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The Morphogenetic Code and Colon Cancer Development

2007; Cell Press; Volume: 11; Issue: 2 Linguagem: Inglês

10.1016/j.ccr.2007.01.003

1878-3686

Gijs R. van den Brink, G. Johan A. Offerhaus,

Digestive system and related health

The initiating genetic lesion in sporadically occurring cancers is impossible to identify. The existence of rare inherited cancer syndromes has helped to uncover some of the mutations that can initiate tumorigenesis. Most of these initiating lesions affect genes belonging to morphogenetic signaling pathways. We review the evidence that the cellular fate of individual epithelial cells in the adult is nonautonomous and depends on extrinsic information, just like cells in a developing embryo. Cancer stem cells need to disrupt these extrinsic restraints to gain an autonomous clonal proliferative advantage over neighboring stem cells. The initiating genetic lesion in sporadically occurring cancers is impossible to identify. The existence of rare inherited cancer syndromes has helped to uncover some of the mutations that can initiate tumorigenesis. Most of these initiating lesions affect genes belonging to morphogenetic signaling pathways. We review the evidence that the cellular fate of individual epithelial cells in the adult is nonautonomous and depends on extrinsic information, just like cells in a developing embryo. Cancer stem cells need to disrupt these extrinsic restraints to gain an autonomous clonal proliferative advantage over neighboring stem cells. Multicellular organisms are built from subsets of specialized cell types that are allocated to different organs in a predefined location and quantity. To develop and maintain such a level of complexity, individual cells are subject to stringent conditions that determine when and where they live and what they do (i.e., cell fate). It is therefore a hallmark of cells in multicellular organisms that cell fate is not regulated at the level of the individual cell but at the population level by extracellular signals. As a result, it is not surprising that mutations that disturb this type of intercellular signaling have now been established as important initiators of the genetic cancer cascade in epithelial tissues. Our current understanding of the histogenesis of a colon carcinoma suggests that this process starts with a genetically mutated cancer stem cell that gains a competitive advantage in proliferative capacity over neighboring stem cells. To understand how a colonic stem cell is able to gain such a competitive advantage, it is first important to recognize that cellular fate is controlled by environmental cues in the intestine. The fate of a cell in a developing embryo is determined by its position. The mechanisms used to communicate positional information during development have been the subject of experimental biology for more than a century (Wolpert, 1996Wolpert L. One hundred years of positional information.Trends Genet. 1996; 12: 359-364Abstract Full Text PDF PubMed Scopus (218) Google Scholar). It is much less appreciated that there is important evidence for a similar mechanism of cell-fate regulation in the adult, especially in rapidly dividing tissues. Similar to that of cells in a developing organ, the cellular fate of epithelial cells in the adult gut is regulated by extrinsic signals (extracellular information that is nonautonomous to the cell). This information patterns epithelial-cell fate along two important axes that are addressed separately below. Each stem cell in the gastrointestinal tract generates a variety of more committed precursor cells that undergo a transient phase of cellular proliferation. Each descendant of these precursor cells exits the cell cycle, is allocated to a specific cell lineage, and undergoes a process of maturation into a differentiated epithelial cell that lives a short life before it dies. This cycle of life and death is a linear process from the stem-cell niche toward the compartment of differentiated cells. A cell that migrates along this vertical axis has to make important decisions regarding its cell-cycle status, lineage commitment, maturation status, and death. In a hallmark study by the Gordon laboratory, it was shown that the fate of epithelial cells is regulated in a nonautonomous way (Hermiston et al., 1996Hermiston M.L. Wong M.H. Gordon J.I. Forced expression of E-cadherin in the mouse intestinal epithelium slows cell migration and provides evidence for nonautonomous regulation of cell fate in a self-renewing system.Genes Dev. 1996; 10: 985-996Crossref PubMed Scopus (205) Google Scholar). This group has made use of the fact that each stem cell along the longitudinal axis of the gut produces its offspring as monoclonal