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Seminars in Pediatric Surgery (2006) 15, 284-292

tem cells: Tissue regeneration and cancer

onika Tataria, MD, Scott V. Perryman, MD, Karl G. Sylvester, MD

rom the Department of Surgery, Stanford University School of Medicine and Lucile Packard Children’s Hospital,

alo Alto, California.

Regenerative medicine is the promised paradigm of replacement and repair of damaged or senescenttissues. As the building blocks for organ development and tissue repair, stem cells have unique andwide-ranging capabilities, thus delineating their potential application to regenerative medicine. Therecognition that consistent patterns of molecular mechanisms drive organ development and postnataltissue regeneration has significant implications for a variety of pediatric diseases beyond replacementbiology. The observation that organ-specific stem cells derive all of the differentiated cells within agiven tissue has led to the acceptance of a stem cell hierarchy model for tissue development,maintenance, and repair. Extending the tissue stem cell hierarchical model to tissue carcinogenesis mayrevolutionize the manner in which we conceptualize cancer therapeutics. In this review, the clinicalpromise of these technologies and the emerging concept of “cancer stem cells” are examined. A basicunderstanding of stem cell biology is paramount to stay informed of this emerging technology and theaccompanying research in this area with the potential for clinical application.© 2006 Elsevier Inc. All rights reserved.

INDEX WORDSStem cells;Cancer stem cells;Tissue regeneration

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Throughout development from conception through post-atal growth, organogenesis proceeds as an orderly progres-ion of cellular division, differentiation, and senescence.rom totipotent gametes to pluripotent embryonic stem cellsESC) through multipotent postnatal tissue-specific stemells, each successive generation of more committed cells iserived from and carries the same genetic material. How-ver, each of these examples of a stem cell serves to illus-rate the point that, despite possessing identical DNA, cellsecome programmed to vastly different capabilities for dif-erentiation and proliferation by varying their expressionattern of specific genes. Many of these dominant genes ororphogens encode signaling pathways that are conserved

hroughout phylogeny and ontogeny. Interestingly, as wille illustrated, many of these same pathways that drivembryogenesis and organogenesis are re-activated in an

Address reprint requests and correspondence: Karl G. Sylvester,D, Stanford University School of Medicine, Division of Pediatric Sur-

ery, Department of Surgery, Lucille Packard Children’s Hospital, 257ampus Drive, Stanford, CA 94305.

qE-mail: sylvester@stanford.edu.

055-8586/$ -see front matter © 2006 Elsevier Inc. All rights reserved.oi:10.1053/j.sempedsurg.2006.07.008

rgan-specific manner, resulting in dysregulated cell prolif-ration during carcinogenesis. Thus, it is observations suchs these that demonstrate that normal development andissue homeostasis depend on a careful balance between celloss and cell renewal. Accordingly, to maintain this balance,tem cells must possess a capacity for self-renewal as wells differentiation in a tightly controlled system of genectivation and silencing. Stem and progenitor cells performhese biologic processes as the functional units of embryo-enesis and organogenesis and during both tissue homeosta-is and repair.

The defining characteristics of progenitor cells are theirarying capacities for proliferation and differentiation. Thetrictest definition of a stem cell is a cell that must be capablef self-renewal and asymmetric mitosis1-3 (Figure 1). Self-enewal implies the derivation of an exact replica of the orig-nating or founder cell such that the potential of the parent cells not lost through subsequent rounds of cell division.4 Ifelf-renewal is impeded, the ability of postnatal organs, likehe gut, to undergo epithelial renewal every several days is

uickly lost as the stem cell pool becomes rapidly depleted.

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285Tataria et al Stem Cells

n addition, a true stem cell must also be capable of asym-etric mitosis to derive progeny cells with more restricted

otential, usually capable of differentiation to a specializedell function, such as a secretive goblet cell or an absorptiventerocyte, in keeping with the example provided for theut.

