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It is poorly understood how a single protein, p53, can be responsive to so many stress signals and orchestrates very diverse cell responses to maintain/restore cell/tissue functions. The uncovering that TP53 gene physiologically expresses, in a tissue-dependent manner, several p53 splice variants (isoforms) provides an explanation to its pleiotropic biological activities. Here, we summarize a decade of research on p53 isoforms. The clinical studies and the diverse cellular and animal models of p53 isoforms (zebrafish, Drosophila, and mouse) lead us to realize that a p53-mediated cell response is, in fact, the sum of the intrinsic activities of the coexpressed p53 isoforms and that unbalancing expression of different p53 isoforms leads to cancer, premature aging, (neuro)degenerative diseases, inflammation, embryo malformations, or defects in tissue regeneration. Cracking the p53 isoforms' code is, thus, a necessary step to improve cancer treatment. It also opens new exciting perspectives in tissue regeneration.
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p53 Isoforms: Key Regulators of the
Cell Fate Decision
Sebastien M. Joruiz and Jean-Christophe Bourdon
Dundee Cancer Centre, University of Dundee, Ninewells Hospital and Medical School,
Dundee DD1 9SY, United Kingdom
Correspondence: j.bourdon@dundee.ac.uk
It is poorly understood how a single protein, p53, can be responsive to so manystress signals
and orchestrates very diverse cell responses to maintain/restore cell/tissue functions. The
uncovering that TP53 gene physiologically expresses, in a tissue-dependent manner, several
p53 splice variants (isoforms) provides an explanation to its pleiotropic biological activities.
Here, wesummarize a decade of research on p53 isoforms. Theclinical studies and the diverse
cellular and animal models of p53 isoforms (zebrafish, Drosophila, and mouse) lead us to
realize that a p53-mediated cell response is, in fact, the sum of the intrinsic activities of the
coexpressed p53 isoforms and that unbalancing expression of different p53 isoforms leads to
cancer, premature aging, (neuro)degenerative diseases, inflammation, embryo malformations,
or defects in tissue regeneration. Cracking the p53 isoforms’ code is, thus, a necessary step to
improve cancer treatment. It also opens new exciting perspectives in tissue regeneration.
p53 is a central sensor of cell signals and a mas-
ter regulatorof cell response to damage (Lane
and Levine 2010). But how can so many distinct
extracellular and intracellular signals modulate
p53 activity? Howcan onlyone protein bind spe-
cificallyto so manydifferentDNA sequences( p53
response elements)(el-Deiry etal. 1992; Bourdon
et al. 1997; Khoury and Bourdon 2011; Simeo-
nova et al. 2012) and directly regulate expression
of thousands of genes? How does p53 select the
target genes to trigger on time the appropriatecell
responses to so many different cellular damages?
Why is it so difficult to link TP53 mutation status
to prognosis and cancer treatment?
The TP53 gene is highly conserved in mul-
ticellular organisms (Lane et al. 2010). It is lo-
cated on human chromosome 17p13.1 and is
composed of 13 exons of which the first is non-
coding (Fig. 1A). TP53 contains the Hp53Int1
and WRAP53 genes within exon-1/intron-1
(Reisman et al. 1996; Mahmoudi et al. 2009).
It presents multiple genetic polymorphisms de-
fining more than 100 distinct TP53 haplotypes,
some of which are correlated with an increased
risk of cancer (Dumont et al. 2003; Garritano
et al. 2010; Wu et al. 2013). Although it is un-
equivocally established that TP53 is the most
frequently mutated gene in human cancer, it is
still difficult in the clinic to link TP53 mutation
status to cancer treatment and clinical outcome,
suggesting that the p53 pathway is not entirely
understood. The discovery that the TP53 gene
encodes several different splice variants may ex-
plain the discrepancy.
Editors: Guillermina Lozano and Arnold J. Levine
Additional Perspectives on The p53 Protein available at www.perspectivesinmedicine.org
Copyright #2016 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a026039
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Δ40p53γ
Δ40p53α
11
Associated polymorphisms
Polymorphisms affecting p53
internal promoter activity
G-Quadruplex DNA structure
Hp53Int1 gene
A
B
P2 internal promoter
WRAP53 gene
promoters
g
Polymorphism: g12718a
missense V217M
Polymorphism: c14007g
missense T312S
Polymorphism: g16970c
missense G360A
si133a/b
P1
P2
109
9
9
9
9
9
9
9
9
8765432
1
i
ii
iii
iv
v
vi
vii
i2
i2
i2
i4
i4
i4
viii
ix
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
4
4
5
5
5
5
5
5
5
5
6
6
6
6
6
6
6
6
7
7
7
7
7
7
7
7
8
8
8
8
8
8
8
8
10
10
10
10
10
10
10
10
11
11
11
11
11
11
11
11
p53/Δ40 Δ133/Δ160
P1
c11455g
(P72R)
t12081c (intronic)
g12247a (intronic)
g12273a (intronic)
g11338a (silent)
c11298a (intronic)
ic)
acc tggagggctg ggg (1268–1273)
t12239c (intronic)
(missense P47S)
Encoded protein:
Δ133p53γ or Δ160p53γ or
Δ133p53β or Δ160p53β or
Δ133p53α and Δ160p53α
Δ133p53α or Δ160p53α or
9β
9γ
9γ
9γ
9γ
9γ
9γ
9β
9βΔ40p53β
Δ40p53 only
Δ40p53β only
Δ40p53γ only
123 56 789
9β9γ10 11
4
g
siγ
siγsiα
siβ
siSplice
siE7/E8
siE7
siE2
β
β
β
γ
γ
γ
p53γ or Δ40p53γ or p53γ and
p53β or Δ40p53β or p53β and
p53α or Δ40p53α or p53α and
Δ133p53γ and Δ160p53γ
Δ133p53β and Δ160p53β
Figure 1. TP53 locus and p53mRNAs. All introns/exons are represented to scale. Black boxes represent non-
coding sequences, whereas coding sequences are colored. (A) The human TP53 gene’s locus structure. The TP53
gene, which is composed of 11 exons and two cryptic exons (9band 9g), encodes several p53 isoforms
attributable to alternative promoters (BP1 and P2) and alternative retention of the cryptic exons. The non-
coding exon-1 and intron-1 contain different promoters for the WRAP53 gene (antisense-coded) and intron-1
contains the Hp53Int1 gene. A G-quadruplex DNA structure located within intron-3 modulates splicing of
intron-2 and activities of the internal p53 promoter P2. Several polymorphisms (including Pin3 and R72P)
change activities of the internal p53 promoter (P2). (B) p53 mRNAs. The TP53 gene encodes nine different
mRNAs attributable to the alternative promoters (BP1 and P2) and splicing (^). The promoter P1, located
upstream of exon-1, encodes for intron-2 spliced (i, ii, and iii) or intron-2 retained (iv, v, and vi) mRNAs. The
intron-2 spliced mRNAs can encode the full length (ATG1) and/or the D40 (ATG40) proteins, depending on the
cell context, whereas the mRNA retaining intron-2 can only encode the D40 proteins. The P2 initiation tran-
scription site is located in intron-4 and encodes for three transcripts (vii, viii, and ix), which encode the D133
and the D160 forms. Small interfering RNAs (siRNAs) targeting the different p53 isoforms are represented on
top of the corresponding exons or introns.
S.M. Joruiz and J.-C. Bourdon
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p53 splice variants were first identified in the
late 1980s in human and mouse (Matlashewski et
al. 1984; Wolf et al. 1985). Thereafter, an alterna-
tive splicing of TP53 intron-9 has been described
(Arai et al. 1986; Flaman et al. 1996). To date, in
human, nine p53 mRNAs (Fig. 1B) encoding 12
different p53 protein isoforms have been des cribed
(Bourdon et al. 2005; Marcel et al. 2010a), p53a
(also named full-length p53, FLp53, canonical
p53, TAp53a), p53b(or p53i9), p53g,D40p53a
(or DNp53, p44 or p47), D40p53b,D40p53g,
D133p53a,D133p53b,D133p53g,D160p53a,
D160p53b,andD160p53g(Fig. 2A).
