Imaging of Liver Tumors Using Surface-Enhanced Raman Scattering Nanoparticles

Complete surgical resection is the ideal first-line treatment for most liver malignancies. This goal would be facilitated by an intraoperative imaging method that enables more precise visualization of tumor margins and detection of otherwise invisible microscopic lesions. To this end, we synthesized silica-encapsulated surface-enhanced Raman scattering (SERS) nanoparticles (NPs) that act as a molecular imaging agent for liver malignancies. We hypothesized that, after intravenous administration, SERS NPs would avidly home to healthy liver tissue but not to intrahepatic malignancies. We tested these SERS NPs in genetically engineered mouse models of hepatocellular carcinoma and histiocytic sarcoma. After intravenous injection, liver tumors in both models were readily identifiable with Raman imaging. In addition, Raman imaging using SERS NPs enabled detection of microscopic lesions in liver and spleen. We compared the performance of SERS NPs to fluorescence imaging using indocyanine green (ICG). We found that SERS NPs delineate tumors more accurately and are less susceptible to photobleaching. Given the known advantages of SERS imaging, namely, high sensitivity and specific spectroscopic detection, these findings hold promise for improved resection of liver cancer.

Scribble is required for pregnancy-induced alveologenesis in the adult mammary gland

The cell polarity protein scribble (SCRIB) is a crucial regulator of polarization, cell migration and tumorigenesis. Whereas SCRIB is known to regulate early stages of mouse mammary gland development, its function in the adult gland is not known. Using an inducible RNA interference (RNAi) mouse model for downregulating SCRIB expression, we report an unexpected role for SCRIB as a positive regulator of cell proliferation during pregnancy-associated mammary alveologenesis. SCRIB was required in the epithelial cell compartment of the mammary gland. Lack of SCRIB attenuated prolactin-induced activation of the JAK2-STAT5 signaling pathway. In addition, loss of SCRIB resulted in the downregulation of prolactin receptor (PRLR) at cell surface and its accumulation in intracellular structures that express markers of the Golgi complex and the recycling endosome. Unlike its role in virgin gland as a negative regulator cell proliferation, SCRIB is a positive regulator of mammary epithelial cell proliferation during pregnancy.

Coordinated Tumor Suppression by Chromosome 8p

In this issue of Cancer Cell, Cai et al. use genome editing to study 8p deletions in a mammary epithelial cell model and show that 8p loss of heterozygosity (LOH) attenuates the action of several genes that collectively promote cell invasion and enhance cellular sensitivity to autophagy inhibitors.

BRD4 Connects Enhancer Remodeling to Senescence Immune Surveillance

Oncogene-induced senescence is a potent barrier to tumorigenesis that limits cellular expansion following certain oncogenic events. Senescent cells display a repressive chromatin configuration thought to stably silence proliferation-promoting genes while simultaneously activating an unusual form of immune surveillance involving a secretory program referred to as the senescence-associated secretory phenotype (SASP). Here, we demonstrate that senescence also involves a global remodeling of the enhancer landscape with recruitment of the chromatin reader BRD4 to newly activated super-enhancers adjacent to key SASP genes. Transcriptional profiling and functional studies indicate that BRD4 is required for the SASP and downstream paracrine signaling. Consequently, BRD4 inhibition disrupts immune cell-mediated targeting and elimination of premalignant senescent cells in vitro and in vivo Our results identify a critical role for BRD4-bound super-enhancers in senescence immune surveillance and in the proper execution of a tumor-suppressive program.

SIGNIFICANCE

This study reveals how cells undergoing oncogene-induced senescence acquire a distinctive enhancer landscape that includes formation of super-enhancers adjacent to immune-modulatory genes required for paracrine immune activation. This process links BRD4 and super-enhancers to a tumor-suppressive immune surveillance program that can be disrupted by small molecule inhibitors of the bromo and extra terminal domain family of proteins. Cancer Discov; 6(6); 612-29. ©2016 AACR.See related commentary by Vizioli and Adams, p. 576This article is highlighted in the In This Issue feature, p. 561.

