Genetically Engineered Mouse Research Articles

The KRASG12C Inhibitor MRTX849 Reconditions the Tumor Immune Microenvironment and Sensitizes Tumors to Checkpoint Inhibitor Therapy.

KRASG12C inhibitors, including MRTX849, are promising treatment options for KRAS-mutant non-small cell lung cancer (NSCLC). PD-1 inhibitors are approved in NSCLC; however, strategies to enhance checkpoint inhibitor therapy (CIT) are needed. KRASG12C mutations are smoking-associated transversion mutations associated with high tumor mutation burden, PD-L1 positivity, and an immunosuppressive tumor microenvironment. To evaluate the potential of MRTX849 to augment CIT, its impact on immune signaling and response to CIT was evaluated. In human tumor xenograft models, MRTX849 increased MHC class I protein expression and decreased RNA and/or plasma protein levels of immunosuppressive factors. In a KrasG12C -mutant CT26 syngeneic mouse model, MRTX849 decreased intratumoral myeloid-derived suppressor cells and increased M1-polarized macrophages, dendritic cells, CD4+, and CD8+ T cells. Similar results were observed in lung KrasG12C -mutant syngeneic and a genetically engineered mouse (GEM) model. In the CT26 KrasG12C model, MRTX849 demonstrated marked tumor regression when tumors were established in immune-competent BALB/c mice; however, the effect was diminished when tumors were grown in T-cell-deficient nu/nu mice. Tumors progressed following anti-PD-1 or MRTX849 single-agent treatment in immune-competent mice; however, combination treatment demonstrated durable, complete responses (CRs). Tumors did not reestablish in the same mice that exhibited durable CRs when rechallenged with tumor cell inoculum, demonstrating these mice developed adaptive antitumor immunity. In a GEM model, treatment with MRTX849 plus anti-PD-1 led to increased progression-free survival compared with either single agent alone. These data demonstrate KRAS inhibition reverses an immunosuppressive tumor microenvironment and sensitizes tumors to CIT through multiple mechanisms.

Abstract IA012: Engineering precision cancer immunotherapy

Abstract The treatment of cancer has been revolutionized by the development of immune-based therapies designed to boost the number and function of cytotoxic T cells that kill tumor cells. However, it is already apparent that this strategy alone will not benefit all patients, as the majority of cancers generate a highly immunosuppressive tumor microenvironment (TME) that shuts down even the most potent T cells. Furthermore, as a consequence of ramping up the immune system, these immune stimulating drugs instigate significant autoimmune toxicity. We aim to develop a radical new strategy to circumvent both challenges by identifying immunosuppressive regulatory mechanisms unique to cancerous tissues, thereby uncovering pathways active only in tumors that can be targeted to generate precision cancer immunotherapies that preserve immune tolerance in healthy tissues. The focus will be on Regulatory T cells (Tregs), a subset of immunosuppressive CD4+T cells that lie at the fulcrum of immunity, directing cells of the immune system to go or stop. In cancer, Tregsinfiltrate tumors and dampen anti-tumor immune responses, but Tregs also play an essential role in preventing autoimmunity. This proposal will test the hypothesis that human cancers generate an immunosuppressive TME by enforcing a unique epigenetic program in intratumoral Tregsthat, if identified, could provide new targets to selectively modulate tumor-infiltrating Tregsand limit autoimmunity. To do this, the research will employ genetically engineered mouse (GEM) cancers, an accurate and flexible platform to test the full complement of factors that make up the TME of human cancers, combined with sophisticated genetic tools to track Treg entry and activity in the TME. Our investigations thus far have led us to the investigation of Ezh2 activity, a H3K27 methyltransferase, in defining Treg behavior selectively in the TME. Citation Format: Michel DuPage. Engineering precision cancer immunotherapy [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr IA012.

Preclinical Models of Pancreatic Ductal Adenocarcinoma and Their Utility in Immunotherapy Studies

Simple SummaryImmune checkpoint blockade has provided durable clinical responses in a number of human malignancies, but not in patients with pancreatic cancer. Efforts to understand mechanisms of resistance and increase efficacy of immune checkpoint blockade in pancreatic cancer require the use of appropriate preclinical models in the laboratory. Here, we discuss the benefits, caveats, and potentials for improvement of the most commonly used models, including murine-based and patient-derived models.The advent of immunotherapy has transformed the treatment landscape for several human malignancies. Antibodies against immune checkpoints, such as anti-PD-1/PD-L1 and anti-CTLA-4, demonstrate durable clinical benefits in several cancer types. However, checkpoint blockade has failed to elicit effective anti-tumor responses in pancreatic ductal adenocarcinoma (PDAC), which remains one of the most lethal malignancies with a dismal prognosis. As a result, there are significant efforts to identify novel immune-based combination regimens for PDAC, which are typically first tested in preclinical models. Here, we discuss the utility and limitations of syngeneic and genetically-engineered mouse models that are currently available for testing immunotherapy regimens. We also discuss patient-derived xenograft mouse models, human PDAC organoids, and ex vivo slice cultures of human PDAC tumors that can complement murine models for a more comprehensive approach to predict response and resistance to immunotherapy regimens.

