Cell Cycle Regulation and Oncogenesis Flashcards
Cyclin B-CDK1 Regulation and Function
In G2, Cyclin B-CDK1 is constitutively repressed until it is phosphorylated by Cyclin A-CDK1.
Active Cyclin B-CDK1 degrades the lamins in nuclear envelope, and enables condensin to condense chromosomes.
Degraded by APC and mitotic exit occurs.
Anaphase promoting complex
Degrades Cyclin B to remove cell from mitosis and indirectly leads to the separation of sister chromatids by cohesin cleavage.
Cell Cycle Checkpoints
DNA synthesis checkpoint (in G2). 2 DNA damage checkpoints at the beginning of M and S phases. Incomplete spindle formation checkpoint (during M). Restriction point (G1).
Restriction Point
A cell cycle checkpoint in G1 that connects extracellular signaling and cell cycle control. Growth factor signaling leads to the upregulation of Cyclin D, causing Rb phosphorylation and E2F release –>DNA synthesis.
This regulation is lost in cancer.
DNA Damage Checkpoint
P53 is normally bound to MDM2 (a ubiquitin ligase), which usually targets P53 for degradation. Normally P53 levels are low.
When DNA damage occurs, numerous acetylases and kinases disrupt the interaction between MDM2 and P53, preventing P53 degradation. Concentration of P53 increases, which acts as a transcription factor for P21, which inhibits CDK activity and inhibits phosphorylation of Rb. Cell arrests prior to S phase. P21 also inhibits CyclinB-CDK1, so DNA damage can also arrest cell prior to M.
Also, P53 increases transcription of pro-apoptotic factors PUMA and NOXA.
DNA synthesis checkpoint
CDC25 not able to remove inhibitory Wee1 phosphates until it is phosphorylated as a result of complete DNA synthesis.
Spindle Checkpoint
The cell can sense unoccupied kinetochores. When it does, it keeps the anaphase promoting complex inactive, so cohesin doesn’t split. Loss of a single MT is enough to stop M, so only low doses of anti-MT drugs like Taxol are needed.
Immortalized Cells
Phenotypically normal, unlimited proliferation
Transformed Cells
Cells have started to accumulate certain properties of tumorigenic cells:
Cell looks more like a fibroblast than epithelial cell.
Cell depends heavily on glycolysis (Warberg effect).
Cell is no longer dependent upon integrin signaling.
Loss of contact inhibition.
Loss of growth factor dependence.
Changes in ploidy.
Angiogenesis.
Senescence
M1. When a cell does not divide, but does not die. Cells look like fried eggs. Senescence is irreversible (growth factors don’t cause growth), as opposed to quiescence. This occurs when telomeres in somatic cells get too short and the cell believes that DNA damage has occurred. P53 will be upregulated and the cell cycle will be arrested.
If cells have p53 or Rb ablation…
They will continue to grow until M2 (crisis), where the cells will die. If, however, cells are treated with telomerase, M2 can be overcome.
Do cancer cells have greater or fewer telomeric repeats than germ cells and senescent cells?
Fewer. Cancer cells have already hit crisis, but then overcome cell death.
ALT
Alternate means of telomere maintenance. Tumor cells will continue to maintain telomeres in the presence of telomerase inhibitor. Something must be happening here…
How do RNA tumor viruses cause cancer?
1) Viral genome contain the oncogene (i.e. Rous Sarcoma Virus)
2) Insertional mutagenesis- if viral genome sits next to proto-oncogene, overexpression can occur.
Proto-oncogenes
Normal genes that can become oncogenes if overexpressed.
Methods of proto-oncogene activation
Deletion or point mutation in the coding sequence (hyperactive protein generation). Regulatory mutation (overexpression). Gene amplification (overexpression). Chromosome rearrangement (overexpression or fusion protein).
Double Minutes
Products of gene amplification
3 ways that gene amplification leads to oncogenic activity
1) A limiting protein is greatly overexpressed (Cyclin D, CDK4).
2) Growth factors receptors are overexpressed and dimerize spontaneously (Her2, EGF-R).
3) Autocrine Loop, where a product is made in high amounts in a cell and causes endogenous activation of receptors.
2 ways that gene rearrangement leads to oncogenic activity
1) Regulatory regions are moved around, causing new expression levels (Burkett’s Lymphoma: t(8,14))
2) Novel fusion proteins (t(9,22) causes CML by generating BCR-ABL, which signals cell proliferation).
Gleevec
Blocks BCR-ABL’s ability to phosphorylate a substrate protein that causes cell proliferation.
Cooperative Oncogenes
Some oncogenes only trigger transformation if they are expressed at the same time (myc+ras).
Experiment suggesting existence of tumor suppressors
Tumor cells fused with normal cells. Resulting cells were normal.
Inheritance pattern of inherited tumor syndromes
Generally autosomal dominant, but incompletely penetrant due to an inheritance of just susceptibility.
Difference between RNA and DNA tumor viruses
DNA tumor viruses contain novel oncogenes that don’t have human cell counterparts.
SV40
Simian virus 40. Expresses large T antigen that inactivates p53 and pRb.
Adenoviruses
Express E1A and E1B that bind to pRb and p53.
HPV
Human Papilloma Virus. Expresses E7 that inactivates pRb and E6 that targets p53 for degradation.
Sporadic vs Hereditary Retinoblastoma, and why.
Sporadic is associated with single eye defects and no secondary tumors. Hereditary RB is bifocal and causes secondary tumors. This occurs because of the two hit hypothesis. In familial Rb, there is an inherited susceptibility on one RB locus. Early on, a second mutation develops. Both of these hits cause many tumors. In sporadic, the two hits occur spontaneously and are localized to one tissue.
Difference between oncogenes and tumor suppressors
Oncogenes generally represent “gain of function.” Only requires one mutated allele. Tumor suppressors are “loss of function,” meaning that loss of both alleles is needed for tumorigenesis.
ARF
Alternate reading frame protein. ARF inhibits MDM2, preventing p53 degradation and leading to cell death and CDK inhibition.
Most common p53 mutation
Missense mutation
What happens to a protein after a frameshift mutation?
Generally is truncated.
Adenomatous Polyposis Coli mutation
APC protein susceptible to frameshifts. Normally binds to beta-catenin, phosphorylates it, and causes its degradation. If APC is truncated, beta-catenin is not degraded, and it promotes cell proliferation.
Most common pRb mutation
Deletion
Loss of heterozygosity
When there’s a missense or frameshift mutation on one allele of a tumor suppressor, the remaining wild-type allele is deleted.
Haplo-insufficiency
An exception to the two-hit hypothesis, where loss of one allele is enough to cause a tumor. Three ways:
1) Reduced expression below some threshold.
2) Dominant negative effects
3) Epigenetic silencing of the other allele.
