The p53 family: guardians of maternal reproduction

The p53 protein family includes three transcription factors — p53, p63 and p73 — that each function in the DNA damage response, which is highly impor‑tant for the prevention of cancer1–4. These proteins share a common overall domain structure and can be transcribed as several different isoforms (FIG. 1). For all three members of the p53 family, the use of two different promoters at the amino terminus of the gene locus results in the expression of two isoforms with different N‑terminal domains: an isoform containing a com‑plete N‑terminal transactivation domain (TA‑isoform); and an N‑terminally trunc‑ated ΔN‑isoform that lacks the transactiva‑tion domain and acts, at least in part, as a dominant‑negative isoform. In addition, p53 family transcripts undergo extensive 3′ alternative splicing, thereby leading to a large number of protein isoforms for each family member. p53, p63 and p73 pro‑teins have the ability to homo tetramerize through their oligomerization domains (ODs) and to compete for binding to DNA through their DNA‑binding domains (DBDs). This means that the overall activit y of p53, p63 or p73 is a function of the ratio between their different isoforms, rather than of the isolated activity of any one form.

p53 (REFS 5,6), p63 (REF. 7) and p73 (REFS 8,9) each have a primary role in the DNA damage response in cells, although with distinct mechanisms. Indeed, all three members are able to induce cell‑cycle arrest and cell death following DNA damage, sug‑gesting a potential therapeutic response in treating the frequent cases of mutated p53, in which p63 or p73 can overcome the missing p53 function. However, these fac‑tors each have distinct effects: whereas the p73‑knockout has revealed an unexpected role for p73 in the nervous system, p63 affects the development of the epidermis. p53, p63 and p73 are powerful transcription factors10 and, consequently, small variations in their activation status can have profound effects on the expression of the genes that they transcribe11, and thus modify the fate of the cell12. The global impact of the p53 family members is mainly evident in their cellular functions (described in TABLE 1) and their effects on cancer13, development, stemness14–17, natural immunity18, oxidative stress19 and ageing20. Through this wide range of functions, the p53 family members represent one of the most powerful gene families.

Since its first appearance in vertebrates, p53 has undergone many amino‑acid changes (57–80 out of 200 amino acids).

In particular, the p53 protein’s relatively conserved DNA‑binding domain, which determines its gene targets, has changed substantially. However, the genes encod‑ing p63 and p73 have remained relatively conserved through this period of vertebrate evolution (undergoing only 7–11 changes and 18–21 changes out of 200 amino acids, respectively)21,22. What causes this evolu‑tionary pressure, and is it specific to p53 within this family? Here, we propose that this is due to the role of p53 in preserva‑tion of the female germ line. Observations made over the past few years have revealed a role for all p53 family members in human maternal reproduction control. In flies and worms, the common ancestor of p63 and p73 is found predominately in the germ line, where it protects against DNA damage and stress signals21; this fidelity function prevents mistakes in the germ line from producing abnormal embryos. As such, p63 and p73 seem to have conserved functions from invertebrates to humans in protecting the quality of egg development.

This concept has now been reinforced by the characterization of genetically engine ered mouse models with dele‑tions in p53 (REF. 23), p63 (REF. 24) or p73 (REF. 25) proteins or genetic disruption of specific isoforms, namely TAp73 (REF. 8), ΔNp73 (REF. 9) or TAp63 (REF. 26). These mouse models not only have uncovered new insights as to how these proteins affect cancer, development and longevity, but also have unexpectedly highlighted a central role for each of these proteins in the female reproductive tract. Whereas p63 may control the quality and survival of the oocyte pool27–28, p73 guarantees that the dividing early blastocyst undergoes normal mitotic entry8 and p53 regulates the implantation of the fertilized egg29 (FIG. 2). In this Opinion article, we discuss each of these functions of the p53 family proteins and their importance for mater‑nal fertility. These maternal roles have increased our understanding of how the primordial oocyte pool is maintained and how implantation and fertility are control‑led, and we propose that they represent the ancestral, evolutionarily conserved functio n of the p53 protein family.

