p53 and its mutants on the slippery road from stemness to carcinogenesis
p53 and its mutants on the slippery road from stemness to carcinogenesis
Alina Molchadsky 0
Varda Rotte 0
0 Department of Molecular Cell Biology, Weizmann Institute of Science , Rehovot 76100 , Israel
Normal development, tissue homeostasis and regeneration following injury rely on the proper functions of wide repertoire of stem cells (SCs) persisting during embryonic period and throughout the adult life. Therefore, SCs employ robust mechanisms to preserve their genomic integrity and avoid heritage of mutations to their daughter cells. Importantly, propagation of SCs with faulty DNA as well as dedifferentiation of genomically altered somatic cells may result in derivation of cancer SCs, which are considered to be the driving force of the tumorigenic process. Multiple experimental evidence suggest that p53, the central tumor suppressor gene, plays a critical regulatory role in determination of SCs destiny, thereby eliminating damaged SCs from the general SC population. Notably, mutant p53 proteins do not only lose the tumor suppressive function, but rather gain new oncogenic function that markedly promotes various aspects of carcinogenesis. In this review, we elaborate on the role of wild type and mutant p53 proteins in the various SCs types that appear under homeostatic conditions as well as in cancer. It is plausible that the growing understanding of the mechanisms underlying cancer SC phenotype and p53 malfunction will allow future optimization of cancer therapeutics in the context of precision medicine.
The processes underlying normal development, tissue homeo- germ layer. Whereas, oligopotent SCs are able to give rise to a
stasis and regeneration upon injury include replication of exis-t more limited subset of cell lineages than the multipotent ones.
ing cells, expansion and differentiation of stem and progenitor Finally, unipotent SCs are able to produce only one mature
cells, as well as trans-differentiation or dedifferentiation of cell type. Therefore, oligopotent and unipotent SCs are termed
cells from one cell type to another. The hallmark character-is progenitor cells (3,4). Importantly, preservation of genomic
tics of stem cells (SCs) are self-renewal, the ability to proliferate fidelity in SCs is of key importance for normal development
indefinitely throughout life and the capacity to undergo d-if and regeneration. On the contrary, lack of SCs integrity may
ferentiation into various cell types, upon specific signals1,(2). result in developmental abnormalities and the rise of cancer
SCs persisting throughout both embryogenesis and postnatal stem cells (CSCs) that greatly contribute to various aspects of
period can be classified according to their distinct different-ia tumorigenesis.
tion potency. Totipotent SCs, found in a zygote are able to ge-n The p53 protein (encoded by the human geneTP53) is a
piverate all embryonic and extra-embryonic cell types. Pluripotent otal tumor suppressor, designated ‘the guardian of the genome’
SCs, such as embryonic stem cells (ESCs) and induced pluripo- (5), because of its major role in maintenance of genomic st-a
tent stem cells (iPSCs) have the capacity to differentiate into all bility and fidelity, thereby preventing cancer development6)(.
cell types of the embryo proper. Multipotent SCs, such as me-s Indeed, recent studies demonstrated that elephants who are
enchymal stem cells (MSCs) and hematopoietic stem cells pr-e resistant to cancer containing multiple copies of the p53 gene
dominantly found in adult tissues are capable of contributing (7). Notably, p53 function is disrupted in the majority of human
to various cell types usually derived from the same embryonic tumors. While mutations in the p53 gene were detected in more
adipose-derived stem cell
cancer stem cell
embryonic stem cells
gain of function
hematopoietic stem cell
induced pluripotent stem cell
loss of heterozygosity
mesenchymal stem cell
Thus, while the possibility of prolongedin vitro propagation
sets ESCs as valuable resources for regenerative medicine, it also
may acquire ESCs with malignant potential, thereby comprom-is
ing their safety for cellular therapies. Additional disadvantages
of ESCs hindering their use for basic research and transplan-ta
tion therapy are the possibility of tissue rejection and ethical
concerns referring toex vivo destruction of human embryos.
Characterization of iPSCs
A decade ago Takahashi and Yamanaka introduced the groun-d
breaking technology allowing the generation of iPSCs from
mouse somatic cells by ectopic co-expression of the transcri-p
tion factors Oct4, Sox2, Klf4 and Myc (OSKM)2(1,22). The
availthan half of human cancer cases, its activity may be abrogated ability of man-made iPSCs provided new opportunities for the
by alternative mechanisms in the other cases8(,9). development of autologous personalized medicine and disease
In this review, we will describe the fundamental roles of wild modeling by overriding the ethical concerns associated with the
type (WT) p53 and its mutants in the biology of the various SCs use of ESCs. Despite the initial observation that iPSCs succe-ss
types. fully mimic ESCs, sharing a similar morphology, proliferation
and pluripotent features, subsequent reports demonstrated that
Bridging between SCs, CSCs and cancer the mRNA and miRNA expression signatures, epigenetic
landscape as well as protein expression and protein phosphoryl-a
Characterization of ESCs tion of iPSCs are distinguishable from ESCs2(3–26). Interestingly,
ESCs derived from the inner cell mass of the blastocyst have iPSCs can be generated by introduction of variable combination
the potential to give rise to cells that comprise the three germ of reprogramming factors, besides OSKM 2(7,28). For example,
layers: endoderm, mesoderm and ectoderm 1(0). ESC can be it was recently found that ectopic expression of Sall4, Nanog,
cultivatedin vitro for a prolonged period of time without lo-s Esrrb and Lin28 in mouse embryonic fibroblasts generated
highing their pluripotency and self-renewal capacity, when suppl-e quality iPSCs more efficiently than other combinations of factors
mented with reagents that prevent induction of differentiation including OSKM (29). In addition, it was shown that iPSCs could
(11). The balance between ESCs self-renewal and differentiation be derived from a wide repertoire of cell types such as blood,
is maintained by reciprocal regulatory networks governed by the stomach and liver cells, keratinocytes, melanocytes, pancreatβic
transcription factors OCT4, SOX2 and NANOG that function as cells and neural progenitors 2(8). Of note is the observation that
master regulators of ESCs identity12(,13). Preservation of proper cell origin influences the molecular and functional properties of
genomic integrity is fundamental for maintenance of rapid pr-o the established iPSCs. This is attributed to the findings that iPSCs
liferation and normal embryogenesisin vivo. As ESCs are the retain a transient epigenetic memory of their original somatic
precursors of all adult organ systems, they must engage mech-a cells (30,31). Furthermore, high resolution studies indicated that
nisms that prevent the propagation of mutations, generated during reprogramming the genetic integrity of the iPSCs may
as a consequence of DNA damage, to their descendant mature be occasionally impaired. Indeed, iPSCs may gain genetic
abersomatic cells. Indeed, it was found that compared with somatic rations including copy-number variations and protein-coding
cells ESCs display lower mutation frequencies, as well as they point mutations 3(
). Notably, it is now well-accepted the
possess extreme sensitivity to DNA damage and exhibit el-e molecular mechanisms that underlie the generation of iPSCs are
vated DNA repair capacity. Alternatively, ESCs with faulty DNA remarkably similar to those that are deregulated in cance3r4)(.
