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Chapter 21The Genetic (Cell Biology)

Basis of Cancer

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Chapter OutlineCancer: A Genetic Disease

Oncogenes

Tumor Suppressor Genes

Genetic Pathways to Cancer

Tumors in Plants

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Cancer: A Genetic Disease

Mutations in genes that control

cell growth and division areresponsible for cancer.

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(cell proliferation and differentiation)

Carcinogens DNA mutations

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CancerCancers arise when critical genes are

mutated, causing unregulated proliferation ofcells.

These rapidly dividing cells pile up on top of

each other to form a tumor.

When cells detach from the tumor and invadesurrounding tissues, the tumor ismalignant

and may form secondary tumors at otherlocations in a process called metastasis.

A tumor whose cells do not invade

surrounding tissues is benign.

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Tumor is a condition where there is abnormal cellular growth thusforming a lesion or in most cases, a lump in some part of your body.

Benign tumor grows in confined area

Malignant tumor capable ofinvading surrounding tissues

Cancer degenerative disease with a cellular condition where there is uncontrolledgrowing mass of cells capable of invading neighboring tissues and spreading via body fluids to

other parts of the body.

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Named for site of origin

Carcinomas epithelial cells; cover external & internal body

surfaces (90%)

Sarcomas supporting tissue; bone, cartilage, fat, connective

tissue, pancreas, Liver.

Lymphoma & leukemias blood & lymphatic tissue(leukemia reserved for cancers that reside in bloodstream

not as solid tissue)

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Comparison of Normal and TumorGrowth in the Epithelium of the Skin

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Comparison of Normal and TumorGrowth in the Epithelium of the Skin

Location/distribution

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Growth properties of normal andcancerous cells

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Hematoxylin (nucleus) and Eosin (cytoplasm) stain

N l ll C ll

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Normal cells vs. Cancer cells

Normal cell proliferation Cancer cell proliferation

Anchorage dependent Anchorage independentDensity-dependent inhibition Can grow on top of one

another

Limited number of celldivisions

Immortal

Telomere shortening Telomere maintenance

Proliferation dependent upon

extracellular signals

Constant signal to divide

Checkpoints activated atappropriate times

Loss of checkpoint

Apoptosis functional Apoptosis inhibited

independent

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Basic Properties of a Cancer Cell

In culture, normal cells can be transformed bychemicals or viruses.

Different types of cancer cells share a numberof similarities:

Aberrant chromosome numbers (aneuploidy)

High metabolic requirements Unregulated growth

Synthesis of unusual cell surface proteins

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Invasion

Metastasis

Why?

How?

Basal lamina

Matrix

Stages in the Process of Invasion and

Metastasis

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Loss of cell surface proteins involve in cell-cell adhesion

E-cadherin

Increased Motility

signaling molecules,

chemoattractants,

protease activator (plasminogen plasmin)

Basal lamina

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Some cells are more capable than others

99%

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Some preferential sites

blood flow patterns: capillaries

(5-10 um of diametervs 20 x 25 um)

seed and soil

Surrounding environment

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Cancers

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Cell Cycle Checkpoints

Transitions between different phases ofthe cell cycle (G1, S, G2, and M) are

regulated at checkpoints.

A checkpoint is a mechanism that halts

progression through the cycle until acritical process is completed.

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Cyclins and CDKs

Important checkpoint proteins are the cyclins and thecyclin-dependent kinases (CDKs); complexesformed between cyclins and CDKs cause the cellcycle to advance.

The CDKs phosphorylate target proteins but areinactive unless they are associated with a cyclinprotein.

Cell cycling requires the alternate formation anddegradation of cyclin/CDK complexes.

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The STARTCheckpoint

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Mitotic M-cyclinsMitotic M-cdks

S cyclins

/A

Cdc2 (Cell Division Cycle ) = CDK (Cyclin-dependent kinase)

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Checkpoints in Tumor Cells

In tumor cells, cell cycle checkpoints are oftenderegulated due to genetic defects in the machinerythat alternately raises and lowers the abundance

of the cyclin/CDK complexes.

