Cancer Cell 12, October 2007 ©2007 Elsevier Inc. 303
Somatic p53 missense mutations are found in approxi-
mately 50% of human cancers (Soussi et al., 2006), but
it is generally assumed that the p53 pathway is also
inactivated in wild-type (WT) p53-carrying tumors via
indirect mechanisms such as MDM2/MDMX ampli-
fication leading to p53 destabilization (Marine et al.,
2007; Brooks and Gu, 2006). An increasing number of
studies indicate that a subset of p53 mutant proteins
are oncogenic and actively participate in neoplastic
transformation (Weisz et al., 2007; Peart and Prives,
2006). These observations have several implications,
including a possible heterogeneous clinical phenotype
depending on whether p53 itself is mutated and on the
site of mutations or whether p53 function is modified
indirectly (Prives and Manfredi, 2005).
Analysis of the spectrum of p53 mutations in human
cancer demonstrates a link between exposure to various
types of carcinogens and the development of specific
cancers (Hussain et al., 2000). In skin tumors, a high
frequency of p53 mutations is observed at dipyrimidine
sites, a molecular signature of mutagenesis by solar UV
rays (Ziegler et al., 1994). The relationships between G
→ T transversion and lung cancer in smokers or muta-
tion of codon 249 observed in aflatoxin B1-induced liver
cancers are also very demonstrative (Staib et al., 2003).
This spectrum related to carcinogen-induced adducts
can be modulated qualitatively and quantitatively by
various factors that act as positive or negative filters
(Figure 1). These factors include genetic heterogeneity
of the activation process that transforms an unreactive
carcinogen into a form that binds DNA or heterogene-
ity of the detoxification process that eliminates the acti-
vated molecule. Similarly, SNP leading to variations in
the activity of DNA repair genes associated with base
excision repair (BER) or nucleotide excision repair (NER)
have been detected in a normal population and can also
shape the pattern of mutations (Hung et al., 2005). Many
studies have focused on the analysis of these various
upstream filters that control the transformation of a DNA
adduct into a stable mutation. Although mutations can
occur at multiple positions in the genome, only those
that lead to a selective advantage for the cell will be
identified on analysis of tumors.
So far, analyses of the spectrum of p53 mutations and
inactivation have not taken into account another set of
selection events that can be referred to as downstream
filters. They include tissue specificity, functional poly-
morphisms in genes associated with the p53 pathway, or
other oncogenic modifications occurring in the tumor. In
this review, we will discuss how these filters can quanti-
tatively and qualitatively shape p53 alterations;how they
can act by selecting specific mutant p53 whose proper-
ties, either loss or gain of function, are associated with
a transforming phenotype in a given genetic and physi-
ologic environment;and how these observations can be
translated into clinical practice.
p53 Mutant Heterogeneity
The main activity of p53 is to act as a transcription fac-
tor that binds DNA via a domain localized in the core
region of the protein (CR; residues 100 to 300). The DNA
sequence found in the various p53 response elements
(p53 RE) is markedly degenerated, and the affinity of
p53 for these various binding sites is highly heteroge-
neous, an important feature in the regulation of the p53
response, as it imposes a hierarchy in the occupancy of
the various p53 response elements (Qian et al., 2002;
Inga et al., 2002). Several thousand genes have been
shown to be directly regulated by p53, but as all these
studies were performed in cell lines and only limited data
are available about tissue specificity, the spatiotemporal
response of p53 in vivo and the way in which promoter
heterogeneity regulates p53 response remain unclear.
Furthermore, specific cofactors, some of which have tis-
sue-specific expression, can bind to various regions of
Shaping Genetic Alterations in Human Cancer:
The p53 Mutation Paradigm
Thierry Soussi1,2,* and Klas G. Wiman2
1Université P.M. Curie, 4 place Jussieu, 75005 Paris, France
2Karolinska Institute, Department of Oncology-Pathology, Cancer Center Karolinska (CCK), SE-171 76 Stockholm, Sweden
p53 mutations are found in 50% of human cancers. Molecular epidemiology has shown strong
correlations between the spectrum of p53 mutations and exposure to exogenous carcinogens.
This spectrum is influenced quantitatively and qualitatively by various upstream genetic filters that
modulate carcinogen activation, detoxification, and/or DNA repair. In this review, we will discuss
how other factors such as tissue specificity, SNP of genes associated with the p53 pathway, other
genetic alterations, or p53 mutant heterogeneity can act as a second set of downstream filters that
also have a profound impact on the spectrum of p53 mutations.
