Download Replication of the DNA

Chapter 14. Cancer and
Aging
Prepared by Woojoo Choi
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Somatic mutation
1) If a mutation occurs in somatic cells, a variety of possibilities may result.
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Somatic mutation
2) A mutation which occurs early on in embryonic development may be
highly detrimental.
3) If the single precursor cell that divides, eventually giving rise to a major
organ, suffers a serious mutation, the results may be serious or fatal.
4) Most somatic mutations occurring later in development will affect only
one or a few cells and will be of little significance except for cancers.
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Some somatic mutation cause cancer
1) Some somatic mutations occurring after the organism has reached
maturity are still dangerous.
2) Cancer: disease due to unplanned of mutant somatic cells that damage
the regulatory system controlling cell growth and division
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Some somatic mutation cause cancer
3) Cancer occurs in several stage and requires several mutations.
① First, a cell is mutated and normal control of cell division is lost.
② Second, the mutant cell divides to form a tumor.
• Benign: when a tumor stays in one place
③ Third, a cancer may gain the ability to invade other tissues and form
secondary tumors.
• Malignant: a cancer that has the capability of dispersing
throughout the body
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Environmental factors and cancer
1) Those chemicals and radiation which cause mutations also cause cancer.
– Carcinogen: any agent that causes cancer
2) Most cancers (approximately 80 percent) are derived from epithelial cells,
the cells forming the outer covering of tissues.
3) Because the outermost layers are constantly worn away, the underlying
layer must keep dividing.
4) The surface cells are much more likely to suffer exposure to dangerous
chemicals and harmful radiation.
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Genes that affect cancer
1) Two general types of genes that affect cancer
① Oncogene: mutant gene that promotes cancer
• Mutant oncogenes have a positive effect and promote the
development of cancer cells.
• Mutations in oncogenes are dominant and the second, wild-type
copy of the gene cannot make up for the defect.
② Anti-oncogene (tumor-suppressor gene): gene acting to prevent
unwanted cell division
• Tumor suppressor genes have a negative effect on cancer
development (suppress division of cancer cells).
• To allow cancers to grow, both copies of a tumor suppressor
gene must be inactivated by mutation (recessive mutations).
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Oncogenes and proto-oncogenes
1) Although oncogenes were first discovered on cancer-causing viruses,
they are found in all normal cells, too.
2) Oncogenes exist in three forms (the good, the bad and the ugly)
① Proto-oncogene (the good)
• Original, healthy and unmutated form of gene that may give rise
to an oncogene
• Necessary for the growth and division of the cell to the right size
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Oncogenes and proto-oncogenes
② The bad oncogenes
• Mutations in genes that control cell division are a bad thing.
• These mutant versions are the bad oncogenes.
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Oncogenes and proto-oncogenes
③ The ugly going on due to retroviruses
• These viruses occasionally pick up an oncogene from their host
cells and the result is a cancer-causing virus.
• v-onc: oncogene carried by a virus
• c-onc: oncogene carried on a cell’s chromosome
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Oncogenes and proto-oncogenes
3) Although quite a few cancer viruses are known, most human cancers are
not due to viruses, but are due to new mutations of the cellular protooncogene to its oncogene form.
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Detection of oncogenes
1) If oncogene or the suspect DNA is extracted and is then inserted into
healthy cells, it can change them into cancer cells.
2) Transformation: conversion of a normal cell to a cancer cell
3) Normal cultured animal cells usually grow as a thin monolayer on the
surface of a culture dish.
4) Contact inhibition: when normal cells prevent their neighbors from
dividing by touching them
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Detection of oncogenes
5) Cancer cells are uninhibited and they keep on dividing and piling into
heaps.
6) These can be seen when DNA containing an oncogene is added to
normal cells in culture.
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Detection of oncogenes
7) Cancer cells cause tumors in mice.
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Creating an oncogene
1) The wild type duplicate copy of the proto-oncogene does not overcome
the effect of the oncogene.
2) This is because oncogenic mutations result not from loss of activity but
from increased activity of the oncogene.
