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Cancer
What is Cancer?
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uncontrolled cell growth (as
opposed to steady-state
replacement of cells)
usually accompanied by dedifferentiation of cells
cancerous mass = tumor or
neoplasm
Natural selection: cells which grow
faster than others will take up
more and more space.
– Our cells have multiple
defenses against cells
overgrowing their allotted
locations.
– Cancer occurs when those
defenses have been removed.
– starts with one transformed
cell
Genetic Phenomenon
• Cancer involves changes in DNA sequence i.e. it
is genetic
• Cancer is not epigenetic i.e. changes in patterns
of gene expression without DNA changes.
• If cancer were epigenetic, it might be easier to
reverse.
• Epigenetic changes, such as DNA methylation
and histone modification, do occur in cancer, but
they are rarely or never the underlying cause.
Cancer is a progressive disease
• Needs 5-6 mutations for full-blown cancer.
• Involves natural selection--in a slowgrowing tumor, a faster growing mutant
will take over.
Stages of Cancer
1.
2.
3.
Initiation: A mutation that transforms
the cell, leaving it capable of
unrestrained growth.
Promotion: No growth unless cell
enters S phase (many cells are
arrested and need a promoter, a
mitogen, to get them started)
Progression:
A. Angiogenesis--invasion of tumor
by blood vessels
B. Invasiveness--ability to
penetrate basal membranes.
Tumors that can't do this are
benign, those that can are
malignant
C. Metastasis--ability to go through
the blood and colonize other
tissues
Gene Transfer Experiments
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Gene Transfer experiments are an approach to
identifying oncogenes
Normal fibroblasts will multiply in Petri dishes,
but they have 2 specific properties of interest:
1. contact inhibition: they stop growing when they
touch, leading to a monolayer.
2. finite number (50-60) of cell divisions before death
Partially Transformed Cells
• When transformed, cells lose
contact inhibition (they pile up)
and become immortal.
• NIH 3T3 mouse cells are
partially transformed: immortal
but still contact inhibited. That
is, they grow in a monolayer
• However, mutagens, etc. will
create foci (plural of focus) of
piled up cells starting with a
single transformed cell.
• Basis for oncogene assay.
GENES PLAYING ROLE IN
CANCER DEVELOPMENT
• Oncogenes
• Tumor suppressor genes
• DNA repair genes
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What are the genes responsible for tumorigenic
cell growth?
Normal
Proto-oncogenes
+
Tumor suppressor genes
-
Cell growth
and
proliferation
Cancer
Mutated or “activated”
oncogenes
Loss or mutation of
Tumor suppressor genes
++
Malignant
transformation
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ONCOGENES
• Oncogenes are mutated forms of
cellular proto-oncogenes.
• Proto-oncogenes code for cellular
proteins which regulate normal cell
growth and differentiation.
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Activating Oncogenes
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Normally, cellular oncogenes are proto-oncogenes: they have
a regular cellular function and aren’t involved with cancer.
Two basic ways of converting proto-oncogenes into
oncogenes:
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2.
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mutate the protein
make lots of the normal protein
There are a variety of ways to accomplish these events.
Tumor suppressor genes
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Normal function - inhibit cell proliferation
Absence/inactivation of inhibitor --> cancer
Both gene copies must be defective
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Tumor Suppressor genes
• A distinction:
– Oncogenes act in a dominant fashion: one mutant copy plus one
normal copy gives a tumor.
– Tumor suppressor genes are recessive: one mutant and one
normal is still wild type--need both copies mutant to give a tumor.
• Wild-type oncogenes (proto-oncogenes) promote cell
proliferation; mutant versions enhance this property.
• On the other hand, tumor suppressors regulate and
inhibit cell proliferation; mutant versions remove controls
on proliferation.
• Tumor suppressor genes mostly found by cloning familial
cancer genes and chromosome regions commonly
deleted in tumor cells.
What are oncogenes and tumor
suppresors?
