1、Squamous cell carcinomas(SCCs)are among the most fre-quent solid cancers in humans1 and represent a major cause of death worldwide.Their incidence is sharply rising owing to increased exposure to carcinogens,such as ultraviolet radiation related to sun exposure,smoking,alcohol consumption or human p
2、apilloma virus(HPV)infection1,2.SCCs are classified according to the loca-tion where they appear,being frequently found in skin,head and neck,oesophagus,lung and cervix26 and more rarely in pancreas,thyroid,bladder and prostate710.During the past decades,great efforts have been made to elucidate the
3、 cell of origin of different malignancies11.Lineage tracing studies enabled the identification of the cellular hierarchies and lineage segregation that mediate homeostasis and repair of the different tissues from which cancer arises12(FiG.1).Many cancers arise from tissues maintained by the presence
4、 of stem cells and progenitors that self-renew and differentiate into the different cell lineages that compose these tissues.Depending on the turnover,differentiated cells and progenitors usually pres-ent a shorter lifespan,whereas stem cells reside long term,sometimes throughout the life of the ani
5、mals.Upon tissue damage,the cellular hierarchy that governs epithelial tissue homeostasis can be altered,and more committed progenitors and even differentiated cells can acquire stem cell potential and contribute to tissue repair13.With oncogenic hits,both stem cells and progenitors can serve as the
6、 cells of origin in cancer(Box1).The nature of the cells at the cancer origin has also been proposed to determine the differentiation charac-teristics and aggressiveness of tumours.Tumours arising from progenitors may show lineage-restricted differen-tiation,whereas tumours arising from stem cells c
7、ould present multilineage differentiation potential.However,multipotent differentiation of a tumour could also result from plasticity of tumour cells induced by their onco-genic mutations or by their microenvironment or by neighbouring cells.In this Review,we discuss recent studies that define the c
8、ell of origin of cutaneous,head and neck,oesopha-geal and lung SCCs,which represent the most common SCCs.We first describe the architecture and cellular hierarchy present in the different epithelia from which the different SCCs arise.Then,we discuss how lineage tracing strategies have been instrumen
9、tal to identifying the cell of origin in these SCCs.Finally,we discuss how oncogenic mutations and the cell of origin cooperate in determining the differentiation,aggressiveness and metastatic potential of SCCs.Architecture of tissues of SCC originSCCs arise from epithelial tissues that can be class
10、ified as stratified squamous epithelium(which includes epithelia of skin,oesophagus and oral cavity)and non-squamous epithelia(which include airway epithelium).The differ-ent types of SCCs have common histological features,such as the presence of squamous differentiation visible by the formation of
11、keratin pearls.Skin compartments are maintained by their own res-ident stem cells.The skin epidermis acts as the first defensive line to isolate and protect our bodies from the external environment.The mammalian epidermis is a stratified epithelium composed by the interfollicular epi-dermis(IFE),hai
12、r follicles,sebaceous glands and sweat glands14.Lineage tracing experiments in mice have shown that during homeostasis,the distinct skin compartments are maintained by their own pool of lineage-restricted stem cells1519(FiG.2).However,during tissue regeneration such as wound healing,epithelial cells
13、 acquire plasticity Squamous cell carcinomas(SCCs).Cancers that present squamous differentiation,which is visible by the presence of keratin materials.Lineage tracingA method involving experiments that enable the labelling of a cell or a group of cells and assess the fate of these labelled cells and
14、 their progeny over time.Stem cellsCells that are at the top of the cellular hierarchy and are characterized by long-term self-renewing capacity and give rise to progenitors,transit-amplifying cells and differentiated cells.Deciphering the cells of origin of squamous cell carcinomasAdrianaSnchez-Dan
15、s1 and CdricBlanpain1,2*Abstract|Squamous cell carcinomas(SCCs)are among the most prevalent human cancers.SCC comprises a wide range of tumours originated from diverse anatomical locations that share common genetic mutations and expression of squamous differentiation markers.SCCs arise from squamous
16、 and non-squamous epithelial tissues.Here,we discuss the different studies in which the cell of origin of SCCs has been uncovered by expressing oncogenes and/or deleting tumour suppressor genes in the different cell lineages that compose these epithelia.We present evidence showing that the squamous
17、differentiation phenotype of the tumour depends on the type of mutated oncogene and the cell of origin,which dictate the competence of the cells to initiate SCC formation,as well as on the aggressiveness and invasive properties of these tumours.1Universit Libre de Bruxelles,Laboratory of Stem Cells
