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The mig a ion abili y o s em cells can explain he exis ence o
cance o unknown p ima y si e. Re hinking me as asis.
Miguel López-Láza o1
1 Depa men o Pha macology, Facul y o Pha macy, Uni e si y o Se ille, Spain
Co espondence o: Miguel López-Láza o, email: [email p o ec ed]
Keywo ds: Cance o unknown o igin, Cance s em cells, Cance he apy, Cells o o igin in cance , S em cell model o cance
Recei ed: Ap il 5, 2015 Accep ed: Ap il 28, 2015 Published: May 1, 2015
This is an open-access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion License, which pe mi s un es ic ed use,
dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal au ho and sou ce a e c edi ed.
ABSTRACT
Cance s o unknown p ima y si e a e me as a ic cance s o which p ima y
umo s a e no ound a e de ailed in es iga ions. In many cases, he si e o o igin
is no iden i ied e en on pos mo em examina ion. These cance s a e he ou h mos
common cause o cance dea h. The biological e en s in ol ed in he de elopmen o
his ype o cance s emain unknown. This manusc ip discusses ha , like me as a ic
cells, s em cells ha e a na u al abili y o mig a e. A cance o unknown p ima y si e
would o m when de egula ed, p emalignan o cance ous s em cells mig a ed away
om hei na u al issue and ga e ise o a cance in a new si e be o e o wi hou
gene a ing a umo in hei o iginal issue. I is impo an o ealize ha o ming
a umo in a issue is no a p e equisi e o s em cells o mig a e away om ha
issue. This iew is in acco dance wi h ecen obse a ions ha s ongly suppo he
umo igenesis model in which cance a ises om no mal s em cells. E idence has
accumula ed ha cance s em cells may play a key ole in cance p og ession and
esis ance o he apy. Success ul ea men o cance , including ha o unknown
p ima y si e, may he e o e equi e he de elopmen o he apies agains cance
s em cells.
INTRODUCTION
Me as asis is conside ed o be he p ocess by
which cance cells lea e a p ima y umo and o m one
o se e al seconda y umo s in o he pa s o he body;
hese seconda y umo s a e called me as a ic umo s o
me as ases [1]. His ological analyses commonly show
ha cells om he p ima y and seconda y umo s a e
simila and esemble hose o he no mal issue o o igin.
In mos cance pa ien s, he p ima y umo is diagnosed
be o e o a he same ime han i s me as ases. Some imes,
me as a ic umo s a e ound be o e s anda d diagnos ic
s udies e eal he loca ion o he p ima y si e. Bu o he
imes, me as a ic umo s a e ound, and a p ima y umo
is no iden i ied a e de ailed diagnos ic in es iga ions.
These umo s gene ally con ain poo ly di e en ia ed o
undi e en ia ed cells, which makes he iden i ica ion o
he issue o o igin di icul . In some cases, he his ology
o loca ion o he umo s ongly sugges a speci ic
p ima y si e; howe e , exhaus i e in es iga ions also
ail o iden i y a p ima y umo . These cance s a e called
cance s o unknown p ima y (CUP), cance s o unknown
p ima y si e, cance s o unknown o igin, o occul p ima y
cance s [2-4].
Cance o unknown p ima y si e is a he e ogeneous
g oup o cance s o which he ana omical si e o o igin
emains occul a e de ailed in es iga ions. In 15-
25% o cases, he p ima y si e is no iden i ied e en on
pos mo em examina ion. Cance o unknown p ima y si e
accoun s o app oxima ely 3-5% o all human cance s. I
is he se en h o eigh h mos equen malignancy, and i
is he ou h mos common cause o cance dea h in bo h
sexes. In a hi d o pa ien s diagnosed wi h CUP, h ee
o mo e o gans a e in ol ed a he ime o diagnosis.
The pa e n o me as asis ( equency and loca ion o
me as ases) in pa ien s diagnosed wi h CUP may be
signi ican ly di e en om ha in pa ien s wi h known
p ima y umo s. Mos cance s o unknown p ima y si e
show an agg essi e beha io and a e incu able [2-4].