populations. The authors produced a series of chimeric mice in which two fertilized zygotes (one wild-type and the other transgenic) or one (transgenic) embryonic stem cell and a fertilized zygote are fused and the clonal offspring of normal and genetically engineered cells can be juxtaposed. In one such chimeric mouse, the cell-cell adhesion molecule E-cadherin was specifically overexpressed in maturing small-intestinal epithelial cells derived of E-cadherin transgenic 129/SV mouse ES cells, whereas cells derived from the wild-type C57Bl/6 blastocyst were normal. As a result, E-cadherin-overexpressing cells produced by transgenic stem cells were migrating side by side with normal epithelial cells on a single villus. Overexpression of E-cadherin considerably slowed migration of transgenic cells. Even though the speed of migration of transgenic cells was much reduced, their maturation status along the vertical axis was completely position dependent and the same as their normal counter parts. This experiment nicely demonstrated that the intestinal epithelial life cycle is extrinsically regulated. Cell fate is not only coupled to the topography of the epithelial life cycle but also dependent on cellular position along a proximal-to-distal axis of the gut tube. Regulation along this second axis ensures that precursor cells generate a module of specialized cell types that suits that particular region of the gut. Cell-fate regulation along the longitudinal axis is a topic that is less well investigated in the adult, especially when compared to our understanding of the regulation along the vertical axis. Two lines of evidence suggest, however, that cell-fate specification along the longitudinal axis may require continuous patterning by epithelial-cell nonautonomous information similar to the vertical axis. The ongoing nature of precursor-cell programming along the longitudinal axis and its dependence on extrinsic factors may be best exemplified by recent findings in patients and animals receiving bone-marrow transplants. Bone-marrow transplants contain stem cells that are able to produce cell lineages specific for many of the recipient's tissues. Bone-marrow stem cells are able to colonize small-intestinal crypts in a clonal manner that is typical for gut stem cells and stably populate adjacent villi with cells of all intestinal epithelial-cellular lineages that appear histologically normal (Jiang et al., 2002Jiang Y. Jahagirdar B.N. Reinhardt R.L. Schwartz R.E. Keene C.D. Ortiz-Gonzalez X.R. Reyes M. Lenvik T. Lund T. Blackstad M. et al.Pluripotency of mesenchymal stem cells derived from adult marrow.Nature. 2002; 418: 41-49Crossref PubMed Scopus (4992) Google Scholar, Rizvi et al., 2006Rizvi A.Z. Swain J.R. Davies P.S. Bailey A.S. Decker A.D. Willenbring H. Grompe M. Fleming W.H. Wong M.H. Bone marrow-derived cells fuse with normal and transformed intestinal stem cells.Proc. Natl. Acad. Sci. USA. 2006; 103: 6321-6325Crossref PubMed Scopus (216) Google Scholar). The interpretation of some of the work performed with bone-marrow-derived stem cells remains controversial, and it is still debated whether stem-cell recruitment and transdifferentiation really occurs, since fusion of recruited bone-marrow-derived stem cells with resident stem cells would be an alternative scenario (Marx, 2004Marx J. Medicine. Bone marrow cells: The source of gastric cancer?.Science. 2004; 306: 1455-1457Crossref PubMed Google Scholar, Pauwelyn and Verfaillie, 2006Pauwelyn K.A. Verfaillie C.M. 7. Transplantation of undifferentiated, bone marrow-derived stem cells.Curr. Top. Dev. Biol. 2006; 74: 201-251Crossref PubMed Scopus (12) Google Scholar). The distinction between fusion and transdifferentiation is an important one because a recruited stem cell that fuses with a resident stem cell would acquire information that is intrinsic to the resident stem cell, a scenario that does not support a nonautonomous control of patterning along the longitudinal axis (although it would not necessarily argue against it). In recent experiments in irradiated mice, most of the bone-marrow-derived stem cells that contributed to intestinal epithelial tissue seemed to do so through fusion with resident stem cells (Rizvi et al., 2006Rizvi A.Z. Swain J.R. Davies P.S. Bailey A.S. Decker A.D. Willenbring H. Grompe M. Fleming W.H. Wong M.H. Bone marrow-derived cells fuse with normal and transformed intestinal stem cells.Proc. Natl. Acad. Sci. USA. 2006; 103: 6321-6325Crossref PubMed Scopus (216) Google Scholar). However, in a study by Houghton et al., 2004Houghton J. Stoicov C. Nomura S. Rogers A.B. Carlson J. Li H. Cai X. Fox J.G. Goldenring J.R. Wang T.C. Gastric cancer originating from bone marrow-derived cells.Science. 