Chapter Ten - The Central Role of Wnt Signaling and Organoid Technology in Personalizing Anticancer Therapy
Introduction
Stem cell-derived three-dimensional (3D) replicas of organs grown in tissue culture, termed organoids, have led to remarkable advances in stem cell and developmental biology, human disease, and regenerative medicine.1 “Organoid” is a term originally used by developmental biologists working with tissue explants to unravel the mechanisms of organogenesis. The term literally means “organ-like.” More recent use of the term organoid is defined as a 3D structure established from stem cells and consisting of organ-specific cell types that self-organize to mimic their tissue of origin.2, 3 Organoids can be initiated from two main types of stem cells (summarized in Table 1). The first stem cell type is the pluripotent embryonic stem (ES) cell or the induced pluripotent stem cell (iPS). For iPS, adult cells are artificially reprogrammed to pluripotency,4 and then differentiated toward different organ cell types using cues that have been identified to orchestrate the development of those organs during embryogenesis and organogenesis.3, 5 Diverse tissue and organ cell types can be derived from a pluripotent stem cell.
By contrast, the second type of stem cells, the tissue restricted adult stem cells, have a “memory” of their tissue of origin and self-organize and differentiate into structures that contain the different tissue-specific cell types; they recapitulate the characteristics of tissue function and architecture.1 The culture conditions that were developed to establish adult tissue stem cell-derived organoids were then adapted to growing organoids from diseased tissues, such as cancers (Table 1). This innovation has led to one of the most important advances in cancer research—high throughput drug prescreening, in a clinically relevant time frame, on patient-derived tumor organoids to personalize treatment.6, 7 In this chapter we briefly summarize the discoveries that led to adult stem-cell-derived organoid technology, the central role of Wnt signaling in this advance and how this advance is poised to revolutionize anticancer treatment. We also highlight the next challenges for patient-derived tumor organoid technology in the quest for curative anticancer treatment. Improved survival is clearly a great outcome for anticancer treatment, but the ultimate goal is curative treatment where the tumor cells are eliminated.
Section snippets
WNT signaling pathway
The Wnt signal transduction pathway has several branches that are β-catenin-dependent and β-catenin-independent (i.e., the calcium and planar cell polarity pathways) and the core components of these are highly conserved through evolution.8, 9 Here we give a brief overview of the Wnt/β-catenin branch10 as it is critical to stem cell function and organoid formation. In the absence of a Wnt ligand, β-catenin is primarily engaged in cell-cell adherens junctions. Any free newly synthesized β-catenin
Organoids derived from adult epithelium
Adult stem cell-derived organoids were first established from the epithelium lining the mouse intestine.26 Several discoveries led to this game-changing achievement. The first was the demonstration in 1998 by Korinek and coworkers that Wnt signaling is critical for intestinal stem cells. Deletion of the gene that codes for Tcf4, the downstream effector of Wnt signaling (Fig. 1), from the developing mouse intestine led to a depletion of the putative epithelial stem cell compartment and postnatal
Organoids derived from patient tumors
Once the culture techniques for growing intestinal organoids were established, variations on the same culture protocol led to the establishment of organoids from several other gastrointestinal tissues,41, 48 as well as many other stem-cell maintained adult tissues (several comprehensive recent reviews49, 50, 51). Clevers and coworkers also adapted the organoid protocols to grow patient-derived colon cancer tumor organoids. The mini-tumor organoids similarly recapitulate the features of actively
Modeling dormant tumor cells: the next frontier for tumor organoids
One limitation for mini-tumor organoids as a drug screen is that the ever-expanding tumor organoids mimic the actively growing tumor cells. The mini-tumor organoids do not mimic the dormant tumor cell state. The key to curative cancer treatment is to therapeutically target and eliminate the disseminated dormant tumor cells that eventually reestablish tumors at secondary sites and are ultimately the cause of death. Some therapies will target actively dividing, as well as dormant tumor cells; but
Conclusions
In this chapter we have tried to highlight the current state of the tumor organoid field, which is advancing at an astronomical rate. Mini-gut and mini-tumor organoids have revolutionized our understanding of the molecular mechanisms that underlie transition from normal epithelial stem cell to cancer cell, identify the molecular drivers of cancer cells, and predict their susceptibility to anticancer drugs. The ability to conduct drug screens on dormant patient tumor cells is the next frontier
Acknowledgments
We thank Gavin Mitchell for generating the artwork, and Nancy Amin and Jordane Malaterre for the confocal images. Funding is gratefully acknowledged from the National Health and Medical Research Council of Australia (566679 & APP1099302); Melbourne Health project grants (605030 & PG-002) and early career researcher grant (GIA-033); Cancer Council Victoria project grant (APP1020716) and Fellowship; and Cardiff University.
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