Designing and generating a mouse model: frequently asked questions.

Genetically engineered mouse (GEM) models are commonly used in biomedical research. Generating GEMs involve complex set of experimental procedures requiring sophisticated equipment and highly skilled technical staff. Because of these reasons, most research institutes set up centralized core facilities where custom GEMs are created for research groups. Researchers, on the other hand, when they begin thinking about generating GEMs for their research, several questions arise in their minds. For example, what type of model(s) would be best useful for my research, how do I design them, what are the latest technologies and tools available for developing my model(s), and finally how to breed GEMs in my research. As there are several considerations and options in mouse designs, and as it is an expensive and time-consuming endeavor, careful planning upfront can ensure the highest chance of success. In this article, we provide brief answers to several frequently asked questions that arise when researchers begin thinking about generating mouse model(s) for their work.

The Genetic Basis of Reporter Mouse Strains.

Genetically engineered mouse (GEM) models have been revolutionizing the biomedical studies on deciphering the physiological roles of genes in vivo. In addition to deactivating a gene in mice, diverse strategies have been created to monitor gene expressions and molecular dynamics of specific proteins in vivo. Although gene targeting in mouse embryonic stem (ES) cells was essential for the precise engineering of the mouse genome over almost three decades, this process is a time-consuming, expensive, and laborious one. These days, new technologies that directly apply engineered endonucleases, such as zinc-finger nucleases (ZFNs), Transcription Activator-Like Effector (TALE) Nucleases (TALENs), and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system, into the mouse zygotes are enabling us to rapidly replace conventional gene targeting in mouse ES cells. In this chapter, we will describe the principles of reporter mouse strains and the recent advances in generating them using engineered endonucleases.

A genetic mouse model with postnatal Nf1 and p53 loss recapitulates the histology and transcriptome of human malignant peripheral nerve sheath tumor.

BackgroundMalignant peripheral nerve sheath tumors (MPNST) are aggressive sarcomas. Somatic inactivation of NF1 and cooperating tumor suppressors, including CDKN2A/B, PRC2, and p53, is found in most MPNST. Inactivation of LATS1/2 of the Hippo pathway was recently shown to cause tumors resembling MPNST histologically, although Hippo pathway mutations are rarely found in MPNST. Because existing genetically engineered mouse (GEM) models of MPNST do not recapitulate some of the key genetic features of human MPNST, we aimed to establish a GEM-MPNST model that recapitulated the human disease genetically, histologically, and molecularly.MethodsWe combined 2 genetically modified alleles, an Nf1;Trp53 cis-conditional allele and an inducible Plp-CreER allele (NP-Plp), to model the somatic, possibly postnatal, mutational events in human MPNST. We also generated conditional Lats1;Lats2 knockout mice. We performed histopathologic analyses of mouse MPNST models and transcriptomic comparison of mouse models and human nerve sheath tumors.ResultsPostnatal Nf1;Trp53 cis-deletion resulted in GEM-MPNST that were histologically more similar to human MPNST than the widely used germline Nf1;Trp53 cis-heterozygous (NPcis) model and showed partial loss of H3K27me3. At the transcriptome level, Nf1;p53-driven GEM-MPNST were distinct from Lats-driven GEM-MPNST and resembled human MPNST more closely than do Lats-driven tumors.ConclusionsThe NP-Plp model recapitulates human MPNST genetically, histologically, and molecularly.

MPA/DMBA-driven mammary carcinomas.

The polycyclic aromatic hydrocarbon 7,12-dimethylbenz[a]anthracene (DMBA, D) administered per os to wild-type female mice bearing slow-release medroxyprogesterone (MPA, M) pellets s.c. drives the formation of mammary carcinomas that recapitulate numerous immunobiological features of human luminal B breast cancer. In particular, M/D-driven mammary carcinomas established in immunocompetent C57BL/6 female mice (1) express hormone receptors, (2) emerge by evading natural immunosurveillance and hence display a scarce immune infiltrate largely polarized toward immunosuppression, (3) exhibit exquisite sensitivity to CDK4/CDK6 inhibitors, and (4) are largely resistant to immunotherapy with immune checkpoint blockers targeting PD-1. Thus, M/D-driven mammary carcinomas evolving in immunocompetent female mice stand out as a privileged preclinical platform for the study of luminal B breast cancer. Here, we provide a detailed protocol for the establishment of M/D-driven mammary carcinomas in wild-type C57BL/6 female mice. This protocol can be easily adapted to generate M/D-driven mammary carcinomas in female mice with most genetic backgrounds (including genetically-engineered mice).

