RB Governor of Proliferation

Slide 1 · RB: Governor of Proliferation

Tumor suppression, G₁/S control and retinoblastoma genetics

Molecular basis of cancer

Pathology lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives: From cell-cycle brake to cancer

Explain how phosphorylation switches RB from an E2F repressor to a permissive cell-cycle state.

Distinguish inherited predisposition from biallelic loss in the developing tumor.

Connect cyclin D, CDK4/6, p16 and viral oncoproteins to RB dysfunction.

Schematic

Describe the G₁/S RB–E2F checkpoint

Contrast two routes to RB inactivation

Apply the two-hit model to retinoblastoma

Identify upstream and viral mechanisms

Slide 3 · RB is a central negative regulator of proliferation

RB regulates the critical G₁/S transition.

The normal pathway responds to both mitogenic and growth-inhibitory signals.

Direct RB mutation is only one way to disable its antiproliferative function.

Schematic

Functional RB restrains G₁ → S

Loss of the brake permits inappropriate cell-cycle entry

Many cancers inactivate RB directly or indirectly

Slide 4 · RB1 and RB protein are related but distinct entities

The RB locus is mapped to chromosome 13q14 in the retinoblastoma model.

A functioning RB gene supplies the RB protein that represses E2F-dependent transcription.

A retained RB allele or detectable protein does not, by itself, prove normal pathway activity.

Schematic

RB1 gene: Chromosome 13q14

RB protein: Tumor-suppressive pocket protein

Binding E2F limits S-phase gene expression

Loss of function permits neoplastic growth

Slide 5 · RB belongs to the tumor suppressor class

Tumor suppressors normally oppose one or more hallmarks of cancer.

Loss of tumor suppressor function can permit excessive cell proliferation.

RB illustrates how a growth-regulatory checkpoint becomes a cancer vulnerability.

Schematic

Proto-oncogene activation

Tumor suppressor loss

Gain of growth-promoting activity

Examples: Cyclin D, CDK4

Pushes cell-cycle progression

Loss of growth-restraint activity

Example: RB loss

Releases cell-cycle restraint

Slide 6 · The mammalian cell cycle provides the setting

Quiescent cells are in G₀ and may re-enter the cycle after appropriate stimuli.

G₁ precedes DNA synthesis; S is the DNA-replication phase.

The G₁/S and G₂/M checkpoints provide distinct safeguards.

Schematic

G₀

G₁

S

G₂

M

G₁ is the major RB-regulated decision interval

S phase involves DNA synthesis

Slide 7 · The G₁ restriction point is a commitment decision

The G₁ restriction point marks commitment to continued cycle progression.

Before this decision, growth-factor availability helps determine whether the cell proceeds.

RB integrates the opposing inputs that shape the G₁/S decision.

Schematic

G₀

G₁

S

G₂

M

Restriction point: Cell can proceed without further initiating signal

Decision is regulated by growth-promoting versus inhibitory cues

Slide 8 · Why the G₁/S checkpoint matters in cancer

Defects at G₁/S release proliferative restraint.

Checkpoint defects can also favor genomic instability.

Dysregulated growth and a mutator phenotype reinforce neoplastic progression.

Schematic

G₁/S control fails

Inappropriate DNA replication proceeds

DNA repair safeguards may be impaired

Mutated clones can accumulate

Slide 9 · Mitogenic drive and cell-cycle restraint are opposed

Mitogens generally activate cyclin–CDK complexes.

Antiproliferative inputs induce CDK inhibitors.

The phosphorylation state of RB reflects the net effect of these pathways.

Schematic

Growth-factor inputs

Growth-inhibitory inputs

Cyclin–CDK activity ↑

CDK inhibitors ↑

RB integrates opposing signals

Slide 10 · Cyclins activate cyclin-dependent kinases

Cyclin levels vary over the cell cycle through regulated synthesis and degradation.

Cyclin binding activates the associated CDK.

The resulting kinase activity drives stage-specific transitions by phosphorylating target proteins.

