Download MUTATION, DNA REPAIR AND CANCER

MUTATION, DNA REPAIR AND CANCER
1
Mutation
A heritable change in the genetic material
 Essential to the continuity of life

 Source
of variation for natural selection
New mutations are more likely to be
harmful than beneficial
 DNA repair systems reverse DNA damage
 Cancer is a disease caused by gene
mutations

2
Gene mutations alter the DNA sequence


Point mutation affects only a single base
pair
2 basic alterations
1.
2.
Change base sequence
Add or remove nucleotide(s)
3
Examples

Change base sequence
 Base
substitution
5’ – CCCGCTAGATA – 3’
→
3’ – GGGCGATCTAT – 5’

5’ – CCCGCGAGATA – 3’
3’ – GGGCGCTCTAT – 5’
Add or remove a single base pair
5’ – GGCGCTAGATC – 3’
3’ – CCGCGATCTAG – 5’
→
5’ – GGCAGCTAGATC – 3’
3’ – CCGTCGATCTAG – 5’
4
Gene mutations may affect amino acid sequences

Silent or neutral mutations
 Do
not alter the amino acid sequence
 Genetic code is degenerate

Missense mutation
 Changes
a single amino acid in a polypeptide
 May be neutral if substituted amino acid
similar in chemistry to original
 Sickle-cell anemia is the result of a single
amino acid substitution
5
6

Nonsense mutation
 Change
from a normal codon to a stop or
termination codon
 Produces a truncated polypeptide

Frameshift mutation
 Addition
or deletion of nucleotides that are not
multiples of 3
 Completely different amino acid sequence
downstream from mutation
7
Gene mutations outside of coding sequences

Promoter
 Up
promoter mutations or down promoter
mutations

Transcriptional response element/
operator site
 May

alter regulation of transcription
Splice junctions
 Mutations
at the boundaries between introns
and exons can prevent proper splicing

Translational response elements
 May
prevent proper translational regulation
8
The Lederbergs used replica plating to show that
mutations are random events

How do mutations occur?
 Result
of pre-existing circumstances?
 Unplanned random events?
Joshua and Ester Lederberg tested
opposing views in bacteria using replica
plating
 Only rare cells that had mutated survived
selection
 Supported idea that mutations are random
events

Germ-line or somatic cell mutations
Exact time and location of a mutation is
critical to severity of effect and to the
ability to pass on the mutation
 Germ-line cells give rise to gametes

 Can
occur in a sperm or egg cell or in cell that
gives rise to eggs and sperm

Somatic cells are all other body cells
 Can

occur early or late in development
Genetic mosaic results from patches of mutated
tissue
11
12
Spontaneous or induced mutations
Spontaneous mutations result from
abnormalities in biological processes
 Rates vary from species to species and
from gene to gene
 Expected rate of background mutation
approximately 1 mutation for every 1
million genes

13
14
Induced mutations are brought on by
environmental agents
 Mutation rate higher than spontaneous
mutation rate
 Mutagens are chemical or physical agents

 Can
alter DNA in various ways
15
16
Mutagens

Disrupt pairing by…
 Modifying

nucleotide structure
Nitrous acid deaminates bases so that modified
bases do not pair with appropriate nucleotide
 Using
base analogue similar to particular
bases
 Alkylating bases

Interfering with replication
 Acridine
dyes insert between bases and
distort the helix
17
18

Ionizing radiation has high energy and can
penetrate deeply into biological materials
to create free radicals
X
rays and gamma rays
 Base deletions, breaks in 1 or both DNA
strands

Nonionizing radiation has less energy and
can only penetrate the surface
 UV
rays can cause formation of thymine
dimer causing gaps or incorporation of
incorrect bases
19
20
Ames test

Uses Salmonella typhimurium that cannot
synthesize histidine due to a point
mutation
 Bacteria
cannot grow unless histidine added
to medium
 OR mutation occurs allowing synthesis of
histidine

Test monitors rate at which second
mutation occurs
21
22
DNA repair
All living organisms must have the ability
to repair changes that occur in DNA in
order to minimize mutation
 Requires…

 DNA
damage be detected
 Repair of DNA damage
23
Types of repair

Direct repair
 Enzyme
removes a modification (for example
an alkyl group)