3) This can be due either to alteration of the sequence (structure) of protein
encoded by the proto-oncogene or due to increased production of the
protein.
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Creating an oncogene
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The ras oncogene- hyperactive protein
1) ras oncogene is the result of a single base change in the structural
region of the gene.
2) Only a few very specific mutations can create a ras oncogene from the
proto-oncogene.
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The ras oncogene- hyperactive protein
3) ras protein
– A protein involved in cell
proliferation which, when
mutated, can cause cancer
– Involved in transmitting
signals concerning cell
division
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The ras oncogene- hyperactive protein
4) The cancer-causing form of the ras protein is locked permanently into
the signal emitting mode and never splits its GTP.
5) It constantly floods the cell with signals urging cell division, even when
none are being received from outside.
6) The consequence is uncontrolled cell division and the beginnings of a
possible cancer.
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The Myc oncogene – overproduction of protein
1) Some oncogenes are created by changes that affect regulation and suffer
changes that vastly increase the amount of the protein formed, although
the protein itself is not changed.
2) Myc protein is a transcription factor.
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The Myc oncogene – overproduction of protein
3) A Myc protein overdose can occur in two ways.
4) Some myc-dependent cancers result from chromosomal changes in
which the myc gene is duplicated many times.
- The Myc protein will be overproduced.
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The Myc oncogene – overproduction of protein
5) It is possible to have the
standard two copies of the
myc gene but alter their
regulation.
- When myc structural gene
is translocated into another
regulatory region, the Myc
protein is produced
continuously in substantial
amounts instead of being
strictly regulated as before.
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Normal role of proto-oncogenes
1) The pathway for activating cell growth and division has several stages.
2) The proto-oncogenes encode the proteins taking part in message
transmission.
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Normal role of proto-oncogenes
3) Signal transmission proteins: These pass on the signal from outside the
cell to enzymes or genes needed in cell division (eg, protein kinases)
– Protein kinases: an enzyme that switches other enzymes in an
inactive form on or off by attaching a phosphate group to them
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Tumor suppressor genes or anti-oncogenes
1) Tumor suppressor genes normally suppress uncontrolled cell division, so
it is necessary to inactivate both copies in order to initiate cancerous
growth.
– Null mutation: a mutation that fully inactivates a gene
– Nullizygous: when both copies of a gene are fully inactivated
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Tumor suppressor genes or anti-oncogenes
2) There are two ways to end up with both copies of a gene inactivated.
① During division of the cells which form the body, two successive
somatic mutations may occur.
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Tumor suppressor genes or anti-oncogenes
② You inherit one defective gene and acquire a second by mutation.
– The first mutation occurs in one copy of the gene in a germ line cell
of one of your ancestors.
– A somatic mutation which inactivates the second copy may then
occur as your cells divide.
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Tumor suppressor genes or anti-oncogenes
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How do anti-oncogenes work?
1) They encode proteins whose job is to inhibit growth or prevent cell
division.
2) Originally it was thought that chemical signals to start growing all came
from outside the cell (hormones).
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How do anti-oncogenes work?
3) More recently, in addition to hormone control, many somatic cell lines
are also pre-programmed.
4) Some anti-oncogenes are part of this system and when they are
defective the cell fails to stop dividing.
5) Many of the proteins encoded by anti-oncogenes are DNA-binding
proteins with zinc fingers.
– Zinc-finger: finger-like bulge on a protein that binds and read a
short DNA sequence (three bases per each zinc finger) in the
regulatory region of the genes
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Normal cell division: the cell cycle
1) To understand further how antioncogenes work, we need to
consider the process of normal
cell division.
2) Cell cycle: the sequence of events
required for growth and division
of a cell
– G1 phase: cell growth
– S phase: duplication of
chromosomes
– G2 phase: preparation for
division
– M phase: cell division or
mitosis
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Normal cell division: the cell cycle
3) To move from one stage to
another it requires the permission
of proteins called cyclins, one for
each major stage.
– Cyclin
• protein that controls the
cell cycle
• The cyclins have
subordinates, the cyclin
dependent kinases, or CDK
protein.
