TP53 Guardian of the Genome

Slide 1 · TP53: Guardian of the Genome

Stress sensing • genome integrity • tumor suppression

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain how p53 integrates DNA damage and oncogenic stress signals.

Trace p53-mediated G₁ arrest, repair, senescence, and apoptosis.

Distinguish genetic TP53 loss from functional inactivation of p53.

Interpret inherited TP53 predisposition and therapy-response implications.

Slide 3 · Why “guardian of the genome”?

P53 prevents propagation of cells with potentially oncogenic genomic damage.

A temporary pause creates an opportunity for repair.

Persistent or irreversible damage may trigger permanent arrest or cell death.

Loss of this restraint allows genetically abnormal clones to expand.

Slide 4 · Four protective activities of p53

Activated p53

Cell-cycle arrest

Restricts G₁→S progression

DNA repair

Induces repair-associated genes

Senescence

Establishes permanent arrest

Apoptosis

Eliminates damaged cells

These responses together constrain malignant transformation.

Slide 5 · TP53 gene and p53 protein

TP53 is a tumor suppressor gene located at chromosome 17p13.1.

TP53 encodes the p53 transcription factor.

The gene is distinguished from its protein product in mechanistic discussions.

The central function is cellular stress-responsive control of gene expression.

Slide 6 · TP53 is a frequent cancer target

The source describes TP53 as the most frequently mutated gene in human cancer.

Loss-of-function mutations are reported in more than 50% of cancers.

The statement refers to the textbook’s overall cancer estimate—not a uniform rate across tumor types.

Slide 7 · Cancer types with TP53 abnormalities

TP53 mutations occur across a very broad spectrum of malignancies.

The Robbins examples include carcinomas of lung, colon, and breast.

Tumor type alone does not establish a specific TP53 genotype.

Different cancers can also impair p53 through regulators rather than the TP53 gene itself.

Slide 8 · Somatic and inherited TP53 mutations

Somatic setting

Alteration acquired in tumor-lineage cells

Most cancers have somatic TP53 abnormalities

Loss of both functional alleles is typical model

Inherited setting

One mutated allele is present in the germline

Remaining normal allele is susceptible to an additional hit

Creates predisposition rather than a tumor at birth

Both settings converge on loss of a protective p53 response.

Slide 9 · The two-hit tumor suppressor model

Normal cell

Two functional alleles

First hit

One allele impaired

Second hit

Remaining function lost

Tumor risk

Checkpoint protection fails

Schematic of biallelic inactivation; Inherited carriers begin with the first hit.

Slide 10 · Genetic loss is not the only route

TP53 mutation

Coding gene itself is altered

P53 response may be absent or defective

Common route in many carcinomas

P53 pathway suppression

TP53 sequence may remain normal

Regulatory proteins suppress p53 abundance/activity

Illustrated by MDM2 excess or viral E6

A normal TP53 coding sequence does not guarantee normal p53 function.

Slide 11 · Constitutive p53 turnover in healthy cells

Non-stressed cell

P53 production continues

MDM2 association

MDM2 ubiquitinates p53

Proteasome

P53 degraded

Low basal p53

Limited stress-response transcription

Rapid degradation keeps p53 protein at very low levels during ordinary growth.

Slide 12 · MDM2 is a key negative regulator

MDM2 is an enzyme that ubiquitinates p53 and promotes proteasomal degradation.

The p53–MDM2 interaction restrains p53 in the absence of stress.

To activate p53, a stressed cell must relieve this inhibitory interaction.

Slide 13 · MDM2 gene amplification and p53 dysfunction

MDM2 amplification

More MDM2 regulator

Enhanced p53 loss

Excess degradation

Weak p53 output

Deficient stress response

Cancer permissiveness

Reduced tumor suppression

MDM2 amplification in 33% of human sarcomas; This is a source-specific figure.

Slide 14 · High-risk HPV E6 inactivates p53

High-risk HPV

Viral E6 oncoprotein

P53 targeting

Promotes p53 degradation

P53 defense falls

Stress responses weakened

Oncogenic effect

Supports malignant transformation

Examples in the source: Cervical carcinoma and head-and-neck squamous carcinoma.

Slide 15 · Mechanisms that lower p53 function

Route

Primary alteration

Functional consequence

TP53 loss

Gene mutation/inactivation

Defective p53-dependent transcription

MDM2 excess

Regulator overexpression/amplification

Enhanced p53 degradation

Viral E6

High-risk HPV oncoprotein

Targeted p53 degradation

Distinct upstream causes can converge on defective p53-mediated tumor suppression.

