DNA polymerase arrest by adducted trivalent chromium

Area of Science:

• Environmental Science
• Molecular Biology
• Toxicology

Background:

• Hexavalent chromium (Cr6+) is a known carcinogen that enters cells and is reduced to trivalent chromium (Cr3+).
• Trivalent chromium (Cr3+) readily forms adducts with DNA, potentially leading to genotoxicity.
• The precise mechanisms by which chromium adducts affect DNA replication in vitro are not fully understood.

Purpose of the Study:

• To investigate the impact of trivalent chromium (Cr3+) DNA adducts on in vitro DNA synthesis.
• To analyze the role of chromium-mediated DNA cross-linking in polymerase arrest.
• To elucidate the potential genotoxic mechanisms of chromium exposure.

Main Methods:

• Utilized a polymerase-arrest assay with synthetic DNA replication templates treated with varying concentrations of chromium(III) chloride.
• Separated prematurely terminated replication products using DNA sequencing gels.
• Employed Sequenase version 2.0 T7 DNA polymerase, DNA polymerase I large (Klenow) fragment, and unmodified T7 DNA polymerase for replication studies.

Main Results:

• Dose-dependent DNA polymerase arrest was observed at biologically relevant chromium adduct levels (6 and 21 adducts/1,000 nucleotides).
• Polymerase arrest sites predominantly occurred upstream of guanine residues.
• Interstrand DNA cross-linking was detected at higher adduct levels, and low-level cross-linking was observed even at the lowest dose, correlating with polymerase arrest.

Conclusions:

• Chromium-mediated interstrand DNA-DNA cross-links are a likely cause of DNA polymerase arrest during replication.
• This obstruction of DNA replication by chromium adducts represents a potential mechanism for chromium-induced genotoxicity in vivo.
• The findings highlight the critical role of DNA replication interference in the carcinogenic effects of chromium.