Hindawi Publishing CorporationInternational Journal of
DentistryVolume 2012, Article ID 540561, 7
pagesdoi:10.1155/2012/540561Review ArticleOral Leukoplakia as It
Relates to HPV Infection: A ReviewL. Feller and J. LemmerDepartment
of Periodontology and Oral Medicine, University of Limpopo, Medunsa
Campus, Medunsa, South AfricaCorrespondence should be addressed to
L. Feller, [email protected] 13 July 2011; Revised 2
September 2011; Accepted 19 December 2011Academic Editor: Neil S.
NortonCopyright 2012 L. Feller and J. Lemmer.
ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properlycited.Leukoplakia
is the most commonpotentiallymalignant lesionof the oral
cavityandcanbe categorisedaccordingtoitsclinical
appearanceashomogeneousornonhomogenous. Tobaccoandarecanutuse,
eitheraloneorincombinationarethemost commonriskfactors for oral
leukoplakia, but someoral leukoplakias areidiopathic.
Someleukoplakias arisewithineldsof precancerizedoral
epitheliuminwhichthekeratinocytesmaybeat dierent stagesof
cytogenetictransformation.Leukoplakias may unpredictably regress,
may remain stable, or may progress to carcinoma. There is a greater
risk of carcinomatoustransformation of idiopathic leukoplakia, of
non-homogenous leukoplakia, of leukoplakia aecting the oor of the
mouth; theventrolateral
surfaceofthetongueandthemaxillaryretromolarandadjoiningsoftpalate(collectivelycalledhigh-risksites),of
leukoplakia with high-grade epithelial dysplasia, and of
leukoplakia in which the keratinocytes carry cytogenetic
alterationsassociated with carcinomatous transformation. Although
there appears to be some link between human papillomavirus (HPV)and
oral leukoplakia, there is little evidence to support a causal
relationship either between HPV infection and oral leukoplakia
orbetween HPV-infected leukoplakic keratinocytes and their
carcinomatous transformation.1. IntroductionLeukoplakia is the most
common potentially malignantlesion of the oral cavity [1, 2].
Leukoplakia is a term describ-ing a white lesion of the oral mucosa
that cannot be
char-acterizedclinicallyormicroscopicallyasanyotherdenedoraldiseaseentity[3,4].AtaWorldHealthOrganisation(WHO)workshopheldin2005,
itwasrecommendedthatoral leukoplakia be dened as a white plaque of
questionablerisk having excluded (other) known diseases or
disorders
thatcarrynoincreasedriskforcancer[57].Oralleukoplakianeedstobedistinguishedfromotherpredominantlywhitekeratotic
lesions including frictional keratosis and stomatitisnicotina,
which do not have malignant potential [1, 2, 58].About 7090% of
oral leukoplakias are related to smok-ing and areca nut use, either
alone or in combination, andthere is a direct
relationshipbetweenthe frequency andthedurationof cigarette, pipe,
or cigar smokingandtheprevalence of oral leukoplakia [8, 9]. The
factors implicatedin the pathogenesis of idiopathic leukoplakia are
unknown.However it is possible that infection of the oral
epithe-liumwith human papillomavirus (HPV) and excessiveconsumption
of alcoholic beverages may be associated withoral leukoplakia, but
there is little evidence of a
causalrelationshipbetweeneitherHPVinfectionoralcohol, andoral
leukoplakia [6].It has been suggested that a denitive diagnosis
oforal leukoplakiamust beestablishedbyhistopathologicalexclusion of
other keratotic oral lesions that are recognisedas specic entities,
and by exclusion of any aetiological agentsother than tobacco/areca
nut use [7].It istheopinionof theauthorsthat
thesecriteriaareunrealistically limiting, ignoring as they do the
possibleroles of HPV, alcohol, chronic inammation, and low-grade
chronic frictional trauma in the pathogenesis of oralleukoplakia.