304 Cancer Cell 12, October 2007 ©2007 Elsevier Inc.
the p53 protein and modulate its activity. Biochemical
and structural studies have shown that the CR of p53
is highly flexible and cycles rapidly between folded and
unfolded states (Joerger and Fersht, 2007). This fragil-
ity explains the high concentration of missense muta-
tions in the 600 bp that codes for this domain. Each
residue in this region has been found to be mutated in
human tumors with frequencies ranging from twice for
infrequent mutants to more than 1000 times for hot spot
mutants (UMD p53 database 2007_R1; http://p53.free.
fr/). Biochemical and functional studies have clearly
demonstrated that different p53 mutants in the CR have
a marked heterogeneity in terms of loss of structure and
function (Soussi and Lozano, 2005). Using a library of
2500 different p53 mutants, Kato et al. showed that,
while hot spot mutants found in human cancers dis-
play total loss of transactivating capacity, other mutants
retain a partial transactivation activity for a subset of tar-
get genes, leading to a wide range of possible mutant
activities (Figure 2) (Kato et al., 2003). This observation
is of importance, as hot spot mutants at codons 175,
248, and 273 represent only 10% of all somatic p53
mutations found in human tumors. It is also noteworthy
that the fragility of the p53 core domain and susceptibil-
ity of p53 to inactivating point mutations have apparently
not been selected against during the course of evolution,
presumably because loss of p53 function and develop-
ment of cancer usually occur long after reproduction and
the raising of offspring. p53 CR is also critical for inter-
actions with various cofactors. Cellular proteins such as
Bcl-Xl or 53BP2/ASPP2 interact with WT p53, but each
protein requires a specific set of partially overlapping
p53 residues of the CR (Samuels-Lev et al., 2001; Mihara
et al., 2003). p53 interaction with Bcl-Xl is specifically
associated with transcription-independent p53-induced
apoptosis, whereas 53BP2/ASPP binding specifically
enhances transactivation of proapoptotic genes such as
BAX and PIG 3 but has no effect on WAF1 (Samuels-Lev
et al., 2001; Mihara et al., 2003). This intricate promiscu-
ity of various functions in the CR of p53 raises a num-
ber of questions concerning the interplay between the
loss of these functions and p53 point mutations (Soussi,
2007). Some mutants, such as R175H, are totally defec-
tive for transactivation and do not bind to 53BP2/ASPP2,
whereas other mutants such as R273H retain a normal
conformation and/or the capacity to bind 53BP2/ASPP2
despite loss of their transactivation activity (Tidow et al.,
2006). Whether loss of the transcriptional transactiva-
tion function of p53 via impaired DNA binding is the only
consequence of p53 mutations selected during tumor
evolution remains an open question (Soussi, 2007).
The impact of the mutation on the various biologi-
cal functions of p53 is therefore an important factor
in the selection process that will act together with the
other filters discussed below such as tissue specificity
or genetic background. Mouse models support these
observations on the heterogeneity of p53 mutations.
Knockin mice expressing two hot spot p53 mutations
(R172H and R270H corresponding to human R175H and
R273H, respectively) have a different phenotype from
p53−/− mice (Olive et al., 2004; Lang et al., 2004). They
develop a different spectrum of tumors with 50% of car-
cinomas with a high metastatic potential, while both of
Figure 1. Shaping p53 Mutations in Human Cancer
In the exposure step, exogenous or endogenous carcinogens lead to
the generation of a wide range of DNA adducts (blue half-ring shapes).
These adducts are influenced qualitatively and quantitatively by sev-
eral factors, including the efficiency of the activation and detoxifica-
tion process, the affinity of the carcinogen for a given nucleotide, the
chromatin structure, and any other factors that can modify the inter-
action of an active carcinogen with DNA. In a second step, most of
the lesions are eliminated, but the cellular contexts (phases of the
cell cycle, cell type) have an impact on the efficiency of DNA repair.
In the fixation step, nonrepaired lesions are transformed into stable
mutations (red stars) that are transmitted to daughter cells after cell
division, resulting in three possible outcomes. First, if the mutation
leads to a lethal phenotype, the cell dies and this counterselection
prevents propagation of that specific alteration. A second outcome,
which is the most common, is that the mutation does not result in
any particular phenotype. This is the case for the majority of inter-
genic or intronic alterations, but also for certain intragenic mutations
that target nonessential amino acid residues. Mutations targeting the
third base of nucleotide codons also fall into this category, although it
should be kept in mind that they can lead to defects in RNA stability
or splicing. These “neutral mutations” are not involved in any selec-
tion process and are expanded at the rate of normal cell division of
the original tissue. Nevertheless, if this particular cell is the target of
a second alteration that leads to its clonal expansion, the first muta-
tion will be coselected despite the absence of associated phenotype.
Such mutations, called “passenger mutations,” as opposed to the
“driving mutations” that lead to clonal expansion, are quite common,
and their frequency is related to the efficiency of the mechanism con-
trolling genetic stability. The presence of passenger mutations is a
real problem for molecular diagnosis, as the increased throughput and
sensitivity of genetic analysis will reveal a wide variety of passenger
mutations that will be difficult to distinguish from true driving muta-
tions in the absence of functional assays. The third possible outcome
for stable somatic mutations is a functional modification that results
in a new phenotype that contributes to the neoplastic process. These
true driving mutations are fortunately rare, but a selection process can
be modulated by factors such as tissue specificity, the presence of
other somatic genetic modifications, or the genetic background of the
individual, as discussed in the text.
