DNA Damage and Carcinogenesis

Slide 1 · DNA Damage and Carcinogenesis

Ionizing radiation • DNA repair fidelity • genomic instability

DNA injury

Repair and checkpoints

Cell fate

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify the principal DNA lesions produced by ionizing radiation.

Compare homologous recombination with nonhomologous end joining.

Explain how checkpoints prevent propagation of chromosomal damage.

Connect persistent damage to genomic instability and delayed neoplasia.

Slide 3 · The radiation-to-cancer sequence

Ionizing radiation

DNA lesions

Repair / checkpoint response

Surviving abnormal clone

Additional transforming events

A multistep pathway: Exposure alone does not imply inevitable malignancy.

DNA injury is an initiating event, not a cancer diagnosis.

Damage outcome depends on repair fidelity and cell-cycle control.

Persistence and selection of abnormal clones permit later transformation.

Slide 4 · Ionizing radiation as a genotoxic agent

Ionizing radiation

Direct interaction

With DNA

Indirect injury

Through free radicals

Ionization transfers sufficient energy to disrupt molecular structures.

DNA can be damaged by direct energy deposition or indirect chemical reactions.

Radiation is used medically, yet can induce mutagenesis and carcinogenesis.

Slide 5 · Direct DNA injury

Energy is deposited in DNA or its immediate molecular environment.

Ionizing events can alter bases or disrupt the sugar–phosphate backbone.

The biological effect is determined by the lesion and its subsequent processing.

Schematic DNA lesion

Direct energy deposition may alter DNA

Slide 6 · Indirect DNA injury through water radiolysis

Ionizing radiation

Water radiolysis

Free-radical formation

DNA injury

Radiation generates highly reactive species through radiolysis of water.

Free radicals then react with nearby macromolecules, including DNA.

This indirect route is prominent in the Robbins radiation-injury model.

Slide 7 · Oxygen amplifies indirect injury

Tissue oxygenation

Oxygenated tissue

More susceptible

Hypoxic tissue

Relatively resistant

Reactive oxygen species contribute substantially to radiation-related DNA damage.

Well-oxygenated tissue is generally more radiosensitive than hypoxic tissue.

Oxygen status therefore changes the biological consequences of exposure.

Slide 8 · Multiple lesions can follow one exposure

DNA lesion

Primary consequence

Base damage

Altered nucleotide chemistry

Single-strand break

One interrupted backbone

Double-strand break

Both strands disrupted

DNA–protein crosslink

Protein covalently associated with DNA

Single-base lesions affect individual nucleotides.

Slide 9 · Single-base damage

Damage limited to one base need not disrupt both DNA strands.

Enzymatic repair systems can correct many simpler lesions.

Uncorrected base injury may be copied as a sequence alteration.

Schematic DNA lesion

Base lesion

Slide 10 · Single-strand breaks

A single-strand break interrupts one backbone while the partner remains intact.

Many simple lesions can be repaired by mammalian enzymatic systems.

The context and completion of repair determine residual genomic injury.

Schematic DNA lesion

One backbone interrupted

Slide 11 · DNA–protein crosslinks

Ionizing radiation can produce DNA–protein crosslinking.

The covalent attachment can impede DNA transactions.

Crosslinks belong to the radiation lesion spectrum; They are not synonymous with DSBs.

Schematic DNA lesion

Protein

DNA–protein linkage

Slide 12 · Double-strand breaks are especially consequential

Both strands of the helix are severed.

An incorrect rejoining event can change chromosomal structure.

DSBs as the most serious radiation-induced DNA lesions.

Schematic DNA lesion

Both strands broken

Slide 13 · Why DSBs threaten chromosome integrity

Double-strand break

Free DNA ends

Correct or incorrect reunion

Preserved or altered chromosome

Broken ends must be reconnected in the correct configuration.

End joining between inappropriate partners can rearrange chromosomes.

The danger persists if damaged cells continue cycling.

Slide 14 · Two major mechanisms repair DSBs

DNA double-strand break

Homologous

Recombination

Nonhomologous

End joining

Homologous recombination is a template-guided pathway.

Nonhomologous end joining directly rejoins DNA ends.

The pathways differ in dependence on homology and potential fidelity.

Slide 15 · Homologous recombination: Core principle

Double-strand break

Homologous template

Sequence-directed repair

Chromosome integrity

A homologous DNA sequence provides a reference for repair.

This strategy can restore lost information with high fidelity.

Homologous recombination defects with cancer predisposition.

Slide 16 · Homology supplies the missing information

A homologous template can guide restoration of the disrupted sequence.

Template use distinguishes homologous recombination from simple end joining.

