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Cancer
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• Unregulated cell growth
• Family of >100 related diseases
– Cells multiplying out of control
• Tumor: mass of cells from repeated cell division
– A tumor can be benign (not called “cancer”):
• Grows slowly
• Tissue remains differentiated
• Enclosed in a capsule
– A benign tumor can still be dangerous
• E.g. a brain tumor, slowly crushes brain
• Cells from tumor spread elsewhere: metastasis
– Tumor that metastasizes = malignant
Cancer cells are different
• In culture, are rounded and float rather than
stick to surfaces.
• Become de-differentiated
• Immortal
• Do not respond to cell signals
– Do not stop growing when crowded or touching
• No contact inhibition
– No dependence on normal growth signals
• Loss of normal number and structure of
chromosomes
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Types of cancer
Cancer is named from the type of
tissue in which it arises. There are
various historical classifications.
Carcinomas: from epithelial tissue; 90% of all cancers.
Leukemias, lymphomas, and myelomas: from bone
marrow and blood cells; 8% of all cancers.
Sarcomas: solid connective tissue such as muscle and
bone; 2% of all cancers.
Cells that multiply often are more likely to form cancers.
http://www.willamette.edu/~stas/physiology/labs/lab1/epithelial2a.jpg
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Cancer and aging
As people age,
more
mutations
accumulate,
increasing the
risk of cancer.
Also, as cells
age, they too
can become
cancerous.
http://ovid.iss.it/html/lecture/pc0161/img003.GIF
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Cancer is a genetic disease
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• Predisposition to cancer can be inherited
– Breast cancer, retinoblastoma, Li-Fraumeni
• Clonal expansion: all cells in a tumor descend
from the abnormal cell, all inherit bad genes.
• Agents that damage, mutate DNA cause cancer
– Chemicals, UV and ionizing radiation
• Certain genes brought into DNA by viruses
cause cancer
• Cancer cells show chromosomal loss, damage
Review: The Cell Cycle
•G1: a time of cell
growth and general
functioning.
•S: all the DNA in the
cell is doubled to
prepare for division.
•G2: cell prepares for
division.
•M: mitosis, actual dividing up of the copied
chromosomes and distribution to daughter cells.
http://www.med.unibs.it/~marchesi/cellcycle.gif
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Review: Cell cycle, continued
• G0: cells not dividing;
may never divide again,
or may re-enter cycle
when needed.
Some cells divide nearly
continuously; others
enter G0 once mature.
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Cell cycle (continued)
• From cancer research, we have learned that the cell
cycle is tightly regulated!
– Checkpoints exist: at G1/S, at G2/M, and late in
Mitosis (the “M” checkpoint)
• At each checkpoint, progress evaluated.
– G1/S checkpoint: is DNA in good condition?
– Mutations that allow progression past this
checkpoint with damaged DNA lead to downward
spiral of inherited mutations.
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Molecular regulation
• Two kinds of proteins work together:
– cdc kinases and cyclins
– Kinases: proteins that phosphorylate other proteins
• Adding a PO4 turns molecules “on” or “off”
• Kinases always present in the cell
– Cyclins are proteins that change in amounts during
the cell cycle
• Specific cyclins accumulate at different times.
• Kinase combines with cyclin
– Kinase is activated, given directions
– Kinase phosphorylates proteins controlling cell
division
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Kinase/cyclin combination acts during checkpoint
G1/S checkpoint
important in cell
cycle regulation.
Important “tumor
suppressor” genes
involved in
regulation at this
step.
Picture based on Hartl & Jones, 5th edition.
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The multi-hit hypothesis
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• Cancer results from at least 2 “hits”, 2 instances
of damage to the DNA.
– Some cancers involve several.
• The gas pedal/ brake analogy
– For the car to move, gas pedal doesn’t do much if
brake is on; taking brake off doesn’t get car to
move if gas pedal isn’t pressed.
