ATR inhibition disrupts rewired homologous recombination and fork protection pathways in PARP inhibitor-resistant BRCA-deficient cancer cells

Yazinski et al. show that the functions of BRCA1 in homologous recombination and replication fork protection are sequentially bypassed during the acquisition of PARP inhibitor (PARPi) resistance. Despite the lack of BRCA1, PARPi-resistant cells regain RAD51 loading to DNA double-stranded breaks and stalled forks, enabling two distinct mechanisms of PARPi resistance.

Poly-(ADP-ribose) polymerase (PARP) inhibitors (PARPis) selectively kill BRCA1/2-deficient cells, but their efficacy in BRCA-deficient patients is limited by drug resistance. Here, we used derived cell lines and cells from patients to investigate how to overcome PARPi resistance. We found that the functions of BRCA1 in homologous recombination (HR) and replication fork protection are sequentially bypassed during the acquisition of PARPi resistance. Despite the lack of BRCA1, PARPi-resistant cells regain RAD51 loading to DNA double-stranded breaks (DSBs) and stalled replication forks, enabling two distinct mechanisms of PARPi resistance. Compared with BRCA1-proficient cells, PARPi-resistant BRCA1-deficient cells are increasingly dependent on ATR for survival. ATR inhibitors (ATRis) disrupt BRCA1-independent RAD51 loading to DSBs and stalled forks in PARPi-resistant BRCA1-deficient cells, overcoming both resistance mechanisms. In tumor cells derived from patients, ATRis also overcome the bypass of BRCA1/2 in fork protection. Thus, ATR inhibition is a unique strategy to overcome the PARPi resistance of BRCA-deficient cancers.

Expression of β-globin by cancer cells promotes cell survival during blood-borne dissemination

Metastasis-competent circulating tumour cells (CTCs) experience oxidative stress in the bloodstream, but their survival mechanisms are not well defined. Here, comparing single-cell RNA-Seq profiles of CTCs from breast, prostate and lung cancers, we observe consistent induction of β-globin (HBB), but not its partner α-globin (HBA). The tumour-specific origin of HBB is confirmed by sequence polymorphisms within human xenograft-derived CTCs in mouse models. Increased intracellular reactive oxygen species (ROS) in cultured breast CTCs triggers HBB induction, mediated through the transcriptional regulator KLF4. Depletion of HBB in CTC-derived cultures has minimal effects on primary tumour growth, but it greatly increases apoptosis following ROS exposure, and dramatically reduces CTC-derived lung metastases. These effects are reversed by the anti-oxidant N-Acetyl Cysteine. Conversely, overexpression of HBB is sufficient to suppress intracellular ROS within CTCs. Altogether, these observations suggest that β-globin is selectively deregulated in cancer cells, mediating a cytoprotective effect during blood-borne metastasis.

Circulating tumour cells contribute to metastatic spread. Here the authors find that beta-chain of haemoglobin is overexpressed in those cells and protects them from oxidative stress, prolonging their survival in circulation and thereby increasing the likelihood of metastasis formation.

ACTL6A Is Co-Amplified with p63 in Squamous Cell Carcinoma to Drive YAP Activation, Regenerative Proliferation, and Poor Prognosis

Loss-of-function mutations in SWI/SNF chromatin-remodeling subunit genes are observed in many cancers, but an oncogenic role for SWI/SNF is not well established. Here, we reveal that ACTL6A, encoding an SWI/SNF subunit linked to stem cell and progenitor cell function, is frequently co-amplified and highly expressed together with the p53 family member p63 in head and neck squamous cell carcinoma (HNSCC). ACTL6A and p63 physically interact, cooperatively controlling a transcriptional program that promotes proliferation and suppresses differentiation, in part through activation of the Hippo-YAP pathway via regulators including WWC1. Ectopic ACTL6A/p63 expression promotes tumorigenesis, while ACTL6A expression and YAP activation are highly correlated in primary HNSCC and predict poor patient survival. Thus, ACTL6A and p63 collaborate as oncogenic drivers in HNSCC.

Abstract B16: Loss of SIRT6 reactivates the RNA-binding protein Lin28b to drive pancreatic cancer

Chromatin remodeling proteins are frequently dysregulated in human cancer; however, little is known about how they control tumorigenesis. The NAD+ dependent histone deacetylase Sirtuin 6 (SIRT6) is a nutrient sensor that reprograms the epigenome in response to nutrient stress and shows reduced expression in human pancreatic ductal adenocarcinoma (PDAC) relative to normal tissue. Thus, we aimed to determine the role of SIRT6 in PDAC and to determine how the loss of this chromatin modifier could influence PDAC formation, progression and metastasis.