Schematic

Cyclin expression

Cyclin binds its CDK partner

Active cyclin–CDK complex

Target protein phosphorylation

Slide 11 · RB restrains E2F by physical sequestration

RB is a pocket protein that binds E2F transcription factors.

When bound by functional RB, E2F cannot freely induce its target genes.

This interaction is an immediate molecular basis for RB-mediated growth inhibition.

Schematic

Active, hypophosphorylated RB

RB binds E2F

E2F-driven S-phase transcription restrained

RB acts upstream of the G₁/S gene-expression program

Slide 12 · Hypophosphorylated RB is the active brake

RB is active in its hypophosphorylated state.

Quiescent cells are characterized by this growth-restraining RB state.

The central effect is inhibition of E2F-regulated transcription.

Schematic

Hypophosphorylated RB

RB binds E2F

E2F bound → G₁/S progression restrained

Quiescent cells have active RB

Slide 13 · Hyperphosphorylation functionally inhibits RB

Cell-cycle entry is accompanied by RB hyperphosphorylation.

Hyperphosphorylated RB no longer effectively sequesters E2F.

E2F transcription factors can then stimulate genes required for S phase.

Schematic

Cyclin–CDK-mediated phosphorylation

RB becomes hyperphosphorylated

E2F release → S-phase gene expression

Slide 14 · A state change can mimic genetic RB deletion

RB1 loss-of-function is one route to failed growth control.

Persistent upstream kinase activation can inhibit intact RB protein.

Function must not be inferred from DNA sequence alone.

Schematic

Genetic pathway loss

Functional pathway loss

Both RB copies inactivated

RB tumor suppressor function lost

E2F restriction abolished

RB gene may remain normal

RB phosphorylation increased

E2F restriction weakened

Slide 15 · Cyclin D–CDK4 is an RB-directed kinase complex

Cyclin D–CDK4 phosphorylates RB.

Excessive activity can tip RB toward its inhibited state.

Gain-of-function alterations affecting this complex can drive oncogenic proliferation.

Schematic

Growth-factor signal

Cyclin D–CDK4 activity rises

RB phosphorylation increases

G₁/S progression favored

Slide 16 · Cyclin D–CDK6 acts alongside CDK4

Cyclin D can form active complexes with CDK6 as well as CDK4.

Both cyclin D–CDK4 and cyclin D–CDK6 participate in controlling G₁ progression.

CDK inhibition can preserve a growth-inhibitory RB state.

Schematic

Cyclin D binds CDK6

Cyclin D–CDK6 complex

RB phosphorylation

Release of E2F restraint

Slide 17 · Cyclin E–CDK2 supports passage into S phase

Cyclin E–CDK2 is another complex contributing to G₁/S progression.

High activities of the G₁-associated complexes promote RB hyperphosphorylation.

The effect converges on E2F-dependent S-phase gene expression.

Schematic

G₁ cyclin–CDK activation

Cyclin E–CDK2 activity

Further RB phosphorylation

Transition toward DNA synthesis

Slide 18 · Multiple kinase complexes converge on RB

CDK4–cyclin D, CDK6–cyclin D and CDK2–cyclin E all contribute.

The biochemical endpoint is increased RB phosphorylation.

Inhibited RB permits the transcriptional program needed for G₁/S transit.

Schematic

Cyclin D–CDK4/6

Cyclin E–CDK2

RB phosphorylation

RB phosphorylation

Inhibition of RB → release of E2F

Slide 19 · E2F links RB activity to S-phase genes

E2F is the key transcription-factor output of the RB pathway.

Available E2F stimulates genes required for cell-cycle progression.

This explains how an RB phosphorylation event becomes a proliferative phenotype.

Schematic

RB–E2F binding decreases

E2F transcription factors available

S-phase target genes expressed

DNA replication program activated

Slide 20 · Growth-factor signaling pushes the RB gate open

Mitogenic signals normally stimulate cyclin–CDK activity.

The activated complexes promote RB hyperphosphorylation.

This is a regulated physiological route through G₁/S in a responding cell.