Altered DNA strand removed and new
segment synthesized
 More
common
 Nucleotide excision repair
 Methyl-directed mismatch repair
24
Nucleotide Excision Repair (NER)
Most common DNA repair system
 Region encompassing several nucleotides
in the damaged strand is removed from
the DNA
 Found in all eukaryotes. Intact
undamaged strand is used as a template
for resynthesis of a normal complementary
strand

25
26
Methyl-directed mismatch repair system
Base mismatch is an abnormality where
the AT/GC rule is not obeyed
 DNA polymerase may make a mistake

 May
correct itself with proofreading ability
Methyl-directed mismatch repair systems
exist in all species
 Detect mismatch and specifically remove
segment from newly made strand
 Specifically repairs the new strand rather
than the parental template strand

27
28
Cancer
Disease of multicellular organisms that is
characterized by uncontrolled cell division
 Over 1 million Americans are diagnosed
with cancer each year, and about half that
number will die from the disease
 In about 10% of cancers, a higher
predisposition to develop the disease is an
inherited trait
 Most cancers, about 90%, do not involve
genetic changes that are passed from
parent to offspring

29
Carcinogens
About 80% of all human cancers are
related to exposure to carcinogens
 Agents that increase the likelihood of
developing cancer
 Most carcinogens, such as UV light and
certain chemicals in cigarette smoke, are
mutagens that promote genetic changes in
somatic cells
 DNA alterations can lead to effects on
gene expression that ultimately affect cell
division, and thereby lead to cancer

30
Cancers originate from a single cell
 Mutates so that cell grows abnormally
 Tumor- an overgrowth of cells with no
useful purpose
 Tumor may begin as a benign or precancerous

31

Cancerous stage
 Malignant
- lost normal growth regulation
 Invasive- can invade healthy tissue
 Metastatic- can migrate to other parts of the
body

Left untreated, malignant cells will cause
the death of the organism
32
33
Cancer-causing genes

Oncogene
 When
a mutation causes this gene to be
overactive, uncontrolled cell growth occurs.

Tumor-suppressor gene
 Normally,
this gene encodes a protein that
prevents cancer
 If a mutation eliminates this function, cancer
can occur
34
Oncogenes




Cell division regulated by hormones called growth
factors
Bind to cell surface and initiate cascade leading to cell
division which includes activating specific genes
Mutations in genes producing cell growth signaling
proteins can change them into oncogenes producing
abnormally high level of activity in some proteins
An oncogene may promote cancer by keeping the cell
division signaling pathway in a permanent “on” position
In some cancers the amount of gene product is abnormally high
 In others the gene produces a functionally hyperactive protein

35
36
Proto-oncogene


1.
2.
3.
4.
Normal gene that, if mutated, can
become an oncogene
4 common genetic changes
Missense mutations
Gene amplifications
Chromosomal translocations
Retroviral insertions
37
Missense mutations
 Chemical mutagens have been shown to
cause missense mutations leading to cancer

38
39

Gene
amplifications


Abnormal
increase in copy
number results in
too much of the
encoded protein
Many human
cancers are
associated with
the amplification
of particular protooncogenes
40

Chromosomal
translocations
 Two
different
chromosomes break,
and the ends of the
broken chromosomes
fuse with each other
incorrectly
 Very specific types of
chromosomal
translocations have been
identified in certain types
of tumors
 Chimeric genes are
composed of two gene
fragments fused together
41
42

Retroviral insertions
 Viral DNA may insert into
a host chromosome in
such a way that a viral
promoter and response
elements are next to a
proto-oncogene
 May result in the
overexpression of the
proto-oncogene,
thereby promoting
cancer
 Alternatively, a virus may
cause cancer because it
carries an oncogene in the
viral genome
43
Majority of cancers are caused by
mutagens
 A few viruses are known to cause cancer
in plants, animals, and humans
 Some viruses may cause cancer by
modifying genes in the host cell
 Others carry oncogenes in the viral
genome

44
Tumor-suppressor genes


Role to prevent cancerous growth
Proteins encoded by tumor-suppressor genes
usually have one of two functions
1.
Proteins that maintain the integrity of the genome by
monitoring and/or repairing alterations in the
genome

2.
Checkpoint proteins check the integrity of the genome and
prevent a cell from progressing past a certain point in the
cell cycle
Proteins that are negative regulators or inhibitors of
cell division