Here, we uncover an epigenetic program mediated by SIRT6 that is critical for the suppression of PDAC. We demonstrate that SIRT6 inactivation cooperates with oncogenic KRAS to greatly accelerate the development of lethal pancreatic tumors regardless of p53 status. In addition to developing PDAC and high-grade pancreatic intraepithelial neoplasia (PanIN) at an earlier age, SIRT6-deficient tumors had a greater propensity to metastasize to the lung, compared to their SIRT6 wild-type (WT) counterparts. Through genome-wide profiling of SIRT6 WT and knock-out (KO) PDAC cell lines, we identified the oncofetal RNA-binding protein Lin28b as a target of Sirt6-mediated transcriptional repression. Loss of SIRT6 in both murine and human PDAC cell lines resulted in hyperacetylation of histone 3 lysine 9 and lysine 56 at the Lin28b promoter, MYC recruitment, and pronounced induction of Lin28b expression. Knocking down Lin28b resulted in potent suppression of cell proliferation and tumor sphere formation in Sirt6null/low murine and human cell lines, while murine and human Sirt6WT/high PDAC lines were completely insensitive to the same treatment. Functionally, Lin28b binds to and induces the degradation of the let-7 family of microRNA, thus resulting in increased expression of let-7 target genes such as the IGF2BPs and HMGA2, which have been correlated with increased agressiveness and metastasis in pancreatic tumors. Furthermore, Gene Set Enrichment Analysis (GSEA) comparing PDAC tumors and cell lines with high versus low expression of LIN28B revealed that LIN28Bhigh tumors were strongly enriched for the expression of let-7 targets. Finally, either loss of SIRT6 or the reactivation of Lin28b or the let-7 target genes IGF2BP3 and HMGA2 correlated with a worse overall survival in human PDAC patients.

Thus, our critical findings define SIRT6 as a tumor suppressor in PDAC, working as a transcriptional repressor of the oncofetal RNA-binding protein Lin28b. We demonstrate that loss of Sirt6 creates a permissive hyperacetylated chromatin state, allowing for MYC-dependent transactivation of Lin28b. This work not only provides new insights into the epigenetic mechanisms governing the reactivation of oncofetal proteins in cancer but also demonstrates that this pathway is well-conserved in human PDAC, where reduced SIRT6 expression defines a specific subset of tumors which may benefit from novel therapeutic approaches aimed at targeting components of the LIN28B/let-7 pathway.

This abstract is being presented as a short talk in the scientific program. A full abstract is printed in the Proffered Abstracts section PR09 of the Conference Proceedings.

Citation Format: Sita Kugel, Carlos Sebastián, Julien Fitamant, Kenneth N. Ross, Supriya K. Saha, Esha Jain, Adrianne Gladden, Kshitij S. Arora, Yasutaka Kato, Miguel N. Rivera, Sridhar Ramaswamy, Ruslan I. Sadreyev, Alon Goren, Vikram Deshpande, Nabeel Bardeesy, Raul Mostoslavsky.{Authors}. Loss of SIRT6 reactivates the RNA-binding protein Lin28b to drive pancreatic cancer. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2016 May 12-15; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(24 Suppl):Abstract nr B16.

Genomic Instability Is Induced by Persistent Proliferation of Cells Undergoing Epithelial-to-Mesenchymal Transition

TGF-β secreted by tumor stroma induces epithelial-to-mesenchymal transition (EMT) in cancer cells, a reversible phenotype linked to cancer progression and drug resistance. However, exposure to stromal signals may also lead to heritable changes in cancer cells, which are poorly understood. We show that epithelial cells failing to undergo proliferation arrest during TGF-β-induced EMT sustain mitotic abnormalities due to failed cytokinesis, resulting in aneuploidy. This genomic instability is associated with the suppression of multiple nuclear envelope proteins implicated in mitotic regulation and is phenocopied by modulating the expression of LaminB1. While TGF-β-induced mitotic defects in proliferating cells are reversible upon its withdrawal, the acquired genomic abnormalities persist, leading to increased tumorigenic phenotypes. In metastatic breast cancer patients, increased mesenchymal marker expression within single circulating tumor cells is correlated with genomic instability. These observations identify a mechanism whereby microenvironment-derived signals trigger heritable genetic changes within cancer cells, contributing to tumor evolution.

JARID1B Enables Transit between Distinct States of the Stem-like Cell Population in Oral Cancers

The degree of heterogeneity among cancer stem cells (CSC) remains ill-defined and may hinder effective anti-CSC therapy. Evaluation of oral cancers for such heterogeneity identified two compartments within the CSC pool. One compartment was detected using a reporter for expression of the H3K4me3 demethylase JARID1B to isolate a JARID1B(high) fraction of cells with stem cell-like function. JARID1B(high) cells expressed oral CSC markers including CD44 and ALDH1 and showed increased PI3K pathway activation. They were distinguished from a fraction in a G0-like cell-cycle state characterized by low reactive oxygen species and suppressed PI3K/AKT signaling. G0-like cells lacked conventional CSC markers but were primed to acquire stem cell-like function by upregulating JARID1B, which directly mediated transition to a state expressing known oral CSC markers. The transition was regulated by PI3K signals acting upstream of JARID1B expression, resulting in PI3K inhibition depleting JARID1B(high) cells but expanding the G0-like subset. These findings define a novel developmental relationship between two cell phenotypes that may jointly contribute to CSC maintenance. Expansion of the G0-like subset during targeted depletion of JARID1B(high) cells implicates it as a candidate therapeutic target within the oral CSC pool. Cancer Res; 76(18); 5538-49. ©2016 AACR.