Schematic

Growth factor engages receptor

Pro-growth signaling increases

Cyclin–CDK activity rises

RB inactivated → E2F release

Slide 21 · Growth-inhibitory signals preserve RB activity

Growth-inhibitory inputs can induce CDK inhibitors.

Reduced kinase activity helps keep RB in its active form.

Active RB maintains repression of E2F-dependent proliferation.

Schematic

Growth-inhibitory signaling

CDK inhibitor expression ↑

Cyclin–CDK activity restrained

Hypophosphorylated RB persists

Slide 22 · TGF-β signals can oppose G₁/S passage

TGF-β as a growth-inhibitory input in the RB checkpoint diagram.

CDK inhibitors are part of the response to these inhibitory cues.

The pathway protects the active RB state rather than directly replacing RB.

Schematic

TGF-β growth-inhibitory input

CDK inhibitor response

Less phosphorylation of RB

Cell-cycle entry restrained

Slide 23 · P53 can restrain the RB gate through p21

P53 can induce the CDK inhibitor p21.

P21 inhibits cyclin–CDK complexes and favors RB-dependent arrest.

This is a point of functional interaction, not identity, between RB and p53.

Schematic

P53-mediated stress response

P21 (CDKN1A) increases

Cyclin–CDK activity decreases

RB-mediated growth restraint maintained

Slide 24 · INK4 inhibitors selectively target CDK4/6

The INK4 family comprises p15, p16, p18 and p19.

These proteins selectively restrain CDK4 and CDK6 complexes.

The downstream consequence is preservation of functional RB.

Schematic

INK4 proteins: P15, p16, p18, p19

Cyclin D–CDK4/6 inhibited

RB phosphorylation is limited

E2F repression maintained

Slide 25 · The CIP/KIP family inhibits broader CDK activity

P21, p27 and p57 belong to the CIP/KIP CDK-inhibitor family.

They have broader activity than INK4 inhibitors.

Loss of CDK inhibitor function can compromise checkpoints despite an intact RB gene.

Schematic

P21, p27, p57

Broad cyclin–CDK inhibition

Less permissive phosphorylation

Cell-cycle progression restrained

Slide 26 · P16 and p21 restrain the same gate differently

Both proteins suppress kinase activity upstream of RB.

P16 is an INK4 family inhibitor; P21 belongs to CIP/KIP.

They arise from distinct regulatory programs and must not be conflated.

Schematic

P16/INK4a

P21/CIP1

Encoded by CDKN2A

Targets cyclin D–CDK4/6

Promotes active RB

Encoded by CDKN1A

Induced by p53

Broad CDK inhibitor

Slide 27 · Growth and inhibitory inputs meet at RB

Extracellular growth cues promote kinase activity.

Growth-inhibitory and stress pathways oppose those kinases.

RB converts these competing signals into control of the E2F program.

Schematic

Growth factors

TGF-β / p53

Cyclin–CDK activity ↑

CDK inhibitors ↑

RB state determines E2F access

Slide 28 · Normal cell-cycle entry is conditional, not autonomous

Normal progression requires a balance favoring cell-cycle advance.

Checkpoint signaling can prevent inappropriate entry into S phase.

Oncogenic lesions undermine this conditionality.

Schematic

Adequate growth signal

Strong inhibitory signal

Cyclin–CDK drive ↑

RB becomes inactive

E2F genes turn on

CDK inhibitors ↑

RB remains active

E2F remains constrained

Slide 29 · Two major classes of RB pathway failure

Both RB alleles can acquire loss-of-function alterations.

Alternatively, upstream regulatory lesions can drive excessive RB phosphorylation.

The shared downstream consequence is loss of E2F restraint.

Schematic

Direct RB loss

Indirect RB inactivation

Biallelic loss of RB

No effective RB brake

Retinoblastoma prototype

Cyclin/CDK gain

CDK inhibitor loss

Hyperphosphorylated RB

Slide 30 · Biallelic RB loss is a cellular genetic event

Two loss-of-function hits must affect the RB alleles in the retinoblastoma model.

One altered allele alone does not normally abolish cellular growth control.

Loss of the remaining functional allele removes the suppressive brake.

