There is much interest in understanding the mechanisms underlying the complex patterns displayed in mutational spectra, because these spectra will help to illuminate the molecular etiology of genetic diseases, such as cancer. The lambda gpt delta C57BL/6J mouse is an extraordinarily useful model for the probing underlying mechanisms of human cancer, and the mutational spectra of dozens of environmental carcinogens have been characterized using this transgenic animal model. Responding to the need for a high-throughput cell culture model derived from this mouse, Dr. Pennapa Thongararm from the Essigmann lab used lentiviral transformation to produce mouse embryo fibroblast cell lines. She then tested the utility of these cells for studies of the kind of DNA damage (specifically, methylation damage) that is produced by N-nitrosamines, environmental carcinogens that are the focus of MIT SRP research.

Studies by Dr. Thongaram et al., recently published in Chemical Research in Toxicology, show a clear induction of methylation damage-induced mutations, with the predominant mutation being G to A. Based on the chemistry of the DNA damage that is formed by methylating agents, the team predicted that these mutations might be coming from O⁶-methylguanine, which can mispair efficiently with thymine. To test this possibility, the team used a chemical inhibitor of the protein that repairs O⁶MeG and queried the resulting mutation pattern. They saw a clear enhancement of the original signature, providing direct evidence that O⁶MeG is the main driver of mutagenicity from methylating agents, and calling attention to the importance of DNA repair as a key player in suppressing methylation-induced mutations. This work opens doors to studies of methylation damage-induced mutations in vivo where ongoing collaborative work is uncovering the importance of gene-environment interactions as modulators of the risk of cancer from exposure to environmental contaminants. Publication DOI: 10.1021/acs.chemrestox.9b00444