O P I N I O N

The p53 family: guardians of maternal reproductionArnold J. Levine, Richard Tomasini, Frank D. McKeon, Tak W. Mak and Gerry Melino

Abstract | The p53 family of proteins consists of p53, p63 and p73, which are transcription factors that affect both cancer and development. It is now emerging that these proteins also regulate maternal reproduction. Whereas p63 is important for maturation of the egg, p73 ensures normal mitosis in the developing blastocyst. p53 subsequently regulates implantation of the embryo through transcriptional control of leukaemia inhibitory factor. Elucidating the cell biological basis of how these factors regulate female fertility may lead to new approaches to the control of human maternal reproduction.

PERSPECTIVES

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p63 control of primordial folliclesIt was the evolutionary biologist J. B. S. Haldane (1892–1964) (see Supplementary information S1 (box)) who first provided evidence that the female germ line contri‑butes an extremely low rate of mutations to the genome compared with the male germ line. He also introduced the concept that there are protective mechanisms at play in the female germ line.

Meiosis — the cell division that is necessary for sexual division and which ensures the production of haploid cells each containing one copy of each chromo‑some — includes distinct steps: meiosis I (leptotene, zygotene, pachytene, diplotene and diakinesis), metaphase I, anaphase I and telophase I, followed by an interphase leading to meiosis II. Oocytes exist as a limited population and are arrested in the potentially vulnerable, tetraploid state of

prophase I for an extraordinary length of time in order to protect germ line fidelity. When stimulated by specific hormones secreted from the follicle cells that surround the oocytes, fully grown oocytes mature and become receptive to fertilization. During their maturation, immature oocytes resume meiosis from the first prophase and pro‑ceed to the first or second metaphase, at which point they can be naturally insemin‑ated (FIG. 2), a process that is under strict control of mechanisms that are still largely unclear, such as the post‑pachytene check‑point. p63 seems to be important for the protection of the female germ line during meiotic arrest27. Indeed, the gene encoding p63 is constitutively expressed in female germ cells during meiotic arrest and, more specifically, it has been shown in vitro that phosphorylation of the TAp63 isoform by ABL1 (also known as c‑Abl) is essential

for DNA‑damage‑induced oocyte death, independen t of p53 (REFS 27,28) (FIG. 3a).

The regulatory process that maintains the quality of the primordial follicle pool is also responsible for the depletion of this follicle population in cancer patients under‑going radiation or chemotherapy treatment: p63 has been implicated as a major regu‑lator of oocyte death following treatment with irradiation and chemotherapeutic drugs28. TAp63 is expressed in the nucleus of oocytes after meiotic double‑stranded DNA break repair28, 30, and a null mutant in p63 prevents oocyte apoptosis induced by ionizing radiation30. Upon treatment with the DNA‑damage‑inducing agent cisplatin, a common anti‑cancer chemotherapeutic, TAp63 is stabilized by ABL1‑dependent phosphoryl ation of Tyr149, Tyr171 and Tyr289, resulting in the transcriptional activation by TAp63 of genes encoding pro‑apoptotic factors, which results in oocyte cell death28. Consequently, treatment with the ABL1 kinase inhibitor imatinib counter acts these cisplatin‑induced effects to in activate TAp63 and allow cell survival, which has a long‑term impact on prema‑ture ovarian failure28. This has potentially important medical implications, as it seems to indicate that imatinib or other ABL1 inhibitors could be used to preserve female fertility during chemotherapy31. However, this requires further investigation, and the possibility of side effects on any subsequent embryo development should also be a cru‑cial consideration. Indeed, the question of whether these damaged oocytes should be allowed to die remains; it is of little use to preserve female fertility if this causes fetal malformation.