Since he si e o he p ima y umo usually dic a es
he ea men and expec ed ou come, he inabili y o
iden i y a p ima y si e aises conce n among oncologis s
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and pa ien s. Cu en esea ch and clinical e o s a e made
o de elop and use knowledge and echnology o loca e
he p ima y umo o , a leas , o eliably p edic he issue
o o igin. P ima y and me as a ic umo s commonly ha e
simila exp ession p o iles and molecula signa u es. This
makes immunohis ochemical es ing and issue-o -o igin
molecula p o iling use ul ools o p edic ing he issue
o o igin [3]. Recen ly, Polak e al. compa ed mu a ion
densi ies o epigene ic p o iles o no mal and cance cells
om di e en issues, and epo ed indings sugges ing
ha he issue o o igin o a cance may be accu a ely
p edic ed based on he dis ibu ion o mu a ions along i s
genome [5]. P edic ing he issue o o igin is pa icula ly
impo an o he ypes o CUP ha espond ela i ely
well o speci ic he apies (app oxima ely 20% o CUP).
When hese cance s a e uled ou , i usually becomes
less impo an o ind o p edic he p ima y si e. Fo
example, in an analysis o se e al pos -mo em coho
s udies, he po en ial p ima y umo was iden i ied in
73% o pa ien s, and he mos common p ima ies we e
lung (27%) and panc ea ic (24%) umo s [6]. The i e-
yea ela i e su i al a e o pa ien s wi h lung cance and
panc ea ic cance wi h dis an me as ases is 4% and 2%,
espec i ely [7]. The e o e, he iden i ica ion o a p ima y
si e in pa ien s wi h hese wo ypes o cance would no
ha e changed much hei ou come.
Unde s anding he exis ence o cance s o
unknown p ima y si e and he mechanisms in ol ed in
hei o ma ion may lead o he de elopmen o be e
ea men s. These ea men s may be use ul no only o
pa ien s wi h hese cance s, bu also o pa ien s wi h o he
me as a ic cance s. Cu en ly, i is gene ally accep ed ha
CUP exis s because o ou inabili y o iden i y he p ima y
umo due o clinical o echnological ine iciencies, o
because he p ima y umo eg esses o s ays do man a e
sp eading he cance cells ha gene a e he me as ases [2-
4]. Some s udies ha e shown molecula ea u es sha ed
by cance s o unknown p ima y o igin. Fo example,
a ecen analysis o 1806 cases o cance o unknown
p ima y si e has e ealed ha TP53 is he mos commonly
mu a ed gene in hese cance s [8]. Howe e , he biological
e en s ha allow he p ima y si e o emain occul a e he
de elopmen o me as ases emain unknown [2-4].
S em cells ha e a na u al abili y o mig a e
Recen e idence sugges s ha he biological e en s
occu ing du ing he de elopmen o me as asis a e a he
simila o hose occu ing du ing emb yonic de elopmen .
Du ing emb yogenesis, s em cells can in ade issues,
mo e h ough he in e io o he emb yo, a el long
dis ances, and es ablish in new places o pa icipa e in he
o ma ion o o gans and issues [9-11]. Du ing me as asis,
cance cells can in ade issues, mo e h ough he
lympha ic and ci cula o y sys ems, a el long dis ances,
and es ablish in new issues o o m umo s [1,12]. The
ac i a ion o he epi helial-mesenchymal ansi ion (EMT)
and he mesenchymal-epi helial ansi ion (MET) seems
o play a c i ical ole in he mig a ion abili y o bo h
emb yonic s em cells and me as a ic cells. The ac i a ion
o hese ansi ion p og ams in ol es p o ound changes
in cell mo phology and beha io , including changes in
cy oskele on s uc u e, cell pola i y, cell-cell con ac and
ex acellula ma ix deg ada ion. Recen da a sugges
ha EMT ac i a ion is an ea ly e en in ca cinogenesis,
and ha he e is a c oss alk among EMT ac i a ion, he
acquisi ion o molecula and unc ional ai s o cance
s em cells, and he inac i a ion o mu a ion o p53
[9,10,13-15].
