Lecture 14 - Cell cycle checkpoints Flashcards
Cell cycle is under Tight Regulation
What are the key features of the unperturbed cell cycle?
Timely
Sequential
Complex feedback loops
What happens when the cell cycle progression is perturbed?
Regulatory pathways in the eukaryotic cell cycle triggers DELAYS in cell cycle progression until conditions are suitable for the cell to progress to the next stage - Checkpoints
How are checkpoint sensors are very sensitive?
Detect even the smallest cell cycle defect
+ A single DNA strand break triggers DNA damage checkpoint
+ A single unattached kinetochore blocks entry into anaphase
Once the defect is detected, the cell cycle is immediately halted
Can the cell cycle operate without checkpoints?
Yes, but the daughter cells might be defective
What are some examples of checkpoints in the cell cycle?
DNA damage
DNA replication
Spindle assembly checkpoint
Example: DNA Damage checkpoints
Depending on DNA lesions and context, damaged cells with activated checkpoints can be:
- allowed to survive and resume cell cycle progression upon repair of damage and checkpoint termination, or
- eliminated by apoptosis, or
- silenced by cellular senescence
Double-strand breaks (DSB) pose a particularly serious threat to genomic integrity:
- disrupt cellular processes such as transcription, replication and chromosome segregation, therefore leading to chromosome loss or anuploidy. Possibly the deadliest type of DNA lesion
How are DNA double-strand breaks (DSBs) dealt with?
Detection of breaks -> activation of checkpoint components -> downstream effectors
To protect genomic information from such threats, DNA repair proteins are activated or
transcriptionally induced, and are physically recruited to sites of DNA lesions
What are the 3 components in each checkpoint system?
Sensor (detects cell cycle defects)
Signal
Effector (enables defects to be remedied)
Which two protein kinases are key component of the DNA damage checkpoint?
ATM = Ataxia Telenglectasia Mutated
ATR = ATM-Related
These two protein kinases are members of the P13Kinase related kinase family
DNA damage, eg double-stranded breaks, activates protein kinases ATM & ATR
G1 and S phases
How does DNA damage affect Cdc25?
The activation of ATM & ATR causes Cdc25, an important phosphatase, is phosphorylated and destroyed.
G2 phase
How does DNA damage affect Cdc25?
DNA damage activates ATM & ATR
Which activates Chk1, Chk2, phosphorylate Cdc25
Cdc25 exported out of nucleus into cytoplasm
Export of Cdc25 prevents it from activating it as its substrates (eg CDKs) that are in the nucleus
What is the slow, sustained responses to DNA damage (hours, days)?
DNA Damage
ATM and/or ATR
p53 (slow action as it is a transcription factor)
p21CIP/KIP - block entry into S phase
How do cells keep p53 levels low?
MDM2 (Mouse Double Minute 2) is a ubiquitin ligase that ubiquitinates suppressor P53 triggering its degradation
MDM2 (a ubiquitin ligase) was first discovered as a proto-oncogene which was highly amplified in DOUBLE MINUTE mini-chromsomes in certain tumours.
MDM2 causes p53 degradation
MDM2 amplification leading to over-expression - associated with cancers (eg lung cancers)
What is the effect of phosphorylation on p53?
Unphosphorylated p53 has low transcription activity
Transcription activity of p53 is enhanced upon phosphorylation
What is the function of tumor-suppressor genes, and what is the result of loss-of-function mutations in these genes?
Tumor-suppressor genes - encode proteins that inhibit cell proliferation.
Loss of one or more of these “brakes” contributes to the development of many cancers.
so the loss-of-function mutations in these tumour-suppressor genes will lead to increase chances of errors in cell division
How come loss-of-function mutations in tumor-suppressor genes are oncogenic?
Tumor-suppressor genes in many cancers have deletions or point mutations
such mutations either lead to
- an inability of cells to produce the gene product
- the production of a non-functional gene product
generally one copy of a tumor-suppressor gene suffices to control cell proliferation
-therefore, both alleles of a tumor-suppressor gene must be lost or inactivated in order to promote tumor development.
- single allele mutation = recessive
Why do mutations in the p53 gene allow cancer cells to survive and proliferate despite DNA damage?
Its involvement in cell-cycle control, apoptosis, and in maintenance of genetic stability
- all aspects of the fundamental role of the p53 protein in protecting the organism against cellular damage and disorder.
The loss of p53 activity
- it allows faulty mutant cells to continue through the cell cycle.
- it allows them to escape apoptosis.
This could lead to the genetic instability characteristic of cancer cells, allowing further cancer-promoting mutations to accumulate as they divide.
What are oncogenes?
Oncogene is any gene that encodes a protein able to transform cells in culture or to induce cancer in animals.
Of the many known oncogenes, all but a few are derived from normal cellular genes (i.e., proto-oncogenes) whose products participate in cellular growth-controlling pathways.
Because most proto-oncogenes are basic to animal life, they have been highly conserved over eons of evolutionary time.
The gain-of-function mutations in oncogenes would normally mean that gene products promoting cell division could for instance
- increase in abundance
- increase in activity
which would mean that cells are driven more into dividing
How do cancer cells arise say of the colon if there are no dividing cells?
Adult stem cells
cells that are specialized to provide an indefinite supply of fresh differentiated cells here these are lost, discarded, or needed in greater numbers.
Adult stem cells proliferate
How are stem cells are capable of regenerating a particular tissue for the life of an organism?
They are considered self-renewing in that some of their daughters themselves become stem cells.
Asymmetric division
Stem cell –> stem cell (self-renewing) + differentiated cell
Assuming that the rate of mutation is roughly constant during a lifetime, then the incidence of most types of cancer would be independent of age if only one mutation were required to convert a normal cell into a malignant one.
However, the incidence of most types of human cancers increases markedly and exponentially with age. Why?
Although many explanations of this phenomenon have been considered, the incidence data are most consistent with the notion that multiple mutations are required for a cancer to form
“Multi-hit” model