– Cell must get “ON” signals AND ignore “OFF”
signals
• On: growth factor stimulation
• Off: tumor suppressors
New insights into off/on regulation
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• Recent research: effect of estrogen on entire genome
of ovarian tissue
– Estrogen is a growth factor, promotes cell division
– Genes for promoting cell division AND stopping cell
division both turned on: a balance
• Like “riding the brake” to keep car under control
Genetics of on and off
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• “On signal” mutations are dominant for cancer
– If other mutations present, one mutated copy of the
gene is enough for cancer.
• “Off signal” mutations are recessive for cancer.
– But are dominant for a susceptibility to cancer.
– Both copies of tumor suppressor genes must be
bad for cancer to occur.
– Familial predispositions: usually one copy is already
mutated, much more likely to get a mutation in one
copy of the gene than both.
Genetics visualized
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What are the “on” and “off” signals?
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• “Off” signals: tumor suppressor genes
– Genes (and their proteins) which act to prevent the
cell from proceeding through the cell cycle.
• pRB, p53, BRCA1, BRCA2
• “On” signals: proto-oncogenes
– Genes that stimulate cell division.
– Mutated forms called oncogenes
• Through various malfunctions, stimulate cell
division when inappropriate.
Tumor suppressor genes
• Retinoblastoma
– Cancer of the retina of the eye
– Occurs in about 1 in 15,000, usually ages 1 – 3.
– 40% inherited susceptibility; 60% sporadic
• RB1 gene codes for pRB, a tumor suppressor
– Functions at the G1/S checkpoint
• Protein is active during G0 and G1
– Binds to E2F, a transcription factor
• Prevents E2F from working, prevents
transcription of genes that aid cell division.
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RB continued
At G1/S, cyclin
complex
phosphorylates
pRB, inactivating it
and releasing E2F.
E2F is a transcription factor controlling 30 genes needed
for progression through cell cycle.
Mutation in both copies of pRB allows unregulated
passage through G1/S checkpoint.
p53: the Guardian of the Genome
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• An important tumor suppressor; various signals
including DNA damage activate p53 which then
– Activates DNA repair
– Blocks cell division
• Prevents replication of damaged DNA
– Triggers apoptosis in damaged cells.
• Cells beyond repair are killed to avoid mutations
• Mutations in p53 account for >50% of cancers.
– Single mutation is dominant negative because of
tetrameric structure.
Action of p53
• Continually made but
continually degraded
– Also Inactive because NOT
phosphorylated
DNA damage activates protein kinases
Results in phosphorylation (thus activation)
Degradation of p53 stops.
Active p53 accumulates in cell, carries out its duties
http://www.accelrys.com/webzine/01/q1/appnotes/geneatlas_japan/p53.jpg
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BRCA genes
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• 10% of breast cancers involve a familial
susceptibility; mutations in BRCA1 or BRCA2.
– Woman with a faulty BRCA1 allele has an 85%
chance of developing mutation in 2nd allele, thus
getting breast cancer (with risk of ovarian cancer)
• Proteins coded for by BRCA1 and 2 in nucleus
– Abundant during S phase
– Work with other proteins in DNA repair, etc.
– Failure of these proteins to repair may result in the
mutations that lead to cancer.
Cancer continued
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• Proto-oncogenes to oncogenes
– Proto-oncogenes are normal cell genes that turn on
cell division.
– A mutated or over-expressed proto-oncogene
becomes an oncogene: gene leading to cancer
• Given other appropriate mutations:
– These are dominant mutations for a cancer
phenotype; that is, only one bad copy is enough.
• Two gas pedals: one turned on makes it go.
Control of cell division
Proto-oncogene proteins found in membrane,
cytoplasm, and nucleus.
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Cancer results from too much “ON”
Effect of mutations:
•Cancer cells produce their own
growth factors.
•Increase in growth factor
receptors.
•ON signal sent to nucleus no matter what.
•Example: ras, described in text.
•Mutation in transcriptional activator; wrong genes
turned on.
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Oncogenes: the rude surprise
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• Peyton Rous, 1966 Nobel Laureate
– Discovered that viruses could cause cancer in
chickens (e.g. Rous sarcoma virus)
• Molecular biologists identified genes in the
viruses that were responsible: oncogenes.