The mig a ion abili y o s em cells is ep essed a e
emb yonic de elopmen , bu p obably eappea s du ing
pa hological condi ions. Adul s em cells (also known
as issue s em cells) can inc ease hei mig a o y ac i i y
when hei mic oen i onmen is al e ed [16]. These cells
play a key ole in issue damage epai ; issue inju y
ac i a es de elopmen al p og ams ha ac i a e adul s em
cell mig a ion o he si e o damage [17]. In e es ingly,
issue inju y may inc ease cance isk [18-21]. Adul
s em cells may also inc ease hei mig a o y po en ial
a e accumula ing speci ic DNA al e a ions. These
p emalignan s em cells may also acqui e addi ional DNA
al e a ions and become cance s em cells (CSCs). These
cance cells seem o play a key ole in umo me as asis
[12,22]. I has been p oposed ha CSCs can be s a iona y
(which es ablish umo g ow h) o mobile (which lead o
umo me as asis) [23], and hese wo popula ions o CSCs
ha e been ound in human cance issues [24].
The mig a ion abili y o s em cells can explain he
exis ence o cance s o unknown p ima y si e. A cance
o unknown p ima y si e would o m when de egula ed,
p emalignan o cance ous s em cells mig a ed away om
hei na u al issue and ga e ise o a cance in he new
si e be o e o wi hou gene a ing a umo in hei o iginal
issue. I is impo an o ealize ha o ming a umo in a
issue is no a p e equisi e o s em cells o mo e away
om ha issue; his is p obably he key o unde s and he
exis ence o cance o unknown p ima y si e. S em cells
can mig a e om hei na u al issue and ini ia e a cance
in he new si e be o e gene a ing a de ec able umo in
hei na u al issue. In his case, he p ima y umo could
be iden i ied a e some ime. Howe e , s em cells can also
mig a e away om hei na u al issue wi hou gene a ing
a cance he e. In his case, he “p ima y umo ” would
ne e exis . This can explain why he p ima y si e is
no iden i ied e en on pos mo em examina ion in many
pa ien s wi h hese cance s (Figu e 1).
Cance may a ise om no mal s em cells
This explana ion o he exis ence o cance s o
unknown p ima y si e implies ha hese cance s a ise
ei he om no mal s em cells o om non-s em cells ha
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ha e ac i a ed s em cell p og ams. He e I discuss e idence
sugges ing ha cance , including ha o unknown p ima y,
may o igina e in no mal s em cells.
Nume ous expe imen al in es iga ions ha e been
conduc ed o iden i y he cells o o igin in cance [25].
These in es iga ions bo h challenge and suppo he idea
o cance a ising om s em cells [25-27]. Fo example,
expe imen al da a sugges ha p ogeni o cells can
dedi e en ia e and de elop s em-like p ope ies; his has
led o he p oposal ha cance may s a in p ogeni o
cells [26]. Expe imen al da a ha e also shown ha long-
e m passaged s em cells become cance cells in cul u e
and gene a e solid umo s when injec ed in o oden s
[28]. I is impo an o no e ha all hese in es iga ions
in ol e expe imen al manipula ion. As discussed
elsewhe e [28], s em cells a e de ined in e ms o hei
unc ional p ope ies: long- e m sel - enewal capaci y
and di e en ia ion po en ial. This p esen s an inhe en
p oblem because hese p ope ies can only be assessed
by expe imen al manipula ion, and expe imen al
manipula ion p obably al e s he unc ional p ope ies o
hese cells. Jus cul u ing s em cells in i o may induce
p o ound changes in hei p oli e a i e a es, cellula a e
o beha io . In i o, s em cells a e es ablished in niches.
S em cell niches a e mic oen i onmen s ha p o ec he
s em cells and egula e he way hey beha e. The s em
cell niche p o ides suppo and signals ha egula e
hei p oli e a i e a es, sel - enewal, di e en ia ion and
mig a ion ac i i ies [16]. S em cells cul u ed in i o a e
dep i ed o hei niches, which may comple ely change
hei biological beha io . This may explain, o example,
ha s em cells ha a e es ing in i o may become
p oli e a i e in cell cul u e. In addi ion, he s udies
aimed a iden i ying he cells o o igin in cance usually
in ol e a gene ic o chemical manipula ion o he cells
[25]. We should he e o e in e p e he indings o hese
expe imen al s udies cau iously.