Another important question is whether the DNA damage response is different in the primordial follicle pool and, if so, what the basis of this is. Moreover, is p63 involved in the physiological post‑pachytene check‑point? The post‑pachytene checkpoint is of crucial importance in preventing meiotic nuclear division in cells that fail to complete meiotic recombination; hence, it is absent in plants, which are compatible with poly‑ploidy and have no meiotic recombination32, as first pointed out by Haldane33. Indeed, this control mechanism prevents chromo‑some missegregation that would lead to the production of aneuploid gametes, and it requires mitotic DNA damage checkpoint proteins and meiosis‑specific chromosomal proteins. A role for p63 in this checkpoint pathway would be extremely important and might help clarify this poorly understood control point.

Figure 1 | ‘La famiglia’ of p53, p63 and p73. The genes encoding members of the p53 protein family, Trp53 (REFS 34–36), Trp63 (REF. 52) and Trp73 (REF. 53), are transcribed as distinct protein isoforms from two alternate promoters (P1 or P2). For Trp63 and Trp73, this generates either the transactiva‑tion domain (TA) isoforms (TAp63 and TAp73) or the amino‑terminally truncated versions (ΔNp63 and ΔNp73), which lack the TA‑domain and can exert dominant‑negative effects on the TA forms54. Δ40p53 is generated by a second ATG from the first promoter, whereas Δ133p53 is generated by the second promoter. ΔNp63 and ΔNp73, generated by their second promoters, are encoded from a 3′‑exon, with a distinct sequence, shown in red. Alternate 3′‑end‑splicing and posttranslational cleav‑age55–56 increase further the spectrum of family complexity (not shown); additionally, the various protein isoforms can synergize or compete with each other57, suggesting that the ratio of the indi‑vidual isoforms dictates the functional outcome. Percentages represent residue identity for individual domains comparing p53 with p63, or p63 with p73, from the top. For example, 23% and 27% represent residue identity against the TA domain of p53 versus p63 or p63 versus p73, respectively; the corre‑sponding identity comparing the DNA‑binding domain (DBD) of p53 versus p63 or p63 versus p73 is 63% and 86%. The sterile α‑motif (SAM) domain, absent in p53, shows 53% identify between p63 and p73. CT, carboxy-terminal domain; NLS, nuclear localization signal; OD, oligomerization domain; PR, Pro domain; TI, transcriptio n inhibitory domain.

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p53 and female fertilityThe p53 protein, and its encoding gene, was first identified in 1979, and its associa‑tion with cancer34–36 and its functions as a tumour suppressor were recognised37–39. The TP53 gene has been under significant evolutionary selective pressures that can be seen both in the selection of a single nucleo‑tide polymorphism in codon 72 and in the haplotype diversity index of this protein (see Supplementary information S2 (box)). This is unexpected, as we would not normally think of a disease such as cancer as exerting evo‑lutionary selection pressures on a gene such as TP53 because onset is often after child‑bearing age. This was also first discussed by Haldane, who explained why late‑onset diseases, such as Huntington’s disease or cancers, do not exert selective pressure on evolutionar y processes.

Insight into this question came from analysis of Trp53 –/– mice (FIG. 2b), which give birth to small litter sizes, even after taking into account that some female offspring were

runted and others were born with the brains outside of the skull (exencephalic brains)29. This reduction in fecundity was sexually dimorphic. Male Trp53–/– mice crossed with female wild‑type mice had normal litter sizes (10–12 mice), whereas male wild‑type mice crossed with female Trp53–/– mice had small litter sizes (1–2 mice), depending upon the inbred strain of the female. Both ovulation and fertilization appeared to occur normally in these mice; however, implantation of these fertilized eggs into the uterus was inefficient and it was possible to flush out many of them from the uterine cavity29,40,41. This phenotype seems to result from regulation of leukaemia inhibitory factor (Lif) gene transcription by p53; LIF is secreted by the glandular cells of the uterus and is required for the implanta‑tion of fertilized eggs into the uterus at 4 days after fertilization in mice (12 days in humans) (FIG. 3b). p53 expression levels reach a maxi‑mum in the glandular cells of the uterus at this time, as do levels of LIF protein and several other p53‑regulated transcripts in

the uterus. In female Trp53–/–mice, LIF levels at 4 days after fertilization are reduced29,40, and injection of LIF into a pregnant Trp53–/– mouse at 4 days after fertilization can res‑cue implantation defects29,40. Thus, p53 is required for efficient LIF production in the uterus, which in turn is required for efficient implantatio n of fertilized eggs.

