Page 569 - The Toxicology of Fishes

Chemical Carcinogenesis in Fishes 549

concluded that epigenetic events were the initiating source of the cancers observed (Koehler, 2004),
although none was characterized.
One class of nongenotoxic epigenetic carcinogens is selected pesticides. Pesticide examples include
chlorothalonol and acetochlor, which promote induction of CYP1A, formation of ROS, and peroxisome
proliferation (Freeman and Rayburn, 2006; Rakitsky et al., 2000). Acetochlor is used as a herbicide and
is a contaminant found in the aquatic environment at levels of 700 ng/L to highest concentrations of 2.7
µg/L (Helbing et al., 2006). In cell-cycle studies, it was found to significantly reduce the number of cells
in the G phase of the cell cycle (Freeman and Rayburn, 2006). Future studies will likely identify many
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more examples of epigenetic carcinogens and hopefully elucidate their mechanisms of action in fish.

DNA Repair
Overview of Repair Mechanisms
DNA repair is fundamentally important to all cells and organisms. In humans, defects in normal DNA
repair are associated with diseases including xeroderma pigmentosum, Cockayne’s syndrome, hereditary
nonpolyposis colorectal cancer, and Fanconi’s anemia; high cancer incidences are commonly associated
with defective DNA repair processes (Lehmann, 2003). Different mechanisms or pathways are respon-
sible for repairing different types of DNA damage, and in mammals one type of damage may be repaired
by a preferred pathway with high activity, as well as by a different pathway with lower activity for that
damage. Such a system operates to ensure the efficient removal of damaged DNA. A brief overview of
DNA repair pathways is presented before discussing DNA repair in fishes. Readers are referred to an
excellent review of DNA repair for additional information (Sancar et al., 2004).

Direct Reversal (Pyrimidine Dimers)
Direct reversal pathways utilize proteins with a high specificity for a specific type of damage, and the
protein directly repairs the damage without synthesizing new DNA. Examples of direct reversal repair
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proteins are DNA photolyase and O -alkylguanine-DNA transferase (ADT). Photolyase recognizes
pyrimidine dimers that arise from exposure to ultraviolet (UV) light (adjacent pyrimidine bases in a
DNA strand covalently joined following exposure to UV light). Photolyase scans DNA, binds pyrimidine
dimers, and utilizes energy from photoreactivating light to break covalent bonds joining bases, thus
directly returning the DNA to its original state. ADT in Escherichia coli removes methyl groups from
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the O of guanine and from the O of thymine, and some larger alkyl groups are also removed by this
protein. DNA-alkyltransferases function as suicide enzymes; that is, each protein molecule can remove
one alkyl group from DNA, the alkyl group becomes attached to the DNA-alkyltransferase, and the
protein cannot remove additional alkyl groups.

Deficient/Low Excision Repair (Especially PAH Adducts)
Excision repair is a different type of mechanism for repairing damaged DNA and involves removal of
damaged nucleotides followed by synthesis of new DNA. Excision repair can be classified as base
excision repair or nucleotide excision repair. In base excision repair, enzymes (DNA glycosylases)
recognize damaged bases and initiate the repair process by cleaving the N-glycosidic bond connecting
damaged base and sugar, creating an apurinic/apyrimidimic (AP) site. AP endonuclease then removes
the base-free sugar by cleaving phosphodiester bonds, creating a one-nucleotide gap in the DNA; DNA
polymerase replaces the missing nucleotide using the complimentary strand as a template, and DNA
ligase forms a phosphodiester bond with the new nucleotide and the original strand, completing the
repair process. Different DNA glycosylases recognize different types of DNA damage, including glyc-
osylases specific for uracil, 5-methylcytosine, G–T mispairs, 3-methyladenine, formamidopyrimidine
moieties, and pyrimidine dimers. In the case of nucleotide excision repair, a multi-enzyme complex
containing endonucleases recognizes damaged DNA, often containing bulky adducts or distorted sec-
ondary conformation. Repair is initiated by endonuclease incision on either side of the damaged site,