Molecular Characterization of Constipation Disease as Novel Phenotypes in CRISPR-Cas9-Generated Leptin Knockout Mice with Obesity.

(1) Background: We characterized a novel animal model with obesity-induced constipation because constipation is rarely known in genetically engineered mice (GEM); (2) Methods: The changes in the constipation parameters and mechanisms were analyzed in CRISPR-Cas9-mediated leptin (Lep) knockout (KO) mice from eight to 24 weeks; (3) Results: Significant constipation phenotypes were observed in the Lep KO mice since 16 weeks old. These mice showed a significant decrease in the gastrointestinal motility, mucosal layer thickness and ability for mucin secretion as well as the abnormal ultrastructure of Lieberkühn crypts in the transverse colon. The density or function of the enteric neurons, intestinal Cajal cells (ICC), smooth muscle cells, and the concentration of gastrointestinal (GI) hormones for the GI motility were remarkably changed in Lep KO mice. The downstream signaling pathway of muscarinic acetylcholine receptors (mAChRs) were activated in Lep KO mice, while the expression of adipogenesis-regulating genes were alternatively reduced in the transverse colon of the same mice; (4) Conclusions: These results provide the first strong evidence that Lep KO mice can represent constipation successfully through dysregulation of the GI motility mediated by myenteric neurons, ICC, and smooth muscle cells in the transverse colon during an abnormal function of the lipid metabolism.

Methodology Aspects of Colony Maintain for a Murine Model of Amyotrophic Lateral Sclerosis (ALS) TDP-43 Proteinopathy.

Simple SummaryRodent models of central nervous system (CNS) disorders are widely used to explore pathology and molecular mechanisms of disease. These models are a valuable tool for understanding the genetic basis of amyotrophic lateral sclerosis (ALS) research, a CNS disease that affects motor neurons in the brain and spinal cord, causing loss of voluntary muscle control. There are two different types of ALS: Sporadic (sALS), accounts for 90 to 95 percent of all cases, which means the disease seems to occur at random with no clearly associated risk factors and no family history of the disease, and familial (fALS), which is inherited and accounts for 5 to 10 percent of all cases. One example of a gene mutation in fALS is the A315T mutation to the gene encoding TDP-43. The congenic Prp-TDP43A315T Tg model of ALS disease expresses full-length human TARDBP containing this mutation. The effects of the highly aggressive phenotype of this Tg model of human ALS, and its premature sudden death prior to full development of neurodegenerative symptoms, makes it essential to determine their reproductive pattern in order to guarantee proper colony maintenance, which is the main goal of this study.The use of genetically engineered mouse (GEMs) models provides an unprecedented opportunity to study the genetic basis of diseases and gene function, therefore it is paramount to determine reproductive parameters that guarantee proper colony maintenance. We studied the reproductive parameters of mice hemizygous for TDP-43A315T transgene, which are viable, fertile, and express a mutant human TAR DNA binding protein (hTDP-43) cDNA harboring an amino acid substitution associated with familial amyotrophic lateral sclerosis (fALS). TDP43A315T mice were backcrossed to a C57Bl6/J pure background for four consecutive generations. The Tg offspring genotype were then confirmed by PCR assays. Our statistical analysis indicated there were no differences in the sex and number of pups per offspring when hemizygous female and male TDP43A315T mice were backcrossed to C57Bl6/J mice. Interestingly, our results showed significant differences in the number of offspring expressing the transgene when hemizygous TDP43A315T male mice were used as breeders. Therefore, our findings suggest that male TDP43A315T mice transfer the transgene with a greater genetic strengths. Such is an important breeding consideration to ensure the principle of reduction in animal experimentation considering most basic research with models focuses on males and excludes female mice.

