Rb Tumor Suppressor Pathways Research Articles

Abstract 1564: Novel combination of TRIP13 and Aurora kinase A inhibition demonstrated extensive DNA damage and immunogenic cell death in RB-deficient cancers

Abstract Many lethal cancer types have inactivation of the retinoblastoma (Rb) tumor suppressor pathway including cancers caused by human papillomavirus (HPV), which causes >5% of all cancers worldwide. There are no therapies that uniquely target Rb-deficient cancers. Biomarker-selected, molecular targeted therapy for Rb-deficient cancers represents both an unmet need and a translational knowledge gap. To address this gap, we recently published data that support a model in which Rb-deficient cancers, having high levels of Mad2, survive owing to TRIP13-mediated inhibition of Mad2 and Aurora A-mediated mitotic progression. The combination of Aurora A inhibition (alisertib) plus TRIP13 depletion caused extensive apoptosis in Rb-deficient, but not in Rb-proficient, cancer cells. Both Aurora A and TRIP13 are involved in the cell’s response to DNA damage and apoptosis that can lead to immunogenic cell death (ICD), prompting us to investigate the mechanism underlying the synthetic lethality of TRIP13 and Aurora A in Rb-deficient cancer cells. We transfected two HPV+ cell lines with TRIP13 or control siRNA and incubated with 100nM alisertib. We also generated two stable HPV+ cell lines with TRIP13KO. In both these systems, TRIP13 KD or KO alone did not cause apoptosis. Incubation with alisertib resulted in significant cell death, that validated our previous findings. We then tested the effect of the combination on DNA damage. Alisertib caused robust increases in γH2AX, pTIF1β (KAP1, TRIM28), and DNA PKcs, measured using immunoblotting, in both TRIP13 KD and KO HPV+ cell lines. We also observed enhanced γH2AX staining using confocal microscopy. ICD leads to activation of immune cells by the dying cancer cells in the tumor microenvironment involving the release of antigenic fragments that are engulfed, processed and presented by antigen presenting cells for the induction of antigen-specific T-cell responses. Studies have shown DNA damage stimulates the cGAS/STING pathway leading to ICD. In our system, we saw that TRIP13KO and KD in HPV+ cells treated with alisertib, resulted in marked activation of the cGAS/STING pathway and release of cytochrome C and IL1α in the cell supernatant, which are markers of pyroptosis. Additionally, we also observed gasdermin-E cleavage. The combination further led to increased cancer cell secretion of GM-CSF, IL6, and IL18 that can activate macrophages, NK cells and T-cells. The selective effect of the combination in Rb-deficient cancer cells allowed us to significantly reduce the concentration of alisertib. By sparing normal cells that express Rb, this combination can potentially reduce treatment toxicity in patients with Rb-deficient cancers. The ICD induced by this combination may lead to host T-cell engagement, thereby further enhancing tumor elimination. This work can shape future clinical trials that can change current treatment regime. Citation Format: Soma Ghosh, Tuhina Mazumdar, Lacin Yapindi, Pragya Sinha, Jagannadha Sastry, Faye M. Johnson. Novel combination of TRIP13 and Aurora kinase A inhibition demonstrated extensive DNA damage and immunogenic cell death in RB-deficient cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1564.

Cellular senescence limits translational readthrough.

ABSTRACTThe origin and evolution of cancer cells is considered to be mainly fueled by DNA mutations. Although translation errors could also expand the cellular proteome, their role in cancer biology remains poorly understood. Tumor suppressors called caretakers block cancer initiation and progression by preventing DNA mutations and/or stimulating DNA repair. If translational errors contribute to tumorigenesis, then caretaker genes should prevent such errors in normal cells in response to oncogenic stimuli. Here, we show that the process of cellular senescence induced by oncogenes, tumor suppressors or chemotherapeutic drugs is associated with a reduction in translational readthrough (TR) measured using reporters containing termination codons withing the context of both normal translation termination or programmed TR. Senescence reduced both basal TR and TR stimulated by aminoglycosides. Mechanistically, the reduction of TR during senescence is controlled by the RB tumor suppressor pathway. Cells that escape from cellular senescence either induced by oncogenes or chemotherapy have an increased TR. Also, breast cancer cells that escape from therapy-induced senescence express high levels of AGO1x, a TR isoform of AGO1 linked to breast cancer progression. We propose that senescence and the RB pathway reduce TR limiting proteome diversity and the expression of TR proteins required for cancer cell proliferation.

