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Human Genetics: concepts and applications
6th edition
Ricki Lewis
Chapter 18
The Genetics of Cancer
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Cancer
Cancer is a group of diseases caused by
loss of cell cycle control.
Cancer is associated with abnormal
uncontrolled cell growth.
Carcinogens are substances which cause
cancer by mutating DNA.
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Many genes that can mutate to cause
cancer control the cell cycle or DNA
maintenance (repair).
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Origin of cancer
Cancer begins from the growth of a single abnormal cell.
• A mutation occurs allowing a cell to undergo cell
division when it would not normally divide.
• Division produces more abnormal cells.
Mutations can occur:
• In somatic cells => sporadic cancer only affecting the
individual
• In germline cells => mutations that are inherited
• Germline mutations usually require second somatic
mutation also.
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Telomeres affect the cell cycle
Telomerase is the protein and enzyme complex that adds
telomere sequences to the ends of chromosomes.
Presence of telomerase and telomeres allows cells to
pass a cell cycle checkpoint and divide.
18-5
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Germline versus sporadic cancer
18-6
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Cancer can progress slowly over years
18-9
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Types of cancer genes
Type of gene
Normal function
Mutated function Types of proteins
Oncogene
Promotes
division
Promotes
Growth factors
division abnormal time or
cell type
Tumor
suppressor
gene
Suppresses cell
division
Fails to suppress Checkpoint
division
molecules
DNA repair
gene
mutation
Repair DNA
mutations
Fail to repair
DNA mutations
18-10
Enzymes for
mismatch or
excision repair
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Characteristics of cancer cells
• Divide continually (given space and nutrients)
• Heritable mutations: cells with mutations have daughter
cells which inherit the same mutations.
• Transplantable
• Dedifferentiated: cells lose their specialized identity
• Different appearance: reflects dedifferentiation
• Lack contact inhibition: will divide in a crowd of cells
and pile on top of each other
• Induce angiogenesis (local blood vessel formation)
• Increased mutation rate
• Invasive: squeeze into any space available
• Metastasize: cells move to new location in the body
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Oncogenes
• Proto-oncogenes are normal versions of genes which
promote cell division.
• Expression at the wrong time or in the wrong cell type
leads to cell division and cancer.
• Proto-oncogenes are called oncogenes in their
mutated form.
• One copy of an oncogenic mutation is sufficient to
promote cell division.
18-12
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Oncogenes:
overexpression of a normal function
• Viruses integrated next to a proto-oncogene can cause
transcription when the virus is transcribed.
• Moving a proto-oncogene to a new location can separate
the coding region from regulatory regions of the gene
leading to incorrect expression.
•
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Moving a proto-oncogene next to a highly transcribed gene
can lead to erroneous transcription of the proto-oncogene.
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Tumor suppressor genes
• Cancer can be caused by loss of genes that
inhibit cell division.
• Tumor suppressor genes normally stop a cell
from dividing.
• Mutations of both copies of a tumor suppressor
gene is usually required to allow cell division.
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p53 coordinates cell cycle regulation
• p53 acts as a cell cycle protein which determines
if a cell has repaired DNA damage. If damage
cannot be repaired, p53 can induce apoptosis.
• More that 50% of human cancers involve an
abnormal p53 gene.
• Rare inherited mutations in the p53 gene cause a
disease called Li-Fraumeni syndrome in which
family members have many different types of
cancer at early ages.
18-18
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Colon cancer results from genetic
alterations in multiple genes
18-22
Inherited mutations in the APC gene
dramatically increase risk of colon cancer
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Environment impacts cancer
Exposure to carcinogens
• Carcinogens in tobacco smoke are correlated
with lung cancer incidence.
Exposure to radiation
• Burns from overexposure to sunlight can cause
skin cancer.
Variation in diet
• Fatty diets are correlated with increased estrogen
and increased breast cancer.
18-23
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Cruciferous vegetables can lower
cancer risk
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