced by inactivation of the pRb family proteins (pRb/p107/p130). In contrast, loss of p19Arf greatly accelerates the progression of prostate cancer induced by inhibition of pRb family proteins. Moreover, using ectopically expressed oncogenic H-Ras we demonstrate that p53 response remains intact in the Mdm2(C305F) mutant MEF cells. Thus, unlike the p19Arf-Mdm2-p53 pathway, which is considered a general oncogenic response pathway, the RP-Mdm2-p53 pathway appears to specifically suppress tumorigenesis induced by oncogenic c-Myc.

Recurrent chromosomal copy number alterations in sporadic chordomas

The molecular events in chordoma pathogenesis have not been fully delineated, particularly with respect to copy number changes. Understanding copy number alterations in chordoma may reveal critical disease mechanisms that could be exploited for tumor classification and therapy. We report the copy number analysis of 21 sporadic chordomas using array comparative genomic hybridization (CGH). Recurrent copy changes were further evaluated with immunohistochemistry, methylation specific PCR, and quantitative real-time PCR. Similar to previous findings, large copy number losses, involving chromosomes 1p, 3, 4, 9, 10, 13, 14, and 18, were more common than copy number gains. Loss of CDKN2A with or without loss of CDKN2B on 9p21.3 was observed in 16/20 (80%) unique cases of which six (30%) showed homozygous deletions ranging from 76 kilobases to 4.7 megabases. One copy loss of the 10q23.31 region which encodes PTEN was found in 16/20 (80%) cases. Loss of CDKN2A and PTEN expression in the majority of cases was not attributed to promoter methylation. Our sporadic chordoma cases did not show hotspot point mutations in some common cancer gene targets. Moreover, most of these sporadic tumors are not associated with T (brachyury) duplication or amplification. Deficiency of CDKN2A and PTEN expression, although shared across many other different types of tumors, likely represents a key aspect of chordoma pathogenesis. Sporadic chordomas may rely on mechanisms other than copy number gain if they indeed exploit T/brachyury for proliferation.

Hdac1 and Hdac2 act redundantly to control p63 and p53 functions in epidermal progenitor cells

Epidermal and hair follicle development from surface ectodermal progenitor cells requires coordinated changes in gene expression. Histone deacetylases alter gene expression programs through modification of chromatin and transcription factors. We find that deletion of ectodermal Hdac1 and Hdac2 results in dramatic failure of hair follicle specification and epidermal proliferation and stratification, phenocopying loss of the key ectodermal transcription factor p63. Although expression of p63 and its positively regulated basal cell targets is maintained in Hdac1/2-deficient ectoderm, targets of p63-mediated repression, including p21, 14-3-3σ, and p16/INK4a, are ectopically expressed, and HDACs bind and are active at their promoter regions in normal undifferentiated keratinocytes. Mutant embryos display increased levels of acetylated p53, which opposes p63 functions, and p53 is required for HDAC inhibitor-mediated p21 expression in keratinocytes. Our data identify critical requirements for HDAC1/2 in epidermal development and indicate that HDAC1/2 directly mediate repressive functions of p63 and suppress p53 activity.

Targeted delivery of NRASQ61R and Cre-recombinase to post-natal melanocytes induces melanoma in Ink4a/Arflox/lox mice

We have developed a somatic cell gene delivery mouse model of melanoma that allows for the rapid validation of genetic alterations identified in this disease. A major advantage of this system is the ability to model the multi-step process of carcinogenesis in immune-competent mice without the generation and cross breeding of multiple strains. We have used this model to evaluate the role of RAS isoforms in melanoma initiation in the context of conditional Ink4a/Arf loss. Mice expressing the tumor virus A (TVA) receptor specifically in melanocytes under control of the dopachrome tautomerase (DCT) promoter were crossed to Ink4a/Arf(lox/lox) mice and newborn DCT-TVA/Ink4a/Arf(lox/lox) mice were injected with retroviruses containing activated KRAS, NRAS and/or Cre-recombinase. No mice injected with viruses containing KRAS and Cre or NRAS alone developed tumors; however, more than one-third of DCT-TVA/Ink4a/Arf(lox/lox) mice injected with NRAS and Cre viruses developed melanoma and two-thirds developed melanoma when NRAS and Cre expression was linked.

