Download LESSON 3.2 WORKBOOK How do normal cells become

LESSON 3.2 WORKBOOK
How do normal cells become
cancer cells?
DEFINITIONS OF TERMS
For a complete list of defined
terms, see the Glossary.
The key factor that determines whether a normal cell will become a tumor is the
kind of mutations it acquires. Most mutations do not affect cell function, and those
that do usually lead to cell death. However, a minority of mutations can both affect
cell function and allow the cell to live. Of those mutations, only a minority leads
to cancer. In this lesson, we will explore how cells acquire random mutations and
how carcinogens increase the chance that very rare cancer-causing mutations will
accumulate.
DNA mutations and tumor formation
In Unit 2, we learned how critical it is for cells in tissues to work together as a community, and for tissue
communities to work together to regulate organ function. Tumor formation occurs when cells lose their
ability to participate as members of a tissue community, and this may eventually disrupt organ function too.
The key change in cellular behavior that precipitates this loss of function occurs when cells accumulate
mutations in their DNA that modify critical proteins. This change in a normal cell’s DNA so that the cell
forms a tumor is called transformation.
Transformation – the process
by which a cell acquires characteristics of a tumor cell.
DNA acquires random mutations for a variety of reasons, but the most common include:
■■ DNA polymerase errors — the DNA polymerase enzyme duplicates DNA during S phase as the
cell prepares for mitosis. DNA polymerase makes a mistake once in every 10 million bases it copies.
■■ Mistakes during mitosis — for mitosis to occur cleanly chromosomes must be divided equally
between the daughter cells. This does not always occur.
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Lesson 3.2
■■ Effects of environmental agents — environmental agents called mutagens can modify the chemical structure of the DNA bases themselves, or promote errors DNA polymerase and/or mitosis. Most
mutagens are also carcinogens, but not every carcinogen is a mutagen.
MC Questions:
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1. Transformation describes which of
the following types of cell?
aa. A cell that has acquired
mutations
bb. A cell acquiring spreading traits
cc. A cell that can replicate
indefinitely
dd. A cell entering the blood stream
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LESSON READINGS
DEFINITIONS OF TERMS
Mutagen – any chemical or
agent that is capable of mutating
DNA sequence.
Somatic cell – any cell that
forms the body of an organism
that is not a germ cell.
Germline mutation – any
detectable mutation or variation
of DNA present within germ cells
that is inherited by offspring of
that individual.
BRCA1– a tumor suppressor
gene involved in DNA repair,
whose mutated form is associated with breast and ovarian
cancer as well as other cancers.
BRCA2 – another tumor suppressor gene involved in DNA
repair, whose mutated form is associated with breast and ovarian
cancer as well as other cancers.
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Lesson 3.2
As we know, there are two types of cell in the body: Germ cells (eggs and sperm) and somatic cells.
Germ cells contain one copy of the genome on 23 chromosomes, while somatic cells contain two copies
of the genome on 23 pairs of
chromosomes. When germ
cells fuse they form a zygote
with two copies the genome
on 23 pairs of chromosomes.
Whatever mutations the
germ cells have acquired will
therefore be inherited by the
zygote, so that each somatic
cell in the offspring will also
Figure 1: Pictures of normal epithelial cells of the mammary
have the mutation. Inheritable
duct (left) compared to transformed epithelial cells of the
mutations like this are called
mammary duct (right). Normal cells are generally more strucgermline mutations. If the
tured and ordered within the tissue compared to transformed
cells.
germline mutations have the
potential to cause the cell to
transform into a tumor they can genetically predispose individuals that have them to develop cancer.
