Homologous recombination contributes to the repair of acetaldehyde-induced DNA damage

Transcription blocking properties and transcription-coupled repair of N2-alkylguanine adducts as a model for aldehyde-induced DNA damage

The N2 position of guanine is a preferential reaction site in DNA for numerous dietary and environmental carcinogens or their electrophilic metabolites, aldehydes arising from lipid peroxidation as well as reactive by-products of normal metabolism. However, DNA repair mechanisms of the resulting covalent adducts in mammalian cells are not well understood, with nucleotide excision repair (NER), base excision repair, and a dioxygenase-mediated damage reversal being discussed as likely pathways. Considering fundamentally different damage recognition principles between the global genome NER and the transcription-coupled (TC)-NER, we here assessed transcription blocking capacities of four synthetic deoxyguanosine (dGuo) adducts of variable size and geometry, using a transfection-based reporter assay. Notably, adducts as different as the aliphatic N2-ethylguanine, the exocyclic 1,N2-ethenoguanine, and the bulky polycyclic 3-(deoxyguanosin-N2-yl)-2-acetylaminofluorene, displayed robust DNA strand-specific transcription-blocking properties. The specific TC-NER components ERCC8/CSA and ERCC6/CSB were consistently required for the removal of all transcription-blocking N2-dGuo adducts, whereas the absence of XPC or DDB2/XPE (both specific to global genome NER) did not compromise the repair capacities in the isogenic human cell models. In contrast, no inhibition of the gene expression was detected for reporter constructs carrying N2-methylguanine even in the NER-deficient XP-A cell line, suggesting that this adduct is either bypassed with very high efficiency during transcription or repaired by a mechanism different from NER. Collectively, the results identify N2-dGuo adducts bigger than methylguanine as a structural subclass of transcription-blocking DNA lesions whose repair heavily relies on the TC-NER pathway.

Loss of Heterozygosity associated with ubiquitous environments in yeast

The effect of ubiquitous environmental conditions on mutational mechanisms, particularly loss of heterozygosity (LOH) remains poorly understood. Environment induced LOH can rapidly alter the genome and promote disease progression. Using mutation accumulation (MA) lines, we analysed the effect of ubiquitous environmental conditions on mutational mechanisms in a diploid hybrid (S288c/YJM789) baker's yeast strain. These included blue light, low glucose (calorie restriction), oxidative stress (H2O2), high temperature (37°C), ethanol, and salt (NaCl). The frequency of LOH increased significantly in all environments including calorie restriction relative to the control (YPD). Interestingly, the percentage of the genome covered by LOH varied significantly depending on the condition. For example, the LOH tracts seen in calorie restriction conditions were significantly shorter than those observed in blue light exposure that rapidly homozygotized the genome. We also report a unique mutational signature of blue light exposure comprising LOH, small indels, large deletions and transversion mutations (G:C > T:A; G:C > C:G), with the latter likely to result from the photooxidation of guanine bases. Our results suggest ubiquitous environmental conditions cause LOH but result in distinct mutational signatures due to the type of damage induced and the pathways used to repair them.

The mutagenic properties of formaldehyde and acetaldehyde: Reflections on half a century of progress

Formaldehyde and acetaldehyde are reactive, small compounds that humans are exposed to routinely, variously from endogenous and exogenous sources. Both small aldehydes are classified as human carcinogens. Investigation of the DNA damaging properties of these two compounds began some 50 years ago. In this review, we summarize progress in this field since its inception over half a century ago, distilling insights gained by the collective efforts of many research groups while highlighting areas for future directions. Over the decades, general consensus about aspects of the mutagenicity of formaldehyde and acetaldehyde has been reached. But other characteristics of formaldehyde and acetaldehyde remain incompletely understood and require additional investigation. These include crucial details about the mutational signature(s) induced and possible mechanistic role(s) during carcinogenesis.