PTEN loss compromises homologous recombination repair in astrocytes: implications for glioblastoma therapy with temozolomide or poly(ADP-ribose) polymerase inhibitors

Glioblastomas (GBM) are lethal brain tumors that are highly resistant to therapy. The only meaningful improvement in therapeutic response came from use of the S(N)1-type alkylating agent temozolomide in combination with ionizing radiation. However, no genetic markers that might predict a better response to DNA alkylating agents have been identified in GBMs, except for loss of O(6-)methylguanine-DNA methyltransferase via promoter methylation. In this study, using genetically defined primary murine astrocytes as well as human glioma lines, we show that loss of phosphatase and tensin homolog deleted on chromosome 10 (PTEN) confers sensitivity to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), a functional analogue of temozolomide. We find that MNNG induces replication-associated DNA double-strand breaks (DSB), which are inefficiently repaired in PTEN-deficient astrocytes and trigger apoptosis. Mechanistically, this is because PTEN-null astrocytes are compromised in homologous recombination (HR), which is important for the repair of replication-associated DSBs. Our results suggest that reduced levels of Rad51 paralogs in PTEN-null astrocytes might underlie the HR deficiency of these cells. Importantly, the HR deficiency of PTEN-null cells renders them sensitive to the poly(ADP-ribose) polymerase (PARP) inhibitor ABT-888 due to synthetic lethality. In sum, our results tentatively suggest that patients with PTEN-null GBMs (about 36%) may especially benefit from treatment with DNA alkylating agents such as temozolomide. Significantly, our results also provide a rational basis for treating the subgroup of patients who are PTEN deficient with PARP inhibitors in addition to the current treatment regimen of radiation and temozolomide.

Insertional mutagenesis in mice deficient for p15Ink4b, p16Ink4a, p21Cip1, and p27Kip1 reveals cancer gene interactions and correlations with tumor phenotypes

The cyclin dependent kinase (CDK) inhibitors p15, p16, p21, and p27 are frequently deleted, silenced, or downregulated in many malignancies. Inactivation of CDK inhibitors predisposes mice to tumor development, showing that these genes function as tumor suppressors. Here, we describe high-throughput murine leukemia virus insertional mutagenesis screens in mice that are deficient for one or two CDK inhibitors. We retrieved 9,117 retroviral insertions from 476 lymphomas to define hundreds of loci that are mutated more frequently than expected by chance. Many of these loci are skewed toward a specific genetic context of predisposing germline and somatic mutations. We also found associations between these loci with gender, age of tumor onset, and lymphocyte lineage (B or T cell). Comparison of retroviral insertion sites with single nucleotide polymorphisms associated with chronic lymphocytic leukemia revealed a significant overlap between the datasets. Together, our findings highlight the importance of genetic context within large-scale mutation detection studies, and they show a novel use for insertional mutagenesis data in prioritizing disease-associated genes that emerge from genome-wide association studies.

Olig2-regulated lineage-restricted pathway controls replication competence in neural stem cells and malignant glioma

Recent studies have identified stem cells in brain cancer. However, their relationship to normal CNS progenitors, including dependence on common lineage-restricted pathways, is unclear. We observe expression of the CNS-restricted transcription factor, OLIG2, in human glioma stem and progenitor cells reminiscent of type C transit-amplifying cells in germinal zones of the adult brain. Olig2 function is required for proliferation of neural progenitors and for glioma formation in a genetically relevant murine model. Moreover, we show p21(WAF1/CIP1), a tumor suppressor and inhibitor of stem cell proliferation, is directly repressed by OLIG2 in neural progenitors and gliomas. Our findings identify an Olig2-regulated lineage-restricted pathway critical for proliferation of normal and tumorigenic CNS stem cells.

Loss of p53 and Ink4a/Arf cooperate in a cell autonomous fashion to induce metastasis of hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related death worldwide. HCC patients frequently present with disease that has metastasized to other regions of the liver, the portal vein, lymph nodes, or lungs, leading to poor prognoses. Therefore, model systems that allow exploration of the molecular mechanisms underlying metastasis in this disease are greatly needed. We describe here a metastatic HCC model generated after the somatic introduction of the mouse polyoma virus middle T antigen to mice with liver-specific deletion of the Trp53 tumor suppressor locus and show the cell autonomous effect of p53 loss of function on HCC metastasis. We additionally find that cholangiocarcinoma also develops in these mice, and some tumors display features of both HCC and cholangiocarcinoma, suggestive of origin from liver progenitor cells. Concomitant loss of the Ink4a/Arf tumor suppressor locus accelerates tumor formation and metastasis, suggesting potential roles for the p16 and p19 tumor suppressors in this process. Significantly, tumor cell lines isolated from tumors lacking both Trp53 and Ink4a/Arf display enhanced invasion activity in vitro relative to those lacking Trp53 alone. Thus, our data illustrate a new model system amenable for the analysis of HCC metastasis.