One example of a germline mutation that plays a critical role in predisposition to a number of different
cancers occurs to the tumor suppressor protein BRCA1 that is involved in DNA repair Mutated forms
of BRCA1, which stands for ‘BReast CAncer susceptibility protein’, have been conclusively linked to
predisposition to developing breast cancer. When mutations to BRCA1 are seen together with mutations
to another tumor suppressor protein, BRCA2 the likelihood of developing breast cancer by the age of 70
increases to 50-65% while the likelihood of developing ovarian cancer increased to 35-46%. Mutations
to BRCA2 alone increase the likelihood of developing breast cancer to 40-57% and ovarian cancer to
13-23%. Mutations in BRCA1/2 are also risk factors for colon, prostate, and pancreatic cancer.
Unlike germline mutations that are found in eggs and sperm and therefore inherited by every zygote
produced when the germ cells fuse, somatic mutations are found in somatic, not germ cells. As a result
they will only affect the individual who acquired the mutation, but will not be inherited by their offspring.
Somatic mutations also only affect the cell that acquired that mutation, and no other cell in the body.
Somatic mutations that lead to tumor formation and hence cancer are therefore found only in those tumor/
cancer cells and not throughout the body.
MC Questions:
2. Which of the following can lead
to cell transformation? (Circle all
correct.)
aa. DNA polymerase errors.
bb. Mistakes of mitosis.
cc. Exposure to mutagens.
dd. Exposure to carcinogens.
3. Which of the following is true of
BRCA1? (Circle all correct.)
aa. It is a tumor suppressor gene.
bb. It is mostly active in breast and
ovarian cancers.
cc. Mutations in the gene cannot be
passed to offspring.
dd. Mutations in the gene increase
cancer risk.
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LESSON READINGS
Types of DNA mutations
DEFINITIONS OF TERMS
Somatic mutation – a change
in DNA sequence of a cell that is
not inherited by the offspring.
Neutral mutations – mutations
that do not affect the ability of a
cell to function or survive. These
include any mutation in a noncoding sequence, or synonymous
DNA mutations that do not affect
protein sequence.
Synonymous mutations –
mutations within a protein coding
sequence that do not affect the
amino acid sequence.
Nonsynonymous mutations –
mutations within a protein coding
sequence that alter the amino
acid sequence.
Wo r k b o o k
Lesson 3.2
Mutations occurring during DNA replication and mitosis
Mutations caused by the errors that occur during DNA replication or mitosis are limited to cells that are
actively dividing. Most cells that are terminally differentiated are no longer dividing, and are therefore
protected from these kinds of errors. However, stem cells and progenitor cells that are still dividing are
capable of developing mutations each time DNA
is replicated.
Normally, DNA polymerase is very accurate and
the DNA repair proteins are vigilant to identify
mistakes of DNA replication. As a result, the
normal mutation rate is only approximately 175
mutations total per duplication of the genome.
Most often, cells that accumulate mutations will
die, because they prevent the cell from being
a productive member of the cell community.
Most of the remaining mutations will have no
observable effect on the cell. These neutral
Figure 2: A gene fusion is formed when
mutations typically occur in sequences that do
the ends of chromosome 9 and chromosome
not encode proteins, or, if they do occur in coding
22 recombine, forming a longer chromosome
sequences do not alter the protein sequence.
9 and shorter chromosome 22. This forms
These so-called synonymous mutations are
the bcr-abl gene fusion, which is a hyperactive
possible because each amino acid has more
form of two proto-oncogenes.
than one codon. Most mutations a cell acquires
will be neutral: Only 2% of the genome encodes
proteins, so the chances of a mutation changing amino acid sequence (so-called nonsynonymous
mutations) and affecting cell behavior are very low. Moreover, the types of acceptable errors in DNA
replication is very limited. Hence development of mutations In a cell is a very slow and rare process.
However, if a random mutation compromises cell cycle control mechanisms (e.g. DNA repair proteins,
Rb, or p53), then the cell will begin to divide rapidly, thereby increasing the number of mutations that the
cell can accumulate. In fact, the mutations accumulated in tumors can increase by as much as 100,000
mutations per cell compared to normal cells.