Retraction for Bischof et al., "Human Papillomavirus Oncoprotein E7 Targets the Promyelocytic Leukemia Protein and Circumvents Cellular Senescence via the Rb and p53 Tumor Suppressor Pathways".

This article refers to:Human Papillomavirus Oncoprotein E7 Targets the Promyelocytic Leukemia Protein and Circumvents Cellular Senescence via the Rb and p53 Tumor Suppressor Pathways

Pan-cancer molecular analysis of the RB tumor suppressor pathway

The retinoblastoma tumor suppressor gene (RB1) plays a critical role in coordinating multiple pathways that impact cancer initiation, disease progression, and therapeutic responses. Here we probed molecular features associated with the RB-pathway across 31 tumor-types. While the RB-pathway has been purported to exhibit multiple mutually exclusive genetic events, only RB1 alteration is mutually exclusive with deregulation of CDK4/6 activity. An ER+ breast cancer model with targeted RB1 deletion was used to identify signatures of CDK4/6 activity and RB-dependency (CDK4/6-RB integrated signature). This signature was prognostic in tumor-types with gene expression features indicative of slower growth. Single copy loss on chromosome 13q encompassing the RB1 locus is prevalent in many cancers, yielding reduced expression of multiple genes in cis, and is inversely related to the CDK4/6-RB integrated signature supporting a cause-effect relationship. Genes that are positively and inversely correlated with the CDK4/6-RB integrated signature define new tumor-specific pathways associated with RB-pathway activity.

IA26 Stage-Specific Roles of RB Constrain Tumor Progression, Lineage Fidelity, and Metastasis

Mutations in the Rb tumor suppressor pathway are a hallmark of cancer and a prevalent feature of lung adenocarcinoma. Additionally, recent clinical successes with cyclin-dependent kinase inhibitors have reinvigorated interest in reactivating the Retinoblastoma (Rb) pathway to treat lung adenocarcinoma and other tumor types. Remarkably, though, Rb’s role in suppressing lung adenocarcinoma remains unclear and whether Rb pathway reactivation would be efficacious in this disease remains unknown. To model Rb pathway reactivation as a treatment strategy in lung adenocarcinoma and to shed light on its role in this disease, we established an RbXTR allele that enables Cre-dependent inactivation of Rb in developing tumors and allows Flp recombinase-inducible reactivation of Rb after tumors are established. In the KrasLox-Stop-Lox-G12D/+;p53flox/flox (KP) mouse model of lung adenocarcinoma, we show that Rb inactivation facilitates the bypass of two molecularly distinct barriers to tumor progression and dramatically accelerates malignant conversion and the development of metastatic disease. Although in the presence of Rb, malignant conversion requires amplification of the Raf/Mek/Erk (MAPK) signaling pathway beyond that normally activated by the Kras oncogene, we find that this requirement is abrogated when Rb is inactivated. Mechanistically, we identified Cdk2 as an important effector downstream of amplified MAPK signaling and that this activity suppresses Rb’s ability to limit the adenoma-to-carcinoma transition. Importantly, inactivation of Cdk2 reduces cell proliferation in Rb wild-type cells and confers sensitivity to Cdk4/6 inhibition in both human and mouse lung adenocarcinoma cell lines that were intrinsically resistant. Acquiring metastatic competency in Rb wild-type tumors is causally linked to epigenetic changes resulting in loss of lung lineage cell fate-determining transcription factors and concomitant derepression of factors normally restricted to embryonic cell types. However, inactivation of Rb uncouples the onset of metastatic competency from the loss of lung lineage factors, facilitates the early derepression of prometastatic factors, and significantly enhances metastatic proclivity. Finally, we demonstrate that reactivation of Rb in metastatic disease settings reprograms these tumors toward a less aggressive cell state and improves overall survival. Our study highlights an unappreciated role for Rb in regulating metastasis-promoting programs, and the potential of Rb restorative therapies to treat lung adenocarcinoma. Further, we suggest a renewed investment in the development of specific Cdk2 inhibitors may be necessary for Rb pathway reactivation in certain cancer types.

