Role of the amino-terminal domain of simian virus 40 early region in inducing tumors in secretin-expressing cells in transgenic mice

Diverse landscape of genetically engineered mouse models: Genomic and molecular insights into prostate cancer

Prostate cancer (PCa) is a significant health concern for men worldwide and is particularly prevalent in the United States. It is a complex disease presenting different molecular subtypes and varying degrees of aggressiveness. Transgenic/genetically engineered mouse models (GEMMs) greatly enhanced our understanding of the intricate molecular processes that underlie PCa progression and have offered valuable insights into potential therapeutic targets for this disease. The integration of whole-exome and whole-genome sequencing, along with expression profiling, has played a pivotal role in advancing GEMMs by facilitating the identification of genetic alterations driving PCa development. This review focuses on genetically modified mice classified into the first and second generations of PCa models. We summarize whether models created by manipulating the function of specific genes replicate the consequences of genomic alterations observed in human PCa, including early and later disease stages. We discuss cases where GEMMs did not fully exhibit the expected human PCa phenotypes and possible causes of the failure. Here, we summarize the comprehensive understanding, recent advances, strengths and limitations of the GEMMs in advancing our insights into PCa, offering genetic and molecular perspectives for developing novel GEMM models.

Functional Domains of the Early Proteins and Experimental and Epidemiological Studies Suggest a Role for the Novel Human Polyomaviruses in Cancer

As their name indicates, polyomaviruses (PyVs) can induce tumors. Mouse PyV, hamster PyV and raccoon PyV have been shown to cause tumors in their natural host. During the last 30 years, 15 PyVs have been isolated from humans. From these, Merkel cell PyV is classified as a Group 2A carcinogenic pathogen (probably carcinogenic to humans), whereas BKPyV and JCPyV are class 2B (possibly carcinogenic to humans) by the International Agency for Research on Cancer. Although the other PyVs recently detected in humans (referred to here as novel HPyV; nHPyV) share many common features with PyVs, including the viral oncoproteins large tumor antigen and small tumor antigen, as their role in cancer is questioned. This review discusses whether the nHPyVs may play a role in cancer based on predicted and experimentally proven functions of their early proteins in oncogenic processes. The functional domains that mediate the oncogenic properties of early proteins of known PyVs, that can cause cancer in their natural host or animal models, have been well characterized and we examined whether these functional domains are conserved in the early proteins of the nHPyVs and presented experimental evidence that these conserved domains are functional. Furthermore, we reviewed the literature describing the detection of nHPyV in human tumors.

T antigen transgenic mouse models

The study of polyomavirus has benefited immensely from two scientific methodologies, cell culture and in vitro studies on one side and the use of transgenic mice as experimental models on the other. Both approaches allowed us to identify cellular products targeted by the viruses, the consequences of these interactions at the phenotypic and molecular level, and thus the potential roles of the targets within their normal cellular context. In particular, cell culture and in vitro reports suggest a model explaining partially how SV40 large T antigen contributes to oncogenic transformation. In most cases, T antigen induces cell cycle entry by inactivation of the Rb proteins (pRb, p130, and p107), thus activating E2F-dependent transcription and subsequent S-phase entry. Simultaneously, T antigen blocks p53 activity and therefore prevents the ensuing cell-cycle arrest and apoptosis. For the most part, studies of T antigen expression in transgenic mice support this model, but the use of T antigen mutants and their expression in different tissue and cell type settings have expanded our knowledge of the model system and raised important questions regarding tumorigenic mechanisms functioning in vivo.

The genomic revolution and endocrine pathology

The genome has been sequenced. However, the functions of each gene remain to be elucidated through phenotypic analysis. This analysis has been called phenogenomics. That part of phenogenomics related to disease can be called pathogenomics or Genomic Pathology. The initial phases of disease analysis will use genetically modified mice. The proliferation of ambitious programs designed to use mice for phenogenomics has been met with alarm by comparative pathologists who note the lack of qualified genomic pathologists and of training programs in genomic pathology. While endocrine pathology offers a number of excellent examples of the contributions made by pathologists to the scientific literature, it also contains examples of the hazards of working with untrained, unwary personnel.

Enterocyte proliferation and intestinal hyperplasia induced by simian virus 40 T antigen require a functional J domain

Transgenic mice expressing the simian virus 40 large T antigen (TAg) in enterocytes develop intestinal hyperplasia that progresses to dysplasia with age. This induction requires TAg action on the retinoblastoma (Rb) family of tumor suppressors and is independent of the p53 pathway. In cell culture systems, the inactivation of Rb proteins requires both a J domain in TAg that interacts with hsc70 and an LXCXE motif that directs association with Rb proteins. Together these elements are sufficient to release E2Fs from their association with Rb family members. We have generated transgenic mice that express a J domain mutant (D44N) in villus enterocytes. In contrast to wild-type TAg, the D44N mutant is unable to induce enterocyte proliferation. Histological and morphological examination revealed that mice expressing the J domain mutant have normal intestines without loss of growth control. Unlike mice expressing wild-type TAg, mice expressing D44N do not reduce the protein levels of p130 and are also unable to dissociate p130-E2F DNA binding complexes. Furthermore, mice expressing D44N in a null p130 background are still unable to develop hyperplasia. These studies demonstrate that the ectopic proliferation of enterocytes by TAg requires a functional J domain and suggest that the J domain is necessary to inactivate all three pRb family members.

