Ch. 7: Tumor Suppressor Genes Flashcards
Do oncogenes act dominantly or recessively?
dominantly
What function do proto-oncogenes normally have in a cell?
proliferation, protein synthesis, and survival
What types of mutations convert proto-oncogenes to oncogenes?
- changes to gene expression
- changes to protein sequence
- increase protein activity
What function do tumor suppressor genes normally promote in a cell?
inhibition of cell growth, promote apoptosis
What types of mutations in
proto-oncogenes convert them to oncogenes?
Loss of fxn mutations: decreased expression, decreased function
at the protein level
Are tumor suppressor genes dominant or recessive fashion?
recessive
What is a tumor suppressor gene?
a gene whose inactivation leads to increased likelihood of cancer development
Are cancer-causing genes dominant or recessive? (experiment)
1970s-1980s
- conducted cell fusion and injected it into an animal
* Results:
* Most cells from tumors that were caused by
viruses – dominant
* Most cells from tumors that were not caused
by viruses – recessive
What was one of the first studied tumor suppressor genes?
retinoblastoma
What is retinoblastoma?
rare childhood eye tumor
What are the two types of retinoblastoma?
- sporadic
- familial
Sporadic retinoblastoma
mild version (tumor in 1
eye, no cancer recurrence after tumor removal)
Familial retinoblastoma
aggressive version, parent
had RB, (tumors multiple sites of both eyes, still change of cancer recurrence in eyes and other tissues after tumor removal)
What is thought to give rise to retinoblastomas?
precursor cells that give rise to rods/cones are thought to give rise to retinoblastomas
Individuals with familial RB have increased risk of…
other tumors and giving to their children
Bilateral
increased risk of tumors in other areas
Genetics of Familial RB
- 1 mutant allele arose in germline (all cells in organism have this)
- 2nd mutant allele arose early development/ childhood in precursor
RB cells à poised to proliferate - Once the mutation is in the germ lineage (egg or sperm), it can be given to offspring
genetics of sporadic RB
- Both mutant alleles occurred in somatic cells, in precursor RB cells, during early development/ childhood. Separately.
Will individuals with sporadic RB have increased chance of passing this on to their offspring?
No, because the mutant RB allele is not created in early development when all of their cells are created and it is localized in eye cells (so eggs/sperm are not affected)
RB types can also be classified as ____
hits
“Hit”
refers to a somatic mutation
How many hits are needed for familial and sporadic RB transformation?
- familial: only need 1 additional hit for transformation
- sporadic: need 2 hits (more rare)
What are the types of alteration that can affect tumor suppressor genes?
- regular mutation rate
- loss of heterozygosity
- promoter methylation
Alteration- regular mutation rate
changes to nucleotides based on errors in DNA replication or exposure to mutagens
Alteration- loss of heterozygosity
a genetic event when one of two alleles at a heterozygous locus is lost.
Alteration- promoter methylation
methylation of cytosines in the promoter region of a gene that subsequently causes the inhibition of
transcription
Are mutation rates common or rare?
rare
* 10 -6 per cell generation
* Likely will get first mutation in some cells based on regular mutation
(Rb-/Rb+)
* Not likely to get a second mutation in the same gene in the same cell
via regular mutation
* Second hit -> mechanism not dependent on mutation rate (Rb-/Rb-)
What is loss of heterozygosity?
- a genetic event when 1 or 2 alleles at a heterozygous locus is lost
- proved to be the case for some RBs
Mechanisms that contribute to loss of heterozygosity
- Mitotic recombination
- Gene conversion
- Chromosomal non-disjunction
What is promoter methylation?
The cell methylates DNA to control gene expression
- methyl groups are added to Cs
- Generally, lots of DNA methylation in a promoter region, tends to inhibit transcription of a gene
Promoter methylation of some TSGs progressively
increases with _______
cancer progression
Hypermethylated promoter of p16 is associated with
____________________
pre-cancerous tissues
Promoter methylation of some TSGs may be…
an early step in cancer progression
note:?
1) Chance for early detection
2) Target for cancer prevention
Tumor suppressor genes
- promote cancer when inactivated
- prevent cancer when active
What is one common familial cancer syndrome (like Rb)?