Deletions linked to TP53 loss drive cancer through p53-independent mechanisms

Mutations disabling the TP53 tumour suppressor gene represent the most frequent events in human cancer and typically occur through a two-hit mechanism involving a missense mutation in one allele and a 'loss of heterozygosity' deletion encompassing the other. While TP53 missense mutations can also contribute gain-of-function activities that impact tumour progression, it remains unclear whether the deletion event, which frequently includes many genes, impacts tumorigenesis beyond TP53 loss alone. Here we show that somatic heterozygous deletion of mouse chromosome 11B3, a 4-megabase region syntenic to human 17p13.1, produces a greater effect on lymphoma and leukaemia development than Trp53 deletion. Mechanistically, the effect of 11B3 loss on tumorigenesis involves co-deleted genes such as Eif5a and Alox15b (also known as Alox8), the suppression of which cooperates with Trp53 loss to produce more aggressive disease. Our results imply that the selective advantage produced by human chromosome 17p deletion reflects the combined impact of TP53 loss and the reduced dosage of linked tumour suppressor genes.

The histone chaperone CAF-1 safeguards somatic cell identity

Cellular differentiation involves profound remodeling of chromatic landscapes, yet the mechanisms by which somatic cell identity is subsequently maintained remain incompletely understood. To further elucidate regulatory pathways that safeguard the somatic state, we performed two comprehensive RNAi screens targeting chromatin factors during transcription factor-mediated reprogramming of mouse fibroblasts to induced pluripotent stem cells (iPSCs). Remarkably, subunits of the chromatin assembly factor-1 (CAF-1) complex emerged as the most prominent hits from both screens, followed by modulators of lysine sumoylation and heterochromatin maintenance. Optimal modulation of both CAF-1 and transcription factor levels increased reprogramming efficiency by several orders of magnitude and facilitated iPSC formation in as little as 4 days. Mechanistically, CAF-1 suppression led to a more accessible chromatin structure at enhancer elements early during reprogramming. These changes were accompanied by a decrease in somatic heterochromatin domains, increased binding of Sox2 to pluripotency-specific targets and activation of associated genes. Notably, suppression of CAF-1 also enhanced the direct conversion of B cells into macrophages and fibroblasts into neurons. Together, our findings reveal the histone chaperone CAF-1 as a novel regulator of somatic cell identity during transcription factor-induced cell fate transitions and provide a potential strategy to modulate cellular plasticity in a regenerative setting.

Immunofluorescent Staining of Mouse Intestinal Stem Cells

Immunofluorescent staining of organoids can be performed to visualize molecular markers of cell behavior. For example, cell proliferation marked by incorporation of nucleotide (EdU), or to observe markers of intestinal differentiation including paneth cells, goblet cells, or enterocytes (see Figure 1). In this protocol we detail a method to fix, permeabilize, stain and mount intestinal organoids for analysis by immunofluorescent confocal microscopy.

Isolation, Culture, and Maintenance of Mouse Intestinal Stem Cells

In this protocol we describe our modifications to a method to isolate, culture and maintain mouse intestinal stem cells as crypt-villus forming organoids. These cells, isolated either from the small or large intestine, maintain self-renewal and multilineage differentiation potential over time. This provides investigators a tool to culture wild type or transformed intestinal epithelium, and a robust assay for stem cell tissue homeostasis in vitro.

Surface-enhanced resonance Raman scattering nanostars for high-precision cancer imaging