2004; 306: 1568-1571Crossref PubMed Scopus (1008) Google Scholar in the Helicobacter-infected stomach, it was shown that when female mice were transplanted with green fluorescent protein (GFP)-expressing male bone-marrow donor cells the donor-derived epithelial cells (GFP-positive, negative for hematopoietic marker CD45, positive for epithelial marker cytokeratin) contained single X and Y chromosomes, indicating that they truly transdifferentiated without cell fusion upon recruitment. The fact that such recruitment and transdifferentiation can occur (however rare it may be) is a proof of principle and a strong argument per se in favor of the existence of a stem-cell niche in adult epithelial tissues. It would mean that the niche contains signals that are nonautonomous to the stem cell and inform and guide the recruited stem cell to ensure the generation of the appropriate cell lineage depending on its position along the longitudinal axis (Figure 1A). More evidence is needed to further clarify this notion. (A) Bone-marrow-derived stem cells have the capability to generate a module of cell types that is typical for a given location along the longitudinal axis of the gut. This illustrates the presence of extrinsic signals from the mesenchyme specifying cell fate along the longitudinal axis at the level of the stem-cell niche. Here these mesenchymal cells are schematically depicted as secreting a mix of extracellular factors specific for that particular location along the longitudinal axis. (B) In areas of metaplasia, cell types are generated that are not appropriate for that specific location along the longitudinal axis. In Barrett's esophagus (B), the esophageal multilayered squamous epithelium is replaced by intestinal-type columnar cell epithelium (note presence of goblet cells) in the setting of chronic inflammation (asterisks indicate inflammatory infiltrate). (C) Such a phenotypic switch in cell-type generation could conceivably be caused by the influence of the inflammatory infiltrate on the composition of extracellular signals in the stem-cell niche. The second example of the dynamic nature of precursor-cell programming along the longitudinal axis is the existence of metaplasias (Slack, 1986Slack J.M. Epithelial metaplasia and the second anatomy.Lancet. 1986; 2: 268-271Abstract PubMed Scopus (62) Google Scholar, Tosh and Slack, 2002Tosh D. Slack J.M. How cells change their phenotype.Nat. Rev. Mol. Cell Biol. 2002; 3: 187-194Crossref PubMed Scopus (343) Google Scholar). Epithelium in areas of inflammation can change into a phenotype that is inappropriate for the location along the longitudinal axis. Such change is known as metaplasia and is easily recognized histologically by the presence of a cell population that is not normally present in the original tissue (e.g., intestinal goblet cells in Barrett's esophagus; Figure 1). Examples in the gastrointestinal tract include epithelial columnar cell and/or intestinal metaplasia of the esophagus in Barrett's esophagus (Figures 1B and 1C), intestinal metaplasia and pancreatic metaplasia of the stomach in atrophic gastritis, gastric mucin cell metaplasia of the duodenum in peptic duodenitis, pseudopyloric metaplasia of the small intestine in Crohn's disease, and Paneth cell metaplasia of the colon in inflammatory bowel disease. It has been suggested that metaplasias might result from somatic DNA mutations. However, a monoclonal proliferation of metaplastic cells would be expected if metaplasia were due to a somatic mutation and metaplasias were multifocal lesions (Thompson et al., 1983Thompson J.J. Zinsser K.R. Enterline H.T. Barrett's metaplasia and adenocarcinoma of the esophagus and gastroesophageal junction.Hum. Pathol. 1983; 14: 42-61Abstract Full Text PDF PubMed Scopus (184) Google Scholar) that were polyclonal in nature (Nomura et al., 1998Nomura S. Kaminishi M. Sugiyama K. Oohara T. Esumi H. Clonal analysis of isolated intestinal metaplastic glands of stomach using X linked polymorphism.Gut. 1998; 42: 663-668Crossref PubMed Scopus (30) Google Scholar). An epigenetic mechanism for the development of metaplasias is therefore more likely. We would hypothesize that differentiation of gut precursor cells is most likely determined by extracellular signals that are altered in the above situations of inflammation due to factors that alter the microenvironment of the precursor-cell niche (Figure 1C). Thus, substantial evidence indicates that extrinsic signals generate the spatial information that is necessary for the generation of the appropriate cell-type repertoire by stem cells along the longitudinal axis and the subsequent maturation and cell death of their descendants along the axis of renewal. Since cellular renewal in the adult gut is regulated by extrinsic information (information that is nonautonomous