In contradistinction to stem cells, progenitor cells areormally more restricted in their growth and differentiationotential, and thus specifically lack the ability for self-enewal. The ESC is a prototypical pluripotent stem cell inhat given the appropriate environment it is capable ofndless self-renewal4 (Figure 1A). In addition, the pluripo-ency of ESC reflects their capacity to derive progeny cellshat can in turn derive specialized cells of each of the threembryonic germ layers: endoderm, ectoderm, and meso-erm3,5,6 (Figure 1A). The totipotent gametes are distin-uished from pluripotent ESC by their capacity to alsoerive the extra-embryonic membranes, including amnion,lacenta, and umbilical cord (Figure 1). Most postnatalissue-specific stem cells are considered multipotent givenheir lineage restriction to the specialized cells of the organf origin (Figure 1B). A closer examination of recent ad-ances in ESC biology will illustrate the promise and con-roversy that surround this potent cell type.

The potential of stem cell biology came acutely into

igure 1 Embryonic and postnatal stem cells. (A) An oocyte isertilized and becomes a totipotent zygote that is capable of dif-erentiating to all possible cell types, including the extra-embry-nic tissue of the placenta and cord. The zygote undergoes cellivision until it becomes a blastocyst. Pluripotent ESC are derivedrom the inner cell mass of the blastocyst and are capable ofifferentiating into all three germ layers. ESC undergoelf-renewal and asymmetric mitosis to derive more committedrogenitor cells. Progenitor cells proliferate and terminally differ-ntiate to effector cells. (B) Postnatal stem cells are also charac-erized by self-renewal and asymmetric mitosis. Postnatal stemells include tissue-specific stem cells, such as neural stem cellsNSC), hematopoietic stem cells (HSC), and mesenchymal stemells (MSC). These cells give rise to terminally differentiated cellypes. (Color version of figure is available online.)

ocus following the widely publicized first large mammal a

loning experiment in 1997. In the case of Dolly, an entiredult ewe was successfully cloned as an exact phenotypicnd genetic match of its founding organism.7 This achieve-ent was the proof of principle experiment that DNA is

onserved during the development of complex multicellularrganisms, and could potentially be used to regeneratehole tissue and organ systems. Not only did this bring

tem cell research to the attention of the public, but it alsorovided new impetus to the prospect of regenerativeedicine through stem cell research.7 Scientists began to

onceptualize that nuclear transfer, the technology thatreated Dolly, could be utilized to create the raw materialo replace defective or senescent tissue8-11 (Figure 2).he ability to recapitulate an adult organism from aingle cell has also raised a heated political and ethicalebate regarding the use of nuclear transfer to createutologous ESC.12,13

igure 2 Stem cell nuclear transfer and reproductive and ther-peutic cloning. (A) This diagram demonstrates Nuclear Transfer,ene Therapy, and Cell Transplantation as a possible clinically

pplicable paradigm for genetic and subsequent phenotypicorrection.16 (B) This diagram illustrates the divergent pro-esses of Reproductive and Therapeutic Cloning. The commonteps of somatic cell nuclear isolation and injection to annucleated oocyte are demonstrated. (Color version of figure is

vailable online.)

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286 Seminars in Pediatric Surgery, Vol 15, No 4, November 2006

SC and nuclear transfer

SC are totipotent cells that can be derived from the innerell mass (ICM) of a blastocyst during gastrulation14 (Fig-res 1 and 2). When separated from the remainder of thelastocyst, with concomitant inherent arrest of further em-ryonic development, the ICM can be maintained in aargely undifferentiated state through the addition of spe-ific growth factors. In culture, ESC often form small ag-regates called embryoid bodies (EB) in which early differ-ntiation that parallels initial germ layer formation of thembryo can develop.11,14 ESCs represent a potential sourcef cells with almost unlimited self-renewal and differentia-ion capacity. These cells are able to give rise to all of theomatic and germ line cells of the fully developed organ-sm.10,11,14,15 The ESC is the prototypical stem cell as de-ned by its ability to indefinitely expand, self-renew (un-ifferentiated progeny), and give rise to more specializedrogeny cells.