SV40 large T antigen targets multiple cellular pathways to elicit cellular transformation

DNA tumor viruses such as simian virus 40 (SV40) express dominant acting oncoproteins that exert their effects by associating with key cellular targets and altering the signaling pathways they govern. Thus, tumor viruses have proved to be invaluable aids in identifying proteins that participate in tumorigenesis, and in understanding the molecular basis for the transformed phenotype. The roles played by the SV40-encoded 708 amino-acid large T antigen (T antigen), and 174 amino acid small T antigen (t antigen), in transformation have been examined extensively. These studies have firmly established that large T antigen's inhibition of the p53 and Rb-family of tumor suppressors and small T antigen's action on the pp2A phosphatase, are important for SV40-induced transformation. It is not yet clear if the Rb, p53 and pp2A proteins are the only targets through which SV40 transforms cells, or whether additional targets await discovery. Finally, expression of SV40 oncoproteins in transgenic mice results in effects ranging from hyperplasia to invasive carcinoma accompanied by metastasis, depending on the tissue in which they are expressed. Thus, the consequences of SV40 action on these targets depend on the cell type being studied. The identification of additional cellular targets important for transformation, and understanding the molecular basis for the cell type-specific action of the viral T antigens are two important areas through which SV40 will continue to contribute to our understanding of cancer.

Energy homeostasis and gastrointestinal endocrine differentiation do not require the anorectic hormone peptide YY

The gastrointestinal hormone peptide YY is a potent inhibitor of food intake and is expressed early during differentiation of intestinal and pancreatic endocrine cells. In order to better understand the role of peptide YY in energy homeostasis and development, we created mice with a targeted deletion of the peptide YY gene. All intestinal and pancreatic endocrine cells developed normally in the absence of peptide YY with the exception of pancreatic polypeptide (PP) cells, indicating that peptide YY expression was not required for terminal differentiation. We used recombination-based cell lineage trace to determine if peptide YY cells were progenitors for gastrointestinal endocrine cells. Peptide YY(+) cells gave rise to all L-type enteroendocrine cells and to islet partial differential and PP cells. In the pancreas, approximately 40% of pancreatic alpha and rare beta cells arose from peptide YY(+) cells, suggesting that most beta cells and surprisingly the majority of alpha cells are not descendants of peptide YY(+)/glucagon-positive/insulin-positive cells that appear during early pancreagenesis. Despite the anorectic effects of exogenous peptide YY(3-36) following intraperitoneal administration, mice lacking peptide YY showed normal growth, food intake, energy expenditure, and responsiveness to peptide YY(3-36). These observations suggest that targeted disruption of the peptide YY gene does not perturb terminal endocrine cell differentiation or the control of food intake and energy homeostasis.

Sudden onset of colitis after ablation of secretin-expressing lymphocytes in transgenic mice

Though secretin mRNA was demonstrated in mouse lymphoid organs, its role in the immune system is unknown. Here, secretin gene-expressing cells were ablated by ganciclovir infusion in mice transgenic for the rat secretin promoter (Sec) directing the expression of herpesvirus thymidine kinase (Sec-HSVTK). Thymus, spleen, blood, and colon were investigated by histology. Lymphoid cells were extracted and quantified, and CD19+ B-cells and CD3+, CD103+, CD4+, and CD8+ T-cells were analyzed by flow cytometry. Protein extracts from spleen and thymus were assayed for secretin by Western blotting, and isolated lymphocytes were investigated for HSVTK, secretin, and secretin receptor (Sec-R) mRNA by reverse transcription-polymerase chain reaction (RT-PCR). Ablation of secretin-expressing cells produced severe colitis with morphological features similar to those observed in graft-versus-host (GVH) disease. Profound lymphoid depletion was observed in spleen, thymus, and peripheral blood. The relative percentage of B- and T-cell subsets were unaffected. Analysis of colonic lymphocytes revealed a marked depletion of CD4+ T lymphocytes. Colitis and lymphoid depletion were not reversed by secretin cotreatment. Immunoblot analysis of protein extracts from spleen and thymus identified secretin-like immmunoreactant. RT-PCR of lymphocyte mRNA from spleen and thymus identified secretin and secretin receptor transcripts. We conclude that GVH-like colitis in ganciclovir-treated Sec-HSVTK mice arises from depletion of secretin gene-expressing lymphoid cells and not from the failure of secretin production.