Abstract PR01: Modeling Rb loss and pathway reactivation in lung adenocarcinoma

Abstract Mutations in the Rb tumor suppressor pathway are a hallmark of cancer and a prevalent feature of lung adenocarcinoma. Additionally, recent clinical successes with cyclin dependent kinase inhibitors have reinvigorated interest in reactivating the Retinoblastoma (Rb) pathway to treat lung adenocarcinoma and other tumor types. Remarkably though, Rb’s role in suppressing lung adenocarcinoma remains unclear, and whether Rb pathway reactivation would be efficacious in this disease remains unknown. To model Rb pathway reactivation as treatment strategy in lung adenocarcinoma and to shed light on its role in this disease, we established an RbXTR allele that enables Cre-dependent inactivation of Rb in developing tumors, and allows Flp recombinase-inducible reactivation of Rb after tumors are established. In the KrasLox-Stop-Lox-G12D/+;p53flox/flox (KP) mouse model of lung adenocarcinoma, we show that Rb inactivation facilitates the bypass of two molecularly distinct barriers to tumor progression and dramatically accelerates malignant conversion and the development of metastatic disease. Although, in the presence of Rb, malignant conversion requires amplification of the Raf/Mek/Erk (MAPK) signaling pathway beyond that normally activated by the Kras oncogene, we find that this requirement is abrogated when Rb is inactivated. Mechanistically, we identified Cdk2 as an important effector downstream of amplified MAPK signaling and that this activity suppresses Rb’s ability to limit the adenoma-to-carcinoma transition. Importantly, inactivation of Cdk2 reduces cell proliferation in Rb wild-type cells and confers sensitivity to Cdk4/6 inhibition in both human and mouse lung adenocarcinoma cell lines were intrinsically resistant. Acquiring metastatic competency in Rb wild-type tumors is causally linked to epigenetic changes resulting in loss of lung lineage cell fate-determining transcription factors and concomitant derepression of factors normally restricted to embryonic cell types. However, inactivation of Rb uncouples the onset of metastatic competency from the loss of lung lineage factors, facilitates the early derepression of prometastatic factors, and significantly enhances metastatic proclivity. Finally, we demonstrate that reactivation of Rb in metastatic disease settings reprograms these tumors toward a less aggressive cell state and improves overall survival. Our study highlights an unappreciated role for Rb in regulating metastasis-promoting programs, and the potential of Rb restorative therapies to treat lung adenocarcinoma. Further, we suggest that a renewed investment in the development of specific Cdk2 inhibitors may be necessary for Rb pathway reactivation in certain cancer types. This abstract is also being presented as Poster A27. Citation Format: Travis Yates, Caroline Kim-Kiselak, Walter Wang, David M. Feldser. Modeling Rb loss and pathway reactivation in lung adenocarcinoma [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr PR01.

A phase II trial of LEE011 in combination with everolimus in the treatment of advanced well differentiated neuroendocrine tumors of foregut origin.