Errors in mitosis are also relatively rare in normal cells, but these also will increase if cell cycle control
mechanisms are damaged. These errors of mitosis may lead to chromosomes inappropriately mixing
together, a process called chromosome recombination. When chromosomes recombine, they may
MC Questions:
4. Which of the following is the MAIN
difference between germline and
somatic mutations?
aa. Germline mutations cannot
affect gene expression.
bb. Germline mutations do not
cause transformation.
cc. Somatic mutations cannot be
inherited.
dd. Somatic mutations cannot alter
sequence of tumor suppressor
genes.
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5. True or False: Most mutations that
occur in a cell cause cell death.
aa. True.
bb. False.
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LESSON READINGS
DEFINITIONS OF TERMS
Chromosome recombination
– the process by which portions
of different chromosomes are
mixed together.
Gene fusion – a mutant gene
that is formed by two genes that
were previously separate were
joined through chromosome
recombination.
Point mutation – a change
in a single nucleotide of a DNA
sequence.
Single nucleotide
polymorphism (SNP) – a type
of mutation where only a single
nucleotide is altered – either
inserted, deleted, or substituted.
Wo r k b o o k
Lesson 3.2
create new types of genes by merging portions of
two genes together into one gene. A notable example
of this process, called a gene fusion, involves
chromosomes 9 and chromosome 22 swapping
portions of their chromosomes, producing a fusion of
two proto-oncogenes bcr and abl. This fusion (bcr-abl)
is responsible for causing a specific type of leukemia, as
well as being involved in the development of many other
types of cancer.
Mutations caused by agents in the environment
While mutations caused by DNA replication and mitosis
are limited to dividing cells, environmental mutagens can
cause mutations in any cells that are exposed to them
(i.e. both dividing and terminally differentiated cells).
While both replicating and non-replicating (terminally
Figure 3: Ultimate carcinogens
can chemically bind to DNA bases.
differentiated) cells are susceptible, replicating cells are
This affects how DNA polymerase
particularly vulnerable since environmental mutagens
detects the sequence, and leads to
may also promote errors of DNA replication and mitosis.
mutation of DNA.
Most exposed surfaces of our body are composed of
terminally differentiated epithelial cells; if these cells
are mutated by mutagens, they are easily killed and replaced by the epithelial progenitor and stem cells
buried just beneath them deeper in the tissue. Stem and progenitor cells are harder to replace if they
are mutated, so mutagens that can penetrate deeper into the tissue can have more profound effects in
causing cancer.
Perhaps the most common chemical mutations are stimulated by the reactive oxygen species (ROS)
that are produced when oxygen is metabolized in cells. When DNA bases such as guanidine (G) are
exposed to ROS they undergo a chemical reaction called oxidation, which produces 8-oxo-guanine. DNA
repair proteins incorrectly identify this base as a thymidine nucleotide and convert the G to a T. This type
of mutation in one nucleotide is called a point mutation. Another name for a point mutation is a single
nucleotide polymorphism (SNP). SNPs are any single base mutation such as when a single base is
added, removed, or substituted in a DNA sequence. UV radiation causes SNPs by chemically linking
thymidines together. DNA repair proteins then replace these thymidines with an adenine nucleotide.
Carcinogens, such as those found in tobacco, can bind DNA and cause damage (see Figure 3). Modification of DNA sequences by carcinogens leads to errors in DNA repair, which may make the cell more
susceptible to more mutations, particularly if that mutation occurs in a DNA repair protein.
MC Questions:
6. If random mutations are rare, why
do cancer cells typically have
thousands of mutations?
aa. Cancer cells are exposed to
more carcinogens.
bb. Epithelial cells have less efficient
DNA repair proteins.
cc. Most mutations are neutral
mutations.
dd. One key mutation leads
to accumulation of many
mutations.