Somatic cell nuclear transfer (SCNT) is a processherein the nucleus and genetic material of a postmitotic

omatic cell is injected into an unfertilized, enucleated oo-yte7,11,16 (Figure 2A). Through this nuclear manipulation,

blastocyst can be achieved which may realize one ofeveral fates. If the blastocyst is transferred to a receptiveaternal surrogate, fully replicated progeny can be

chieved in a process of reproductive cloning7,17 (FigureB). Alternatively, if the inner cell mass is isolated andeparated from the blastocyst, then undifferentiated ES cellsan be derived.2,16 This pluripotent ball of cells is capable ofeproducing individual cells and, therefore, tissues of theostnatal organism from which it was derived in a processnown as therapeutic cloning11,14,15 (Figure 2B). This pro-ess could potentially allow each individual to have anutologous source of their own fully immune-compatibleSC created. It is important to distinguish the divergentrocesses and outcomes of reproductive and therapeuticloning from the same technique of nuclear transfer. Stillncertainty persists as to the true histocompatibility oflonally derived ESC, given the persistence of maternalitochondrial DNA in the recipient enucleated oocyte.17,18

In 2002, the proof of principle experiment for therapeuticloning was provided by an experiment to correct the geneefect in Rag2 immune-deficient mice (complete lack of Bnd T cells)19 (Figure 2A). Through a combination of nu-lear transfer to create MHC-compatible ESC, homologousecombination for genetic correction, in vitro differentiationo hematopoietic stem cells (HSC), and transplantation toag2 recipients, mice were successfully genetically andhenotypically corrected19 (Figure 2A). Whereas this set ofxperiments demonstrates the utility of nuclear transfer forherapeutic cloning, there are equally numerous reports nowemonstrating the frailty of nuclear transfer for reproduc-ive cloning. Reproductive cloning has been found to beostly a highly inefficient process, moreover producing

bnormal progeny in what is becoming known as the “large

ffspring syndrome.”20 In whole organism cloning, there e

ppears to be epigenetic derangements resulting in fetalvergrowth, placental defects, and a myriad of commonkeletal abnormalities.20-22 For example, the regulated ex-ression of several imprinted genes whose pattern of ex-ression is determined by the parental gamete that contrib-ted them may be disrupted.17,20 Examples of dysregulatedmprinting are the IGF2 (paternal allele expressed, maternalllele repressed) and IGF2r (maternal allele expressed, andaternal allele repressed) genes, that when either loci man-fests maternal and paternal allele coexpression may lead tovergrowth syndromes and cancer. Interestingly, expressionf both copies of the imprinted genes IGF2/ IGF2r has anown association with both Beckwith-Wiedeman Syn-rome and Wilms’ tumor.23,24

Despite the unquestioned totipotency of ESC, there areumerous unanswered biologic questions as to the regula-ion of their growth and differentiation. The safety profile ofnselected ESC for transplantation has demonstrated dys-egulated cell growth with transplantation to the immune-ompromised host, resulting in teratoma formation.25 Thisxample speaks to the need to explore strategies of ESCredifferentiation or selection for lineage specification be-ore attempts at in vivo use. Current strategies under inves-igation seek to predifferentiate ESC in vitro before func-ional in vivo testing.26-28 Furthermore, gene profiling ofany stem cell lines for master transcription factors, like thect4 and Nanog genes, are underway in an effort to under-

tand the signals controlling cell proliferation and differen-iation in both ESC and postnatal stem cell sources (ie,SC).29-31 Given the wide variety of both genetic variabil-

ty and epigenetic changes that occur in ESC, large numbersf cell lines in addition to those with current federal ap-roval for study are needed to successfully study and un-erstand these complex biologic processes.21,22,32 Beyondhe use of ESC for replacement biology is their potentialtility for the study of the genetic basis of human disease.he current United States (US) government moratorium on

ederal funding for human ES studies stands to significantlympede the pace at which this critical work can proceed.

In addition to the ethical concerns over the potential usef discarded embryos for the derivation of ESC, both prac-ical and biologic barriers to wide applicability can be fore-een. For example, culture techniques for human ESC haventil recently required a xeno-culture feeder cell system,hich would meet with considerable FDA restrictions.16,33

rom a practical standpoint, even long-term batch culturesf allogeneic cells would be beset by histocompatibilityarriers to effective and widespread ESC transplantationtrategies. This issue further speaks to a current shortcomingf strategies to “scale up” ESC production for clinical usenless autologous ESCs are created on an individual needasis via nuclear transfer for example. An alternate strategyould be to create large-scale banks of ESC lines, most

ikely derived from in vitro fertilization blastocysts, forLA typing and cell matching to potential patient recipi-

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ostnatal somatic stem cells

issue-specific stem cells have long been recognized to existn postnatal and adult animals.15 In their tissue of residence,hese cells function as lineage-committed progenitors that giveise to cells capable of highly specialized tasks that make uphe tissue. Tissue-specific stem cells were first described in theematopoietic system by Gilbert and Lajtha in 1964.34 Theyntroduced the concept of a multipotent stem cell capable ofoth self-renewal and giving rise to more differentiated prog-ny.34 These long-term repopulating stem cells are, by def-nition, capable of the complete reconstitution of all theifferentiated cell types in the hematopoietic system35-37

Figure 3A). From this initial discovery has grown theoncept of a “cellular hierarchy” within tissues (Figure 3).

