AMC-HC 3 + 4: Multi-step & Maintainance Flashcards
dag 2
How long does it take for most cancers to develop? (rough estimation)
Years/decades
Because of the late onset of cancer, curing it averagely has a minor effect on ….
life expectancy
Risk of death to cancer
Incedence = age^4-7
> 5-8 independent events must occur for cancer to form
Transformation to some degree does always occur. True or False?
True, at a later age
It is possible for different tumor progression stages to …
Coexist in the same tissue
Which treatment most effectively reduces risk of disease with cancer?
Surgical removal of early-stage lesions (if detected early-stage)
Increasing neoplastic phenotypes correlate with ….
Increasing number of altered genetic loci
Tumour stimulating genetic alterations
Proto-oncogene activation and tumor-suppressor gene inactivation
The number of inactivated tumor suppressor genes </> (?) number of activated oncogenes in human tumor cell genome
>
Loss of heterozygosity (LOH)
Loss of one of the both alleles of a gene which can expose a faulty allele if it was masked (or the second allele was already broken) > losing the functionality of both alleles.
Only if the person was already heterozygous for the e.g. tumor suppressor gene (after a first hit)
Cancer cells often underwent multiple gene …
LOH
Chromosome translocation is a … for cancer
inducer
Why does a single mutation in a gene or loss or gain of function of the gene generically not lead to carcinogenesis?
The entire pathway of proliferation/growth/checkpoint is not effected (inactivated or overexpressed). Often, multiple gene routes can lead to a similar outcome (reaction pathways)
Why does tumor progression resemble evolution?
-Survival of the fittest
-Competition between cancer cells for limited resources
-The best adaptes cancercells survive and proliferate > advantageous traits.
-Clonal evolution > clonal expansion of the strongest cells after independent mutations
How does genome instability increase with tumor progression?
The mutation rates increase with tumor progression
Why do different subclones exist within the same tumor?
The selection can’t keep up with genetic diversification
How are distinct subclones able to coexist and help each other?
Costimulation with growth factors which they secrete (paracrine signaling and autocrine signaling)
Plasticity
the adaptability of an organism to changes in its environment or differences between its various habitats.
Which plasticity adds another layer of complexity to tumor progression?
Epigenetic plasticity
Transformation requires at least … mutant genes
2
> several thousand mutated ras oncogenes are created every day
> truly normal primary cells are resistant to single-hit transformations
Why are human fibroblast cells diffucult to transform?
Passaged human cells become senescent (non-proliferative)
Peto’s Paradox: The connection
Within one species, cancer risk and body size are positively correlated (more cell proliferation)
Peto’s Paradox: the counterpoint
Larger organisms have bigger and slowly dividing cells with a lower energy turnover which reduces the risk of cancer initiation.
> there are large differences between species
Mitogen
Enhances mitosis
Mutagen
Enhances mutations
Non-mutagenic or toxic agents which promote tumorigenesis
-Tabacco smoke > genotoxic chemicals > initiating agent
-Alcohol: weak mutagen but very toxic to epithelial cells > cells die > repair needed > causes proliferation and enhances risk of tumorigenesis
Mitogenic agents which promote tumorigenesis
-Steroid hormones promote cell growth > estrogen and progesterone stimulate proliferation of cells in reproductive tissue in menstrual cycle
The more mentrual cycles a female undergoes, the higher …
the risk of breast cancer (inducing cell proliferation)
Which kind of drugs reduce cancer risk?
Anti-inflammation drugs
Why is cell proliferation essential for tumor progression?
For increasing the number of mutations which the cells are carrying.
Why does cell proliferation (cell divisions) lead to new mutations?
DNA replications can lead to new mutations
Important: A carcinogen is not always a …
Mutagen, it can also be a mitogenic agent which doesn’t damage the DNA, but induces mitosis.
Complete carcinogens
Initiate and promote tumorigenesis
How do the cells of the gastrointestinal lining minimalise the accumulation of mutations?
Paneth cells at the bottom of the crypt. rapidly dividing stem cells between the Paneth cells. In the zone of proliferation until pushed out of the crypt into the zone of differentiation and they are moved to the top of the villus and released into the lumen of the small intestine to be removed by poop. > discarded from the body.
Do the stem cells in the bottom of the crypts divide a lot?
No, after asymmetrical division, one daughter cell becomes a Transit-amplifying cell which divides a lot (pushing the cells before it to the villus). When in the zone of differentiation, only two cell divisions are needed to get a highly differentiated cell (which is dicarded very fast)
How are the stem cells in the bottom of the crypt protected against mutations?
-Not replicating a lot
-Being localized in the bottom of the crypt, away from the hazardous intestine lumen.
- Asymmetric stem cell division: only one daughter cell stays a stem cell, the other differentiates and is pushed out
Risk with the intestinal stem cells
When there is a shortage of stem cells, the transit-amplifying cells (which underwent a lot of divisions) are able to de-differentiate and switch to a stem cell
> risk of a stem cell with higher rate of transformation
Cytotoxic agents
Kill (stem) cells
Correlation cancer risk and stem cells
Number of stem cell division in a given tissue is positively correlated to lifetime risk of cancer in that tissue
Protective mechanisms of stem cells
-Apoptosis: intestinal stem cells with DNA lesions will undergo apoptosis and not attempt repair
-Drug pumps: stem cell pumps out certain drugs more effectively than differentiated cells (because of high expression of Mdr1 (multi-drug resistance 1)
> pumping out the dangerous stuff
Cancer stem cells and apoptosis
Cancer stem cells have a reduced potential for apoptosis and a increased DNA repair capacity.
