Genetically engineered mouse models in oncology research and cancer medicine

Abstract

Genetically engineered mouse models (GEMMs) have contributed significantly to the field of cancer research. In contrast to cancer cell inoculation models, GEMMs develop de novo tumors in a natural immune-proficient microenvironment. Tumors arising in advanced GEMMs closely mimic the histopathological and molecular features of their human counterparts, display genetic heterogeneity, and are able to spontaneously progress toward metastatic disease. As such, GEMMs are generally superior to cancer cell inoculation models, which show no or limited heterogeneity and are often metastatic from the start. Given that GEMMs capture both tumor cell-intrinsic and cell-extrinsic factors that drive de novo tumor initiation and progression toward metastatic disease, these models are indispensable for preclinical research. GEMMs have successfully been used to validate candidate cancer genes and drug targets, assess therapy efficacy, dissect the impact of the tumor microenvironment, and evaluate mechanisms of drug resistance. In vivo validation of candidate cancer genes and therapeutic targets is further accelerated by recent advances in genetic engineering that enable fast-track generation and fine-tuning of GEMMs to more closely resemble human patients. In addition, aligning preclinical tumor intervention studies in advanced GEMMs with clinical studies in patients is expected to accelerate the development of novel therapeutic strategies and their translation into the clinic.

Keywords: cancer; genetically engineered mouse models; metastasis; therapy; tumor microenvironment.

Figure 1. Applications of GEMMs in basic cancer research and translational oncology

Figure 2. Schematic overview of transplantation‐based mouse models and germline GEMMs

(A) Cancer cell line transplantation models are based on (orthotopic) inoculation of cultured human or mouse cancer cells in immunodeficient or syngeneic mice, respectively. (B) Patient‐derived tumor xenograft models or GEMM‐derived tumor allograft models are based on direct (orthotopic) implantation of human or mouse tumor fragments in immunodeficient or syngeneic mice, respectively. (C) In oncomice, de novo tumorigenesis is induced by transgenic expression of an oncogene (ONC) from a tissue‐specific promoter. (D) In tumor suppressor gene (TSG) knockout mice, de novo tumorigenesis is induced by germline inactivation of a TSG. (E, F) In conditional GEMMs, de novo formation of sporadic tumors is induced by tissue‐specific Cre‐loxP‐mediated inactivation of conditional TSG alleles (E) and/or activation of conditional oncogenes (F). Tissue‐specific expression of the Cre‐recombinase is achieved by crossbreeding with Cre transgenic mice, tamoxifen‐inducible Cre‐ERT transgenic mice, or by local administration of Cre‐encoding lenti‐ or adenoviruses. (G, H) Oncogene addiction can be studied using GEMMs with tamoxifen‐ or doxycycline‐inducible gene expression. (G) Administration of tamoxifen to transgenic mice carrying an oncogene‐ERT (ONC‐ERT) fusion will induce tumors that may undergo no, temporal, or durable regression upon tamoxifen withdrawal. (H) Similar studies can be performed by administration of doxycycline to bi‐transgenic mice with tissue‐specific expression of the reverse tetracycline‐controlled transactivator (rtTA) and carrying an oncogene or shRNA under transcriptional control of a tetracycline response element (TRE).

Figure 3. Schematic overview of non‐germline GEMMs

(A) Embryonic stem cell (ESC)‐derived non‐germline GEMMs. ESCs from wild‐type mice or established GEMMs can be used to introduce single or multiple mutations using CRISPR/Cas9‐based gene editing and/or mutant alleles using recombinase‐mediated cassette exchange (RMCE). The resulting ESCs can be injected into host blastocysts, which are implanted into pseudo‐pregnant females to produce chimeric mice. (B, C) CRISPR/Cas9‐based non‐germline GEMMs. CRISPR/Cas9‐mediated in situ gene editing can be achieved by local administration of sgRNA‐encoding lentiviruses in transgenic mice with tissue‐specific Cas9 expression (B), or by local administration of lentiviruses that encode both Cas9 and sgRNA in wild‐type mice (C). The latter approach may require immunodeficient or Cas9‐tolerant mice to avoid Cas9‐specific immune responses.

Figure 4. Applications of mouse models in metastasis research

This overview summarizes the utility of different preclinical mouse models of experimental and spontaneous metastasis to study the different steps of the metastatic cascade. Conventional GEMMs represent oncomice and TSG knockout mice. Next‐generation GEMMs represent conditional mouse models that are genetically engineered to accurately mimic sporadic human cancer. For some models, the utility for studying specific steps in the metastatic cascade has yet to be determined, as indicated by a question mark. Moreover, several studies have shown that components of the adaptive immune system contribute to the various steps of the metastatic cascade. These aspects cannot be studied in models based on xenografting of human cancer cells or tumor fragments in immunodeficient hosts (indicated by an asterisk). To circumvent this, humanized mice can be used as hosts.

Figure 5. Applications of mouse models in cancer drug development

Development of novel treatment strategies in oncology requires preclinical studies in mouse cancer models to identify and validate novel cancer drivers and therapeutic targets, to determine in vivo drug pharmacokinetics and pharmacodynamics (PK/PD), and to evaluate in vivo anti‐cancer efficacy of novel therapeutics. When promising preclinical results are obtained, the tolerability and anti‐cancer efficacy of these drugs are evaluated in human patients in phase I–III clinical trials. A proportion of patients will show poor response due to intrinsic or acquired resistance, which may be studied mechanistically in preclinical mouse models to identify response biomarkers and combination therapies to prevent or overcome resistance. The close alignment of mouse studies and human clinical trials will lead to better patient stratification, identification of novel biomarkers, and development of optimal combination therapies, culminating in improved cancer patient care.