Slide 16 · Cellular stresses detected by the p53 network

DNA damage is a major activating stimulus.

Other stresses include hypoxia and shortened telomeres.

Excessive signaling from oncogenes such as RAS and MYC can also activate p53.

Different inputs converge on stabilization of the p53 protein.

Slide 17 · Two routes to p53 stabilization

Cellular stress

DNA damage

ATM/ATR kinase pathway

Hypoxia

ATM/ATR-associated response

Oncogenic signaling

P14/ARF–MDM2 pathway

Different sensors converge on greater availability of p53.

Slide 18 · ATM and ATR: Stress-response kinases

ATM denotes ataxia-telangiectasia mutated.

ATR denotes ataxia-telangiectasia and Rad3-related.

They detect differing DNA lesion/stress contexts, but both can stabilize p53.

Their activation leads to phosphorylation of p53 and MDM2.

Slide 19 · DNA damage activates p53

DNA damage

Kinase sensing

ATM / ATR

Phosphorylation signals

P53 + MDM2

Binding disrupted

P53 increases

Transcriptional response

Phosphorylation disrupts p53 degradation and increases its cellular abundance.

Slide 20 · Why phosphorylation matters

Stabilization precedes effective p53-driven transcription.

Stress-induced phosphorylation interferes with the p53–MDM2 degradation axis.

The resulting increase in p53 abundance enables induction of downstream targets.

Slide 21 · ATM and inherited cancer predisposition

ATM was first identified in patients with ataxia-telangiectasia.

The textbook associates germline ATM mutations with defects in repair of some DNA lesions.

These individuals have an increased incidence of cancer.

This illustrates why upstream DNA damage sensing also protects against neoplasia.

Slide 22 · Oncogenic RAS signaling evokes p53

Aberrant RAS

Persistent progrowth signaling

Stress response

P14/ARF expression rises

MDM2 restrained

P53 is displaced

P53 accumulates

Antiproliferative outputs

The intermediate mechanisms as incompletely understood.

Slide 23 · P14/ARF links oncogenic stress to p53

The CDKN2A tumor suppressor locus encodes p14/ARF.

Oncogenic growth signals can increase p14/ARF expression.

P14/ARF associates with MDM2 and displaces p53 from inhibitory regulation.

This is a different route to p53 stabilization from ATM/ATR signaling.

Slide 24 · DNA-damage versus oncogenic-stress inputs

DNA damage / hypoxia

Input: Lesion or oxygen-related stress

Mediator: ATM and ATR kinases

Effect: P53 and MDM2 phosphorylation

Aberrant growth signals

Input: Excessive RAS or MYC signaling

Mediator: P14/ARF from CDKN2A

Effect: P53 released from MDM2 restraint

Both pathways release p53 from its basal inhibitory state.

Slide 25 · Activation converts p53 into a gene regulator

P53 stabilization

Protein accumulation

DNA binding

Sequence-specific recognition

Gene transcription

Induction of target programs

Cell fate

Arrest, repair, senescence or death

The source states that p53 controls hundreds of target genes.

Slide 26 · Three broad classes of p53 target genes

One group induces cell-cycle arrest.

Another promotes apoptosis.

A third reduces anabolic metabolism or enhances catabolic functions.

DNA-repair programs are also induced as part of the protective stress response.

Slide 27 · P53 coordinates proliferation and metabolism

Cellular stress

P53 is activated

Arrest program

DNA synthesis postponed

Metabolic reset

Biosynthesis restrained

Protection

Growth and division are deferred

The source links reduced macromolecule synthesis to cessation of proliferation.

Slide 28 · Three major cell-fate routes

Sustained p53 response

Transient arrest

Allows lesion repair

Senescence

Permanent arrest

Apoptosis

Removes damaged cells

The selected outcome depends on context; The mechanism is not a rigid universal switch.

Slide 29 · P53 initiates the G₁ checkpoint

P53 transcription

CDKN1A upregulated

P21 increases

CDK inhibitor

Cyclin D–CDK4 inhibited

RB remains active

G₁→S restrained

DNA replication delayed

A causal link between p53 and RB-mediated inhibition of cell-cycle progression.

Slide 30 · CDKN1A encodes p21

P21 is a cyclin-dependent kinase inhibitor induced by p53.

