Pancreatic Cancer Modeling using Retrograde Viral Vector Delivery and IN-Vivo CRISPR/Cas9-mediated Somatic Genome Editing
Curators: Larry H. Benstein, MD, FCAP and Aviva Lev-Ari, PhD, RN
Pancreatic cancer modeling using retrograde viral vector delivery and in vivo CRISPR/Cas9-mediated somatic genome editing
Little is known about how these alterations contribute to the development of metastatic and therapy-refractory PDAC. Given the inability to test gene function in human cancers in vivo, genetically engineered mouse models represent tractable and biologically relevant systems with which to interrogate the molecular determinants of each stage of pancreatic cancer development.
Identification of the mutations that drive the development of human pancreatic cancer combined with the ability to alter gene function in mice has enabled the development of genetically engineered murine PDAC models. Transgenic expression of Cre-recombinase in pancreatic cells of loxP-Stop-loxP (LSL) KrasG12Dknock-in mice (KrasLSL-G12D/+) results in deletion of the transcriptional/translational Stop element, expression of oncogenic KrasG12D, and development of lesions that closely resemble early stage human pancreatic intraepithelial neoplasms (PanINs) (Hingorani et al. 2003). Concomitant expression of a point mutant p53 allele, deletion of p53, deletion of Cdkn2a, and/or deletion of Smad4 allow(s) for the development of invasive and metastatic PDAC
(Aguirre et al. 2003; Hingorani et al. 2005; Bardeesy et al. 2006a,b; Gidekel Friedlander et al. 2009; Whittle et al. 2015).
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526740/
Pancreatic ductal adenocarcinoma (PDAC) is a genomically diverse, prevalent, and almost invariably fatal malignancy. Although conventional genetically engineered mouse models of human PDAC have been instrumental in understanding pancreatic cancer development, these models are much too labor-intensive, expensive, and slow to perform the extensive molecular analyses needed to adequately understand this disease. Here we demonstrate that retrograde pancreatic ductal injection of either adenoviral-Cre or lentiviral-Cre vectors allows titratable initiation of pancreatic neoplasias that progress into invasive and metastatic PDAC. To enable in vivo CRISPR/Cas9-mediated gene inactivation in the pancreas, we generated a Cre-regulated Cas9 allele and lentiviral vectors that express Cre and a single-guide RNA. CRISPR-mediated targeting of Lkb1 in combination with oncogenic Kras expression led to selection for inactivating genomic alterations, absence of Lkb1 protein, and rapid tumor growth that phenocopied Cre-mediated genetic deletion of Lkb1. This method will transform our ability to rapidly interrogate gene function during the development of this recalcitrant cancer.
Pancreatic ductal adenocarcinoma (PDAC) is an almost uniformly lethal tumor type that is projected to become the second leading cancer killer in the United States by 2030 (Rahib et al. 2014). PDAC patients have a 5-year survival rate of ∼5%, underscoring the need for novel approaches to accelerate the molecular characterization of this disease. Although high-prevalence mutations have been identified in pancreatic cancer, these tumors also incur low-frequency mutations and genomic alterations, interact with their extensive and complex stromal environment, and undergo poorly characterized changes in their gene expression programs (Biankin et al. 2012; Waddell et al. 2015). Despite the potential importance of these molecular and cellular changes, very little is known about how these alterations contribute to the development of metastatic and therapy-refractory PDAC. Given the inability to test gene function in human cancers in vivo, genetically engineered mouse models represent tractable and biologically relevant systems with which to interrogate the molecular determinants of each stage of pancreatic cancer development.
Identification of the mutations that drive the development of human pancreatic cancer combined with the ability to alter gene function in mice has enabled the development of genetically engineered murine PDAC models. Transgenic expression of Cre-recombinase in pancreatic cells of loxP-Stop-loxP (LSL)KrasG12D knock-in mice (KrasLSL-G12D/+) results in deletion of the transcriptional/translational Stop element, expression of oncogenic KrasG12D, and development of lesions that closely resemble early stage human pancreatic intraepithelial neoplasms (PanINs) (Hingorani et al. 2003). Concomitant expression of a point mutant p53 allele, deletion of p53, deletion of Cdkn2a, and/or deletion of Smad4 allow(s) for the development of invasive and metastatic PDAC (Aguirre et al. 2003; Hingorani et al. 2005; Bardeesy et al. 2006a,b;Gidekel Friedlander et al. 2009; Whittle et al. 2015).
These in vivo models have been instrumental in our understanding of the genetic determinants of cancer progression as well as the functional interactions of neoplastic cells with the immune system and stromal environment. However, using conventional genetically engineered autochthonous mouse models of PDAC to interrogate gene function is complicated by inherent practical and biological limitations of these systems. Existing mouse models typically fail to model the adult onset of pancreatic cancer and induce genomic alteration in nearly every cell in the pancreas. In the decade since the first genetically engineered PDAC models were developed, few technical advances have been made, and generating the mice required to investigate a gene of interest in the established PDAC models remains a time-consuming and costly endeavor (Aguirre et al. 2003; Hingorani et al. 2003,2005; Saborowski et al. 2014).