The inability to visualize the true extent of cancers represents a significant challenge in many areas of oncology. The margins of most cancer types are not well demarcated because the cancer diffusely infiltrates the surrounding tissues. Furthermore, cancers may be multifocal and characterized by the presence of microscopic satellite lesions. Such microscopic foci represent a major reason for persistence of cancer, local recurrences, and metastatic spread, and are usually impossible to visualize with currently available imaging technologies. An imaging method to reveal the true extent of tumors is desired clinically and surgically. We show the precise visualization of tumor margins, microscopic tumor invasion, and multifocal locoregional tumor spread using a new generation of surface-enhanced resonance Raman scattering (SERRS) nanoparticles, which are termed SERRS nanostars. The SERRS nanostars feature a star-shaped gold core, a Raman reporter resonant in the near-infrared spectrum, and a primer-free silication method. In genetically engineered mouse models of pancreatic cancer, breast cancer, prostate cancer, and sarcoma, and in one human sarcoma xenograft model, SERRS nanostars enabled accurate detection of macroscopic malignant lesions, as well as microscopic disease, without the need for a targeting moiety. Moreover, the sensitivity (1.5 fM limit of detection) of SERRS nanostars allowed imaging of premalignant lesions of pancreatic and prostatic neoplasias. High sensitivity and broad applicability, in conjunction with their inert gold-silica composition, render SERRS nanostars a promising imaging agent for more precise cancer imaging and resection.

Apc Restoration Promotes Cellular Differentiation and Reestablishes Crypt Homeostasis in Colorectal Cancer

The adenomatous polyposis coli (APC) tumor suppressor is mutated in the vast majority of human colorectal cancers (CRC) and leads to deregulated Wnt signaling. To determine whether Apc disruption is required for tumor maintenance, we developed a mouse model of CRC whereby Apc can be conditionally suppressed using a doxycycline-regulated shRNA. Apc suppression produces adenomas in both the small intestine and colon that, in the presence of Kras and p53 mutations, can progress to invasive carcinoma. In established tumors, Apc restoration drives rapid and widespread tumor-cell differentiation and sustained regression without relapse. Tumor regression is accompanied by the re-establishment of normal crypt-villus homeostasis, such that once aberrantly proliferating cells reacquire self-renewal and multi-lineage differentiation capability. Our study reveals that CRC cells can revert to functioning normal cells given appropriate signals and provide compelling in vivo validation of the Wnt pathway as a therapeutic target for treatment of CRC.

Abstract 2866: Defining epigenetic vulnerabilities in synovial sarcoma

Synovial sarcoma constitutes 10% of all soft-tissue sarcomas and arises most frequently in adolescents and young adults. The defining genetic event of synovial sarcoma and main driver of the disease is the translocation of the SS18 gene on chromosome 18q11 to either the SSX1 and SSX2 genes located on chromosome Xp11. The resulting SS18-SSX fusion oncogene lacks a DNA binding domain, but is thought to exert its function via interaction with transcription factors and chromatin remodelers such as polycomb-group chromatin remodeling factors, and components of the SWI/SNF complex. The fact that the fusion oncogene is the defining event in this disease suggests that, on its own, it is able to generate an epigenetic profile that drives tumorigenesis.

In this study we expressed SS18-SSX2 in C2C12 myoblasts to recreate the epigenetic events driving synovial sarcoma. C2C12 myoblasts expressing SS18-SSX1/2 did not show significant differences in cell growth however became completely refractory to differentiation to myotubes. In addition, knockdown of SS18-SSX2 in a mouse synovial sarcoma cell line (derived from a mouse model in which the human SS18-SSX2 fusion oncogene is expressed in the myogenic progenitor compartment) led to strong growth arrest and induction of induction of p16 and p21 cell cycle regulators. This results support the notion that the SS18-SSX oncogene drives tumorigenesis by blocking differentiation programs and by repressing tumor suppressor genes to maintain self-renewal of synovial sarcoma cells. To identify epigenetic vulnerabilities that are specifically generated by the SS18-SSX fusion protein we screened two cell lines using an shRNA library targeting epigenetic modulators: the synovial sarcoma cell line mentioned above and C2C12 myoblasts. Three control shRNAs targeting the SS18-SSX2 oncogene were within the top depletion hits in the synovial sarcoma cell line but were neutral in the C2C12 myoblasts, validating the screen performance. To uncover SS18-SSX specific hits we concentrated in genes within this list that specifically depleted in the synovial sarcoma cell line but not in myoblasts. Genes within this list and to which at least two independent shRNAs scored were subjected to extensive one-by one validation. Our results may uncover epigenetic mechanisms required to maintain self-renewal in synovial sarcoma.