TPS546 Background: Changes in the retinoblastoma (Rb) tumor suppressor pathway are believed to contribute to the development of well differentiated neuroendocrine tumors (WDNETs). In the pre-clinical setting, loss or downregulation of proteins that normally inhibit the cyclin dependent kinases Cdk4 and Cdk6 have contributed to NET development. Separately, rigorous investigation of everolimus in WDNETs has demonstrated a survival benefit in this patient (pt) population. Pre-clinical data suggests that the Cdk4/Cdk6 inhibitor LEE011 is synergistically active with everolimus. The aim of this study is to evaluate the efficacy and safety of LEE011 in combination with everolimus in pts with advanced WDNETs of foregut origin (thymic, bronchopulmonary, gastric, duodenal, pancreatic). Methods: This study is a multicenter, non-randomized, open-label phase II clinical trial using a Simon two stage optimal design. Main inclusion criteria include: adult patients with WDNET of foregut origin, low to intermediate grade, unresectable and/or metastatic, disease progression ≤ 12 months prior to enrollment, ECOG 0-1. Between 15 and 43 patients will be enrolled from three sites across the US. LEE011 300mg daily, 3 weeks on and 1 week off, in combination with everolimus 2.5mg daily (final dosing based on phase 1b clinical trial performed in metastatic breast cancer; LEE011X2106) will be administered orally until disease progression, unacceptable toxicity, investigator decision, or pt withdrawal. All enrolled pts will be followed by telephone contact for overall survival until death or consent withdrawal. The primary endpoint, progression free survival, will be assessed based on radiographic review by RECISTv1.1. Main secondary endpoints include establishing the safety of this drug combination in this patient population, objective response rate, clinical benefit rate, and overall survival. Correlative objectives include exploring the effect of this drug combination on biomarkers related to the Rb pathway and/or WDNET pathogenesis. This trial began enrollment in 2/27/2017, with 10 patients enrolled to date. Clinical trial information: NCT03070301.

OCT4 controls mitotic stability and inactivates the RB tumor suppressor pathway to enhance ovarian cancer aggressiveness.

OCT4 (Octamer-binding transcription factor 4) is essential for embryonic stem cell self-renewal. Here we show that OCT4 increases the aggressiveness of high-grade serous ovarian cancer (HG-SOC) by inactivating the Retinoblastoma tumor suppressor pathway and enhancing mitotic stability in cancer cells. OCT4 drives the expression of Nuclear Inhibitor of Protein Phosphatase type 1 (NIPP1) and Cyclin F (CCNF) that together inhibit Protein Phosphatase 1 (PP1). This results in pRB hyper-phosphorylation, accelerated cell proliferation and increased in vitro tumorigenicity of ovarian cancer cells. In parallel, OCT4 and NIPP1/CCNF drive the expression of the central Chromosomal Passenger Complex (CPC) components, Borealin, Survivin and the mitotic kinase Aurora B, promoting the clustering of supernumerary centrosomes to increase mitotic stability. Loss of OCT4 or NIPP1/CCNF results in severe mitotic defects, multipolar spindles and supernumerary centrosomes, finally leading to the induction of apoptosis. These phenotypes were recapitulated in different cancer models indicating general relevance for human cancer. Importantly, activation of these parallel pathways leads to dramatically reduced overall survival of HG-SOC patients. Altogether, our data highlights an unprecedented role for OCT4 as central regulator of mitotic fidelity and RB tumor suppressor pathway activity. Disrupting this pathway represents a promising strategy to target an aggressive subpopulation of HG-SOC cells.

Coadministration of Trametinib and Palbociclib Radiosensitizes KRAS-Mutant Non-Small Cell Lung Cancers In Vitro and In Vivo.

To investigate the potential roles that p16 (CDKN2A) and RB activation have in sensitization to MEK inhibitor in resistant KRAS-mutant non-small cell lung cancer cells (NSCLC) in vitro and in vivo. Cell viability was measured with MTS assays. Effects of administration of radiation and combination drug treatments were evaluated by clonogenic assay, flow cytometry, and Western blots. DNA repair was assessed using immunofluorescent analysis. Finally, lung cancer xenografts were used to examine in vivo effects of drug treatment and radiation therapy. In this study, we showed that sensitivity to MEK inhibitor correlated to the RB/p16/CDK4 pathway and knockdown of RB induced resistance in cell lines sensitive to MEK inhibitor. Also, overexpression of p16 and inhibition of CDK4 had the ability to sensitize normally resistant cell lines. Our data indicated that the MEK inhibitor (trametinib, GSK112012) cooperated with the CDK4/6 inhibitor (palbociclib, PD0332991) to strongly reduce cell viability of KRAS-mutant NSCLCs that were resistant to the MEK inhibitor in vitro and in vivo. In addition, we report for the first time that resistance of KRAS-mutant NSCLCs to MEK inhibitor is, at least partly, due to p16 mutation status, and we described a drug combination that efficiently reactivates the RB tumor suppressor pathway to trigger radiosensitizing effects, apoptosis, and cell-cycle arrest. Our findings suggest that MEK inhibitor in combination with CDK4/6 inhibitor has significant anti-KRAS-mutant NSCLC activity and radiosensitizing effect in preclinical models, potentially providing a novel therapeutic strategy for patients with advanced KRAS-mutant NSCLCs.