may be eradicated from the self-renewing SC pool by induction Among these are dedifferentiation, gain of accelerated prolifer-a
of either cell death or differentiation1(4). Nevertheless, multiple tion accompanied by senescence bypass, cancer-like changes in
line of evidence indicated that long-term culturing of ESCs may metabolic profile and dynamic changes in epigenetic
modificalead to gain of genomic abnormalities manifested by extensive tions that may acquire genetic alterations in cancer associated
acquirement of point mutations, numerical and structural ch-ro genes. Accordingly, while on one hand the resemblance of iPSCs
mosomal aberrations as well as variation of telomere length and generation and tumor development places the reprogramming
alterations in epigenetic status, thereby conferring ESCs with technology as a valuable tool for cancer modeling, on the other
tumorigenic potential1(5–17). hand the risk that iPSC may attain malignant characteristics
Indeed, it has been already declared that ESCs share cellular hampers their safety for regenerative therap3y5().
and molecular phenotypes with cancer cells. Among these are
rapid proliferation rate, lack of contact inhibition, high ac-tiv Characterization of adult SCs
ity of telomerase, as well as high expression of oncogenes such Adult SCs usually reside in specific niches of self-renewing
tisas c-MYC and KLF4 (17). Upon injection into immune-compro- sues including intestine, skin, muscle, fat, bone marrow and
mised mice, bona fide ESCs give rise to teratomas, fully differe-n nervous system. Normally, adult SCs are quiescent, however,
tiated benign tumors1(0). However, several studies have shown upon specific activation signals, they significantly promote
that transplantation of culture-adapted ESCs resulted in g-en either normal tissue turnover or repair and regeneration foll-ow
eration of aggressive tumors, such as teratocarcinom1a7(–19). ing injury (
). Notably, adult SCs number and function declines
Concomitantly, it is was demonstrated that various types of with age in various tissues as a result of reduction in the act-iv
poorly differentiated aggressive human tumors, exhibiting the ity of gate keeping tumor suppressors, DNA damage accum-u
worst prognoses, display an enriched ESCs-like gene expression lation, changes in cellular physiology as well as environmental
signature. Moreover, activation targets of Nanog, Oct4, Sox2 and changes in tissues (
). These, age-related changes in the
c-Myc are observed more frequently in poorly differentiated function of SCs and other progenitors may contribute to
agetumors than in well-differentiated tumor2s0(). associated degenerative diseases 3(7).
Multipotent SCs such as MSCs and hematopoietic stem cells SC niche, a microenvironment that is crucial for the main-te
also present in neonatal tissues such as umbilical cord and pl-a nance of SCs properties 4(4).
centa. Regardless of their origin MSCs were shown to be able to Thirdly, it has been proposed that endogenous
reprogramdifferentiatein vitro into chondrocytes, osteocytes, muscle cells, ming or residues of the embryo might serve as the source of
adipocytes or even neurons and glia 3(9). Considering the afore- CSCs (
mentioned concerns regarding the usage of ESCs and iPSCs for The major difference between normal SCs and CSCs lies
clinical purposes, being multi-potent, hypoimmunogenic and on the ability to regulate stemness pathways including Wnβt-/
easy to isolate MSCs represent promising candidates for auto-lo catenin, JAK-STAT, TGF-β, Notch, Hippo, etc. In normal SCs,
gous transplantation3(6). Nevertheless, one should bear in mind these pathways are tightly controlled with intact genetics and
that duringin vitro culturing adult SCs including MSCs tend to epigenetics. In CSCs deregulation of these pathways along with
undergo replicative senescence that is typically accompanied improper interactions between them may represent key events
with progressive loss of proliferation and differentiation ability, for the propagation and pathogenesis of CSCs4(
morphological changes, high expression of tumor suppressors Investigation of CSCs in multiple tumor types have identified
and increased telomere shortening 4(0). Moreover, despite the dozens of markers specific to these cells. However, several mar-k
fact that adult SCs typically divide less frequently than plu-ri ers are shared between CSCs and normal SCs4(9). CD133 is one
potent SCs, they are still prone to acquire chromosomal aber-ra of the most common cell surface markers used to identify CSCs
tions during expansion in culture4(1). in breast, prostate, pancreas and lung tumor4s9(
surface molecules that were proposed for CSCs isolation are
Characterization of CSCs CD34+/CD38−, CD44+/CD24−, CD44 alone or in combination with
Carcinogenesis can be explained by two major models, the ‘st-o other antigens, and ephrin receptors (review in55). Aldehyde
chastic’ model and the ‘CSCs’ model. According to the stochastic dehydrogenase 1 (ALDH1) is considered an internal marker for
model all tumor cells result from clonal evolution through the normal and malignant SCs and progenitor cells. High ALDH1
acquisition of genetic mutations and epigenetic changes and activity is associated with chemo-resistance, aggressiveness
are equipped with a similar tumorigenic potential. Whereas, the and poor clinical prognosis in many cancer types 4(
CSCs model suggests that similarly to normal tissues, tumors main stemness transcription factors, Nanog, OCT4 and SOX2
show population hierarchy, where a subpopulation of CSCs has were also found to be highly expressed in CSCs (
evithe greatest tumorigenic activity compared to the other cells dence suggests that YAP1 and BMI1 are important intra-cellular
comprising the tumor bulk 4(
). CSCs are defined as rare, CSCs markers that endow SCs characteristics in multiple gast-ro
malignant cells within the tumor, which are able to self-renew intestinal origin and head and neck cancers, respectively5(8,59).
and differentiate and thus can restore the entire original cell Elucidation of the mechanism underlying CSCs features is
pool of the tumor when injected into immune-deficient mice expected to permit development of novel cancer therapeutics
). Being relatively quiescent, CSCs are resistant to chem-o targeting specifically CSCs, thereby eradicating the tumor and
therapies, which mainly target rapidly dividing cells43(). The avoiding recurrence and drug resistance.