Abstract B11: Targeting mesenchymal cells in the tumor stroma by GPX4 inhibition

Fibroblast-like mesenchymal cells are the most abundant component of tumor stroma and have been shown to promote tumor progression. However, there are no therapeutic agents that target these cells. Developing therapeutics against mesenchymal cells has been challenging because they lack clear “druggable” targets. Here we used a phenotypic screening approach to identify therapeutic targets in bone-marrow mesenchymal stem cells (MSCs), precursors of cancer-associated fibroblasts, that are reported to induce a metastatic phenotype in breast cancer cells (Karnoub et al, Nature 2007). Using the in vitro phenotype of enhanced migration of breast cancer cells (GFP-labeled MDA-MB-231; MDA) induced by MSCs (3-fold faster than MDA alone), we identified inflammatory signaling and glutathione peroxidase 4 as potential targets in MSCs.

To validate this screening model, we first confirmed that MSCs can enhance metastasis using orthotopic xenografts of luciferase-labeled MDA cells co-injected with primary human MSCs in NOD-SCID mice. Mice with MSC+MDA tumors had 5-fold greater thoracic bioluminescence (corresponding to lung metastasis) than mice with MDA tumors alone. We then performed gene-expression profiling of MSC+MDA co-cultures in vitro compared to cells grown alone. The top pathway upregulated in co-cultures was the interferon pathway, suggesting that an inflammatory response follows MSC-MDA interactions. Using publicly available gene-expression datasets, we prioritized transcripts that are expressed in patient stroma and correlate with poor survival in a meta-analysis of 20 whole-tumor datasets. To determine if these transcripts are necessary for MSC-induced metastatic behavior, we performed shRNA knockdown and measured effects on in vitro migration of MSC+MDA co-cultures compared to normal endothelial cells. Knockdown of 9 genes was specific for MSC+MDA migration but the effects were weak. This suggested functional redundancy and indicated that targeting individual upregulated genes is insufficient to block the phenotype.

We then performed a small-molecule screen on the MSC+MDA migration phenotype, counter-screening hits on endothelial cell migration. Only one compound, RSL3, showed a large and selective inhibition of MSC+MDA migration. RSL3 was selectively toxic to MSCs with no effect on MDA cells. The target of RSL3 was recently identified to be glutathione peroxidase 4 (GPX4) that metabolizes lipid peroxides. To identify lipid mediators of RSL3 toxicity, we profiled lipid levels in RSL3-treated MSC+MDA co-cultures and found that the top changes were lipoxygenase products of arachidonic acid. Pre-treatment with zileuton or PD146176, that inhibit 5- and 15-lipoxygenase respectively, abrogated RSL3's toxic effects on MSCs. We found that MSCs contain >10-fold higher lipoxygenase products than MDA cells, highlighting an inflammatory role of MSCs. Fibroblasts from different tissues (lungs, spleen, breast, skin) were similarly sensitive to RSL3, identifying a previously unrecognized vulnerability in these mesenchymal cells.

Since RSL3 is a tool compound with poor plasma stability, we used GPX4 shRNA knockdown (KD) to determine in vivo effects of depleting MSCs in MSC+MDA xenografts. Unexpectedly, we found accelerated tumor growth in GPX4 KD tumors compared with controls. However, this is consistent with previous reports showing that mesenchymal cells restrain tumor growth and can have both pro- and anti-tumor effects. Histological examination revealed increased myeloid cell infiltration into GPX4 KD tumors reflecting increased inflammation mediated by lipoxygenase products like leukotriene B4.

In summary, phenotypic screening has identified GPX4 inhibition as a novel approach to target mesenchymal cells. This approach may be particularly effective at recruiting immune/ inflammatory cells into tumors with the exciting possibility of synergizing with cancer immunotherapy.

Citation Format: Shrikanta Chattopadhyay, Cherrie Huang, Ninib Baryawno, Nicolas Severe, Vasanthi Viswanathan, Zarko Boskovic, Siddhartha Mukherjee, Jeff Gentry, Ben Wittner, Sridhar Ramaswamy, Alykhan Shamji, David Scadden, Stuart Schreiber. Targeting mesenchymal cells in the tumor stroma by GPX4 inhibition. [abstract]. In: Proceedings of the AACR Special Conference: Function of Tumor Microenvironment in Cancer Progression; 2016 Jan 7–10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2016;76(15 Suppl):Abstract nr B11.