Referring to the WHO denition of 2005 above,it is dicult
tounderstandhowsucha denitioncouldgainanyuseful currencysinceit is
soexclusivethat itleaves no rational guidance for the everyday
diagnosis of oralleukoplakia.2. Oral Leukoplakia: Clinical
AspectsAccordingtoits clinical appearance, oral
leukoplakiacanbecategorisedintotwomainclinical types: homogeneous2
International Journal of Dentistryand non-homogeneous. Either type
may occur as an isolatedlesion or as multiple lesions. The
leukoplakic lesion can varyin size from a few millimetres to
several centimeters [1, 2, 512].Homogeneous leukoplakia is a
uniformly white atplaquewithasmoothor relativelysmoothsurface;
non-homogeneous leukoplakia may be nodular or verrucoushaving a
wrinkled or corrugated surface or may be a minglingof white and red
areas termed erythroleukoplakia [7, 10, 11].The clinical appearance
of oral leukoplakia may changeover time. Some homogeneous lesions
may become larger,
ornon-homogeneous,butmostoralleukoplakiaswillremainstable or will
regress, while some few will undergo carcino-matous transformation
[1315].Oral erythroplakia, of all precancerous oral
lesions,carriesthegreatest threat of malignant transformation.
Ithas avelvety-redappearance, andabout 50%of all casesof
erythroplakiaarealreadysquamouscell carcinomataatthe time of
diagnosis. The erythroplakic component of oralerythroleukoplakia is
identical to erythroplakia [16].Proliferative verrucous
leukoplakia, considered to beeitheraclinical subtypeof
non-homogeneousoral leuko-plakia or to be a distinct clinical
entity, is not strongly associ-ated with smoking, is characterized
by multiple leukoplakiclesions that aect wide areas of the oral
epithelium, and mayprogress either to verrucous carcinoma or to
squamous cellcarcinoma[1720]. Inmost cases,
proliferativeverrucousleukoplakiaisrecognisedonlylateinitscoursesinceinitsinitial
stages it is identical to an isolated leukoplakia [1724].3.
Epidemiology of Oral
LeukoplakiaThedatafromepidemiologicalstudiesonoralleukoplakiaisinconsistent,
mostprobablyowingtodierencesincaseselection criteria
(house-to-house surveys, hospital surveys,age, gender, race,
ethnicity, and tobacco use) and in method-ology (diagnostic
criteria, time of follow-up and whether ornot the leukoplakia had
previously been treated) [1, 10, 11].Estimates of theglobal
prevalenceof oral leukoplakiarange from 0.5% to 3.46%, and of the
rates of carcinomatoustransformation of oral leukoplakia from 0.7%
to 2.9% [25].OralleukoplakiaismoreprevalentinIndiawherepersonssmoke
andpractice the habit of tobaccoandareca nutchewing more than
elsewhere [26].Oral leukoplakia is usually diagnosed in middle age,
andits prevalence increases with age. About 10% of oral
leuko-plakias are idiopathic and the greater part of the
remaining90%isassociatedwiththeuseof tobacco/arecanut [26].Males
are more often aected than females probably
owingtothegreaterprevalenceoftobaccousebymales[8].
Thebuccalmucosaisaectedin25%ofcases,themandibulargingiva in 20%,
the tongue in 10%, the oor of the mouth in10%, and other oral sites
account for the remainder [13].The literature on the relationship
between race and oralleukoplakiaissparse.
InaSouthAfricanstudyofarchivedhistopathological material, 86%of
oral leukoplakias werefrom whites, 9% from blacks, and 5% from
Asians,
despitethefactthatthevastmajorityofSouthAfricansareblack[27].