Availability of an appropriate template constrains pathway choice.

Schematic DNA lesion

Homologous template (conceptual)

Slide 17 · Homologous repair protects genome stability

Intact HR factors

Faithful DSB repair

Fewer persistent aberrations

Genome protection

Accurate restoration limits formation of rearrangements.

Defective homologous repair shifts damaged cells toward more error-prone repair.

Cancer risk rises when high-fidelity repair pathways fail.

Slide 18 · Nonhomologous end joining: Core principle

Broken DNA ends

End alignment / processing

End joining

Possible sequence change

Broken DNA ends are reunited without a homologous template.

NHEJ as the most common DSB-repair pathway.

Rejoined DNA can contain new sequence or structural errors.

Slide 19 · NHEJ repairs damage but may introduce mutations

End processing or joining can change the original sequence.

Short deletions and duplications are examples of possible outcomes.

The existence of a repaired chromosome does not prove accurate restoration.

Schematic DNA lesion

Both strands broken

Slide 20 · Short deletion following end joining

Loss of bases during break repair yields a deletion.

A short deletion can change coding or regulatory sequence if it occurs there.

Robbins explicitly identifies short deletions among NHEJ-associated changes.

Schematic chromosomal segment order

Before

After

A

B

C

A

C

The B segment is deleted at the repair junction.

Slide 21 · Short duplication following end joining

A segment may be repeated at the repaired junction.

The result is a local change in copy number or sequence arrangement.

This is one described mutational outcome of NHEJ.

Schematic chromosomal segment order

Before

After

A

B

C

A

B

B

C

An extra copy of segment B appears.

Slide 22 · Chromosomal translocation

Misjoining ends from different chromosomes can produce a translocation.

The rearrangement may alter gene regulation or generate abnormal fusion contexts.

Translocations among radiation-associated repair aberrations.

Schematic DNA lesion

Joined fragments from different chromosomes

Slide 23 · Chromosomal inversion

A DNA segment may reintegrate in the reverse orientation.

Sequence order is altered even if DNA content is not obviously lost.

Inversions are another chromosomal aberration linked to NHEJ.

Schematic chromosomal segment order

Before

After

A

B

C

A

B ←

C

B reverses orientation; A and C retain their positions.

Slide 24 · Deletion and duplication versus rearrangement

Alteration

Illustrative effect

Short deletion

Sequence removed near a repaired junction

Short duplication

Sequence repeated near a junction

Translocation

Segments joined between chromosomes

Inversion

Segment orientation reversed

Small insertions/deletions change local sequence length.

Slide 25 · Misjoining changes chromosome topology

Two DNA breaks

Multiple free ends

Incorrect end pairing

Structural aberration

Two independent breaks generate several free ends.

Pairing the wrong ends creates structural rearrangement.

Preserved cell viability is compatible with a persistently abnormal genome.

Slide 26 · Comparing the two DSB repair strategies

Feature

Homologous recombination

NHEJ

Repair principle

Template-guided

Direct end joining

Template

Homologous sequence

No homologous template required

Fidelity

Potentially high

May be error-prone

Cancer relevance

Defects predispose to cancer

Aberrant joining can alter chromosomes

Both pathways can resolve a DNA double-strand break.

Slide 27 · Repair success is not a binary concept

Post-repair cell status

Cell survives with

Accurate repair

Cell survives with

Aberrant repair

A chromosome may be reconnected while its sequence is altered.

Mutagenic repair may support cell survival but retain carcinogenic potential.

The relevant endpoints are survival, sequence accuracy, and chromosome architecture.

Slide 28 · Cell-cycle checkpoints prevent error propagation

DNA damage

Checkpoint activation

Cell-cycle pause

Repair or elimination

A cell with unrepaired DSBs should not proceed unchecked through replication.

Checkpoint controls create time for repair or direct a protective cell fate.

Failure lets chromosomal abnormalities persist in daughter cells.

Slide 29 · The DNA damage response activates p53

DNA damage

ATM / ATR response

P53 stabilization

Protective cell fate

ATM and ATR are stress-responsive protein kinases upstream of p53.

Kinase signaling stabilizes p53 following DNA damage.

Activated p53 directs arrest, repair-associated programs, senescence, or apoptosis.

Slide 30 · ATM is linked directly to DSB surveillance

Double-strand breaks

ATM signaling

P53 response

Genome protection

ATM is a kinase in the DNA damage response.

Defects in ATM impair responses to DNA double-strand breaks.

Ataxia-telangiectasia illustrates the link between DNA-repair failure and cancer.

