Analysis of formaldehyde-induced Adh mutations in Drosophila by RNA structure mapping and direct sequencing of PCR-amplified genomic DNA

Toxic effects of formaldehyde and the protective effect of docosahexaenoic acid in Drosophila

Formaldehyde (FA) is a commercially important chemical applied in industry and scientific research. However, FA has a distinct impact on learning and memory. Although the mechanisms of FA toxicity have been well studied, additional research is required to establish the mechanisms of neuroprotection in cases of FA exposure. Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid with a variety of health benefits, including the enhancement of learning and memory. In this study, we investigated the neuroprotective effects of DHA in Drosophila melanogaster that had ingested FA. Our data suggested that DHA enhanced reproductive processes, leading to an increase in the number of eggs, larvae, and adults. Surprisingly, we found that DHA had a mild protective effect against FA-induced impairments in learning and memory.

The importance of distinct metabolites of N-nitrosodiethylamine for its in vivo mutagenic specificity

Although N-nitrosodiethylamine (NDEA) is a potent carcinogen in rodents and a probable human carcinogen, little attempts were made to characterize its mutation spectrum in higher eukaryotes. We have compared forward mutation frequencies at multiple (700) loci with the mutational spectrum induced at the vermilion gene of Drosophila, after exposure of post- and pre-meiotic male germ cells to NDEA. Among 30 vermilion mutants collected from post-meiotic stages were 12 G:C-->A:T transitions (40%), 8 A:T-->T:A transversions (27%), and 4 structural rearrangements (13%). The remainder were three A:T-->G:C transitions, two G:C-->C:G transversions and one G:C-->T:A transversion. The results show that although NDEA induces predominantly transitions (40% G:C-->A:T and 10% A:T-->G:C), the frequencies of transversions (37%, of which 27% of A:T-->T:A transversions) and especially of rearrangements (13%) are remarkably high. This mutation spectrum differs significantly from that produced by the direct-ethylating agent N-ethylnitrosourea (ENU), although the relative distribution of ethylated DNA adducts is similar for both carcinogens. These differences, in particular the occurrence of rearrangements, are most likely the result of the requirement of NDEA for bioactivation. Since all four rearrangements were collected from non-metabolizing spermatozoa (or late spermatids), it is hypothesized that they derived from acetaldehyde, a stable metabolite of NDEA. Due to its cytotoxicity, attempts to isolate vermilion mutants from NDEA-exposed pre-meiotic cells were largely unsuccessful, because only two mutants (one A:T-->G:C transition and one 1bp insertion) were collected from those stages. Our results show that NDEA is capable of generating carcinogenic lesions other than base pair substitutions.

Pharmacodynamics of formaldehyde: applications of a model for the arrest of DNA replication by DNA-protein cross-links

A variety of evidence suggests that formaldehyde (HCHO)-induced DNA-protein cross-links (DPX) are genotoxic as a result of their ability to arrest DNA replication. Although DPX can be removed and the DNA can be repaired, failure to remove the blockage prior to cell division or excision followed by incomplete repair could cause cell death or a mutation. To characterize the concentration and time dependence of this mechanism, a biologically based model for DNA replication in the presence of DPX was developed based on the assumptions that (1) DPX are formed randomly in the DNA and (2) a replication fork can advance up to but not past a DPX. Using a combination of Poisson and binomial statistics, a quantitative relationship between the amount of newly synthesized DNA and the concentration of DPX was derived, which predicts that the rate of DNA replication should decrease nonlinearly with increasing concentrations of DPX. Because the latter is a nonlinear function of the airborne concentration of HCHO, an inverse sigmoidal relationship is predicted between the rate of DNA replication and the concentration of inhaled formaldehyde. The model was parameterized using data derived from a study of the incorporation of [methyl-(14)C]thymidine monophosphate into the DNA of the nasal respiratory mucosa of Fischer-344 rats exposed to (3)HCHO and H(14)CHO (6 ppm, 6 h). The model was then applied to measurements of DNA replication in the nasal mucosa of experimental animals exposed to wide ranges of H(14)CHO (rats: 0.7, 2, 6, or 15 ppm, 3 h; rhesus monkeys: 0.7, 2, or 6 ppm, 6 h). The results indicate that, at airborne concentrations above 6 ppm in rats, there is a marked decrease (ca. 62% at 15 ppm) in the amount of newly synthesized DNA due to DPX formation during a single 6-h exposure to HCHO. The arrest of DNA replication at high HCHO concentrations could result in cytolethality or genotoxicity, both of which are critical factors in the induction of rat nasal cancer by HCHO. However, at concentrations below 2 ppm in monkeys or 1 ppm in rats, the decrease in the rate of DNA replication is predicted to be <1% after a 6-h exposure. This small decrease is probably undetectable using currently available techniques. The parameterized model suggests that the arrest of DNA replication by DPX is mainly a high-dose phenomenon and that at ambient exposure concentrations it is unlikely to be a major risk factor.

Formaldehyde mechanistic data and risk assessment: endogenous protection from DNA adduct formation

Exposures of rodents to airborne formaldehyde (FA) produce dose-related toxicity, enhanced cell proliferation and squamous cell carcinomas in the nasal passages. The mechanism of FA-induced tumor formation involves DNA-protein crosslink formation and enhanced cell proliferation secondarily to cytotoxicity. The mucociliary apparatus and glutathione protect against low-dose FA-induced effects. Consequently, the mechanistic information is consistent with a very sublinear dose-response curve for tumor formation. The sublinear dose-response of nasal DNA-protein crosslinks levels in rodents and monkeys has been used in the risk assessment of FA.