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A lot of fundamental cell
processes have been
investigated as part of
understanding oncogenes.
Several basic types:
1. growth factors
2. growth factor receptors at the
cell surface
3. signal transduction proteins
4. transcription factors
5. cell cycle regulatory proteins
6. DNA damage detection and
repair proteins
Cell Cycle Control
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Complex and not fully understood yet. Many overlapping
control systems
In general a cell can: stay in interphase, divide, or undergo
programmed cell death (apoptosis).
Checkpoints: the cell cannot proceed past them until certain
conditions are met.
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G1 -> S
G2 -> mitosis
metaphase spindle attachment
G1-S checkpoint. The main control point for cells with
damaged DNA
G2-M checkpoint. Cells must have completed DNA repair to
pass this point
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Mitosis is initiated by the MPF (maturation promoting factor) protein
complex, composed of cyclins and CDKs which have built up over
the course of the cell cycle.
Genome Integrity
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Mutation is a constant problem. Many
mechanisms prevent cells with seriously
mutated DNA from dividing.
Malignant cells usually undergo
chromosomal rearrangements, leading to
new fused genes and loss of heterozygosity.
Spindle checkpoint. During mitosis, cells
can only proceed into anaphase when all of
the chromosomes are properly attached to
the spindle. A protein complex on the
kinetochore is displaced when the
kinetochore is attached to the spindle.
Telomerase. This enzyme prevents the loss
of DNA at the ends of chromosomes, an
inevitable consequence of replication. It is
inactive in most cells, which results in them
dying after 60 or so cell divisions. However,
it is re-activated in 85% of successful tumor
cells, resulting in cellular immortality. This is
one of the most common markers of cancer.
DNA Damage Detection
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Several DNA repair systems are tumor suppressor
genes.
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BRCA1 and BRCA2: genes implicated in breast
cancer. Also ATM, the ataxia telangiectasia protein.
Part of a multi-protein BASC complex that scans the
DNA for damage
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Xeroderma pigmentosum: DNA damage caused by
sunlight isn't repaired, leading to skin tumors.
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Several forms, involving nucleotide excision DNA repair
enzymes.
Hereditary non-polyposis colon cancer (doesn't form
polyps).
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Autosomal dominant
When the genomes of HNPCC patients were scanned
for LOH, many micro-satellite loci had changes in
repeat number, all over the genome.
Related to E. coli mutator system MutHLS,
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Mutations in these genes increase mutation rate in E.
coli up to 1000 x.
Mismatch repair system: removes mismatched DNA
bases on newly synthesized strand and resynthesizes that stretch of DNA.
Human analogue of MutS gene mapped to region of
HNPCC gene, and turned out to be mutant in HNPCC
patients.
One human homologue of MutL is also responsible for
much of HNPCC
Apoptosis: Programmed Cell Death
• Programmed Cell
death
• Cancer cells
escape apoptosis
• After a certain
number of
generations (mitosis
& cytokinesis) a
normal cell exits the
cell cycle and dies.
• Instead of
proliferating,
differentiating, and
functioning normally
until they die, cancer
cells proliferate
rapidly and do not
make the transition to
apoptosis.
Cellular equilibrium
Proliferation
Differentiation
Death
Transit
Renewing
Proliferating
Exiting
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Cancer: disruption of
cellular equilibrium
Proliferation
Differentiation
Death
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Cancer as Multi-step Process
Progression of colorectal cancer
(familial adenomatous cancer)
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Adenomas are polyps seen in
the colon. They start as
abnormal crypt cells, progress to
benign polyps, and finally
become cancerous.
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The following shows one way an
FAP case may develop:
1.
2.
3.
4.
Normal colon epithelium has
mutation in APC tumor
suppressor gene causing rapidly
proliferating epithelium.
Activation by mutation of KRAS
(ras-K) leads to polyp formation.
Loss of heterozygosity tumor
suppressor gene in 18q (exact
gene not clear) leads to late
stage polyp.
Mutation in p53 leads to
carcinoma.