These mutations may be:

in the genes encoding the cyclins or CDKs,

in genes encoding the proteins that respond tospecific cyclin/CDK complexes

in genes encoding proteins that regulate theabundance of these complexes.

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Cancer and Programmed Cell Death

Apoptosis is part of the normal developmentalprogram in animals and is important in the preventionof cancer.

The caspases, a family of proteolytic enzymes, areinvolved in apoptosis and cleave many targetproteins.

If apoptosis is impaired, a cell that should be killedcan survive and proliferate, potentially forming aclone that could become cancerous.

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Major Steps in Apoptosis

Necrosis= injury

Apoptosis= program for cell death

bubble

I d ti f A t i b C ll D th Si l

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Induction of Apoptosis by Cell Death Signals

or by Withdrawal of Survival Factors

IGFR=insulin-like growth factor receptor

INSR= insulin receptor

Autoproteolysis

ATP

proteolysis

Killer lymphocytes

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Evidence of a Genetic Basis for Cancer

The cancerous state is clonally inherited.

Some types of viruses can induce the formation oftumors in experimental animals.

Cancer can be induced by mutagens.

Certain types of white blood cell cancers areassociated with particular chromosomal abnormalities.

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Cancer and Genes

Oncogenes are genes that, when mutated,actively promote cell proliferation.

Tumor suppressor genesare genes that,when mutated, fail to repress cell division.

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Oncogenes the overexpression of certain

genes

the abnormal activity of certaingenes

theirmutant protein products.

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Tumor-Inducing Retroviruses

and Viral OncogenesRetroviruses have an RNA genome.

The Rous sarcoma virus, the first tumor-inducing virus, contains four genes gag encodes the capsid protein of the virus

polencodes the reverse transcriptase

envencodes a viral envelope protein

v-srcencodes a protein kinase that inserts intothe plasma membranes of infected cells. The v-src gene is an oncogene that is responsible for theviruss ability to induce abnormal cell growth.

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Proteins Encoded by Viral

OncogenesGrowth factors similar to those encoded by

cellular genes

Proteins similar to growth-factor and hormonereceptors

Tyrosine kinases that do not span the plasma

membraneTranscription factors homologous to cellular

proteins

Any protein

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Human?

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Proto-OncogenesThe proteins encoded by viral oncogenes are

similar to cellular proteins with importantregulatory functions.

These cellular homologues are called proto-oncogenes or normal cellular genes.

The normal c-oncogeneshave introns; the

viral v-oncogenesoften lack introns.

From c-onco to v-onco.. Replication-defective viruses

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http://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrwhttp://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrwhttp://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrwhttp://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrwhttp://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrwhttp://www.google.com/url?q=http://www.ncbi.nlm.nih.gov/pubmed/1655896&sa=U&ei=pPD8T5i8DcaU2AW02u3nDw&ved=0CBcQFjAB&usg=AFQjCNF9p-L2PZw9xM9CWMyXE3HiedBFrw

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Replication-defective virus

Normal gene

Cell-oncogene(c-onc)

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TheTransfection

Test to Identify

Mutant CellularOncogenes

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Viral Oncogenes and Cancer

Some viral oncogenes produce moreprotein than their cellular counterpart.

Other viral oncogenes express theirproteins at inappropriate times.

Other viral oncogenes express mutantforms of the cellular proteins.

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The c-ras GeneThe c-H-rasoncogene was identified by the

transfection test (homologue to the Harveystrain of the rat sarcoma virus)

The mutant c-H-ras protein has a mutationthat impairs its ability to hydrolyze GTP. Thiskeeps the mutant protein in an active

signaling mode and causes it to stimulate celldivision.

Mutant versions of c-rashave been found in

many types of tumors. John Wiley & Sons, Inc.

