DNA damage
mutagenicity and genotoxicity
lDNA:
- principal molecule for life of the cell
- structure and function carefully checked
- changes rapidly repaired
- irreversible changes -> cell death (apoptosis)
l
lMutagenesis - MUTATIONS
l - changes in the sequences of deoxynucleotides
l - natural mutations (billions of nucleotides/day)
: variability in genoms; reparations
l
- chemical-induced mutagenesis

DNA damage



DNA repair
lDamage of DNA is carefully controlled
l constitutively expressed proteins
l
l Changes in DNA
l induction of reparation enzymes ("SOS-repair")
l = biomarker of DNA damage







Induced mutations
lMUTAGENS
l
l- ionizing radiation and UV
l
l- chemicals
l
–Base analogs - inserted into the DNA strand during replication in place of the substrates.
–
–Agents reacting with DNA - structural changes leading to miscopying of the template strand
–
–Indirect mutagens - affect cells that synthesize chemicals with direct mutagenic effect

Point mutations
BASE - EXCHANGE:
•Silent mutations:
•   code for the same amino acid.
•
•Missense mutations:
•   code for a different amino acid.
•
•Nonsense mutations:
•   which code for a stop

Point mutation
INSERTION
DELETION
Change of the
reading frame

Large scale mutations / chromosomal



Physical factors & DNA damage
lIonizating radiation
l - direct interaction with hydrogen atoms in water (and bases)
-> OH* radicals; H2O2, O2-
l - oxidation of bases; dimerization ...
l
lUV radiation
l - interaction with aromatic cycles (bases)
- base dimerization (T=T)

Ionizing radiation effects on DNA



Chemical induced DNA damage
lBases analogs
l - incorporation
into DNA
during replication
l(5-Br-Uracil: AT -> GC)

Chemical induced DNA damage
lHNO2, HSO3-,Hydroxylamine, Methoxyamine
ldeamination of bases
(GC -> AT)
l

Chemical induced DNA damage
lAlkylsulphates, N-nitroso-alkyles, cis-platinum
- alkylation of bases; crosslinks of dsDNA
l
cisplatin
cyclophosphamide

Chemical induced DNA damage
–INTERCALATION & ADDUCT FORMATION
lPolycyclic aromatic hydrocarbons (PAHs) & derivatives (N-acetyl-2-aminofluorene (AAF),
benzo[a]pyrene)
lMycotoxins (aflatoxins) aduct formation with DNA (biomarkers)
C:\Documents and Settings\Ludek Blaha\Plocha\E3.GIF

C:\Documents and Settings\Ludek Blaha\Dokumenty\katedra\vyuka\Biomarkery-Mechanismy\_molekularni
biologie\598-599.jpg
Psoralen
DNA intercalation




http://www.bioscience.org/1998/v3/d/trosko/fig2.jpg
Trosko and Ruch 1998, Frontiers in Bioscience 3:d208
AhR
ER
GJIC
Oxidative stress
IMPORTANT PROCESSES IN CANCEROGENESIS




Does chemically-induced genotoxicity results in in vivo effects
- adducts from mitochondrial DNA ?
- distance between „source of radicals“ and nuclear DNA ?
- protection mechanisms (mutation -> death/apoptosis)
Rubin (2002) Oncogene 21:7392
Thilly (2003) Nature Genetics 34(3):255
Mutations are not „primarily“ caused by chemicals
Chemicals only allow „unveil“ previously existing mutations in nuclear DNA (non-genotoxic events
cause cancer !!!)