THE HUMAN p53 ISOFORMS
For decades, it was thought that one gene en-
codes one protein. The sequencing of the human
genome changed this dogma and revealed that
98% of the human genes are alternatively
spliced and contain multiple initiation sites of
transcription (promoter). The TP53 gene is no
exception. To date, it is reported that human
TP53 differentially expresses in normal tissue at
least nine mRNAs in a tissue-dependent manner.
They are a result of alternative promoter usage
(P1 and P2) and alternative splicing of intron-2
and intron-9 (Fig. 1B). Furthermore, depending
on the cell type, the translation of the p53
mRNAs can be initiated at different codons.
For the mRNAs transcribed from the proximal
promoter (P1), translation can be initiated at
codons 1 and/or 40, whereas the mRNAs tran-
scribed from the internal promoter (P2) trans-
lation can be initiated at codons 133 and/or 160.
The fully spliced p53 transcript (i) encodes
the canonical p53 protein (p53a) but also en-
codes the D40p53aisoform thanks to an inter-
nal ribosomal entry site (IRES) (Yin et al. 2002;
Candeias et al. 2006; Ray et al. 2006). This tran-
script also exists with two different alternative
splicings of exon-9 retaining thus the exon-9b
or -9g(ii/iii) and encoding, respectively, the
p53band/or D40p53b, and the p53gand/or
D40p53g. Both exon-9band exon-9gcontain
stop codons so that exon-10 and -11 are non-
coding in band gp53 mRNA splice variants.
The second type of transcript, also expressed
from the promoter P1, conserves intron-2
(iv/v/vi). Retention of this intron leads to sev-
eral stop codons when translation is initiated
from codon 1, thus preventing synthesis of full-
length p53 proteins. However, translation can
still be initiated at codon 40 so that this group
of mRNAs encodes exclusively D40p53a,
D40p53b,orD40p53g. The mRNAs vii/viii/ix
are transcribed from the internal promoter P2
located within intron-4. Their translation can
start either at the codons 133 or 160 encoding,
thus, the D133 and D160 p53 protein isoforms
(a,b,org, respectively).
At the protein level, p53a(canonical p53)
was the first p53 protein isoform to be identi-
fied. It is a 393-amino-acid-long protein with
seven functional domains (Fig. 2A,B). The ami-
no terminus is composed of two transactivation
(TA) domains, TA-1 and TA-2, which are re-
quired to induce a distinct subset of p53-target
genes. p53aalso contains a proline-rich domain
(PRD), a DNA-binding domain (DBD), and a
hinge domain (HD). The carboxyl terminus is
composed of an oligomerization domain (OD)
and a negative regulation domain (a). The neg-
ative regulation domain, rich in lysine (K-rich),
undergoes many different posttranslational
modifications (phosphorylation, methylation,
acetylation, sumoylation, ubiquitinylation, ned-
dylation, etc.), which regulate p53aactivity and
stability (Meek and Anderson 2009). In addi-
tion, the hinge and adomains contain several
nuclear localization signals (NLSs). The p53
DBD protein sequence is highly conserved
through evolution. It contains several conserved
cysteines and histidines that coordinate Zn
2þ
or
Mg
2þ
essential for p53 conformation and DNA-
binding activity (Fig. 2B) (Pavletich et al. 1993;
Cho et al. 1994; Xue et al. 2009).
The D40, D133, and D160 protein isoforms
are, respectively, lacking the first 39, 132, and
159 amino acids. The D40 isoforms have lost
the TA-1 but still retain the TA-2 and the com-
plete DBD. The D133 isoforms lack both TA
domains and a small part of the first conserved
cysteine box of the DBD, whereas the D160 iso-
forms lack both TA domains and the entire
first conserved cysteine box of the DBD, retain-
ing the three other conserved cysteine boxes of
the DBD.
From the TP53 Gene to p53 Protein Isoforms
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12
A
γ
β
356 7810 11i44i2
p53α
TA-1
TA-1
TA-2
TA-2 PRD
PRD
SAPUSAPUSAPU
CM1
DNA-binding domain
DNA-binding domain
HD
HD OD
ODα
α
p53γ
p53β
Δ40p53α
Δ40p53β
Δ133p53β
Δ160p53β
Δ40p53γ
Δ133p53γ
Δ160p53γ
Δ133p53α
Δ160p53α
2
B3
60
393
331
300
240
180
120
4
9β
9γ
332
332 341
346
332
5
IRGRERFEM
MEEPQSDPSV
DEAPRMPEAA PRVAPAPAAP TPAAPAPAPS WPLSSSVPSQ KTYQGSYGFR LGFLHSGTAK
SVTCTYSPAL NKMFCQLAKT CPVQLWVDST PPPGTRVRAM AIYKQSQHMT EVVRRCPHHE
EPPLSQETFS DLWKLLPENN VLSPLPSQAMDDLMLSPDDI EQWFTEDPGP
DQTSFQKENC
MLLDLRWCYFLINSS
FRELNEALEL KDAQAGKEPG GSRAHSSHLK SKKGQSTSRH KKLMFKTEGP DSD
7
11
Conserved regions
6
8
9
10
TIHYNYMCNS
VGSDCT
RCSDSDGLAP PQHLIRVEGN LRVEYLDDRN TFRHSVVVPY EPPE
β
γ
PGSTKRALPN NTSSSPQPKK KPLDGEYFTL Q β
γ
α
α
SCMGGMNRRP KKGEPHHELPDRRTEEENLREVRVCACPGR
SGNLLGRNSF
ILTIITLEDS
140
160133
NLS
Oligomerization domain
GEYFTLQ
GEYFTLQIRG...AG KE...HSS H...KKGQSTSR...DSD
GEYFTLQIRG...AG KE...HSS H...KKGQSTSR...DSD
GEYFTLQIRG...AG KE...HSS H...KKGQSTSR...DSD
GEYFTLQIRG...AG KE...HSS H...KKGQSTSR...DSD
DQ TSFQKENC
DQ TSFQKENC
DQ TSFQKENC
DQ TSFQKENC
MLLDLRWCYFLINSS
MLLDLRWCYFLINSS
MLLDLRWCYFLINSS
MLLDLRWCYFLINSS
GEYFTLQ
GEYFTLQ
GEYFTLQ
GEYFTLQ
GEYFTLQ
GEYFTLQ
GEYFTLQ
DO-12
KDa
3933573252919462401
421/BP53.10
26
29
42
48
26
29
42
47
31
35
47
53
DO-1 1801
DO-7 DO-11 240
9β9γ9
CM1
Figure 2. Human p53 protein isoforms. All exons and domains are represented to scale. (A) Schematic of the 12
p53 isoforms. The color of the protein domains matches the corresponding exon. p53ais composed of two
transactivation domains (TA-1 and TA-2), a proline-rich domain (PRD), the DNA-binding domain (DBD), the
hinge domain (HD), the oligomerization domain (OD), and the aregulatory domain. Compared with p53a,
D40 forms lack the TA-1 because of alternative translation initiation at ATG40. The D133 and D160 protein
isoforms are transcribed from P2 and lack TA-1, TA-2, PRD, and part of the DBD. Regarding the carboxy-
terminal isoforms, band gforms are attributable to an alternative splicing of intron-9 (brown and pink). As they
contain the entire exon-9, they encode the beginning of the OD. The theoretical molecular weight of each protein
isoform is indicated. The epitope boundaries of different p53 antibodies are represented by dotted lines. It is
important to note that Sapu and CM1 polyclonal antibodies contain multiple epitopes within the first 80 amino
acids of p53aand several epitopes in the a-carboxy-terminal regulatory domain, explaining the enhanced
detection of p53acompared with the other p53 protein isoforms. (B) Amino-acid sequence of human p53
isoforms. The color of the amino-acid sequence matches the encoding exon. The translation initiation methi-
onines (M) are indicated in red. The a,b,orgpeptide sequences are indicated. The underlined amino-acid
sequences relate to the conserved cysteine domains in the DBD. Three nuclear localization sequences (NLS) are
represented in black boxes. One is common to all isoforms (exon-9) and two are specific of the aforms. The OD
encoded by exon-9 and -10 is shown in a red box.