hallmark traits of CSCs include their heterogeneity, inter-ac
tCiSoCns walistohpmosiscersosentvhieropnrmopeenrttyaonfd‘rpolbausstticnietsys4’6,()w.Ihnichadidnictliuodne,s p53 in health and disease
a number of features such as the capacity to perform efflux of Under normal physiological conditions p53 is maintained in low
cytotoxic agents, resistance to oxidative stress and a fast DNA levels, due to continuous ubiquitination by its predominant ne-g
damage repair, all of which facilitate drug resistance43(
). ative regulator, MDM2 that mediates its degradation by nuclear
Furthermore, tumors with CSCs entity have been shown to p-os and cytoplasmic proteasomes6(0). Activation of p53 is driven by
sess a higher propensity for metastasis in several cancers49(). various cellular insults including DNA damage, oncogene act-i
The precise origin of CSCs is a debatable issue at present. vation, telomere erosion and hypoxia. These stress signals lead
There are three main theories for the acquisition of the CSCs to p53 protein stabilization mainly by blocking its interaction
). The first theory postulates that CSCs originate with MDM2 and by a series of post-translational modifications
from malignant transformation of normal adult SCs that have (
). Once activated, aiming to eliminate the proliferation of
undergone accumulation of epigenetic and genetic alterations. aberrant cells, p53 can determine the cellular fate by induction
In this respect, a controversial report recently suggested that of cell cycle arrest, senescence or various types of cell death
the variation in lifetime cancer risk for several tissues is p-re such as apoptosis, autophagy or ferroptosis, largely dependent
dominantly attributed to random, ‘bad luck’ mutations arising on the severity and type of damage, as well as on the context
during DNA replication in normal, noncancerous SCs maintai-n of cellular microenvironment 6(
). The p53 protein structure
ing that tissue’s homeostasis. Utilizing meta-analysis, Tomasetti consists of the amino-terminal transactivation domain, a p-ro
and Vogelstein established that two-third of the human cancer line rich domain, a core DNA-binding domain, oligomerization
types examined originate from SCs 5(0). However, the ‘bad luck’ and a carboxy-terminal regulatory domain66(). p53 functions
hypothesis was later undermined by the study of Wuet al., who primarily as a sequence-specific tetrameric transcription factor,
provided multiple lines of evidence, obtained from mathemat-i which regulates a multitude of downstream target gene6s7(,68).
cal, epidemiological and molecular models, that cancer risk is In addition, accumulating data suggest that protein–protein
profoundly influenced by extrinsic factors, while intrinsic risk interactions play an essential role in fine-tuning p53 activity at
factors contribution to carcinogenesis is rather modes5t1,(
). different levels of regulation 6(9,70). Notably, p53 may exert its
The second theory pertaining the origin of CSCs relies on the effects not only inside a given cell, rather it can act in a
nonnotion that tumors can acquire plasticity, which allows prog-eni cell-autonomous manner, thereby affecting the cell surroun-d
tor or differentiated cells to undergo dedifferentiation acco-m ings (71–73).
panied by epithelial–mesenchymal transition (EMT) and hence Although, the hallmark of p53 is its tumor suppressor act-ivi
to become CSCs (
). Such phenotypic plasticity may result ties, ample data collected during more than three decades of
from dynamic epigenetic changes regulated by signals from the research indicates that p53 functions a homeostatic regulator
that coordinates a wide variety of additional cellular processes mice knocked in with either conformational mut-p53R172H or
including metabolism (74,75), inflammation (76), aging (63,77), the DNA contact mut-p53R270H, equivalent to the codons 175
reproduction 7(8) differentiation and development 7(9,80), and 273 in humans. These mice displayed a wider tumor range
regeneration 8(1) and others. and higher metastatic frequency compared with p53 null mice.
The majority of p53 mutations found in human tumors result Correspondingly, mut-p53 homozygous primary cells derived
from missense substitutions. As a consequence, a dysfunc- from these mouse models exhibited accelerated cell prolifer-a
tional full-length p53 protein containing a different amino acid tion, DNA synthesis and high transformation potentia9l4(,95).
in the mutation site is translated82(,83). Although analysis of Over the years of p53 research, great efforts have been de-di
numerous human tumors indicated that mutations may occur cated to elucidate the mechanisms underlying mut-p53 GOF.
in diverse locations along the p53 coding sequence, the distr-i Ample data obtained from variousin vitro and in vivo models
bution of p53 mutations is not random. Most of the mutations indicated that the effect of mut-p53 GOF has been implicated in
in p53 are concentrated within the sequence encoding for the various processes associated with tumorigenesis9(,92). These
DNA-binding domain of the p53 protein, resulting in the mutant include deregulation of cell cycle and enhanced cellular
prolifp53 (mut-p53) protein that is incapable of binding the respo-n eration (
), protection from cell death
sive elements in the DNA of its classical target genes and exert and increased survival 1(
), maintenance of chronic
inflamtheir trans-activation9)(. Approximately 40% of p53 missense mation (
) invasion and metastasis 1(04,105) as well as
mutations involve the following six ‘hot spot’ residues: R175H, drug resistance 1(
). Of note, the various p53 mutants utilize
G245S, R248Q/W, R249S, R273H and R282W (84). p53 mutations distinct mechanisms to execute their oncogenic GOF. Moreover,
may be categorized into two main classes encompassing the the effect and the degree of oncogenic GOF exhibited by the p53
‘DNA contact’ mutations and ‘conformational’ mutations. While mutants is dependent on the type of nucleotide substitution,
‘DNA contact’ mutations referred to substitutions in amino specific position in the p53 gene DNA sequence and the
particuacids that involve sequence-specific DNA binding and is repr-e lar cellular conditions; thus further entangling the relationship
sented by R248Q and R273H, ‘conformational’ mutations such as between mut-p53 genotype and phenotype8(
R175H, G245S, R249S and R282W cause modifications in protein Importantly, it has been generally considered that the ability
structure, thereby compromising the capacity to bind DN8A5(). of mut-p53 to accumulate in the cells serves as a prerequisite
Apparently, a considerable variation in the frequency of p53 for exhibition of its GOF 1(
). In particular, the conclusion
mutations is observed in diverse cancer types. For example, deduced from animal models was that mut-p53 proteins do not
while it was found that only 10–20% of hematopoietic maligna-n accumulate in normal tissues, rather they are highly expressed
cies exhibit p53 mutations 8(6), strikingly it was demonstrated predominantly in tumor cells 9(
). Similarly to the WT
that p53 is mutated in 96% of high grade serous ovarian car-ci p53 protein, upon cellular stress mut-p53 undergoes post-tra-ns
noma cases (87). lational modifications and becomes stabilized, however unlike
Unlike p53 somatic mutations that promote the devel-op the WT protein, mut-p53 is not targeted for degradation by
ment of sporadic tumors, p53 germline mutations are associated MDM2, rather remains constitutively active10(9). Interestingly,
with the rare familial cancer predisposition termed Li-Fraumeni a recent elegant study using a novel mut-p53 mouse model
Syndrome (LFS). LFS patients inherit one copy of p53 mutated expressing the R248Q hotspot mutation demonstrated that
gene in every cell of their body and tend to develop early onset constant expression of stabilized mut-p53 is indispensable for