Citation Format: Ana Banito, Nilgun Tasdemir, Marc Ladanyi, Scott W. Lowe. Defining epigenetic vulnerabilities in synovial sarcoma. [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 2866. doi:10.1158/1538-7445.AM2015-2866

Direct and indirect targeting of MYC to treat acute myeloid leukemia

Purpose

Acute myeloid leukemia (AML) is the most common acute leukemia in adults and is often resistant to conventional therapies. The MYC oncogene is commonly overexpressed in AML but has remained an elusive target. We aimed to examine the consequences of targeting MYC both directly and indirectly in AML overexpressing MYC/Myc due to trisomy 8/15 (human/mouse), FLT3-ITD mutation, or gene amplification.

Methods

We performed in vivo knockdown of Myc (shRNAs) and both in vitro and in vivo experiments using four drugs with indirect anti-MYC activity: VX-680, GDC-0941, artemisinin, and JQ1.

Results

shRNA knockdown of Myc in mice prolonged survival, regardless of the mechanism underlying MYC overexpression. VX-680, an aurora kinase inhibitor, demonstrated in vitro efficacy against human MYC-overexpressing AMLs regardless of the mechanism of MYC overexpression, but was weakest against a MYC-amplified cell line. GDC-0941, a PI3-kinase inhibitor, demonstrated efficacy against several MYC-overexpressing AMLs, although only in vitro. Artemisinin, an antimalarial, did not demonstrate consistent efficacy against any of the human AMLs tested. JQ1, a bromodomain and extra-terminal bromodomain inhibitor, demonstrated both in vitro and in vivo efficacy against several MYC-overexpressing AMLs. We also confirmed a decrease in MYC levels at growth inhibitory doses for JQ1, and importantly, sensitivity of AML cell lines to JQ1 appeared independent of the mechanism of MYC overexpression.

Conclusions

Our data support growing evidence that JQ1 and related compounds may have clinical efficacy in AML treatment regardless of the genetic abnormalities underlying MYC deregulation.

Electronic supplementary material

The online version of this article (doi:10.1007/s00280-015-2766-z) contains supplementary material, which is available to authorized users.

A common functional consequence of tumor-derived mutations within c-MYC

The relevance of changes to the coding sequence of the c-MYC oncogene to malignancy is controversial. Overexpression of a pristine form of MYC is observed in many cancers and is sufficient to drive tumorigenesis in most contexts. Yet missense changes to MYC are found in ~50% of Burkitt's lymphomas, aggregate within an amino-terminal degron important for proteasomal destruction of MYC, and where examined profoundly enhance the tumorigenic properties of MYC in vitro and in vivo. Much of the controversy surrounding these mutants stems from the limited number of mutations that have been evaluated and their clustering within a single region of the MYC protein; the highly-conserved Myc box I (MbI) element. Here, by analysis of extant genomic data sets, we identify a previously unrecognized hotspot for tumor-associated MYC mutations, located in a conserved central portion of the protein. We show that, despite their distal location in MYC, mutations in this region precisely phenocopy those in MbI in terms of stability, in vitro transformation, growth-promoting properties, in vivo tumorigenesis and ability to escape p53-dependent tumor surveillance mechanisms. The striking parallels between the behavior of tumor-derived mutations in disparate regions of the MYC protein reveals that a common molecular process is disrupted by these mutations, implying an active role for these mutations in tumorigenesis and suggesting that different therapeutic strategies may be needed for treatment of lymphomas expressing wild type versus mutant forms of MYC protein.