• Cyclin dependent kinase
(CDK): subordinate proteins
that transmit orders from a
cyclin by adding
phosphate groups to the
enzymes they control
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The p53 anti-oncogene
1) p53 gene
– A notorious anti-oncogene, often mutated in cancer cells
– Involved in a very large number of diverse cancers
2) The reason why p53 is so often involved is that its behavior differs from
that of the standard anti-oncogene.
3) A single defective p53 allele does show effects unlike the typical antioncogene.
4) Why is this??
– The protein encoded by the p53 gene assembles in groups of four
(tetramers).
– When a cell has one bad copy, it will produce a mixture of good and
bad p53 proteins.
– These will assemble into mixed tetramers and the bad ones will mess
up the whole assembly.  loose activity
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The p53 anti-oncogene
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The p53 anti-oncogene
5) If a cell’s DNA is damaged in any way, the p53 protein activates the gene
for p21.
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The p53 anti-oncogene
6) The p21 protein then blocks the action of all of the cyclins and freezes
the cell wherever it is in the cell cycle until the damage can be repaired.
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The p53 anti-oncogene
7) p53 protein is not necessary for normal cell division.
8) The role of p53 is to shut down cell division in emergencies, when, for
example, the DNA is damaged.
9) Defective p53 protein cannot stop cell division.
10)Over half of all human cancer are defective in p53.
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Formation of a tumor
1) The actual generation of a real
tumor requires several steps.
2) Cancer development goes
through a series of four or
more mutational stages before
a full blown tumor result.
3) Many colon cancers carry
① Inactivation of the APC
anti-oncogene
② Activation of the ras
oncogene
③ Inactivation of the DCC
anti-oncogene
④ Mutational loss of the p53
gene
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Formation of a tumor
4) Metastasis: spreading of cancer cells from their original site to form new
secondary cancers
5) Other mutations, that are not fully understood, are necessary for cancer
cells to start traveling.
6) These include mutations which result in
– Loss of adhesion to neighboring cells in the home tumor
– Ability to bind to and penetrate the membranes surrounding other
tissues of the body
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Inherited susceptibility to cancer
1) Many of the genes involved in inherited cancer are poorly understood,
but we should briefly mention three general categories.
① It is possible to inherit one dud copy of an anti-oncogene (eg, Rb
gene)
• This means that every one of your somatic cells starts life with
one faulty copy and only a single somatic mutation is needed to
completely inactivate the pair of anti-oncogenes.
• Homozygous recessives for anti-oncogenes are generally lethal
when inherited.
② Mutations in certain special genes affect the rate at which mutations
occur during cell division.
• Mutator gene: a gene that will cause an increased rate of
mutation if it is mutated
③ There are indirect effects due to genetic differences between races
or population (eg, skin cancer).
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Cancer-causing viruses
1) Very few human cancers are due to retroviruses.
2) The first cancer-causing retroviruses to be discovered was Rous Sarcoma
Virus.
– Rous Sarcoma Virus (RSV): a retrovirus that causes cancer in chickens
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Cancer-causing viruses
3) In fact on about 15 percent of
human cancers are due to viruses
and most of these are due to
DNA viruses.
– Papillomavirus: family of DNAcontaining viruses that
sometimes cause tumor
– Herpesvirus: family of DNAcontaining viruses causing a
variety of diseases and
sometimes tumors
4) Only rarely do they cause
dangerous tumor.
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Aggressive gene therapy for cancer: today and tomorrow
1) Two plans of attack may be used, direct and indirect.
① Direct plan: a gene that helps killing cancer tissue is used.
• Tumor necrosis factor (TNF): short protein that kills cancer cells
• Tumor-infiltrating lymphocyte (TIL): white blood cell which secretes
TNF
• TNF is quite often effective at snuffing out small cancers.
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Aggressive gene therapy for cancer: today and tomorrow
• To attack a large
cancer that is out of
control, we can try to
hype up the TNF
system.
i) Clone the TNF gene.
ii) Introduce extra copies
of the TNF gene or
improved TNF gene into
the white blood cells.
iii) Inject them back into
the patient.
