How genetically engineered mouse tumor models provide insights into human cancers

Abstract

Genetically engineered mouse models (GEMMs) of human cancer were first created nearly 30 years ago. These early transgenic models demonstrated that mouse cells could be transformed in vivo by expression of an oncogene. A new field emerged, dedicated to generating and using mouse models of human cancer to address a wide variety of questions in cancer biology. The aim of this review is to highlight the contributions of mouse models to the diagnosis and treatment of human cancers. Because of the breadth of the topic, we have selected representative examples of how GEMMs are clinically relevant rather than provided an exhaustive list of experiments. Today, as detailed here, sophisticated mouse models are being created to study many aspects of cancer biology, including but not limited to mechanisms of sensitivity and resistance to drug treatment, oncogene cooperation, early detection, and metastasis. Alternatives to GEMMs, such as chemically induced or spontaneous tumor models, are not discussed in this review.

Fig 1.

Schematic representation of DNA constructs used to generate genetically engineered mouse models (GEMMs) of human cancer. (A) In conventional transgenic animals, transcription of a cDNA encoding an oncogene or a sequence encoding a short hairpin RNA (shRNA; to knockdown expression of a target gene) is driven by a tissue-specific or ubiquitous promoter. (B) In this example of conventional gene targeting, the gene is disrupted by replacing its first exon with an antibiotic selection gene (eg, a phosphoglycerate kinase-neomycin [PGK-neo] cassette). (C,D) To generate conditional knock-outs and knock-ins, recombinases (bacteriophage P1-derived Cre recombinase and Saccharomyces Cerevisiae–derived Flp recombinase) are used to eliminate or activate expression of a functional gene at a specific time. For example (C), an exon of a target gene is cloned between specific short direct repeats (loxP sites for Cre, and Frt sites for Flp). The target gene is functional until it is exposed to the recombinase, at which time the flanked DNA is excised by the recombinase. Expression of the recombinase can be directed to a defined tissue by generating transgenic mice that express the enzyme under the control of a tissue-specific promoter or by using viruses to deliver the enzyme to a tissue of interest. (D) The Cre/loxP and Flp/FRT systems can also be used to conditionally express a mutant in a tissue of interest from its endogenous promoter or an oncogene/shRNA from a housekeeping promoter.^, The main advantage of using recombinase-based systems is that they allow for tissue-specific expression of the sequence of interest. A potential disadvantage is that, once recombination has occurred, the excision is irreversible. The recombinase can also be delivered using a viral vector (eg, Adenovirus-cre), allowing the event to occur only in a small subset of cells within an otherwise normal tissue, which recapitulates the scenario that most frequently occurs in human tumors. (E) An avian retrovirus–based method to deliver oncogenes somatically to subsets of cells within a tissue of interest has been used in mice. Here, a transgenic mouse is generated that carries the receptor for the avian leukosis virus (ALV) under the control of a tissue-specific promoter. The mouse is then infected with an ALV-pseudotyped retrovirus (RCAS) carrying an oncogene or, as shown recently, a microRNA. Mouse cells do not normally express the viral receptor; thus, normal cells are not affected. In addition to ensuring that a subset of cells in the defined organ is infected, this method allows oncogenes to be conditionally expressed in different tissues. It is particularly suited to highly proliferative tissues, because the replication-competent RCAS retrovirus only infects cycling cells. (F) One of the most widely used methods to inducibly express oncogenes or shRNAs is to express them under the control of a tetracycline-inducible promoter. Activation of the tetracycline-inducible promoter in the Tet-ON system occurs when the animal is exposed to tetracycline (or the tetracycline analog doxycycline) and expresses the reverse tetracycline transactivator (rtTA) in the tissue(s) of interest. Withdrawal of tetracycline causes expression of the transgene to shut off. On the contrary, in the Tet-OFF system, the tetracycline transactivator (tTA) induces expression from the tetracycline regulated promoter in the absence of tetracycline. Expression is then repressed on addition of the drug. The Tet systems have been widely used in tumor maintenance studies to determine whether tumors have become dependent on continuous expression of an oncogene or loss of a tumor suppressor gene. An alternative strategy to inducibly regulate the function of a protein encoded by a transgene in vivo is to modify the protein to contain an estrogen-responsive moiety so that its activity can be induced by addition of the estrogen analog, tamoxifen. tTa, tetracycline transactivator.

Fig 2.

Multiple uses of genetically engineered mouse models (GEMMs) of human cancer. Most clinically relevant problems in cancer biology studied by using GEMMs include treatment, drug resistance, metastasis, early detection, and cancer prevention (the last is not discussed in this review).