Systems that enable in vivo functional interrogation of genes in pancreatic cancer without the financial and temporal cost of generating new mouse alleles and incorporating them into increasingly complex mouse models could have an extremely broad impact on pancreatic cancer research. To functionally investigate the molecular changes that drive each step of pancreatic cancer development, it would be desirable to have a system in which the timing of tumor initiation and the number of lesions that form in the adult pancreas can be controlled, the number of germline-encoded alleles is minimized, and genes of interest can be eliminated without having to generate a conditional allele and breed it into a complex genetically engineered mouse model.
Here we describe methods for the direct delivery of viral vectors to the pancreas and transgenic mouse lines to allow CRISPR/Cas9-mediated genomic alterations in pancreatic cells in vivo. These systems allow titratable initiation of pancreatic tumors in adult mice and functional interrogation of candidate genes in pancreatic cancer in vivo.
Somatic genome engineering enables rapid generation of genetically defined pancreatic cancer mouse models
To assess the impact of Lkb1 deletion on pancreatic cancer, we performed retrograde ductal injections of KT;H11LSL-Cas9/+, control KT, and KT;Lkb1flox/flox mice with Lenti-sgLkb1/Cre as well as KT;H11LSL-Cas9/+ mice with lentiviral vectors containing negative control sgRNAs. Control KT mice infected with Lenti-sgLkb1/Cre as well as KT;H11LSL-Cas9/+ mice infected with a Lenti-sgRNA/Cre vector containing either a nontargeting sgRNA (sgNT) or an sgRNA targeting an inert region of the genome (sgNeo) formed only rare Tomatopositive lesions (Fig. 3F–H). Both Lenti-sgLkb1/Cre-infected KT;H11LSL-Cas9/+ and KT;Lkb1flox/flox mice had extensive tumor growth as early as 2 mo after tumor initiation (Fig. 3I,J, respectively).
All three groups of negative control mice developed only rare PanIN lesions within almost completely normal pancreata (Fig. 4A–C; Supplemental Fig. 9A; data not shown). The tumors that formed in Lenti-sgLkb1/Cre-infectedKT;H11LSL-Cas9/+ mice were cystic lesions comprised of Tomatopositive, CK19positive tall cuboidal to columnar epithelial cells with otherwise bland cytological features (Fig. 4D; Supplemental Fig. 9B). Some smaller lesions also contained high levels of mucin (Supplemental Fig. 9B). Importantly, these features were histologically indistinguishable from those found in KT;Lkb1flox/flox mice infected with Lenti-sgLkb1/Cre (Fig. 4E; Supplemental Fig. 9C). Lenti-sgLkb1/Cre-infected KT;H11LSL-Cas9/+ mice also had a substantially higher tumor burden when compared with all three groups of negative control mice (Fig. 4F).
The development of systems that accelerate our ability to investigate pancreatic carcinogenesis at the molecular level will be a critical step toward overcoming the dismal rate of successful treatment and low survival rate of patients with this recalcitrant cancer. The ability to induce pancreatic cancer using viral vectors will be instrumental in understanding the mechanisms that sustain tumor growth, lead to metastatic spread, and drive drug resistance. Our CRISPR/Cas9-based model should allow any gene of interest to be inactivated in pancreatic cancer in vivo without the need to generate any new mouse alleles. The method developed in this study could have a profound impact on both basic and translational pancreatic cancer research. Collectively, these methods will enable a more rapid and complete understanding of the molecular regulators of all aspects of pancreatic tumorigenesis and complement the strength of existing genetically engineered models, human patient-derived xenograft models, and studies on human and murine cell lines.
Retrograde ductal injection of Adeno-Cre and Lenti-Cre vectors allows titratable pancreatic tumor initiation in the adult pancreas. This removes the requirement for the transgenic Cre(ER) alleles used in conventional genetically engineered pancreatic cancer models and enables sparse rather than widespread expression of oncogenic Kras and deletion of tumor suppressor genes, more closely recapitulating the initiating events in human PDAC. The inclusion of a fluorescent Cre reporter in transgenic Cre(ER)-induced PDAC models leads to the fluorescent labeling of not only the neoplastic cells but also most of the nontransformed pancreatic epithelial cells, making unequivocal distinction of neoplastic from normal cells difficult.
The ability to induce PDAC in Cre-lox models without having to include a transgenic Cre line will make the molecular investigation of pancreatic cancer more rapid and less expensive (Supplemental Table 2). Additionally, with the ease of generating pancreatic tumors now approaching that of lung tumors (via intranasal or intratracheal injection), comparing the impact of the same genetic alterations on each cancer type should become standard practice and will uncover the commonalities and differences between these lethal adenocarcinomas.
PDAC models often generate tumor masses of unknown clonal origin. By incorporating a multicolor fluorescent reporter, we were able to mark individual clonal lesions and identify the relationship between primary tumors and metastases. Unexpectedly, some late time point mice had only one or two large cancers, underscoring the dramatic heterogeneity in expansion potential of pancreatic lesions initiated with identical engineered genetic events.
2012pharmaceutical
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