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Aggressive gene therapy for cancer: today and tomorrow
② Indirect approach
• It is to rely on the body’s natural defenses which are really quite
effective at killing cancers, provided they find them in time.
• We insert into the tumor cells a gene that betrays the tumor as
an alien intruder to be eliminated.
• Then immune system kills the tumor.
• HLA gene is an example.
 Human leukocyte antigens (HLA): proteins found on the cell
surface that allow cells to be recognized by the immune
system
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Aggressive gene therapy for cancer: today and tomorrow
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Getting genes in by lipofection
1) In lipofection, hollow microscopic spheres of fat (liposomes) are filled
with DNA.
2) These tiny fat globules will merge with the membranes surrounding most
normal animal cells and whatever was inside the liposome will end up
inside the cell.
3) However, it is non-specific as the liposomes tend to glop on to any cells.
4) Nonetheless, “armed” liposomes can be injected directly into tumor
tissue (drug delivery).
5) They are probably of
more use in delivering
proteins, something not
feasible when using
viruses as genetic
engineering vectors.
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Immortality and the philosophers’ stone: Aging
1) The ability to repair DNA declines with the passing years.
2) Cells that get several mutations in growth regulating genes will give rise
to tumors.
3) If declining of the ability to repair DNA could be somehow remedied,
perhaps the number of mutations could be reduced and tumor
formation could be put off a few years longer.
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“The end is nigh” (again): Telomeres
1) Another link between aging and cancer concerns telomeres.
2) For a cell to become cancerous it must regain the ability to divide.
3) This means that the gene encoding telomerase must be reactivated by
one or other of the mutations responsible for causing cancer.
4) The way to inhibit telomerase
- Antisense RNA: an RNA molecule which is complementary in
sequence to and so will base pair with, a target RNA molecule with
some functional role in the cell
- Antisense RNA should bind to and block the telomerase RNA.
5) Whether telomerase inhibitors will prove useful in treating cancer is for
the future to decide.
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“The end is nigh” (again): Telomeres
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Older and wiser worms
1) Science has managed to lengthen the life of certain primitive worms.
2) daf-2 gene
– gene involved in life length determination in nematode worms
– When defective, this lowers the metabolic rate.
3) A lower metabolic rate results in less oxidative damage, especially in the
mitochondria.
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Programmed cell death
1) In the case of a frog, the cells that are no longer needed kill themselves.
– Apoptosis: programmed suicide of unwanted cells for the good of
the whole animal
2) Necrosis
– unplanned death of cells as the result of injury
– It causes inflammation that attracts cells of the immune system to
the trouble spot.
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Programmed cell death
3) Apoptosis is still important for us.
4) For example, it prevents human from being born with the webbed
fingers found in early fetuses.
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Death is only a phone call away
1) Even more amazing is that many animal cells need a constant signal to
stay alive.
2) Once they no longer receive permission to stay alive, they commit
suicide.
3) A cell’s fate hangs in the balance, depending on the function of just a
single gene (eg, ced genes).
– ced genes: genes which control apoptosis in nematode worms
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Death is only a phone call away
4) Humans are more complex than worms and rather than just a single
gene that decides whether our cell live or die, we may have a sort of
molecular jury system which takes account of a variety of factors.
– bcl-2 gene: a gene that probably controls apoptosis in humans and
may contribute to cancer when mutated and resembles the ced-9
gene in some ways
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Mitochondria are maternal, not eternal
1) We do not yet understand the aging process.
2) About 75 percent of human mitochondrial DNA encodes proteins
involved in energy generation.
3) Mutation accumulate over the years in mitochondrial DNA as in the
chromosomal DNA.
4) Gradual decrease in the energy generation, less active and alert: aging
5) Some people age faster than others. It is due to their starting life with
some of their mitochondria already mutated.
6) Mitochondria are inherited only from your mother.
7) Your life expectancy depends mostly on your mother.
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Mitochondria are maternal, not eternal
8) Mutations to mitochondria do not cause cancer since mitochondria are
not in charge of their own multiplication.
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