MBRS-46. CHARTING NEOPLASTIC AND IMMUNE CELL HETEROGENEITY IN HUMAN AND GEM MODELS OF MEDULLOBLASTOMA USING scRNAseq

We explored cellular heterogeneity in medulloblastoma using single-cell RNA sequencing (scRNAseq), immunohistochemistry and deconvolution of bulk transcriptomic data. Over 45,000 cells from 31 patients from all main subgroups of medulloblastoma (2 WNT, 10 SHH, 9 GP3, 11 GP4 and 1 GP3/4) were clustered using Harmony alignment to identify conserved subpopulations. Each subgroup contained subpopulations exhibiting mitotic, undifferentiated and neuronal differentiated transcript profiles, corroborating other recent medulloblastoma scRNAseq studies. The magnitude of our present study builds on the findings of existing studies, providing further characterization of conserved neoplastic subpopulations, including identification of a photoreceptor-differentiated subpopulation that was predominantly, but not exclusively, found in GP3 medulloblastoma. Deconvolution of MAGIC transcriptomic cohort data showed that neoplastic subpopulations are associated with major and minor subgroup subdivisions, for example, photoreceptor subpopulation cells are more abundant in GP3-alpha. In both GP3 and GP4, higher proportions of undifferentiated subpopulations is associated with shorter survival and conversely, differentiated subpopulation is associated with longer survival. This scRNAseq dataset also afforded unique insights into the immune landscape of medulloblastoma, and revealed an M2-polarized myeloid subpopulation that was restricted to SHH medulloblastoma. Additionally, we performed scRNAseq on 16,000 cells from genetically engineered mouse (GEM) models of GP3 and SHH medulloblastoma. These models showed a level of fidelity with corresponding human subgroup-specific neoplastic and immune subpopulations. Collectively, our findings advance our understanding of the neoplastic and immune landscape of the main medulloblastoma subgroups in both humans and GEM models.

Got Milk? Identifying and Characterizing Lactation Defects in Genetically-Engineered Mouse Models.

The ability to produce and expel milk is important for the health and survival of all mammals. Nevertheless, our understanding of the molecular events underlying the execution of this process remains incomplete. Whilst impaired mammary gland development and lactational competence remains the subject of focused investigations, defects in these events may also be an unintended consequence of genetic manipulation in rodent models. In this technical report, we outline established and emerging methods to characterize lactation phenotypes in genetically-engineered mouse models. We discuss important considerations of common models, optimized conditions for mating and the importance of litter size and standardization. Methods for quantifying milk production and quality, as well as protocols for wholemount preparation, immunohistochemistry and the preparation of RNA and protein lysates are provided. This review is intended to help guide researchers new to the field of mammary gland biology in the systematic analysis of lactation defects and in the preparation of samples for more focused mechanistic investigations.

Abstract PO-046: Dissecting the role of tumor-intrinsic Cxcl1 in mediating immune exclusion in Ras-p53 cooperative pancreatic cancer

Abstract INTRODUCTION: Ras-p53 cooperative mutations, which define >50% of human pancreatic ductal adenocarcinoma (PDAC), promote an immune-excluded tumor microenvironment. In Ras-p53 murine models that phenocopy human PDAC, overexpression of the myeloid chemo-attractant Cxcl1 is associated with T-cell exclusion. We sought to dissect the role of tumor-intrinsic Cxcl1 in mediating immune exclusion in Ras-p53 PDAC. METHODS: Single-cell immune deconvolution and Cxcl1 expression were queried in TCGA PDAC samples (n=150). Single-cell RNA sequencing (scRNAseq) data in KrasG12D/+;Trp53fl/+;Pdx1Cre (KPC WT) genetically engineered mice (GEM) were interrogated. CRISPR/Cas9-based genome editing was used to silence Cxcl1 in KPC tumor cells (KPC-Cxcl1KO). Multiparameter flow cytometry-based immunophenotyping of tumor-infiltrating and circulating myeloid/T-cell populations was performed in in-vivo orthotopic models utilizing KPCWT / KPC-Cxcl1KO cells. RESULTS: In human TCGA samples, Cxcl1 overexpression was strongly associated with expression of its cognate receptor CXCR2, reduced CD4+/CD8+ T-cells, and increased CD33+ myeloid-derived suppressor cells (MDSCs). In KPC GEMs, scRNAseq revealed that CXCR2 expression was exclusively present on the polymorphonuclear MDSC subset (PMN-MDSC). Exogenous Cxcl1 conditioning in murine bone marrow-derived cells induced PMN-MDSC polarization and upregulated CXCR2. Genetic ablation of Cxcl1 in KPC tumor cells disrupted transwell migration of splenocyte-enriched MDSC in-vitro; this effect was dependent on CXCR2 expression on MDSCs. There was a dramatic reduction in tumor weights and metastatic outgrowth (p<0.0001), as well as circulating PMN-MDSC and CXCR2+-MDSC populations in KPC-Cxcl1KO compared with KPCWT mice (p=0.0006). Significantly fewer PMN-MDSC and M2-like macrophages infiltrated KPC-Cxcl1KO tumors, which was accompanied by dramatically higher CD4+/CD8+ T-cell infiltration (p<0.0001). While cytolytic effector CD8+ T-cell populations (CD44+62L-CD107a+) remained unchanged, non-cytolytic “exhausted” PD-1+CD8+ T-cells (PD-1+CD107a-) increased in KPC-Cxcl1KO vs. KPCWT tumors, providing rationale for addition of checkpoint blockade to reverse T-cell exhaustion. CONCLUSION: Tumor-intrinsic Cxcl1 appears a major contributor to immune exclusion in Ras-p53 cooperative PDAC. Targeting Cxcl1 represents a promising strategy to overcome MDSC-mediated immunosuppression and improve susceptibility to checkpoint blockade in PDAC. Citation Format: Anna Bianchi, Siddharth Mehra, Daisy Dai, Iago de Castro Silva, Prateek Sharma, Antonio Colaprico, Austin R. Dosch, Samara Singh, Nagaraj S. Nagathihalli, Nipun B. Merchant, Jashodeep Datta. Dissecting the role of tumor-intrinsic Cxcl1 in mediating immune exclusion in Ras-p53 cooperative pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-046.