Thisisnoteasytoexplain. Bearinginmindthatthestudy was on
histopathological material, an explanation
maybethatblackstendtopostponeseekingmedicaltreatmentuntil the
leukoplakia has already undergone carcinomatoustransformation, thus
skewingthestatistics awayfromtheleukoplakia[27],
orbecauseblackpeopleinSouthAfricamay smoke less than white
people.4. Epithelial Dysplasia and Oral LeukoplakiaThe reported
prevalence of epithelial dysplasia in oral leuko-plakia ranges
from5%to 25%[8]. Dysplasia is more frequentin non-homogeneous than
in homogeneous leukoplakia[11], and it is probable that dysplasia
is the histopathologicalexpression of genomic and molecular
alterations in a eld ofkeratinocytes [28, 29].Thepresenceof
epithelial dysplasiaisamarkerof themalignant potential of oral
leukoplakia, andthe risk ofanindividual
leukoplakiclesiontoprogresstocarcinomaincreases withthe increase of
the grade of the epithelialdysplasia [11, 12, 14,
30].However,somedysplasticoralleukoplakiascanremainstable or even
regress, while some oral leukoplakias withoutepithelial
dysplasiawill indeedprogress tocarcinoma[5,11, 12]. Inonestudy36%of
dysplasticoral leukoplakiasprogressedtosquamous cell carcinoma, but
asubstantialproportionof 16%of oral leukoplakiaswithout
epithelialdysplasia at the time of initial
biopsyalsoprogressedtocarcinoma[31]. Theriskof
progressiontocarcinomaofleukoplakias with moderate and severe
dysplasia is estimatedtobetwiceas great as for oral leukoplakias
withsimpleepithelial hyperplasia or with mild dysplasia
[14].Treatment of dysplastic oral leukoplakia by excision,
bylaserorbycryosurgery, orbytopical orsystemicchemo-therapy does
not eliminate either the risk of relapse orrecurrence, or the risk
of carcinomatous
transformation[1,5,6].Theestimatedrecurrencerateoforalleukoplakiamay
be as high as 30% [32], and squamous cell carcinomadevelops at
12%of sites of treated leukoplakia [15]. In a studyinvestigating
the pattern of carcinomatous transformation oforal leukoplakia and
oral erythroplakia, 36% of carcinomatadeveloped at the same site,
49% at contiguous sites, and 15%at oral sites remote from the
preexisting lesions [33].It isevident fromthisdatathat somecasesof
treatedoral leukoplakia are unpredictably destined to recur
ortoundergocarcinomatous transformation, andthat thereare not yet
any diagnostic methods available (clinical,histological or
molecular), to condently identify these cases[1].Epithelial
dysplasia in oral leukoplakia is a useful markerof the risk of
carcinomatous transformation and is an impor-tant guide to clinical
management [1, 11, 28]. However,
sincedysplasiacanremainstableforlongperiods, itcannotbeused with
condence as a predictor of carcinomatous trans-formation [14, 29].
Moreover, as the histological exercise
ofgradingofepithelialdysplasiaishighlysubjectivewith
lowinterpersonal and intrapersonal reproducibility [16, 34,
35],andasanincisional biopsycannotberepresentativeofanInternational
Journal of Dentistry 3entire lesion [34, 36], a histopathological
report of any degreeof epithelial dysplasia or of the absence of
epithelial dysplasiamust be viewed with caution.5. The Natural
Course and the MalignantPotential of Oral LeukoplakiaThe
progression of oral leukoplakia to carcinoma is unpre-dictable but
is relatively infrequent with an estimated
overallriskoflessthan2%peryear[5, 6, 13, 26]; ifprogressionoccurs,
it maytakeafewmonthsormanyyears[8]. Thecarcinomatous
transformationof oral leukoplakia is
notpredictablyassociatedwithtobaccosmoking[33], andthefrequency of
carcinomatous transformationof idiopathicleukoplakia is higher than
that of tobacco-associated leuko-plakia [11, 26].Inpopulations
where smoking, the use of smokelesstobacco, reverse smoking, and
the use of areca nut arevery prevalent, most squamous cell
carcinomata arise
frompreexistingleukoplakias;whileinpopulationswith alowerprevalence
of these habits, most squamous cell carcinomataarise de novo
innormal-looking epithelium[26]. It hasbeensuggestedthat squamous
cell carcinoma arising
denovousuallyrunsamoreaggressivecourseandhasalessfavourable
prognosis than squamous cell carcinoma
arisingfrompreexistingleukoplakia[12, 26], but arecent studyhas
demonstrated little dierence [37]. Sometimes squamouscell carcinoma
arises de novo inclose proximity tooralleukoplakias
[8].Non-homogeneous leukoplakia has a greater risk of
car-cinomatous transformation(2025%) thanhomogeneousleukoplakia
(0.65%) [11, 13]. Most leukoplakias eitherremain stable or will
regress [13, 15]. However, if proliferativeverrucous leukoplakia is
considered as a distinct entity, mostsuch cases progress to
carcinoma [18, 24].The rates of progression of large oral