Cancer Cell 12, October 2007 ©2007 Elsevier Inc. 305
these features are absent in p53−/− mouse. Furthermore,
the tumor spectra are different in mice expressing the
two p53 mutants in the same genetic background. More
relevant to the human population, expression of the
same mutant p53 (R175H) in two different genetic back-
grounds results in a different tumor spectrum. How tis-
sue specificity and genetic background can contribute
to the heterogeneity of p53 mutations both quantitatively
(frequency of mutations) and qualitatively (diversity of
p53 mutations) is discussed below.
The p53 Family Network and Tissue Specificity
After discovering the p53 family members p63 and p73,
it has become clear that all three proteins are intertwined
in a complex crossregulation network (Yang and McKeon,
2000; Deyoung and Ellisen, 2007). Both p73 and p63 are
expressed as several protein isoforms that either contain
a p53-like transactivation domain (TA isoforms) or that
lack this domain (∆N isoforms). Each p53, p63, and 73
isoform possesses a homologous DNA-binding domain
and shares transcriptional targets but with opposite
effects, as the binding of DN isoforms can block trans-
activation (Stiewe, 2007). Recent reports indicate that
p53 is also expressed as multiple isoforms (Rohaly et al.,
2005; Bourdon et al., 2005). Although p53 expression is
relatively ubiquitous, p73 expression and p63 expression
are more restricted to specific tissue with various ratios of
TA and ∆N isoforms. p63 is essential for the proliferation
and differentiation cascade in stratified epithelia, and the
∆N isoforms play a major role in this function (Perez and
Pietenpol, 2007). TAp73 has a strong proapoptotic activ-
ity after DNA damage induced by drugs used in chemo-
therapy such as cisplatin or adriamycin (Yuan et al., 1999;
Gong et al., 1999; Agami et al., 1999). It is therefore not
surprising that deregulation of p73 or p63 will depend
on tissue type and the ratio of the various isoforms. The
patterns of deregulation of this p63/73 network in human
cancer have been recently discussed in several reviews
(Deyoung and Ellisen, 2007; Ratovitski et al., 2006). In the
present review, we will focus on how p53 mutations can
contribute to deregulation of this network and their rela-
tionship to tissue specificity. WT p53 does not oligomerize
with p73, but some mutant p53 proteins bind to p73 and
inhibit its apoptotic activity (Strano et al., 2000; Di Como
et al., 1999). This interaction occurs specifically via the CR
of both proteins and is restricted to structural mutant p53
that display a change of conformation such as R249S or
R175H (Gaiddon et al., 2001). Patients with head and neck
SCC presenting these mutant p53 have a poor response
to therapy associated with a lack of p73-associated apop-
tosis (Bergamaschi et al., 2003). More recently, Leong et
al. reported that patients with the triple-negative subset of
breast cancer (Basal-like, negative for estrogen and pro-
gesterone receptor and negative for ERBB2 amplification)
display a high frequency of p53 inactivation, overexpres-
sion of ∆Np63 associated with inhibition of p73 apoptotic
activity and loss of chemosensitivity (Leong et al., 2007).
The p53 Response Is Tissue Specific
Analysis of p53 response in cell lines after various types
of stress has led to the general belief that stabilization
and activation of p53 always occur regardless of the cell
type. However, this universal p53 response does not
apply to whole animals (Toledo and Wahl, 2006; Soussi,
2007). As early as 1995, Midgley et al. demonstrated a
marked tissue-specific restriction of the p53 response
after gamma irradiation (Midgley et al., 1995). Accumu-
lation of p53 protein following whole-body irradiation
was associated with a strong apoptotic response in the
spleen and thymus, while no response was observed
in hepatocytes. Using in situ hybridization with probes
corresponding to various p53 target genes, Fei et al.
extended these observations and also showed strong
tissue specificity with distinct regulation of various p53
target genes in different tissues (Fei et al., 2002). In liver
cells, only the Waf1 gene involved in growth arrest was
induced, whereas none of the proapoptotic p53 target
genes were upregulated. This observation is the inverse
of what occurs in the spleen with specific induction of the
proapoptotic gene Puma, whereas Waf1 or other apop-
totic genes such as Noxa or Dr5 were barely detectable.