TAp73 protection of oocyte qualityMice that are null for p73 showed defects in sensory and hormonal pathways, which contribute to behavioural phenotypes that preclude mating (FIG. 4; TABLE 1)25. But mice that are specifically null for TAp73, which mate normally and have a normal menstrual cycle, still display female infertility8. The phenotypes observed in this mouse model include poor oocyte quality and are in‑line with some parameters that are correlated with human infertility as well as with embryonic defects observed in patients undergoing in vitro fertilization (IVF), particularly older patients.

Table 1 | Cellular and physiological roles of p53 family proteins

Isoforms Biological roles Major mouse phenotype

Basis of fertility defects

Genes with similar phenotype

Refs

Cell cycle arrest

Apoptosis Development DNA Damage

Maternal reproduction

p53

p53 +++ +++ ± +++ +++ Cancer Implantation defects

Lif 58

Δ40p53 – – +++ ++ Impaired neural development

59

Δ133p53 ++ – ++ 60

Δp53 – – ++ ++ 61

p63

TAp63 ++ ++ ++ +++ +++ Impaired skin development

Post‑pachytene checkpoint

Abl1 (also known as c-Abl)

28

ΔNp63 – ± +++ ++ Absent skin 62,63

p73

TAp73 ++ ++ +++ +++ +++ Neural defects Ovulation rates Bmp15, Bmpr1b, GD9 64

Follicle pool Pten, Pdk1, Nobox 65–67

Androgen receptor, mTORC1 components

68,69

Pkbα (also known as Akt1)

70

Oocyte quality Bub1, BubR1, AuKB 43–45

Ex2p73 – – –

Ex2/3p73 – – –

ΔNp73 – – +++ ++ – Neural defects 9

ΔN’p73 – –

ΔN, amino‑terminally truncated; Bmp15, bone morphogenetic protein 15; Bmpr1b, BMP receptor 1b; Lif, leukaemia inhibitory factor; Nobox, newborn ovary homeobox; Pdk1, 3-phosphoinositide-dependent kinase 1; Pkbα, protein kinase Bα; Pten, phosphatase and tensin homologue; TA, transactivation domain. +++, ++ and ± indicate the severity of the major phenotypes observed in the mouse mutant. – indicates that no phenotype is found.





The oocyte quality of TAp73‑deficient female mice significantly contributes to the infertility observed (FIGS 2c,3b). Germinal vesicle or primary stage oocytes that have not begun to mature, as well as ovulated oocytes from TAp73‑deficient mice induced to mature in vitro, showed the normal rate of arrest in metaphase II. However, TAp73‑deficient oocytes exhibited a strikin g increase in spindle abnormalities, which included multipolar spindles, spindle relax‑ation and spindle scattering accompanied by varying degrees of chromosome misalign‑ment. Although the fertilization rate was unaltered by the selective loss of TAp73, the majority of embryos obtained from TAp73‑deficient oocytes showed impaired developmental competence in vitro: they arrested during early cleavage, resulting

in embryos with multinucleated blasto‑meres, and blastocysts of inferior quality and abnormal cell number8. Therefore, a lack of TAp73 in developing oocytes leads to a failure of preimplantation embryonic develop ment, suggesting that Trp73, through its TAp73 isoform, is essential for maternal reproduction.