Neurogenin 3-expressing progenitor cells in the gastrointestinal tract differentiate into both endocrine and non-endocrine cell types

Mice deficient for the transcription factor neurogenin 3 (ngn3) fail to develop endocrine cells in the intestine and pancreas and show partial endocrine differentiation in the stomach. We expressed Cre recombinase under control of a ngn3 BAC to achieve high fidelity cell lineage tracing in vivo to determine whether endocrine cells in these organs differentiate from NGN3+ precursor cells. Our results indicate that all small intestinal enteroendocrine cells arise from ngn3-expressing cells and confirm that NGN3+ cells give rise to all pancreatic endocrine cells as noted previously. By examining mice at a developmental stage when all of the cell types in the stomach have differentiated, we have delineated region-associated differences in endocrine differentiation. A much smaller fraction of endocrine cells populating the acid-producing region of the stomach is derived from NGN3+ precursor in contrast to the antral-pyloric region. Unexpectedly, ngn3 is expressed in cells that adopt non-endocrine cell fates including significant fractions of goblet and Paneth cells in the intestine and a small number of duct and acinar cells in the pancreas. Rarely, ngn3 was expressed in pluripotent cells in intestinal crypts with resultant labeling of an entire crypt-villus unit. Thus, ngn3 expression occurs in mixed populations of immature cells that are not irreversibly committed to endocrine differentiation.

'Designer' tumors in mice

We have developed and tested successfully a general method based on Cre-mediated recombination that can be used for ubiquitous or tissue-specific expression of protein products, including tumor-inducing oncoproteins. Depending on the specificity of a chosen promoter driving cre expression, tumors develop by design in bitransgenic mouse progeny derived by crossing Cre-producing mice with partners carrying a dormant oncogenic transgene (targeted into the 3' noncoding region of the cytoplasmic beta-actin locus) that becomes functional after excision of a 'floxed' DNA segment. To provide proof-of-principle, we have used as models transgenes encoding the polyomavirus middle T antigen (PVMT) and the T antigens of the SV40 early region (SVER). Cre-dependent activation of widespread SVER expression resulted in hyperplasias or invasive tumors affecting particular visceral smooth muscles, whereas Cre-dependent, mammary gland-specific expression of PVMT-induced adenocarcinomas, according to plan. Unexpectedly, we also encountered spontaneous (Cre-independent) oncogene expression occurring as a rare event, which simulates the initiation of sporadic tumors and leads to PVMT-induced hemangiomas and mammary carcinomas or SVER-induced disseminated sarcomas, thus, revealing particular tissue susceptibilities to the actions of these oncoproteins.

A mouse model of uterine leiomyosarcoma

We are using an approach that is based on the cre/loxP recombination process and involves a binary system of Cre-producing and Cre-responding transgenic mice to achieve ubiquitous or tissue-specific expression of oncoproteins. To develop mouse models of tumorigenesis, Cre-producers are mated with responder animals carrying a dormant oncogene targeted into the 3' untranslated region of the locus encoding cytoplasmic beta-actin (actin cassette). Production of oncoprotein from a bicistronic message is accomplished in bitransgenic progeny by Cre-mediated excision of a segment flanked by loxP sites that is located upstream from the oncogenic sequence. Widespread Cre-dependent activation and expression of an actin-cassette transgene encoding the T antigens of the SV40 early region (SVER) commencing in embryos was compatible with normal development and did not impair viability. However, at approximately 3 months of age, all female animals developed massive uterine leiomyosarcomas, whereas practically all males exhibited enormously enlarged seminal vesicles because of pronounced hyperplasia of the smooth muscle layers. In addition, because of smooth muscle hyperproliferation, marked dilation of the gallbladder was observed in mice of both sexes. To begin exploring aberrant signaling events in the SVER-triggered tumorigenic pathways, we analyzed the expression profile of leiomyosarcomas by DNA microarray analysis.

T antigens of simian virus 40: molecular chaperones for viral replication and tumorigenesis

Simian virus 40 (SV40) is a small DNA tumor virus that has been extensively characterized due to its relatively simple genetic organization and the ease with which its genome is manipulated. The large and small tumor antigens (T antigens) are the major regulatory proteins encoded by SV40. Large T antigen is responsible for both viral and cellular transcriptional regulation, virion assembly, viral DNA replication, and alteration of the cell cycle. Deciphering how a single protein can perform such numerous and diverse functions has remained elusive. Recently it was established that the SV40 T antigens, including large T antigen, are molecular chaperones, each with a functioning DnaJ domain. The molecular chaperones were originally identified as bacterial genes essential for bacteriophage growth and have since been shown to be conserved in eukaryotes, participating in an array of both viral and cellular processes. This review discusses the mechanisms of DnaJ/Hsc70 interactions and how they are used by T antigen to control viral replication and tumorigenesis. The use of the DnaJ/Hsc70 system by SV40 and other viruses suggests an important role for these molecular chaperones in the regulation of the mammalian cell cycle and sheds light on the enigmatic SV40 T antigen-a most amazing molecule.