Schematic

Normal cell

RB+ | RB+

One hit

RB− | RB+

Two hits

RB− | RB−

Complete loss of RB suppressor function in a cell

Slide 31 · Knudson’s model explains retinoblastoma initiation

The two-hit hypothesis arose from inherited and sporadic retinoblastoma patterns.

Both RB copies must lose function within a tumor-founding retinal cell.

The model distinguishes inherited cancer predisposition from tumor-cell genotype.

Schematic

Two hits in the same retinal lineage

At least one RB allele functions initially

Separate loss-of-function events disable both alleles

Retinal cell escapes RB-mediated suppression

Slide 32 · Familial retinoblastoma starts with a germline hit

A germline pathogenic RB allele is inherited from a carrier parent.

Every somatic cell begins with one defective and one functional copy.

A later somatic alteration of the remaining allele can initiate retinal tumorigenesis.

Schematic

Inherited predisposition

Conception: One RB allele is already defective

All somatic cells carry that first RB alteration

A retinal second hit creates biallelic loss

Slide 33 · Sporadic retinoblastoma requires two somatic hits

Neither RB hit is inherited in the classical sporadic model.

Two independent somatic events must accumulate in the same retinal lineage.

This lower-probability sequence helps explain sporadic disease frequency.

Schematic

Sporadic pathway

Conception: Both RB alleles functional

First somatic hit in one retinal cell

Second somatic hit in its lineage → RB loss

Slide 34 · The pathways converge on an identical cell state

The origin of the first mutation differs between the two routes.

Both result in tumor-founding cells with biallelic RB inactivation.

The affected cell, not the family history alone, determines the tumor phenotype.

Schematic

Familial predisposition

Sporadic initiation

First hit: Germline

Second hit: Retinal cell

Both RB copies lost

First hit: Retinal cell

Second hit: Retinal cell

Both RB copies lost

Slide 35 · Dominant inheritance coexists with recessive cellular loss

An inherited RB defect produces an autosomal dominant predisposition to cancer.

At the level of an individual cell, a second loss-of-function hit is needed.

This resolves the apparent contradiction between family transmission and the two-hit mechanism.

Schematic

Patient/family level

Individual cell level

Predisposition is inherited

Autosomal dominant pattern

One inherited abnormal allele

Growth suppression often retained

Both RB copies must fail

Cellular loss is recessive

Slide 36 · Why a heterozygous carrier can have normal cells

Most carrier cells retain one functional RB allele.

This remaining copy usually maintains adequate cellular RB activity.

Tumor formation follows additional loss in a susceptible retinal cell.

Schematic

Carrier cell

RB− | RB+

Remaining RB

RB− | RB+

Retinal second hit

RB− | RB−

Normal growth persists until the remaining allele is lost

Slide 37 · Inherited RB defects increase opportunities for a second hit

Every retinal cell in a germline carrier begins with a first RB hit.

A second hit can occur independently in separate retinal cells.

This helps explain the tendency toward bilateral or multiple retinal lesions.

Schematic

First mutation present from conception

Large number of retinal cells at risk

Second hits arise in individual cells

One or multiple retinal tumors may develop

Slide 38 · Retinoblastoma patterns differ between clinical groups

Approximately 40% of retinoblastomas as familial and 60% as sporadic.

Bilateral involvement strongly fits a germline-predisposition mechanism.

Laterality is a pattern, not a definitive molecular test.

Schematic

Familial/heritable

Sporadic

≈40% text

Often affects both eyes

Germline predisposition

≈60% text

Virtually always one eye

Two somatic RB hits

Slide 39 · A germline RB defect expands cancer susceptibility

Carriers are at greatly increased risk of retinoblastoma.

Robbins also associates germline RB alterations with osteosarcoma.

Other soft-tissue sarcomas are part of the described hereditary risk spectrum.

Schematic

Germline RB alteration

Increased retinoblastoma risk

Elevated osteosarcoma risk

Other soft-tissue sarcoma susceptibility

Slide 40 · The two-hit model predicts somatic counterparts

Study of familial retinoblastoma established a general tumor-suppressor paradigm.