Reciprocal interaction of Wnt and RXR-α pathways in hepatocyte development and hepatocellular carcinoma

Genomic analysis of human hepatocellular carcinoma (HCC) is potentially confounded by the differentiation state of the hepatic cell-of-origin. Here we integrated genomic analysis of mouse HCC (with defined cell-of-origin) along with normal development. We found a major shift in expression of Wnt and RXR-α pathway genes (up and down, respectively) coincident with the transition from hepatoblasts to hepatocytes. A combined Wnt and RXR-α gene signature categorized HCCs into two subtypes (high Wnt, low RXR-α and low Wnt, high RXR-α), which matched cell-of-origin in mouse models and the differentiation state of human HCC. Suppression of RXR-α levels in hepatocytes increased Wnt signaling and enhanced tumorigenicity, whereas ligand activation of RXR-α achieved the opposite. These results corroborate that there are two main HCC subtypes that correspond to the degree of hepatocyte differentation and that RXR-α, in part via Wnt signaling, plays a key functional role in the hepatocyte-like subtype and potentially could serve as a selective therapeutic target.

A targetable GATA2-IGF2 axis confers aggressiveness in lethal prostate cancer

Elucidating the determinants of aggressiveness in lethal prostate cancer may stimulate therapeutic strategies that improve clinical outcomes. We used experimental models and clinical databases to identify GATA2 as a regulator of chemotherapy resistance and tumorigenicity in this context. Mechanistically, direct upregulation of the growth hormone IGF2 emerged as a mediator of the aggressive properties regulated by GATA2. IGF2 in turn activated IGF1R and INSR as well as a downstream polykinase program. The characterization of this axis prompted a combination strategy whereby dual IGF1R/INSR inhibition restored the efficacy of chemotherapy and improved survival in preclinical models. These studies reveal a GATA2-IGF2 aggressiveness axis in lethal prostate cancer and identify a therapeutic opportunity in this challenging disease.

KRAS mutation in colorectal cancer and its association with a stromal-derived gene signature

628

Background: KRAS mutation in colorectal cancer (CRC) is characterized by an altered transcriptional profile when compared to wild-type KRAS tumors. The list of differentially expressed genes overlaps significantly with a stromal fibroblast activation (SFA) signature present across multiple carcinomas. We have reported low expression of SFA genes in KRAS mutant CRC compared to KRAS wild type tumors. Here we sought to confirm the variation of the SFA signature with KRAS mutation and infer its origin in the stromal component of the tumor using experimental models. Methods: The SFA signature was assessed in an inducible-KRAS murine CRC model using RNA-sequencing, and in a CRC cell line with and without a transduced KRAS mutant vector by microarray analysis. Finally, RNA-sequencing of CRC patient-derived xenografts (PDXs) was used to determine whether the SFA signature was being expressed in the tumor epithelium or the surrounding stroma by leveraging the ability to align sequenced reads to the mouse and human genomes separately. Results: The SFA signature was identified in the inducible-KRAS mouse model, matching human cohort observations of decreased SFA gene expression in KRAS mutant CRC. On the other hand, KRAS transduction did not recapitulate the SFA signature in a CRC cell line, suggesting that the presence of stroma may be required for the expression of the SFA signature. Finally, RNA-seq reads for SFA signature genes in CRC PDXs immediately after implantation aligned primarily to the human genome but in later passages of the same PDXs aligned only to the mouse genome. These data suggest that the SFA transcriptional program is associated with the stroma rather than the epithelial tumor cells. Conclusions: KRAS mutation in CRC is associated with a gene expression signature derived from the tumor stroma. These findings suggest that KRAS mutation in the epithelial tumor cells may impact the tumor microenvironment in CRC.

