Cancer

• A disease of the genome– Genomic instability

• A disease of the cell cycle– Checkpoint failures

• A disease of the aged

• Benign vs. Malign• What makes a cancer cell so special?

– Interaction with basal lamina– Interactions with other cells– Blood Supply

DysplasticACF

EarlyAdenoma

IntermediateAdenoma

LateAdenoma Carcinoma MetastasisNormal

Epithelium

APCother changes?

DCCDPC4/JV18?

K-Ras

MMRDeficiency

Cancer Progression Example: Colorectal Cancer

• APC mutations initiate the neoplastic process.• Patients with FAP inherit APC mutations and develop numerous dysplastic aberrant

crypt foci ACF, which then progress• MMR deficiency speeds up this process• K-RAS is an oncogene that requires only one genetic event for its activation.• The other specific genes indicated are tumor suppressor genes that require two

genetic events (one in each allele) for their inactivation• Chromosome 18q21 may contain several different tumor suppressor genes

involved in colorectal neoplasia, with DCC, DPC4, and JV18–1 genes proposed as candidates

Cancer

• The pathway to carcinogenesis consists of 4-7 rate-limiting events.

• Tumor development proceeds by a process analogous to Darwinian evolution.

• The multiple lines of cellular defense may explain why cancer is not more frequent during an average human lifetime.

• If you live long enough, you will develop cancer.

Cell, Vol. 100, 57–70, January, 2000

Cancer• The pathway to carcinogenesis consists of 4-7 rate-

limiting events.– Recently, as little as 2: activation of a pro-growth (c-myc)

factor and a survival factor (Bcl-XL) or suppression of a proapoptotic p53 or p19ARF

• Tumor development proceeds by a process analogous to Darwinian evolution.

• The multiple lines of cellular defense and the maintainence of DNA fidelity may explain why cancer is not more frequent during an average human lifetime.

• If you live long enough, you will develop cancer.

Cell, Vol. 100, 57–70, January, 2000

Cell, Vol. 100, 57–70, January, 2000

General Charactistics of Cancer

• All cancers must acquire several of the same six hallmark capabilities

• Means and order of acquisition vary significantly.

The catalyst of acquiring these is Genomic instability.

Cancer: A disease of the Genome

Genomic Stability• Given the standard mutation rate in dividing cells,

coincident with the fidelity of DNA replication, the time it would take to achieve a sufficiently mutated state for cancer would far surpass the human lifespan.– A normal error frequency of 1 base-pair change in roughly 109

base pairs for each cell generation (1 in a billion).– A single gene that encodes an average-sized protein (~103 base

pairs) suffers a mutation once in about 106 cell generations. – This number is roughly consistent with the evolutionary estimate -

one mutation appears in an average gene in the germ line every 200,000 years.

Genomic Instability

DNA damage/mutation Apoptosis

Mitosis

ArrestRepair

AttemptExtensiv

e

damage

CorrectionNormal Cell

Damage toApoptoticPathway

MitosisDamage

Cancer Cell

Inability toSense Damage

DamagedRepair

Mechanisms

Inability toarrest

Rapid accumulationof genomic damage.

leads to accumulation of more key cancer cellcharacteristics

Perpetuation oferrors to daughter

cells.

DNA damage/mutation

Environmental Causes of Genomic Instability and Aneuploidy

• Oncogenic Viruses• Chemical carcinogens• Ionizing radiation

Loss of checkpoints +

circumvention of apoptosis =

genomic instability

Cell, Vol. 100, 57–70, January, 2000

General Charactistics of Cancer

• All cancers must acquire the same six hallmark capabilities

• Means and order of acquisition vary significantly.

• The catalyst of acquiring these is Genomic instability.

Characteristics of Tumor Cells

Genome Instability

• Given the normal mutation rate in dividing cells, the time it would take to achieve a sufficiently mutated state for cancer would far surpass the human lifespan.

• A normal error frequency of 1 base-pair change in roughly 109 base pairs for each cell generation (1 in 1,000,000,000, or one in a billion).

• A single gene that encodes an average-sized protein (~103 base pairs) suffers a mutation once in about 106 cell generations.

• This number is roughly consistent with the evolutionary estimate - one mutation appears in an average gene in the germ line every 200,000 years.

Cancer: A Disease of the Genome

Genomic Stability• Given the standard mutation rate in dividing cells, coincident

with the fidelity of DNA replication, the time it would take to achieve a sufficiently mutated state for cancer would far surpass the human lifespan

– A normal error frequency of 1 base-pair change in roughly 109 base pairs for each cell generation (1 in a billion)

– A single gene that encodes an average-sized protein (~103 base pairs) suffers a mutation once in about 106 cell generations

– This number is roughly consistent with the evolutionary estimate – one mutation appears in an average gene in the germ line every 200,000 years.