Abstract 2656: Loss of SIRT6 reactivates the oncofetal RNA-binding protein Lin28b to drive pancreatic cancer

Chromatin remodeling proteins are frequently dysregulated in human cancer, however little is known about how they control tumorigenesis. The NAD+-dependent histone deacetylase Sirtuin 6 (SIRT6) is a nutrient sensor that reprograms the epigenome in response to nutrient stress and shows reduced expression in human pancreatic ductal adenocarcinoma (PDAC) relative to normal tissue. Thus, we aimed to determine the role of SIRT6 in PDAC and to determine how the loss of this chromatin modifier could influence PDAC formation, progression and metastasis.

Here, we uncover an epigenetic program mediated by SIRT6 that is critical for suppression of PDAC. We demonstrate that SIRT6 inactivation cooperates with oncogenic KRAS to greatly accelerate the development of lethal pancreatic tumors regardless of p53 status. In addition to developing PDAC and high-grade pancreatic intraepithelial neoplasia (PanIN) at an earlier age, SIRT6-deficient tumors had a greater propensity to metastasize to the lung, compared to their SIRT6 wild-type (WT) counterparts. Through genome-wide profiling of SIRT6 WT and knock-out (KO) PDAC cell lines, we identified the oncofetal RNA-binding protein Lin28b as a target of Sirt6-mediated transcriptional repression. Loss of SIRT6 in both murine and human PDAC cell lines resulted in hyperacetylation of histone 3 lysine 9 and lysine 56 at the Lin28b promoter, MYC recruitment, and pronounced induction of Lin28b expression. Knocking down Lin28b resulted in potent suppression of cell proliferation and tumor sphere formation in Sirt6null/low murine and human cell lines, while murine and human Sirt6WT/high PDAC lines were completely insensitive to the same treatment. Functionally, Lin28b binds to and induces the degradation of the let-7 family of microRNA, thus resulting in increased expression of let-7 target genes such as the Igf2bps, which have been correlated with increased agressiveness and metastasis in pancreatic tumors. Furthermore, Gene Set Enrichment Analysis (GSEA) comparing PDAC tumors and cell lines with high versus low expression of LIN28B revealed that LIN28Bhigh tumors were strongly enriched for the expression of let-7 targets.

Thus, our critical findings define SIRT6 as a tumor suppressor in PDAC, working as a transcriptional repressor of the oncofetal RNA-binding protein Lin28b. We demonstrate that loss of Sirt6 creates a permissive hyperacetylated chromatin state, allowing for MYC-dependent transactivation of Lin28b. This work not only provides new insights into the epigenetic mechanisms governing the reactivation of oncofetal proteins in cancer but also demonstrates that this pathway is well-conserved in human PDAC, where reduced SIRT6 expression defines a specific subset of tumors which may benefit from novel therapeutic approaches aimed at targeting components of the LIN28B/let-7 pathway.

Citation Format: Sita Kugel, Carlos Sebastián, Julien Fitamant, Kenneth N. Ross, Supriya K. Saha, Alon Goren, Sridhar Ramaswamy, Vikram Deshpande, Nabeel Bardeesy, Raul Mostoslavsky. Loss of SIRT6 reactivates the oncofetal RNA-binding protein Lin28b to drive pancreatic cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2656.

Abstract 2679: Induction of β-globin protects circulating tumor cells from oxidative stress during dissemination

Identification of candidate metastasis genes has traditionally resulted from comparison of primary and metastatic tumor specimens. However, Circulating Tumor Cells (CTCs) contain metastatic precursors that are present transiently in the bloodstream and their analysis may reveal additional pathways that are induced for a limited time, as they invade and survive within the vasculature. By comparing transcriptome profiles of CTCs from breast, prostate and lung cancers with their primary tumor of origin, we observed consistent and significant induction of the β-globin gene (HBB) within CTCs. In contrast, expression of α-globin, its binding partner within hematopoietic cells, is not coordinately upregulated. The tumor-specific origin of HBB was further confirmed by analysis of human xenografts-derived CTCs in mice, where human-specific HBB polymorphisms are readily distinguishable in the murine background. In cultured cancer cells, we further demonstrate that induction of HBB is triggered by exposure to reactive oxygen species (ROS), and we identify KLF-family transcriptional regulators that mediate this effect. To investigate the function of aberrant β-globin expression within CTCs, we performed shRNA-mediated knockdown of HBB in breast CTC-derived cultures. Cells with depleted HBB expression displayed elevated intracellular ROS levels, increased sensitivity to hydrogen peroxide, and impaired metastatic potential in mouse models. Taken together, these observations suggest that β-globin, a component of functional hemoglobin in red blood cells, is deregulated in disseminated tumor cells, where it may function as a ROS scavenger, reducing oxidative stress and facilitating cancer metastasis.

Citation Format: Yu Zheng, David T. Miyamoto, Ben S. Wittner, James P. Sullivan, Nicola Aceto, Nicole Vincent Jordan, Min Yu, Nezihi Murat Karabacak, Valentine Comaills, Robert Morris, Rushil Desai, Niyati Desai, Erin Emmons, Richard J. Lee, Chin-Lee Wu, Lecia V. Sequist, Wilhelm Haas, David T. Ting, Mehmet Toner, Sridhar Ramaswamy, Shyamala Maheswaran, Daniel A. Haber. Induction of β-globin protects circulating tumor cells from oxidative stress during dissemination. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2679.