of wide spectrum of cancer types, including bone and soft-tissue tumor maintenancein vivo. It was found that HSP90/HDAC6
sarcomas, acute leukemia, breast cancer as well as brain cancers chaperone machinery is a major determinant of mutp53 st-a
such as glioblastoma and adrenocortical tumors88(,89). bilization. Ablation of mut-p53 or long-term HSP90 inhibition
Similarly to the state of LFS patients, in the course of s-po that triggers mut-p53 destabilization led to diminished tumor
radic tumorigenesis, a mutation in p53 gene initially arises only growth and extended survival1(13). This observation is
consistin one of the alleles, resulting in p53 heterozygous cells that co-n ent with the oncogene addiction concept1(14) postulating that
comitantly express both WT and mut-p53 proteins. In this case, although tumors display various genomic aberrations, some of
mut-p53 may counteract the WT p53 tumor suppressor act-ivi them are dependent on single-specific oncogene, in this case
ties via a dominant-negative mechanism, manifested by he-t mut-p53, which is crucial for both maintenance of the mal-ig
ero-oligomerization with wild-type p53 through the C-terminal nant phenotype and cell survival. Additional relatively new
tetramerization domain9(0). Eventually, during cancer progre-s mechanism suggested for mut-p53 both dominant-negative and
sion, the remaining WT allele is lost, in a process termed loss of oncogenic GOF activities is its ability to aggregate into prion-like
heterozygosity (LOH). Interestingly, recently performed genomic amyloid oligomers and fibrils. These aggregates sequester the
and transcriptomic meta-analyses revealed that more than 93% p53 protein, thereby abolishing its transactivation function and
of sporadic tumors harboring mut-p53 undergo p53 LOH91(). anti-proliferative effects1(
). It was lately shown that p53
These findings support the notion that p53 is a recessive tumor protein aggregation promotes platinum resistance in ovarian
suppressor and loss of the remaining WT allele is essential for cancer (
). Moreover, Soragniet al. (
), have demonstrated
tumor development. that a small cell penetrating peptide, ReACp53 designed to int-er
Nowadays, it is well-accepted that besides abolishing the rupt mut-p53 aggregation into amyloid-like state rescues p53
tumor suppressive functions of the remaining WT protein, mut- tumor suppression in ovarian carcinomas.
p53 proteins acquire new oncogenic functions that significantly All in all, this data suggest that depletion, destabilization or
contribute to the malignant properties of the cells. This p-he disaggregation of mut-p53 might be potential therapeutic st-rat
nomenon is referred as mut-p53 gain of function (GOF) 9(,92). egy to suppress tumor progression.
The first report launching the concept of mut-p53 GOF was done
bmyuDt-ipt5t3mperroett eainl.s(9in3t),owp5h3oddeefimcioennsttcrealltseednthhaantceidnttrhoedirutcutimono-roi f The role of p53 in ESCs
genic potential. The most compelling evidence for mut-p53 GOF ESCs possess robust mechanisms to preserve their genomic
was provided later following the establishment of transgenic integrity aiming to avoid transferring genetic alterations to their
progeny. Therefore, it is not surprising that p53 ‘the guardian of Albeit the fundamental role of p53 in ESCs differentiation
the genome’ was found to play an important role in maintaining programs, it was found that p53 null and p53 mutant
knockedESCs genomic stability by regulating their differentiation and in mice are born viable and undergo quite normal development
apoptotic pathways. with only minor abnormalities, although they develop a variety
Unlike human ESCs in which p53 is expressed at very low of malignancies, later in postnatal life7(9,94,95,134–137). The
levels due to negative regulators, HDM2 and TRIM24, that tr-ig relatively normal development of p53 knockout mice may be
ger p53 degradation 1(
), several lines of evidence indicated explained by existence of distinct compensatory mechanisms
that mouse ESCs display high levels of the p53 protein, located executed by p53 family members, such as p63 and p73, as well
mainly in the cytoplasm 1(
). These increased p53 levels in as by other tumor suppressor genes7(9,80). Nevertheless, we
undifferentiated mouse ESCs are mediated by low expression of found that p53-deficient mouse ESCs display a high incidence
miRNA-125a and 125b (
). Interestingly, despite the abundance of karyotype abnormalities, including cells with increased nu
mof p53 in ESCs, at first glance it seemed like p53 does not tran-s ber of chromosomes and translocations. Interestingly, mouse
locate to the nucleus in response to DNA damage and does not ESCs knocked-in with conformational mut-p53 R172H remain
induce apoptosis in ESCs (
). However, later in depth studies pluripotent and benign and have a relatively normal karyotype
demonstrated that dependent on the type of cellular insult p53 compared with ESCs knocked out for p53. Strikingly, we disco-v
may induce apoptosis of mouse ESC, both while it remains in ered that in these ESCs mut-p53 is stabilized into a WT p53 co-n
the cytoplasm or translocates to the nucleus. For, instance, in formation. High-content RNA sequencing analysis of mut-p53
response to reactive oxygen species (ROS) cytoplasmic p53 tri-g ESCs indicated that in response to DNA damage p53 is transcr-ip
gers mitochondrial-dependent apoptosis in mouse ESCs 1(25). tionally active and able to elevate the levels of classical WT p53
Alternatively, following ultra violet (UV) radiatioγni,rradiation or target genes, such as p21 and btg2. Utilizing mass
spectrometrychemotherapy treatment p53 is able to enter to the nucleus and based technology we identified a network of proteins, including
trans-activate its classical target genes including p21, mdm2, the chaperonin CCT complex, USP7, Aurora kinase, Nedd4 and
puma and noxa and induce apopotosis 1(
). In con- Trim24, that bind mut-p53 and thus may shift its conformation
trast to differentiated cells, upon exposure to damaging agents to a WT form. Such conformational shift is a novel mechanism
ESCs do not undergo the G1/S cell cycle arrest due to functional of maintenance of genomic integrity in ESCs 1(
accorduncoupling of p53-p21 signaling (128). Apparently, ESCs possess ance with these findings it was reported by Trinidaedt al., that
an alternative mechanism underlying p53-dependent mai-n proper folding of WT p53 is promoted by an interaction with the
tenance of genetic stability, manifested by inducing the diffe-r CCT complex. Moreover, mut-p53 proteins unable to interact
entiation into other cell types. More specifically, a land mark with CCT exhibited conformational instability, facilitated in-va
report by Linet al., provided evidence that following DNA dam- sion and random motility that is similar to the activity of
tumorage induced by UV radiation or doxorubicin treatment activated derived p53 mutants 1(
p53, phosphorylated on serine 315, suppresses the expression of Taken together, p53 significantly contributes to preservation
the master regulator of pluripotentcy, Nanog, by direct binding to of ESCs integrity by inducing cell death or differentiation of cells
its promoter, thereby inducing mouse ESCs differentiation12(6). with faulty genome. It might be suggested that the identified
Interestingly, later by integrating global gene expression and proteomic network responsible for the stabilization of the
mutcomputational analyses, Leeet al., discovered that phosphoryla- p53 protein into WT conformation, leading to the maintenance
tion of p53 on serine 212 and serine 312 by Aurka leads to upreg-u of genomic integrity in pluripotent cells, can serve as the basis
lated p53 activity that triggers ESC differentiatio1n29(). However, for development of future anticancer therapeutics.