Proofreading
-By DNA polymerase
-3’ -> 5’ exonuclease activity when errors occur
-Low error rate of polymerases
Mismatch repair
-Not part of DNA polymerase, but repairs are done during replication
-MMR enzymes monitors recently synthesized DNA and recognize altered DNA structures like loops or nicks
-The enzymes make a nick downstream and excise a strand fragment, DNA polymerases and ligases repair the gap
Which enzymes are involved in MMR
-DNA damage detection enzymes
-nucleases
-polymerases
-ligases
Does the mechanism of proofreading and mismatch repair prevent any mutations from arising?
No: the mechanism still allows evolutionary mutations, but the risk of carcinogenesis is decreased.
Endogenous processes for mutagenesis
Depurination, depyrimidation, deanimation of bases
Causes of endogenous mutagenesis
-Oxidation by reactive oxygen species (ROS) with metabolic by-products
> induce single and double stranded
> apurinic and apyrimidinix sites (abasic sites)
-Base mispairing during replication
Exogenous agents as a source for mutagenesis
-Ionizing radiation
-UV-light
-Alkylating agents
-Cellular processes
-Alcohol
> infrequent
Ionizing radiation like X-rays as exogenous agent
Can hit DNA directly but mostly strips the electrons from watermolecules > creating ROS > creating SSBs (single strand breaks) and DSBs
UV-light as exogenous agent
Is ubiquitous
-covalent bonds between adjacent pyrimidine bases create pyrimidine dimers
Why does skin cancer occur a lot in Australia
Not very well pigmented humans live with a lot of UV radiation which increases the incidence of skin carcinomas
Alkylating agents as exogenous agent
Lead to a loss of purine or pyrimidine bases
Cellular processes as exogenous agent
Convertion of procarcinogens to carcinogens > formation of DNA adducts by for example tabacco smoke
Alcohol as exogenous agent
Can be a mutagen, but not directly. Toxic effect kills cells, thereby inducing proliferation and chance on DNA damage during replication
Cell protection of DNA
-Physical shielding (like melanine)
-ROS scavangers (vitamin C, bilirubin, urate) > react with ROS
…[]
Base excision repair (BER)
-Cleave the bond linking the modified base to the deoxyribose sugar
-Mostly repair non-helix distorting lesions
Nucleotide-excision repair (NER)
-Repair helix-distorting lesions
-Removes the entire nucleotide and repairs the gap
-Transcriptional coupled repair or global coupled repair
…..[]
Xinoderma pigmentosum
A disease which leads to a 2000-fold increased risk of skin cancer before the age of 20 > even without being in contact with the skin, skin carcinomas will appear
Double strand breaks repair
-Homologous recombination
-Non-homologous end joining
Where are DSBs localized
Arise at stalled replication forks and can be induced by ionizing radiation
Homologous recombination (HR)
-Active in S and G2 phase > second copy of the same chromosome is available and is used as a scaffold to repair the DNA strands at the site of the break
-Involves BRCA1 and BRCA2 mutations
-Used in genome editing
Non-homologous end joining (NHEJ)
-Takes the two end and ligates them together > loss of bases > deletion and possibly frameshift
-Quick and dirty
-No way to perserve perfect information at the break site
Changes in karyotype often occurs in tumor cells. What does this mean?
Changes in number and/or structure of chromosomes
What are changes in chromosome structure called?
Chromosomal aberrations
Examples of chromosomal aberrations
Deletions, amplifications, translocations
What kind of aberrations are detected in many tumor types?
recurrent aberrations
What can these recurrent aberrations lead to?
Loss of heterogeneity (LOH)
> can expose a faulty gene if it was dominated by the healthy allele
Chromothripsis
Localized, massive chromsome fragmentations, followed by multiple rejoining between resulting fragments
Polyploidy
Due to errors in mitosis, duplication of the entire chromosome set > haploid, triploid or tetraploid state
Aneuploidy
Changes in number of individual chromosomes > caused by mitotic errors or errors in DSB repair
Chromosome instability
A state of susceptibilty to change in chromosome number (in >80% of carcinomas)
Are chromosomal instability and chaotic karyotypes required for transformation?
No
Can chromosome instability lead to full transformation and stimulation of initial mutations?
Yes
Chromosome instability is cancer dependent. Name the scenarios for solid tumors andf hematopoietic tumors
-Solid tumors: characterized by instability
-Hematopoietic tumors: cells often have diploid karyotypes with one or two translocations
What is the most common cause of changes in chromosome number?
Mitotic missegregation of chromosomes
Nondisjunction
both sister chromatids in a pair are pulled to one centrosome and end up in the same daughter cell
Merotely
A single chromatid is pulled by both centrosomes
> often in cancer cells
> result of disfunctional centrosomes
Genetic instability in cancer
-Dysfunction of repair mechanisms make cancer cells vulnerable
-Allows cancer to accumulate genomic changes which are needed for transformation > accelerated evolution