Developmental programming of polycystic ovary syndrome (PCOS): prenatal androgens establish pancreatic islet α/β cell ratio and subsequent insulin secretion

Exogenous androgenic steroids applied to pregnant sheep programmes a PCOS-like phenotype in female offspring. Via ultrasound guidance we applied steroids directly to ovine fetuses at d62 and d82 of gestation, and examined fetal (day 90 gestation) and postnatal (11 months old) pancreatic structure and function. Of three classes of steroid agonists applied (androgen - Testosterone propionate (TP), estrogen - Diethystilbesterol (DES) and glucocorticoid - Dexamethasone (DEX)), only androgens (TP) caused altered pancreatic development. Beta cell numbers were significantly elevated in prenatally androgenised female fetuses (P = 0.03) (to approximately the higher numbers found in male fetuses), whereas alpha cell counts were unaffected, precipitating decreased alpha:beta cell ratios in the developing fetal pancreas (P = 0.001), sustained into adolescence (P = 0.0004). In adolescence basal insulin secretion was significantly higher in female offspring from androgen-excess pregnancies (P = 0.045), and an exaggerated, hyperinsulinaemic response to glucose challenge (P = 0.0007) observed, whereas prenatal DES or DEX treatment had no effects upon insulin secretion. Postnatal insulin secretion correlated with beta cell numbers (P = 0.03). We conclude that the pancreas is a primary locus of androgenic stimulation during development, giving rise to postnatal offspring whose pancreas secreted excess insulin due to excess beta cells in the presence of a normal number of alpha cells.

SIRT6 Suppresses Pancreatic Cancer through Control of Lin28b

Chromatin remodeling proteins are frequently dysregulated in human cancer, yet little is known about how they control tumorigenesis. Here, we uncover an epigenetic program mediated by the NAD(+)-dependent histone deacetylase Sirtuin 6 (SIRT6) that is critical for suppression of pancreatic ductal adenocarcinoma (PDAC), one of the most lethal malignancies. SIRT6 inactivation accelerates PDAC progression and metastasis via upregulation of Lin28b, a negative regulator of the let-7 microRNA. SIRT6 loss results in histone hyperacetylation at the Lin28b promoter, Myc recruitment, and pronounced induction of Lin28b and downstream let-7 target genes, HMGA2, IGF2BP1, and IGF2BP3. This epigenetic program defines a distinct subset with a poor prognosis, representing 30%-40% of human PDAC, characterized by reduced SIRT6 expression and an exquisite dependence on Lin28b for tumor growth. Thus, we identify SIRT6 as an important PDAC tumor suppressor and uncover the Lin28b pathway as a potential therapeutic target in a molecularly defined PDAC subset. PAPERCLIP.

Artificial sweeteners as a sugar substitute: Are they really safe?

Nonnutritive sweeteners (NNS) have become an important part of everyday life and are increasingly used nowadays in a variety of dietary and medicinal products. They provide fewer calories and far more intense sweetness than sugar-containing products and are used by a plethora of population subsets for varying objectives. Six of these agents (aspartame, saccharine, sucralose, neotame, acesulfame-K, and stevia) have previously received a generally recognized as safe status from the United States Food and Drug Administration, and two more (Swingle fruit extract and advantame) have been added in the recent years to this ever growing list. They are claimed to promote weight loss and deemed safe for consumption by diabetics; however, there is inconclusive evidence to support most of their uses and some recent studies even hint that these earlier established benefits regarding NNS use might not be true. There is a lack of properly designed randomized controlled studies to assess their efficacy in different populations, whereas observational studies often remain confounded due to reverse causality and often yield opposite findings. Pregnant and lactating women, children, diabetics, migraine, and epilepsy patients represent the susceptible population to the adverse effects of NNS-containing products and should use these products with utmost caution. The overall use of NNS remains controversial, and consumers should be amply informed about the potential risks of using them, based on current evidence-based dietary guidelines.

ID: 42: FACTORS ASSOCIATED WITH TREATMENT SEEKING IN VETERANS WITH LATE-ONSET PTSD

Background and Significance

Post-traumatic stress disorder (PTSD) is a chronic anxiety disorder with a lifetime prevalence of 6.8% in the general population and up to 30% among Vietnam War veterans. While presentation of PTSD is typically acute, delayed onset of PTSD (i.e., 6 months post trauma) is considered to be relatively rare. There is anecdotal evidence supporting reactivation of trauma memories and symptoms of PTSD among aging veterans who previously had no symptoms or treatment for PTSD. Clinically this type of presentation appears to be different from those presenting with symptoms earlier in life. It is unclear what might precipitate or reactivate these symptoms many years following trauma exposure. The Veterans Health Administration has a growing population of aging veterans exposed to combat during their military service. Current treatment guidelines provide limited guidance towards the management of late-onset PTSD. The prevalence of PTSD among OEF/OIF veterans is very high and as this cohort ages, we need to be better equipped to manage clinical changes seen across the lifespan. Understanding the clinical phenomenology, risk factors and potential biomarkers of this condition could pave the way for better screening methods and treatment interventions for clinicians.