in human ESCs the mechanism underlying p53-dependent
iinndduuccetiodniffeorfednitfifaetreionntiiantihounmisandiEfSfeCrse,npt5.3Itacwetayslastheodwonntlhysaitneto p53 as a barrier to iPSCs generation
373 activates expression of miR-34a and miR-145, which in turn Reprogramming of mature somatic cells into iPSCs is an inefficient
repress the pluripotency factors OCT4, KLF4, LIN28A1(19). In process due to existence of multiple barriers. Thus, many stu-d
concordance, a whole-genome study of p53-mediated DNA dam- ies were aimed to decipher the molecular mechanisms that may
age signaling in ESCs indicated that in response to DNA damage, improve the quality and efficiency of reprogramming1(
p53 affects the status of ESCs through activating differentiation- generation of iPSCs and malignant transformation share many
associated genes and repressing ESCs-enriched genes (130). common characteristics3(5) it is not surprising that studies exa-m
Interestingly, while it was observed that in human ESCs p53 l-ev ining the role of p53 found that WT p53 serves as a barrier of the
els increase upon differentiation induction1(19), it was demon- reprogramming process. In accordance, p53 diminution markedly
strated that in mouse ESCs differentiation led to a decrease in increased the efficiency of iPSCs generation 1(
examthe levels of p53 and to a shift in its conformational status to the ple, Utikalet al. and Li et al., referred to the observation that onset
mutant form, with a concomitant loss of functional activi1t2y1(). of senescence, which often results from increase in expression
Of note, the pro-differentiation effect of p53 aiming to eliminate of the cell cycle inhibitors such as p16INK4a, ARF and p21Cip1
genomically damaged self-renewing cells is in line with the well as well as activation of p53, decrease reprogramming potential. It
accepted notion that p53-dependent induction of different-ia was found that immortal fibroblasts deficient in components of
tion in cancer cells blocks cancer progression 7(9,80). Moreover, the Arf-Trp53 pathway exhibited increased reprogramming kin-et
it was observed that in somatic cells p53 may either promote or ics and a significantly higher efficiency than WT cells, endowing
attenuate differentiation in a given cell type depending on c-el almost every somatic cell that normally fail to reprogram with the
lular fate and distinct micro-environmental signals79(,131,132). potential to form iPSCs1(42,146). Notably, in contrast Hannaet al.,
In accordance, using a genome-wide study Lee et al., revealed reported that the majority of the cells underwent reprogramming
an anti-differentiation function of p53 in mouse ESCs through without depleting p53 or immortalizing the cells. They suggested
directly regulating the Wnt signaling pathway. Thus suggesting that p53 deficiency only increased the kinetics of reprogramming,
that p53 has a dual function also in mouse ESCs, regulating both without affecting the overall efficiency of the reprogramming
pro-differentiation and anti-differentiation program1s33(). process (148).
Others demonstrated that introduction of reprogramming DNA damage and chromosomal aberrations, that contribute to
factors can activate the p53 pathway. Importantly, down--reg enhanced tumorigenic potential. Hence, while cells with abr-o
ulation of p53 enabled fibroblasts to give rise to iPSCs using gated p53 pathway are not suitable for therapeutic applications,
only Oct4 and Sox2, instead of OSKM group. However, constant recent findings demonstrate the plausibleness to utilize
mutsuppression of p53 led to lower quality of iPSCs that displayed p53 expressing iPSCs in studying LFS associated malignancies.
genomic instability 1(
). Concomitantly, Marion et al.,
reported that p53 is crucial to prevent generation of iPSCs -car The role of p53 in adult SCs
rying various types of DNA damage, including short telomeres,
DNA repair deficiencies or exogenously inflicted DNA damage. Ample reports have set p53 as a regulator of adult SCs diff-er
Reprogramming in the presence of pre-existing, but tolerated, entiation. Apparently, p53 may either induce or suppress cel-lu
DNA damage is aborted by the activation of a DNA damage lar differentiation not only by restricting proliferation, but also
response and p53-dependent apoptosis 1(
). by controlling master differentiation regulators. In this section,
Later studies uncovering the mechanisms by which p53 we will briefly describe the most prominent studies highligh-t
counteracts the reprogramming process showed that miR- ing the p53 regulation and function in neural stem cells (NSCs)
34a, a p53 target, plays an essential role in restraining somatic and MSCs.
reprogramming by repression of pluripotency genes, including
Nanog, Sox2 and N-myc (149). Among other p53-regulated ta-r The function of p53 in NSCs
get genes, it was shown that p21, but not Puma played a partial Deranged neuronal development resulting in exencephaly in
role in inhibition of iPSCs formation probably by attenuating cell approximately a quarter of p53 null embryos, was one of the
division. Moreover, reactivation of p53 at any time point during initial evidences that p53 plays a prominent role in this process.
the reprogramming process not only interrupted the formation This neural tube malformation is a result of either extensive cell
of iPSCs, but also induced newly formed iPSCs to differentiate outgrowth or reduced apoptosis in the neural tissu1e3(7,156).