At the apex of this hierarchy is a resident tissue stem cell.his cell is undifferentiated yet capable of an asymmetricitosis, producing a replica cell and a more committed

rogeny cell.38 To date, the hematopoietic system remainshe best characterized postnatal stem cell compartment and

igure 3 Cellular hierarchy within the hematopoietic and intes-inal tissue compartments. (A) In the hematopoietic system, theong-term and short-term repopulating stem cells give rise to earlyrogenitor cells, which are highly proliferative and are known asransit amplifying cells. They are able to divide a limited numberf times before terminally differentiating to effector cells (eryth-ocytes, macrophage, platelets etc). (B) In the gut, stem cells areocated near the base of the Crypts of Leiberkuhn. Gut stem cellsifferentiate as they migrate upwards to the surface epithelium.efore terminally differentiating into absorptive enterocytes or

ecretory goblet cells, progenitor cells give rise to transit ampli-ying cells, the immediate precursors to the effector cells. (Color

oersion of figure is available online.)

hus serves as a model system to which other stem cellystems are often compared. As such, it is instructive toave a working knowledge of the cellular hierarchy thatxists within the bone marrow38 (Figure 3A). The mostrimitive cell types at the pinnacle of the hematopoieticystem are the long-term and short-term repopulating HSChat give rise to more differentiated progeny; specifically,he common myeloid (CMP) and common lymphoid pro-enitor (CLP) cells. These cells, known as transit amplify-ng (TA) cells, are highly proliferative and can, in turn, giveise to fully differentiated blood cells. In contradistinction tohe HSC, TA cells are not capable of self-renewal. The HSCs therefore necessary and sufficient for long-term reconsti-ution of ablated bone marrow.

A similar concept of tissue stem cell hierarchy exists inhe gut, where enteric crypt progenitor cells are also able toive rise to the multitude of differentiated gut epithelial cellypes (Figure 3B). In this system, stem cells are found nearr at the base of invaginations within the epithelial lining ofhe gut, termed Crypts of Lieberkuhn.39 Similar to the HSC,he gut stem cell is capable of self-renewal and the deriva-ion of TA progenitor cells. In contrast to the hematopoieticystem, the subsequent divergence of lineage-committedA cells in the gut has not been defined. As enteric pro-enitor cells migrate upward through the epithelium towardrojections known as villi in the small intestine or towardhe surface epithelium of the colon, they differentiate intounctional absorptive and secretory intestinal cells.40 Theseffector cells are eventually shed into the gut lumen and areeplaced by new cells as the cycle of migration, functionalifferentiation, and loss is initiated repeatedly by the nichef renewing enteric progenitors. Whereas cells within aingle crypt may be derived from a single parent stem cell,ultiple crypts contribute to the surface epithelium of a

ingle villus, resulting in a multiclonal population of termi-ally differentiated effector cells lining the enteric surfacepithelium. Therefore, as in the hematopoietic system, theut stem cell is necessary and sufficient for the regenerationf the intestine following injury and its maintenance duringeriods of homeostasis.

In addition to the hematopoietic system and intestine, stemells are also thought to exist in mesenchymal tissue. Specif-cally, there is a second population of unique progenitor cells inbone marrow) BM called the mesenchymal stem cellsMSC).9,41-43 The MSC was originally believed to representhe supportive cell substrate for the HSC, but was later foundo have the capacity to derive multiple mesenchymal line-ges.44 Much work has been done in an effort to isolate androspectively define the cell type that derives from BM andives rise in vitro to adipocytes, chondrocytes, and osteo-lasts.44-56 MSC hold the promise of becoming a highly ben-ficial biologic tool with potential clinical applications for theegeneration of connective tissue. Work to isolate and expandhe MSC has led to the finding that a similar cell type may alsoxist in adipose tissue, trabecular bone, skeletal muscle, amni-