Solid biological concep s may p o ide mo e eliable
in o ma ion on he cells o o igin in cance . The long
li espan and sel - enewal capaci y o s em cells suppo he
idea o cance a ising om hese cells. These biological
p ope ies indica e ha s em cells ha e he oppo uni y o
accumula e su icien DNA al e a ions o gene a e cance
cells [29]. Th ee easons a e commonly used o challenge
his idea. Fi s , cance p obably s a s in cells wi h high
p oli e a i e a es, and he adul s em cells o some
issues do no di ide o en. Second, s em cells ep esen a
small popula ion wi hin a issue, and he p obabili y o a
s ochas ic ca cinogenic hi is lowe in a small popula ion
han in a la ge popula ion. Thi d, ca cinogenesis depends
on he acquisi ion o ad an ageous cell pheno ypes,
and hese pheno ypes a e less likely o occu when a
cell sel - enews han when a cell di e en ia es [26].
Recognizing ha li e s a s be o e bi h makes he i s
wo challenging easons di icul o accep . Fi s , s em
cells di ide e y ac i ely du ing emb yonic de elopmen .
Figu e 1: The mig a ion abili y o s em cells can explain he exis ence o cance o unknown p ima y si e. A cance o
unknown p ima y si e would o m when de egula ed s em cells, p emalignan s em cells o cance s em cells mig a ed away om hei
na u al issue (p ima y si e) and o med a umo in ano he issue (seconda y si e) be o e o wi hou gene a ing a umo in hei p ima y
loca ion. Thick a ows ep esen he p obably mos common pa hway o me as asis. Cu ed a ows ep esen sel - enewal capaci y. See
ex o u he de ails.
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Du ing his ela i ely sho pe iod, one cell (zygo e)
gi es ise o he billions o cells ha o m a de eloped
emb yo [30]. The genome o he adul s em cells a he
ime o bi h has al eady been copied many imes, and
p obably con ains many he i able al e a ions in ela ion
o ha o he zygo e. In some adul s em cells, hese
al e a ions may con ibu e o a possible ca cinogenesis
p ocess occu ing la e in li e. They may also inc ease
hei p oli e a i e a es du ing adul hood. Second, s em
cells a e he only cell popula ion a he ini ial s ages o
emb yonic de elopmen . S em cells a e he e o e he only
cells exposed o he acquisi ion o ca cinogenic damage
du ing his pe iod o in ense mi o ic ac i i y. Thi d,
he chance o a ca cinogenic hi du ing cell di ision is
p obably simila in a cell ha sel - enews o p oduce wo
undi e en ia ed daugh e cells han in a cell ha gi es ise
o wo di e en ia ed daugh e cells. The limi ed ideli y o
DNA polyme ases is known o be an impo an sou ce o
DNA mu a ions du ing cell di ision. DNA polyme ases
p obably inse simila le els o inco ec nucleo ides in
he DNA o he daugh e cells when copying he DNA o
a s em cell ha sel - enews han when copying he DNA
o a p ogeni o cell ha di e en ia es. The acquisi ion o
ad an ageous cell pheno ypes is p obably ele an a la e
s ages o ca cinogenesis; howe e , his ac o is unlikely
o de e mine wha ype o cell is mo e p one o acqui e he
i s ca cinogenic hi . O e all, he idea o cance a ising
om no mal s em cells has a solid biological basis.
Figu e 2: The s em cell model o cance can explain he age dis ibu ion o cance . A) Age is he mos impo an cance isk
ac o , and cance incidence inc eases wi h age. B) S em cells sel - enew, ha e a long li espan, and a e he only cells ha ansmi he DNA
o he zygo e o he cells we ha e a he ime o dea h. S em cells can di e en ia e in o p ogeni o cells and e en ually in o di e en ia ed
cells. C) S em cells can acqui e and accumula e DNA al e a ions du ing ou whole li e. Because p ogeni o cells lack an inna e abili y
o sel - enew and ha e a ela i ely sho li espan (un il hey ully di e en ia e), hey can acqui e and accumula e DNA al e a ions only
du ing ela i ely sho pe iods o ou li e. The model in which cance a ises om s em cells i s he age dis ibu ion o cance be e han
he model in which cance ini ia es in p ogeni o cells. Cu ed a ows ep esen sel - enewal capaci y. SC: s em cell; PC: p ogeni o cell;
DC: di e en ia ed cell.
