p53 Polymorphisms: Cancer Implications

Catherine Whibley; Paul D. P. Pharoah; Monica Hollstein

The p53 Pathway

Regulation and Activation of p53

The levels of p53 are key to its activity and are tightly controlled in the cell, largely by covalent modifications.^[17] Numerous stress sensors that converge at p53 result in the phosphorylation, acetylation, ubiquitylation and methylation of specific p53 residues.^{[17,18,19,20]} This range of modifications elicits downstream p53 responses that counteract the deleterious consequences of DNA damage, hypoxia, metabolic stress or oncogene activation (Fig. 1). Polymorphisms at loci that alter the activity of any single upstream event that activates p53 should not entirely abrogate the p53 response, owing to the high level of redundancy in stress responses, but cellular responses could be attenuated by altering one or more of the triggers for p53 activation.

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Figure 1.

The p53 pathway. The p53 pathway is complex. At least 50 different enzymes can covalently modify p53 to alter its stability, cellular location or activity.^[17] Under normal cellular conditions, MDM2 represses p53 by binding and sequestering p53, and by ubiquitylating p53, targeting it for degradation. DNA damage, oxidative stress and oncogene activation are among the processes that can activate p53 by a range of regulators. Basal levels of p53 and p53 in cells that are undergoing low levels of stress can also affect cell physiology. Under high levels of stress, the interactions between MDM2, MDM4 and p53 are disrupted by post-translational modifications of these proteins. This allows activated p53 to act as a transcription factor, activating or repressing genes involved in apoptosis, cell cycle arrest and senescence. p53 can also move to the mitochondria, where it physically interacts with members of the Bcl-2 family to form pores in the mitochondrial membrane, leading to the release of cytochrome c and subsequent apoptosis. Some of the more intensively studied activators, regulators and effectors of p53 are shown in the figure. ATM = ataxia-telangiectasia mutated; BAX = BCL-2-associated X; HIPK2 = homeodomain-interacting protein kinase 2; JNK = Jun N-terminal kinase; KAT5 = K (lysine) acetyltransferase 5; MLH1 = MutL protein homologue 1; PRMT5 = protein arginine methyltransferase 5; SESN1 = sestrin 1; SMYD2 = SET and MYND domain-containing 2.

Under normal, unstressed conditions, a key negative regulator of p53 is MDM2, which binds to the transactivation domain of p53 and ubiquitylates the protein, targeting it for degradation.^[21] Because p53 transcriptionally activates MDM2, the expression levels of p53 and MDM2 are balanced through a negative feedback loop, which is altered by an increase in p53 levels following stresses such as DNA damage.^[22] The related protein MDM4 (also known as MDMX) also modulates p53 activity,^[23] and the interplay between p53, MDM2 and MDM4 at the molecular level is complex. For example, MDM2 binds to TP53 mRNA, controlling the rate of translation,^[24] and MDM2 regulates the levels of itself, MDM4 and p53.^[23,25,26] The pivotal role of MDM2 and MDM4 in the control of p53 function argues that polymorphisms at these loci should be scrutinized for potential modulation of p53 function.

The range of post-translational modifications and p53 upregulation elicited by stress are well-studied features of the p53 network. Less is known about the physiological roles of p53 that are not necessarily linked to tumour suppression and that can be executed by low levels of p53 and in the absence of a severe insult or stress (Table 1). These homeostatic activities might also be affected by polymorphisms in p53, although there are limited data on this subject (not discussed in this Review).

Events Following p53 Activation

p53 controls multiple cellular functions by inducing or repressing target genes with p53 response elements^[27] (Fig. 2). Several hundred p53-responsive genes have been identified, more than 100 of which have been verified by several different methods to be directly regulated by p53.^[27] These include genes involved in cell cycle arrest, such as CDKN1A , which encodes p21; genes involved in apoptosis, such as BBC3 (BCL-2-binding component 3, also known as PUMA), BAX (BCL-2-associated X) and PERP (p53 apoptosis effector related to PMP22); genes involved in the inhibition of angiogenesis, such as THBS1 (thrombospondin 1); and genes involved in cellular senescence, such as CDKN1A and PML (promyelocytic leukaemia). p53 binding to response elements is regulated by the structure and conformation of p53, post-translational modifications of p53, and the structure and sequence of the response element.^[28]

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Figure 2.

Sequences of p53 response elements. The canonical p53 response element is displayed in the box. This comprises two palindromic 10 base pair sequences separated by a spacer varying between 0 and 13 base pairs.^[27] The p53 response elements of several p53 target genes are listed beneath the consensus sequence, with bases matching the consensus sequence listed in the appropriate colour, non-matching bases are not coloured. Several p53 responsive genes contain multiple p53 response elements; the two MDM2 response elements are shown as an example. BAX = BCL-2-associated X; BBC3 = BCL-2-binding component 3; PERP = p53 apoptosis effector related to PMP22; PML = promyelocytic leukaemia; R = purine; SESN1 = sestrin 1; TIGAR = TP53-induced glycolysis and apoptosis regulator; W = adenine or thymine; Y = pyrimidine.

