# Thymines opposite to bulky aristolactam-DNA adducts in duplex DNA are not targeted by human thymine-DNA glycosylase

## TL;DR

This study shows that human thymine-DNA glycosylase does not efficiently repair DNA damage caused by aristolochic acids, which are linked to urothelial cancer.

## Contribution

The study reveals that TDG does not target thymine opposite bulky aristolactam-DNA adducts, clarifying its role in aristolochic acid-induced DNA repair.

## Key findings

- TDG does not excise thymine opposite dA-AL or dG-ALII adducts efficiently in duplex DNA.
- TDG efficiently excises thymine opposite guanine and hypoxanthine in mismatched DNA pairs.
- These findings suggest TDG is not involved in repairing aristolochic acid-induced DNA damage.

## Abstract

Consumption of aristolochic acids (AA) from the plant Aristolochia results in the formation of bulky aristolactam-dA (dA-AL) and aristolactam-dG (dG-AL) adducts in cellular DNA ultimately leading to the development of urothelial cancer. Intriguingly, the dA-AL adducts induce A•T→T•A transversions in tumor cells preferentially in CpA*→TpG context. The human mismatch-specific thymine-DNA glycosylase (TDG) protects cells against mutagenesis induced by spontaneous deamination of 5-methylcytosine (5mC) by removing thymine opposite to guanine in a CpG context in the base excision repair (BER) pathway. Nevertheless, challenges for DNA glycosylases to the faithful discrimination between non-damaged and damaged DNA strands do exist, such as mismatched pairs between two canonical bases, which may result due to DNA polymerase errors during replication. Previously, we demonstrated that TDG is prone to aberrant excision of T opposite to damaged adenine in duplex DNA in CpA*/TpG context.

In the present work, using in vitro reconstitution assays, we investigated whether TDG participates in the aberrant removal of thymine opposite to dA-AL adducts in duplex DNA.

We have demonstrated that TDG either does not excise thymine or does so with extremely low efficiency when it is paired with dA-AL or dG-ALII adducts in duplex DNA. At the same time, TDG excises with high efficiency thymine opposite to guanine and hypoxanthine in T•G and T•Hx mispairs.

These findings strongly suggest that the human TDG is not involved in the aberrant DNA repair of AA-induced DNA damage.