Chapter 21
The Genetic (Cell Biology)
Basis of Cancer
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Chapter Outline
Cancer: 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 are
responsible for cancer.
(cell proliferation and differentiation)
Carcinogens  DNA mutations
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Cancer
Cancers arise when critical genes are
mutated, causing unregulated proliferation of
cells.
These rapidly dividing cells pile up on top of
each other to form a tumor.
When cells detach from the tumor and invade
surrounding tissues, the tumor is malignant
and may form secondary tumors at other
locations in a process called metastasis.
A tumor whose cells do not invade
surrounding tissues is benign.
Tumor – is a condition where there is abnormal cellular growth thus
forming a lesion or in most cases, a lump in some part of your body.
Benign tumor – grows in confined area
Malignant tumor – capable of invading surrounding tissues
Cancer – degenerative disease with a cellular condition where there is uncontrolled
growing mass of cells capable of invading neighboring tissues and spreading via body fluids to
other parts of the body.
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)
Comparison of Normal and Tumor
Growth in the Epithelium of the Skin
Comparison of Normal and Tumor
Growth in the Epithelium of the Skin
Location/distribution
Growth properties of normal and
cancerous cells
Hematoxylin (nucleus) and Eosin (cytoplasm) stain
Normal cells vs. Cancer cells
Normal cell proliferation
Cancer cell proliferation
Anchorage dependent
Anchorage independent
Density-dependent inhibition
Can grow on top of one
another
Immortal
Limited number of cell
divisions
Telomere shortening
Proliferation dependent upon
extracellular signals
Checkpoints activated at
appropriate times
Apoptosis functional
Telomere maintenance
Constant signal to divide
independent
Loss of checkpoint
Apoptosis inhibited
Basic Properties of a Cancer Cell
– In culture, normal cells can be transformed by
chemicals or viruses.
– Different types of cancer cells share a number
of similarities:
•
•
•
•
Aberrant chromosome numbers (aneuploidy)
High metabolic requirements
Unregulated growth
Synthesis of unusual cell surface proteins
Stages in the Process of Invasion and
Metastasis
Basal lamina
Invasion
Metastasis
Why?
How?
Matrix
Basal lamina
Loss of cell surface proteins involve in cell-cell adhesion
E-cadherin
Increased Motility
signaling molecules,
chemoattractants,
protease activator (plasminogen
plasmin)
Some cells are more capable than others
99%
Some preferential sites
blood flow patterns: capillaries
(5-10 um of diameter vs 20 x 25 um)
“seed and soil”
Surrounding environment
Cancers
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Cell Cycle Checkpoints
Transitions between different phases of
the cell cycle (G1, S, G2, and M) are
regulated at checkpoints.
A checkpoint is a mechanism that halts
progression through the cycle until a
critical process is completed.
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Cyclins and CDKs
 Important checkpoint proteins are the cyclins and the
cyclin-dependent kinases (CDKs); complexes
formed between cyclins and CDKs cause the cell
cycle to advance.
 The CDKs phosphorylate target proteins but are
inactive unless they are associated with a cyclin
protein.
 Cell cycling requires the alternate formation and
degradation of cyclin/CDK complexes.
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The START Checkpoint
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/A
Mitotic M-cyclins
Mitotic M-cdks
S cyclins
Cdc2 (Cell Division Cycle ) = CDK (Cyclin-dependent kinase)
Checkpoints in Tumor Cells
 In tumor cells, cell cycle checkpoints are often
deregulated due to genetic defects in the machinery
that 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 to
specific cyclin/CDK complexes
 in genes encoding proteins that regulate the
abundance of these complexes.
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Cancer and Programmed Cell Death
 Apoptosis is part of the normal developmental
program in animals and is important in the prevention
of cancer.
 The caspases, a family of proteolytic enzymes, are
involved in apoptosis and cleave many target
proteins.
 If apoptosis is impaired, a cell that should be killed
can survive and proliferate, potentially forming a
clone that could become cancerous.
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Major Steps in Apoptosis
‘bubble”
Necrosis= injury
Apoptosis= program for cell death
Induction of Apoptosis by Cell Death Signals
or by Withdrawal of Survival Factors
Killer lymphocytes
Autoproteolysis
ATP
proteolysis
IGFR=insulin-like growth factor receptor
INSR= insulin receptor
Evidence of a Genetic Basis for Cancer
The cancerous state is clonally inherited.