How is Fidelity Normally Maintained?

• Protein level: Wobble Effect.• Genetic level: Distinct DNA repair mechanisms.– DNA polymerase exonuclease activity– post-replicative recognition, excision, and repair of

mismatches

• Other strategy Affecting stability:• 5% exons• Extensive introns

Genomic Instability

DNA damage/mutation Apoptosis

Mitosis

Arrest RepairAttempt

Extensive

damage

CorrectionNormal Cell

Damage toApoptoticPathway

MitosisDamage

Cancer Cell

Inability toSense Damage

DamagedRepairMechanisms

Inability toarrest

Rapid accumulationof genomic damage.

leads to accumulation of more key cancer cellcharacteristics

Perpetuation oferrors to daughtercells.

DNA Damage

Arrest

Normal Cell

Cancer Cell

RepairDamage

Apoptosis

Mitosis

DamagedRepair

Mechanism

InabilityTo SenseDamage

Inability to Arrest

DNADamage

Damage to Apoptotic Pathway

Mitosis Rapid accumulationof genetic damage

Perpetuation ofErrors to

Daughter Cells

Genomic Instability

DNA Damage

Aneuploidy• Euploidy: Having a chromosome number that is an exact multiple

of the monoploid number.• Aneuploidy: Having a chromosome number that is not an exact

multiple of the usually haploid number.• Develops from defects in the process of chromosome segregation.• Most benign tumors are diploid.• All malignant tumors are aneuploid; this is one of the identifying

characteristics of malignant tumor cells.• Is required for cell immortalization; it is a critical rate-limiting step

of tumorigenesis – therefore, it develops early in the progression.

• Caused by the same factors which cause genomic instability.

Causes of Genomic Instability and Aneuploidy

• Oncogenic Viruses• Chemical carcinogens• Ionizing radiation• Spontaneous occurrence in p53-/- cell lines

derived from Li-Fraumeni patients (germline mutation in the p53 gene)

Once genomic instability is in effect, cancer cells then follow a quicker path towards the accumulation of necessary

mutations required to reach full-blown carcinogenesis.

Cell 2000 Jan 7;100(1):57-70

Acquired Characteristics of Tumor Cells

1. Self-Sufficiency in Growth Signals2. Insensitivity to Antigrowth Signals3. Evading Apoptosis4. Limitless Replicative Potential

(Immortalization)5. Sustained Angiogenesis6. Tissue Invasion and Metastasis

Characteristics of Tumor Cells

I: Self-Sufficiency in Growth Signals• many oncogenes mimick normal growth signaling.

• 3 strategies: – 1: Alteration of extracellular growth signals (Autocrine stimulation)

• PDGF (platelet-derived growth factor) • TGFa (tumor growth factor alpha)

– 2: Signal Transducer alteration• A: wild-type GF receptors are overexpressed: hypersensitivity

– EGF-R/erbB• B: Structural alteration of receptors can result in ligand-independent signaling

– EGF receptor lacking most of the cytoplasmic domain fire constitutively – 3: Intracellular circuits

• RAS - 25% of human tumors, constitutive mutants

Cell, Vol. 100, 57–70, January, 2000

II: Insensitivity to Antigrowth Signals

• Normal tissue: – Antiproliferative signals operate to maintain

cellular quiescence and tissue homeostasis.• Soluble growth inhibitors • Immobilized inhibitors embedded

– the extracellular matrix – the surfaces of nearby cells

– These growth-inhibitory signals, like their positively acting counterparts, are received by transmembrane cell surface receptors coupled to intracellular signaling circuits.

E Cadhedrin

CAMs

antigrowth signals

b-catenin

Lef/Tcf transcription factor

N-CAM

Wilms tumor, neuroblastoma, and small cell lung cancer

Epithelial Cancers

antigrowth signals

Growth Inhibitory Signal via binding

Extracellular Matrix

Integrins

Apoptotic signals

Apoptotic Signals via detachment

III: Evading Apoptosis: future lecture• Bcl-2: Oncogene that doesn’t promote growth, but inhibits death. usually

found mutated in conjunction with c-myc, which is a powerful activator of the cell cycle.

• p53: 50% of human tumors have this gene mutated.• Decoy death receptors• etc.

DR-4/5

TRAIL

DcR

TRAIL

Decoy Receptor

Can still bind ligand, but unable to transduce signal

DD

IV: Limitless Replicative Potential: Immortalization

• Even cells which are free to divide at their whim eventually stop growing.