Bmi1 and cell of origin determinants of brain tumor phenotype

Glioblastomas frequently express oncogenic EGFR and loss of the Ink4a/Arf locus. Bmi1, a positive regulator of stem cell self renewal, may be critical to drive brain tumor growth. In this issue of Cancer Cell, Bruggeman and colleagues suggest that brain tumors with these molecular alterations can be initiated in both neural precursor and differentiated cell compartments in the absence of Bmi1; however, tumorigenicity is reduced, and tumors contain fewer precursor cells. Surprisingly, tumors appear less malignant when initiated in precursor cells. Bmi1-deficient tumors also had fewer neuronal lineage cells, suggesting a role for Bmi1 in determination of cell lineage and tumor phenotype.

Bmi1 controls tumor development in an Ink4a/Arf-independent manner in a mouse model for glioma

The Polycomb group and oncogene Bmi1 is required for the proliferation of various differentiated cells and for the self-renewal of stem cells and leukemic cancer stem cells. Repression of the Ink4a/Arf locus is a well described mechanism through which Bmi1 can exert its proliferative effects. However, we now demonstrate in an orthotopic transplantation model for glioma, a type of cancer harboring cancer stem cells, that Bmi1 is also required for tumor development in an Ink4a/Arf-independent manner. Tumors derived from Bmi1;Ink4a/Arf doubly deficient astrocytes or neural stem cells have a later time of onset and different histological grading. Moreover, in the absence of Ink4a/Arf, Bmi1-deficient cells and tumors display changes in differentiation capacity.

p15Ink4b is a critical tumour suppressor in the absence of p16Ink4a

The CDKN2b-CDKN2a locus on chromosome 9p21 in human (chromosome 4 in mouse) is frequently lost in cancer. The locus encodes three cell cycle inhibitory proteins: p15INK4b encoded by CDKN2b, p16INK4a encoded by CDKN2a and p14ARF (p19Arf in mice) encoded by an alternative reading frame of CDKN2a (ref. 1). Whereas the tumour suppressor functions for p16INK4a and p14ARF have been firmly established, the role of p15INK4b remains ambiguous. However, many 9p21 deletions also remove CDKN2b, so we hypothesized a synergistic effect of the combined deficiency for p15INK4b, p14ARF and p16INK4a. Here we report that mice deficient for all three open reading frames (Cdkn2ab-/-) are more tumour-prone and develop a wider spectrum of tumours than Cdkn2a mutant mice, with a preponderance of skin tumours and soft tissue sarcomas (for example, mesothelioma) frequently composed of mixed cell types and often showing biphasic differentiation. Cdkn2ab-/- mouse embryonic fibroblasts (MEFs) are substantially more sensitive to oncogenic transformation than Cdkn2a mutant MEFs. Under conditions of stress, p15Ink4b protein levels are significantly elevated in MEFs deficient for p16Ink4a. Our data indicate that p15Ink4b can fulfil a critical backup function for p16Ink4a and provide an explanation for the frequent loss of the complete CDKN2b-CDKN2a locus in human tumours.

Loss of xeroderma pigmentosum C (Xpc) enhances melanoma photocarcinogenesis in Ink4a-Arf-deficient mice

Despite an extensive body of evidence linking UV radiation and melanoma tumorigenesis, a clear mechanistic understanding of this process is still lacking. Because heritable mutations in both INK4a and the nucleotide excision repair (NER) pathway predispose individuals to melanoma development, we set out to test the hypothesis that abrogation of NER, by deletion of the xeroderma pigmentosum C (Xpc) gene, will heighten melanoma photocarcinogenesis in an Ink4a-Arf-deficient background. Experimentally, we generated a strain of mice doubly deficient in Xpc and Ink4a-Arf and subjected wild-type, Xpc-/-Ink4a-Arf+/+, Xpc-/-Ink4a-Arf-/-, and Xpc+/+Ink4a-Arf-/- mice to a single neonatal (day P3) dose of UVB without additional chemical promotion. Indeed, there was a significant increase in the development of dermal spindle/epithelioid cell melanomas in Xpc-/-Ink4a-Arf-/- mice when compared with Xpc+/+Ink4a-Arf-/- mice (P = 0.005); wild-type and Xpc-/-Ink4a-Arf+/+ mice failed to develop tumors. These neoplasms bore a striking histologic resemblance to melanomas that arise in the Tyr-vHRAS/Ink4a-Arf-/- context and often expressed melanocyte differentiation marker Tyrp1, thus supporting their melanocytic origination. All strains, except wild-type mice, developed pigmented and non-pigmented epidermal-derived keratinocytic cysts, whereas Xpc+/+Ink4a-Arf-/- mice exhibited the greatest propensity for squamous cell carcinoma development. We then screened for NRas, HRas, Kras, and BRaf mutations in tumor tissue and detected a higher frequency of rare Kras(Q61) alterations in tumors from Xpc-/-Ink4a-Arf-/- mice compared with Xpc+/+Ink4a-Arf-/- mice (50% versus 7%, P = 0.033). Taken together, results from this novel UV-inducible melanoma model suggest that NER loss, in conjunction with Ink4a-Arf inactivation, can drive melanoma photocarcinogenesis possibly through signature Kras mutagenesis.