Slide 31 · P53 stabilization changes transcriptional programs

DNA damage signals

P53 escapes MDM2 control

P53 accumulates

Target-gene expression

MDM2 normally promotes p53 degradation in unstressed cells.

DNA damage signaling reduces MDM2-mediated suppression.

Accumulated p53 acts as a transcription factor for protective responses.

Slide 32 · P21 links p53 to G1 arrest

P53

P21 / CDKN1A

CDK activity restrained

Active RB: G1 arrest

P53 induces CDKN1A, encoding the CDK inhibitor p21.

P21 inhibits cyclin D–CDK4 activity.

RB remains active and restricts passage from G1 into S phase.

Slide 33 · The G1/S pause limits replication of damaged DNA

Damage detected

G1/S transition held

Repair attempt

Normal cycle resumes if resolved

The transition into S phase commits DNA to replication.

A checkpoint pause delays copying of a damaged genome.

Repair can return the cell to normal proliferation after damage is resolved.

Slide 34 · Successful repair ends the checkpoint signal

DNA injury

Arrest + successful repair

P53 falls

Cell-cycle resumption

Resolution of damage reduces signals stabilizing p53.

Declining p53 releases the reversible cell-cycle block.

The cell may re-enter a normal proliferative state.

Slide 35 · Persistent damage triggers additional protection

Unrepaired DNA damage

Persistent arrest

(senescence)

Programmed cell death

(apoptosis)

Uncorrected DNA damage can maintain p53 activation.

Possible outcomes include permanent growth arrest or apoptosis.

Which outcome predominates varies with damage and cell context.

Slide 36 · Senescence prevents replication of an unstable genome

Persistent cellular stress

P53-dependent response

Permanent cell-cycle arrest

No clonal expansion

Senescence is a durable, essentially permanent cell-cycle arrest.

Cells may survive but lose proliferative capacity.

This prevents transmission of persistent genome damage to a clone.

Slide 37 · Apoptosis removes irreparably damaged cells

Irreparable DNA injury

P53 proapoptotic targets

Apoptosis

Damaged cell eliminated

P53 induces proapoptotic genes, including BAX and PUMA.

The intrinsic apoptotic pathway can eliminate cells with serious damage.

Cell elimination is protective against malignant transformation.

Slide 38 · Three p53-dependent responses

Response

Consequence

Genomic benefit

Reversible arrest

Pause in cell cycling

Time for DNA repair

Senescence

Permanent growth arrest

No propagation of damage

Apoptosis

Cell elimination

Damaged genome removed

Transient arrest permits repair and continued survival.

Slide 39 · Checkpoint loss converts damage into inherited instability

DNA damage

Ineffective checkpoint

Replication continues

Abnormal clone expands

Failure of p53-mediated arrest allows proliferation despite DNA injury.

Chromosomal aberrations then persist across cell generations.

Additional driver mutations can accumulate in surviving clones.

Slide 40 · Wild-type versus defective checkpoint

Checkpoint intact

Checkpoint impaired

Replication can pause

Division may continue

Repair or elimination is promoted

Mutated genomes can persist

Lower opportunity for clonal fixation

More opportunity for additional drivers

Functioning response: Stop, repair, senesce, or die.

Slide 41 · Genomic instability accelerates carcinogenesis

Faulty genome maintenance

Aberration accumulation

Cancer-gene alteration

Clonal selection

Persistent DNA repair or checkpoint defects increase acquisition of genomic abnormalities.

Aberrations can affect oncogenes or tumor suppressor genes.

Additional mutations allow selection and progression of transformed clones.

Slide 42 · Chromosome damage can affect cancer genes

Cancer-gene alterations

Proto-oncogene

Activation

Tumor suppressor

Loss of function

A rearrangement is consequential when it changes important gene function.

Proto-oncogenes may become dysregulated.

Tumor suppressor function may be lost or reduced.

Most DNA damage does not, by itself, establish malignant transformation.

Slide 43 · Radiation carcinogenesis is a multistep process

Initial radiation lesion

Persisting mutation

Clonal expansion

Additional drivers

Malignant neoplasm

A surviving damaged cell can initiate a lineage with an abnormal genome.

Additional transforming events are required for malignant progression.

The extended sequence helps explain long clinical latency.

Slide 44 · Latency reflects delayed clonal evolution

Exposure

Surviving altered cell

Long interval

Additional changes

Cancer

Chromosomal damage can remain in cells that survive irradiation.

Years may pass before additional events produce overt cancer.

Absence of immediate malignancy does not exclude later carcinogenic effects.

Slide 45 · Only a surviving lineage can propagate damage

Irradiated cell

Cell eliminated

No clonal propagation

Cell survives

Potential clonal persistence

Many severely injured cells die rather than form tumors.