This poor oocyte quality, leading to abnormal embryonic development, is also associated with maternal ageing, which is one of the most important prognostic fac‑tors in human infertility. The mechanism through which TAp73 affects maternal reproduction was unexpected: TAp73 acts through the spindle assembly checkpoint (SAC). This checkpoint prevents anaphase onset until all chromosomes are prop‑erly attached to the spindle and the two

kinetochores on each pair of sister chroma‑tids are attached to opposite spindle poles. The SAC stops the cell cycle by negatively regulating CDC20, thereby preventing the polyubiquitylating activity of the anaphase‑promoting complex (APC) and maintaining genomic stability42. Deficiency in TAp73 thus results in an increased aneuploidy rate13, the presence of which in oocytes is a marker of maternal ageing. Moreover, defects in SAC proteins, such as BUBR1, BUB1 or Aurora kinase B (which also result in aneuploidy), have also been implicated as causes of early lethality, poor oocyte quality and infertility43–45. Taking all these data into account, we can hypothesize that TAp73, acting through this SAC pathway, maintains genomic stability. However, an absence of TAp73 induces genomic instability in

Figure 2 | p53, p63 and p73 regulate distinct steps of maternal repro-duction. Sequence of events from the physiological maturation of oocytes to the correct implantation of the blastocyst, showing the ovary (primor‑dial, secondary and tertiary follicles), fallopian tube (oocyte ovulation, fertilization and zygote) and the uterus (implantation and blastocyst). Although all three p53 family members are involved in this same physio‑logical process, the underlying molecular events that they regulate are distinct and occur at different stages. a | The transactivation domain (TA) isoform of p63 (TAp63) is specifically expressed in oocytes, where it regu‑lates the post‑pachytene checkpoint and the propensity of cells to undergo apoptosis. Death of oocytes after DNA damage requires p63. Images show staining of MSY2, an oocyte marker, in wild‑type and TAp63‑deficient mouse ovaries after ionizing radiation. b | p53 affects the implan‑tation of the embryo via direct transactivation of the leukaemia inhibitory

factor (Lif ) gene. Wild‑type mice show a high number of implantation sites, whereas p53‑deficient mice show only 1–2 implantation sites, result‑ing in a very low fertility rate. Injection of LIF in p53‑deficient mice res‑cues the phenotype, resulting in normal fertility. c | TAp73 affects the spindle assembly complex (SAC) and therefore its absence results in poor blastocyst quality and defects in kinetochore–microtubule associations. In vitro fertilized eggs from superovulated female TAp73‑deficient mice show poor quality, with multinuclear cells and morphological defects (such as sperm heads) when compared to wild‑type eggs. Images in part a are reproduced, with permission, from REF. 27 © (2006) Macmillan Publishers Ltd. All rights reserved. Images in part b are reproduced, with permission, from REF. 29 © (2007) Macmillan Publishers Ltd. All rights reserved. Images in part c are reproduced, with permission, from REF. 8 © (2008) Cold Spring Harbor Laboratory Press.




oocytes, which leads to aneuploidy in the developing embryos, contributing to the infertility observed in TAp73‑deficient female mice.

There is also a significant decrease in the numbers of primordial (quiescent) and primary (early growing) follicle cells in TAp73‑deficient ovaries of adolescent mice8. On the basis of these data, we hypothesize that, with only two‑thirds as many pri‑mordial follicles as wild‑type adolescent mice, TAp73‑deficient mice will undergo reproductive failure faster than wild‑type animals. Another phenotype that is observed in TAp73‑deficient female mice is a reduced ovulation rate. Furthermore, the few ovu‑lated oocytes are trapped under the ovarian bursa (a membranous sac of peritoneum that forms a chamber along with the ovary) and do not show normal progression towards the fallopian tubes. These data suggest that, in addition to its effects on the follicular pool, TAp73 also regulates the ovulation rate of oocytes and their localization. Finally, the genes in TABLE 1 are all expressed in oocytes, highlighting the importance of oocyte‑secreted factors in regulating ovulation rate. TAp73, which is highly expressed within oocytes, is not a secreted factor but regulates the transcription of 79 genes, such as paternally expressed gene 10 (PEG10) and CTR9, that show an altered