CD43 promotes cell growth and helps to evade FAS-mediated apoptosis in non-hematopoietic cancer cells lacking the tumor suppressors p53 or ARF

CD43 is a highly glycosylated transmembrane protein expressed on the surface of most hematopoietic cells. Expression of CD43 has also been demonstrated in many human tumor tissues, including colon adenomas and carcinomas, but not in normal colon epithelium. The potential contribution of CD43 to tumor development is still not understood. Here, we show that overexpression of CD43 increases cell growth and colony formation in mouse and human cells lacking expression of either p53 or ARF (alternative reading frame) tumor-suppressor proteins. In addition, CD43 overexpression also lowers the detection of the FAS death receptor on the cell surface of human cancer cells, and thereby helps to evade FAS-mediated apoptosis. However, when both p53 and ARF proteins are present, CD43 overexpression activates p53 and suppresses colony formation due to induction of apoptosis. These observations suggest CD43 as a potential contributor to tumor development and the functional ARF-p53 pathway is required for the elimination of cells with aberrant CD43 expression.

Chromosome 9p21 gene copy number and prognostic significance of p16 in ESFT

Chromosome 9p21 gene copy number in Ewing's sarcoma family of tumour (ESFT) cell lines and primary ESFT has been evaluated using Multiplex Ligation-dependent probe amplification, and the clinical significance of CDKN2A loss and p16/p14^ARF expression investigated. Homozygous deletion of CDKN2A was identified in 4/9 (44%) of ESFT cell lines and 4/42 (10%) primary ESFT; loss of one copy of CDKN2A was identified in a further 2/9 (22%) cell lines and 2/42 (5%) tumours. CDKN2B was co-deleted in three (33%) cell lines and two (5%) tumours. Co-deletion of the MTAP gene was observed in 1/9 (11%) cell lines and 3/42 (7%) tumours. No correlation was observed between CDKN2A deletion and clinical parameters. However, co-expression of high levels of p16/p14^ARF mRNA predicted a poor event-free survival (P=0.046, log-rank test). High levels of p16/p14^ARF mRNA did not correlate with high expression of p16 protein. Furthermore, p16 protein expression did not predict event-free or overall survival. Methylation is not a common mechanism of p16 gene silencing in ESFT. These studies demonstrate that loss (homozygous deletion or single copy) of CDKN2A was not prognostically significant in primary ESFT. However, high levels of p16/p14^ARF mRNA expression were predictive of a poor event-free survival and should be investigated further.

Expression of p16Ink4a Compensates for p18Ink4c Loss in Cyclin-Dependent Kinase 4/6–Dependent Tumors and Tissues

Cell cycle progression from G(1) to S phase depends on phosphorylation of pRb by complexes containing a cyclin (D type or E type) and cyclin-dependent kinase (e.g., cdk2, cdk4, or cdk6). Ink4 proteins function to oppose the action of cdk4/6-cyclin D complexes by inhibiting cdk4/6. We employed genetic and pharmacologic approaches to study the interplay among Ink4 proteins and cdk4/6 activity in vivo. Mouse embryo fibroblasts (MEF) lacking p16(Ink4a) and p18(Ink4c) showed similar growth kinetics as wild-type MEFs despite increased cdk4 activity. In vivo, germline deficiency of p16(Ink4a) and p18(Ink4c) resulted in increased proliferation in the intermediate pituitary and pancreatic islets of adult mice, and survival of p16(Ink4a-/-);p18(Ink4c-/-) mice was significantly reduced due to aggressive pituitary tumors. Compensation among the Ink4 proteins was observed both in vivo in p18(Ink4c-/-) mice and in MEFs from p16(Ink4a-/-), p18(Ink4c-/-), or p16(Ink4a-/-);p18(Ink4c-/-) mice. Treatment with PD 0332991, a specific cdk4/6 kinase inhibitor, abrogated proliferation in those compartments where Ink4 deficiency was associated with enhanced proliferation (i.e., islets, pituitary, and B lymphocytes) but had no effect on proliferation in other tissues such as the small bowel. These data suggest that p16(Ink4a) and p18(Ink4c) coordinately regulate the in vivo catalytic activity of cdk4/6 in specific compartments of adult mice.