Abstract 3786: Novel regulation of cyclin D1 stability and the DNA damage response

Abstract Deregulation of the cell division cycle is critical for cancer development. Transition from G1 to S phase of the cell cycle is promoted by D-type cyclins (D1, D2 and D3). Cyclin D1 is a critical regulator of cell cycle progression due to its ability to activate cyclin dependent kinases (CDK) 4 and 6, which phosphorylate and inactivate the retinoblastoma protein (pRb) and activates the transcription factor E2F1 to promote the expression of genes essential for G1/S progression. Importantly, mutations in the tumor suppressor gene RB1 or inactivation of the RB tumor suppressor pathway occurs in almost all human malignancies and one such mechanism involves cyclin D1 overexpression. Even in cancer cells with amplified or elevated cyclin D1 mRNA, the Cyclin D1 protein rapidly declines as cells progress through the G1 phase of the cell cycle to reach undetectable levels in S-phase, and this is thought to be dependent on the Skp1-Cul1-F-box protein (SCF) E3 ubiquitin ligase complex. The identity of the E3 ubiquitin ligase that is specifically involved in downregulating cyclin D1 during S-phase of the cell cycle or the functional significance of cyclin D1 downregulation however, remains unknown, as recent genetic studies in mice demonstrated that the steady state levels or stability of cyclin D1 protein, are not altered following the inactivation of the known regulators of cyclin D1 stability. Using a genetic si-RNA-based screen, we have identified a novel E3 ubiquitin ligase; SCF-FBXW5, which specifically interacts with and downregulates cyclin D1 protein, both in unperturbed proliferating cancer cells and following DNA damage. Importantly, inactivation of this E3 ubiquitin ligase inhibits the proliferation of cancer cells primarily through cyclin D1-dependent inhibition of S-phase progression and sensitizes cancer cells to DNA damage induced by UV or ionizing radiation. These findings identify a novel axis of regulating cyclin D1 stability in cancer cells with implications for targeting this axis to inhibit cyclin D1-driven cancers. Citation Format: Fadila Guessous, Jinho Heo, Naga Vaddadi, Tarek Abbas. Novel regulation of cyclin D1 stability and the DNA damage response. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3786. doi:10.1158/1538-7445.AM2015-3786

Identification of novel target genes specifically activated by deregulated E2F in human normal fibroblasts.

The transcription factor E2F is the principal target of the tumor suppressor pRB. E2F plays crucial roles not only in cell proliferation by activating growth-related genes but also in tumor suppression by activating pro-apoptotic and growth-suppressive genes. We previously reported that, in human normal fibroblasts, the tumor suppressor genes ARF, p27(Kip1) and TAp73 are activated by deregulated E2F activity induced by forced inactivation of pRB, but not by physiological E2F activity induced by growth stimulation. In contrast, growth-related E2F targets are activated by both E2F activities, underscoring the roles of deregulated E2F in tumor suppression in the context of dysfunctional pRB. In this study, to further understand the roles of deregulated E2F, we explored new targets that are specifically activated by deregulated E2F using DNA microarray. The analysis identified nine novel targets (BIM, RASSF1, PPP1R13B, JMY, MOAP1, RBM38, ABTB1, RBBP4 and RBBP7), many of which are involved in the p53 and RB tumor suppressor pathways. Among these genes, the BIM gene was shown to be activated via atypical E2F-responsive promoter elements and to contribute to E2F1-mediated apoptosis. Our results underscore crucial roles of deregulated E2F in growth suppression to counteract loss of pRB function.