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All p53 isoforms, whether transcribed from
the P1 or P2 promoter, can alternatively splice
exon-9band -9gto produce the b- and g-car-
boxy-terminal protein domains, replacing part
of the oligomerization domain and the entire
adomain. The b-carboxy-terminal protein is
composed of 10 amino acids, although the
g-carboxy-terminal protein domain is com-
posed of 15 amino acids. Of note, the first seven
amino acids of the oligomerization domain are
present in the a,b, and gp53 isoforms.
SCIENTIFIC TOOLS TO INVESTIGATE
p53 ISOFORM BIOLOGICAL ACTIVITIES
(RT-q)PCR
The most reliable method to identify and quan-
tify p53 splice variants is (RT-q)PCR (quanti-
tative reverse transcription polymerase chain
reaction). Several protocols and a set of primers
have been designed and published to detect and
quantify p53 isoform mRNAs in cell lines and
tumor samples (Khoury et al. 2013). However,
there is not always a strict correlation between
the expression levels of p53 mRNAvariants and
p53 protein isoforms because p53 protein iso-
forms are also regulated at the posttranscrip-
tional level.
Small Interfering RNAs (siRNAs)
We and others have developed several siRNAs
and shRNAs targeting specifically distinct exons
or introns of human TP53, which enable knock-
down differentially and specifically subsets of
human p53 splice variants (Fig. 1B; Table 1).
The siRNAs are essential tools to determine
the biological activities of endogenous p53 pro-
Table 1. p53 isoform-specific small interfering RNAs (siRNAs)
Position on
sequence (nt)
GenBank
accession number
(NG_017013) Exon location Exon sequence Targeted forms
si E2 15968–15986 Exon 2 GCAGUCAGAUCCUAGCGUC p53a, p53b, p53g,
D40p53a,D40p53b,
D40p53g
si 133a 1721217230 Intron 4 GGAGGUGCUUACACAUGUU D133p53a,D133p53b,
D133p53g,D160p53a,
D160p53b,D160p53g
si 133b 17271 17289 Intron 4 CUUGUGCCCUGACUUUCAA D133p53a,D133p53b,
D133p53g,D160p53a,
D160p53b,D160p53g
si E7 18322–18340 Exon 7 GCAUGAACCGGAGGCCCAU All
si E7/E8 18363–18370/
18714–18724
Exon 7 and 8 GACTCCAGTGGTAATCTAC All
si b19211–19229 Exon 9bGGACCAGACCAGCUUUCAA p53b,D40p53b,
D133p53b,D160p53b
si g19009–19016/
19285–19295
Exon 9 and 9gCCCUUCAGAUGCUACUUGA p53g,D40p53g,
D133p53g,D160p53g
si Splice 19284–19302 Exon 9band 9gGAUGCUACUUGACUUACGA p53b,D40p53b,
D133p53b,D160p53b,
p53g,D40p53g,D
133p53g,D160p53g
si a21846–21864 Exon 10 GUGAGCGCUUCGAGAUGUU p53a,D40p53a,
D133p53a,D160p53a
From the TP53 Gene to p53 Protein Isoforms
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tein isoforms (Bourdon et al. 2005; Fujita et al.
2009; Aoubala et al. 2011; Terrier et al. 2011,
2012, 2013; Marcel et al. 2012, 2014; Wei et al.
2012; Bernard et al. 2013; Mondal et al. 2013).
Antibodies
Several mouse monoclonal antibodies are avail-
able to detect endogenous p53 protein isoforms
by immunofluorescence, immunohistochem-
istry (IHC) on paraffin-embedded tissue, and
western blotting (Marcel et al. 2013). The epi-
tope of each p53 antibody has been determined
by ELISA on an epitope-mapping peptide li-
brary (Figs. 2A, 3B; Table 2). The mouse mono-
clonal antibodies DO-1 and DO-7 recognize the
same epitope within the TA-1 domain and can
only detect p53a, p53b, and p53g. The mouse
monoclonal antibody 1801 recognizes an epi-
tope within the TA-2 domain and, thus, detects
p53a, p53b, p53g,D40p53a,D40p53b, and
D40p53g. In some cell lines and tissue, 1801
cross-reacts with a protein not related to p53.
The mouse monoclonal 421 and BP53.10 rec-
ognize a similar epitope within the adomain
and, thus, detect p53a,D40p53a,D133p53a,
and D160p53a. The other mouse monoclonal
antibodies (DO-11, 240, DO-12, and 1620) rec-
ognize distinct epitopes within the DBD and
should, thus, theoretically detect all p53 pro-
tein isoforms. Because p53 protein isoforms
can be modified by posttranslational modifica-
tions, which alter their migration on SDS-PAGE
(sodium dodecyl sulfate polyacrylamide gel
electrophoresis), it is recommended to transfect
cells with siRNA targeting distinct p53 splice
variants to validate that the bands detected
are p53 protein isoforms (Marcel et al. 2013).
Table 2. p53 isoform-specific antibodies
Epitope
Amino acids Exon Sequence Recognized forms
DO-1 20–25 2 SDLWKL p53a, p53b, p53g
DO-7 20–25 2 SDLWKL p53a, p53b, p53g
1801 46–55 4 SPDDIEQWFT p53a, p53b, p53g,
D40p53a,D40p53b,
D40p53g
1620 145157 þ
201–212
5/6 LWVDST PPPGTRV þLRVEYLDDRN TF ND
DO-11 181–190 5/6 RCSDSDGLAP All
240 211–220 6 TFRHSVVVPY All
DO-12 256–267 7/8 TLEDSSGNLLGR All
SAPU 11–65/
290–301/
363–393
2/3/4/
8/9/
10/11
EPPLSQ-DPGPDEAPR/RKKGEPHHELPP/
RAHSSHLKSKKGQSTSRHKKLMFKTEGPDSD
All
CM1 1–90/
379–393
2/3/
4/11
MEEPQSDPSV......TPAAPAPAPS/
RHKKLMFKTEGPDSD
p53a, p53b, p53g,
D40p53a,D40p53b,
D40p53g,D133p53a,
D160p53a
421 372–379 11 KKGQSTSR p53a,D40p53a,
D133p53a,
D160p53a
BP53.10 374–378 11 GQSTS p53a,D40p53a,
D133p53a,D160p53a
ND, not determined.
S.M. Joruiz and J.-C. Bourdon
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Furthermore, posttranslational modifications
within epitopes can prevent binding of p53
antibodies. Detection by DO-1 is impaired
by phosphorylation of serine-20 (Craig et al.
1999). Similarly, it is well established that bind-
ing of 421 antibody is prevented by posttrans-
lational modification of lysine-372, serine-376/
378, threonine-377, and histidine-380 (Pospı
´si-
lova
´et al. 2000). The antibody BP53-10 could
be used to circumvent this problem as it is less
sensitive to posttranslational modifications sur-
rounding its epitope.
The rabbit (CM1) and sheep (SAPU) poly-
clonal antibodies have been raised against bac-
terially produced recombinant human p53a.