Convergent loss of PTEN leads to clinical resistance to a PI(3)Kα inhibitor

The feasibility of performing broad and deep tumour genome sequencing has shed new light into tumour heterogeneity and provided important insights into the evolution of metastases arising from different clones^1,2. To add an additional layer of complexity, tumour evolution may be influenced by selective pressure provided by therapy, in a similar fashion as it occurs in infectious diseases. Here, we have studied the tumour genomic evolution in a patient with metastatic breast cancer bearing an activating PIK3CA mutation. The patient was treated with the PI3Kα inhibitor BYL719 and achieved a lasting clinical response, although eventually progressed to treatment and died shortly thereafter. A rapid autopsy was performed and a total of 14 metastatic sites were collected and sequenced. All metastatic lesions, when compared to the pre-treatment tumour, had a copy loss of PTEN, and those lesions that became refractory to BYL719 had additional and different PTEN genetic alterations, resulting in the loss of PTEN expression. Acquired bi-allelic loss of PTEN was found in one additional patient treated with BYL719 whereas in two patients PIK3CA mutations present in the primary tumour were no longer detected at the time of progression. To functionally characterize our findings, inducible PTEN knockdown in sensitive cells resulted in resistance to BYL719, while simultaneous PI3Kp110β blockade reverted this resistance phenotype, both in cell lines and in PTEN-null xenografts derived from our patient. We conclude that parallel genetic evolution of separate sites with different PTEN genomic alterations leads to a convergent PTEN- null phenotype resistant to PI3Kα inhibition.

Abstract IA15: Accelerating translational research using ESC-GEMM models and inducible RNAi

Cancer arises through an evolutionary process whereby normal cells acquire mutations that erode growth controls, leading to the expansion of aberrantly proliferating cells. Such mutations activate oncogenes or inactivate tumor suppressors, each bestowing new capabilities to developing leukemic cells. Our research is based on the premise that the path of cancer evolution dictates a tumor's subsequent response to therapy and creates unique vulnerabilities that represent therapeutic opportunities. Hence, we in use cancer genomics and functional studies in a coordinated manner to identify cancer drivers and dependencies, the then study their role of key genes in therapy response. To facilitate our research, we frequently employ mouse cancer models based on the genetic manipulation of stem and progenitor cells ex vivo followed by transplantation of the altered cells into the appropriate organ of syngeneic recipient mice. This approach allows us to rapidly study the impact of many genes and gene combinations on tumorigenesis and distinguish driver and passenger events from many candidates. Furthermore, we have developed powerful methods for using RNA interference to suppress gene function in vivo in either a stable and reversible manner, enabling us to study the gene function in established disease. Recently, we have combined these approaches to identify and validate candidate driver genes, and then used the animal models obtained from these studies as preclinical model to test novel therapeutic approaches. In addition, we have used inducible RNAi to study the role of tumor suppressor genes and cancer targets in normal and neoplastic tissues.

Citation Format: Scott W. Lowe. Accelerating translational research using ESC-GEMM models and inducible RNAi. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr IA15.

In vivo engineering of oncogenic chromosomal rearrangements with the CRISPR/Cas9 system

Chromosomal rearrangements play a central role in the pathogenesis of human cancers and often result in the expression of therapeutically actionable gene fusions¹. A recently discovered example is a fusion between the Echinoderm Microtubule-associated Protein-like 4 (EML4) and the Anaplastic Lymphoma Kinase (ALK) genes, generated by an inversion on the short arm of chromosome 2: inv(2)(p21p23). The EML4-ALK oncogene is detected in a subset of human non-small cell lung cancers (NSCLC)² and is clinically relevant because it confers sensitivity to ALK inhibitors³. Despite their importance, modeling such genetic events in mice has proven challenging and requires complex manipulation of the germline. Here we describe an efficient method to induce specific chromosomal rearrangements in vivo using viral-mediated delivery of the CRISPR/Cas9 system to somatic cells of adult animals. We apply it to generate a mouse model of Eml4-Alk-driven lung cancer. The resulting tumors invariably harbor the Eml4-Alkinversion, express the Eml4-Alk fusion gene, display histo-pathologic and molecular features typical of ALK+ human NSCLCs, and respond to treatment with ALK-inhibitors. The general strategy described here substantially expands our ability to model human cancers in mice and potentially in other organisms.