Organoid Cultures Derived from Patients with Advanced Prostate Cancer

The lack of invitro prostate cancer models that recapitulate the diversity of human prostate cancer has hampered progress in understanding disease pathogenesis and therapy response. Using a 3Dorganoid system, we report success in long-term culture of prostate cancer from biopsy specimens and circulating tumor cells. The first seven fully characterized organoid lines recapitulate themolecular diversity of prostate cancer subtypes, including TMPRSS2-ERG fusion, SPOP mutation, SPINK1 overexpression, and CHD1 loss. Whole-exome sequencing shows a low mutational burden, consistent with genomics studies, but with mutations in FOXA1 and PIK3R1, as well as in DNA repair and chromatin modifier pathways that have been reported in advanced disease. Loss of p53 and RB tumor suppressor pathway function are the most common feature shared across the organoid lines. The methodology described here should enable the generation of a largerepertoire of patient-derived prostate cancer lines amenable to genetic and pharmacologic studies.

MiR-106a Represses the Rb Tumor Suppressor p130 to Regulate Cellular Proliferation and Differentiation in High-Grade Serous Ovarian Carcinoma

The degree of differentiation in human cancers generally reflects the degree of malignancy, with the most undifferentiated cancer being also the highest grade and the most aggressive. High-grade serous ovarian carcinomas (HGSOC) are poorly differentiated and fast-growing malignancies. The molecular mechanisms underlying the poor differentiation of HGSOC has not been completely characterized. Evidence suggests that miRNA, miR are dysregulated in HGSOC. Therefore, we focused on those miRNAs that are relevant to tumor differentiation. Expression profiling of miRNAs in HGSOC, indicated miR-106a and its family members were significantly upregulated. Upregulation of miR-106a was further validated by real-time reverse transcriptase PCR (qRT-PCR) and miRNA in situ hybridization in a large cohort of HGSOC specimens. Overexpression of miR-106a in benign and malignant ovarian cells significantly increased the cellular proliferation rate and expanded the side-population fraction. In particular, SKOV3 cells with miR-106a overexpression had significantly higher tumor initial/stem cell population (CD24- and CD133-positive cells) than control SKOV3 cells. Among many miR-106a predicated target genes, p130 (RBL2), an retinoblastoma (Rb) tumor suppressor family member, was not only confirmed as a specific target of miR-106a but also related to tumor growth and differentiation. The importance of mir-106a and RBL2 was further demonstrated in vivo, in which, SKOV3 cells overexpressing miR-106a formed poorly differentiated carcinomas and had reduced RBL2 levels. To our knowledge, this is the first study of miR-106a mediating proliferation and tumor differentiation in HGSOC. The current study suggests that the RB tumor suppressor pathway is a critical regulator of growth and differentiation in HGSOC.

In the race for protection, ARF comes second

In the race for protection, ARF comes second

Alteration of the p53 and Rb tumor suppressor pathways by p16/INK4a and p14/ARF promoter methylation and loss of protein expression in recurrent and nonrecurrent meningiomas.

2064 Background: Promoter methylation inactivates tumor suppressor genes (TSG). The INK4a/ARF locus encodes p16INKa and p14ARF cell cycle regulatory proteins, which control Rb and p53 TSG pathways. Most meningiomas are slow growing tumors, but in spite of complete surgical removal, the recurrence rate at 5 years is 5%, rising to 19% in long-term follow-up. However, there are no markers predictive of this evolution. Epigenetic changes in low-grade meningiomas have not been previously addressed. To get insights into the possible role of p16INK4a and p14ARF TSG alterations in grade 1 meningiomas, we study the methylation status and protein expression of these genes in 140 specimens of meningiomas: 29 nonrecurrent and 57 recurrent in one, two or three times. Methods: Methylation specific PCR and bisulfate modification followed by bisulfate genomic sequencing of CpG islands and staining with p16INKa and p14ARF antibodies (Ab’s). Results: Our data show p16INK4a and p14ARF methylation in 43.4% and14.2% meningiomas respectively. Methylation of p16INK4a is found in a similar proportion in non-recurrent meningiomas (37.9%) and the first biopsy of recurrent cases (38.8%) and increases to a 52.3% in successive biopsies of recurrent cases. Methylation of p14ARF occurs in 13.8% of nonrecurrent vs. 9.6% recurrent meningiomas (first biopsy) and 19.6% of successive recurrent meningiomas. Loss of p16INK4a and p14ARF protein expression was shown in 52.7% and 18.6% of meningiomas respectively. p16INK4a and p14ARF methylation was associated with loss of protein expression in 54.7% and 18.8% of meningiomas. Loss of p16INK4a and p14ARF expression was associated with unmethylated promoters in 52.9% and 17.6% of cases respectively. Conclusions: Epigenetic changes of p16INK4a and p14ARF genes and loss of protein expression leading to Rb and p53 TSG pathways alterations, may have a pathogenic role in human meningiomas. Loss of p16INK4a and p14ARF protein expression associated with unmethylated promoters, could be due to loss of heterozigosity or gen mutation. Increase of p16INK4a and p14ARF methylation along the following biopsies of recurrent cases suggests a possible role of methylation in tumor progression.

