Modeling metastasis in mice: a closer look

Abstract

Unraveling the multifaceted cellular and physiological processes associated with metastasis is best achieved by using in vivo models that recapitulate the requisite tumor cell-intrinsic and -extrinsic mechanisms at the organismal level. We discuss the current status of mouse models of metastasis. We consider how mouse models can refine our understanding of the underlying biological and molecular processes that promote metastasis, and we envisage how the application of new technologies will further enhance investigations of metastasis at single-cell resolution in the context of the whole organism. Our view is that investigations based on state-of-the-art mouse models can propel a holistic understanding of the biology of metastasis, which will ultimately lead to the discovery of new therapeutic opportunities.

Keywords: genetically engineered mouse models (GEMMs); metastasis; syngeneic models; xenograft models.

Figure 1:. Schematic of the metastasis cascade.

The metastasis cascade is comprised of many steps that require tumor cells to display gradients of cellular plasticity to adapt to external conditions and unique physiological environments. First, a tumor becomes locally invasive (step 1), which allows them to intravasate into the blood vessels (step 2). The molecular and cellular changes that affect this early phase of the metastatic cascade are still not well understood, although it is known that EMT is important for tumor dissemination. Next, tumor cells circulate in the bloodstream, called CTCs (step 3), often transiting with other cells. It is well known that only a few CTCs are able to extravasate in distant organs (step 4) and this may be affected by their intrinsic predisposition to a preferred site (organotropism). Once they colonize a new site (step 5), some cells become dormant and can remain in this quiescent phase for many years. Once activated, metastatic cells interact with the local microenvironment, called the metastatic niche, to form metastatic locus.

Figure 2:. Model organisms for studying metastasis.

Among the various model organisms have been used to study metastasis, C. Elegans and Drosophila are the simplest genetic organisms. Their analysis enables mechanistic studies of early steps of metastasis, including invasion and migration. Another key model is Zebrafish, which is amenable both for genetic manipulation and implantation of tumor cells, thus allowing genetic screenings of metastatic drivers as well as analysis of human xenografts. Zebrafish is versatile for intravital imaging in vivo to study in real time the interaction of metastatic cells and their microenvironment. Among other simple models, the Chicken embryo is easy to manipulate and therefore enables analysis of basic processes of metastasis, including angiogenesis and invasion. Its chorioallantoic membrane is immunodeficient, thus allowing implantation of human tumors. Analyses of mouse models can inform on all or most aspects of the metastatic cascade. Transplantation models enable the implantation of tumors, organoids or tumor cells from human (xenografts models) or mice (syngeneic models), thus enabling studies of candidate drivers of metastasis, mechanism of organotropism, and forward genetic screens. While xenografts are grown in immunodeficient mouse hosts and therefore enable investigations of cell intrinsic features of human cancer associated with metastasis, syngeneic models are grown in immunocompetent hosts which makes it possible to study the microenvironment associated with metastasis. Genetically engineered mouse models enable investigations of de novo metastasis as occurs in the whole organism. They can model specific epigenetic and genomic alterations associated with human metastasis, and enable investigations of evolutionary processes associated with metastasis, particularly with the benefit of lineage tracing. Other applications include investigations of organotropism and preclinical studies.