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Normal Ras Protein Signaling

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Mutant Ras Protein (V12 or G12V) isUnregulated

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Mutations in c-ras are Dominant

A single mutant c-ras allele is dominant in itsability to bring out the cancerous state.

Mutations in c-ras and other oncogenes aredominant activatorsor uncontrolled cellgrowth.

Most dominant activating mutations in cellularoncogenes occur spontaneously in somaticcells, not in the germline.

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Cancer is the Result of

Several MutationsA single mutation usually does not

result in cancer.

Usually several genes that regulate cellgrowth are mutated before a cancerous

state results.

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Chromosome Rearrangements: The

Philadelphia ChromosomeThe Philadelphia chromosome is the result of a

reciprocal translocation between

chromosomes 9 and 22 with breakpoints in thec-ablgene on chromosome 9 and the c-bcrgene on chromosome 22.

The fusion gene created by this rearrangementencodes a tyrosine kinase that promotescancer in white blood cells.

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The Philadelphia Chromosome

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Chromosomal Rearrangements:

Burkitts LymphomaBurkitts lymphoma is associated with reciprocal

translocations involving chromosome 8 and achromosome carrying an immunoglobulin gene (2,

14, or 22).

The translocations juxtapose c-myc to the genes forthe immunoglobulin genes, causing overexpressionof c-myc in B cells.

The c-myc gene encodes a transcription factor thatactivates genes for cell division.

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A Reciprocal Translocation

Involved in Burkitts Lymphoma

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8p21.1

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Tumor Suppressor Genes

Many cancers involve theinactivation of genes whose

products play important roles in

regulating the cell cycle.

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C-ras and c-mycgenes required for regulation cell cycle.

-increase activity and/or concentration-----oncogene----may form tumors.

-decrease activity and/or concentration----anti-oncogene----not tumor formation

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Knudsons Two-Hit Hypothesis

When tumor suppressor genes are mutated,a predisposition to develop cancer oftenfollows a dominant pattern of inheritance.

The mutation is usually a loss-of-functionmutation in the tumor suppressor gene.

Cancer develops only if a second mutation insomatic cells knocks out the function of thewild-type allele.

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V ifi i f h T Hi

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Verification of the Two-HitHypothesis for Retinoblastoma

Several cases of retinoblastoma are associated with a smalldeletion in chromosome 13q. Mapping refined the RB locus to13q14.2.

Positional cloning was used to isolate a candidate RB gene thatencodes a protein that interact with transcription factors thatregulate the cell cycle.

In retinoblastoma cells, both copies of this gene were

inactivated. In cell culture, expression of a wild-type RBallele could revert

the phenotype of cancer cells.

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Cellular Roles of Tumor Suppressor Proteins

The proteins encoded by tumorsuppressor genes are involved in

cell division,

cell differentiation,

programmed cell death,

DNA repair.

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RB R l t th C ll C l

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pRB Regulates the Cell Cycle

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--Retinoblastoma, small-cell lung carcinomas, osteosarcomas,

bladder, cervical and prostate carcinomas.--Essential for life.

--105 KDa.

--Nuclear Protein.

--Alter cell cycle.

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53 R l t C ll C l d

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p53 Regulates Cell Cycle andApoptosis

The p53 tumor suppressor protein is encodedby the TP53 gene (53 KDa).

Inherited mutations in TP53 are associatedwith Li-Fraumeni syndrome.

Somatic mutations that inactivate both copiesof TP53 are associated with the majority ofcancers.

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53 i T i ti F t

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p53 is a Transcription Factor

Most mutations in that inactivate p53 are in the DNA-binding domain (DBD) and impair its ability to bindenhancer sequences in its target genes. Mutations inthis domain are lost-of-function.

OD: homo-oligomerazation domain. Mutations in thisdomain are dominant negative.

TAD: transcriptional activation domain John Wiley & Sons, Inc.