Another study based on transgenic mice expressing a
lacZ reporter gene fused to the Mdm2 promoter showed
that the pattern of p53 transcriptional response is fairly
homogeneous in the early embryo but becomes more
restricted with increasing embryo age and with differen-
tiation of the tissues (Gottlieb et al., 1997). A predomi-
nant p53 response after DNA damage was observed in
Figure 2. Heterogeneity of the DNA-Binding Activity of
Due to the marked degeneracy of the p53 DNA response element,
there is a wide variation in the affinity of wild-type p53 to DNA, with two
extremes represented by p21/WAF1 (strong binding) and bax (weak
binding) (vertical axis). p53 mutation generates a large number of p53
variants with unique properties depending on the remaining affinity for
DNA that will dictate transcription of a subset of p53-responsive genes
(horizontal axis). Highly penetrant mutants such as R175H show a total
loss of DNA binding with no transcriptional activity. However, the R175P
mutant displays only a partial loss of activity with no binding to the bax
promoter but normal binding to the p21/WAF promoter. This mutant is
deficient for induction of apoptosis but retains wild-type activity with
respect to induction of growth arrest. The observation that a subset
of p53 mutants has also lost the ability to bind coactivators such as
ASSP2 adds another level of complexity. Red and green represent wild-
type and mutant p53 activity, respectively.
306 Cancer Cell 12, October 2007 ©2007 Elsevier Inc.
the spleen, thymus, and small intestine. Recently, Ring-
shausen et al., using an inducible p53 mouse model,
showed that p53 activation leads to specific apoptosis
of radiosensitive tissue such as bone marrow, thymus,
or spleen, whereas radioresistant tissues only displayed
marked growth arrest (Ringshausen et al., 2006). All
these observations reveal that the phenotype of a cell
line has only a limited relation to the original tissue from
which it is derived. Although this is not unexpected, as
p53 is a pleiotropic stress sensor and in vitro cell culture
in plastic dishes is quite unnatural, there is a general ten-
dency to overinterpret in vitro studies and transfection
experiments (Toledo and Wahl, 2006).
The observed tissue-dependent p53 responses also
raise some questions concerning whether tissue origin
could specify the selection of particular p53 mutations.
This issue is complex, as several confounding factors
such as the tissue specificity of several carcinogens or
the presence of other genetic alterations (see below)
could influence the outcome. Nevertheless, in hepato-
cellular carcinoma, several findings support the notion
that the R249S mutation is specifically selected in liver
cells. Epidemiologic studies unambiguously show that
the p53 mutation at codon 249 in liver cancer is linked
to aflatoxin B1 (AFB1) exposure (Staib et al., 2003). In
vitro studies exposing human liver cells to AFB1 also
revealed the same R249S hot spot (Puisieux et al.,
1991). Two nonexclusive explanations can account for
the high prevalence of this particular mutation in liver
cancer. First, codon 249 may be highly sensitive to the
mutagenic effect of AFB1, as indicated by several stud-
ies. Second, it is possible that this mutant provides a
special growth advantage to liver cells. The second
hypothesis is supported by several findings. (1) Although
AFB1 binds specifically to codon 249 in vitro, it is not
the major site of AFB1 adducts, and stronger binding
has also been observed at other codons, such as 245
and 248, at which mutations are found in other cancer
types (Puisieux et al., 1991; Denissenko et al., 1998).
The fact that these mutants are not found in liver cancer
indicates a preferential selection for R249S. (2) Mutant
R249S properties such as dominant-negative activity or
increased survival effects are more potent in liver cells
than those of other mutants (Ponchel et al., 1994). There
is no explanation for this liver specificity of the R249S
mutant, but a better understanding of the various gain-
of-function activities of mutant p53 and identification
of the tissue specificity of p53 isoforms could uncover
some novel functions for this mutant. Another asso-
ciation between tissue specificity and a particular p53
mutant has been found in adrenal cortical carcinoma
(ACC). In south Brazil, a germline mutation at codon 337
(R337H) has been specifically associated with pediat-
ric ACC, with several cases in one family (Ribeiro et al.,
2001). This mutation is localized in the oligomerization
domain but analysis of classical functions such as cell
cycle arrest or apoptosis failed to reveal any defect of
this mutant despite its strong association with pediatric
ACC. This paradox has been resolved by structural anal-
ysis demonstrating a very high sensitivity of the R337H
mutant to pH-induced denaturation as compared to WT
p53 (DiGiammarino et al., 2002). As the adrenal gland is
known to undergo extensive apoptosis during pre- and
postnatal development, it has been postulated that an
increased intracellular pH may lead to p53 inactivation
and impair apoptosis specifically in these cells.
Several cancers display a very low frequency of p53
mutation, indicating that the upstream or downstream
p53 pathways could be impaired. Indeed, amplification of
MDM2 is observed in sarcoma (Michael and Oren, 2003),
and Laurie et al. recently demonstrated that amplification
of MDMX disrupts the p53 pathway and suppresses the
apoptotic response in retinoblastoma (Laurie et al., 2006).
