cancer genetics

1. Genetics of cancer
 Cell cycle and cancer
 Two-hit mutation model
 Oncogenes and RNA/DNA tumor viruses
 Tumor suppressor genes
 Mutator genes
 Carcinogens: chemicals and radiation

2. Some basic terminology:
Oncogenesis = process of initiation of tumors (cancer) in an organism
(onkos = mass; genesis = birth)
Tumor = tissue composed of cells that deviate from normal program of
cell division and differentiation.
Benign tumor = tumor cells remain together in a single mass and do not
invade or disrupt surrounding tissues
Malignant tumor = tumor cells invade and disrupt surrounding tissues
(diagnosed as cancer, and such cells can transform other cells to the
cancerous state).
Metastasis = spread of malignant tumor cells throughout the body
(typically through the blood and lymphatic system)

3. How do we define cancer?
Cancer is a group of disorders that causes cells to escape
normal controls on cell division
-cancer cells divide more frequently
-cancer cells are not inhibited by contact with
other cells and can form tumors
-cancer cells can invade other tissues, a
process called metastasis

4. Non-cancerousNon-cancerous
cells form sheets.cells form sheets.
Cancer cellsCancer cells
grow into tumors.grow into tumors.
Cancer cells canCancer cells can
invade other tissues.invade other tissues.

5. Oncogenesis arises from:
1. Spontaneous gene or chromosome mutations.
2. Exposure to mutagens or radiation.
3. Activity of genes introduced by tumor viruses.
4. Some cancers are inherited (individuals may be predisposed).

6. Cell cycle and cancer:
Cell differentiation occurs as cells proliferate to form tissues.
• Cell differentiation correlates with loss of ability to proliferate;
highly specialized cells are terminally differentiated.
• Terminally differentiated cells have a finite life span, and are
replaced with new cells produced from stem cells.
• Stem cells are capable of self-renewal; cells divide without
undergoing terminal differentiation.
Cell death (apoptosis) is equally important.
• Apoptosis is the normal outcome for most cells, and the sequence of
events must be programmed correctly.
• Otherwise cells don’t die when they should, and uncontrolled cell
division can result in cancer.

7. Chemical Signals that Control the Cell Cycle
1. Cyclin and Kinase
-proteins that initiate mitosis
-requires buildup of cyclin to pair with kinase
2. Hormones
-chemical signals from specialized glands
that stimulate mitosis
3. Growth Factors
-chemical factors produced locally that stimulate mitosis

8. Normal cell cycle is controlled by signal transduction:
• Growth factors bind to surface receptors on the cell; transmembrane
proteins relay information to the cell by signal transduction.
• Two types of growth factors:
1. Growth factors stimulate cell division.
2. Growth-inhibiting factors inhibit cell division.
• Healthy cells divide only when growth factor and growth-inhibiting
factor balance favors cell division.
• Cancer cells divide without constraint
• Cancer is primarily caused by mutations in growth and growth-
inhibiting factor genes, and pathways that inhibit the normal
sequence of events associated with apoptosis.

9. Fig. 1 Regulation of cell division by signal transduction.

10. Ras Proto-OncogeneRas Proto-Oncogene
In response to growth factor
binding at receptor, the Ras
gene product combines with
GTP to promote cell division
In cancer cells, the RAS
gene product is locked into
its GTP-binding shape and
does not require a signal at
the receptor in order to
stimulate cell division

11. Two-hit mutation model for cancer:
• Most cancers result from mutations in cellular genes.
(other cancers are caused by viruses)
• Two types of cancer:
• Sporadic more frequent, no hereditary cause
• Familial less frequent, hereditary
• Retinoblastoma (OMIM-180200) shows sporadic and hereditary forms
and fits the pattern of a two-hit model.
• Most common eye tumor of children.
• Occurs from birth to 4 years of age.
• Early treatment with gamma radiation is 90% effective.

12. Two-hit mutation model for retinoblastoma (OMIM-180200):
Sporadic retinoblastoma
• 60% of retinoblastoma cases.
• Develops in children with no family history.
• Occurs in one eye.
Hereditary retinoblastoma
• 40% of retinoblastoma cases.
• Onset typically is earlier than sporadic cases.
• Multiple tumors involving both eyes.
• Consistent pedigrees; siblings and offspring develop the same
type of tumors.