tic fluid, and umbilical cord perivascular tissue.45-48 The rev-

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288 Seminars in Pediatric Surgery, Vol 15, No 4, November 2006

lation that an undifferentiated “stem cell” may exist in mul-iple locations speaks to the relevance of the niche inaintaining stem cells in a relatively undifferentiated state

ntil cued to provide their requisite specialized progeny cells.dditionally, because connective tissue is relatively mitotically

nd metabolically quiescent, in contrast to the gut, structuralissue engineering applications for MSC have garnered earlyttention. Since MSC demonstrate sufficient cellular pheno-ypic plasticity, their potential for use as raw material for tissuengineering would seem a logical extension of this biology.

rospective clinical applications ofellular therapy

ematopoeietic stem cell transplantation has been used toreat a number of diseases and exemplifies the use of tissue-pecific stem cells in clinical application. Three separate trialsor autologous transplantation of purified HSC were performedn patients with multiple myeloma, non-Hodgkins lymphoma,nd metastatic breast cancer, respectively. The purification andransplantation of HSC markedly reduced tumor burden in allf these patients, and short-term outcomes were favorable.49-51

dditionally, preclinical trials and animal studies havehown that HSC transplantation may be a promising therapyo treat a number of autoimmune diseases.52,53 Following

yeloablation and the deletion of self-reactive clonal Tells, hematopoietic reconstitution through HSC transplan-ation may provide a reasonable alternative to immune sup-ressive therapies for either autoimmune illnesses or postolid organ transplantation.

Depending on the specific characteristics of the stem cellopulation under study, they may have significant utility invariety of clinical applications.41,54-56 Several therapeutic

trategies are immediately apparent which may exploit thenique capacity for self-renewal, proliferation, differentia-ion, and wide distribution or homing. For example, itould be very appealing to adopt a gene transfer strategy

nto HSC or MSC, followed by transplantation and tissueepopulation.55 This same strategy has been successfully dem-nstrated in murine systems as proof of principle (see Rag2xample above and Figure 2) and for the treatment of theuman genetic disease Osteogenesis Imperfecta (OI).19,57,58 Inhe latter, human MSC transplantation was performed in OIatients with a resultant decrease in the overall fracture rate.umerous heritable gene defects and other acquired dis-

ases would be seemingly amenable to this approach.54,55

dditionally, postnatal somatic stem or progenitor cellsould also be utilized in a form of cellular therapy for localissue repair and regeneration.8,59,60 MSC could, for exam-le, be implanted locally to promote or augment repair oregeneration of a fractured or osteoporotic bone.41,61,62

If cellular therapy is to become a reality, many of theame hurdles of graft immunology that face solid organransplantation will have to be addressed. Most experiments

hat have been done with HSC as a cellular source have thus n

ar involved a marrow ablative regimen such that hemato-oietic chimera are created.63-65 This strategy has providedhe parallel benefit of tolerance to the intended therapeuticellular transplant. If a cell source other than HSC is in-ended to be used, this same marrow replacement strategyuickly becomes ineffective.

tem cells and cancer

espite a great deal of excitement regarding the potentialor clinical application of stem cells, caution before imple-entation must be exercised given the implications of un-

egulated cell growth and differentiation upon transplanta-ion to humans. For example, undifferentiated embryonictem cells form teratomas when transplanted to an immune-ompromised host.66 This observation was one of the first tomply a possible relationship between the molecular prop-rties of stem cells that render them simultaneously highlyroliferative and multipotent with the potential for tumorormation. There is accumulating evidence that loss of con-rol over normal tissue repair or renewal mechanisms mayead to malignant transformation.

The association between cancer and persistent inflamma-ory or regenerative states may be a reflection of this possibil-ty.67-69 Moreover, highly conserved morphogen signalingathways, such as Wnt/�-catenin, Hedgehog, Jagged/Notch,GFs, and the BMP/TGF-� superfamilies, are all recapitulated

n the process of postnatal tissue restoration, and when dys-egulated, perhaps initiate carcinogenesis (Figure 4). Interest-ngly, consistent with this relationship between tissue repair

igure 4 Interplay of dominant morphogen pathways in stemell maintenance and cancer development. Wnt, Hh, and Notchignaling in the intestinal crypt controls progenitor cell prolifera-ion and differentiation. The Wnt pathway leads to increased pro-iferation of progenitor cells which is inhibited by Hh signaling.otch signaling leads to increased differentiation of crypt progen-

tors. Loss of the careful balance between these signals leads toncontrolled progenitor cell proliferation and may lead to malig-

ancy. (Color version of figure is available online.)