Some types of viruses can induce the formation of
tumors in experimental animals.
Cancer can be induced by mutagens.
Certain types of white blood cell cancers are
associated with particular chromosomal abnormalities.
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Cancer and Genes
Oncogenes are genes that, when mutated,
actively promote cell proliferation.
Tumor suppressor genes are genes that,
when mutated, fail to repress cell division.
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Oncogenes
… the overexpression of certain
genes
…the abnormal activity of certain
genes
…their mutant protein products.
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Tumor-Inducing Retroviruses
and Viral Oncogenes
Retroviruses have an RNA genome.
The Rous sarcoma virus, the first tumorinducing virus, contains four genes
–
–
–
–
gag encodes the capsid protein of the virus
pol encodes the reverse transcriptase
env encodes a viral envelope protein
v-src encodes a protein kinase that inserts into
the plasma membranes of infected cells. The vsrc gene is an oncogene that is responsible for the
virus’s ability to induce abnormal cell growth.
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Proteins Encoded by Viral
Oncogenes
Growth factors similar to those encoded by
cellular genes
Proteins similar to growth-factor and hormone
receptors
Tyrosine kinases that do not span the plasma
membrane
Transcription factors homologous to cellular
proteins
Any protein
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Human?
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Proto-Oncogenes
The proteins encoded by viral oncogenes are
similar to cellular proteins with important
regulatory functions.
These cellular homologues are called protooncogenes or normal cellular genes.
The normal c-oncogenes have introns; the
viral v-oncogenes often lack introns.
From c-onco to v-onco….. Replicationdefective viruses
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Normal gene
Cell-oncogene
(c-onc)
Replication-defective virus
The
Transfection
Test to Identify
Mutant Cellular
Oncogenes
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Viral Oncogenes and Cancer
Some viral oncogenes produce more
protein than their cellular counterpart.
Other viral oncogenes express their
proteins at inappropriate times.
Other viral oncogenes express mutant
forms of the cellular proteins.
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The c-ras Gene
The c-H-ras oncogene was identified by the
transfection test (homologue to the Harvey
strain of the rat sarcoma virus)
The mutant c-H-ras protein has a mutation
that impairs its ability to hydrolyze GTP. This
keeps the mutant protein in an active
signaling mode and causes it to stimulate cell
division.
Mutant versions of c-ras have been found in
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many types of tumors.
Normal Ras Protein Signaling
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Mutant Ras Protein (V12 or G12V) is
Unregulated
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Mutations in c-ras are Dominant
A single mutant c-ras allele is dominant in its
ability to bring out the cancerous state.
Mutations in c-ras and other oncogenes are
dominant activators or uncontrolled cell
growth.
Most dominant activating mutations in cellular
oncogenes occur spontaneously in somatic
cells, not in the germline.
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Cancer is the Result of
Several Mutations
A single mutation usually does not
result in cancer.
Usually several genes that regulate cell
growth are mutated before a cancerous
state results.
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Chromosome Rearrangements: The
Philadelphia Chromosome
The Philadelphia chromosome is the result of a
reciprocal translocation between
chromosomes 9 and 22 with breakpoints in the
c-abl gene on chromosome 9 and the c-bcr
gene on chromosome 22.
The fusion gene created by this rearrangement
encodes a tyrosine kinase that promotes
cancer in white blood cells.
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The Philadelphia Chromosome
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Chromosomal Rearrangements:
Burkitt’s Lymphoma
 Burkitt’s lymphoma is associated with reciprocal
translocations involving chromosome 8 and a
chromosome carrying an immunoglobulin gene (2,
14, or 22).
 The translocations juxtapose c-myc to the genes for
the immunoglobulin genes, causing overexpression
of c-myc in B cells.
 The c-myc gene encodes a transcription factor that
activates genes for cell division.
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A Reciprocal Translocation
Involved in Burkitt’s Lymphoma
8p21.1
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Tumor Suppressor Genes
Many cancers involve the
inactivation of genes whose
products play important roles in
regulating the cell cycle.
C-ras and c-myc……genes 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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Knudson’s Two-Hit Hypothesis
When tumor suppressor genes are mutated,
a predisposition to develop cancer often
follows a dominant pattern of inheritance.
The mutation is usually a loss-of-function
mutation in the tumor suppressor gene.