They recognize several distinct epitopes within
the amino-terminal and adomains so that
the detection of p53ais strongly enhanced
compared with the other p53 isoforms, thus
preventing any expression-level comparisons
between p53aand the other isoforms. CM1
and SAPU antibodies have a lower affinity for
the D133/D160 and b/gforms that lack the
amino terminus or car boxyl-terminus epitopes,
respectively. Only SAPU recognizes an epitope
within the HD, which is common to all p53 pro-
tein isoforms. It can, thus, detect D133p53b/g
and D160p53b/g, whereas CM1 cannot.
THE p53 ISOFORM RELEVANCE IN CLINIC
In recent years, numerous studies have shown
that p53 isoforms are abnormally expressed in
breast cancer, melanoma, renal cell carcinoma
(RCC), acute myeloid leukemia (AML), colon
carcinoma, head and neck tumors (HNSSCs),
hepatic cholangiocarcinoma, ovarian cancer,
lung tumor, and glioblastoma (Bourdon et al.
2005, 2011; Anensen et al. 2006; Boldrup et al.
2007; Avery-Kiejda et al. 2008; Marabese et al.
12
P1
2
B
A
3
60
120
180
240
300
331
393
4
7
1801
1620
1620 240
BP53.10421
DO-12
DO-1/DO-7
DO-11
9
10
9β
9γ
332
332 341
367
346
332
6
5
8
11
P2 γ
siγsiαsiβ
β
siSplice
siE7/E8
siE7
siE2
356 7899β9γ10 11i44i2
si133a/b
SCMGGMNRRP
IRGRERFEM
PGSTKRALPN
MEEPQSDPSV
DEAPRMPEAA PRVAPAPAAP TPAAPAPAPS WPLSSSVPSQ KTYQGSYGFR LGFLHSGTAK
SVTCTYSPAL NKMFCQLAKT CPVQLWVDST PPPGTRVRAM AIYKQSQHMT EVVRRCPHHE
EPPLSQETFS DLWKLLPENN VLSPLPSQAM DDLMLSPDDI EQWFTEDPGP
NTSSSPQPKK KPLDGEYFTL Q
DQTSFQKENC
MLLDLRWCYFLINSS
FRELNEALEL KDAQAGKEPG GSRAHSSHLK SKKGQSTSRH KKLMFKTEGP
KKGEPHHELPDRRTEEENLREVRVCACPGR
SGNLLGRNSF
DSD
ILTIITLEDS
TIHYNYMCNS
VGSDCT
RCSDSDGLAP PQHLIRVEGN LRVEYLDDRN TFRHSVVVPY EPPE
β
β
γ
γ
α
α
Figure 3. Localization of the human p53 small interfering RNA (siRNA) and p53 antibodies’s epitopes. (A) p53
isoform-specific siRNAs are represented on the human TP53 gene. (B) p53 antibodies’ epitopes on human p53
protein. The underlined amino-acid sequences correspond to the epitopes of the named antibodies (mAb1801,
mAb240, mAb DO-11, and mAb DO-12). mAb DO-1 and DO-7 have the same epitope. mAb1620 antibody has
a conformational epitope that is represented by green boxes. The mAb 421 epitope is shown in a red box, whereas
the mAb BP53.10 epitope is underlined.
From the TP53 Gene to p53 Protein Isoforms
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2008; Fujita et al. 2009; Song et al. 2009; Hof-
stetter et al. 2010; Nutthasirikul et al. 2013;
Takahashi et al. 2013).
Several studies investigated whether p53
isoforms were associated with cancer patient
prognosis. First, it was reported that the evolu-
tion from colorectal adenoma to carcinoma
was potentially driven by an imbalance of
p53b/D133p53aratio in favor of the latter (Fu-
jita et al. 2009). In AML, distinct p53 isoform
biosignatures correlate with clinical outcome
(A
˚nensen et al. 2012), whereas in cholangio-
carcinoma, down-regulation of the TAp53
isoforms (p53a/p53b/p53g) combined with
up-regulation of the D133 forms is associated
with a shortened overall survival (Nutthasirikul
et al. 2013).
Importantly, several studies in different hu-
man cancers have reported that the prognostic
value of TP53 mutation status is improved when
combined with p53 isoform expression. Wild-
type (WT) TP53 mucinous or serous ovarian
cancer patients expressing D40p53ahave a bet-
ter clinical outcome than WT TP53 mucinous
or serous ovarian cancer patients not expressing
D40p53a. Reciprocally, mutant TP53 serous
ovarian cancer patients expressing D133p53a
have a better disease-free and overall survival
than mutant TP53 serous ovarian cancer pa-
tients not expressing D133p53a(Hofstetter et
al. 2011, 2012; Chambers and Martinez 2012).
Furthermore, in WT TP53 ovarian cancers,
p53bexpression is associated with higher risk
of recurrence because it is a marker of serous
and poorly differentiated cancer (Hofstetter
et al. 2010).
In breast cancer, it was reported that p53b
expression is associated with smaller tumors
and longer disease-free survival in mutant
TP53 tumors, whereas D40p53 isoforms expres-
sion was found to be associated with the aggres-
sive triple negative subtype (Avery-Kiejda et al.
2014). In another study, it was shown that
mutant TP53 breast cancer patients expressing
p53ghave as a good prognosis as WT TP53
breast cancer patients, whereas mutant TP53
breast cancer patients devoid of p53gexpres-
sion have a particularly poor prognosis (Bour-
don et al. 2011).
Altogether, it suggests that the prognostic
values of p53b, p53g,D40p53a,o
rD133p53a
depend on the TP53 mutations status and the
cancer type.
Other studies have investigated whether p53
isoforms play active roles in cancer formation
and treatment. Silent TP53 mutation or muta-
tions in noncoding regions, such as IRES se-
quences, introns, or splicing sites, are associated
with cancer formation probably because they
lead to unbalanced p53 isoform expression de-
spite expressing WT p53 proteins (Hofstetter
et al. 2010; Grover et al. 2011; Khan et al.
2013). Furthermore, the pathogenic bacteria
Helicobacter pylori has recently been shown to
induce expression of D133 and D160 isoforms
in gastric epithelial cells, increasing their sur-
vival and probably promoting cancer formation
(Wei et al. 2012). Moreover, p53 isoforms are
involved in response to chemotherapy in vivo in
AML and melanoma (Anensen et al. 2006;
Avery-Kiejda et al. 2008; A
˚nensen et al. 2012).
In conclusion, p53 isoforms expression is
associated to clinical outcome of cancer pa-
tients. For a given cancer type, the accuracy of
patient prognosis is greatly improved by com-
bining p53 isoforms expression and TP53 mu-
tation status, which may be used to predict
response to treatment for some cancer types.
ANIMAL MODELS: IN SEARCH OF THE
PHYSIOLOGICAL RELEVANCE OF
ISOFORMS
The structure of the TP53 gene is highly con-
served through evolution (Bourdon et al. 2005;
Chen et al. 2005). Several p53 isoform animal
models were, thus, generated to investigate their
biological relevance and roles in carcinogenesis.
Zebrafish
The zebrafish TP53 gene (Zp53) has a dual gene
structure as its human counterpart (Storer and
Zon 2010). In addition to the Zp53aprotein,
corresponding to the human p53a(Cheng et al.
1997), the gene also encodes Zp53b,ZDNp53a,
and ZD113p53a. In addition, in some zebrafish
strains, the Zp53 gene expresses from the inter-
S.M. Joruiz and J.-C. Bourdon
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nal promoter TA2Zp53, TA3Zp53, TA4Zp53,
and TA5Zp53 because of a polymorphism
(Fig. 4A).
ZDNp53ais produced through retention of
intron-2, similarly to the human D40p53 forms.
However, ZDNp53ais not completely identical
to the human form as the translation is initiated
in intron-2 and substitutes the 38 first amino
acids of canonical p53 by 33 different amino
acids constituting a different TA domain (Da-
vidson et al. 2010). ZDNp53aaccumulates in
response to g-ray and is able to form a pro-
tein complex with Zp53athrough the OD.