In neuroblastoma, inactivation of p53 can often occur
through nuclear exclusion via amplification of the Parc
protein (Nikolaev et al., 2003; Moll et al., 1995). Whether
this high frequency of indirect p53 inactivation, predomi-
nantly observed in nonepithelial tumors, has any biologi-
cal significance remains to be analyzed.
The importance of the tissue specificity of p53 altera-
tion was recently highlighted by analysis of p53 restora-
tion in animal models (Martins et al., 2006; Ventura et al.,
2007; Xue et al., 2007). Although these three studies show
that restoring WT p53 activity in a p53-deficient tumor
leads to p53-dependent regression of the tumor, the out-
come differs according to tumor type. In solid tumors, p53
induced marked growth arrest, whereas in lymphoma,
p53 induced intense and rapid apoptosis (Martins et al.,
2006; Ventura et al., 2007; Xue et al., 2007).
p53 Mutations and Genetic Background
Exonic p53 SNP
Thirty-seven SNP have been identified in the p53 gene,
18 of which have a frequency higher than 5% in the
SNP500 cancer populations. The first exonic SNP dis-
covered results in a proline-to-arginine substitution at
codon 72 (Harris et al., 1986). A striking bias in the dis-
tribution of this SNP in the human population was noted
(Beckman et al., 1994). The frequency of the Pro/Pro
haplotype is 16% in Scandinavian populations and 63%
in the Nigerian population. The reason for this North/
South gradient is unknown at the present time. Residue
72 is localized in the proline-rich domain (PRD) of the
protein that has been associated with a regulatory func-
tion of p53 apoptosis. Transfection studies have shown
that deletion of the PRD motif in both human and mouse
p53 leads to specific abrogation of the apoptotic activ-
ity of p53, maintaining an intact growth arrest function
(Walker and Levine, 1996; Sakamuro et al., 1997; Venot
et al., 1998). These observations were not reproduced
in MEF from mouse lacking the PRD that are unable to
induce a cell cycle arrest but display a weak apoptotic
activity (Toledo et al., 2006). The Arg72 p53 variant has
a more potent proapoptotic capacity compared to the
Pro72 variant (Dumont et al., 2003; Bergamaschi et al.,
2006). Several nonexclusive explanations have been
Cancer Cell 12, October 2007 ©2007 Elsevier Inc. 307
proposed to interpret this observation. Dumont et al.
showed that a stronger interaction of the R72 variant
with the nuclear-export protein CRM1 leads to enhanced
nuclear export and greater accumulation in mitochon-
dria (Dumont et al., 2003). This differential interaction is
specific to human cells and absent in mice, indicating
that an appropriate environment is important to display
the specificity of each human p53 variant (Phang and
Sabapathy, 2007). Another explanation has been pro-
posed by Bergamaschi et al., who showed that iASPP, a
specific cellular inhibitor of p53, binds to the PRD of p53
and interacts more strongly with the P72 variant, leading
to a more pronounced inhibitory effect on its apoptotic
activity (Bergamaschi et al., 2006). Epidemiologic stud-
ies have been performed to determine whether this SNP
is associated with an increased risk of cancer. Results
of these studies are very contradictory and have not
demonstrated any highly significant correlations, sug-
gesting that, if such an association exists, it may not be
very strong or may be due to other as yet unidentified
genetic factors (Pietsch et al., 2006). Similarly, analy-
sis of the prognostic value of codon 72 SNP in various
types of cancer has not been fully conclusive. A possible
explanation is that p53 mutant gain of function differs for
the two p53 variants. As mentioned in a previous sec-
tion, some structural p53 mutants bind and inactivate
the apoptotic activity of p73. The binding of mutant p53
for p73 is stronger for the R72 variants compared to
mutants with the Pro72 variants and generates mutant
p53 with a strong gain of function (Marin et al., 2000;
Bergamaschi et al., 2003). Patients with head and neck
cancer who display these structural p53 mutants asso-
ciated with Arg72 variants have a very poor outcome.
Therefore, two confounding effects should be consid-
ered: the lower apoptotic activity of the P72 variant and
the enhanced gain of function of mutant p53 carrying
the R72 polymorphism, both of which can be associated
with a poor response to therapy or poor outcome (Figure
3). In tumors with a high frequency of TP53 mutation, the
R72 variant can be associated with a poor prognosis,
whereas in tumors with a low frequency of p53 mutation,
the low apoptotic variant P72 could be associated with a
poorer outcome. It is unclear at the present time whether
this model can be generalized to other settings.
A second exonic polymorphism at codon 47 chang-
ing a proline to serine has also been described (Felley-
Bosco et al., 1993). The frequency is very low, ranging
from 0.5% to 5% in various studies. This polymorphism
is close to serine 46, phosphorylation of which is a key
event for the apoptotic function of p53. In vitro studies
have shown that the S47 variant is a poorer substrate for
S46 phosphorylation and has an impaired proapoptotic
ability (Li et al., 2005). The clinical significance of this
p53 variant is not known.