Loss of Arf causes tumor progression of PDGFB-induced oligodendroglioma

In a subset of gliomas, the platelet-derived growth factor (PDGF) signaling pathway is perturbed. This is usually an early event occurring in low-grade tumors. In high-grade gliomas, the subsequent loss of the INK4a-ARF locus is one of the most common mutations. Here, we dissected the separate roles of Ink4a and Arf in PDGFB-induced oligodendroglioma development in mice. We found that there were differential functions of the two tumor suppressor genes. In tumors induced from astrocytes, both Ink4a-loss and Arf-loss caused a significantly increased incidence compared to wild-type mice. In tumors induced from glial progenitor cells there was a slight increase in tumor incidence in Ink4a-/- mice and Ink4a-Arf-/- mice compared to wild-type mice. In both progenitor cells and astrocytes, Arf-loss caused a pronounced increase in tumor malignancy compared to Ink4a-loss. Hence, Ink4a-loss contributed to tumor initiation from astrocytes and Arf-loss caused tumor progression from both glial progenitor cells and astrocytes. Results from in vitro studies on primary brain cell cultures suggested that the PDGFB-induced activation of the mitogen-activated protein kinase pathway via extracellular signal-regulated kinase was involved in the initiation of low-grade oligodendrogliomas and that the additional loss of Arf may contribute to tumor progression through increased levels of cyclin D1 and a phosphoinositide 3-kinase-dependent activation of p70 ribosomal S6 kinase causing a strong proliferative response of tumor cells.

Loss of tumour suppressor p16 expression in initial stages of oral oncogenesis

BACKGROUND

The p16 tumour suppressor gene is known to be involved in regulation of the cell cycle. p16 expression in sequential histological stages of oral squamous cell carcinoma (OSCC) formation was investigated using an experimental model of induced oral carcinogenesis in Syrian golden hamsters.

MATERIALS AND METHODS

Thirty-seven animals were divided into one control group (N = 7) and three experimental groups (N = 10 each) which were treated with a carcinogen and sacrificed at 10, 14 and 19 weeks after treatment. Tumour sections were studied immunohistochemically using monoclonal antibodies against p16 protein.

RESULTS

p16 was found significantly increased in hyperplasia, sharply decreased in dysplasia and in the subsequent stages of oral carcinogenesis.

CONCLUSION

Inactivation of p16 occurs at the early stage of oral mucosal dysplasia in the multistep process of oral tumourigenesis. Therefore, p16 may be considered as a useful prognostic marker for the progression of oral cancer.

Therapeutic anti-EGFR antibody 806 generates responses in murine de novo EGFR mutant-dependent lung carcinomas

Activating EGFR mutations occur in human non-small cell lung cancer (NSCLC), with 5% of human lung squamous cell carcinomas having EGFRvIII mutations and approximately 10%-30% of lung adenocarcinomas having EGFR kinase domain mutations. An EGFR-targeting monoclonal antibody, mAb806, recognizes a conformational epitope of WT EGFR as well as the truncated EGFRvIII mutant. To explore the anticancer spectrum of this antibody for EGFR targeted cancer therapy, mAb806 was used to treat genetically engineered mice with lung tumors that were driven by either EGFRvIII or EGFR kinase domain mutations. Our results demonstrate that mAb806 is remarkably effective in blocking EGFRvIII signaling and inducing tumor cell apoptosis, resulting in dramatic tumor regression in the EGFRvIII-driven murine lung cancers. Another EGFR-targeting antibody, cetuximab, failed to show activity in these lung tumors. Furthermore, treatment of murine lung tumors driven by the EGFR kinase domain mutation with mAb806 also induced significant tumor regression, albeit to a less degree than that observed in EGFRvIII-driven tumors. Taken together, these data support the hypothesis that mAb806 may lead to significant advancements in the treatment of the population of NSCLC patients with these 2 classes of EGFR mutations.

Hyperactivation of Ha-ras oncogene, but not Ink4a/Arf deficiency, triggers bladder tumorigenesis