Carcinogenesis requires continued viability and potential proliferation.

The fate of damaged cells matters as much as the initial lesion.

Slide 46 · Radiation injury does not equal malignancy

Post-radiation finding

Interpretive caution

Enlarged or pleomorphic nuclei

Can reflect radiation effect

Abnormal mitotic figures

Not automatically a new tumor

Giant or multinucleated cells

May persist after exposure

True malignancy

Requires appropriate diagnostic evidence

Radiation can produce cellular atypia and chromosomal abnormalities.

Slide 47 · Chromosomal injury produces nuclear alterations

DSBs and chromosome damage

Mitotic abnormalities

Altered ploidy / nuclear form

Radiation atypia

DNA breaks are associated with deletions, translocations, and fragmentation.

Spindle disorganization can accompany polyploidy and aneuploidy.

These alterations can create striking abnormal nuclear morphology.

Slide 48 · Cellular radiation atypia can mimic cancer

Atypical irradiated sample

Radiation-related

Reactive atypia

Residual or new

Malignant tumor

Radiation may induce pleomorphic nuclei and giant cells.

Abnormal mitoses can appear in previously irradiated tissue.

A known radiation field is vital context when reviewing tissue sections.

Slide 49 · Diagnostic pitfall in a treated tumor bed

Finding

Question for interpretation

Pleomorphic cells

Treatment effect or viable tumor?

Giant nuclei

Radiation-associated change?

Abnormal mitoses

Is there independent evidence of neoplasia?

Stromal fibrosis

Expected post-treatment injury?

Postirradiation atypia may resemble malignant cytology.

Slide 50 · Early cell killing and late cancer are different endpoints

Endpoint

Dominant mechanism

Timing concept

Acute tissue injury

Loss of sensitive cells

Early

Fibrosis

Vascular / parenchymal injury

Delayed

Cancer

Persisting mutations + progression

Often years later

Acute injury reflects death or dysfunction in sensitive tissues.

Slide 51 · Inherited repair defects reveal the pathway’s importance

Germline repair defect

Faulty DNA damage response

Genome instability

Cancer predisposition

Disorders involving repair genes provide natural evidence of genome protection.

The diseases vary in phenotype and affected repair proteins.

Cancer susceptibility rises when chromosomal damage is not handled correctly.

Slide 52 · Ataxia-telangiectasia: Defective ATM signaling

ATM defect

Impaired DSB response

Radiation hypersensitivity

Cancer susceptibility

Autosomal recessive ATM defects impair the DSB damage response.

Clinical features include ataxia, immunodeficiency, and radiation hypersensitivity.

Predisposition particularly includes lymphoid malignancies.

Slide 53 · BRCA1 and BRCA2 support homologous repair

BRCA pathway defect

Reduced HR capacity

Aberrant end-joining risk

Chromosomal instability

BRCA proteins are required for homologous recombination repair.

Defective pathway activity predisposes to several malignancies.

Failure of the high-fidelity route favors aberrant repair and instability.

Slide 54 · Fanconi anemia: Crosslink repair and marrow failure

Fanconi pathway defect

Crosslink repair failure

Marrow vulnerability

Leukemia risk

Multiple Fanconi genes act in a pathway repairing DNA crosslinks.

The pathway cooperates with homologous recombination machinery.

Affected patients can develop marrow aplasia and leukemia.

Slide 55 · Bloom syndrome: Homologous repair vulnerability

Helicase defect

Impaired recombination repair

Genome instability

Diverse cancer risk

Bloom syndrome is associated with loss of a repair-associated helicase.

Developmental abnormalities accompany a broad cancer predisposition.

It reinforces the importance of chromosome maintenance in tumor prevention.

Slide 56 · Repair-deficiency syndromes in comparison

Syndrome / genes

Repair link

Teaching association

Ataxia-telangiectasia / ATM

DNA damage signaling

Radiation sensitivity; lymphoid cancer

BRCA1/BRCA2

Homologous recombination

Hereditary cancer predisposition

Fanconi anemia

Crosslink / HR-associated repair

Marrow failure and leukemia

Bloom syndrome

Repair-associated helicase

Broad malignancy predisposition

Different inherited disorders compromise different parts of DNA maintenance.

Slide 57 · Radiotherapy exploits incomplete tumor recovery

Radiation delivery

DNA injury in tumor / normal cells

Interfraction recovery

Different survival outcomes

Radiation-induced DNA damage is essential to its antitumor action.

Repair capacity and cell survival determine treatment response.

Fractionation may allow normal tissue repair between exposures.