Functional SNP in the p53 Pathways
As discussed in a previous section, the p53 RE rec-
ognized by WT p53 is highly degenerated. The con-
sensus sequence is composed of two copies of the
5′PuPuPuC(A/T)(T/A)GPyPyPy-3′ motif separated by a
spacer of 0 to 13 bases (Funk et al., 1992; el-Deiry et
al., 1992). It has been clearly established that variation
in this sequence can drastically affect the efficiency of
transactivation (Qian et al., 2002). Several studies have
been performed to determine whether certain natural
SNP are present in functional p53 RE. Although a diffi-
cult challenge, using p53 as a paradigm for these studies
is facilitated by the large number of genes regulated by
p53 (more than a thousand), the length and degeneracy
of the p53 RE, and our good knowledge of the p53 net-
work. Two types of analysis were used for these studies,
a candidate gene approach performed by Bonds et al.
and a global unbiased screening of the whole genome
performed by Tomso et al. (Bond et al., 2004; Tomso et
al., 2005). The first type of analysis led to the detection
of SNP309 in the MDM2 gene (Bond et al., 2004). This
polymorphism changes a T to G at nucleotide 309 of
intron 1 close to the p53 RE and creates a higher-affinity
DNA-binding site for Sp1 that leads to increased levels
of MDM2 RNA and protein in cells. Cell lines homozy-
gous for the G allele express high levels of MDM2 and
have an impaired p53 response after DNA damage with
poor induction of p53 and weak p53-induced apoptosis
(Bond et al., 2004). The frequency of individuals with the
G/G genotype is around 1%, which is sufficiently high
Figure 3. Complexity of p53 R72P Polymorphism
In normal cells (gray shade), p53 72R has a more potent apoptotic
activity compared to p53 P72, suggesting that individuals with this
SNP will have a higher cancer susceptibility compared to R72 in-
dividuals, as symbolized by the thickness of the arrows. In tumors
(orange shade) the situation is more complex. In tumors with a low
frequency of p53 mutations (red shade, right), p53 polymorphism P72
is more frequent, and tumors are less sensitive to apoptosis-inducing
treatment. On the other hand, in tumors with a high frequency of p53
mutations (red shade, left, mutation symbolized by asterisk), mutant
p53 encoded by the R72 allele may bind to p73 and inhibit its apop-
totic activity, and are therefore preferentially selected during tumor de-
velopment. This is supported by the observation that head and neck
tumors bearing p53 mutations with the R72 allele are more resistant to
chemotherapy, and these patients have a shorter survival.
308 Cancer Cell 12, October 2007 ©2007 Elsevier Inc.
for population studies. Indeed, individuals with the Li-
Fraumeni syndrome carrying the G allele (either in the
heterozygous or homozygous state) have an earlier
age of tumor onset. SNP309 is found in a region of the
MDM2 promoter that is regulated by hormonal signaling
pathways, and recent studies have shown that the clini-
cal penetrance of this SNP is stronger in women than in
man. The onset of cancer was found to be significantly
earlier in patients with sarcoma, breast cancer, colorec-
tal cancer, or lymphoma (Bond et al., 2006; Bond and
Levine, 2007). A second SNP (SNP 354) in exon 12 of
the MDM2 gene has been recently associated with an
increased breast cancer incidence, but its biological sig-
nificance is unknown (Boersma et al., 2006).
A specific combination of SNP in the AKT gene has
been associated with an increased amount of AKT
protein in cells. A large body of data indicate that the
IGF-1-AKT survival pathway is strongly integrated in the
p53 network (Levine et al., 2006). AKT protein phos-
phorylates and stabilizes MDM2 and downregulates
p53 (Zhou et al., 2001; Mayo and Donner, 2001). AKT
is negatively regulated by the phosphatase PTEN, a
p53 response gene that is inactivated in a wide variety
of cancers. AKT also phosphorylates and inactivates
TSC2, which is encoded by a p53 target gene, that neg-
atively regulates the mTOR pathway after glucose star-
vation. AKT is therefore able to impair p53 response
at multiple points in various pathways. Several SNP
associated with variable expression of AKT have been
identified. Analysis of p53-dependent apoptosis after
irradiation in 113 lymphoblastoid cell lines showed that
individuals expressing high levels of AKT displayed a
reduced apoptosis compared to individuals with lower
AKT expression (Harris et al., 2005). Whether or not this
population with high AKT activity will be less respon-
sive to radiotherapy or more prone to cancer has yet to
Using a global genome analysis, Menendez et al. iden-
tified a SNP in a putative p53 response element on the
promoter of the FLT-1 gene (Fms-like tyrosine kinase 1).