(150). Interestingly, our findings indicated that p53 restricted NSCs are self-renewing cells in the nervous system that
mesenchymal-to-epithelial transition (MET) during the early can generate both neurons and glia1(
). p53 was shown to be
phases of reprogramming and that this effect was primarily involved in the regulation of proliferation and differentiation of
mediated by the ability of p53 to inhibit Klf4-dependent activ-a neural stem and progenitor cells, promotion of neuronal mat-u
tion of epithelial genes (151). ration and axonal growth and regeneration following neuronal
Furthermore, we found that mut-p53 R172H exhibited GOF injury, at large 1(
). In particular, it was reported that p53 is
by markedly enhancing the efficiency of the reprogramming expressed in the NSCs in the adult brain and negatively reg-u
process, compared with p53 deficiency. Although mut-p53 iPSCs lates NSCs self-renewal (159). Furthermore, it was demonstrated
maintained their pluripotent capacitiny vitro, following injection that in response to genotoxic insults such as X-ray or etoposide
to immune suppressed mice, the reprogrammed cells expressing treatment, p53 maintained chromosomal stability in NSC1s6(0).
mut-p53 lost this capability and gave rise to malignant tumors, Using transcriptomic profiling and functional studies of murine
instead of teratomas generated by iPSCs expressing WT15(2). NSCs Zheng et al., established that cooperative actions of p53
Later it was reported that distinct mut-p53 proteins have di-ffer and Pten down-regulating the expression of Myc are crucial for
ent reprogramming efficiencies 1(
). Interestingly, we observed the regulation of normal and malignant stem/progenitor cell
that the reprograming kinetics of mut-p53 heterozygous (p53 differentiation, self-renewal and tumorigenic potential16(1).
R172H/WT) mouse embryonic fibroblasts into iPSCs are comp-a Another mechanism suggested for the control of p53 in NSCs
rable to WT p53 homozygous cells and similarly to WT p53 iPSCs, proliferation involves the inhibition of the activity and nuclear
mut-p53 heterozygous iPSCs gave rise to teratomas. Unlike he-te localization of GLI1, which expression induces an increase in
rozygous mouse embryonic fibroblasts that robustly underwent the number of NSCs. In turn, GLI1 represses p53 through activ-a
LOH in culture, the majority of the iPSCs retained heterozygosity tion of Mdm2, establishing a homeostatic inhibitory loop that
for prolonged periodin vitro. Nevertheless, a small percentage of normally controls the number of NSCs. Accordingly, primary
iPSCs that underwent p53 LOH gave rise to malignant tumorins, glioblastoma SCs cultures expressing mut-p53 display increased
vivo. This observation suggests that p53 LOH is attenuated d-ur levels of GLI1, consistent with GLI1-driven SCs expansion and
ing the reprogramming process 1(
). Intriguingly, in a recent tumourigenesis (162). Moreover, NSCs harboring mut-p53 di-s
study Lee et al., utilized iPSCs generated from fibroblasts of played impaired TGF-β induced growth inhibition, which
conLFS patients heterozygous for mut-p53 G245D to investigate tributes to their increased proliferation rat1e63(). Alternatively,
the role of mut-p53 in a model of osteosarcoma development it was reported that p53 regulates NSCs proliferation and -dif
(154). The LFS-derived iPSCs were induced to differentiate first ferentiation via the BMP-Smad1 pathway and its target gene Id1.
into MSCs and then into osteoblasts, which are the main cell Interestingly, Armesilla-Diazet al., demonstrated that differe-n
type sustaining osteosarcoma1(
). It was found that LFS- tiation of p53 knockout-derived neurospheres, that are enriched
iPSCs-derived osteoblasts recapitulated osteosarcoma features for NSCs was biased toward neuronal precursors, compared to
including defective osteoblastic differentiation and tumorigenic WT p53 neurospheres that were able to undergo differentiation
potential. In addition, integration of global transcriptional and into astrocytes, oligodendrocytes and neurons16(0). However,
computational analyses indicated that mut-p53 exerted GOF Zhou et al., reported that depletion of p53 alone could convert
by suppressing H19 expression and its associated imprinted fibroblasts into astrocytes, oligodendrocytes and functional
gene network component, decorin that is responsible for LFS- neurons. Apparently, knockdown of p53 upregulates neurogenic
associated osteosarcoma development1(
). transcription factors, which in turn boosts fibroblast-neuron
All in all, p53 serves as a roadblock to reprogramming, faci-li conversion (164). Moreover, a recent study demonstrated that
tating cell cycle arrest, senescence or cell death in cells with pre- suppression of p53, in conjunction with cell cycle arrest and
existing DNA damage, thereby preventing the reprogramming appropriate cell culture environment markedly increases the
of aberrant cells. Although p53 deficiency or mutation allow efficiency in the trans-differentiation of human fibroblasts to
efficient generation of iPSCs, such iPSCs may carry persistent induced dopaminergic neurons by Ascl1, Nurr1, Lmx1a and
miR124 (165). These findings underscore the complex regulation compared to their WT p53 expressing counterparts. Moreover,
of p53 in the neuronal differentiation process. p53 may diffe-r MSCs lacking p53 displayed genomic instability, changes in
entially regulate a the plasticity of diverse cell types, i.e. NSCs the expression of c-myc and anchorage-independent growth
and fibroblasts, depending on the cell of origin and extracellular (166,177). Interestingly, it was shown that MSCs lacking the p21
surroundings. gene and bearing only one functional allele of WT p53
underwent p53 LOH during prolonged culturing that conferred them
The role of p53 in MSCs with tumorigenic potential, manifested in formation of sar-co
Multipotent MSCs reside mainly in the bone marrow but can mas upon injection into immune-compromised mice 1(78). In
also be found in other tissues including bone, adipose tissues, agreement, our data showed that MSCs heterozygous for
mutmuscle and many more. MSCs play an important role in no-r p53 predominately undergo p53 LOH conferring them with the
mal tissue turnover as well as in regeneration and repair f-ol high carcinogenic potential 1(
lowing injury (
). Nonetheless, it was reported that MSCs Taken together, multiple mechanisms have evolved to ensure
may contribute to tumor development. They were suggested proper p53 regulation of the various aspects of MSCs biology
to be a major component of tumor stroma able of modifying including differentiation, proliferation as well as immune-mo-d
tumor growth, or alternatively serve as an origin of certain ulatory functions, to avoid deterioration towards carcinogenesis.