zygo e dea h
emb yo e us bi h
RISK OF DEVELOPING CANCER
SC SC SC SC SC
PC
DC
DC
PC
Unlike p ogeni o cells, s em cells can acqui e and accumula e DNA al e a ions du ing ou whole li e
S em cells ha e a long li espan, di ide many imes, and a e he only cells ha ansmi he
DNA o he zygo e o he cells we ha e a he ime o dea h
Cance isk inc eases h oughou li e (age dis ibu ion o cance )
SC SC
SC DNA al e a ions
DNA al .PC
CANCER
PC
DC
DC
PC
PC
DC
DC
PC
DC
DC
PC
PC
DC
DC
PC
DC
DC
PC
CANCER
PC
CANCER
CANCER
DNA al .DNA al .
A
B
C
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I is impo an o ecognize ha he i s ca cinogenic
hi does no necessa ily ha e o be he i s mu a ion
occu ing in one o he genes whose in ol emen in cance
de elopmen is unde s ood a a gi en momen . The i s
ca cinogenic hi should be e e ed o as he i s he i able
change (e.g., DNA mu a ion, epigene ic change o
ch omosome al e a ion) ha pa icipa es in he o ma ion
o a cance . The i s hi may be a DNA al e a ion wi h
biological implica ions no ye unde s ood, and may occu
in a non-coding DNA sequence [31].
Obse a ional s udies do no in ol e expe imen al
manipula ion and may p o ide eliable in o ma ion
ega ding he cells o o igin in cance . Nume ous
obse a ional s udies ha e shown ha age is he mos
impo an cance isk ac o [7,32-35]. Fo example,
ecen da a show ha app oxima ely 4.4% o people
unde 50 yea s old a e diagnosed wi h cance in he US;
his pe cen age inc eases o 6.3% in people in hei 50s,
o 12.5% in people in hei 60s, and o 31.2% in people
olde han 70 yea s old [7]. These s udies indica e ha ou
cance isk inc eases du ing ou whole li e. S em cells a e
he only cells ha ansmi he genome o he zygo e o he
cells we ha e a he ime o dea h and, he e o e, a e he
only cells ha can acqui e and accumula e DNA al e a ions
du ing ou whole li e. P ogeni o cells lack an inna e
abili y o sel - enew and ha e a ela i ely sho li espan.
P ogeni o cells, he e o e, can acqui e and accumula e
DNA al e a ions only du ing ela i ely sho pe iods o ou
li e. I cance a ose om p ogeni o cells, a p ogeni o cell
p oduced when we a e 10 would ha e he same p obably
o ini ia e a cance han a p ogeni o cell p oduced when
we a e 30 o 50. A e a lag pe iod o e.g. 20 yea s, ou
isk o ha ing cance would be simila in ou 30s, 50s
o 70s. Cance s a is ics show o he wise. The s em cell
model o cance i s he age dis ibu ion o cance be e
han he p ogeni o cell model o cance (Figu e 2). Cance
s a is ics also show ha cance incidence decele a es la e
in li e, i.e., cance incidence ises wi h age, bu he ise
occu s mo e slowly in la e yea s. In e es ingly, an o e all
decline in issue egene a i e po en ial also occu s in
la e yea s, which is a ibu ed o a decline in s em cell
unc ionali y wi h age [36].