TSPO-targeted PET and Optical Probes for the Detection and Localization of Premalignant and Malignant Pancreatic Lesions.

Pancreatic cancer is among the most aggressive malignancies and is rarely discovered early. However, pancreatic "incidentalomas," particularly cysts, are frequently identified in asymptomatic patients through anatomic imaging for unrelated causes. Accurate determination of the malignant potential of cystic lesions could lead to life-saving surgery or spare patients with indolent disease undue risk. Current risk assessment of pancreatic cysts requires invasive sampling, with attendant morbidity and sampling errors. Here, we sought to identify imaging biomarkers of high-risk pancreatic cancer precursor lesions. Translocator protein (TSPO) expression, which is associated with cholesterol metabolism, was evaluated in premalignant and pancreatic cancer lesions from human and genetically engineered mouse (GEM) tissues. In vivo imaging was performed with [18F]V-1008, a TSPO-targeted PET agent, in two GEM models. For image-guided surgery (IGS), V-1520, a TSPO ligand for near-IR optical imaging based upon the V-1008 pharmacophore, was developed and evaluated. TSPO was highly expressed in human and murine pancreatic cancer. Notably, TSPO expression was associated with high-grade, premalignant intraductal papillary mucinous neoplasms (IPMNs) and pancreatic intraepithelial neoplasia (PanIN) lesions. In GEM models, [18F]V-1008 exhibited robust uptake in early pancreatic cancer, detectable by PET. Furthermore, V-1520 localized to premalignant pancreatic lesions and advanced tumors enabling real-time IGS. We anticipate that combined TSPO PET/IGS represents a translational approach for precision pancreatic cancer care through discrimination of high-risk indeterminate lesions and actionable surgery.

TMOD-31. NOVEL GENETIC CHARACTERIZATION OF MURINE MEDULLOBLASTOMA CELL LINES

Abstract Pre-clinical research in medulloblastoma (MB) is conducted either with human-derived cell lines or tumors arising in genetically engineered mice (GEM). Murine MB cell lines are not readily available. To begin in vitro immuno-oncology MB research, we identified murine MB cell lines from research laboratories to determine their fidelity to human cell lines and GEM models. Using PubMed, we identified murine MB cell lines of potential interest. Seven cell lines form two independent laboratories were identified. We characterized the cell lines’ behavior in culture and performed next generation sequencing (NGS) on four of them (two from each group) to determine the extent to which these cell lines resemble both human and GEM MB models. DNA and RNA were extracted from MB cell lines, quality and quantity assessed. Whole-exome sequencing was performed and post-capture libraries sequenced. The cell lines identified were derived from either FVB/N TP53 null mice carrying the N-MYC oncogene (GTML3 and GTML5) or C57Bl6/J TP53 null heterozygous ptch (-/+) mice (MM1 and MM3). GTML cells grow in suspension and form clusters. MM cells are adherent, growing in monolayers. Whole exome sequencing of the 4 lines revealed similarities between both GEM and human MB cell lines, demonstrating 25% of sixteen genes known to be affected in both GEM tumors and human MB cell lines. Furthermore, the four cell Iines shared five deleterious mutations previously not described in MB, suggesting they might play a role in MB tumorigenesis: Pde4d, Whrln, Olr156, RIKEN cDNA D630045J12, and Olfr159. These are the first data to describe these murine MB cell lines and demonstrates 1) they resemble both GEM tumors and human MB cell lines at the genome level, and 2) they offer the potential to identify heretofore unrecognized mechanisms in MB tumorigenesis.