Cancer develops only if a second mutation in
somatic cells knocks out the function of the
wild-type allele.
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Verification of the Two-Hit
Hypothesis for Retinoblastoma
 Several cases of retinoblastoma are associated with a small
deletion in chromosome 13q. Mapping refined the RB locus to
13q14.2.
 Positional cloning was used to isolate a candidate RB gene that
encodes a protein that interact with transcription factors that
regulate the cell cycle.
 In retinoblastoma cells, both copies of this gene were
inactivated.
 In cell culture, expression of a wild-type RB allele could revert
the phenotype of cancer cells.
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Cellular Roles of Tumor Suppressor Proteins
The proteins encoded by tumor
suppressor genes are involved in
cell division,
cell differentiation,
programmed cell death,
DNA repair.
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pRB Regulates the Cell Cycle
--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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p53 Regulates Cell Cycle and
Apoptosis
The p53 tumor suppressor protein is encoded
by the TP53 gene (53 KDa).
Inherited mutations in TP53 are associated
with Li-Fraumeni syndrome.
Somatic mutations that inactivate both copies
of TP53 are associated with the majority of
cancers.
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p53 is a Transcription Factor
 Most mutations in that inactivate p53 are in the DNAbinding domain (DBD) and impair its ability to bind
enhancer sequences in its target genes. Mutations in
this domain are “lost-of-function.”
 OD: homo-oligomerazation domain. Mutations in this
domain are “dominant negative.”
 TAD: transcriptional activation domain
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The Cellular Function of p53
Expression of p53 is very low in normal cells.
Expression of p53 increases in response to
DNA damage due to a decrease in
degradation.
p53 can inhibit cell division or induce
apoptosis.
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[ increase ]
p-p53
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pAPC controls proliferation and
differentiation of cells.
 pAPC mutations are associated with adenomatous
polyposis coli, which often leads to colorectal cancer.
 pAPC regulates the renewal of cells in the epithelium
of the large intestine. Loss of pAPC function results in
the formation of polyps.
 pAPC binds to catenin, which binds to transcription
factors. Cells with mutations in pAPC lose their ability
to control catenin levels.
 Familial adenomatous polyposis (FAP):rare
autosomal dominant dissease.
pAPC
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phMSH2 regulate genomewide instability
The phMSH2 protein is a homologue of the bacterial
and yeast MutS protein, which is involved in DNA
repair.
Mutations in the hMSH2 gene are associated with
hereditary nonpolyposis colorectal cancer (HNPCC), a
dominant 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) have
been implicated in hereditary breast and
ovarian cancer.
Both genes encode proteins that are localized
in the nucleus and have putative
transcriptional activation domains.
pBRCA1 and pBRCA2 may be involved in
DNA repair in human cells.
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Genetic Pathways to Cancer
Cancers develop through an
accumulation of somatic (not a
single) mutations in protooncogenes and tumor suppressor
genes.
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Multiple Mutations in Cancer
Most malignant tumors cannot be attributed
to mutation of a single gene.
Tumor formation, growth, and metastasis
depend on the accumulation of mutations in
several different genes.
The genetic pathways to cancer are diverse
and complex.
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Pathway to Metastatic
Colorectal Cancer
Carcinoma-epithelial cells.
Adenoma-glandular cells.
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Pathway to AndrogenIndependent Prostate Cancer
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Hallmarks of Pathways to
Malignant Cancer
1. Cancer cells acquire self-sufficiency in the
signaling processes that stimulate division
and growth.
2. Cancer cells are abnormally insensitive to
signals that inhibit growth.
3. Cancer cells can evade programmed cell
death (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 other
tissues and colonize them.
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Somatic Mutation and Cancer
Somatic mutation is the basis for the
development and progression of all
types of cancer.
As mutations accumulate and cells
become unregulated, genetic instability
increases the likelihood that the cells
will develop the hallmarks of cancer.
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Interaction of Ti plasmid DNA with the plant
genome
Bacteria genetically engineer plants to control
their differentiation (tumorigenic) and
production of opines that can only be
catabolized by the infecting Agrobacterium
strain.
R1
R2
HOOC-C-NH-C-COOH
H
H
T-DNA transfer, single strand invasion
Transfer of T-DNA resembles bacterial
conjugation
T-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).
T-DNA transfer to host