18、and Cancer,Brussels,Belgium.2WELBIO,Universit Libre de Bruxelles,Brussels,Belgium.*e-mail:Cedric.blanpainulb.ac.behttps:/doi.org/10.1038/s41568-018-0024-5 CANCER ORIGINSNaTuRe RevIeWS|CANCErReviews volume 18|SePTemBeR 2018|549such as hair follicles and infundibulum stem cells that get activated,migr
19、ate towards the wounded region and differentiate into suprabasal cells of the IFE1922.The IFE is a stratified squamous epithelium maintained by the existence of basal cells with high self-renewing capacities that balance proliferation and differentiation.How precisely the balance between renewal and
20、 differentiation is achieved to sustain the homeostasis of the epidermis remains a matter of dis-cussion and may vary between the different parts of mouse skin(ear,paw,tail,ventral and dorsal skin)2329.It was initially hypothesized that the epidermis is maintained by the existence of many small unit
21、s of proliferation called epidermal proliferative units,which contain slow-cycling stem cells that generate transit-amplifying cells,which,after a defined number of cell divisions,give rise to terminally differentiated cells30.Lineage tracing using AhCreER,a construct that combines the cytochrome P4
22、50 1A1(Cyp1a1)promoter with a Cre recombinase fused to the oes-trogen receptor(CreER)and leads to expression in a largely ubiquitous manner following the administra-tion of beta-naphthoflavone,has been used to assess the mode of epidermal homeostasis28,29.The CreER ena-bles control of the activity o
23、f Cre by the administration of tamoxifen,which mediates the translocation of Cre to the nucleus,the recombination of loxP sites and the expression of the reporter gene(FiG.1).Clonal analysis of AhCreER lineage tracing data in the IFE demon-strated that the clone size does not converge on a pre-cise
24、number of basal cells,suggesting that the unit of proliferation has no predefined fixed size.The clone size distribution could be explained by the presence of a single population of equipotent progenitors that balance renewal and differentiation in a stochastic manner28,29.More recent studies have d
25、emonstrated that epidermal cells are more heterogeneous than ini-tially anticipated and have illustrated that different CreER mice target different stem and progenitor cells with distinct proliferation rates and survival capacities,different long-term renewing capacities and different abilities to m
26、ediate long-term skin repair2427.The pop-ulation of progenitor cells targeted by the involucrin(Ivl)CreER in the mouse tail IFE is identical to the one targeted by the AhCreER25,28.By contrast,keratin 14(K14;also known as Krt14)CreER(K14CreER)targets progenitors and stem cells,the latter of which ha
27、ve long-term survival and divide asymmetrically to give rise to progenitors25,27(FiG.2a,b).Distinct anatomical regions compose the hair follicles,including the infundibulum,isthmus,sebaceous glands and lower hair follicle regions that produce the hair shaft.These different epidermal regions are main
28、tained by their own pool of resident stem cells1519,31(FiG.2a).Mouse?Fig.1|Lineage tracing and the cells of origin of cancer.Many cancers arise from epithelial tissues,which are maintained by stem cells and their progeny.Stem cells are at the top of the cellular hierarchy and have the ability to sel
29、f-renew and generate progenitors,which can self-renew and give rise to progenitors and terminally differentiated cells(part a).Tamoxifen-induced activation of the Cre recombinase under a cell lineage-specific promoter,for example,a stem cell-specific or progenitor-specific promoter,leads to the elim
30、ination of the STOP cassette between loxP sites,resulting in the expression of the reporter gene(part b),the expression of the oncogene or the deletion of the tumour suppressor gene(TSG)between loxP sites(part c)in the respective cell population and its progeny(parts b and c).If a progenitor-specifi
31、c promoter is used,the reporter gene is expressed in progenitors and their progeny and in differentiated cells but not in the stem cell population.A progenitor usually has a limited lifetime,and thus,the reporter expression may be lost over time(part b,right panel).Conditional activation of oncogene
32、s or deletion of TSGs has enabled the identification of the cells of origin in different mouse tumour models(part d).ER,oestrogen receptor.ProgenitorsCells that can self-renew and give rise to terminally differentiated cells.Depending on the proportion of asymmetric and symmetric divisions,progenito
33、rs can live long term or short term.Stratified squamous epitheliumEpithelium composed of a layer of basal proliferative cells and several suprabasal layers of differentiated cells that express keratins and progressively flatten near the surface,eventually presenting as enucleated cells that are shed