The obse a ion ha some issues gi e ise o
cance s millions o imes mo e o en han o he issues
has long puzzled cance esea che s. This obse a ion
canno be explained by di e en le els o exposu e
o ca cinogens. Fo example, melanocy es and basal
epide mal cells o he skin a e exposed o simila le els o
he same ca cinogen (ul a iole adia ion); howe e , basal
cell ca cinomas a e much mo e common han melanomas
[37]. C. Tomase i and B. Vogels ein [37] ha e ecen ly
p o ided an explana ion o his obse a ion. They ound
ha he e exis s a highly posi i e co ela ion (Spea man’s
ho = 0.81; P < 3.5 × 10−8) be ween he numbe o no mal
s em cell di isions in a issue and he li e ime isk o
de eloping cance in ha issue. This s iking co ela ion
applied o 31 cance ypes and ex ended ac oss i e o de s
o magni ude. No en i onmen al o inhe i ed ac o is
known o co ela e so s ongly wi h cance incidence
ac oss issues [37]. In simple e ms, he da a p o ided by
C. Tomase i and B. Vogels ein indica e ha i he no mal
s em cells om one o ou issues di ide once, ou cance
isk in ha issue is app oxima ely 1X. I hey di ide
10 imes, ou cance isk is 10X. I hey di ide 1,000
imes, ou cance isk is 1,000X. And i he no mal s em
cells om one o ou issues di ide 100,000 imes, ou
cance isk in ha issue is app oxima ely 100,000X. This
s ongly sugges s ha he main eason we ha e cance is
ha ou no mal s em cells di ide. A easonable way o
in e p e his is ha cance a ises om no mal s em cells.
The o he op ion would be di icul o accep . I cance
a ose and de eloped in non-s em cells, we would ha e
o admi ha s em cell di ision is he main de e minan
o he neoplas ic ans o ma ion o non-s em cells. This
would mean ha any ime a pa icula cell di ides, ano he
cell is ans o med. I makes much mo e sense o hink ha
no mal s em cells acqui e and accumula e ca cinogenic
damage when hey di ide han o hink ha hei di ision
in lic s ca cinogenic damage on non-s em cells.
CONCLUDING REMARKS
Cu en ly, me as asis is conside ed o ollow he
nex sequence: in asion and in a asa ion o cance
cells om he p ima y umo , dissemina ion h ough he
ci cula ion, ex a asa ion in di e en o gans, su i al on
a i al, se lemen in o la ency, eac i a ion, and o e
coloniza ion wi h gene a ion o a new mac oscopic
umo [1,12]. I me as asis is he p ocess by which cance
cells lea e a p ima y umo and o m seconda y umo s
in o he loca ions [1,12], he exis ence o cance s o
unknown p ima y si e can be seen as a biological mys e y
[4]. Howe e , i we challenge his p e ailing iew and
conside ha he cells ha lea e he p ima y issue do
no necessa ily ha e o be cance cells om a umo , he
exis ence o cance s o unknown p ima y si e becomes a
plain biological p ocess. The injec ion o mo phologically
no mal b eas cells om gene ically enginee ed mice in o
he ail eins o o he emale mice esul ed, a e oncogene
induc ion, in he o ma ion o umo s in he lungs bu no
in he b eas [38]. Al hough hese cells we e gene ically
and chemically manipula ed, hese expe imen s show ha
he o ma ion o a p ima y umo is no a equisi e o he
o ma ion o me as a ic umo s. Me as asis should be seen
as he p ocess by which cells om a issue o m umo s
in o he issues. All cance s, including hose o unknown
p ima y si e, i in his simple de ini ion (Figu e 1).
The e m occul p ima y umo should be a oided
in he absence o a p ima y umo , because i implies
ha he umo exis s and is hidden. This e m may cause
anxie y among clinicians and pa ien s, who may hink ha
hei e alua ion has been de icien . In many cance s o

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unknown p ima y si e, p ima y umo s a e ne e ound.
In an analysis o pos -mo em coho s udies, p ima y
umo s we e no iden i ied in 27% o pa ien s [6]. In some
cance s o unknown p ima y si e, he his ology o loca ion
o he umo s ongly sugges a speci ic p ima y si e. Fo
example, he iden i ica ion o adenoca cinomas in isola ed
unila e al axilla y lymph nodes in women wi h CUP
s ongly sugges s ha hese umo s o igina e in he b eas .
B eas umo s commonly me as asize o his si e, and
his ype o CUP is ound almos exclusi ely in women.
Howe e , b eas umo s we e no iden i ied in 28% o
446 women who we e diagnosed wi h his ype o CUP
and unde wen mas ec omy and subsequen his ological
analysis [2,39]. The e ms p ima y umo and seconda y
umo should also be a oided in he absence o a p ima y
umo . In CUP pa ien s, he e ms p ima y si e and umo
o unknown p ima y si e should be used ins ead, a leas
un il a possible p ima y umo is e en ually ound.
