, . 181: 127–129 (1997)

EDITORIAL

FIELD CHANGE, CLONALITY, AND EARLYEPITHELIAL CANCER: POSSIBLE LESSONS FROM p53

There are considerable data suggesting that p53abnormalities are the most prevalent molecular changesin human (and probably other animal) neoplasia. Fre-quently there are mutations in the central region of thep53 gene encoding the unique sequence-specific DNAbinding domain, such that the transcriptional regulatoryproperties of p53 are lost and with it the critical controlfunctions of p53 in growth arrest and apoptosis. Possibleother actions of p53 that do not involve transcriptionalregulation must also be considered, but there is aclear predilection for mutations to occur in the regionencoding the DNA binding domain.1 That this is ofmajor significance in neoplasia is now well established,although the possible role of p53 in other regulatorysystems is of potential importance. Although manyexceptions are now known, there is in general an associ-ation between abnormal (i.e., increased) p53 proteinexpression and neoplasia, and in many (but not all) casesthis is due to mutation.2 In certain situations, thedetection of p53 protein may be of some diagnosticutility3 and has been argued to be of prognostic signifi-cance (reviewed in Dowell and Hall4). A further criticalpoint is that the induction of increased normal p53protein expression occurs physiologically as a responseto genotoxic, hypoxic, and possibly other forms ofstress.5,6 This may act as a measurable and objectivemarker to recent exposure of cells to damaging insults.The paper by Mandard et al.7 in the current issue of

the Journal of Pathology raises interesting issues con-cerning the basic biology of early epithelial changes inneoplasia. This study, which is paralleled by other recentstudies in the squamous epithelium of the aerodigestivetract,8–10 indicates that there are alterations in theexpression of p53 protein that may precede overt neo-plastic change and that may be widespread, often occur-ring at some distance from any tumour. Furthermore,abnormalities of p53 expression can occur in the absenceof overt neoplasia (ref. 11 and Ogden, unpublishedobservations). Other biochemical evidence of fieldchange has been reported.12,13 The mechanistic basis andsignificance of this field change are at present notunderstood, but the idea of field change associated withcancer is not new.Based on his extensive morphological observations,

Willis14 argued that epithelial tumours generally arisefrom fields (of various size) and enlarge by cellularproliferation and progressive neoplastic conversion of

adjacent populations within those fields. This is in starkcontrast to the earlier unicentric views of Cohnheim andRibbart (see ref. 14, pp. 105–124 and pp. 204–205).Willis cited, for example, the multicentric nature ofmany basal cell carcinomas and cutaneous and oralsquamous tumours as support for this ‘field origin oftumours’. Furthermore, he discounted the experimentaldata of Berenblum (the seminal observations on initia-tion and promotion in mouse skin), which he arguedwere ‘at variance with the histological facts’! Withregard to the aerodigestive tract, Danely Slaughterargued for multicentric tumour origin in relation to bothspace and time.15The possible origins of multiple foci of such cells are

diverse. Multiple independent origins are of coursepossible, from multiple initiated cells. Support for thiswould come from molecular studies indicating differentprofiles of objective molecular changes. For example,Chung et al.16 have analysed the mutations present inprimary head and neck tumours and second primarytumours. They found frequent discordance, such thatthe presence or location of the mutations in the initialprimary tumours differed from that seen in subsequenttumours. Alternatively, it is quite conceivable for mul-tiple separate foci to have originally developed from asingle clone. Evidence for this comes from the modellingdata of Liu and Wright17 and also from molecularstudies,18 indicating that in separate islands of in situtumour there can be identical molecular profiles. Thus, itis quite conceivable for multiple separate foci to haveoriginally developed from a single clone.While the ideas of Willis and Slaughter were widely

held, the studies of tumour clonality during the 1960sand 1970s led to the widespread belief that tumours arisefrom a single transformed cell.19,20 A concern aboutmany of these studies relates to the issue of ‘patch size’,which confounds the analysis of clonality using allelicmarkers in heterozygous females. X-chromosome in-activation occurs relatively early in development, suchthat there may be large patches of cells derived from acommon progenitor cell, all having the same X chromo-some inactivated. In the absence of clear data on patchsize in human adult tissues, the clonality data employingX-inactivation approaches should be considered criti-cally. Furthermore, the idea that multiple cells aretransformed in fields and that the data on clonalityreflect the subsequent outgrowth of a dominant cloneremain largely untested,21 although some evidence forclonal evolution exists.22 Indeed, these ideas now takeon more credence with the recent observation thatcolonic adenomas are frequently polyclonal.23 These

Addressee for correspondence: P. A. Hall, Department ofMolecular and Cellular Pathology, University of Dundee, DundeeDD1 9SY, U.K.