leukoplakias(>5 mm)andofleukoplakiasatsitesinthemouthknownto be
at most risk of developing carcinoma (oor ofmouth, ventrolateral
surface of tongue, and maxillaryretromolar/soft
palateregion)aregreaterthanforsmallerleukoplakias or for
leukoplakias at other sites in the mouth[1, 11, 13, 33, 38]. The
increasedriskof carcinomatoustransformationoforal
leukoplakiaathigh-risksitesisnotentirely a functionof the degree of
dysplasia. It is alsodependent upon as yet undened characteristics
of
thelocationoftheleukoplakiasincetherateofcarcinomatoustransformation
of dysplastic leukoplakias at high-risk sites isgreater than the
rate of transformation of equally dysplasticleukoplakias at other
sites [38].There is evidence that clearly suggests that some
leuko-plakiasarisefromcytogeneticallyalteredtransformedker-atinocytes
withinelds of precancerizedoral epithelium.Keratinocytes of oral
leukoplakia show cytogenetic changesincludingalterations
inthep53tumour suppressor gene,aberrations intheir DNAcontent,
andloss of heterozy-gosity (LoH) at chromosomal regions of
candidate tumoursuppressor genes [20, 33, 3945]. LoHat either 3por
at9poccurs frequentlyinkeratinocytes of oral leukoplakiaand is
associated with carcinomatous transformation ofthese lesions [33,
41, 42, 44, 45]. Additional cytogeneticalterations
tothekeratinocytes intheprecancerizedeldreferredtoabove may result
inthe evolutionof one orseveral keratinocytes containing a complete
set of cytogeneticalterations of a cancerous phenotype, and in the
subsequentdevelopment of squamous cell carcinoma [41, 46,
47].However, some precancerous oral leukoplakias in
whichcytogenetic alterations in the keratinocytes cannot
bedemonstrated, nevertheless undergocarcinomatous trans-formation
[41, 42]. The pathogenic mechanisms that bringabout the progressive
transformation of these keratinocytestocarcinomatouscellsareyet
tobeelucidated. Most oralleukoplakias arebenigninnatureandwill
remainstableorwill regress[28, 32].
Theseleukoplakiasprobablyhaveadierent aetiopathogenesis
toprecancerous leukoplakiasand probably do not have the cytogenetic
characteristics ofprecancerous leukoplakias. What is certain,
however, is thatleukoplakias withmalignant potential andthose
withoutmalignant potential cannot be distinguished clinically
[1].6. HumanPapillomavirus andOral LeukoplakiaHuman papilloma
viruses (HPVs) are strictly epitheliotropicandinfect
eithercutaneousormucosal squamousepithe-lium,
dependingupontheirgenotype[48, 49]. Thosethatinfect mucosal
epithelium have been categorized into high-risk types (e.g.,
HPV-16, 18, 31, 33, and 35) based on theirepidemiological
associationwithcarcinomaof the
cervixuteri,orintolow-risktypes(HPV-6,11, 13,
and32)[50].Thesecategorieshavebeenuniversallyadoptedforuseinstudies
of the oncogenic signicance of HPV infection at allanatomical
regions of the upper aerodigestive tract.Low-riskHPVgenotypes
havebeenimplicatedinthepathogenesis of the benign oral
proliferative epitheliallesions, squamous cell papilloma,
commonwart (verru-cousvulgaris), condylomaacuminatum, andfocal
epithe-lial hyperplasia(Heckdisease);
whilehigh-risktypeshavebeenassociatedwithprecancerousandcancerousoralandoropharyngeal
epithelial lesions [49, 5156].There is an extreme variation in the
reported prevalenceof HPV infection in oral precancerous and
cancerous lesionsranging from0%to 100%[57, 58]. This is owing
todierences insampling andHPVdetectionmethods, todierences in
ethnicity, geographic locations, and sample
sizeofthesubjectsexamined, andtotheinappropriategroup-ingtogetherof
dierent lesionsfromdierent anatomicallocations of the mucosa of the
upper aerodigestive tract[49, 51,
5864].ManystudiesinvestigatingtheassociationofHPVandsquamous cell
carcinoma of the mucosa of the upperaerodigestive tract
usedPCRtechniques for detectionofHPVDNAwithout
alsoquantifyingtheDNAviral load.PCR can detect extremely small
fragments of DNA that mayrepresent either contamination of the
sample or biologicallyinsignicant HPV infection [51, 57, 58]. These
ndings havebeen reported as if they were pathogenically signicant.4
International Journal of
DentistryWhetherthesearelegitimatendingsoraretheresultsofinconsistenciesanderrorsinmethodology,severalHPVgenotypeshavebeendetectedinprecancerousorallesions.High-risk
HPV genotypes, in particular HPV-16, have beenreported to be the
most prevalent in oral leukoplakias,including proliferative
verrucous leukoplakia [55, 65]. Otherreports implicated low-risk
rather than high-risk HPVgenotypes in oral leukoplakia [54, 63],
and yet others assertthat oral leukoplakiais
coinfectedwithavarietyof HPVgenotypes[49, 55, 66].