playEsviadecnecnetsroabltraoilneeidn ftrhoembnioulmogeyrooufsMsStCuds.ieFsoirnedxicaamteplteh,aMtSpC5s3 The role of p53 in CSCs
derived from p53 knockout mice exhibited an enhanced di-f Accumulating data indicates that CSCs and p53 mutant proteins
ferentiation towards the osteogenic and adipogenic lineages, endow the cells with common oncogenic features including
suggesting that p53 plays a negative regulatory role in these accentuated tumorigenic potential, protection from cell death
processes (132,167–169). However, it was shown that in other and drug resistance, capacity to invade and metastasize, as well
circumstances p53 may facilitate osteogenic differentiation. For as cancer-associated metabolic profile 1(06,178). Additionally,
instance, more than 20 years ago Radinskyet al., have shown gain of CSCs phenotype via acquisition of plasticity and de-d-if
that introduction of WT, but not mut-p53, into p53-deficient ferentiation process has been associated with EMT4(4,45), which
osteosarcoma cells was associated within vivo induction of ter- has been also shown to underlie the oncogenic GOF of mut-p53
minal differentiation and apoptosis that inhibited progressive (179–181). Moreover, a compelling evidence linking mut-p53 GOF
growth of metastases 1(70). Utilizing gel shift and chromatin and dedifferentiation is the association of mutations in p53 with
immune-precipitation assays, a more recent study corroborated poorly differentiated tumors, such as thyroid carcinoma1s8(2),
this observation, demonstrating that introduction of WT p53 gliomas (183), squamous cell carcinoma (184) and ovarian carc-i
into osteosarcoma cell line resulted in a direct transactivation nomas (185). Bearing these observations in mind, it is tempting
of the osteocalcin expression, which is produced in the terminal to speculate that mut-p53 GOF may facilitate the rise of cells
stages of osteogenic differentiation 1(71). These observations exhibiting CSCs characteristics. In accordance, it appears that
suggest that p53 may alter its effect on osteogenic differ-en several studies have contradicted the statement that mut-p53
tiation depending on a specific cellular state and tumorigenic stabilization is unique for cancer cells. Accumulation of mut-p53
potential. was observed in sub-populations of non-cancerous cells such as
Numerous evidence derived from in vitro and in vivo models human keratinocytes 1(86), human epithelial cells of distal fa-l
support the notion that p53 plays a restrictive a role in the di-ffer lopian tubes (187) as well as in a subpopulation of highly proli-f
entiation towards the adipogenic lineage in MSCs as well as in erating cells in the small intestine of mut-p53 knocked-in mice
other more committed adipogenic precursors79(,172). Adipose- (188). Thus, it might be plausible that upon further perturbations
derived stem cells (ASCs) possess multilineage differentiation and appropriate environmental conditions these cells will serve
capacity that offers the potential for the repair or regeneration as primary candidates to acquire CSCs phenotype.
of various tissues 1(73). Interestingly, while the examination of In line with its tumor suppressor activities, WT p53 was
p53 expression levels in proliferating ASCs and terminally d-if shown to compromise CSCs features by repression of CSCs
ferentiated adipocytes did not reveal any differences1(74), we markers such as CD133 (189) and CD44 (190) either by direct
have shown that knockdown of p53 in hASCs resulted in a si-g binding to the promoter or indirectly, through inducing its target
nificant increase in their adipogenic differentiation capacity, gene miR-34a, respectively. Whereas, alterations in the p53 st-a
supporting the restrictive effect of p53 on white adipogenic d-if tus have been found to contribute to CSCs properties in various
ferentiation 1(31). cancer types. Several examples are described below.
Within the tumor microenvironment, MSCs acquire the By using mouse models Fleshkin-Nikitin et al. (191),
identicharacteristics of carcinoma-associated fibroblasts and support fied a cancer-prone SCs residing at the junction area of ovarian
tumor growth. p53 was found to regulate MSCs function in a surface epithelium. These SCs expressing the SCs markers such
tumor microenvironment through different mechanisms. One ALDH1, LGR5, LEF1 and CD133 gained enhanced transformation
of the mechanisms entails p53 dependent inhibition of MSCs potential following alteration of p53 and Rb1 status and were
motility towards tumor cells by decreasing the transcription of suggested to constitute the cells of origin of epithelial ovarian
the chemokine CXCL12 (also known as SDF1), which is necessary carcinomas. Likewise, Wang et al., revealed that initiation of
for MSCs migration in response to tumor cells17(5). Another glioma formation is associated with the appearance of NCSs
mechanism involved p53-mediated reduction of the levels of expressing detectable level of mut-p53 proteins in the subve-n
iNOS, leading to diminished immunosuppressive capacity of tricular zone of the brain and the subsequent expansion of
mutMSCs and as a consequence attenuated tumor growth17(6). p53-expressing Olig2+ transit-amplifying progenitor-like cells
Lack of p53 in MSCs was also shown to augment their ce-l (192). Others reported that NSCs transformation may result from
lular growth as manifested by increased proliferation rate, a inhibition of oxidative metabolism that leads to p53 genetic
shorter doubling time and enhanced colonies formation capa-c inactivation. Moreover, p53 mutations correlate with alterations
ity, loss of p16 expression and lack of senescence response, as in mitochondrial metabolism and genomic instability in primary
glioma-initiating NSCs 1(93). Alternatively, it was demonstrated therapy. One of the major compounds related to mut-p53 re-ac
that brain CSCs generation can result from p53-deficient mouse tivation is CP-31398 that can stabilize the DNA-binding core
astrocytes either by expression of oncogenic Ras 1(94) or via domain of several mut-p53 proteins, induce conformational
Nanog-induced dedifferentiation 1(95). change and restore p53 transcriptional activity and function
Several studies have provided evidence from mouse models (202). Additional prominent compounds refolding mut-p53 into
that tumors with sarcoma features can arise from iPSCs, MSCs WT p53 conformation are PRIMA-1 and its analog PRIMA-1MET
or poorly differentiated osteoblasts bearing at least one end-og that is currently in clinical trials. The mechanisms underlying
enous mut-p53 allele (152–154). In addition, it was observed that their action involve the conversion of these compounds to pro-d
spontaneous transformation of MSCisn vitro is accompanied by ucts, which form adducts with thiol groups in the DNA-binding
concomitant gain of p53 mutation in the DNA binding domain domain of p53 mutant proteins, leading to restoration of WT
and CSCs-like characteristics1(96). In line with these observa- activity and induction of apoptosis in tumor cells20(1,203).