Although ras is a potent mitogenic oncogene, its tumorigenicity depends on cellular context and cooperative events. Here we show that low-level expression of a constitutively active Ha-ras in mouse urothelium induces simple urothelial hyperplasia that is resistant to progression to full-fledged bladder tumors even in the absence of Ink4a/Arf. In stark contrast, doubling of the gene dosage of the activated Ha-ras triggered early-onset, rapidly growing, and 100% penetrant tumors throughout the urinary tract. Tumor initiation required superseding a rate-limiting step between simple and nodular hyperplasia, the latter of which is marked by the emergence of mesenchymal components and the coactivation of AKT and STAT pathways as well as PTEN inactivation. These results indicate that overactivation of Ha-ras is both necessary and sufficient to induce bladder tumors along a low-grade, noninvasive papillary pathway, and they shed light on the recent findings that ras activation, via point mutation, overexpression, or intensified signaling from FGF receptor 3, occurs in 70%-90% of these tumors in humans. Our results highlight the critical importance of the dosage/strength of Ha-ras activation in dictating its tumorigenicity--a mechanism of oncogene activation not fully appreciated to date. Finally, our results have clinical implications, as inhibiting ras and/or its downstream effectors, such as AKT and STAT3/5, could provide alternative means to treat low-grade, superficial papillary bladder tumors, the most common tumor in the urinary system.

Increasing p16INK4a expression decreases forebrain progenitors and neurogenesis during ageing

Mammalian ageing is associated with reduced regenerative capacity in tissues that contain stem cells. It has been proposed that this is at least partially caused by the senescence of progenitors with age; however, it has not yet been tested whether genes associated with senescence functionally contribute to physiological declines in progenitor activity. Here we show that progenitor proliferation in the subventricular zone and neurogenesis in the olfactory bulb, as well as multipotent progenitor frequency and self-renewal potential, all decline with age in the mouse forebrain. These declines in progenitor frequency and function correlate with increased expression of p16INK4a, which encodes a cyclin-dependent kinase inhibitor linked to senescence. Ageing p16INK4a-deficient mice showed a significantly smaller decline in subventricular zone proliferation, olfactory bulb neurogenesis, and the frequency and self-renewal potential of multipotent progenitors. p16INK4a deficiency did not detectably affect progenitor function in the dentate gyrus or enteric nervous system, indicating regional differences in the response of neural progenitors to increased p16INK4a expression during ageing. Declining subventricular zone progenitor function and olfactory bulb neurogenesis during ageing are thus caused partly by increasing p16INK4a expression.

Overexpression of the wip1 gene abrogates the p38 MAPK/p53/Wip1 pathway and silences p16 expression in human breast cancers

Wild-type p53-induced phosphatase (Wip1 or PPM1D) is a serine/threonine protein phosphatase expressed under various stress conditions, which selectively inactivates p38 MAPK. The finding that this gene is amplified in association with frequent gain of 17q21-24 in breast cancers supports its role as a driver oncogene. However, the pathogenetic mechanism of the wip1 gene expression in breast carcinogenesis remains to be elucidated. In this study, we examine Wip1 mRNA and protein expression in 20 breast cancer tissues and six cell lines. We additionally investigate the relationship among Wip1, active p38 MAPK, p53, and p16 proteins. In our experiments, Wip1 mRNA was significantly upregulated in 7 of 20 (35%) invasive breast cancer samples. Overexpression of Wip1 was inversely correlated with that of active (phosphor-) p38 MAPK (P = 0.007). Furthermore, Wip1-overexpressing tumors exhibited no or low levels of p16, which normally accumulates upon p38 MAPK activation (P = 0.057). Loss of p16 expression was not associated with hypermethylation of its promoter or loss of heterozygosity on 9p21. Among the 135 primary breast carcinomas further examined, a significant association was found between the Wip1 overexpression and negative staining for p53 (P value = 0.057), indicating that the tumors are wild-type for p53. This is first report showing that Wip1 overexpression abrogates the homeostatic balance maintained through the p38-p53-Wip1 pathway, and contributes to malignant progression by inactivating wild-type p53 and p38 MAPK as well as decreasing p16 protein levels in human breast tissues.

Frequent loss of chromosome 9, homozygous CDKN2A/p14ARF/CDKN2B deletion and low TSC1 mRNA expression in pleomorphic xanthoastrocytomas

The molecular pathogenesis of pleomorphic xanthoastrocytoma (PXA), a rare astrocytic brain tumor with a relatively favorable prognosis, is still poorly understood. We characterized 50 PXAs by comparative genomic hybridization (CGH) and found the most common imbalance to be loss on chromosome 9 in 50% of tumors. Other recurrent losses affected chromosomes 17 (10%), 8, 18, 22 (4% each). Recurrent gains were identified on chromosomes X (16%), 7, 9q, 20 (8% each), 4, 5, 19 (4% each). Two tumors demonstrated amplifications mapping to 2p23-p25, 4p15, 12q13, 12q21, 21q21 and 21q22. Analysis of 10 PXAs with available high molecular weight DNA by high-resolution array-based CGH indicated homozygous 9p21.3 deletions involving the CDKN2A/p14(ARF)/CDKN2B loci in six tumors (60%). Interphase fluorescence in situ hybridization to tissue sections confirmed the presence of tumor cells with homozygous 9p21.3 deletions. Mutational analysis of candidate genes on 9q, PTCH and TSC1, revealed no mutations in PXAs with 9q loss and no evidence of TSC1 promoter methylation. However, PXAs consistently showed low TSC1 transcript levels. Taken together, our study identifies loss of chromosome 9 as the most common chromosomal imbalance in PXAs and suggests important roles for homozygous CDKN2A/p14(ARF)/CDKN2B deletion as well as low TSC1 mRNA expression in these tumors.