• This is due to telomere shortening.• How does a cell arrive at immortality?

growth in culture

Senescence

Loss of Rb or p53 pathways(mutagens, primary tumor cells)

ImmortalizationCrisis

Massive cell death, karyotypic disarray associated with end-to-end fusion of chromosomes (telomeres no longer protect ends)

1 in 107 cells emerges,and is now able to perpetually grow.

Pathway to Cellular Immoralization

ALT mechanism: maintains telomeres through recombination-based interchromosomal exchanges of sequence information Telomerase is activated

or

Telomerase expression

artificial Ras expression

Inhibition of p16 OR p53

telomere shortening50-100 bp/cycle

growth in

cultureSenescence

Loss of Rb or p53 pathways(mutagens, primary tumor cells)

ImmortalizationCrisis

Massive cell death, karyotypic disarray

associated with end-to-end fusion of chromosomes

(telomeres no longer protect ends)

1 in 107 cells emerges,

and is now able to perpetually grow.

Pathway to Cellular Immoralization

ALT mechanism: maintains telomeres through recombination-based interchromosomal exchanges of sequence information Telomerase is activated

or

Telomerase expression

artificial Ras expression

Inhibition of p16 OR p53

telomere shortening

50-100 bp/cycle

HDACs & HATS: Chromatin Remodelling Overview

B

Pathways to Cellular Immortalization

GrowthIn

cultureSenescence

Many divisionsCRISIS Immortalization

Ectopic Ras expressionInhibition of p16 or p53

Massive cell death; karyotypic disarray associated with end-to-end fusion of chromosomes (telomeres no longer protect ends)

1 in 107 cellsEmerges

And is now able toPerpetually grow

Telomerase isActivated

ALT Mechanism: MaintainsTelomeres through recombination-based interchromasomal exchanges of sequence information

Telomerase expression

Loss of Rb / p53 pathways

V: Sustained Angiogenesis (growth of new blood vessels)

• All cells, normal and cancer alike, must reside within 100 µm of a capillary blood vessel.

• A tumor which fails to activate angiogenesis can only grow to ~2mm in diameter.

5. Sustained Angiogenesis

• All cells, normal and cancer alike, must reside within 100 mm of a capillary blood vessel.

• A tumor which fails to activate angiogenesis can only grow to ~2mm in diameter

Simplistic model of TumorAngiogenesis

endothelial cells

VI: Tissue Invasion and Metastasis

• 90% of human cancer deaths are due to metastatic tumors.

• Invasion and metastasis are exceedingly complex processes, and their genetic and biochemical determinants remain incompletely understood.

Molecular Defects in Cancer:Two Paradigms

Tumor SuppressorsLoss of function• p53• p16• p53• p16• Rb• pTEN• p27• etc. etc.

OncogenesGain of function• c-myc• Ras• Cyclin D• CDK4• Bcl-2• Survivin• Oncogenic Viral Proteins

Each protein is resopnsible in evading one or more of the core six characteristics necessary for tumorigenesis

Fighting Cancer

Chemotherapy• The idea behind chemotherapy is simple: kill cells which are

proliferating. • Chemotherpeutic agents induce apoptosis (specific to the

cycling cells) and necrosis (nonspecific and therefore less effective)

• The Failure of Chemotherapy– unfortunately, many cancers have damaged apoptotic response

pathways, leading to ineffective treatment– 50% of cancers have a mutation in p53, which is crucial for

apoptotic response.– Many normal cells proliferate at the same rate (if not faster) than

tumor cells– Many tumor cells don’t necessarily grow faster as they simply just

don’t die.

Chemotherapy• The idea behind chemotherapy is simple: kill cells that are proliferating

through inducing death pathways.• Chemotherapeutic agents induce apoptosis (specific to the cycling cells)

and necrosis (nonspecific and therefore less effective)

• The failure of chemotherapy:– unfortunately, many cancers have damaged apoptotic response pathways,

leading to ineffective treatment.– 50% of cancers have mutated p53, crucial for the apoptotic response.– Many normal cells proliferate at the same rate (if not faster) than tumor cells– Many tumor cells don’t necessarily grow faster as they simply just don’t die

– Strategy: Reactivate death pathways, then hit them with a toxic stress (synergy between chemotherapy and a sensitizing agent).

Targeting Apoptosis in Cancer Therapy

• Cancer can be distilled to two fundamental genetic lesions: – excessive proliferation– constitutively active survival signaling pathway

• To compensate, pro-growth signals are also capable of inducing pro-apoptotic signals (c-myc)– Provides a failsafe mechanism to offset oncogenic capacity– irony: creates a pressure for tumor cell selection.

Myc

Growth Apoptosis

Lesion 1 Lesion 2

current chemotherapeuticapproaches

Novelapproaches

Strategy: Exploit cancer’s inactivated apoptotic pathways