carcinogens

Question 1

what are carcinogens?

Answer 1

chemical or physical sources that initiate or promote carcinogenesis

Question 2

what is a DNA damage?

Answer 2

abnormal chemical structure in DNA which causes changes

Question 3

what is a genotoxin?

Answer 3

a physical or chemical agent that can induce DNA damages

Question 4

what is a mutation?

Answer 4

an alteration of the nucleotide sequence in the genome

Question 5

what is a mutagen?

Answer 5

a physical or chemical agent that can induce mutations

Question 6

what types of genotoxins are there? What do they cause?

Answer 6

Mutages (DNA adducts and mutations)

clastogens (chromosome breaks)

Aneugens (mitotic spindle)

Question 7

what are the key characteristics of carcinogens?

Answer 7

is genotoxic
alters DNA repair
causes genomic instability
induces oxidative stres
induces chronic inflammation
is immunosuppressive
models receptor mediated effects
causes immortalization
alters cell proliferation/ cell death/ nutrient supply

Question 8

what is the difference between genotixic carcinogens and non-genotoxic carcinogens?

Answer 8

genotoxic: initiate tumor formation by DNA damage and DNA mutagenesis. Is not concentration dependent. DNA damage is reversible but mutation is not. mutagenic

non-genotoxic: promote tumor formation by pathways other than DNA damage –> cell proliferation, cytotoxicity, hormonal effects
is concentration dependent, reversible and non-mutagenic –> provides growth advantage

Question 9

Give examples for genotoxins and non-genotoxins

Answer 9

genotoxin: Aflatoxin B1, Formaldehyde, Nitrosamine, Vinyl Chloride

non-genotoxic: Asbestos, Dioxins, Estradiol, Phenobarbital

Question 10

what are common types of DNA damage?

Answer 10

8-oxo-2’-deoxyguanosine: oxidative lesion in duplex DNA

ß-N-glycosidic bond between base and deoxyribose is hydrolyzed

Single Strand break: : directly or indirectly (BER)

double strand breaks

O6-MG: alkylation of oxygen antom of guanine –> repaired by suicide enzyme MGMT

spontaneous hydrolysis reaction of cytosine into Uracil

Question 11

What is MGMT and how does it work?

Answer 11

is a suicide enzyme that is synthesized if DNA damage is detected: Transfers CH3 group of DNA to its self and is then degraded

Question 12

how can genotoxic carcinogens be activated? what are direct and indirect genotoxins?

Answer 12

many genotoxins are not genotoxic by themselves but can form genotoxic metabolites through cellular metabolism: eg cytochrome P450 enzymes

direct genotoxins: active without metabolic activation

indirect: carcinogenic after metabolic activation

Procarcinogens are concerted by phase I enzymes. They then either form inactive metabolites which are secreted or ultimate carcinogens which are active and can lead to DNA damage

Question 13

is DNA damage random?

Answer 13

No, each carcinogen has its specific pattern how it reacts with the DNA bases

Question 14

how can DNA adducts be detected?

Answer 14

32P Postlabeling
Immunoassay
CG/MS and LC/MS

Question 15

what is the relation between DNA damage, mutation and cell division?

Answer 15

cell division is required to fix a DNA damage

once a DNA damage is fixed in a mutation, it cannot be repaired

Question 16

which mutations are more likely: transition mutation or transversions of bases?

Answer 16

transition more likely than transversion

Question 17

what can lead to specific mutational patterns?

Answer 17

chemical exposures and DNA repair defects

Question 18

why is tumor formation a rare event even though a lot of DNA damages occur per day per cell?

Answer 18

-efficient DNA repair systems
-many mutations are silent (introns, third base of codon)
-possibility of neutral mutation
-most gnees are not involved in cancer

Question 19

is DNA damage sufficient for carcinogenesis?

Answer 19

no, but it is a prerequisite

Question 20

what are the effects of genotoxic and non-genotoxic mechanisms?

Answer 20

genotoxic: DNA adducts; chromosome fusion, deletions, breackage, mis- segregation, non-disjunction

non-genotoxic: inflammation, immunosuppression, ROS, receptor activation, epigenetic silencing –> altered signal transduction

Question 21

what is assesed in safety testing of drugs?

Answer 21

carcinogetic properties are evaluated in mutagenicity and carcinogenicity studies –> most time and cost consuming

Question 22

how are carcinogens regulated? What is paracelsus principle?

Answer 22

paracelsus principle: any posin can be non-toxic if the dose is below an appropiate threshold

–> but no threshold at any exposure level for genotoxic carcinogens

–> drug development stopped if genotoxicity etsts are positive, but non-genotoxic carcinogens may be used

Question 23

how does carcinogenicity testing work?

Answer 23

2-years rodent bioassay:
-rats
-male and female
-400 to 600 animals per study
-one control and three dosis groups

Question 24

how does genotoxicity testing work?

Answer 24

several in vivo and in vitro tests

assessing genotoxicity via detection of mutations, chromosome abnormalities or DNA breaks

–>bacteria: Ames Test
–>rodent/human cells: MLA/HPRT, Micronucleus test, chromosomal aberrations

–> in vivo test as follow up

Question 25

how does the Ames test work?

Answer 25

salmonella or e coli which are auxotrophic for an essential amino acid

incubation of tester strains with a potential genotoxin

induced mutations lead to increase in number of revertants –> are able to grow on minimal essential amino acid medium

Question 26

how does the mammalian mutagenicity tesing (MLA TK test) work?

Answer 26

in vitro gene mutation assay:

nucleoside analogues are incorporated into DNA in TK-positive cells –> cytotoxic

genotoxin induced mutaions in TK lead to resistance –> survival

Question 27

how does the micronucleus test work?