ROS fusion tyrosine kinase activates a SH2 domain-containing phosphatase-2/phosphatidylinositol 3-kinase/mammalian target of rapamycin signaling axis to form glioblastoma in mice

Glioblastoma multiforme is the most common and lethal form of primary brain cancer. Diagnosis of this advanced glioma has a poor prognosis due to the ineffectiveness of current therapies. Aberrant expression of receptor tyrosine kinases (RTK) in glioblastoma multiformes is suggestive of their role in initiation and maintenance of these tumors of the central nervous system. In fact, ectopic expression of the orphan RTK ROS is a frequent event in human brain cancers, yet the pathologic significance of this expression remains undetermined. Here, we show that a glioblastoma-associated, ligand-independent rearrangement product of ROS (FIG-ROS) cooperates with loss of the tumor suppressor gene locus Ink4a;Arf to produce glioblastomas in the mouse. We show that this FIG-ROS-mediated tumor formation in vivo parallels the activation of the tyrosine phosphatase SH2 domain-containing phosphatase-2 (SHP-2) and a phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin signaling axis in tumors and tumor-derived cell lines. We have established a fully penetrant preclinical model for adult onset of glioblastoma multiforme in keeping with major genetic events observed in the human disease. These findings provide novel and important insights into the role of ROS and SHP-2 function in solid tumor biology and set the stage for preclinical testing of targeted therapeutic approaches.

p16Ink4a or p19Arf loss contributes to Tal1-induced leukemogenesis in mice

Analysis of the INK4A/ARF locus in human T-ALL patients revealed frequent deletions in exon 2, the exon common to both p16(INK4A) and p14(ARF). Other studies have described selective deletion of exon 1beta of p14(ARF) or methylation of the p16(INK4A) promoter. Therefore, it is unclear from these studies whether loss of p16(INK4A) and/or p14(ARF) contributes to the development of T-ALL. To elucidate the relative contribution of the ink4a/arf locus to T-cell leukemogenesis, we mated our tal1 transgenic mice to ink4a/arf-/-, p16(ink4a)-/-, and p19(arf)-/- mice and generated tal1/ink4a/arf+/-, tal1/p16(ink4a)+/-, and tal1/p19(arf)+/- mice. Each of these mice developed T-cell leukemia rapidly, indicating that loss of either p16(ink4a) or p19(arf) cooperates with Tal1 to induce leukemia in mice. Preleukemic studies reveal that Tal1 expression stimulates entry into the cell cycle and thymocyte apoptosis in vivo. Interestingly, mice expressing a DNA-binding mutant of Tal1 do not exhibit increases in S phase cells. The S phase induction is accompanied by an increase in thymocyte apoptosis in tal1 transgenic mice. Whereas apoptosis is reduced to wild-type levels in tal1/ink4a/arf-/- mice, S phase induction remains unaffected. Thus, Tal1 stimulates cell cycle entry independent of the ink4a/arf locus, but its ability to induce apoptosis is Ink4a/Arf-dependent.

Role of INK4a locus in normal eye development and cataract genesis

The murine INK4a locus encodes the critical tumor suppressor proteins, p16(INK4a) and p19(ARF). Mice lacking both p16(INK4a) and p19(ARF) (INK4a-/-) in their FVB/NJ genetic backgrounds developed cataracts and microophthalmia. Histopathologically, INK4a-/- mice showed defects in the developmental regression of the hyaloid vascular system (HVS), retinal dysplasia, and cataracts with numerous vacuolations, closely resembling human persistent hyperplastic primary vitreous (PHPV). Ocular defects, such as retinal fold and abnormal migration of lens fiber cells, were observed as early as embryonic day (E) 15.5, thereby resulting in the abnormal differentiation of the lens. We also found that ectopic expression of p16(INK4a) resulted in the induction of gammaF-crystallin, suggesting an important role of INK4a locus during mouse eye development, and also providing insights into the potential genetic basis of human cataract genesis.