Neurofibromatosis
What is neurofibromatosis?
benign tumors, neurofibromas in sheaths surround nerves
- can occasionally become malignant
- common mutation in NF1 tumor suppressor gene
Peripheral nervous system
nerves that branch from the
brain and spinal cord; make
connection between central
nervous system and body
parts
myelin sheath
made of proteins and
phospholipids, insulates nerve cells, increases speed of electrical impulses
Schwann cells
helps form myelin sheaths
What kind of growth are neurofibromas?
complex growths
- contain multiple different cell types
What is NF1?
Neurofibromin- GTPase activating protein (GAP)
- catalyzes the hydrolysis of GTP to GDP + P
- it puts RAS in its inactive state
T/F Loss of NF1 function has the same result on the cell as hyper-activation of RAS.
True
How does loss of NF1 contribute to neurofibromas? (experiment) (lecture 7 part 2, slide 14+)
- Scientists use mouse models to understand function of cancer related genes
- Strategy: Gene knockout. Delete 1 or more copy of the gene and see the effects on the animal.
- Step 1: Use homologous recombination to replace
gene of interest (NF1) with neomycin resistance gene - Step 2: Introduce stem cells with knockout allele into
a blastocyst (early mouse embryo). - Step 3: Transplant early mouse embryos into a
surrogate mom mouse, she has chimeric babies - Step 4: Breed the chimeric progeny with one another
- Step 5: Use the NF1 +/- (heterozygous) mice for experiments
- Results: NF1 homozygous mutant embryos are not
viable
So, how can you determine the result of loss of both NF1 copies in an adult mouse?
Some creative genetics
- conditional gene knockouts (Cre-ER system)
Steps of Cre-ER system
- Step 1: Use homologous recombination to add LoxP
sites to gene of interest (NF1) - Step 2: Introduce stem cells with floxed allele into a
blastocyst (early mouse embryo). - Step 3: Transplant early mouse embryos into a
surrogate mom mouse, she has chimeric babies - Step 4: Breed the chimeric progeny with one another
- Step 5: Keep breeding in such a way that you end up
with homozygous NF1 Fl/Fl and TSP-Cre
You suspect gene X is involved in thyroid cancer. You breed mice to try to make a homozygous knockout mouse but find it is an essential gene. What is an experiment you can do to test the effect of gene X in adult mice?
* A) Make a mouse with the following genotype: GeneX Fl/Fl + Thyroid-CreER)
and add tamoxifen during development.
* B) Make a mouse with the following genotype: GeneX Fl/Fl + Thyroid-CreER)
and add tamoxifen during adulthood.
* C) Make a mouse with the following genotype: GeneX Fl/Fl + Myeloid-CreER)
and add tamoxifen during development.
* D) Make a mouse with the following genotype: GeneX Fl/Fl + Myeloid-CreER)
and add tamoxifen during adulthood
B
How does loss of NF1 contribute to neurofibromas?
- Loss of NF1 in Schwann cells can only cause neurofibromas when
mast cells are heterozygous for NF1 KO allele - Role for paracrine signals. NF1 -/- Schwann cells secrete growth factors
which act on nearby NF1 -/+ mast cells. Get complex neurofibromas which
include various cell types.
Other RAS-GAP’s (like NF1) act as ______________ in other cancers
tumor suppressor genes
APC is a tumor suppressor gene that contribute to
____________
colon cancers
APC
- Follows the same genetic
pattern as Rb - Involved in WNT signaling
pathway - Loss of APC causes stabilization of B-catenin, which can enter the nucleus and cause transcription of target genes
- -> constitutive activation of WNT signaling pathway
- -> subsequent activation of
oncogene leads to cancer
KEAP1 is a tumor suppressor gene
in _________
lung and other cancers
KEAP1
- In normal conditions, KEAP1 allows an E3 ubiquitin (Ub) ligase to add Ub to
NRF2, and NRF2 is degraded by the proteasome - In stressful conditions, KEAP1 ROS or carcinogens bind to KEAP1 and release NRF2, NRF2 can then
accumulate and enter the nucleus and cause the txn of antioxidant genes - In cancers, loss of KEAP1, causes constitutive activation of detox genes