When coexpressed with Zp53a, overexpres-
sion of ZDNp53ainduces developmental mal-
formations (hypoplasia, head malformation,
somites, and eyes). This phenotype does not
appear after overexpression of ZDNp53amutat-
ed in the oligomerization domain or in Zp53
morpholino-depleted embryo, suggesting that
ZDNp53aacts through oligomerization with
Zp53 isoforms during development.
Mp53
DTAp53
Zp53
ZΔNp53
ZΔ113p53
Zp53β
DΔNp53
TA-1 TA-2 DNA-binding domain C-ter
TA-1
TAD
TAD
DNA-binding domain
DNA-binding domain
C-ter
C-ter
1
1 123
138
33
61 113 260 275 373
368
260
294
13
187 388 495
385
41 87 218 286
242
361 390
349
381
i10
i2
Mp53AS
MΔ41p53
Mp53Ψ
Ψ peptide:
β peptide:
AS peptide:
i6
i8
SKFAATLLKNILPYNRRVM
Mouse isoforms
Drosophila isoforms
Zebrafish isoforms
B
C
A
LQPRAFQALIKEESPNC
VTCSEVGSDFTWILCLPPGRL
Figure 4. p53 isoforms in animal models. Like human p53 protein isoforms, zebrafish, Drosophila, and mouse
p53 proteins are also composed of several functional domains: the transactivation domain (TAD, yellow), the
DNA-binding domain (DBD, green) and the carboxy-terminal regulatory domain containing the oligomeri-
zation domain (C-ter, orange). For each species, all domains are represented to scale compared with the full-
length protein of the corresponding species. Hatched protein domains are encoded by cryptic exons within
introns (the corresponding intron is indicated). (A) Zebrafish p53 isoforms. In the ZDNp53 isoform, the first 33
amino acids are encoded bya cryptic exon in intron-2. In Zp53b, the last 19 amino acids are encoded by a cryptic
exon in intron-8. Of note, Zp53bis not homologous to human p53b.(B)Drosophila p53 protein isoforms. Like
in the human TP53 gene family, DDNp53 (also named Dp53) protein is encoded by an mRNA transcribed from
an internal promoter. DDNp53 is homologous to D40p53/D133p53/DNp63/DNp73 proteins. Only DTAp53
protein contains the conserved box I (FxxLW) corresponding to the transactivation domain (TA-1) of the p53
protein family. (C) Mouse p53 isoforms. In Mp53AS protein (alternatively spliced [AS]), the last 26 amino acids
of Mp53aare substituted by 17 others encoded by a cryptic exon in intron-10. Mp53AS protein is homologous
to human p53b.MD41p53 produced by alternative splicing of intron-2 is homologous to human D40p53a. The
Mp53Cis produced by alternative splicing of intron-6 adding 21 new amino acids as indicated. Up to now, no
homologous protein to Mp53Chas been found in other organisms or in human cells expressing the WT TP53
gene.
From the TP53 Gene to p53 Protein Isoforms
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ZD113p53ais homologous to human
D133p53a. It is produced from the internal pro-
moter, which is transactivated by Zp53a(Chen
et al. 2005; Marcel et al. 2010b). ZD113p53a
oligomerizes with Zp53ainhibiting apoptosis
by differentially modulating Zp53 target gene
expression (Ou et al. 2014). ZD113p53ainduc-
es expression of cell-cycle arrest genes, such as
p21 and cyclin-G
1
and of antiapoptotic genes,
such as Bcl-XL, while inhibiting expression of
proapoptotic genes like Reprimo or Bax (Chen
et al. 2009). Furthermore, ZD113p53ais strong-
ly induced after DNA double-strand break
(DSB) and activates the DNA –DSB repair path-
ways, notably by up-regulating the transcrip-
tion of repair genes such as RAD51, RAD52,
or lig4 (Gong et al. 2015).
Recently, Shi et al. reported that a natural
polymorphism, a 4-bp deletion within intron-
4, creates four new upstream translation initia-
tion sites in frame to the ZD113p53 open read-
ing frame. This leads to four protein isoforms
with a putative TA domain named TA2p53,
TA3p53, TA4p53, and TA5p53. They lack the
93 first amino acids of Zp53athat are, respec-
tively, replaced by 65, 45, 18, and 9 amino acids
(Shi et al. 2015). These isoforms coexpressed
with Zp53aconfer resistance to irradiation. In
the same publication, Shi et al. also reported the
alternative splicing of Zp53 intron-8 leading to
Zp53b. However, Zp53bis not homologous to
human p53b.
Drosophila
The Drosophila melanogaster TP53 gene
(Dmp53) is the single Drosophila ortholog of
the mammalian p53 family and has a dual gene
structure with an internal promoter. It encodes
proteins homologous to p53, p63, and p73 (Lu
et al. 2009), and it is thought that, as the unique
member of the p53 family, it is likely that
Dmp53 carries the ancestral functions of all
p53 family genes. Dmp53 transcribes four differ-
ent mRNAvariants but, so far, only two p53 pro-
tein isoforms could be detected: DTAp53 and
DDNp53 (Fig. 4B). DTAp53 contains, in the first
40 amino acids, the highly conserved TA domain
present in mammalian p53a(Bourdon et al.
2005). DDNp53 (also called Dp53), expressed
from the internal promoter, lacks the 123 first
amino acids of DTAp53, which are replaced by
13 different ones encoded by a cryptic exon.
DDNp53 does not contain the conserved TA
domain but is still able to transactivate.
DDNp53 was the first Drosophila p53 isoform
cloned and was thus called Dp53 as the investi-
gators thought they had cloned full-length Dro-
sophila p53 (Brodsky et al. 2000; Jin et al. 2000;
Ollmann et al. 2000). However, DDNp53 is ho-
mologous to the amino-terminally truncated
isoforms of the p53 family proteins (D40p53,
DNp63, or DNp73). The literature on Drosophila
p53 is, thus, confusing as most genetic studies do
not discriminate between DTAp53 and DDNp53
isoforms, reporting phenotype associated with
Dmp53 gene as being mediated by Dp53 protein.
Another source of confusion is that mutant
p53 flies referred both to p53-null flies and to
transgenic flies overexpressing single-point mis-
sense mutant of Dmp53 (UAS-Dmp53R155H
or UAS-Dmp53H159N). However, p53-null
mutation leads to complete loss of p53 protein
expression and, thus, p53 activity, whereas mis-
sense mutation leads to overexpression of mu-
tant p53 proteins that are biochemically and
biologically very active as they directly interact
with numerous proteins modifying, thus, gene
expression and cell responses. p53-null muta-
tion can lead to different phenotypes than mis-
sense p53 mutation as shown in Drosophila and
mammalian models (Brodsky et al. 2000; Oll-
mann et al. 2000; Jassim et al. 2003; Lang et al.
2004; Olive et al. 2004; Wells et al. 2006; de la
Cova et al. 2014; Simo
´n et al. 2014)
The Dmp53 gene is involved in apoptosis
through the Reaper– Hid Grim cascade by di-
rectly regulating Reaper expression in response
to damage (Brodsky et al. 2000, 2004). However,
its physiological activity is not limited to apo-
ptotic pathways. Recent studies unravel the pri-
mordial roles of Drosophila p53 proteinisoforms
in tissue regeneration by directly controlling
apoptosis-induced proliferation (AiP), com-
pensatory proliferation (CP), cell differentia-
tion, organ size, system growth, and cell compe-
tition (Peterson et al. 2002; Jassim et al. 2003;
Wells et al. 2006; Stieper et al. 2008; Mendes et al.
S.M. Joruiz and J.-C. Bourdon
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2009; Fan et al. 2010; Mesquita et al. 2010; Mo-
rata et al. 2011; Wells and Johnston 2012; de la
Cova et al. 2014; Simo
´n et al. 2014). This is con-
sistent with Dmp53 being involved in life-span
control (Bauer et al. 2005, 2010; Waskar et al.