• DNA hybridization studies revealed: we all have
comparable genes: proto-oncogenes.
– In human cancers, viruses can bring oncogenes
into our DNA, or changes can occur in our DNA
without virus involvement.
How oncogenes are made
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• Point mutations
– Ras is a intracellular signal peptide
• Mutation makes it always stimulate cell division
• Translocations
– Burkitts lymphoma: t(8;14) moves myc gene to a
more active promoter, drives proliferation.
• myc is a gene for a transcription factor.
– Philadelphia chromosome: t(9;22) creates a fusion
protein (protein kinase) that activates cell division.
How oncogenes are made-2
• Over-expression & Gene amplification:
– too many copies of receptors, internal signalling
molecules, transcription factors.
– Example: myc. Transcription factor that causes
transcription of genes involved in cell division.
– Virus insertion may put active promoter next to
gene.
– Virus may bring in extra copies of gene.
Cmyc-FISH
www.pathology.unibe.ch/.../ speztech/spez_fish.htm
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Another overview
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• Historically, several models of cancer have
been developed since War on cancer declared
by Nixon, 1970.
• ON/Off switch idea
• Multi-hit hypothesis, textbook: colon cancer
• Gatekeeper and Caretaker genes
– Gatekeepers: tumor suppressor genes. Prevent
cells from dividing when they’re not supposed to.
– Caretakers: protect the DNA, repair damage.
Review of Telomerase
• Because of
discontinuous synthesis
of linear DNA, each
round of DNA would
shorten the
chromosome, eventually
causing chromosome
instability and cell death.
Telomerase adds
protective DNA to end,
making cell “immortal”.
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Cancer and Telomerase
• Stem cells, frequently, rapidly dividing cells,
maintain telomerase.
• Descendents have a life span.
– Telomerase gene is shut off, enzyme not made.
– Lack of telomerase is a cell clock.
– Chromosome becomes unstable, cell undergoes
apoptosis and dies; replaced by new cells.
• Some cells cheat death.
– Cancer cells turn telomerase back on, become
immortal. Area of active study.
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Cancer and the Environment
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• Familial predispositions exist, but sporadic
cancers are more common.
– Environmental factors combine with genetic
background to produce cancer.
– Microbes, chemicals, radiation all implicated.
• Direct experimentation not possible w/ humans.
– Evidence is correlation, not causation.
• Statistical analysis of epidemiological data
• Not “proved” that smoking causes cancer
– Need statistical evidence plus plausible mechanism
Cancer and microbes
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• Retroviruses and other inserting viruses
– RNA to DNA, inserts into chromosome, directs
synthesis of new RNA. HIV, Rous sarcoma virus.
– DNA viruses that insert: Hepatitis B
• Hepatitis B infected: 100x more likely to get liver
cancer
• Epstein Barr Virus, Kaposi’s sarcoma virus
– EBV associated with Burkitt Lymphoma (and mono)
– Herpes virus KSV causes sarcoma in HIV infection
Cancer and microbes-2
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• Sex causes cancer: Papilloma virus and cancer
of the cervix.
– Papilloma virus causes warts, including genital
warts; sexually transmitted disease.
– Subtypes, instead of causing warts, inserts into
DNA, causes increased expression of viral protein
E7. See next slide.
• Helicobacter pylori
– Bacterium now known to cause ulcers.
– Highly correlated with stomach cancer; under study.
Cancer and microbes-3
pRB is a tumor
suppressor protein
that prevents the
action of
transcription factor
E2F by binding to
it. Viral protein E7
bids tightly to pRB,
freeing E2F to
transcribe genes
promoting cell
division.
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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/E7_Rb.gif
Cancer and the environment
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• Relatively few cancers are “inherited”, most
result from interactions with the environment or
“lifestyles”
• Tobacco use
– Benzo-a-pyrene, tar, radioisotopes, formaldehyde
• Diet: natural and produced carcinogens
– Aflatoxins, nitrosoamines
– Fiber, antioxidant compounds helpful.
• Radiation: UV , ionizing