It inhibits cyclin D–CDK4 complexes in the Robbins model.

Its action keeps RB in the active, hypophosphorylated state.

Slide 31 · Active RB versus inactive RB

Active, hypophosphorylated RB

Sequesters E2F transcription factors

Restricts transcription needed for S phase

Supports the p53-induced checkpoint

Inactive, hyperphosphorylated RB

Releases E2F-driven transcription

Supports G₁-to-S transition

Can bypass checkpoint restraint

P53 and RB connect two major tumor suppressor pathways.

Slide 32 · How p21 enforces the cell-cycle pause

P53 target induction

P21 protein rises

CDK restraint

Less RB phosphorylation

E2F inhibited

S-phase program reduced

G₁ pause

Time for DNA repair

The signal restricts entry into S phase while damage is assessed.

Slide 33 · The pause preserves genomic integrity

A damaged cell should not replicate an unrepaired DNA template.

Transient arrest makes repair possible before the next S phase.

When the lesion is resolved, the checkpoint can be released.

Slide 34 · P53 supports DNA repair

Damage sensing

P53 stabilized

P53 targets

GADD45 and others

Repair response

Damage corrected

Signal wanes

Cell cycle resumes

GADD45 is explicitly named as a repair-enhancing p53 target.

Slide 35 · Successful repair terminates the p53 response

Repair reduces the stimuli responsible for p53 stabilization.

The inhibitory MDM2-regulated turnover can again limit p53 abundance.

P21-dependent arrest subsides and cell cycling may resume.

Successful repair restores growth without transmitting the original damage.

Slide 36 · Transient arrest and senescence

Transient arrest

Occurs promptly after p53 activation

Potentially reversible after repair

Preserves the possibility of normal cycling

Senescence

Persistent or permanent cell-cycle block

Associated with changed morphology and gene expression

Prevents future clonal proliferation

The decisive contrast is reversibility, not whether the cell remains alive.

Slide 37 · P53-induced senescence

Senescence represents permanent cell-cycle arrest.

It is accompanied by characteristic morphology and gene-expression changes.

The source distinguishes it from the transient damage-induced pause.

Although senescent cells persist, they cannot continue proliferating.

Slide 38 · One proposed mechanism of senescence

Chronic p53-related stress

Sustained arrest signals

Epigenetic change

Heterochromatin formation

Cell-cycle genes silenced

G₁→S drivers suppressed

Senescence

Stable cell-cycle exit

Robbins explicitly describes the precise fixation mechanism as incompletely understood.

Slide 39 · Stresses that can promote senescence

Unopposed oncogene signaling creates persistent proliferative stress.

Hypoxia can activate the senescence-associated stress response.

Shortened telomeres are another recognized signal.

The ability to remain nonproliferative is an antineoplastic safeguard.

Slide 40 · Why senescence blocks tumor formation

A cell unable to divide cannot itself generate an expanding malignant clone.

Permanent arrest contains cells whose normal growth program cannot be restored.

This is a protective fate distinct from elimination by apoptosis.

Slide 41 · P53 activation can trigger apoptosis

Irreversible injury

Damage persists

P53 target genes

BAX and PUMA rise

Mitochondrial signaling

Intrinsic pathway favored

Apoptosis

Damaged cell removed

Apoptosis is a definitive route to eliminate a cell with severe DNA injury.

Slide 42 · BAX and PUMA: Proapoptotic effectors

BAX and PUMA among p53-induced proapoptotic genes.

Their transcription shifts the balance toward the intrinsic apoptosis pathway.

The mitochondrial route is important when damage cannot safely be repaired.

This response removes a potential source of future driver mutations.

Slide 43 · Senescence versus apoptosis

Senescence

Cell persists but is nondividing

Permanent arrest is the major outcome

Prevents clonal expansion of damaged cell

Apoptosis

Cell undergoes programmed death

Intrinsic mitochondrial pathway is activated

Eliminates the damaged cell

Both are tumor suppressive, but they achieve protection differently.

Slide 44 · How p53 may select a cell fate

Degree / duration of p53 activation

Early / limited

Repair-focused response

Persistent

Senescence may emerge

Strong induction

Apoptotic program may dominate

This is a proposed conceptual model, not a quantitative threshold rule.

Slide 45 · Different p53 targets have different sensitivities

Robbins suggests p53 binds certain repair-gene regulatory sites more avidly.

Promoters/enhancers of proapoptotic genes may require greater p53 accumulation.