2009; for review, see Martı
´n et al. 2009; Kashio
et al. 2014; Mollereau and Ma 2014).
However, the contribution of each Drosoph-
ila p53 protein isoform in those biological phe-
nomena is still poorly understood. Recently,
Dichtel-Danjoy et al. (2013) investigated the
respective roles of DTAp53 and DDNp53 over-
expression in apoptosis and AiP in the develop-
ing wing imaginal disc. AiP is a phenomenon in
which cells undergoing apoptosis after a stress
event (i.e., “undead cells”) secrete mitogens,
such as Wingless (Wg), Decapentaplegic (Dpp),
or Hedgehog (Hh) (homologous to human
Wnt, TGF-b, or Hedgehog, respectively) to pro-
mote proliferation of neighboring cells to re-
store the normal organ and body size. Thus, at
the same time that the stress event eliminates a
large number of cells, it also generates a stimu-
lus to proliferation. AiP is directly controlled by
Dmp53 (Wells et al. 2006; Dichtel-Danjoy et al.
2013; Simo
´n et al. 2014). Both DTAp53 and
DDNp53 protein isoforms were capable of acti-
vating apoptosis but they used different molec-
ular mechanisms. However, only DDNp53 (not
DTAp53) can induce Wg expression in “un-
dead” cells and enhanced proliferation of neigh-
boring cells in Dmp53-null flies, suggesting that
DDNp53 is the main isoform that regulates AiP.
DTAp53 and DDNp53 seem, thus, to have dis-
tinct biological functions.
Mouse
The mouse TP53 gene (MsTP53) has a dual gene
structure as its human counterpart. To date,
four mouse p53 protein isoforms have been
published: Mp53a, Mp53AS, MD41p53a, and
Mp53C(Fig. 4C). Mp53ais the equivalent to
the canonical human p53a. Mp53AS is a car-
boxy-terminal splicing variant because of reten-
tion of part of intron-10. It encodes a 2-kDa
shorter protein with different carboxy-terminal
amino acids, which has a strong homology with
the human bp53 proteins. Mp53AS can bind
DNA and modulate p53-target-gene expression
in a promoter-dependent manner. Mp53AS can
induce apoptosis when expressed in absence of
Mp53a(Almog et al. 1997). However, when
Mp53AS and Mp53aare coexpressed, cells do
not induce apoptosis in response to damage
(Wolf et al. 1985; Almog et al. 2000). Mp53AS
and Mp53aoligomerize together and bind
DNA to regulate gene expression (Wu et al.
1994). Therefore, Mp53AS and Mp53aharbor
completely different/opposite activities when
they are expressed alone or in combination.
Recently, two knockin p53 mice, Mp53
D24
and Mp53
D31
-expressing carboxy-terminal
truncated mutants of p53 have been generated.
The Mp53
D24
mice contain a stop mutation at
codon 367 in the beginning of exon-11 so that
the mice express Mp53
D24
,MD41p53
D24
but
also the WT Mp53AS and Mp53Cproteins, be-
cause alternative splicing of intron-10 can still
occur (Hamard et al. 2013). The Mp53
D31
mice
contain a stop mutation at codon 360 at the end
of exon-10 so that the mice express Mp53
D31
,
MD41p53
D31
, and WT Mp53C. The Mp53
D31
mice do not express Mp53AS (Simeonova et al.
2013). In both studies, the MsTP53 gene has
been extensively sequenced to ensure the ab-
sence of any additional mutations in MsTP53
exons and introns. Interestingly, the Mp53
D24
and Mp53
D31
mice present different pheno-
types despite being of similar mixed genetic
background (BL6/129Sv). The Mp53
D24
and
Mp53
D31
mice have different blood cell counts
and different sizes of heart, thymus, spleen, tes-
tis, and cerebellum. Mp53
D31
mice also develop
oral leukoplakia (100%) and pulmonary fibro-
sis (87%), traits not reported for Mp53
D24
mice.
One of the most intriguing differences is in the
repopulating capabilities of hematopoietic stem
cells (HSCs) from embryos of the Mp53
D24
and Mp53
D31
mice. WT matching mixed genet-
ic background mice (BL6/129Sv) were lethally
irradiated to destroy the bone marrow cells.
The irradiated mice were then transplanted
with HSCs from Mp53
D24
, Mp53
D31
,orWT
MsTP53 mice (BL6/129Sv). In both laborato-
ries, the transplantation of HSC from WT
MsTP53 mice rescued 100% of the irradiated
mice by regenerating the bone marrow, al-
From the TP53 Gene to p53 Protein Isoforms
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though none of the irradiated and nontrans-
planted mice survived, indicating that both
laboratories master this assay. Importantly, the
transplantation of HSC from Mp53
D31
mice
completely fails to repopulate the bone marrow
and to rescue, thus, the irradiated mice (0%
rescue, n¼5), whereas the transplantation of
HSC from Mp53
D24
mice rescue 70% (n¼6)
of the irradiated mice by repopulating the bone
marrow (Hamard et al. 2013; Simeonova et al.
2013).
Knowing the potent transcriptional activi-
ties of Mp53AS and its tissue-specific expres-
sion, it is likely that the different phenotypes
of the Mp53
D24
and Mp53
D31
mice are attribut-
able to differential expression of Mp53AS. Fur-
ther investigations will be required to determine
the physiological roles of Mp53AS in tissue re-
generation in mice.
Recently, an additional isoform attributed
to the alternative splicing of intron-6, Mp53C
(also named p53C) has been described. This
results from the physiological insertion of the
last 55 nucleotides of intron-6, which rapidly
leads to a stop codon. Mp53Cis thus a very
short p53 isoform containing the TA and pro-
line domains but no DBD (Senturk et al. 2014).
Mp53Cmay act as a prometastatic factor in
promoting epithelialmesenchymal transition
and regulating mitochondrial activity togeth-
er with cyclophilin-D. Interestingly, Mp53C
seems to be expressed after damage in specific
stem cells during tissue regeneration in mice.
In liver injured with CCL4, the lesions get
smaller concomitantly to higher expression of
Mp53C. This is a fascinating result as it indi-
cates that the Msp53 gene is involved in tissue
regeneration in mammalians. Furthermore, it
opens new perspectives as p53 isoform expres-
sion could be manipulated to control organ re-
generation in mammalians.
Mp53CmRNA expression is unequivocal
in regenerating lung or liver mouse tissues; fur-
ther investigations will be required to confirm
physiological p53Cexpression in WT human
cells. So far, endogenous human p53Cprotein
could only be detected in one cancer cell line
(Hop62) that bears a mutation in the acceptor-
splicing site of human TP53 intron-6. Such a
mutation abolishes the canonical splicing of in-
tron-6 and promotes the insertion of the last 49
nucleotides of human TP53 intron-6 leading to
a p53C-like protein as shown by the investiga-
tors (Senturk et al. 2014). The development of
human/mouse p53C-specific antibodies and
siRNAwould greatly help to assess p53Cexpres-
sion and study its biological activities.
TheMD41p53aisoform has also been genet-
ically investigated.MD41p53a(also named p44)
is the mouse counterpart of human D40p53a.
MD41p53aoverexpression in WT Mp53 mice
leads to a smaller animal with premature ag-
ing and shorter life span. These effects have
been attributed to abnormal insulin-like growth
factor (IGF) signaling, driving a reduced cellular
proliferation with increased senescence (Maier
et al. 2004; Gambino et al. 2013), inhibition of
embryonic stem cell (ESC) differentiation
(Ungewitter and Scrable 2010), neurodegenera-
tion (Pehar et al. 2010, 2014), and impaired
b-cell proliferation (Hinault et al. 2011), which
suggests that the MD41p53 transgenic mice
have altered tissue-regeneration capabilities.