Objective

To identify the characteristics and risk factors among late-life post-traumatic stress disorder (PTSD) treatment seekers.

Methods

The study is a retrospective chart review of patients diagnosed with PTSD after the age of 55 and a comparison group of veterans diagnosed with PTSD prior to the age of 55 in the VA Nebraska–Western Iowa Health Care System.

Results

Primary variables for analysis included demographic factors, military history, age at treatment seeking, PTSD Checklist (PCL) scores, PTSD treatment, recent stressors/reasons for seeking treatment (e.g., recent life changes, another trauma or reminder), and comorbid medical conditions.

Conclusions

A number of explanations have been proposed for the development of late-onset PTSD, including chronic inflammation and cumulative stress. We are conducting a separate study to determine whether plasma concentration of the inflammatory marker C-reactive protein (CRP) might help in predicting late-onset PTSD. Further studies are needed to evaluate the contributions of other factors (e.g., physical or cognitive decline, sleep disturbances, other traumas) to late-onset PTSD.

Abstract PD3-04: PAK-1 amplified breast cancer cell lines are preferentially sensitive to PAK inhibition with G-5555

The small GTP-binding proteins Rac1 and Cdc42 stimulate activity of the serine/threonine kinase p21-activated kinase-1 (PAK-1) to drive growth factor signaling networks and Ras-driven tumorigenesis. Genomic amplification and over-expression of PAK1 are prevalent in luminal breast cancer and correlate with poor clinical outcome. Here we use a novel and selective small molecule inhibitor, G-5555, of the group I PAKs (PAK1, 2, and 3) to evaluate the importance of PAK1 in promoting growth of PAK1 amplified breast cancer cells. Cell lines with amplification of PAK1 were found to be more sensitive to PAK1 inhibition than non-amplified cell lines. Additionally, reverse phase protein array (RPPA) was used to assess the effects of PAK1 inhibition on a wide range of signaling pathways in both amplified and non-amplified cell lines. Reduced levels of phosphorylation of MEK S298 was observed in all cell lines exposed to G-5555 irrespective of amplification status, consistent with PAK1 inhibition in these cell lines. However, modulation of this downstream PAK1 substrate did not correlate with inhibition of cell proliferation or induction of cell death. Cell lines that showed inhibition of proliferation in response to G-5555 also showed enhanced levels of cell death along with apoptosis. Moreover, G-5555 reduced tumor growth in the PAK1 amplified MDA-MB-175 xenograft tumor model. Finally, we compared the in vitro activity of G-5555 with palbociclib, a recently approved inhibitor of the cyclin-dependent kinases CDK4 and CDK6, in PAK1 amplified luminal breast cancer cell lines. Our data supports PAK1 as an attractive target in PAK1 amplified cells and tumors and suggests that inhibiting PAK1 rather than CDK4/6 in this context may be a more attractive therapeutic strategy.

Citation Format: Ong CC, Blackwood E, Jakubiak D, Daemen A, Ramaswamy S, Heise C, Schmidt M, Sanders L, Wilson TR, Huw L, Ndubaku C, Rudolph J, Hoeflich KP, Friedman L, O'Brien T. PAK-1 amplified breast cancer cell lines are preferentially sensitive to PAK inhibition with G-5555. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr PD3-04.

Mechanistic implications from structures of yeast alcohol dehydrogenase complexed with coenzyme and an alcohol

Yeast alcohol dehydrogenase I is a homotetramer of subunits with 347 amino acid residues, catalyzing the oxidation of alcohols using NAD(+) as coenzyme. A new X-ray structure was determined at 3.0 Å where both subunits of an asymmetric dimer bind coenzyme and trifluoroethanol. The tetramer is a pair of back-to-back dimers. Subunit A has a closed conformation and can represent a Michaelis complex with an appropriate geometry for hydride transfer between coenzyme and alcohol, with the oxygen of 2,2,2-trifluoroethanol ligated at 2.1 Å to the catalytic zinc in the classical tetrahedral coordination with Cys-43, Cys-153, and His-66. Subunit B has an open conformation, and the coenzyme interacts with amino acid residues from the coenzyme binding domain, but not with residues from the catalytic domain. Coenzyme appears to bind to and dissociate from the open conformation. The catalytic zinc in subunit B has an alternative, inverted coordination with Cys-43, Cys-153, His-66 and the carboxylate of Glu-67, while the oxygen of trifluoroethanol is 3.5 Å from the zinc. Subunit B may represent an intermediate in the mechanism after coenzyme and alcohol bind and before the conformation changes to the closed form and the alcohol oxygen binds to the zinc and displaces Glu-67.