expression pattern in the neonatal ovary in the absence of TAp73 (REF. 8). These studies have led to the hypothesis that TAp73 controls ovulation rate through regulation of several oocyte factors. Moreover, macrophage functions have also been implicated in the ability to ovulate46,47, and p53 has been reported to regulate macrophage functions18; this suggests that the p53 family may affect ovulatio n rates in multiple ways.

Implications for human infertilityWhat accounts for the selective pressure on the p53 protein (Supplementary information S2 (box))? In humans, p53 protein con‑taining Arg at codon 72 produces twofold higher levels of LIF transcript and more LIF protein compared with the Pro form of the p53 protein in cultured human cells29,40 (FIG. 3b). When hybrid human–mouse Trp53 genes were inserted into transgenic mice that have no Trp53 gene, the Arg form of the p53 protein induced twofold more Lif mRNA in the uterus of pregnant mice at day 4 than mice with the Pro form. In IVF clinics, the Pro form of the p53 single nucleo tide polymorphism (SNP) has been found at higher frequencies in women who had difficulties with implantation of ferti‑lized eggs than in control populations48,49. SNPs in the LIF gene, as well as in the double

minute 2 (MDM2), MDM4 and herpesvirus‑a ssociate d u biquitin‑specific protease (HAUSP) genes, which produce proteins that regulate the levels of p53 protein, were also identified as having their minor alleles

Figure 3 | The p53 family regulates distinct molecular processes that are important for female fertility. a | The transactivation domain (TA) isoform of p63 (TAp63) controls the survival of oocytes. Upon phosphorylation of Tyr171 by the kinase ABL1 (also known as c-Abl), TAp63 is activated and pro‑motes the transcription of factors that trigger cell death. The inhibition of ABL1 by imatinib blocks the activation of TAp63 and therefore promotes cell survival to protect oocytes upon DNA damage. b | p53 promotes the implan‑tation of the blastocyst via the direct transactivation of leukaemia inhibitory factor (LIF) transcription. Double minute 2 (MDM2), MDM4 and herpesvirus-associated ubiquitin-specific protease (HAUSP) are upstream regulators

of p53 that inhibit this process. The different codon 72 polymorphisms of p53 have different transactivation abilities that affect fertility: p53 with an Arg72 codon results in higher levels of LIF transcription than p53 with a Pro72 codon. c | TAp73 physically binds kinetochore proteins (BUB1, BUB3 and BUBR1), contributing to the proper entry into anaphase. Deregulation of this function in oocytes results in infertility, whereas in TAp73‑deficient cells or in cancer cells this produces aneuploidy. APC, anaphase‑promoting complex; DBD, DNA‑binding domain; NLS, nuclear localization signal; OD, oligomerization domain; PR, Pro domain; p53RE, p53 response element; SAM, sterile α‑motif; TI, transcription inhibitory domain.

Figure 4 | Intrinsic and systemic roles of p73 during evolution. We hypothesize that the ances‑tral primary systemic role of p73 is the regula tion of maternal reproduction, for example through con‑trol of metabolic signalling and trans cription (top). This module has since been adapted for additional processes in other tissues and organs to produce various systemic effects (bottom). The boxes inside the oval represent the main cell‑intrinsic functions of p73, whereas the boxes outside the oval represen t systemic outcomes.




overrepresented in women at the IVF clinic. This suggests that the protein complex that regulates p53 levels is genetically optimized for this step in fecundity. This applied to women under the age of 35; women over the age of 35 no longer had the statistically significant overrepresentation of the Pro iso‑form. At over 35–40 years of age, the qual‑ity of the eggs produced becomes a major factor, with aneuploidy and copy number variations becoming more important than implantatio n failures.