Sildenafil Potentiates the Therapeutic Efficacy of Docetaxel in Advanced Prostate Cancer by Stimulating NO-cGMP Signaling.

Docetaxel plays an indispensable role in the management of advanced prostate cancer. However, more than half of patients do not respond to docetaxel, and those good responders frequently experience significant cumulative toxicity, which limits its dose duration and intensity. Hence, a second agent that could increase the initial efficacy of docetaxel and maintain tolerability at biologically effective doses may improve outcomes for patients. We determined phosphodiesterase 5 (PDE5) expression levels in human and genetically engineered mouse (GEM) prostate tissues and tumor-derived cell lines. Furthermore, we investigated the therapeutic benefits and underlying mechanism of PDE5 inhibitor sildenafil in combination with docetaxel using in vitro, Pten conditional knockout (cKO), derived tumoroid and xenograft prostate cancer models. PDE5 expression was higher in both human and mouse prostate tumors and cancer cell lines compared with normal tissues/cells. In GEM prostate-derived cell lines, PDE5 expression increased from normal prostate (wild-type) epithelial cells to androgen-dependent and castrated prostate-derived cell lines. The addition of physiologically achievable concentrations of sildenafil enhanced docetaxel-induced prostate cancer cell growth inhibition and apoptosis in vitro, reduced murine 3D tumoroid growth, and in vivo tumorigenicity as compared with docetaxel alone. Furthermore, sildenafil enhanced docetaxel-induced NO and cGMP levels thereby augmenting antitumor activity. Our results demonstrate that sildenafil's addition could sensitize docetaxel chemotherapy in prostate cancer cells at much lesser concentration than needed for inducing cell death. Thus, the combinatorial treatment of sildenafil and docetaxel may improve anticancer efficacy and reduce chemotherapy-induced side-effects among patients with advanced prostate cancer.

Tissue-Specific GHR Knockout Mice: An Updated Review.

Growth hormone (GH) signaling plays a key role in mediating growth, development, metabolism, and lifespan regulation. However, the mechanisms of longevity regulation at the cellular and molecular level are still not well-understood. An important area in the field of GH research is in the development of advanced transgenic systems for conditional expression of GH signaling in a cell type- or tissue-specific manner. There have been many recent studies conducted to examine the effects of tissue-specific GHR disruption. This review updates our previous discussions on this topic and summarizes recent data on the newly-made tissue-specific GHR-KO mice including intestinal epithelial cells, bone, hematopoietic stem cells, cardiac myocytes, and specific brain regions. The data from these new genetically-engineered mice have a significant impact on our understanding of the local GH signaling function.

Paradoxical Role of AT-rich Interactive Domain 1A in Restraining Pancreatic Carcinogenesis.

Simple SummaryGenes with deleterious mutations in tumors are widely accepted as tumor suppressors, since, loss of their normal expression often promotes tumor development. However, most tumors develop over a long period, with gradual accumulation of tumor-promoting events. Thus, to comprehend the role of individual genes in this evolving process of tumorigenesis, it is critical to investigate their role in both early precursors and established tumor cells. Despite recurrent mutations in ARID1A in genomes of human cancer, including pancreatic cancer, its role in tumorigenesis is not clear. We aim to understand the role of Arid1a in pancreatic cancer development and maintenance by investigating its role in both early pancreatic precursor cells and established pancreatic cancer cells. Besides novel understanding of context-dependent role of Arid1a in pancreatic cancer, this study will also enable development of therapeutic strategies for pancreatic cancers patients with ARID1A mutations, which is currently a critical unmet need in clinic.Background & Aims: ARID1A is postulated to be a tumor suppressor gene owing to loss-of-function mutations in human pancreatic ductal adenocarcinomas (PDAC). However, its role in pancreatic pathogenesis is not clear despite recent studies using genetically engineered mouse (GEM) models. We aimed at further understanding of its direct functional role in PDAC, using a combination of GEM model and PDAC cell lines. Methods: Pancreas-specific mutant Arid1a-driven GEM model (Ptf1a-Cre; KrasG12D; Arid1af/f or “KAC”) was generated by crossing Ptf1a-Cre; KrasG12D (“KC”) mice with Arid1af/f mice and characterized histologically with timed necropsies. Arid1a was also deleted using CRISPR-Cas9 system in established human and murine PDAC cell lines to study the immediate effects of Arid1a loss in isogenic models. Cell lines with or without Arid1a expression were developed from respective autochthonous PDAC GEM models, compared functionally using various culture assays, and subjected to RNA-sequencing for comparative gene expression analysis. DNA damage repair was analyzed in cultured cells using immunofluorescence and COMET assay. Results: Retention of Arid1a is critical for early progression of mutant Kras-driven pre-malignant lesions into PDAC, as evident by lower Ki-67 and higher apoptosis staining in “KAC” as compared to “KC” mice. Enforced deletion of Arid1a in established PDAC cell lines caused suppression of cellular growth and migration, accompanied by compromised DNA damage repair. Despite early development of relatively indolent cystic precursor lesions called intraductal papillary mucinous neoplasms (IPMNs), a subset of “KAC” mice developed aggressive PDAC in later ages. PDAC cells obtained from older autochthonous “KAC” mice revealed various compensatory (“escaper”) mechanisms to overcome the growth suppressive effects of Arid1a loss. Conclusions: Arid1a is an essential survival gene whose loss impairs cellular growth, and thus, its expression is critical during early stages of pancreatic tumorigenesis in mouse models. In tumors that arise in the setting of ARID1A loss, a multitude of “escaper” mechanisms drive progression.