14. Alfred Knudson’s (1971) model for retinoblastoma (OMIM-180200):
• Two mutations are required for the development of retinoblastoma.
Sporadic retinoblastoma
• Child starts with two wild type alleles (RB+/RB+).
• Both alleles must mutate to produce the disease (RB/RB).
• Probability of both mutations occurring in the same cell is low;
only one tumor forms (e.g., one eye).
Hereditary retinoblastoma
• Child starts with heterozygous alleles (RB/RB+).
• Only one mutation is required to produce disease (RB/RB).
• Mutations resulting in loss of heterozygosity (LOH) are more
probable in rapidly dividing cells, and multiple tumors occur
(e.g., both eyes).

15. Fig. 20.8, Knudson’s 2-hit mutation model for retinoblastoma.

16. Alfred Knudson’s (1971) model for retinoblastoma (OMIM-180200):
• Retinoblastoma was mapped to the long arm of chromosome 13
(13q14.1-q14.2).
• Mutations occur in a gene that encodes a growth inhibitory factors (a
tumor suppressor gene).
• Retinoblastoma is rare among cancers; most cancers result from a
series of mutations in many different genes.
• So retinoblastoma is easier to treat.

17. Cancer and genes:
Three classes of genes frequently are mutated in cancer:
• Proto-oncogenes (⇒ oncogenes)
• Tumor suppressor genes
• Mutator genes

18. Proto-oncogenes ⇒ oncogenes:
Proto-oncogenes
• Proto-oncgenes are genes that possess normal gene products and
stimulate normal cell development.
Oncogenes
• Oncogenes arise from mutant proto-oncogenes.
• Oncogenes are more active than normal or active at inappropriate
times and stimulate unregulated cell proliferation.
Some tumor viruses that infect cells possess oncogenes:
• RNA tumor viruses = possess viral oncogenes
capable of transforming cells to a cancerous state.
• DNA tumor viruses = do not carry oncogenes, but induce cancer
by activity of viral gene products on the cell

19. Proto-oncogene and oncogene protein products:
• ~100 different oncogenes have been identified.
• To understand the cancer, must understand the function of protein
products coded by the proto-oncogenes.
• All known proto-oncogenes are involved in positive control of cell
growth and division.
• Two classes:
• Growth factors, regulatory genes involved in the control of cell
multiplication.
• Protein kinases, add phosphate groups to target proteins,
important in signal transduction pathways.
• Mutations relax cell control of growth, allowing unregulated
proliferation.

20. Retroviruses and oncogenes:
Retrovirus =
• Single-stranded RNA virus that replicates via double-stranded
DNA intermediate.
• RNA is converted to cDNA by reverse transcriptase.
• DNA integrates into host chromosome and is transcribed.
• Retroviruses typically possess:
• 2 copies of a 7-10 kb ssRNA genome
• protein viral core
• glycolipid envelope (glycoproteins recognize host cells)
• Transducing retroviruses possesses oncogenes, which can cause
cancer when integrated into the host chromosome.

21. Fig. 2 Structure of a retrovirus

22. Retroviruses and oncogenes:
• All RNA tumor viruses are retroviruses.
• RNA viral oncogenes are altered forms of normal host genes that
occur in the virus genome.
• Examples of retroviruses include:
• Rous sarcoma virus (RSV), Feline leukemia virus, Mouse
mammary tumor virus
• Human immunodeficiency virus (HIV)
• Not all retroviruses are transducing or cause cancer.

23. Life cycle of a retrovirus:
First characterized in 1910 by F. Peyton Rous from a chicken tumor, later
named the Rous sarcoma virus.
1. ssRNA genome is released from the virus particle and is reverse
transcribed to dsDNA (proviral DNA).
2. Reverse transcriptase occurs in the virus particle and lacks 3’ to 5’
exonuclease activity (no proofreading ⇒ lots of mutations).
3. Proviral DNA and host chromosome DNA cross-over and are joined by
recombination.
4. Host RNA polymerase transcribes proviral DNA and produces viral
mRNAs required for the virus life cycle.

24. Fig. 3
Life cycle of the Rous
sarcoma virus

25. Cell Cycle Checkpoints
G1S
G2
cytoplasm
doubles
chromosomes
replicate
assembly of
components
for division cytokinesis
P M
A
T
Mitosis
DNA DamageDNA Damage
CheckpointsCheckpoints
DNA DamageDNA Damage
CheckpointCheckpoint
ApoptosisApoptosis
CheckpointCheckpoint
SpindleSpindle
AssemblyAssembly
CheckpointCheckpoint

26. Genetic Mutations That Can Cause Cancer
Tumor Suppressor Genes
• Genes that inhibit cell division are inactivated.
– Mutation in a gene that halts the cell cycle in G1 causes
retinoblastoma.
– Mutation in p53, a gene that promotes apoptosis if a cell has
damaged DNA, leads to a variety of cancers.
– Mutation in BRCA1, involved in tumor suppression and DNA
repair, leads to inherited breast cancer.