No all cells can lea e hei na u al issue and o m
umo s in o he loca ions. The cell needs mig a ion abili y
o lea e i s issue and each he new one, p oli e a i e
capaci y o accumula e enough DNA al e a ions o p oduce
a cance cell, and sel - enewal capaci y so ha i s only
a e is no he gene a ion o non-di iding di e en ia ed
cells. S em cells cons i u e he only cell popula ion wi h
hese h ee biological p ope ies unde physiological
condi ions. This implies ha cance s o unknown p ima y
si e o igina e in s em cells, o in non-s em cells ha ha e
acqui ed hese biological p ope ies.
Non-s em cells (e.g., p ogeni o cells) can acqui e
DNA al e a ions. In some cases, hese al e a ions may
esul in he ac i a ion o s em-cell p og ams, and may
e en de e mine whe he o no we will e en ually ha e
a cance . Bu his does no imply ha hese cance s a ise
om non-s em cells, because he s em cells om which
hey de i e may ha e DNA al e a ions ha also pa icipa e
in he o ma ion o hese cance s. Th ee lines o e idence
discussed in his manusc ip sugges ha he majo i y o
human cance s may a ise om no mal s em cells. Fi s ,
cell di ision is a majo sou ce o DNA al e a ions, s em
cells cons i u e he only cell popula ion du ing he ini ial
pe iod o li e (ea ly s ages o emb yonic de elopmen ),
and s em cells di ide e y ac i ely du ing his pe iod.
This means ha i is likely ha s em cells acqui e DNA
al e a ions du ing emb yonic de elopmen , which may
play a ole in a possible ca cinogenic p ocess occu ing
la e in li e. Because he i s ca cinogenic hi may be
a DNA al e a ion wi h biological implica ions no ye
unde s ood, we canno exclude he possibili y ha an
impo an pe cen age o cance s may s a be o e bi h.
Second, s em cells a e he only cells ha ansmi ou DNA
du ing ou whole li e, and he isk o de eloping cance is
known o inc ease du ing ou whole li e. The obse a ion
ha cance isk inc eases wi h age can be be e explained
by a model in which cance ini ia es in s em cells han
by a model in which cance ini ia es in p ogeni o cells
(Figu e 2). Thi d, i cance ini ia ed and de eloped in
non-s em cells, he p esence o biological ac i i y o
s em cells in a issue would no signi ican ly modi y he
isk o de eloping cance in ha issue. Howe e , ecen
obse a ions s ongly sugges ha he numbe o no mal
s em cell di isions in a pa icula issue seems o be he
main de e minan o de eloping cance in ha issue [37].
I cance a ises om no mal s em cells, hese cells
would e en ually become cells wi h he abili y o gene a e
umo s, i.e., cance s em cells (CSCs). In o he wo ds, a
CSC would be a s em cell ha has de eloped a long- e m
abili y o gene a e he e ogeneous umo popula ions. This
iew o CSCs conside s hei o igin (no mal s em cells),
hei sel - enewal capaci y (abili y o copy hemsel es
du ing long pe iods), and hei di e en ia ion po en ial
(abili y o gene a e he e ogeneous cellula popula ions).
These cells would de elop in all ypes o cance , including
hose in which hey ha e no ye been iden i ied. Some
CSCs would sel - enew, and o he s would di e en ia e
and gi e ise o cells wi hou sel - enewal abili y
(p ogeni o cance cells) and wi hou sel - enewal abili y
and p oli e a i e capaci y (di e en ia ed cance cells).
All hese cance cells would con ibu e o he cellula
he e ogenei y o umo s [40]. A CSC loca ed in a issue
di e en om ha o he no mal s em cell om which
i o igina es would be a me as a ic cance s em cell. This
iew implies ha he s em cell can mig a e o he new
issue be o e o a e becoming a CSC. I is impo an o
ealize ha he p ocess by which a no mal cell becomes a
cance cell does no necessa ily ha e o s a and comple e
in he same issue.
CSCs seem o play a c ucial ole in umo g ow h,
me as asis and esis ance o he apy [22,41]. Success ul
cance he apy may he e o e equi e he elimina ion o
hese cells. Expe imen al da a indica e ha non-s em
cance cells may dedi e en ia e in o CSCs [27,42-45].