Their function is necessary to properly halt cell division
otherwise cell division is abnormally accelerated
45
Checkpoint proteins
Proteins called cyclins and cyclindependent protein kinases (cdks) are
responsible for advancing a cell through
the four phases of the cell cycle
 Formation of activated cyclin/cdk
complexes can be stopped by checkpoint
proteins
 p53 - about 50% of all human cancers are
associated with defects in this gene

46




p53 is a G1
checkpoint protein
Expression of the p53
gene is induced when
DNA is damaged
Then, a cell cannot
progress from G1 to
the S, or synthesis,
phase of the cell cycle
If DNA is repaired, a
cell may later proceed
through the cell cycle
47
Alternatively, if the DNA damage is too
severe, the p53 protein will also activate
other genes that promote programmed cell
death or apoptosis
 Caspases function as proteases that
digest selected cellular proteins causing
the cell to break down
 It is beneficial for a multicellular organism
to kill an occasional cell with cancer
causing potential

48
When checkpoint genes are rendered
inactive by mutation, the division of normal
healthy cells may not be adversely
affected
 For example, mice that are missing the
p53 gene are born healthy

 Cell
division leading to normal growth is
regulated properly
 Checkpoint proteins such as p53 are not
necessary for normal cell growth and division

However, these mice are very sensitive to
mutagens and easily develop cancer
 Loss
of checkpoint protein function makes it
more likely that undesirable genetic changes
will occur that could cause cancerous growth 49
Rb gene
Second example of a tumor-suppressor
gene, which is a negative regulator of cell
division
 First tumor-suppressor gene to be
identified in humans by studying patients
with a disease called retinoblastoma
 Some people have an inherited form that
occurs early
 Other forms caused by environmental
agents occur later in life

50
“Two-hit” model for retinoblastoma



According to this idea, retinoblastoma requires
two mutations to occur
People have two copies of the Rb gene, one
from each parent
Individuals with the inherited form of the disease
have one mutant gene from one of their parents
 They
need only one additional mutation to develop
the disease
 Likely to occur early in life

People with the noninherited form of the disease
must have two mutations in the same retinal cell
to cause the disease
 Two
rare events are far less likely than a single event,
so the noninherited form of this disease is expected to
51
occur much later in life, and only rarely



Rb protein negatively controls a regulatory transcription
factor called E2F that activates genes required for cell
cycle progression from G1 to S phase
Binding of the Rb protein to E2F inhibits its activity and
prevents cell division
When both copies of Rb are defective, the E2F protein is
always active resulting in uncontrolled cell division
52
Loss of tumor-suppressor gene function

3 common ways
 Mutation
occurs specifically within a tumorsuppressor gene to inactivate its function
 Chromosome loss may contribute if the
missing chromosome carries one or more
tumor-suppressor genes
 Abnormal methylation of CpG islands near
promoter regions
53
Cancer is a series of changes
Cancer usually requires multiple genetic
changes to the same cell
 Begin with a benign genetic alteration that,
over time and with additional mutations,
leads to malignancy
 Malignancy can continue to accumulate
genetic changes that make it even more
difficult to treat

54
Mutations in Approximately 300 Human
Genes May Promote Cancer



Not all of these mutant genes found in cancers
have been directly shown to affect the growth rate
of cells
But such mutations are likely to be found in tumors
because they provide some type of growth
advantage for the cell population from which the
cancer developed
Over 1% of our genes have the potential to
promote cancer if their function is altered by a
mutation
Lung cancer





Diagnosed in approximately 170,000 men and
women each year in the U.S.
Worldwide, more than 1.2 million cases are
diagnosed
Nearly 90% of these cases are caused by
smoking and are thus preventable
Unlike other cancers for which early diagnosis is
possible, lung cancer is usually detected only
after it has become advanced and is difficult, if
not impossible, to cure
The five-year survival rate for lung cancer
patients is less than 15%
56

Most are carcinomas




Cancers of epithelial cells
Mutations accumulate in basal cells and their numbers
increase - hyperplasia
As additional mutations accumulate, the basal cells
develop more abnormal morphologies - dysplasia
In the early stages, the abnormal basal cells are
precancerous.
If the source of chronic irritation (usually cigarette smoke) is
eliminated, the abnormal cells are likely to disappear
 Alternatively, if smoking continues, these abnormal cells may
accumulate additional genetic changes and lose the ability to
stop dividing
 Such cells have become cancerous—the person has basal cell
carcinoma
Metastasis of these cells to other parts of the body will likely kill the
patient, usually within a year of being diagnosed


57
58