The FLT-1 gene encodes one of the receptors activated
via binding of VEGF, a molecule essential for angiogen-
esis and associated with metastasis. In vitro analysis
showed that only one of the two alleles is induced by
p53, whereas the other is unaffected (Menendez et al.,
2006). Although no clinical data are yet available, this
finding shows that the p53 response in a normal indi-
vidual could be highly heterogeneous. As discussed
in a previous section, animal models also indicate that
genetic background has a strong impact on tumor
heterogeneity, as knockin mouse models expressing
mutant p53 R172H (R175H in human) in different genetic
backgrounds develop different types of tumors (Iwa-
kuma and Lozano, 2007).
Taken together, these data indicate that the p53 net-
work is genetically heterogeneous, a feature that can
lead to a wide variation in the p53 response (Figure 4).
Furthermore, data presented here are certainly the tip
of the iceberg, as only a small component of the p53
pathway has been analyzed. A multitude of SNP affect-
ing the p53 response will undoubtedly be identified in
the near future. Understanding how all these SNP will
collectively define tumor risk or predict tumor behavior,
prognosis, or response to treatment will constitute an
p53 Mutations and Other Tumor-Associated
Although p53 mutations are fairly ubiquitous and can be
found in more than 50% of human tumors, it is now well
known that most types of cancer harbor specific genetic
defects such as mutations in APC in colon cancer, BRCA1
and BRCA2 in breast cancer, and B-RAF in melanoma
(Futreal et al., 2004). As the p53 network is closely linked to
many other cellular pathways, it is likely that defects in any
of these pathways, either inherited or acquired somatically,
could influence p53 function qualitatively or quantitatively.
We have previously discussed how PTEN and p53 muta-
tions are mutually exclusive in breast cancer.
Another example is the relationship between the
BRCA1 and p53 pathways. BRCA1 acts both as a
checkpoint and a DNA damage repair gene that ensures
genome integrity (Venkitaraman, 2002). BRCA1 germ-
line mutations are associated with an increased risk of
developing breast and ovarian carcinoma. Brca1 null
mouse embryos die early during development, but
this phenotype can be partially rescued by p53 defi-
Figure 4. Heterogeneity of the p53 Pathway
AKT kinase, the level of which is controlled by SNP 3-4, phosphorylates
MDM2 protein and enhances MDM2-mediated ubiquitination and degra-
dation of p53. The level of MDM2 expression is modulated by SNP309
SNP. p53 activity is also believed to be controlled by the Arg-Pro poly-
morphism at codon 72. In the worst case scenario, an individual with
strong AKT activity (SNP 3-4 GG), high MDM2 expression (SNP 309 GG),
and a p53 SNP Pro 72 with low apoptotic activity will be at high risk for
cancer and will experience a very low response to DNA damage induced
by chemotherapeutic agents. On the other hand, individuals with the op-
posite genotype will experience the best response. Between these two
possibilities, multiple genotypes in these three genes and other unknown
modifier genes will lead to a marked heterogeneity of p53 responses.
Whether this heterogeneity contributes to cancer incidence or clinical
status and response to treatment remains to be analyzed.
Cancer Cell 12, October 2007 ©2007 Elsevier Inc. 309
ciency. Furthermore, mice carrying a partial deletion
of the carboxy terminus of Brca1 (Brca1∆11) have a
milder developmental defect that is totally rescued in
a p53+/− background, but the resulting mice are prone
to cancer (Cao et al., 2006). In breast cancer families,
p53 mutations arise in a BRCA1-deficient background
associated with genetic instability and DNA repair defi-
ciency, and the high frequency of p53 mutations in this
tumor is therefore not surprising (Crook et al., 1997).
Furthermore, the spectrum of these mutations is dif-
ferent compared to matched sporadic breast tumors of
the same grade. Only a few mutations are localized at
hot spot codons, and several new p53 mutations have
been identified in BRCA1-deficient tumors (Smith et
al., 1999). Functional analysis of these BRCA1-asso-
ciated p53 mutants shows that several of them have
a partial defect of biological activities such as growth
arrest or apoptosis. Other mutants behave in a man-
ner undistinguishable from WT p53 except for a weak
transforming activity when cotransfected with H-RAS
into rat cells (Smith et al., 1999). As animal models sug-
gest that p53 inactivation is a key step for survival of
Brca1-deficient cells, this increased frequency of p53
mutation in BRCA families is not unexpected, and we
can exclude the hypothesis that these mutations are
passenger events randomly coselected in an unstable
genetic background. The observation that many of the
BRCA1-associated p53 mutants retain WT function to
a large extent is interesting. It is possible that they tar-
get a specific functional activity of p53 that has not yet
been identified, such as specific gene transactivation
in mammary glands or cooperation with the BRCA net-
work in DNA repair, as several proteins such as Rad51
can bind to both p53 and BRCA1. Finally, the possibil-
ity that these mutations may affect p53’s transcription-
independent apoptosis function or affect functions of
newly identified p53 isoforms should be considered
(Bourdon et al., 2005; Mihara et al., 2003).