The Retinoblastoma pathway regulates stem cell proliferation in freshwater planarians

Freshwater planarians are flatworms of the Lophotrochozoan superphylum and are well known for their regenerative abilities, which rely on a large population of pluripotent adult stem cells. However, the mechanisms by which planarians maintain a precise population of adult stem cells while balancing proliferation and cell death, remain to be elucidated. Here we have identified, characterized, and functionally tested the core Retinoblastoma (Rb) pathway components in planarian adult stem cell biology. The Rb pathway is an ancient and conserved mechanism of proliferation control from plants to animals and is composed of three core components: an Rb protein, and a transcription factor heterodimer of E2F and DP proteins. Although the planarian genome contains all components of the Rb pathway, we found that they have undergone gene loss from the ancestral state, similar to other species in their phylum. The single Rb homolog (Smed-Rb) was highly expressed in planarian stem cells and was required for stem cell maintenance, similar to the Rb-homologs p107 and p130 in vertebrates. We show that planarians and their phylum have undergone the most severe reduction in E2F genes observed thus far, and the single remaining E2F was predicted to be a repressive-type E2F (Smed-E2F4-1). Knockdown of either Smed-E2F4-1 or its dimerization partner Dp (Smed-Dp) by RNAi resulted in temporary hyper-proliferation. Finally, we showed that known Rb-interacting genes in other systems, histone deacetylase 1 and cyclinD (Smed-HDAC1; Smed-cycD), were similar to Rb in expression and phenotypes when knocked down by RNAi, suggesting that these established interactions with Rb may also be conserved in planarians. Together, these results showed that planarians use the conserved components of the Rb tumor suppressor pathway to control proliferation and cell survival.

Suppression of HPV E6 and E7 expression by BAF53 depletion in cervical cancer cells

Deregulation of the expression of human papillomavirus (HPV) oncogenes E6 and E7 plays a pivotal role in cervical carcinogenesis because the E6 and E7 proteins neutralize p53 and Rb tumor suppressor pathways, respectively. In approximately 90% of all cervical carcinomas, HPVs are found to be integrated into the host genome. Following integration, the core-enhancer element and P105 promoter that control expression of E6 and E7 adopt a chromatin structure that is different from that of episomal HPV, and this has been proposed to contribute to activation of E6 and E7 expression. However, the molecular basis underlying this chromatin structural change remains unknown. Previously, BAF53 has been shown to be essential for the integrity of higher-order chromatin structure and interchromosomal interactions. Here, we examined whether BAF53 is required for activated expression of E6 and E7 genes. We found that BAF53 knockdown led to suppression of expression of E6 and E7 genes from HPV integrants in cervical carcinoma cell lines HeLa and SiHa. Conversely, expression of transiently transfected HPV18-LCR-Luciferase was not suppressed by BAF53 knockdown. The level of the active histone marks H3K9Ac and H4K12Ac on the P105 promoter of integrated HPV 18 was decreased in BAF53 knockdown cells. BAF53 knockdown restored the p53-dependent signaling pathway in HeLa and SiHa cells. These results suggest that activated expression of the E6 and E7 genes of integrated HPV is dependent on BAF53-dependent higher-order chromatin structure or nuclear motor activity.