This cannot be the whole story, how‑ever, as African women with the Pro forms of p53 do not have obvious fertility problems, and this may be because dif‑ferences in genetic backgrounds (SNPs in MDM2 and MDM4) set them apart from Asian and Caucasian women. We probably do not understand all of the selective forces acting upon p53, but it is clear that fecun‑dity is likely to be a major source of this observed selection. Finally, there might be linked SNPs in the p53 Arg haplotype that contribute to these observations and to selection of this haplotype throughout evolution50. Thus, SNPs in p53 have impor‑tant effects on the regulation of implanta‑tion and may account for its effects on fertility. SNPs or mutations in the genes encodin g p63 or p73 may also help to identify women who are having problems producin g high‑quality eggs.

ConclusionsHow can we reconcile these concepts and, particularly, the evolution of members of the p53 family with the multiple effects that have been described for each? In the case of p73, for example, in flies and worms the ancestor of p73 predominately protects the germ line from DNA damage and stress21. Therefore, we could hypo‑thesize that this function was then adapted over time in different organs and tissues to exert control of other processes, including the spindle assembly checkpoint, the cell cycle and hippocampal development (FIG. 4; TABLE 1). This is fully supported by the fact that the ancestral progenitor of the p53 family in Caenorhabditis elegans, CEP‑1, forms dimers via the sterile α‑motif (SAM) domain and resembles mammalian TAp63, which exerts its quality controls on oocytes by switching from dimer to tetramer51. Whereas the ancestral maternal reproduc‑tion role has been retained in p63 as a dimer and tetramer, the subsequent DNA damage role evolved in p53 as a tetramer, indicating a parallel structural–functional evolution.

There is increasing evidence implicating the p53 family as a crucial molecular regu‑lator of female fertility. Indeed, the effects of p53, TAp73 and TAp63 isoforms on the follicle pool, ovulation rates and oocyte quality suggest that p53 family genes are crucial in the control of maternal repro‑duction (FIGS 2,4; TABLE 1). These findings could lead to new insights into the control of maternal reproduction, with profound therapeutic potential for controlling fertil‑ity. We argue that the maternal reproduc‑tion phenotype is the main ancestrally conserved function of p53 family members and this was then adapted for other func‑tions in additional tissues during evolution. This evolutionary pressure is, so far, well‑documented for p53, and, considering that p63 and p73 are more ancestral, we predict that SNPs in p63 and p73 would also affect human infertility as well as other genetic diseases.

Arnold J. Levine is at the Institute for Advanced Studies, Princeton, New Jersey 08540, USA; and at the Cancer Institute of New Jersey, University of Medicine

and Dentistry of New Jersey, New Brunswick, New Jersey 08903‑2681, USA.

Richard Tomasini is at the Institut National de la Sante et de la Recherche Medicale Unite 624, Stress

Cellulaire, 163 Avenue de Luminy, Case 915, Parc Scientifique et Technologique de Luminy,

13288 Marseille Cedex 9, France.

Frank D. McKeon is at the Department of Cell Biology, Harvard Medical School, Boston,

Massachusetts 02115, USA.

Tak W. Mak is at The Campbell Family Institute for Breast Cancer Research, Princess Margaret Hospital,

Toronto, Ontario M5G 2C1, Canada.

Gerry Melino is at the Medical Research Council, Toxicology Unit, Hodgkin Building, Leicester University, Lancaster Road, P.O. Box 138, Leicester LE1 9HN, UK;

and at the Biochemistry IDI‑IRCCS laboratory, Dept Experimental Medicine, University of

Rome Tor Vergata, via Montpellier 1, 00133 Rome, Italy.

Correspondence to G.M. e‑mail: [email protected]

doi:10.1038/nrm3086

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AcknowledgementsWe thank R. A. Knight for helpful discussions and comments.

Competing interests statementThe authors declare no competing financial interests.

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The p53 family: guardians of maternal reproduction