to the individual epithelial cell) along two different axes, it has important similarities with patterning events in the developing embryo. A small number of evolutionarily conserved signaling pathways have emerged in developmental biology as the master regulators of position-dependent cell fate. Most of these master regulators are morphogens, molecules that act by forming concentration gradients through a tissue (Figure 2) (Lawrence and Struhl, 1996Lawrence P.A. Struhl G. Morphogens, compartments, and pattern: Lessons from Drosophila?.Cell. 1996; 85: 951-961Abstract Full Text Full Text PDF PubMed Scopus (490) Google Scholar). Although most morphogens are proteins, this does not mean that concentration gradients necessarily arise by passive diffusion. In fact, proteins of the Hedgehog and WNT families of morphogens are lipid modified and very poorly soluble (Eaton, 2006Eaton S. Release and trafficking of lipid-linked morphogens.Curr. Opin. Genet. Dev. 2006; 16: 17-22Crossref PubMed Scopus (51) Google Scholar). Many different levels of active regulation of gradient formation probably exist, depending on the morphogen, organism, and tissue (Torroja et al., 2005Torroja C. Gorfinkiel N. Guerrero I. Mechanisms of Hedgehog gradient formation and interpretation.J. 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This protein family shares a highly similar intracellular tyrosine kinase signaling pathway through the RAS-RAF-Mitogen-Activated Protein Kinase and PI3 kinase-AKT signaling pathways (Eswarakumar et al., 2005Eswarakumar V.P. Lax I. Schlessinger J. Cellular signaling by fibroblast growth factor receptors.Cytokine Growth Factor Rev. 2005; 16: 139-149Abstract Full Text Full Text PDF PubMed Scopus (1384) Google Scholar, Schlessinger, 2000Schlessinger J. Cell signaling by receptor tyrosine kinases.Cell. 2000; 103: 211-225Abstract Full Text Full Text PDF PubMed Scopus (3351) Google Scholar). The family includes the fibroblast growth factor (FGF), epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and vascular endothelial growth factor (VEGF) tyrosine kinase receptors. Receptor tyrosine kinases share a common signaling pathway and are perhaps better known to many as growth factors in the adult organism, but all families listed above also have important roles as morphogens during development (Bottcher and Niehrs, 2005Bottcher R.T. Niehrs C. Fibroblast growth factor signaling during early vertebrate development.Endocr. Rev. 2005; 26: 63-77Crossref PubMed Scopus (414) Google Scholar, Coultas et al., 2005Coultas L. Chawengsaksophak K. Rossant J. Endothelial cells and VEGF in vascular development.Nature. 2005; 438: 937-945Crossref PubMed Scopus (743) Google Scholar, Hoch and Soriano, 2003Hoch R.V. Soriano P. Roles of PDGF in animal development.Development. 2003; 130: 4769-4784Crossref PubMed Scopus (425) Google Scholar, Shilo, 2005Shilo B.Z. Regulating the dynamics of EGF receptor signaling in space and time.Development. 2005; 132: 4017-4027Crossref PubMed Scopus (152) Google Scholar). The members of this small number of protein families act in varying constellations to form what has previously been termed a "morphogenetic code" (Hogan, 1999Hogan B.L. Morphogenesis.Cell. 1999; 96: 225-233Abstract Full Text Full Text PDF PubMed Scopus (457) Google Scholar). The morphogenetic code encodes information that enables communicating groups of signaling centers to execute basic programs such as the formation of a field of progenitor cells into, for example, a bud or a gland. Although we still have limited information on the role of morphogens in the maintenance of homeostasis in the normal adult colonic epithelium, the first examples of the role of such molecules are slowly starting to emerge. For example, transcriptional activity of WNT-β-catenin signaling acts to maintain a precursor-cell phenotype in intestinal epithelial cells (Gregorieff and Clevers, 2005Gregorieff A. Clevers H. 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Of the FGF family, Keratinocyte Growth factor (KGF/FGF-7) is expressed by myofibroblast-like cells that underlie the colonic enterocytes, and its receptor (a splice variant of FGFR2) is expressed on epithelial cells (Finch et al., 1996Finch P.W. Pricolo V. Wu A. Finkelstein S.D. Increased expression of keratinocyte growth factor messenger RNA associated with inflammatory bowel disease.Gastroenterology. 1996; 110: 441-451Abstract Full Text PDF PubMed Scopus (115) Google Scholar). KGF acts as a positive regulator of precursor-cell proliferation and the goblet cell lineage (Housley et al., 1994Housley R.M. Morris C.F. Boyle W. Ring B. Biltz R. Tarpley J.E. Aukerman S.L. Devine P.L. Whitehead R.H. Pierce G.F. Keratinocyte growth factor induces proliferation of hepatocytes and epithelial cells throughout the rat gastrointestinal tract.J. Clin. Invest. 