Transcript Genetics and Cancer II

Announcements
1. Please let me know if you have a final exam scheduling
conflict; we can reschedule your final now.
2. Reminder - I will calculate your current grade for you if
you are interested. Final exam (20%) can help!
Possible adjustment after final….
3. No labs this week - happy turkey day.
4. Study guide for final (final in 2 weeks) available in class
Wednesday
5. Reminder - look over Ch. 23: 1, 3, 6, 8, 10
Review of lecture 35
1. C. elegans chemotaxis behavior and genetics
2. Statistical analysis of chemotaxis data - 7 t -tests total
So, how exactly will you discriminate
among unknown possibilities?
osm-1 encodes novel protein
osm-3 encodes kinesin
che-3 encodes a dynein
che-3;dpy-13 is short and fat
wt is not short and fat and should behave like wt control!
osm-1, osm-3, che-3, che-3;dpy-13, wild-type
If unknown is wt, will you expect a significant difference bet.
index of unknown and wt control?
Will you be able to distinguish among osm-1, osm-3, and che-3?
Overview of lectures 36/37
Genetics of cancer - Ch. 23
I. cell cycle regulation
II. mutant genes confer predisposition to cancer
III. Tumor suppressor genes - normally halt cell cycle
IV. Oncogenes - normally promote cell cycle
V. Translocations
VI. genomic instability
Mutations play a central role in cancer
-background rate of spontaneous mutation - due to ?
- therefore, always baseline rate of cancer
-above baseline rate, environmental agents that promote
mutation also contribute to cancer = carcinogens
• Which mutant genes are most likely to result in cancer?
• How many mutations are needed to cause cancer?
• How do mutations convert normal cells into malignant tumors? What
are the differences between these cells?
1) uncontrolled growth 2) metastasis
I. The “cell cycle”
Many cells alternate between dividing and “resting” or not dividing
Gap 1; metabolic activity and cell growth
G0 (resting phase)
Mitosis
DNA replication
ex. 1 hour of 16 hour
cell cycle
Gap 2; metabolic activity and cell growth
Three main checkpoints in the cell cycle
•2001 Nobel Prize was awarded to 3 scientists who studied
genes that regulate the cell cycle
1.
1. G1/S
Is cell the correct size?
Is DNA damaged?
2. G2/M
Is DNA fully replicated?
Is DNA damage repaired?
3.
2.
3. M
Have spindle fibers formed?
Have they attached to
chromosomes correctly?
Why are cell cycle checkpoints important?
What might result if DNA repair has not finished?
Uncontrolled cell division could occur - cancerous cell
Example: p53 protein normally targets cells with severe
DNA damage to undergo programmed cell death.
(this removes them from the population)
If the p53 gene is mutated, damaged cells will not
be removed and may continue dividing in an
uncontrolled manner.
Many different types of cancers involve mutations of p53.
Checkpoint Control of Cell Cycle
G1/S
G1
S
G2/M
G2
Cdk-G1 cyclin
M
Cdk- Mitotic (B) cyclin
(MPF)
II. Which mutant genes most likely to
cause cancer? - Retinoblastoma
• Diagnosis: “Cat’s eye”
reflection (leukocoria) in
affected eye.
• Most common cancer of
infants and children
(1/20,000 U.S. live births).
• Survival > 90% with early
diagnosis and treatment.
• Individuals at greater risk of
developing other cancers.
Retinoblastoma: Familial v. Sporadic
+/+
RB/+
RB/+
RB/+
RB/+
RB/+
RB/RB
“Loss of
Heterozygosity”
RB/RB
Common
Rare - Why?
III. Retinoblastoma Gene first ex. of a tumor suppressor gene
• A Tumor Suppressor, which normally suppresses
unregulated cell growth: must be inactive for cell division;
if permanently inactive, no control over division
• Discovered as a regulator of growth of neuroblasts in
developing retina of the eye.
• Inactivation of both copies of the Rb Gene removes a
“brake” on growth, leading to increased incidence of
retinal cancer.
• Since found to be active in all cells.
Rb Protein is Inactivated By CDK-Cyclin
During G1  S
p53 Gene (tumor suppressor)
Normal Functions - controls G1/S
• The “Last Gatekeeper” gene since malignant state
not attained despite the presence of other cancercausing mutations until p53 is inactivated by
mutation.
• 2 possible responses to DNA damage:
1) Acts as a Transcription Factor to activate
expression of p21, which inhibits CDK/G1 cyclin to
halt the cell cycle; then activates DNA repair.
2) Triggers apoptosis (programmed cell death) if
damage can’t be repaired.
Role of p53 in
Cell Cycle
Control
p53 Mutations
• Most commonly mutated gene in cancers (50% of
total).
• When p53 is mutated, DNA-damaged cells are not
arrested in G1 and DNA repair does not take place.
This failure to arrest DNA-damaged cells will be
repeated in subsequent cell cycles permitting other
mutations to accumulate, culminating in neoplastic
transformation... tumor formation and cancer.
Breast Cancer Tumor Suppressors
• A small proportion of breast cancer is heritable. Two
genes are associated with predisposition to breast
cancer.
– BRCA1 on chromosome 17
– BRCA2 on chromosome 13
• Normal function of both is in repair of ds DNA breaks.
IV. Oncogenes
• Arise from mutation in normal gene called a protooncogene; these promote cell division
• Dominant mutation: one copy is sufficient to cause
cancer. (different than tumor-suppressors) - when
switched on permanently, uncontrolled cell division.
• First link between viruses and cancer proposed by
Francis Peyton Rous in 1910 (Nobel Prize, 1966):
cell-free extracts from chicken tumors injected into
healthy chickens caused new tumors.
Rous Sarcoma Virus (RSV)
• Discovered by Harold Varmus and Bishop, 1975-76
(Nobel Prize, 1989).
• A transforming retrovirus (TRV): a cancer-causing
single-stranded RNA virus that uses reverse
transcriptase enzyme to make ssDNA, then ds DNA,
which integrates into host DNA.
• Note: not all retroviruses are TRV’s, not all oncogenes
caused by viruses.
• 100’s of oncogenes now known.
• Human T-cell leukemia virus (HTLV) is only human
TRV known; codes a TF.
Southern Blots Probed with viral src Gene
Revealed Cellular Origin of Oncogenes
Infected chicken #1 Infected chicken #2 Uninfected chicken
(Negative Control)
v-src
c-src
Proto-oncogene
SURPRISE!
Origin of Transforming Retroviruses
Capsid protein Reverse Transcriptase Envelope Protein
Mutation creates oncogene
Ras Proto-oncogene
• Mutated in 30% of all cancers.
• A “molecular switch” in the signal transduction
pathway leading from growth factors to gene
expression controlling cell proliferation: GF 
receptor   Ras    TF  target genes 
growth.
• A single amino acid change in Ras protein can cause
constant stimulation of the pathway, even in the
absence of growth factors.
Cancers Usually Result from a
Series of Mutations in a Single Cell
• Colon Cancer:
oncogene
oncogene
Tumor suppressors
Tumor Progression: Evolution at the
Cellular Level
Benign tumor (polyp in
epithelial cells) is confined
by basal lamina; then
additional mutation occurs.
Malignant tumor (carcinoma
in epithelial cells) grows
very fast, becomes invasive,
and metastasizes.
Cancer Cells Evade Two “Safety”
Mechanisms Built into the Cell Cycle
1. Once p53 is inactivated, cells with DNA damage don’t
arrest from G1 and don’t undergo apoptosis.
2. Telomerase enzyme is activated, avoiding the limit to
cell divisions imposed by telomere shortening.