Slide 58 · Dose rate and fractionation modify cell injury

Delivery and recovery pattern

Time for repair

Between exposures

Insufficient recovery

More cumulative injury

Radiation effects are cumulative but influenced by dose delivery over time.

Intervals can permit repair of some sublethal damage.

Incomplete recovery permits accumulation of biologic injury.

Slide 59 · Proliferating tissues are especially radiosensitive

Feature

Biological consequence

Rapid cell turnover

Early injury from cell loss

Quiescence

Relative resistance to cell killing

Persistent mutant dividing cells

Potential late neoplastic outcome

DNA replication makes dividing cells vulnerable to damage.

Slide 60 · Tumor hypoxia can diminish radiation effectiveness

Poor vascular supply

Lower oxygen tension

Less ROS-mediated damage

Reduced radiosensitivity

Hypoxic regions generate less oxygen-dependent amplification of injury.

Rapidly growing tumors may contain poorly vascularized areas.

Relative radioresistance is one consequence of this microenvironment.

Slide 61 · Irradiated fields can generate second neoplasms

Therapeutic exposure

Surviving altered cell

Latent evolution

Second malignancy

A patient cured of the initial malignancy may later develop a distinct neoplasm.

Persistent DNA damage provides a plausible mechanistic basis.

Myeloid and solid second cancers after radiotherapy.

Slide 62 · Human observations support radiation carcinogenesis

Exposure context

Cancers noted in Robbins

Hiroshima and Nagasaki survivors

Leukemias and multiple solid tumors

Chernobyl accident

Thyroid cancer

Therapeutic radiation

Second myeloid or solid malignancies

Excess leukemias and solid tumors in atomic-bomb survivors.

Slide 63 · Case application: Radiation-treated tissue

Post-treatment biopsy

Treatment-associated

Atypia

Persistent / recurrent

Tumor

A post-treatment biopsy shows giant pleomorphic cells and abnormal nuclei.

The prior specimen was irradiated months earlier.

What prevents immediate classification as recurrent malignancy?

Teaching case: Synthetic scenario; Not a patient record.

Slide 64 · Case resolution: Morphology needs context

Atypical irradiated sample

Check treatment history

Assess overall evidence

Integrated diagnosis

Radiation effects can reproduce several cytologic signs of cancer.

Review field history, timing, architecture, and independent tumor evidence.

Do not equate postirradiation nuclear pleomorphism with neoplasia.

Illustrative teaching interpretation, not a diagnostic rule.

Slide 65 · Case application: Damaged dividing cell

Double-strand break

Incorrect end joining

Checkpoint failure

Proliferating abnormal clone

A dividing cell incurs a DSB and rejoins two incorrect chromosome ends.

Its p53-dependent checkpoint is defective, yet the cell survives.

Explain the two failures that jointly increase oncogenic potential.

Mechanistic teaching case; Hypothetical scenario.

Slide 66 · Case resolution: Mutagenic repair plus escape

Mutagenic NHEJ

Persistent aberration

Unrestrained proliferation

Potential progression

Misrepair creates the heritable chromosomal abnormality.

Checkpoint escape permits replication of the altered genome.

Further cancer-gene changes and selection are still needed for malignancy.

Slide 67 · Integrated model of DNA injury and cell fate

DNA damage response

Accurate repair

Genomic preservation

Irreparable injury

Arrest or apoptosis

Misrepair + checkpoint escape

Instability

Accurate repair preserves genome integrity.

Severe damage may result in senescence or cell death.

Aberrant repair plus failed checkpoint enables persistence and progression.

Slide 68 · Six distinctions that prevent common errors

DNA lesion ≠ fixed mutation.

Successful end joining ≠ restoration of original sequence.

Chromosomal aberration ≠ inevitable cancer.

Radiation atypia ≠ histologic proof of malignant persistence.

Acute radiosensitivity ≠ eventual cancer risk.

Ionizing radiation’s effects include both therapeutic killing and late mutagenesis.

Slide 69 · Mechanistic synthesis

Double-strand break

Repair fidelity

Checkpoint fidelity

Clonal outcome

Delayed cancer risk

Double-strand breaks are the central severe lesion emphasized by Robbins.

NHEJ is common but may generate small mutations and major rearrangements.

ATM–p53 checkpoints protect against propagation of damaged DNA.

Surviving aberrant clones may acquire further events and form cancers after years.

Slide 70 · Principal source and internal cross-reference

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. 2026.

Chapter 9: radiation injury and DNA damage–carcinogenesis pathway.

Chapter 7: p53 responses, genomic instability, and repair-deficiency syndromes.

Illustrations in this deck are original editable schematics; not histology specimens.