This provides a plausible ordering: Repair before elimination.

The explanation does not fully account for all cell-type-specific responses.

Slide 46 · When p53 activation persists

Initial arrest

Repair attempted

Damage unresolved

P53 remains elevated

Chronic response

Long-term growth restraint

Possible endpoint

Senescence or apoptosis

Persistent signaling changes the response from reversible containment to stronger protection.

Slide 47 · Cell type influences the p53 response

Identical stresses need not yield identical p53-mediated fates in every tissue.

Some cell types tend toward rapid apoptosis; Others favor senescence.

This variability as not yet fully explained.

Avoid depicting repair–senescence–death as an inflexible deterministic sequence.

Slide 48 · Intact versus defective p53 after injury

Intact p53

Damage sensed and p53 stabilized

G₁ pause and repair program induced

Unresolved damage may cause senescence or apoptosis

Defective p53

Protective transcription is impaired

Damaged cells may continue cycling

Progressive accumulation of alterations is possible

The essential distinction is the fate of genetically injured cells.

Slide 49 · Loss of p53 supports tumor evolution

Damage arises

Mutations occur

Checkpoint fails

Replication proceeds

Drivers accumulate

Clones gain altered behavior

Transformation

Malignancy may emerge

Schematic causal model; Cancer requires additional alterations.

Slide 50 · Genomic damage is not equivalent to cancer

Loss of p53 permits survival and division despite potentially harmful lesions.

Additional driver mutations in oncogenes and other cancer genes can then accumulate.

Tumor formation is a multistep process—not an automatic consequence of one TP53 hit.

Slide 51 · Mechanism–effector–outcome map

Upstream event

Key effector

Protective result

DNA damage

ATM/ATR → p53

Checkpoint activation

G₁ checkpoint

CDKN1A / p21 / RB

Replication delay

Repair attempt

GADD45-associated program

Lesion correction

Irreparable damage

BAX / PUMA

Apoptosis

Persistent stress

Sustained arrest program

Senescence

All arrows and associations summarize the supplied Robbins text; Each box is editable.

Slide 52 · Li-Fraumeni syndrome: Inherited predisposition

A germline pathogenic TP53 allele predisposes to diverse malignant tumors.

Loss of the remaining protective allele can eliminate p53 tumor suppression.

The syndrome differs from sporadic acquisition of two somatic alterations.

Slide 53 · Inherited TP53 loss and the second hit

Germline first hit

One TP53 allele defective

Somatic second hit

Other functional allele lost

P53 pathway deficient

Tumor protection impaired

Multiple cancer risks

Across tissue types

Conceptual inheritance pathway; Individuals are predisposed, not born with a tumor.

Slide 54 · Li-Fraumeni: Spectrum of malignancy

The Robbins examples include sarcomas and breast cancers.

Leukemias and brain tumors are also common examples.

Adrenocortical carcinomas form another characteristic category.

The breadth reflects the widespread tumor-suppressive role of p53.

Slide 55 · Li-Fraumeni: Age and multiplicity

Affected individuals commonly develop malignant tumors at younger ages.

Multiple primary tumors of differing histologic types may occur.

The family history can be clinically informative when hereditary predisposition is considered.

These are syndrome-level features, not sufficient alone for a molecular diagnosis.

Slide 56 · Textbook risk estimate for Li-Fraumeni

A 25-fold higher likelihood of malignancy by age 50 than the general population.

This is a source-specific comparison, not a population-specific lifetime-risk estimate.

No separate contemporary clinical risk figure is introduced in this source-only lecture.

Slide 57 · Inherited RB and TP53 syndromes

Inherited RB alteration

Strong classically characteristic retinoblastoma association

One inherited defective tumor suppressor allele

Second hit can permit tumor formation

Inherited TP53 alteration

Broad spectrum of malignant tumors

One inherited defective tumor suppressor allele

Second hit can disable p53 protection

The two-hit logic is shared; The tissue distribution differs.

Slide 58 · Clinical application: Multiple primary tumors

A young adult has a history of sarcoma and a second, histologically unrelated primary malignancy. A close relative developed cancer unusually early.

Which hereditary tumor suppressor syndrome should enter the differential?

What explains susceptibility to different tumor types?

Slide 59 · Application: Hereditary TP53 dysfunction

Consider Li-Fraumeni syndrome as a possible explanation for the vignette.

Germline TP53 impairment weakens a broadly used genomic stress checkpoint.