Abstract 1620: MDM2 P1 and P2 promoters have different roles in tumorigenesis

Abstract The murine double minute 2 (MDM2) protein is an E3 ubiquitin ligase most notable for priming tumour-suppressor p53 for degradation. MDM2 contains two promoters - the constitutive P1 and the inducible P2. The p53-independent P1 promoter is responsible for basal levels of MDM2, whilst the p53-dependent P2 promoter is activated upon stress responses. Although the oncogenic functions of MDM2 are well understood, less is known about the individual roles of the promoters and their contribution to tumorigenesis and stress responses. To tease apart the roles of the promoters, we have used genetically engineered mouse (GEM) models null for the P1-promoter and mutated for the P2-promoter in the context of intestinal cancer, lymphoma and hepatocellular carcinoma (HCC). Here, we have shown that mice with heterozygous or homozygous loss of the P1-promoter in the APCMin/+ intestinal model have extended survival and reduced intestinal tumour burden in comparison to the P1- wild type (WT), APCMin/+ mice. Surprisingly, none of the P1-null APCMin/+ mice succumbed to intestinal tumours, but rather to thymic lymphomas. In the P1-null APCMin/+ lymphoma samples, we investigated the p53 functionality by examining the expression levels of p21, a p53 downstream target. Immunohistochemistry analysis clearly revealed that p53 is non-functional as p21 was not expressed, despite variations in p53 expression levels itself. This uncommon occurrence in the APCMin/+ model led us to investigate the role of the P1 and P2 promoters in the Eµ-Myc lymphoma model. Interestingly, in this model, heterozygous loss of the P1 promoter improves survival, while heterozygous loss of the P2 promoter accelerates lymphomagenesis. Our findings thus far indicate a differential role for the P1 and P2 MDM2 promoters in tumorigenesis. In addition to using GEM models, we are investigating the role of the MDM2 promoters in stress-induced HCC. These studies will allow for a better understanding of MDM2 functions, but also perhaps provide an alternative therapeutic strategy for cancers with aberrations in the p53-MDM2 signalling axis. Citation Format: Narisa Phinichkusolchit, Thomas Jamieson, Rachel Ridgway, Dimitris Athineos, Kerryanne Crawford, Nikolina Vlatkovic, Mark Boyd, Owen Sansom, Karen Blyth. MDM2 P1 and P2 promoters have different roles in tumorigenesis [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1620.

Abstract 1016: Inhibition of chemokine-induced myeloid cells potentiates the anti-PD-1 response in KRAS/LKB1 mutant non-small cell lung cancer