The Wnt inhibitory factor 1 (WIF1) is targeted in glioblastoma and has a tumor suppressing function potentially by induction of senescence

Gene expression-based prediction of genomic copy number aberrations in the chromosomal region 12q13 to 12q15 that is flanked by MDM2 and CDK4 identified Wnt inhibitory factor 1 (WIF1) as a candidate tumor suppressor gene in glioblastoma. WIF1 encodes a secreted Wnt antagonist and was strongly downregulated in most glioblastomas as compared with normal brain, implying deregulation of Wnt signaling, which is associated with cancer. WIF1 silencing was mediated by deletion (7/69, 10%) or epigenetic silencing by promoter hypermethylation (29/110, 26%). Co-amplification of MDM2 and CDK4 that is present in 10% of glioblastomas was associated in most cases with deletion of the whole genomic region enclosed, including the WIF1 locus. This interesting pathogenetic constellation targets the RB and p53 tumor suppressor pathways in tandem, while simultaneously activating oncogenic Wnt signaling. Ectopic expression of WIF1 in glioblastoma cell lines revealed a dose-dependent decrease of Wnt pathway activity. Furthermore, WIF1 expression inhibited cell proliferation in vitro, reduced anchorage-independent growth in soft agar, and completely abolished tumorigenicity in vivo. Interestingly, WIF1 overexpression in glioblastoma cells induced a senescence-like phenotype that was dose dependent. These results provide evidence that WIF1 has tumor suppressing properties. Downregulation of WIF1 in 75% of glioblastomas indicates frequent involvement of aberrant Wnt signaling and, hence, may render glioblastomas sensitive to inhibitors of Wnt signaling, potentially by diverting the tumor cells into a senescence-like state.

Abstract 1588: Transformation of primary human fallopian tube secretory epithelial cells: A cell of origin for high-grade serous ovarian carcinoma

Abstract High-grade serous ovarian carcinoma (HGSOC) is a lethal disease for which improved screening and treatment strategies are urgently required. Progress in these areas has been impeded by our poor understanding of HGSOC pathogenesis. The vast majority of ovarian cancer research is been based on the hypothesis that HGSOC develops from ovarian surface epithelial cells. However, recent studies suggest that > 50% of high-grade serous carcinomas involving the ovary likely arose from secretory cells of the fallopian tube epithelium. Consequently, current ovarian-based experimental models of HGSOC are inadequate in representing the full spectrum of the disease. The aim of our study was to develop the first experimental model of fallopian tube secretory epithelial cell (FTSEC) transformation and to use this model to answer three basic questions: 1) Can FTSECs be transformed in vitro?, 2) What genetic alterations are capable of transforming FTSECs?, and 3) Do transformed FTSECs give rise to high-grade serous carcinomas? To this end, we isolated primary human FTSECs from fresh samples of normal fallopian tube fimbriae and immortalized them via transduction with hTERT and SV40 TAg. Immortal FTSECs were transformed by either H-RasV12 or c-Myc expression, resulting in accelerated proliferation, enhanced clonogenicity, anchorage-independent growth, and tumor formation in immunodeficient mice. When SV40 TAg was replaced with equivalent non-viral genetic alterations (sh-p53, CDK4R24C, and sh-PP2A-B56γ), FTSECs remained susceptible to c-Myc-induced transformation and tumorigenesis. These results indicate that hTERT expression plus combined dysfunction of the p53, Rb, and PP2A tumor suppressor pathways is sufficient to support c-Myc-mediated transformation. Importantly, all of the transformed FTSECs in our study produced high-grade serous tumors with Müllerian and epithelial characteristics. Moreover, these tumors were grossly, histologically, and immunophenotypically consistent with human HGSOC. The genetic elements employed by our model represent the most common genomic alterations in human HGSOC – namely, TP53 mutation, RB1 dysfunction, and c-MYC amplification – and are therefore appropriate for modeling this disease. In summary, we have presented the first proof-of-principle evidence that FTSECs can be transformed into high-grade serous carcinomas resembling HGSOC. Our FTSEC transformation model provides a novel framework for assessing the transformative effects of specific genetic alterations on primary FTSECs and thus determining what role they may play in the pathogenesis of HGSOC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1588. doi:10.1158/1538-7445.AM2011-1588

MicroRNAs and senescence

MicroRNAs and senescence