Several genetic pathways leading to cell transforma-
tion have been identified in sporadic colorectal carcino-
mas. In tumors associated with microsatellite instability
(MSI+), the frequency of p53 mutation is low (less than
10%) compared to other sporadic colorectal carcinomas
(around 40%). This observation can be explained by the
propensity of MSI+ tumors to use a pathway for tumor
development that requires mutations in genes contain-
ing a repeated polynucleotide tract such as BAX, TGFβR,
or IGFR (Konishi et al., 1996). It is still unclear whether
the spectrum of p53 mutations is the same in MSI+ and
MSI− tumors. In inflammatory breast cancer and neu-
roblastoma, the frequency of p53 mutation is low, but
immunohistochemical and molecular analysis indicates
accumulation of WT p53 in the cytoplasm of tumor cells.
This abnormal localization of p53 disrupts its function
after DNA damage (Zaika et al., 1999). Whether this par-
ticular mechanism for p53 inactivation is associated with
a specific cell type or with other genetic defects remains
to be determined.
This review puts into perspective several fields of
investigation that have never been previously reviewed
together. It should provide us with a novel working frame-
work to determine how p53 inactivation in human can-
cer should be analyzed to allow a better understanding
of p53 pathways but also to improve our strategy for the
clinical analysis of p53 alterations. The value of p53 muta-
tions as a clinical biomarker in various human tumors has
been the subject of intense investigation (Soussi, 2005).
Despite these efforts, no consensus has been reached,
and p53 mutation analysis is not yet used in clinical prac-
tice. p53 alterations are a very complex issue, and this
review describes how p53 mutations could be shaped
by several factors, including genetic background, other
tumor-associated genetic alterations, and tissue-specific
factors. Other factors that have not been discussed here,
including gender, ethnic background, and age, which
could also have a major impact on p53 mutant selection,
must also be considered. Furthermore, p53 is only one
component of a giant surveillance network whose effi-
ciency is modulated by many other elements including
the other members of the p53 family and several other
signaling pathways. The essential question concerns the
strategy used to assess p53 status in human tumors, as
a mutation is only one of the multiple ways to impair the
p53 pathway. Whether or not these different pathways are
associated with the same tumor phenotype is of impor-
tance. MDM2 amplification observed in a high frequency
of sarcomas is certainly associated with the p53-inde-
pendent oncogenic function of the MDM2 protein. As
the main function of p53 protein is to act as a transcrip-
tion factor with a very broad spectrum of target genes,
comparing expression profiles of tumors with different
p53 status could be very useful to identify a specific p53
signature, as several analyses have shown that p53 muta-
tions are associated with a specific expression signature
in breast cancer (Sorlie et al., 2001; Miller et al., 2005). It
has yet to be determined whether this expression signa-
ture is similar in other tissues or associated with indirect
The recent demonstration that therapy based on p53
reactivation in p53-deficient tumors can lead either to
potent apoptosis or cellular senescence, depending
on the type of tumor, should encourage us to under-
take more global analysis of the p53 status in tumor
cells (Martins et al., 2006; Ventura et al., 2007; Xue et
al., 2007). Several novel strategies are already under-
way, including small molecules that activate WT p53 or
target mutant p53 (Wiman, 2006; Vassilev, 2007). The
effect of mutant p53 rescue by compounds such as CP-
31398 and PRIMA-1 will depend on the specific type of
p53 mutation, as certain mutants appear less amenable
to reactivation. Many other factors, including p53 and
MDM2 polymorphisms and the expression of p53 family
isoforms and other p53-regulating proteins, e.g., iASPP,
can also affect therapeutic efficacy.
310 Cancer Cell 12, October 2007 ©2007 Elsevier Inc.
Activation of WT p53 via inhibition of p53-MDM2 bind-
ing by low-molecular-weight compounds such as Nutlin
3 or RITA is applicable to tumors that retain WT p53 but
also to tumors with mutant p53, and the efficiency of this
type of therapy could be modulated by MDM2 SNP309
(Issaeva et al., 2004; Tovar et al., 2006).
It is therefore reasonable to anticipate that analysis of
the p53 status in human cancer will comprise a combi-
nation of SNP analysis to evaluate the patient’s individ-
ual “p53 network genotype” and precise analysis of the
tumor status at the DNA level (p53 mutation) and/or at
the RNA level (expression signature). Only such studies
will be able to determine whether or not p53 will keep all
of its promises in clinical oncology.
We are grateful to M. Oren for reading this manuscript. K.G.W. is
supported by grants from EU FP6, the Swedish Cancer Society, the
Swedish Research Council (VR), the Cancer Society of Stockholm,
and Karolinska Institutet. T.S. is supported by Cancerföreningen i
Stockholm. K.G.W. is cofounder and shareholder of Aprea AB, a com-
pany that develops p53-based cancer therapy.
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