CCC 0022–3417/97/020127–03? 1997 by John Wiley & Sons, Ltd.

authors took advantage of a unique case in which apatient who was an XO/XY mosaic also had familialadenomatous polyposis (FAP), and used in situ hybridi-zation for Y-chromosome sequences to demonstrate thatthe colonic adenomas were frequently (76 per cent)polyclonal. Using a statistical argument based on thefrequency of polyclonality and the expected frequency ofcollisions between clonal adenomas, it is convincinglyargued that these polyclonal tumours are unlikely to bethe result of collisions. It may be that field effects makethe adenomas cluster with initially multiple clones con-tributing to the tumour and with the later emergence ofdominant clone(s). The fact that transformed cells canelaborate growth factors and survival factors that mightfacilitate growth of nearby cells provides a possiblemechanistic basis for this.12Observations of p53 overexpression and mutation in

fields around tumours make more tractable the issue offield change in the epithelium of the aerodigestive tissuesand elsewhere. Technical advances now allow the analy-sis of the genotype of micro-dissected areas of histologi-cally normal or hyperplastic epithelium as well as areasof overt dysplasia (of varying grades) and malignanttumour, complementing more established methodsof in situ phenotypic analysis. Immunohistochemicalstudies tell us that p53 protein overexpression occurs inhistopathologically normal mucosa surrounding excisedtumours,8,9 even occasionally when the tumour hasproved to be p53-negative.8,10 This may reflect activa-tion of the p53 pathway as a consequence of exposure toenvironmental and other potential aetiological factors.Consequently this phenotype may act as a biomarker toexposure.8,9 Nakanishi et al.10 have suggested that in theupper aerodigestive tract, smoking is a particular stimu-lus to induction of p53 protein in otherwise normalmucosa. Whilst most cases with p53 protein overexpres-sion in the normal aerodigestive mucosa are found tosmoke and/or drink heavily, not all cases are associatedwith alcohol/tobacco (ref. 9 and Ogden, unpublishedobservations). Moreover, not all studies have found p53expression in adjacent normal mucosa, even whensecond primary tumours have developed, or wherepatients take excessive amounts of alcohol or smoketobacco.24 Further studies will be needed in this complexarea. Recent observations in rodents suggest that theremay be genetically determined variation in the p53response to a given inducer of the p53 pathway (Hallet al., unpublished observations). This may influence theprevalence of detectable p53 overexpression. Further-more, dose response and time courses of p53 inductionindicate that it is often a transient phenomenon.Where molecular analysis has been undertaken, p53

mutations have been reported in a surprising number ofcases where histopathologically normal mucosa has beenexamined. Nees et al.25 frequently found both immuno-histochemical and molecular evidence of abnormalitiesof the p53 protein in head and neck cancer patients.More recently, Brennan et al.11 reported an associationbetween molecular evidence of p53 gene mutation innormal mucosa and recurrence, with no recurrences inthe cases where no p53 mutation was identified. How-ever, their length of follow-up was very short and since

they did not undertake analysis of p53 protein expres-sion, we do not know if there was increased stabilizationof wild-type p53. Notwithstanding this, these findingsprobably help to explain why second malignant tumoursare more common in the head and neck region.26Despite progress from the numerous studies of p53

protein and p53 gene mutation, much remains unknownabout the very early events in oncogenesis and themechanisms and significance of field change. There areimportant unresolved issues. At present, there are fewclear definitions that are universally accepted and whichallow communication of results. For example, fewauthors clearly make the distinction between fieldchange occurring adjacent (or at least close) to a pre-existing neoplasm versus field change in the absence ofneoplasia. Such field changes may be continuous ordiscontinuous areas of some objective morphological orbiochemical change which does not amount to dysplasiaor in situ carcinoma, but which may have significance asa precursor lesion or a biomarker of future neoplasticchange. Another critical area is the epidemiology of fieldchange, which is not well defined and thus we do notknow the true frequency of progression or of regression.Molecular studies may help, but without this basicinformation and clear agreement on terminology, ouroverall understanding of early changes in cancer willremain limited.In conclusion, there is a need to investigate the basic

biology of field change, which is at present poorlyunderstood but is now amenable to study. This maybe of real clinical value with regard to understandingthe aetiology and development of neoplasia and theidentification of at-risk groups.

G R. O* P A. H†*Department of Dental Surgery and Periodontology and

†Department of Molecular and Cellular PathologyUniversity of Dundee

DundeeDD1 9SY, U.K.

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