Inameta-analysisofdatafrom94studiesofatotalof4580specimens,
MillerandJohnstone[63] determined that the likelihood of HPV being
detectedinprecancerousoral lesionsis2to3timesgreaterandinoral
squamouscell
carcinomais4to5timesgreaterthaninnormaloralmucosa.TheprevalenceofHPVinnormaloral
mucosa, innondysplastic leukoplakias,
indysplasticleukoplakiasandinotherprecancerousintraepithelial
oralneoplasms, and in oral squamous cell carcinoma is likely tobe
10%, 20.2%, 26.2%, and 46.5%, respectively [63].This suggests that
there may be some link between HPVinfectionandoral
precancerousandcancerouslesions.
AsE6andE7oncoproteinsofhigh-riskHPVgenotypeshavethecapacitytomediatecarcinomatous
transformationofinfectedkeratinocytesbyinactivatingcellularp53andRbtumour
suppressor pathways [50, 52], HPV may play eitheran oncogenic or a
co-oncogenic role in some HPV-infectedprecancerous and cancerous
epithelial neoplasms.In fact, HPV-16 has been found to be causally
associatedprimarily with squamous cell carcinoma of the palatal
tonsils[6770]inasubsetofsubjectswhoareyounger, consumeless tobacco,
are more engaged in high-risk sexual behaviour(great number of
lifetime sexual partners andpracticingoral-genital sex), have
higher HPV-16 serum antibody
titers,andhaveabetterdisease-freesurvival andoverall survivalrates
thansubjects withHPV-cytonegative oropharyngealsquamous cell
carcinoma [51, 64, 6770]. The cells of HPV-cytopositive
oropharyngeal carcinoma in these subjects havea distinct molecular
prole [50]. The cells of squamouscell carcinoma causally associated
with HPV express E6/E7oncoproteins. They frequently demonstrate
viral integrationwithin the cellular genome with the presence of
intactE6gene. Theyexhibit highviral load, reducedexpressionof
Rbproteins, functional overexpressionof p16INK4A,unmutatedp53gene,
andlossof heterozygosity(LoH)atchromosomal loci 3p, 9pand17pis
infrequent [51, 6874]. In contrast, HPV-cytonegative oropharyngeal
squamouscell carcinomaischaracterizedbyp53genemutations, byfrequent
LoHat 3p, 9p, and17p, bydecreasedlevels ofp16INK4A and by normal or
increased levels of Rb proteins[50, 51, 71].Recent meta-analyses
and comprehensive studies [60, 62,67, 75]showlittleornocausal
associationbetweenHPVand oral squamous cell carcinoma in contrast
to the strongassociation between HPV and oropharyngeal squamous
cellcarcinoma. HPV-16cytopositiveoral squamouscell carci-noma is
characterized by a low viral load, by infrequent viralintegration,
andthecancerouscellsseldomcontainactivetranscriptional E6/E7 mRNA
[72, 76]. However, it is possiblethat, inHPV-cytopositiveoral
squamouscell carcinomatathat do not express E6/E7 mRNA, E6/E7
oncoproteins maywell have participated or have had a complementary
role inthe initial transformation, but then phased out
[77].Withoropharyngeal carcinomaasthemodel, acausalassociation
between HPV and cancerization in oral epithe-liumislikelyif
thecellsof thelesioncontainHPVDNAexpressing E6 and/or E7 mRNA [71],
if there is
viralintegrationwithinthecellulargenome[74],andifthereisahighviral
load(>1copiespercell). Alimitedbiologicalsignicanceof thevirus
intheprocess of transformationcan be deduced if there is a low-copy
number (