A confirmed diagnosis requires appropriate clinical and molecular evaluation; The vignette is not diagnostic.

Slide 60 · DNA-damaging therapy engages p53

Radiation / cytotoxic therapy

DNA damage produced

P53 response

Stress signaling activates targets

Apoptotic program

Damaged tumor cell dies

Antitumor effect

Viable tumor burden reduced

A mechanistic rationale; Individual clinical responses have multiple determinants.

Slide 61 · Wild-type versus mutant TP53 and therapy

Wild-type TP53 pathway

Damage can trigger p53-mediated apoptosis

This with greater sensitivity

Effect remains tumor- and context-dependent

Mutated TP53 pathway

DNA damage may fail to induce apoptosis

This with relative resistance

Not a universal resistance rule or stand-alone test

Therapeutic response is an association in the source, not an absolute predictor.

Slide 62 · Examples of p53-associated chemosensitivity

Testicular germ cell tumors often retain wild-type TP53 in the source discussion.

Childhood acute lymphoblastic leukemia is another example given.

Both illustrate contexts with excellent clinical outcomes and generally intact p53.

The treatment response cannot be attributed to TP53 status alone.

Slide 63 · Examples of relative treatment resistance

Lung cancers frequently harbor TP53 mutations at presentation.

Colorectal cancers are also cited as commonly mutated.

Relative resistance to DNA-damaging therapy in these groups.

Mutation status is one mechanistic contributor rather than a complete clinical forecast.

Slide 64 · Therapy can select resistant subclones

Primary tumor

Genetic heterogeneity

Cytotoxic therapy

Selective pressure

P53-deficient cells

Relative survival advantage

Relapse

Resistant clone expands

The source associates relapsed disease with enrichment for TP53-mutant subclones.

Slide 65 · Relapse is a population-level evolutionary event

A treatment-sensitive majority may be eliminated while rare resistant clones survive.

Subclones with TP53 dysfunction may gain a survival advantage under DNA-damaging therapy.

The relapsed lesion may therefore be genetically different from the original mass.

Slide 66 · Clinical application: Wild-type TP53 sarcoma

A sarcoma retains a nonmutated TP53 coding sequence, yet its p53-dependent damage response is suppressed. Molecular analysis reveals MDM2 amplification.

Why can p53 function be impaired without a TP53 sequence mutation?

Which level of the pathway is affected?

Slide 67 · Application: Functional p53 inactivation

Excess MDM2 can increase p53 ubiquitination and degradation.

The p53-dependent program may therefore be reduced despite wild-type TP53.

Gene sequence and pathway function are related but not interchangeable.

Slide 68 · Clinical application: Damage without G₁ arrest

A neoplastic clone with defective p53 continues proliferating after DNA injury. Over subsequent divisions, additional driver lesions appear.

What checkpoint mechanism has been lost?

Why does continued cell division increase oncogenic potential?

Slide 69 · Application: The checkpoint failure chain

P53 inactive

P21 induction weak

RB restraint reduced

G₁→S proceeds

Damage propagated

Mutated descendants survive

More drivers

Higher transformation potential

This teaching schematic joins p53 regulation with the RB cell-cycle gate.

Slide 70 · P53 review: Input → mediator → fate

Stimulus

Principal mediators

Outcome

Routine growth

MDM2-mediated turnover

Low basal p53

DNA damage

ATM/ATR phosphorylation

Stabilized p53

Aberrant RAS signal

p14/ARF–MDM2

Stabilized p53

Potentially reparable damage

p21 / RB; GADD45

Arrest and repair

Persistent/irreparable damage

Senescence and BAX/PUMA pathways

Permanent arrest or apoptosis

Compact pathway summary; Distinctions reflect the supplied Robbins discussion.

Slide 71 · Five conclusions about the genome guardian

TP53 is a commonly inactivated tumor suppressor.

MDM2, stress sensors, and oncogenic signaling determine p53 availability.

P21–RB arrest, repair, senescence, and apoptosis provide complementary defenses.

Inherited TP53 defects confer broad cancer predisposition.

Loss of p53 may contribute to tumor evolution and cytotoxic resistance.

Slide 72 · Reference and further reading

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026).

Neoplasia: tumor suppressor genes and the p53 stress-response pathway.

For further study: regulation of p53, the p21–RB checkpoint, DNA repair, senescence, and intrinsic apoptosis.

Mechanism illustrations in this lecture are explanatory schematics, not histologic specimens or quantitative data.