The same tumor suppressor can also be altered in sporadic malignancies.

Inherited susceptibility and somatic oncogenesis share a molecular endpoint.

Schematic

Heritable RB-predisposition syndrome

RB loss in tumor cells

Somatic RB lesions also found in cancers

A common tumor-suppressor principle

Slide 41 · Somatic RB mutations occur beyond the retina

Somatic RB alterations are reported in several tumor types.

The textbook specifically cites subsets of glioblastoma and lung, breast and bladder carcinomas.

Direct RB mutation frequencies vary; A normal RB gene does not exclude pathway dysregulation.

Schematic

Retinoblastoma and osteosarcoma

Subsets of glioblastoma

Subsets of lung and breast carcinoma

Subsets of bladder carcinoma

Slide 42 · Why retina-predominant susceptibility is unresolved

The retinal predilection is not simply explained by RB expression.

This tissue specificity remains incompletely understood.

Proposed compensation by related proteins is a plausible contributor, not a complete proof.

Schematic

RB is expressed in many cell types

Germline RB defects preferentially lead to retinoblastoma

The complete tissue-selectivity mechanism is not established

Slide 43 · RB family proteins may provide partial compensation

Related RB-family proteins may substitute for RB in certain tissues.

This is one hypothesis explaining the tumor spectrum in germline carriers.

The source does not establish a complete mechanistic explanation.

Schematic

RB is part of a protein family

Related members may replace some functions

Functional redundancy varies by cell type

Retinoblasts may be especially vulnerable

Slide 44 · The four key regulators of the RB axis

P16/INK4a, cyclin D, CDK4 and RB.

Lesions in these functionally connected regulators converge on cell-cycle escape.

The broader network includes CDK6, cyclin E–CDK2 and upstream growth signals.

Schematic

P16/INK4a

Cyclin D

CDK4

RB

Dysregulation of at least one node is common in cancers

Slide 45 · Cyclin D gain enhances RB phosphorylation

D-cyclin genes can undergo gain-of-function changes in cancer.

Increased cyclin D activity promotes the inhibitory RB phosphorylation state.

This is oncogenic drive operating through a tumor-suppressor checkpoint.

Schematic

Cyclin D overactivity

Greater CDK4/6 complex activity

More RB phosphorylation

E2F-mediated proliferation

Slide 46 · CDK4 gain can disable functional RB

CDK4 gene amplification or activating alteration enhances kinase signaling.

An intact RB gene may therefore fail to restrain proliferation.

The causal route is aberrant phosphorylation, not necessarily RB1 mutation.

Schematic

CDK4 activation / amplification

Excess cyclin D–CDK4 activity

Hypophosphorylated RB depleted

E2F-driven G₁/S passage

Slide 47 · P16/INK4a loss removes an upstream inhibitor

P16 normally inhibits cyclin D-associated CDK4/6 activity.

Loss of p16 permits increased phosphorylation and inactivation of RB.

This is a tumor-suppressor defect upstream of the RB protein.

Schematic

CDKN2A / p16 function lost

CDK4/6 inhibition weakened

RB hyperphosphorylation favored

G₁/S entry facilitated

Slide 48 · Upstream oncogenic signals also converge on RB

RB function can be compromised without mutation of any of the four core regulators.

The source cites receptor tyrosine kinases and RAS as upstream examples.

Pathway analysis must therefore extend beyond sequencing RB1 alone.

Schematic

Activated RTK or RAS signaling

Cyclin/CDK expression or activity ↑

RB phosphorylation ↑

E2F-dependent cycle advance

Slide 49 · Distinct mutations can produce an RB-loss phenotype

Mutations in multiple genes can converge on the same endpoint.

Functionally similar tumors need not share the same mutated gene.

This is why the RB pathway is more broadly dysregulated than the RB1 locus alone.

Schematic

Direct loss

Indirect mimic

RB1 biallelic mutation

Functional RB absent

E2F liberated

Cyclin D / CDK4 gain

P16 loss

E2F liberated

Slide 50 · Pathway-level reasoning changes tumor interpretation

Both increased pro-growth drive and diminished inhibition favor RB phosphorylation.