Abstract 2935: RNAi screen identifies therapeutic targets in hepatocellular carcinoma

Liver cancer is the third leading cause of cancer related mortality worldwide. Hepatocellular carcinoma (HCC), the most common type of primary liver cancer, is a very aggressive type of cancer that lacks effective treatment, in part, due to the undruggable nature of its main genetic drivers, such as oncogene MYC amplification or loss of tumor suppressor TP53. The introduction of the multikinase inhibitor Sorafenib represents the biggest therapeutic advance in the past decade, though its activity only increases the life expectancy of liver cancer patients from 8 to 11 months. To identify new therapeutic targets for HCC, we employ RNA interference (RNAi) screens as a non-biased means to identify genes required for tumor maintenance in genetically defined tumors. In the current study, we applied an optimized shRNA platform to perform a negative selection screen in genetically-defined murine hepatocellular carcinoma cell in vitro. Specifically, we have designed a library that contains 2,245 shRNAs targeting 442 “drugged” genes, including genes encoding proteins targeted by FDA-approved drugs, small molecules in clinical trials, or compounds under preclinical development. Countersceening and validation was performed in normal murine cells and human HCC cell lines and xenograft using shRNAs and corresponding small molecular drugs. Pharmacological or shRNA-mediated inhibition of certain targets led to robust antitumor effects, both in vitro and in vivo, and suggest new strategies to inhibit Myc function therapeutically. Our results highlight the utility of RNAi screening for studying cancer vulnerabilities that can be exploited for direct pharmacological intervention.

Citation Format: Chun-Hao Huang, Amaia Lujambio, Johannes Zuber, Thomas Kitzing, Darjus F. Tschaharganeh, Elisa De Stanchina, Scott W. Lowe. RNAi screen identifies therapeutic targets in hepatocellular carcinoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2935. doi:10.1158/1538-7445.AM2014-2935

Abstract LB-40: Inducible in vivo silencing of Brd4 identifies potential toxicities of sustained BET protein inhibition

BET family proteins are novel therapeutic targets for cancer and inflammation and represent the first epigenetic readers against which small-molecule inhibitors have been developed. First generation BET inhibitors have shown some therapeutic efficacy in pre-clinical models, however the consequences of disrupting BET protein function in normal tissues are largely unknown. Using an inducible and reversible transgenic RNAi mouse model targeting the BET family protein Brd4, we show that acute suppression of Brd4 in adult animals has immediate and dramatic effects in multiple tissues. Brd4-depleted mice displayed epidermal hyperplasia, alopecia, and decreased cellular diversity and stem cell depletion in the small intestine. To test whether the loss of intestinal stem cells induced by Brd4 suppression would have consequences in combination with agents that induce acute gut toxicity, we subjected mice to a sub-lethal irradiation. Whereas control mice showed intestinal recovery 4-5 days following irradiation, Brd4-depleted mice showed dramatic and continued crypt-villus atrophy, implying that potent BET protein inhibition in combination with cytotoxic chemotherapy may show significantly enhanced toxicity in the clinic. These findings provide important insight into Brd4 function in normal tissues and importantly, predict several potential outcomes associated with potent and sustained inhibition of BET protein function.

Citation Format: Jessica Bolden, Nilgun Tasdemir, Lukas Dow, Johan H. van Es, John E. Wilkinson, Zhen Zhao, Hans Clevers, Scott W. Lowe. Inducible in vivo silencing of Brd4 identifies potential toxicities of sustained BET protein inhibition. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-40. doi:10.1158/1538-7445.AM2014-LB-40

CDK9-mediated transcription elongation is required for MYC addiction in hepatocellular carcinoma

One-year survival rates for newly diagnosed hepatocellular carcinoma (HCC) are <50%, and unresectable HCC carries a dismal prognosis owing to its aggressiveness and the undruggable nature of its main genetic drivers. By screening a custom library of shRNAs directed toward known drug targets in a genetically defined Myc-driven HCC model, we identified cyclin-dependent kinase 9 (Cdk9) as required for disease maintenance. Pharmacological or shRNA-mediated CDK9 inhibition led to robust anti-tumor effects that correlated with MYC expression levels and depended on the role that both CDK9 and MYC exert in transcription elongation. Our results establish CDK9 inhibition as a therapeutic strategy for MYC-overexpressing liver tumors and highlight the relevance of transcription elongation in the addiction of cancer cells to MYC.