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nd cancer is the observation that long-lived organismsnd/or tissues with high rates of cycling stem cells, such ashe prostate, breast, gut, and blood, also have the highestancer incidence.70 In contradistinction, juvenile cancersccur in rapidly expanding neural tissue (medulloblastoma,euroblastoma) and parenchymal organs (hepatoblastoma,ephroblastoma) that are undergoing tissue expansion as aostnatal extension of the embryonal processes initiateduring organogenesis.

Understanding how tissue regeneration may lead to can-er may require a shift in perspective toward viewing theegenerative processes that govern tissue restoration as pro-iding opportunities for carcinogenesis (Figure 4). Al-hough tumors are populated by a heterogeneous group ofells, only a small subset of cells appears to have the abilityo initiate and maintain this cancerous growth70-73 (Figuresand 5). Previously, tumors were thought possibly to derive

rom any, if not all, of the cells contained therein. Based onhis “stochastic model” of tumor development, any cellithin a heterogeneous tumor population could both initiateew tumors and propagate them as metastases (Figure 5).owever, serial tumor transplant experiments have shown

hat only a few cells are able to fully recapitulate tumors70-72

Figure 5). Out of this changing paradigm was derived theoncept of tissue stem cells serving as cancer stem cells.

The prospective identification of cancer initiating cellsithin the circulating tumors of the hematopoietic system

nd within solid organs, such as the brain and breast, hasroadened our focus to include an understanding of the cellsnvolved in initiating and propagating cancer.71-74 Frombservations such as these, an alternative “hierarchical hy-othesis” of tumorigenesis has been proposed that holds thenitiating cells of tumors are “stem cells” that have been

igure 5 Stochastic versus hierarchical model of carcinogene-is. In the stochastic model of carcinogenesis, cells from a heter-geneous tumor are each able to randomly give rise to new can-ers. In the hierarchical model of carcinogenesis, only a smallroup of cells, known as the cancer-initiating cells, are able to giveise to new tumors. (Color version of figure is available online.)

ransformed (Figures 4 and 5). In this model, the tumor is t

till populated by a heterogeneous group of cells, but only amall subset of cells have the ability to initiate and maintaincancerous growth (Figure 5). In the same way that a tissue

tem cell populates a tissue hierarchy, cancer-initiating cellsr cancer stem cells are the subset of cells that give rise tohe heterogeneous population of cells within a tumor. It iselieved, due to their longevity and self-renewing charac-eristics, that stem cells have a greater propensity to accu-ulate carcinogenic mutations. That is, these cells have

oth the molecular potential and the opportunity to becomeransformed. Thus, tumors, like tissues, may also be gov-rned by a cellular hierarchy that is conveyed on a smallong-lived subset of cells by the gain-of-function ability toelf-renew. This population is often found to represent lesshan 5% of the tumor mass and is necessary and sufficientor regeneration of the tumor.73,75-78

There is growing evidence that the pathogenesis of can-er involves the subversion of normal tissue repair mecha-isms and stem cell signaling pathways. Because stem orrogenitor cell division is triggered during either developmentr persistent inflammatory and regenerative states, it is hypoth-sized that the pathways that govern these processes may alsoncrease the likelihood of transformation (Figure 4). Severalf the major morphogenic pathways, such as HedgehogHh), Notch, and Wingless (Wnt), that are known to benvolved in stem cell proliferation during development andegeneration, have more recently been implicated in severalancers and thus serve to illustrate the point79-85 (Figure 4).n many stem cell compartments, such as the hematopoieticnd enteric systems, Wnt/�-catenin signaling has the netffect of regulating progenitor cell proliferation and multi-otency.86-89 The Hh and Notch pathways balance the pro-iferative potential of the enteric stem cell, providing signalso check proliferation and promote differentiation. When ah or Notch loss-of-function or Wnt/�-catenin gain-of-