Abstract IA09: Single cell RNA-sequencing of circulating tumor cells

Isolation of circulating tumor cells (CTCs) in the blood allows noninvasive tumor sampling from patients with cancer. Molecular analysis of single CTCs may uncover heterogeneous cellular pathways that underlie disease progression and resistance to therapy. Using a microfluidic device, we isolated individual CTCs from patients with metastatic prostate cancer. Single candidate CTCs were micromanipulated after cell surface staining for epithelial (EpCAM) and mesenchymal (CDH11) markers, and then subjected to single cell RNA-sequencing. Digital gene expression profiles of lineage-confirmed CTCs were compared with each other, with primary prostate tumors, and with annotated markers of cellular signaling pathways. Single prostate CTCs displayed considerable heterogeneity in transcriptional profiles, but clustered according to patient of origin, indicating higher diversity in CTCs across different individuals (mean correlation 0.10 for CTCs within patients vs. 0.0014 for CTCs across patients, P=2.0x10E-11). Compared to primary tumors, CTCs were significantly enriched in 37 molecular pathways (FDR <0.1), with the majority implicated in growth factor, cell adhesion, and hormone signaling. Gene mutations and alternative splice variants of the Androgen Receptor (AR) gene were rare in primary prostate tumors and CTCs from untreated patients, but were prevalent in patients with castration-resistant prostate cancer. Distinct AR variants, including AR-V7, were present within different cells of individual patients, as well as within individual CTCs. Together, single cell molecular analysis of human CTCs points to multiple mechanisms of drug resistance in advanced prostate cancer, and suggests the role of heterogeneous signaling pathways that cooperate with co-existing abnormalities in AR in mediating disease progression.

Citation Format: David T. Miyamoto, Yu Zheng, Ben S. Wittner, Richard J. Lee, Huili Zhu, Katherine T. Broderick, Rushil Desai, Brian W. Brannigan, Kshitij S. Arora, Douglas M. Dahl, Lecia V. Sequist, Matthew R. Smith, Ravi Kapur, Chin-Lee Wu, Toshi Shioda, Sridhar Ramaswamy, David T. Ting, Mehmet Toner, Shyamala Maheswaran, Daniel A. Haber. Single cell RNA-sequencing of circulating tumor cells. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Integrating Clinical Genomics and Cancer Therapy; Jun 13-16, 2015; Salt Lake City, UT. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(1_Suppl):Abstract nr IA09.

Pericentromeric satellite repeat expansions through RNA-derived DNA intermediates in cancer

Aberrant transcription of the pericentromeric human satellite II (HSATII) repeat is present in a wide variety of epithelial cancers. In deriving experimental systems to study its deregulation, we observed that HSATII expression is induced in colon cancer cells cultured as xenografts or under nonadherent conditions in vitro, but it is rapidly lost in standard 2D cultures. Unexpectedly, physiological induction of endogenous HSATII RNA, as well as introduction of synthetic HSATII transcripts, generated cDNA intermediates in the form of DNA/RNA hybrids. Single molecule sequencing of tumor xenografts showed that HSATII RNA-derived DNA (rdDNA) molecules are stably incorporated within pericentromeric loci. Suppression of RT activity using small molecule inhibitors reduced HSATII copy gain. Analysis of whole-genome sequencing data revealed that HSATII copy number gain is a common feature in primary human colon tumors and is associated with a lower overall survival. Together, our observations suggest that cancer-associated derepression of specific repetitive sequences can promote their RNA-driven genomic expansion, with potential implications on pericentromeric architecture.

AKT Inhibition Promotes Nonautonomous Cancer Cell Survival

Small molecule inhibitors of AKT (v-akt murine thymoma viral oncogene homolog) signaling are being evaluated in patients with various cancer types, but have so far proven therapeutically disappointing for reasons that remain unclear. Here, we treat cancer cells with subtherapeutic doses of Akti-1/2, an allosteric small molecule AKT inhibitor, in order to experimentally model pharmacologic inhibition of AKT signaling in vitro. We then apply a combined RNA, protein, and metabolite profiling approach to develop an integrated, multiscale, molecular snapshot of this "AKT(low)" cancer cell state. We find that AKT-inhibited cancer cells suppress thousands of mRNA transcripts, and proteins related to the cell cycle, ribosome, and protein translation. Surprisingly, however, these AKT-inhibited cells simultaneously upregulate a host of other proteins and metabolites posttranscriptionally, reflecting activation of their endo-vesiculo-membrane system, secretion of inflammatory proteins, and elaboration of extracellular microvesicles. Importantly, these microvesicles enable rapidly proliferating cancer cells of various types to better withstand different stress conditions, including serum deprivation, hypoxia, or cytotoxic chemotherapy in vitro and xenografting in vivo. These findings suggest a model whereby cancer cells experiencing a partial inhibition of AKT signaling may actually promote the survival of neighbors through non-cell autonomous communication.