The phosphorylated protein can be present but functionally inactive.

A functional pathway readout may differ from a simple mutation inventory.

Schematic

Growth promotion ↑

Growth inhibition ↓

CDK activity ↑

CDKI restraint ↓

Inactive RB state despite normal RB1

Slide 51 · Oncogenic DNA viruses can bind RB directly

Several DNA viruses encode RB-binding oncoproteins.

Binding disrupts a protein–protein checkpoint rather than necessarily altering RB DNA.

Released E2F favors cell-cycle progression.

Schematic

Viral oncoprotein expressed

RB pocket domain bound

RB–E2F sequestration lost

E2F-driven proliferation enabled

Slide 52 · High-risk HPV E7 targets the RB pocket

E7 proteins of high-risk HPV types such as HPV16.

E7 binds the RB pocket used to associate with E2F.

This is a direct nonmutational route to RB inactivation.

Schematic

High-risk HPV E7

Binding to hypophosphorylated RB

E2F no longer sequestered

G₁/S progression promoted

Slide 53 · Polyomavirus large T antigen also inhibits RB

Polyomavirus large T antigen is another viral RB-binding protein.

Its biochemical mechanism illustrates the conserved importance of the RB pocket.

The slide depicts a molecular mechanism, not clinical prevalence or causation for a specific tumor.

Schematic

Polyomavirus large T antigen

Interaction with RB pocket

Loss of RB-mediated E2F restraint

Proliferative transcription allowed

Slide 54 · Viral RB binding and CDK action share an endpoint

Cyclin–CDK signaling and viral oncoproteins disable RB differently.

Both mechanisms weaken RB sequestration of E2F.

This is biochemical convergence, not equivalence of genetic events.

Schematic

Kinase-mediated

Viral-protein-mediated

RB hyperphosphorylation

E2F released

Cell-cycle entry favored

RB pocket bound by E7 / T

E2F released

Cell-cycle entry favored

Slide 55 · RB inactivation is not always an RB1 mutation

RB status must be interpreted on both genetic and protein-functional levels.

Upstream activation and viral binding can alter RB output while RB1 remains intact.

A mutation-negative RB1 result does not demonstrate a normal RB signaling axis.

Schematic

Pathway mechanism

Does RB1 have to mutate?

Biallelic RB loss

Cyclin/CDK activation

CDK inhibitor loss

Yes, for direct RB loss

No

No

Slide 56 · CDK4/6 inhibitors illustrate translational relevance

Clinical effectiveness of CDK4/6 inhibitors in certain forms of breast cancer.

Their pathway target lies upstream of RB phosphorylation.

Do not derive unprovided eligibility criteria or predictive performance from this excerpt.

Schematic

Cyclin D–CDK4/6 phosphorylates RB

CDK4/6 inhibitors are used in certain breast cancers

The supplied source provides no patient-selection algorithm

Slide 57 · Mechanistic diagrams do not establish clinical biomarkers

A mechanistic dependency is not automatically a validated predictive biomarker.

The excerpt does not define a testing assay, threshold or treatment-selection rule.

Clinical decision criteria need independent current verification beyond this source.

Schematic

RB is a key G₁/S regulator

CDK4/6 is a plausible therapeutic target in some contexts

Predictive testing thresholds are not given in this source

Slide 58 · Case: A germline RB alteration precedes retinal tumor

A child carries one pathogenic RB allele in all somatic cells.

Which additional event is necessary for the classic retinoblastoma mechanism?

A second somatic RB loss must remove the remaining functional allele.

Schematic

Explain the second-hit requirement

Child inherits RB− / RB+

A susceptible retinal cell loses RB+

RB− / RB− cell can proliferate abnormally

Slide 59 · Case: Unilateral tumor without inherited predisposition

A child has sporadic retinoblastoma in one eye.

The classical molecular model requires two acquired RB inactivation events.

A single tumor does not itself prove the absence of germline susceptibility.