The p53 response element has been extensively characterized and is one of the key factors that determines the pleiotropy of the p53 stress response. Slight differences in the sequence of response elements can dramatically alter the inducibility of the target gene by p53^[27,29] The numerous splice variant isoforms, and amino-terminal shortened forms of TP53 and p53 family members ( TP63 and TP73) add another layer of complexity to p53 transcriptional activity and the expression of p53 downstream targets.^{[30,31,32,33]} The ability of p53 to repress the transcription of specific genes is also important for its tumour suppressor activity, as shown in a recent report on p53 repression of CD44 expression.^[34]

The effects of p53 on cellular functions are also mediated by protein-protein interactions, which constitute a second axis in the suppression of tumorigenesis by p53. Numerous proteins involved in cell cycle control, DNA repair, signalling or gene transcription can bind to wild-type p53.^[35] Some of these interactions modulate the participation of p53 in the mitochondrial apoptotic programme.^[4,7,8] The p53 protein binds to members of the Bcl-2 family, increasing mitochondrial permeability and the release of mitochondrial intermembrane molecules that trigger downstream apoptotic events.^[36,37] Tumour-associated mutations in the p53 DBD are unable to activate this pathway,^[38] consistent with the importance of these interactions in inhibiting tumorigenesis.

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Table 1. Additional Roles for the p53 Tumour Suppressor
Physiological roles of p53	Examples of proposed mechanisms	Refs
Reproduction	Transcription of LIF is controlled by p53, with enhancement by the p53-R72 variantLevels of LIF affect implantation success	^[65,66]
Prolonged organismal life span	p53 controls stem cell self-renewalIn carriers of the p53-P72 variant, apoptosis can be attenuated, with a reduced age-associated attrition of stem cell populations	^{[85,161,162,163]}
Antioxidant functions	Transcriptional activation of genes encoding enzymes that reduce the cellular levels of reactive oxygen species, including GPX1, SOD2 and sestrins	^[164]
Autophagy	Transcriptional activation of DRAM, a lysosomal membrane protein required for autophagyInhibition of mTOR, a negative regulator of autophagy	^[165,166]
Glucose metabolism	Transcriptional activation of TIGAR, which blocks glycolysis and directs the pathway to the pentose phosphate shunt	^[167,168]
Mitochondrial respiration	Transcriptional activation of SOCS2, a cytokine-induced negative regulator of cytokine signalling	^[169]
Embryonic stem cell pools	Suppression of NANOG by p53 causes embryonic stem cell differentiation	^[170]

DRAM = damage-regulated autophagy mediator; GPX1 = glutathione peroxidase 1; IGF1R = insulin-like growth factor 1 receptor; LIF = leukaemia-inhibitory factor; SOCS2 = suppressor of cytokine signalling 2; SOD2 = superoxide dismutase; [Mn] = mitochondrial; TIGAR = TP53-induced glycolysis and apoptosis regulator.

Table 2. Polymorphisms in p53 Pathway Genes
Gene	Relation to p53	Cellular function	Polymorphisms potentially associated with cancer	Disease association	Potential or associated functional change	Refs
TP53	Encodes p53	Tumour suppressor: induces cell cycle arrest and apoptosis and senescence in response to stress	R72PP47SIntron 3 16 bp insertion	p53 often mutated in cancer Germline TP53 mutations in Li–Fraumeni syndrome Knockout mice are susceptible to tumours	See text	^[1,2,3]
MDM2	Regulator of p53 protein Induced by p53	Core control of p53 levels, activity and localization within the cell	SNP309 (promoter region)	Homozygous knockout in mice is embryonic lethal, but can be rescued by simultaneous knockout of Trp53	Enhanced SP1 binding site (G variant) increases transcription by ER Higher MDM2 levels suppress p53, reducing tumour suppression	^[113]
CDKN1A	Induced by p53	Regulator of cell cycle progression	S31RR84Q	Knockout mice develop normally, but develop tumours early	Reduced G1 arrest in response to DNA damage, which could alter responses to therapy	^{[131,141,142]}
ATM	Phosphorylates (and activates) p53 in response to DNA damage	Recognizes DNA damage and activates p53 and DNA damage response pathways	D1853N	Homozygous mutant ATM results in AT, which confers a 100-fold increased cancer risk	Decreased activation of p53	^[122,171]
BAX	Induced by p53 Interacts with other apoptotic proteins at the mitochondria	Forms a heterodimer with BCL-2 , activating MOMP and the mitochondrial apoptotic pathway	g>a (125 bp upstream of start site)	Lower levels of BAX in multiple tumour types predict poor response to chemotherapy		^[148,172]
TP53I3	Induced by p53	Oxidoreductase-like protein involved in apoptosis	Promoter microsatellite		Increased induction by p53 with increased microsatellite length improves apoptotic response	^[145]
AKT1	Activates MDM2, which inhibits p53	A Ser–Thr kinase induced by growth factors that inhibits apoptosis by activating MDM2	The haplotype (five SNPs in AKT1) in Caucasian populations results in higher levels of AKT1 than in African populations	Some polymorphisms are associated with schizophrenia Knockout mice are smaller (Akt1^-/- )	Lower levels of AKT1 sensitize cells to apoptotic signals	^[173,174]
TP73	Induces p53 Induced by p53 Physically interacts with p53	p53-like protein, shares many p53 functions, including induction of apoptosis and cell cycle arrest	Two linked SNPs in the non-coding region of exon 2		In combination with MDM2 SNP309 increases cancer risk	^[175,176]
NQO1	Stabilizes p53 by an MDM2-independent mechanism	Protects against oxidative stress and carcinogenesis	P187S	NQO1 variant associated with susceptibility to various cancers Altered expression in many tumours and Alzheimer's disease	P187S causes loss of NQO1 activity, and its tumour- suppressive and antioxidative effects, affecting cancer risk and response to therapy	^{[108,177,178]}

AT = ataxia–telangiectasia; ATM = ataxia–telangiectasia mutated; bp = base pair; CDKN1A = cyclin-dependent kinase inhibitor 1; ER = oestrogen receptor; MOMP = mitochondrial outer membrane permeabilization; NQO1 = NADPH dehydrogenase quinone 1; SNP = single nucleotide polymorphism; TP53I3 = tumour protein p53 inducible protein 3.