First International Workshopson Provocative Questions (PQ) in Cancer Research, October-November 2014, New Delhi, Bengaluru, and Thiruvananthapuram, India

In 2011, the National Cancer Institute (NCI, USA) introduced the Provocative Questions (PQ) Initiative, a new approach allowing active researchers to define major unsolved or neglected problems in oncology unaddressed by existing funding. Last year, the U.S. NCI teamed up with the Indian Department of Biotechnology (DBT) to pilot the PQ approach in three cities in India. Workshop outcomes includedthe generation of fundable "PQs" (perplexing questions understudied by the international scientific community), as well as the identification of several non-PQ projects and research-related issues of importance to DBT and other Indian funding groups. The workshops clearly indicated the need to expand beyond crafting "PQs" when considering the best areas for research funding in international settings. Nonetheless, the first set of PQ workshops provided a forum to discuss key issues regarding cancer research in India, including the paucity of cancer research funding, and the lack of relevant human resource training and technology sharing platforms. Continued open debate between researchers, funders and policymakers will be essential to effectively strengthen the cancer research portfolio in India.

RNA-Seq of single prostate CTCs implicates noncanonical Wnt signaling in antiandrogen resistance

Prostate cancer is initially responsive to androgen deprivation, but the effectiveness of androgen receptor (AR) inhibitors in recurrent disease is variable. Biopsy of bone metastases is challenging; hence, sampling circulating tumor cells (CTCs) may reveal drug-resistance mechanisms. We established single-cell RNA-sequencing (RNA-Seq) profiles of 77 intact CTCs isolated from 13 patients (mean six CTCs per patient), by using microfluidic enrichment. Single CTCs from each individual display considerable heterogeneity, including expression of AR gene mutations and splicing variants. Retrospective analysis of CTCs from patients progressing under treatment with an AR inhibitor, compared with untreated cases, indicates activation of noncanonical Wnt signaling (P = 0.0064). Ectopic expression of Wnt5a in prostate cancer cells attenuates the antiproliferative effect of AR inhibition, whereas its suppression in drug-resistant cells restores partial sensitivity, a correlation also evident in an established mouse model. Thus, single-cell analysis of prostate CTCs reveals heterogeneity in signaling pathways that could contribute to treatment failure.

Landscape of Targeted Anti-Cancer Drug Synergies in Melanoma Identifies a Novel BRAF-VEGFR/PDGFR Combination Treatment

A newer generation of anti-cancer drugs targeting underlying somatic genetic driver events have resulted in high single-agent or single-pathway response rates in selected patients, but few patients achieve complete responses and a sizeable fraction of patients relapse within a year. Thus, there is a pressing need for identification of combinations of targeted agents which induce more complete responses and prevent disease progression. We describe the results of a combination screen of an unprecedented scale in mammalian cells performed using a collection of targeted, clinically tractable agents across a large panel of melanoma cell lines. We find that even the most synergistic drug pairs are effective only in a discrete number of cell lines, underlying a strong context dependency for synergy, with strong, widespread synergies often corresponding to non-specific or off-target drug effects such as multidrug resistance protein 1 (MDR1) transporter inhibition. We identified drugs sensitizing cell lines that are BRAF^V600E mutant but intrinsically resistant to BRAF inhibitor PLX4720, including the vascular endothelial growth factor receptor/kinase insert domain receptor (VEGFR/KDR) and platelet derived growth factor receptor (PDGFR) family inhibitor cediranib. The combination of cediranib and PLX4720 induced apoptosis in vitro and tumor regression in animal models. This synergistic interaction is likely due to engagement of multiple receptor tyrosine kinases (RTKs), demonstrating the potential of drug- rather than gene-specific combination discovery approaches. Patients with elevated biopsy KDR expression showed decreased progression free survival in trials of mitogen-activated protein kinase (MAPK) kinase pathway inhibitors. Thus, high-throughput unbiased screening of targeted drug combinations, with appropriate library selection and mechanistic follow-up, can yield clinically-actionable drug combinations.

SETD1A modulates cell cycle progression through a miRNA network that regulates p53 target genes

Expression of the p53-inducible antiproliferative gene BTG2 is suppressed in many cancers in the absence of inactivating gene mutations, suggesting alternative mechanisms of silencing. Using a shRNA screen targeting 43 histone lysine methyltransferases (KMTs), we show that SETD1A suppresses BTG2 expression through its induction of several BTG2-targeting miRNAs. This indirect but highly specific mechanism, by which a chromatin regulator that mediates transcriptional activating marks can lead to the downregulation of a critical effector gene, is shared with multiple genes in the p53 pathway. Through such miRNA-dependent effects, SETD1A regulates cell cycle progression in vitro and modulates tumorigenesis in mouse xenograft models. Together, these observations help explain the remarkably specific genetic consequences associated with alterations in generic chromatin modulators in cancer.

The p53-inducible antiproliferative gene BTG2 is suppressed in many cancers, in the absence of inactivating gene mutations. Here the authors show that the histone lysine methyltransferase SETD1A suppresses the expression of several p53 target genes including BTG2 by inducing a network of microRNAs.