tions, it was shown that in human osteosarcoma cells, mut-p53- In this respect, using innovative approach of screening phage
R248W/P72R oncogenic GOF promoted cancer stem-like features display libraries combined with deep sequencing methodology
reflected by high proliferation rate, sphere formation, clon-o we have recently obtained a large database of mut-p53 pote-n
genic growth, high migration and invasiveness. Furthermore, it tial reactivating peptides. The identified lead peptides were
strongly increased the levels of stemness proteins1(
). able to stabilize the correct conformation of mut-p53, endo-w
In the mammary gland, the physiological function of p53 is ing mut-p53 with WT p53-like activities and cause regression
to maintain a constant number of SCs by imposing an asym- of mut-p53-bearing tumors in several xenograft models. Thus
metric mode of self-renewing divisions. It was demonstrated, suggesting that these peptides might serve as novel agents for
that loss of p53 in mammary SCs increase the frequency of sy m- human cancer therapy 2(04).
metric cell divisions, thereby providing them with CSCs self- As treatment of tumors with multiple independent mod-ali
renewal properties that contribute to mammary tumorigenesis ties appears to yield beneficial anti-tumor responses, intri-gu
(198). In addition it was shown that the p53 isoformΔ133p53β, ingly Zhang et al., explored the possibility to target CSCs with
lacking the transactivation domain, promotes CSCs potential p53 modulators. In this study, CSCs exhibiting high ALDH1 act-iv
in breast cancer cell lines. This is manifested by the expre-s ity from human breast, endometrial and pancreas carcinoma
sion of the key pluripotency factors SOX2, OCT3/4 and NANOG cell lines expressing either WT or mut-p53 were treated with
in a p53 Δ133p53β-dependent manner. Notably, treatment with CP-31398 and PRIMA-1. The small molecule compounds
siga cytotoxic anti-cancer drug, further increased CSCs formation nificantly reduced CSCs content and sphere formation by these
via p53 Δ133p53β, thus potentially increasing the risk of cancer cell lines in vitro. In addition, these agents were more effective
recurrence (199). against CSCs compared to the chemotherapy such as cisplatin
Collectively, perturbations in the p53 pathway seem to co-in and gemcitabine (205).
cide with the generation of CSCs both by malignant transf-or Considering the wide repertoire of p53 mutations in the
mation of normal SCs as well as by acquisition of plasticity by various cancer types and the distinct markers of CSCs, future
mature cells. Hence, providing a strong rationale for combining development of therapeutic strategies should be directed
therapeutic strategies targeting CSCs with compounds which towards the concepts of precision medicine and involve
comcan deplete mut-p53 and restore wild-type p53 activity. bination therapy encompassing drugs targeting CSCs and
mutp53. Design of effective and safe compounds that exclusively
Therapeutic strategies target mut-p53 as well as therapeutic strategies that specifically
hinder CSCs will permit improving clinical efficacy of cancer
Being a cornerstone of the carcinogenic process, CSCs were
found to play fundamental roles in distinct stages of ca-n
cer development and metastasis, acquiring tumors with drug
resistance and sustaining cancer recurrence. To permit more Concluding remarks
efficient cancer treatment, novel therapeutic approaches were Maintenance of genomic fidelity is of paramount importance for
designed aimed to eradicate CSCs. Most common therapies SCs integrity to allow normal development, proper different-ia
aimed to hamper CSCs regeneration and cancer relapse include tion and regeneration, to assure cancer prevention. Therefore,
targeting the specific surface markers and signaling pathways SCs exert stringent mechanisms that prevent the propagation
underpinning the CSCs properties, as well as induction of of genetic alterations to the descendant cells. Ample accum-u
CSCs apoptosis and differentiation. However, since CSCs and lated data indicate that p53, the major tumor suppressor gene,
normal SCs share some similar characteristics, albeit holding plays a fundamental regulatory role in SCs fate decisions with
great promises for cancer therapeutics, some of these therapies an aim to eradicate aberrant SCs form the self-renewing SC
against CSCs may be not specific, thereby destroying normal pool. Importantly, mutations in the p53 endow it with both loss
SCs niches (200). of tumor suppressive function as well as with oncogenic GOF
p53, the most frequently mutated gene in human cancers, that greatly facilitates tumorigenesis. In ESCs, following DNA
has been an attractive target for cancer therapy for many years. damage, WT p53 induces either cell death or differentiation of
Numerous strategies targeting p53 have been developed inclu-d cells with faulty genome. As carcinogenesis and reprogramming
ing gene therapy to restore WT p53 function, drugs aimed at share common molecular mechanisms, p53 serves as a ba-r
disruption of p53-MDM2 interaction, as well as small-molecule rier along the reprogramming process, preventing generation
compounds that specifically target mut-p53, thereby elimin-at of iPSCs from suboptimal somatic cells. Furthermore, p53 may
ing its oncogenic GOF by depleting it or restoring WT p53 tumor either induce or repress differentiation of the various adult SCs
suppressive activities 2(01). Most of the above approaches were and precursor cells, depending on the cell of origin and
microdeveloped either by rational design or by screening of small environmental signaling. Conversely, lack of p53 or expression
molecule chemical libraries. Despite the substantial progress of mut-p53 permits both proliferation of SCs with faulty DNA
in this field, as of today there is still no approved p53-target and in some cases support dedifferentiation processes, thereby
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This study was supported by the Center of Excellence of the 25. Phanstiel, D.H. et al. (2011) Proteomic and phosphoproteomic compa-ri
Flight-Attendant Medical Research Institute (FAMRI), Israel 26. sNoinshoinfoh,uKm. eatn aElS.(a2n01d6i)PDSNcAellms.eNtahty.laMteitohnoddysn, a8m,8i2c1s–i8n27h.uman induced
Science Foundation ISF-MOKED center from the Israeli Academy pluripotent stem cells. Hum. Cell, 29, 97–100.
of Science and The Len and Susan Mark Initiative for Ovarian 27. Rais, Y. et al. (2013) Deterministic direct reprogramming of somatic
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Research Fund (ICRF). V.R. is the incumbent of the Norman 28. Buganim, Y. et al. (2013) Mechanisms and models of somatic cell
reproand Helen Asher Professorial Chair for Cancer Research at the gramming. Nat. Rev. Genet., 14, 427–439.
Weizmann Institute. We are grateful to Tamar Shetzer for the 29. Buganim, Y. et al. (2014) The developmental potential of iPSCs is greatly
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