Both p16(Ink4a) and the p19(Arf)-p53 pathway constrain progression of pancreatic adenocarcinoma in the mouse

Activating KRAS mutations and p16(Ink4a) inactivation are near universal events in human pancreatic ductal adenocarcinoma (PDAC). In mouse models, Kras(G12D) initiates formation of premalignant pancreatic ductal lesions, and loss of either Ink4a/Arf (p16(Ink4a)/p19(Arf)) or p53 enables their malignant progression. As recent mouse modeling studies have suggested a less prominent role for p16(Ink4a) in constraining malignant progression, we sought to assess the pathological and genomic impact of inactivation of p16(Ink4a), p19(Arf), and/or p53 in the Kras(G12D) model. Rapidly progressive PDAC was observed in the setting of homozygous deletion of either p53 or p16(Ink4a), the latter with intact germ-line p53 and p19(Arf) sequences. Additionally, Kras(G12D) in the context of heterozygosity either for p53 plus p16(Ink4a) or for p16(Ink4a)/p19(Arf) produced PDAC with longer latency and greater propensity for distant metastases relative to mice with homozygous deletion of p53 or p16(Ink4a)/p19(Arf). Tumors from the double-heterozygous cohorts showed frequent p16(Ink4a) inactivation and loss of either p53 or p19(Arf). Different genotypes were associated with specific histopathologic characteristics, most notably a trend toward less differentiated features in the homozygous p16(Ink4a)/p19(Arf) mutant model. High-resolution genomic analysis revealed that the tumor suppressor genotype influenced the specific genomic patterns of these tumors and showed overlap in regional chromosomal alterations between murine and human PDAC. Collectively, our results establish that disruptions of p16(Ink4a) and the p19(ARF)-p53 circuit play critical and cooperative roles in PDAC progression, with specific tumor suppressor genotypes provocatively influencing the tumor biological phenotypes and genomic profiles of the resultant tumors.

Induction of Nevi and Skin Tumors in Ink4a/Arf Xpa Knockout Mice by Neonatal, Intermittent, or Chronic UVB Exposures

Nevi and melanomas correlate to childhood and intermittent solar UV exposure, xeroderma pigmentosum patients run increased risk, and p16(Ink4a) expression is often lost in malignant progression. To ascertain the effect of these risk factors, pigmented hairless Ink4a/Arf-, Xpa- knockout mice were subjected to various combinations of neonatal [7,12-dimethylbenz(a)anthracene (DMBA) or UVB exposure] and adult treatments (12-O-tetradecanoylphorbol-13-acetate or subacute daily UVB exposure or intermittent overexposure). Nevi occurred earliest, grew largest, and were most numerous in mice exposed to DMBA followed by intermittent UVB overexposure [effect of six minimal edemal doses (MED), 1 x /2 weeks > 4 MED 1 x /wk]. Neonatal UV exposure enhanced nevus induction but lost its effect after 200 days. The Xpa(-/-) mice proved exquisitely sensitive to UV-driven nevus induction, indicating the involvement of pyrimidine dimer DNA lesions, but Xpa(+/+) mice developed many more nevi (>40 per mouse) at high UV dosages not tolerated by Xpa(-/-) mice. Ink4a/Arf(-/-) mice developed most skin tumors faster, but surprisingly developed nevi slower than their heterozygous counterparts especially after neonatal UV exposure. Despite raising >1,600 nevi, only six melanomas arose in our experiments with Ink4a/Arf knockout mice (five of which in Xpa(+/+) mice at high UV dosages). In contrast to human nevi, these nevi lacked hotspot mutations in Braf or Ras genes, possibly explaining the lack of malignant progression in the Ink4a/Arf(-/-) mice. Hence, although our experiments did not effectively emulate human melanoma, they provided clear evidence that intermittent UV overexposure strongly stimulates and the Ink4a/Arf(-/-) genotype may actually impair nevus development.

Reprogramming of replicative senescence in hepatocellular carcinoma-derived cells

Tumor cells have the capacity to proliferate indefinitely that is qualified as replicative immortality. This ability contrasts with the intrinsic control of the number of cell divisions in human somatic tissues by a mechanism called replicative senescence. Replicative immortality is acquired by inactivation of p53 and p16INK4a genes and reactivation of hTERT gene expression. It is unknown whether the cancer cell replicative immortality is reversible. Here, we show the spontaneous induction of replicative senescence in p53-and p16INK4a-deficient hepatocellular carcinoma cells. This phenomenon is characterized with hTERT repression, telomere shortening, senescence arrest, and tumor suppression. SIP1 gene (ZFHX1B) is partly responsible for replicative senescence, because short hairpin RNA-mediated SIP1 inactivation released hTERT repression and rescued clonal hepatocellular carcinoma cells from senescence arrest.