1994; 94: 1764-1777Crossref PubMed Scopus (254) Google Scholar). 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Cell proliferation and crypt fission are controlled independently.Am. J. Pathol. 1997; 151: 843-852PubMed Google Scholar). The importance of these morphogenetic pathways in the maintenance of epithelial homeostasis is underscored by the accumulating evidence that mutations that disrupt them are the foremost initiators of the genetic cancer cascade identified to date. Although a massive research effort has identified many genes that are mutated in cancer development, we only know relatively few mutations that initiate this process. Some groups have been able to identify a few of these cancer initiators through the study of families with rare inherited cancer syndromes. Generally, affected members of these families are heterozygous for the inherited mutated gene and develop cancer from a stem cell that is affected by a somatic mutation in the remaining wild-type allele. Many of the inherited mutations in cancer syndromes have now been identified. Some of these mutations affect stability genes, such as mutations in DNA mismatch repair genes that have been found in hereditary nonpolyposis colorectal cancer (HNPCC) (Chung and Rustgi, 1995Chung D.C. Rustgi A.K. DNA mismatch repair and cancer.Gastroenterology. 1995; 109: 1685-1699Abstract Full Text PDF PubMed Scopus (132) Google Scholar, Vogelstein and Kinzler, 2004Vogelstein B. Kinzler K.W. Cancer genes and the pathways they control.Nat. Med. 2004; 10: 789-799Crossref PubMed Scopus (3131) Google Scholar). However, a second class of initiating mutations that has emerged from these families involves mutations in pathways that form part of the morphogenetic code. Mutations in at least three distinct morphogenetic signaling pathways cause gastrointestinal polyposis syndromes. First, mutations in the adenomatous polyposis coli (APC) gene, a critical component of the WNT pathway that acts to restrict its activity cause familial adenomatous polyposis (FAP) syndrome (Kinzler et al., 1991Kinzler K.W. Nilbert M.C. Su L.K. Vogelstein B. Bryan T.M. Levy D.B. Smith K.J. Preisinger A.C. Hedge P. McKechnie D. et al.Identification of FAP locus genes from chromosome 5q21.Science. 1991; 253: 661-665Crossref PubMed Scopus (1931) Google Scholar, Nishisho et al., 1991Nishisho I. Nakamura Y. Miyoshi Y. Miki Y. Ando H. Horii A. Koyama K. Utsunomiya J. Baba S. Hedge P. Mutations of chromosome 5q21 genes in FAP and colorectal cancer patients.Science. 1991; 253: 665-669Crossref PubMed Scopus (1561) Google Scholar). Second, mutations that disrupt expression of the BMP receptor 1A and TGFβ family signaling mediators SMAD4 and endoglin have been found in juvenile polyposis syndrome (JPS) (Howe et al., 1998Howe J.R. Roth S. Ringold J.C. Summers R.W. Jarvinen H.J. Sistonen P. 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In mice with conditional inactivation of the TGFβ receptor type II (a mutation often found in microsatellite unstable colon cancers), no histological abnormalities were found in the colon, the mice had a rate of colonic epithelial proliferation that was comparable to controls, and no spontaneous polyposis developed (Biswas et al., 2004Biswas S. Chytil A. Washington K. Romero-Gallo J. Gorska A.E. Wirth P.S. Gautam S. Moses H.L. Grady W.M. Transforming growth factor beta receptor type II inactivation promotes the establishment and progression of colon cancer.Cancer Res. 2004; 64: 4687-4692Crossref PubMed Scopus (123) Google Scholar). The authors showed in the same paper that the conditional TGFβ RII mutation did substantially enhance mutagen-induced carcinogenesis, more consistent with a role for TGFβ pathway mutations in tumor progression. Interestingly it was recently shown in mice that specific ablation of Smad4 expression in T cells resulted in a JPS phenotype, whereas disruption of Smad4 expression from epithelial cells did not (Kim et al., 2006Kim B.G. Li C. Qiao W. Mamura M. Kasperczak B. Anver M. Wolfraim L. Hong S. Mushinski E. Potter M. et al.Smad4 signalling in T cells is required for suppression of gastrointestinal cancer.Nature. 2006; 441: 1015-1019Crossref PubMed Scopus (243) Google Scholar). This could represent an important example of the importance of the stroma in epithelial cancer initiation. However, the promoters that were used to drive the epithelial-specific expression of the Cre transgene and a dominant-negative form of Smad4 are poorly characterized with regard to their use in the intestinal epithelium, and it is not certain that they target intestinal epithelial stem cells, so these results await further confirmation. 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