To genetically investigate the biological ac-
tivities of D133p53 isoforms, Slatter et al. (2011)
have generated knockin mutant TP53 mice
that are deleted of exon-3 and -4 (MsD122p53),
expressing ubiquitously D133p53-like protein
(D122p53a) and probably D122p53AS in a
tissue-dependent manner because of alterna-
tive splicing of intron-10. The homozygote
MsD122p53 mice show enhanced proinflamma-
tory phenotype, features of autoimmune dis-
ease, and early tumor onset (Campbell et al.
2012; Sawhney et al. 2015). It was thus thought
that D122p53 proteins were oncogenic. How-
ever, it was recently genetically shown that
D122p53 proteins enhance the tumor-suppres-
sor activities of an attenuated p53 mutant delet-
ed of the proline domain (MDpro) (Slatter et al.
2015).
Altogether, the different animal models are
consistent in demonstrating that the p53 iso-
forms are potent and essential components of
the p53 pathway, being involved in cancer, ag-
ing, tissue regeneration, glucose metabolism,
embryo development, immune system, and
bacterial infection (Wei et al. 2012). The balance
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between the different p53 isoforms is crucial in
determining cell-fate outcome.
REGULATION OF HUMAN p53 ISOFORM
EXPRESSION/ACTIVITIES
p53 isoform expression is regulated at the
transcriptional level by modulating the TP53
promoter activities and the alternative splicing
of intron-2 and intron-9.
In addition to the epigenetic events that reg-
ulate the tissue-specific activity of the p53 pro-
moters, the internal TP53 promoter activity is
also influenced by several polymorphisms, par-
ticularly the 16-bp insertion in intron-3 ( pin3)
and the R72P polymorphism in exon-4 altering
thus D133p53 isoform expression (Fig. 1A)
(Bellini et al. 2010; Marcel et al. 2012). In addi-
tion, the internal TP53 promoter is transacti-
vated by p53a, diverse p63/p73 isoforms, and
p68 (Moore et al. 2010; Aoubala et al. 2011;
Marcel et al. 2012).
SRSF1 and SRSF3, a highly conserved family
of splicing factors frequently deregulated in can-
cer, regulate the alternative splicing of TP53
intron-9 inhibiting retention of exon-9b/9g
(Tang et al. 2013; Marcel et al. 2014). Interest-
ingly, TG003, a small drug inhibitor of the Cdc2-
like kinases (Clks) that activate SRSF1 and
SRSF3, increases endogenous p53b/gexpres-
sion inhibiting cell proliferation in a p53 iso-
form-dependent manner (Marcel et al. 2014).
The alternative splicing of TP53 intron-2
leads to D40p53. Its expression is influenced
by a G-quadruplex structure within intron-3,
which is strengthened by the pin3 polymor-
phism that contains further G-quadruplex
structures (Marcel et al. 2011).
p53 isoform expression is also regulated at
the translational and posttranslational levels.
The D40p53 isoforms can be translated from
IRES controlled by IRES transactivating factors
(ITAFs), such as polypyrimidine-tract-bind-
ing protein (PTB), dyskerin, DAP5, annexin
A2, and PTB-associated splicing factor (PSF)
(Grover et al. 2008; Sharathchandra et al. 2012;
Weingarten-Gabbay et al. 2014).
Camus et al. (2012) have reported that all
p53 isoforms are ubiquitinated and degraded by
the proteasome. However, although MDM-2
binds to all p53 protein isoforms, it can only
promote the ubiquitination and degradation
of p53b. MDM2 binds probably D133p53 iso-
forms (a,b,org) through the DBD (Wallace
et al. 2006) and/or the a-carboxy-terminal
domain (Poyurovsky et al. 2010) because the
D133p53 isoforms have lost the MDM2-bind-
ing domain present in the amino-terminal end
of p53a, p53b, and p53g.
In addition to proteasome degradation,
D133p53ais also degraded by autophagy dur-
ing replicative senescence. Pharmacological
inhibition of autophagy by bafilomycin A1
restores D133p53aexpression levels. The auto-
phagic degradation of D133p53ainduces senes-
cence and is inhibited by direct interaction
of the chaperone-associated E3 ubiquitin ligase
STUB1/CHIP with D133p53a(Horikawa et al.
2014).
Altogether, expression of each p53 isoform
is tightly and differentially regulated, enabling
dynamic and accurate triggering of the appro-
priate cellular response to damage. The diverse
regulations of p53 isoforms expression could be
used as therapeutic targets to trigger controlled
cell-fate outcome.
p53 ISOFORM BIOLOGICAL ACTIVITIES
p53a, p53b(Mp53AS), p53g,D40p53a
(MD41p53a), and D133p53a(ZD113p53a
and MD122p53) have been shown to differen-
tially regulate gene expression and to be bio-
chemically and biologically active either alone
or in combination. D133p53bhas recently been
shown to regulate cell stemness; however, its
molecular mechanism is still unknown (Arsic
et al. 2015).
Over the past 10 years, using diverse human
cell lines and animal models, all data have con-
sistently indicated that the balance of expression
among the p53 isoforms define the p53-mediat-
ed cell response to trigger after damage, virus/
bacterial infection, or cell signals (Maier et al.
2004; Chen et al. 2009; Fujita et al. 2009; Me-
drano et al. 2009; Davidson et al. 2010; Pehar
et al. 2010, 2014; Ungewitter and Scrable 2010;
Aoubala et al. 2011; Hinault et al. 2011; Slatter
From the TP53 Gene to p53 Protein Isoforms
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et al. 2011, 2015; Terrier et al. 2012; Wei et al.
2012; Bernard et al. 2013; Dichtel-Danjoy et al.
2013; Gambino et al. 2013; Mondal et al. 2013;
Silden et al. 2013; Horikawa et al. 2014; Marcel
et al. 2014; Takahashi et al. 2014; Arsic et al. 2015;
Gong et al. 2015). The p53 isoforms thus play
primordial roles in cell-cycle progression, pro-
grammed cell death, senescence, inflammation,
stem-cell renewal and differentiation, aging,
neurodegeneration, glucose metabolism, angio-
genesis, embryo development, and cancer.
The manipulation of p53 isoforms using
splicing-factor inhibitors, autophagy inhibi-
tors, DNA damage, p53 isoform overexpression,
and/or siRNA targeting, specifically the p53
isoforms, enables the triggering of different
cell-fate outcomes in response to the same dam-
age (Aoubala et al. 2011; Horikawa et al. 2014;
Marcel et al. 2014; Gong et al. 2015). Mechanis-
tically, p53 isoforms oligomerize so that, for
example, the oligomers composed of p53band
p53aregulate different p53-responsive genes
than the ones composed of p53gand p53aor
D133p53aand p53a(Fujita et al. 2009; Aouba
et al. 2011; Bernard et al. 2013; Marcel et al.
2014; Solomon et al. 2014). p53 isoform expres-
sion is cell-type-specific and several p53 iso-
forms are always concomitantly coexpressed or
are mutually exclusive (Fig. 5). Hence, the p53-
mediated cell response would be the sum of the
activities of each oligomer of the coexpressed
p53 isoforms in a given tissue. It implies that
none of the p53 isoforms, including canonical
p53a, is able to aboli sh the activity or expression
of the other coexpressed p53 isoforms. It is thus
impossible to define a p53 isoform as oncogene
or tumor suppressor because its activity de-
pends on the cell context. Hence, although
p53bin T cells or normal human fibroblasts
prevents cell growth and induces senescence
(Fujita et al. 2009; Mondal et al. 2013), Marcel
et al. (2014) showed that endogenous p53b/g
in MCF-7 cells inhibit cell proliferation by pro-
moting G
1
-cell-cycle arrest and cell death under
standard culture conditions, but promote cell
proliferation after treatment with TG003. In
the D133p53-like mouse model (MsD122p53),
although D133p53a/D122p53 inhibits WT p53
tumor suppressor activity, it has recently been
reported that D122p53 proteins enhance the tu-
mor-suppressor activities of an attenuated p53
mutant deleted of the proline domain (MDpro)
(Slatter et al. 2015). This is consistent with clin-
ical studies. Mutant TP53 serous ovarian cancer
patients expressing D133p53ahave better dis-
ease-free and overall survival than mutant TP53
serous ovarian cancer patients not expressing
D133p53a(Hofstetter et al. 2011, 2012; Cham-
bers and Martinez 2012).