unction mutation occurs within the crypt stem cell, controlver proliferation of this compartment is lost, leading to anverall expansion of these cells.87 These cells derive daugh-er cells that have already accumulated the mutation, thusonferring a proliferative advantage to the mutated progen-tors and their progeny over surrounding cells. The cumu-ative effect of the selective expansion of these self-renew-ng, mutated progenitor cells is a larger pool of cellsusceptible to additional mutagenesis and malignant trans-ormation. An illustration of this progression can be seen inhe intestine, where the phenotypic consequence of theseutations is a polyp or adenoma (Figure 4).How then does the stem cell hypothesis of carcinogene-

is fit with common pediatric embryonal tumors? It is be-oming increasingly recognized that there is an inverseelationship between the regenerative capacity of stem cellompartments and the aging or quiescence of a given tis-ue.68 Moreover, this relationship can be extended to theusceptibility of aging tissues to malignancy. Several inves-igators have begun to describe a shift in the balance be-

ween stem cell proliferation and senescence gene pathways

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290 Seminars in Pediatric Surgery, Vol 15, No 4, November 2006

ssociated with aging. For example, adult tissue stem cellsay be more sensitive to senescent pathways that must be

ctivated as a cell compartment ages and accumulates ge-etic or epigenetic abnormalities to prevent progression toncogenesis.90 By the same argument, embryonal tissue, orore specifically, embryonal tumors of the young that arise

rom highly proliferative progenitor cells may be somewhatore sensitive to oncogenic activation, and relatively resis-

ant to dominant tumor suppressor/cell senescence pathwaysike p53, Rb, and PTEN.75-91 This may in part explain theifferences in widespread loss-of-function mutations in the Rbnd p53 pathways frequently seen in adult tumors, yet nor-ally expressed in juvenile embryonal tumors. Taken together,

hese observations would suggest that the late life phenotypearked by a diminished capacity for physiologic regeneration

s an intended and necessary consequence to prevent carci-ogenesis from accumulating mutations. It is therefore noturprising that the increased risk of epithelial cancers tendso accompany the aging process, whereas nonepithelial can-ers (those associated with genetic predisposition or earlyigh-risk exposures) tend to predominate in children.70

The histologic variants of liver tumors in adult (hepato-ellular carcinoma, HCC) and pediatric patients (hepato-lastoma, HB) serve to further illustrate the point. Althoughverall rare, HB is the most common type of pediatric liverumor. HB originates from bipotent liver precursor cellsermed hepatoblasts. The Wnt/�-catenin signaling pathwayhat plays an established role in the growth of the liverrimordia during normal hepatic development is mutated inp to 75% of HBs.92 At the same time, only rare mutationsn the tumor suppressor pathways p53 and RB have beenetected in these embryonal tumors. In contradistinction,CC is one of the most common cancers worldwide, an

ncidence that is linked to the overall prevalence of andssociation with chronic infection by hepatitis B (HBV) and

(HCV) viruses.93 A crucial role for re-activating muta-ions in the Wnt/�-catenin pathway in 30% to 40% of HCCsas been reported in response to the chronic inflammatorynd regenerative stimuli provided by the hepatitis viruses.93

nlike HBs, the HCCs are also accompanied by a muchigher loss of the tumor suppressor pathways p53 and RB.93

hus, perhaps the hepatoblast is more susceptible to gain-f-function mutations for growth and proliferation (Wnt/-catenin), thus obviating the need for tumor suppressor

oss-of-function (p53, RB). These inherent differences maylso have relevance to the choice and effect of varioushemotherapeutic agents. The presence of an intact p53umor suppressor locus is just one example perhaps thatccounts for the high sensitivity of most embryonal tumorso chemotherapeutic agents.

ummary

tem cell biology holds great promise for the potential

reatment of numerous human disease states. From their use

s the building blocks for tissue replacement, to the lessonsf the molecular pathways controlling growth and differen-iation, there remains much to be learned. However, there iseason for optimism that the unexpected discoveries madelong the way may produce previously unforeseen benefits,uch as an improved understanding of tumorigenesis. Witheeper insight may flow more innovative therapies based onsound appreciation for the basic biologic processes under-

ying human development and disease as a function of stemell biology. All those who care for children and the mal-dies that afflict them may benefit from a keener insightained through the lessons of these biologic buildinglocks.

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