7. How does carcinogen exposure
lead to DNA mutations? (Circle all
correct.)
aa. DNA repair proteins cannot
repair chemically altered
nucleotides.
bb. DNA polymerase cannot
recognize chemically altered
nucleotides.
cc. Carcinogens chemically modify
nucleotides.
dd. Carcinogens bind to DNA
polymerase.
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LESSON READINGS
Effects of DNA mutations on gene function
MC Questions:
In many cases, the types of mutations a cell must acquire to become transformed are very specific and
very rare. The random DNA mutations due to carcinogens or errors of DNA replication or mitosis are
rarely preserved in a cell, and the affected cells are usually killed through apoptosis
8. Which of the following mistakes are
caused by carcinogens? (Circle all
correct.)
aa. Gene fusions.
bb. Germline mutations.
cc. Point mutations.
dd. Somatic mutations.
Firstly, the mutations must only occur in a subset of genes and must be just the right types of mutations.
In previous lessons we have discussed how mutations of proto-oncogenes to form oncogenes and of
tumor suppressor are necessary for cellular transformation to occur. Furthermore, they must be just the
right type of mutations – i.e. the mutations must hyperactivate proto-oncogenes and inactivate the tumor
suppressor genes. Mutations that inactivate proto-oncogenes or hyperactivate tumor suppressor genes
will not lead to cancer, and will most likely lead to cell death.
However, if the cell has acquired just the right combination of mutations in tumor suppressor genes and/
or proto-oncogenes, it may be able to avoid death by apoptosis. Accumulation of DNA mutations that
cause a normal cell to become a tumor is just the first step of many physiological changes a transformed
tumor cell has to make on its path to becoming a cancer cell. One relevant DNA mutation is not enough
to cause cell transformation, rather it has been estimated that a minimum of 3-6 key DNA mutations are
necessary. It is important to realize that while all these mutations are required, cells do not all acquire
them in the same order, and differences between the order in which key mutations are acquired can lead
to key differences between individual tumors and cancers, such as how quickly they can spread from the
primary site to different tissues.
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Lesson 3.2
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9. Which of the following is a type
of mutation that will promote
transformation? (Circle all correct.)
aa. Hyperactivation of protooncogene.
bb. Inactivation of proto-oncogene.
cc. Hyperactivation of tumor
suppressor gene.
dd. Inactivation of tumor suppressor
gene.
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STUDENT RESPONSES
Describe 2-3 types of DNA mutations and explain why most DNA mutations result in death of the cell. _____________________________________________________________________________________________________
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TERMS
TERM
DEFINITIONS OF TERMS
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 3.2
DEFINITION
BRCA1
A tumor suppressor gene involved in DNA repair, whose mutated form is associated with breast and ovarian
cancer as well as other cancers.
BRCA2
Another tumor suppressor gene involved in DNA repair, whose mutated form is associated with breast and
ovarian cancer as well as other cancers.
Chromosome
recombination
The process by which portions of chromosomes are mixed together forming variants of sequence within
each chromosome.
Gene fusion
A mutant gene that is formed by two genes that were previously separate were joined through chromosome
recombination.
Germline mutation
Any detectable mutation or variation of DNA present within germ cells that is inherited by offspring of that
individual.
Mutagen
Any chemical or agent that is capable of mutating DNA sequence.
Neutral mutations
Mutations that do not affect the ability of a cell to function or survive.
Nonsynonymous
mutations
Mutations within a protein coding sequence that alter the amino acid sequence.
Point mutation
A change in a single nucleotide of a DNA sequence.
Single nucleotide polymorphism (SNP)
A type of mutation where only a single nucleotide is altered – either inserted, deleted, or substituted.
Somatic cell
Any cell that forms the body of an organism that is not a germ cell.
Somatic mutation
A change in DNA sequence of a cell that is not inherited by the offspring.
Synonymous mutations
Mutations within a protein coding sequence that do not affect the amino acid sequence.
Transformation
The process by which a cell acquires characteristic of a cancer cell.
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