27. In Normal Cells, the Rb Gene ProductIn Normal Cells, the Rb Gene Product
Controls the G1Controls the G1 S TransitionS Transition
Rb = product of
Retinoblastoma gene,
inhibits action of E2F until
chemically modified
People prone to retinoblastoma have one mutated copy of the RbPeople prone to retinoblastoma have one mutated copy of the Rb
gene (Rb-) and one normal copy (Rbgene (Rb-) and one normal copy (Rb++
). Conversion of the Rb). Conversion of the Rb++
copycopy
to Rbto Rb--
by mutation leads to uncontrolled growth of retinal cells.by mutation leads to uncontrolled growth of retinal cells.
E2F = transcription factor
required to activate genes
for DNA synthesis
CDK-cyclin (intracellular
signal) modifies Rb so
the E2F can mediate the
G1S transition

28. In Normal Cells, the p53 Gene ProductIn Normal Cells, the p53 Gene Product
Acts at the G1Acts at the G1 S Checkpoint PreventingS Checkpoint Preventing
Entry Into S Phase If DNA Is DamagedEntry Into S Phase If DNA Is Damaged
p21 inhibits intracellular signals
that would activate EF2
p53 = transcription factor that
causes p21 to be produced
Cells with
damaged DNA
do not pass the
G1S
checkpoint
In cancer cells the mutated p53 gene
product no longer stimulates p21
production. Cells will pass the G1 S
checkpoint even when chromosomal
damage exists.

29. In Normal Cells, the p53 Gene Product StimulatesIn Normal Cells, the p53 Gene Product Stimulates
Apoptosis If DNA Damage Cannot Be RepairedApoptosis If DNA Damage Cannot Be Repaired
p53 gives an internal
signal for apoptosis
In cancer cells, a mutated p53
gene product no longer initiates
self-destruction. Cells with
damaged DNA can divide and
more DNA damage can be
accumulated.
p53 is the most frequently mutated
of all known cancer-causing genes,
contributing to many types of cancer.

30. p53 tumor suppressor genes:
• Mutations in p53 are implicated in ~50% of human cancers,
including cancers of the:
breast, brain, liver, lung, colorectal, bladder, and blood
• Development of tumors requires mutations on two p53 alleles.
• Codes a 393 amino acid protein involved in transcription, cell cycle
control, DNA repair, and apoptosis (programmed cell death).
• p53 binds to several genes and interacts with at least 17 cellular
and viral proteins.

31. Genetic Mutations That Can Cause CancerGenetic Mutations That Can Cause Cancer
DNA Repair GenesDNA Repair Genes
• Genes that promote DNA repair are inactivated.Genes that promote DNA repair are inactivated.
– BRCA1 is a tumor suppressor involved in DNA repair. FaultyBRCA1 is a tumor suppressor involved in DNA repair. Faulty
copies of BRCA1 cause inherited breast cancer.copies of BRCA1 cause inherited breast cancer.
– The disease Xeroderma Pigmentosum results from a defect inThe disease Xeroderma Pigmentosum results from a defect in
excision repair.excision repair.

32. Breast cancer tumor suppressor genes:
• Breast cancer affects 1 in 10 women and represents 31% of
cancers in women (~185,000 women diagnosed each year).
• ~5% of breast cancers are hereditary; age of onset for hereditary
breast cancer is earlier than other forms (mutations at 2 alleles).
• Many genes involved; BRCA1 and BRCA2 are thought to be tumor
suppressor genes.
• BRCA1 is important for homologous recombination, cellular repair
of DNA damage, and transcription of mRNA.
• Mutations in BRCA1 also are involved in ovarian cancer.
• BRCA2 plays a role in timing of mitosis in the cell cycle.

33. Mutator genes:
• Mutator gene increases spontaneous mutation rate of other genes.
• Mutator gene products are involved in DNA replication and repair;
mutations make the cell error prone.
• HNPCC-OMIM 120435, Human non-polyposis colon cancer
• Mutation at any one of 4 genes (hMSH2, hMLH1, hPMS1,
hPMS2) leads to predisposition.
• Tumor formation requires mutation at the second allele.
• All four genes have homologs in yeast.
• DNA blood tests are available for all four genes.