34、 from the surface.These amorphous keratinized ghost cells are known as squames.The inner surface of the body is lined with non-keratinized stratified squamous epithelium,which is characterized by superficial cells that are flattened and 2018|volume 18 hair follicle stem cells can be analysed by usin
35、g the K15 promoter-driven,the K19 promoter-driven or leucine-rich repeat-containing G protein-coupled receptor 5(Lgr5)promoter-driven Cre-inducible constructs K15CrePR18,K19CreER31 and Lgr5CreER15,respectively.In CrePR constructs,Cre is fused to the progesterone receptor(PR),and administration of RU
36、468,a PR ligand,promotes the translocation of Cre to the nucleus and its activity.The oral epithelium is maintained by a proliferative basal compartment.The oral epithelium is composed of areas of keratinized and non-keratinized strati-fied squamous epithelium(FiG.2b).The oral cavity and pharynx are
37、 coated with non-keratinized strati-fied epithelium.The masticatory mucosa is kerati-nized.The larynx is lined by ciliated pseudo stratified columnar epithelium,except the vocal cords,which are covered by stratified squamous epithelium.In all these epithelia,cell proliferation is restricted to the b
38、asal layer.Very little is known concerning the identity of stem cells in head and neck epithelia,the location of stem cell niches or how stem cells and progenitors balance proliferation and differentiation.Pulsechase triti-ated thymidine and 5-bromodeoxyuridine(BrdU)experiments revealed the existenc
39、e of slow-cycling,label-retaining cells in mouse oral epithelia,suggest-ing the existence of a pool of slow-cycling basal stem cells32.Lineage-tracing experiments in mice using the SRY-box 2(Sox2)CreER and K14CreER trans-genic systems,which drive Cre-inducible reporter gene expression in SOX2-expres
40、sing or K14-expressing basal cells,showed that cells located in the basal com-partment present long-term maintenance and can give rise to the different cell types that form the tongue and soft palate33,34.The oesophagus is maintained by basal progenitors.The mouse and rat oesophagus has a keratinize
41、d squa-mous epithelium composed of four to five cell layers with a rapid turnover,which is maintained by the presence of pro-liferative basal cells that are able to self-renew,differentiate and migrate towards the lumen,giving rise to suprabasal layers of terminally differentiated cells35(FiG.2b).Th
42、e cellular hierarchy that mediates the home-ostasis in mouse oesophageal epithelium remains a matter of intense debate33,3642.Clonal analysis using AhCreER transgenic mice suggests that the mouse oesophageal epithelium is maintained by a single,equipotent,committed progenitor cell population that ba
43、lances renewal and differentiation37.By contrast,lineage tracing studies using Sox2CreER or K15CrePR transgenic mice labelled a subpopulation of mouse basal cells with long-term maintenance and the ability to give rise to the differentiated cells,suggest-ing that the cells targeted by Sox2 promoter-
44、inducible or K15 promoter-inducible Cre mark a progenitor and/or stem cell population with higher self-renewal potential than that of committed progenitors33,41.Other studies using fluorescence-activated cell sorting isola-tion,colony forming assays and 3D organoid assays further suggested the prese
45、nce of heterogeneity within oesophageal epithelium in terms of marker expression,proliferation kinetics and ability to reform oesopha-geal epithelium39,40.Side by side comparison using the different Cre systems will be important to resolve this apparent discrepancy.The human oesophagus has a non-ker
46、atinized squa-mous epithelium composed of several layers and two anatomical compartments,the papillae and the interpap-illary region.A study in human oesophageal epithelium suggested the existence of two anatomically different compartments,one populated by a relatively quiescent stem cell population
47、(interpapillary region)and the sec-ond populated by transit-amplifying cells that give rise to terminally differentiated cells(papillae)38.However,in another study,the quiescent cells expressing the stem cell marker CD34 were found at the tip of the papil-lae,and the highly proliferative cells were
48、found at the interpapillary region42.Lung compartments are maintained by their own resident stem cells.Two main compartments consti-tute mammalian lungs:the airways and the alveoli.The respiratory system enables gas exchange and protects lung epithelia from microorganisms and dust particles that are
49、 constantly inhaled.All different lung compart-ments(trachea,bronchi,bronchioles and alveoli)are maintained by their own resident stem cells during homeostasis4347.Upon tissue damage,differentiated cells present some plasticity and can revert back to a basal stem cell fate48.The trachea and bronchi
50、are lined with a pseudo-stratified epithelium49,which consists of basal cells,secretory cells,ciliated cells and rare neuroendocrine cells.The murine bronchioles are lined with a simple columnar epithelium composed mainly of secretory and ciliated cells,containing some neuroendocrine cells and no ba
