The evolution of cancer modeling: the shadow of stem cells

Abstract

Cancer is a complex and highly dynamic process. Genetically engineered mouse models (GEMs) that develop cancer are essential systems for dissecting the processes that lead to human cancer. These animal models provide a means to determine the causes of malignancy and to develop new treatments, thus representing a resource of immense potential for medical oncology. The sophistication of modeling cancer in mice has increased to the extent that now we can induce, study and manipulate the cancer disease process in a manner that is impossible to perform in human patients. However, all GEMs described so far have diverse shortcomings in mimicking the hierarchical structure of human cancer tissues. In recent years, a more detailed picture of the cellular and molecular mechanisms determining the formation of cancer has emerged. This Commentary addresses new experimental approaches toward a better understanding of carcinogenesis and discusses the impact of new animal models.

Fig. 1.

Main molecular mechanisms of human cancer and traditional mouse cancer models. (A) Human cancer is a genetic disease that can originate from several possible types of alterations affecting the structure and/or number of oncogenes or tumor suppressor genes. Independently of the nature of the oncogenic insult, all human tumor cells carry the oncogenic alteration, from the cell of origin to the more differentiated cancer cells, although the role of this oncogene may be different at different stages of tumor differentiation, and these mutations might become carrier mutations rather than driving ones depending on the cellular context. CSC, cancer stem cells. (B) In the classical transgenic mouse models of cancer, the oncogene is expressed under the control of a gene that can be either constitutively expressed or, alternatively, tissue restricted. In both cases, all of the cells in the mouse are genetically modified. In the first case, all of the cells express the oncogene. In the second case, the oncogene is expressed in all the cells of a certain chosen tissue. This rather uncontrolled oncogenic expression leads to the appearance of tumors that do not necessarily reproduce the hierarchical structure of human cancers.

Fig. 2.

Example of the mimicking of a complex human oncogenic alteration in the mouse. (A) Molecular mechanism of a human chromosomal translocation resulting in a chimeric oncogene. (B) Knock-in mouse model of chromosomal translocation. By homologous recombination in ES cells, one allele of gene A is modified to introduce the 3′-elements of gene B in order to mimic the rearrangement seen in humans. These ES cells are injected into wild-type (WT) blastocysts to generate chimeric mice that are composed of WT and genetically modified cells. Therefore, a percentage of the cells in every organ of the chimera carry the oncogenic alteration, which is expressed under the regulatory sequences of gene A, thus generating a model that very closely mimics the human case where tumoral cells are mixed in a background of normal cells. Unfortunately, most of these chimeric mice cannot produce viable knock-in offspring, indicating that fusion proteins are toxic for development. (C) Conditional, Cre-inducible translocation model: genes A and B are modified separately by homologous recombination in ES cells, and loxP sites (or the recently developed Dre-rox sites) are introduced at the precise points where chromosomal translocation happens in humans. F1 mice heterozygous for these modified genes are generated and crossed with a tissue-specific Cre (or Dre) recombinase. These mice carry the modified alleles in all cells and have no phenotype in the absence of recombinase. The oncogene is expressed under the regulatory sequences of gene A in all the cells expressing recombinase and their potential descendants. (The expression of the recombinase under differentiated cell promoters in differentiated cells leads to the appearance of tumors that do not necessarily reproduce the hierarchical structure of human cancers.)

Fig. 3.

New approaches to reproduce the hierarchical structure of human cancer in the mouse. (A) Based on the reprogramming nature of oncogenes, it has been proven that restricting the expression of the oncogenic alterations to the stem cell compartment is all that is needed to recapitulate all the tumoral heterogeneity. Using a stem cell-restricted transgenic expression system, the expression of the oncogene in the reprogramming-prone stem cells and progenitors allows the development of all the cells that compose the tumor mass by a ‘hands-off’ mechanism. The modified gene is present in all the mouse cells but expression of the oncogene is limited to the stem/progenitor compartment. (B) Conditional activation of an oncogenic alteration from the stem cell onwards: by using a conventional transgene that can be activated by recombinase, with the regulatory sequences of a constitutive or tissue-restricted gene (B1); by modifying the locus of an oncogene by introducing a recombinase-inducible activating mutation (B2) or, by modifying the locus of a tumor suppressor to achieve a recombinase-mediated deletion (B3). In these three cases, in combination with a stem/progenitor-restricted recombinase, the oncogenic anomaly is initiated in stem cells and maintained in all their descendants in a manner that is very similar to how it happens in humans.