CONCLUSION AND INSIGHTS
Based on the data gathered over the past 10 years
by several laboratories using diverse human
cell lines and animal models, we can now assert
that p53 isoforms are physiologically active and
potent proteins. However, their activities are
cell-type-dependent. The misregulation of p53
isoform expression leads to cancer, premature
aging, (neuro)degenerative diseases, inflamma-
tion, and embryo malformations. The data
leads us then to realize that a p53-mediated
cell response is not driven by a single protein,
canonical p53, but is in fact the sum of the
activities of the coexpressed p53 isoforms in a
given tissue. Furthermore, a p53-mediated cell
response is broader than just preventing cancer
formation by controlling cell proliferation and
cell death. In fact, the effects of p53 isoform
expression misregulation are consistent with a
physiological role of p53 isoforms in organ
maintenance/regeneration as observed in Dro-
sophila, zebrafish, and mouse p53 isoform ani-
mal models (Jassim et al. 2003; Chen et al. 2005,
2009; Medrano et al. 2009; Davidson et al. 2010;
Dichtel-Danjoy et al. 2013; Hamard et al. 2013;
Simeonova et al. 2013; De la Cova et al. 2014;
Kashio et al. 2014; Senturk et al. 2014; Simo
´n
et al. 2014; Gong et al. 2015). The balance be-
tween the different p53 isoform is crucial in
predicting cell-fate outcome. Therefore, deci-
phering the p53 isoforms combinatorics (p53
isoforms’ code) offers fascinating and impor-
tant new perspectives to treat cancer, degener-
ative diseases and aging.
Cracking the p53 isoforms’ code may seem a
daunting task because of the high number of p53
isoforms and posttranslational modifications.
S.M. Joruiz and J.-C. Bourdon
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p53 isoforms
p53α
p53γ
p53β
Δ133p53α
Δ40p53α
p53α
p53γ
p53β
Δ133p53α
Δ40p53αp53α
p53α
p53α
p53α
p53α
p53α
p53α
p53α
p53α
p53α
p53α
p53αp53γ
p53γ
p53γ
p53γ
p53β
p53β
p53β
p53β
Δ133p53α
Δ133p53β
Δ133p53α
Δ133p53α
Δ133p53α
Δ40p53α
Δ133p53α
Δ133p53α
Δ40p53α
Δ40p53α
Senescence
Depletion of
Δ133p53α, Δ133p53β
Depletion of
p53β, p53γ,
Δ133p53β
siSplice
si133
Cell death
Cell death
Differentiation
Differentiation
Senescence
Senescence
Cell repair
and proliferation
Cell repair
and proliferation
Cell death
Damage
Cell repair
and proliferation
(1)
(2)
AB
C
(3)
Figure 5. Biological model. (A) p53 isoforms define cell-fate outcomes in response to intracellular and extra-
cellular cell signals—cell survival/proliferation, senescence, differentiation, or programmed cell death. (B) The
cell type (epigenetic) defines p53 isoform expression and the possibilities of cell-fate outcomes. (C) In response
to damage, p53 isoforms are differentially regulated and define cell-fate outcome. (1) In this theoretical cell, the
coexpressed p53 isoforms orchestrate cell repair and allow cell proliferation in response to damage. However, it is
important to note that the cell response to the same damage can be drastically changed by manipulating
expression of only a subset of p53 isoforms using p53 isoform-specific small interfering RNA (siRNA). (2)
Hence, after depletion of p53b, p53g, and D133p53busing the siRNA siSplice, the cell response to the same
damage is to induce cell death, (3) although, after depletion of D133p53aand D133p53busing the siRNA si133,
the cell response to the same damage is to induce senescence. (This model is based on data in Wu et al. 1994,
Almog et al. 2000, Yin et al. 2002, Chen et al. 2009, Fujita et al. 2009, Ungewitter and Scrable 2010, Aoubala et al.
2011, Terrier et al. 2012, Marcel et al. 2014, Slatter et al. 2015, and Gong et al. 2015.)
From the TP53 Gene to p53 Protein Isoforms
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However, the p53 isoforms are rarely coex-
pressed all together at once; some isoforms are
mutually exclusive, whereas others are always
coexpressed so that the number of p53 isoform
combinations is limited. Cracking the p53
isoforms’ code seems thus an achievable and
necessary task to accurately predict response
to cancer treatment and to control stem-cell re-
newal and differentiation in the treatment of
degenerative diseases and aging. It offers new
exciting perspectives in regenerative medicine.
ACKNOWLEDGMENTS
J-C.B. is a fellow of Breast Cancer Now (Grant
No. 2012MaySF127). All of the p53 antibodies
described in this review (except SAPU) are com-
mercially available or can be requested from Dr.
Borek Vojtesek (vojtesek@mou.cz). The SAPU
antibody can be requested from J.-C.B.
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... To date, nine TP53 mRNAs are found in humans resulted from alternative promoter usage (promoter 1 and promoter 2) at the TP53 locus and alternative splicing of intron 2 and intron 9, which further encode 12 different P53 protein isoforms via translation initiation at different start codons [1]. Various functional P53 isoforms are also described in Drosophila, zebrafish, and mice [1][2][3]. Although the whole landscape of P53 isoform network in human physiology, cancer, and degenerative diseases remains largely unknown, accumulated evidence based on clinical studies as well as studies with cell lines and animal models supports the notion that depending on the cellular or tissue context, P53 isoforms are differentially co-expressed and work in concert to define cellular responses, suggesting the complication in deciphering the consequence of any alterations at the TP53 locus [3]. ...
... Various functional P53 isoforms are also described in Drosophila, zebrafish, and mice [1][2][3]. Although the whole landscape of P53 isoform network in human physiology, cancer, and degenerative diseases remains largely unknown, accumulated evidence based on clinical studies as well as studies with cell lines and animal models supports the notion that depending on the cellular or tissue context, P53 isoforms are differentially co-expressed and work in concert to define cellular responses, suggesting the complication in deciphering the consequence of any alterations at the TP53 locus [3]. Indeed, it is difficult in the clinic to link TP53 mutation status to cancer treatment and prognosis and TP53 is unfortunately the most frequently mutated gene in human cancers [2][3][4]. ...
... Although the whole landscape of P53 isoform network in human physiology, cancer, and degenerative diseases remains largely unknown, accumulated evidence based on clinical studies as well as studies with cell lines and animal models supports the notion that depending on the cellular or tissue context, P53 isoforms are differentially co-expressed and work in concert to define cellular responses, suggesting the complication in deciphering the consequence of any alterations at the TP53 locus [3]. Indeed, it is difficult in the clinic to link TP53 mutation status to cancer treatment and prognosis and TP53 is unfortunately the most frequently mutated gene in human cancers [2][3][4]. ...
... Additionally, several researchers have found that P 53, a tumour suppressor protein, has a central part in activating apoptosis. P53 influences the mitochondrial pathway by regulating Bax, Bid and Puma proteins, ultimately leading to a cytochrome c release into the cytoplasm, cleaving caspase 9 and then caspase 3 which inevitably and unstoppably initiates apoptosis [44]. The findings also show that proteins involved in the anti-apoptotic pathway (AKT, eIF2A, Erk1/2, hsp27 and NFKB) were downregulated as in Figure 15. ...
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