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The Cellular Function of p53

Expression of p53 is very low in normal cells.

Expression of p53 increases in response toDNA damage due to a decrease indegradation.

p53 can inhibit cell division or induceapoptosis.

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[ increase ]

p-p53

pAPC controls proliferation and

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pAPC controls proliferation anddifferentiation of cells.

pAPC mutations are associated with adenomatouspolyposis coli, which often leads to colorectal cancer.

pAPC regulates the renewal of cells in the epitheliumof the large intestine. Loss of pAPC function results inthe formation of polyps.

pAPC binds to catenin, which binds to transcription

factors. Cells with mutations in pAPC lose their abilityto control catenin levels.

Familial adenomatous polyposis (FAP):rareautosomal dominant dissease.

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pAPC

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phMSH2 regulate genome-

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phMSH2 regulate genomewide instability

The phMSH2 protein is a homologue of the bacterialand yeast MutS protein, which is involved in DNArepair.

Mutations in the hMSH2 gene are associated withhereditary nonpolyposis colorectal cancer (HNPCC), adominant autosomal condition.

Cells in HNPCC tumors exhibit genetic instability.

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pBRCA1 and pBRCA2 regulate

DNA repair.Mutations in the tumor suppressor genes

BRCA1 (Ch17) and BRCA2 (Ch13) havebeen implicated in hereditary breast and

ovarian cancer.

Both genes encode proteins that are localizedin the nucleus and have putativetranscriptional activation domains.

pBRCA1 and pBRCA2 may be involved in

DNA repair in human cells. John Wiley & Sons, Inc.

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Genetic Pathways to Cancer

Cancers develop through anaccumulation of somatic (not a

single) mutations in proto-oncogenes and tumor suppressor

genes.

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Multiple Mutations in Cancer

Most malignant tumors cannot be attributedto mutation of a single gene.

Tumor formation, growth, and metastasisdepend on the accumulation of mutations inseveral different genes.

The genetic pathways to cancer are diverseand complex.

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Pathway to Metastatic

Colorectal Cancer

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Carcinoma-epithelial cells.

Adenoma-glandular cells.

Pathway to Androgen

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Pathway to Androgen-Independent Prostate Cancer

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Hallmarks of Pathways to

Malignant Cancer1. Cancer cells acquire self-sufficiency in the

signaling processes that stimulate divisionand growth.

2. Cancer cells are abnormally insensitive tosignals that inhibit growth.

3. Cancer cells can evade programmed celldeath (apoptosis).

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4. Cancer cells acquire limitless replicate potential.

5. Cancer cells develop ways to grow themselves.

6. Cancer cells acquire the ability to invade othertissues and colonize them.

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Somatic Mutation and Cancer

Somatic mutation is the basis for thedevelopment and progression of alltypes of cancer.

As mutations accumulate and cellsbecome unregulated, genetic instability

increases the likelihood that the cellswill develop the hallmarks of cancer.

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Interaction of Ti plasmid DNA with the plant

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genome

Bacteria genetically engineer plants to control

their differentiation (tumorigenic) and

production of opines that can only becatabolized by the infecting Agrobacterium

strain.

HOOC-C-NH-C-COOH

R1 R2

H H

T-DNA transfer single strand invasion

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T DNA transfer, single strand invasion

Transfer of T-DNA resembles bacterialj i

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conjugationT-DNA is generated when a nick at the right boundary creates a

primer for synthesis of a new DNA strand.

The preexisting single-strand that is displaced by the new synthesis

is transferred to the plant cell nucleus.

Transfer is terminated when DNA synthesis reaches a nick at the

left boundary.

The T-DNA is transferred as a complex of single-stranded DNA

with the VirE2 single-strand binding protein.

The single stranded T-DNA is converted into double-stranded

DNA and integrated into the plant genome.

The mechanism of integration is not known. T-DNA can be used

to transfer genes into a plant nucleus (transformation).

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T-DNA transfer to host