Abstract Dysregulated inflammation dictated by oncogene/tumor suppressor mutations can often lead to an immune suppressive tumor microenvironment (TME). Lung tumors with LKB1 mutations account for 30% of KRAS-mutant non-small cell lung cancer (NSCLC). These tumors are particularly aggressive and resistant to immunotherapy despite a high mutational burden (TMB). The mechanisms of this impaired immunogenicity remain obscure. Here, we report that LKB1 loss leads to a heightened secretion of the ELR(+) CXC chemokines, including CXCL1, CXCL2, CXCL3, CXCL5, and CXCL8, in premalignant and cancerous cells in vitro. Among multiple NSCLC cell lines, cancer cells with both KRAS and LKB1 mutations tend to express higher levels of these ELR(+) CXC chemokines. Consistently, ectopic expression of LKB1 in LKB1-null cancer cells decreases the chemokine production. In a genetically-engineered mouse model, an elevation of these chemokines is also observed in KrasG12D;Lkb1-/- (KL) tumors compared to their KrasG12D (K) or KrasG12D;Tp53-/- (KP) counterparts. Mechanistic studies reveal that the LKB1-MARK axis regulates these chemokines in the premalignant cells. Immune phenotyping of KL or KrasG12D;Tp53+/-;Lkb1-/- (KPL) tumors in vivo demonstrates significantly increased granulocytic myeloid-derived suppressor cells (G-MDSCs), which harbor high levels of reactive oxygen species (ROS). Utilizing a syngeneic murine lung cancer model with a high TMB, we demonstrate that KPL tumors have a minimal response to anti-PD-1 therapy compared to KP tumors. Therefore, we hypothesize that G-MDSCs may cause resistance to anti-PD-1 monotherapy in KPL tumors. We find that inhibition of the G-MDSCs either by depletion or by reduction of ROS can potentiate the anti-PD-1 response and subsequently eliminate the KPL tumors. Investigation of the TME reveals that G-MDSC depletion primes tumor-infiltrating lymphocytes (TILs) for activation: there is an increased percentage of TILs with high Ki67 and PD-1 expression and an increased percentage of antigen presenting cells (macrophages and dendritic cells) with high PD-L1 expression. Combination with anti-PD-1 therapy enhances the function of these TILs, evidenced by IFN-γ and TNF-α secretion. Re-challenge of these mice three months after the initial combination therapy leads to a rapid tumor rejection, suggesting a durable systemic anti-tumor immune response. In conclusion, we find that LKB1 deficiency leads to an increased ELR(+) CXC chemokine production and tumor infiltration of G-MDSCs. Inhibition of G-MDSC enhances the efficacy of anti-PD-1 blockade in LKB1-deficient tumors bearing a high TMB. Citation Format: Rui Li, Ramin Salehi-Rad, Milica Momcilovic, Stephanie Ong, Raymond Lim, Zi Ling Huang, Linh Tran, Zhe Jing, Manash Paul, Michael Teitell, John Minna, David Shackelford, Krysan Kostyantyn, Bin Liu, Steven Dubinett. Inhibition of chemokine-induced myeloid cells potentiates the anti-PD-1 response in KRAS/LKB1 mutant non-small cell lung cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1016.

Abstract 2569: Uncovering metabolic bottlenecks in KEAP1 mutant lung cancer

Abstract During tumorigenesis cancer cells continuously encountermetabolic bottlenecks as a result of accelerated growth, overall increased metabolic demand and increased oxidative stress due to the formation of reactive oxygen species. We use a combination of genetically-engineered mouse models, an accelerated CRISPR/Cas9-based experimentalplatformand biochemical approaches to identify metabolic liabilities that can be exploited using novel targeted therapies.Lung cancer, the leading cause of cancer-related deaths worldwide,is the most common cancer type to acquire mutually exclusive gain-of-function mutations in the anti-oxidant transcription factor NRF2or loss-of-function mutations in its negative regulator KEAP1. Loss of Keap1activates Nrf2, increases antioxidant production and dramatically accelerates KRAS-driven lung cancer. Our data are in line with mounting evidence demonstrating that antioxidants promote cancer progression. Our studies focus on elucidating the mechanisms underlying novel metabolic dependencies in KEAP1mutant tumors and explore the therapeutic potential of targeting metabolism in highly relevant pre-clinical mouse and human lung cancer models. We observe that the ability of KEAP1mutant tumors to divert their metabolism towards antioxidant production comes with a cost, generating multiple metabolic vulnerabilities, including a dependency on glutamine metabolism that can be therapeutically exploited using DRP-104, a novel broad acting glutamine antagonist which inhibits all glutamine-consuming reactions. In addition, we have extended our findings to demonstrate that cancers with high antioxidant capacity exhibit a general dependency on exogenous non-essential amino acids (NEAAs) that is driven by the Nrf2-dependent secretion of glutamate through system xc-(XCT), which limits intracellular glutamate pools that are required for NEAA synthesis. This dependency can be therapeutically targeted by dietary restriction or enzymatic depletion of individual NEAAs. Importantly, limiting endogenous glutamate levels by glutaminase inhibition can sensitize tumors without alterations in Keap1/Nrf2 pathway to dietary restriction of NEAAs. Our findings identify a metabolic strategy to therapeutically target cancers with genetic or pharmacologic activation of the Nrf2 antioxidant response pathway by restricting exogenous sources of NEAAs. Citation Format: Sarah E. LeBoeuf, Warren Wu, Triantafyllia Karakousi, Robert Wild, Thales Papagiannakopoulos. Uncovering metabolic bottlenecks in KEAP1 mutant lung cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2569.