This means ha , i we only elimina e CSCs, non-s em
cance cells may gene a e new CSCs and epopula e he
umo [42]. Success ul he apy may he e o e equi e he
elimina ion o CSCs and hei p ogeny. Clinical ials a e
ongoing o es he e icacy o d ugs a ge ing CSCs, alone
and in combina ion wi h con en ional d ugs a ge ing non-
s em cance cells [46].
An i-CSC he apies migh show e icacy in a
wide ange o CUP pa ien s. His ological analyses o
many umo s om CUP pa ien s show popula ions o
poo ly di e en ia ed o undi e en ia ed cells. Since
s em cells cons i u e he less di e en ia ed cell ype, he
undi e en ia ed cells o hese umo s migh be CSCs;
hese cells would play a ole in he known agg essi e
beha io and he apeu ic esis ance o hese cance s. In
addi ion, he p ognosis o mos CUP pa ien s is poo e en
when he p ima y si e is p edic ed. The iden i ica ion o
a possible p ima y si e allows CUP pa ien s o be ea ed
wi h he he apies used in me as a ic pa ien s wi h known
p ima y umo s. These he apies, howe e , a e gene ally
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no cu a i e. CUP pa ien s migh he e o e bene i om
pa icipa ing in clinical ials e alua ing new an i-CSC
ea men s.
The lack o e ec i e ea men s o mos CUP
pa ien s should no jus i y hei inclusion in clinical s udies
es ing expe imen al he apies no p ope ly alida ed
in p eclinical models. Clinical ials in oncology ha e
he highes ailu e a e compa ed wi h o he he apeu ic
a eas [47]. Mos o he d ugs ha show ema kable
an icance e ec s in p eclinical models ail when es ed
in cance pa ien s. Pa o hese ailu es may be due o
poo p eclinical designs, in which he needs o cance
pa ien s a e o en misunde s ood [48]. Cance pa ien s do
no need new d ugs ha a ge hei cance cells a low
concen a ions i hey also a ge hei no mal cells a
simila concen a ions. Cance pa ien s will p obably no
bene i om d ugs ha induce ma ked umo sh inkages
in animal models ha do no ep esen hei disease.
Cance pa ien s need d ugs ha imp o e he e icacy o
he exis ing ea men s. Selec i i y (in i o) and su i al
a e (in i o) a e p obably he mos eliable pa ame e s
o p edic d ug e icacy in cance pa ien s [48]. In i o,
he new ea men should imp o e he selec i i y o he
exis ing d ugs when es ed in cance cells e sus a a ie y
o no mal cells (non-s em cells and s em cells om a
a ie y o heal hy issues) [48-50]. The e icacy o a
new d ug combina ion should be assessed by es ing in
cance cells e sus non-malignan cells i i imp o es he
selec i i y o he s anda d ea men , and no by es ing in
cance cells i i s cy o oxici y is enhanced (po en ia ion
ac o ) o syne gis ically inc eased (combina ion index)
in ela ion o he cy o oxici y induced by each d ug
indi idually [48]. In i o, he new ea men should
imp o e he su i al a e o he exis ing he apy in
animal models ep esen a i e o he pa ien s who would
e en ually ecei e he new d ugs [48]. Selec ing animal
models o me as asis is essen ial i he new d ugs a e
in ended o CUP pa ien s o o o he pa ien s wi h
me as a ic disease. I should be no ed ha CSCs may
cons i u e a mino i y popula ion wi hin a umo (pe haps
1 o 3% [46]), and ha a ema kable umo sh inkage may
lea e he whole CSC popula ion una ec ed.
In summa y, e idence sugges s ha he majo i y
o human cance s may o igina e in no mal s em cells.
Cance s o unknown p ima y si e p obably exis because
s em cells (de egula ed, p emalignan o cance ous)
mig a e away om hei na u al issues and gene a e
umo s in o he loca ions be o e o wi hou gene a ing
umo s in hei na u al issues. Me as asis should be
conside ed as he p ocess by which cells om a issue gi e
ise o umo s in o he loca ions.
CONFLICT OF INTEREST
The au ho decla es no con lic o in e es .
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