Schematic

Apply the sporadic pathway

Retinal cell begins RB+ / RB+

Two somatic hits occur in one lineage

Biallelic RB loss permits tumor formation

Slide 60 · Case: RB1 intact but cyclin D–CDK4 overactive

A cancer has no identified RB1 mutation but increased CDK4 activity.

Its RB protein can still be functionally disabled by phosphorylation.

This is indirect checkpoint loss rather than biallelic RB1 loss.

Schematic

Cyclin D–CDK4 overactivity

RB becomes hyperphosphorylated

E2F transcription increases

Slide 61 · Case: P16 loss and a preserved RB1 locus

Loss of p16 can compromise the checkpoint while RB1 DNA remains normal.

The immediate disinhibited enzymes are CDK4 and CDK6.

The final molecular effect is a shift toward inactive RB.

Schematic

P16 inhibition is lost

Cyclin D–CDK4/6 restraint falls

RB phosphorylation increases

E2F escape follows

Slide 62 · Case: HPV16 E7 with no RB1 sequence alteration

The viral protein occupies the RB pocket used to bind E2F.

Consequent RB inhibition does not require an RB1 gene mutation.

This shows why pathway-level and gene-level descriptions must be distinguished.

Schematic

HPV16 E7 protein

Direct binding to RB pocket

E2F displacement

Promotion of cycle progression

Slide 63 · Case: DNA stress engages the p53–p21–RB route

P53 can impose cell-cycle arrest through induction of p21.

CDK inhibition protects hypophosphorylated, active RB.

Disturbance at several levels can undermine this restraint.

Schematic

Stress response activates p53

P21 CDK inhibitor increases

RB phosphorylation is restrained

G₁/S arrest becomes possible

Slide 64 · Diagnose the checkpoint lesion, not just the gene name

The same G₁/S escape phenotype can arise through genetically diverse alterations.

Mechanistic classification separates upstream kinase changes from RB protein loss.

Viral RB binding is an additional nonmutational mechanism.

Schematic

RB1 loss

Cyclin D gain

CDK4 activation

P16 loss

Different lesions converge on functional E2F release

Slide 65 · Common interpretive errors in the RB pathway

An intact RB1 coding sequence is not proof of a competent RB checkpoint.

Autosomal dominant transmission does not contradict recessive tumor-cell loss.

A molecular mechanism should not be promoted to an unverified clinical test criterion.

Schematic

RB1 genotype and RB activity are different readouts

Phosphorylation, kinase inputs and viral binding all matter

Do not equate unilateral disease with guaranteed sporadic genotype

Slide 66 · Synthesis: Normal RB limits entry into S phase

RB exerts its brake through E2F binding.

Cyclin–CDK hyperphosphorylation weakens this inhibition.

Normal mitogenic and inhibitory signals tune this molecular switch.

Schematic

Active hypophosphorylated RB

E2F remains bound

S-phase genes repressed

G₁/S progression restrained

Slide 67 · Synthesis: Four routes converge on failed restraint

Direct loss removes the RB protein’s function.

Excess CDK activity or deficient CDK inhibition promotes RB inactivation.

DNA viral oncoproteins directly disrupt RB–E2F sequestration.

Schematic

Biallelic RB loss

Cyclin D gain

CDK4 activation

CDK inhibitor loss

Plus viral E7 / large T binding of RB

Slide 68 · Synthesis: Inherited predisposition versus tumor initiation

An inherited first hit increases opportunities for somatic second hits.

Sporadic retinoblastoma acquires both hits within the same retinal lineage.

In both settings, biallelic loss is the classical cell-level initiating event.

Schematic

Inherited

RB− | RB+

Second hit

RB− | RB−

Tumor

RB− | RB−

Dominant inherited risk; Recessive loss in the tumor cell

Slide 69 · Source material and interpretation boundaries

Primary source

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition (2026), Chapter 7, pp. 256–262; Chapter 1, pp. 24–25.

Source excerpts used

RB: Governor of Proliferation; Insensitivity to Growth Inhibition; Cyclins and Cyclin-Dependent Kinases; cell-cycle checkpoint overview.

The diagrams are original, simplified, editable schematics—not reproduced textbook figures or clinical specimens.