The causal relationship between epigenetic abnormality and cancer development: in vivo reprogramming and its future application

Abstract

There is increasing evidence that cancer cells acquire epigenetic abnormalities as well as genetic mutations during cancer initiation, maintenance, and progression. However, the role of epigenetic regulation in cancer development, especially at the organismal level, remains to be elucidated. Here, we describe the causative role of epigenetic abnormalities in cancer, referring to our in vivo studies using induced pluripotent stem cell technology. We first summarize epigenetic reorganization during cellular reprogramming and introduce our in vivo reprogramming system for investigating the impact of dedifferentiation-driven epigenetic disruption in cancer development. Accordingly, we propose that particular types of cancer, in which causative mutations are not often detectable, such as pediatric cancers like Wilms' tumor, may develop mainly through alterations in epigenetic regulation triggered by dedifferentiation. Finally, we discuss issues that still remain to be resolved, and propose possible future applications of in vivo reprogramming to study cancer and other biological phenomena including organismal aging.

Keywords: cancer; epigenetics; iPS cell; in vivo reprogramming.

Figure 1.

Functional involvement of DNA methylations in murine colon tumor development and progression. Forced reduction of DNA methylation causes chromosomal instability and promotes the loss of Apc heterozygosity, which leads to increased microadenoma formation. The DNA hypomethylation suppresses the transition of early microadenomas into macroscopic tumors. In contrast, the forced expression of de novo DNA methyltransferase Dnmt3b accelerates the transition of colonic microadenomas to macroscopic tumors, while the deletion of Dnmt3b suppresses this progression.

Figure 2.

Epigenetic landscape of cellular differentiation and reprogramming. The developmental process is generally unidirectional and irreversible, like a ball rolling downhill (Waddington’s landscape). Forced expression of the transcription factors (TF) that activate the transcriptional network of the target cells (orange, green and blue ellipsoids represent TFs that constitute key transcriptional network in each cell type) can induce lineage conversion, i.e., reprogramming or direct reprogramming.

Figure 3.

Schematic drawings of in vivo reprogrammable mice. The Yamanaka factors-inducible embryonic stem cell (ESC) possesses the optimized reverse tetracycline-dependent transactivator (M2-rtTA) at the Rosa 26 locus and a polycistronic cassette encoding the four reprogramming factors (Oct3/4, Sox2, Klf4, and c-Myc), followed by ires-mCherry at the Col1a1 gene locus under the tetracycline-dependent promoter (tetOP). In vivo reprogrammable mice were generated via blastocyst injection of these ESCs. Upon treatment with doxycycline (Dox), the cells express the four reprogramming factors in vivo.

Figure 4.

Premature termination of in vivo reprogramming causes kidney cancer development resembling human Wilms’ tumor. (A) Continuous expression of the reprogramming factors results in the development of multiple teratomas in various organs. However, when incomplete reprogramming is induced in these mice by the withdrawal of Dox treatment before teratoma formation, the mice develop cancers showing invasion of surrounding tissues. (B) Left panel: Continuous expression of the reprogramming factors induces teratomas, which consist of differentiated cells of the three germ layers, (a) neural tissue (ectoderm); (b) glandular epithelium (endoderm); (c) cartilage (mesoderm). Middle panel: The kidney tumor developed by incomplete reprogramming consists of epithelial (d), stromal (e), and blastema-like (f) compartments, which are representative histological features of Wilms’ tumors, a common pediatric kidney cancer. Right panel: A histology of human Wilms’ tumor containing epithelial, stromal, and blastema-like compartments.

Figure 5.

Concept for epigenetic cancer. Non-neoplastic cells in the kidney can be reprogrammed into iPSCs in vivo. However, incomplete reprogramming results in the development of a cancer resembling Wilms’ tumor. The cancer cells are readily reprogrammable into iPSCs by additional expression of reprogramming factors in vitro. The kidney tumor-derived iPSCs differentiate into non-neoplastic kidney cells in chimeric mice, demonstrating that kidney cancer cells in this model have not undergone irreversible genetic transformation. We propose that particular types of cancer can develop mainly through the disruption of epigenetic regulation.

Figure 6.

Cellular context-dependent biological consequences of Apc mutations. Colon tumor cells with a loss of Apc function were reprogrammed into iPSC-like cells (reprogrammed tumor cells, RTCs). RTC-derived differentiated cells exhibited neoplastic growth exclusively in the intestine, but not in other cell types in vitro or in vivo. This phenotype suggests that the effect of cancer-related mutations is largely dependent on cellular context, and that cell type-specific epigenetic control is important for the maintenance of cancer properties. Furthermore, the majority of intestinal lesions in RTC-derived mice remained as microadenomas. The results suggest that genetic mutations in tumor cells are not sufficient for full-blown tumor development, thus underscoring the significance of epigenetic regulation during multistage cancer development.