Formation, Accumulation, and Hydrolysis of Endogenous and Exogenous Formaldehyde-Induced DNA Damage

Abstract

Formaldehyde is not only a widely used chemical with well-known carcinogenicity but is also a normal metabolite of living cells. It thus poses unique challenges for understanding risks associated with exposure. N 2-hydroxymethyl-dG (N 2 -HOMedG) is the main formaldehyde-induced DNA mono-adduct, which together with DNA-protein crosslinks (DPCs) and toxicityinduced cell proliferation, play important roles in a mutagenic mode of action for cancer. In this study, N 2 -HOMe-dG was shown to be an excellent biomarker for direct adduction of formaldehyde to DNA and the hydrolysis of DPCs. The use of inhaled [ 13 CD 2 ]-formaldehyde exposures of rats and primates coupled with ultrasensitive nano ultra performance liquid chromatography-tandem mass spectrometry permitted accurate determinations of endogenous and exogenous formaldehyde DNA damage. The results show that inhaled formaldehyde only reached rat and monkey noses, but not tissues distant to the site of initial contact. The amounts of exogenous adducts were remarkably lower than those of endogenous adducts in exposed nasal epithelium. Moreover, exogenous adducts accumulated in rat nasal epithelium over the 28-days exposure to reach steady-state concentrations, followed by elimination with a half-life (t 1/2 ) of 7.1 days. Additionally, we examined artifact formation during DNA preparation to ensure the accuracy of nonlabeled N 2 -HOMe-dG measurements. These novel findings provide critical new data for understanding major issues identified by the National Research Council Review of the 2010 Environmental Protection Agency's Draft Integrated Risk Information System Formaldehyde Risk Assessment. They support a data-driven need for reflection on whether risks have been overestimated for inhaled formaldehyde, whereas underappreciating endogenous formaldehyde as the primary source of exposure that results in bone marrow toxicity and leukemia in susceptible humans and rodents deficient in DNA repair.

N6-Formyllysine as a Biomarker of Formaldehyde Exposure: Formation and Loss of N6-Formyllysine in Nasal Epithelium in Long-Term, Low-Dose Inhalation Studies in Rats

“…A parallel investigation of FA-induced N 2 -hydroxymethyl-dG ( N 2 -HOMe-dG) and DNA-protein cross-links in the same study group revealed strong correlations between the DNA damage 13,14 and FLys. Similar to exogenous FLys, exogenous N 2 -HOMe-dG and DNA-protein cross-links showed formation and accumulation in nasal epithelium and not in tissues distant to the site of initial contact.…”

Measurement of Endogenous versus Exogenous Formaldehyde–Induced DNA–Protein Crosslinks in Animal Tissues by Stable Isotope Labeling and Ultrasensitive Mass Spectrometry

“…A synthesized standard, dG-Me-Cys, was prepared by digestion of dG-Me-Glutathione according to our previous study (28), using the experimental details described in the Supplementary Information. As demonstrated in a previous study, dG-Me-Cys showed a very close UV absorbance spectrum to that of dG (28).…”

Contributions of DNA repair and damage response pathways to the non-linear genotoxic responses of alkylating agents

“…Clearly, the presence of an adduct does not equal a mutation, and the availability of DNA repair activity will play a key role in the ultimate fate of a DNA lesion. The recently published data on formaldehyde, demonstrating that low doses of exogenously inhaled formaldehyde induced an increase in exogenous adducts present at levels below the structurally identical, quantifiable endogenous levels, is leading to a paradigm shift concerning the role of such adducts (endogenous and exogenous) in mutation and cancer induction [6, 25]. …”

“…New in vivo and in vitro data have demonstrated the existence of non-linear/bilinear dose-responses for genotoxic effects ( i.e. a dose–response curve with a slope not significantly different from zero gradient below the estimated threshold or Break Point Dose (BPD)), where there is no significant difference in mutant frequency between the spontaneous background of control and the low-dose exposure region of DNA-reactive agents [2-6]. In recent years, new statistical approaches have also been developed and applied to analyze low-dose results to establish whether the dose-response is linear or non-linear/bilinear, derive a point of departure (PoD), and determine what impact the spontaneous background genotoxicity should have on risk assessment.…”

“…Recent work has begun addressing the potential role of endogenous/background DNA damage in background mutagenesis [5, 6, 8, 16-19]. The Engelward laboratory developed a sensitive mouse model in which HR events at an integrated Fluorescent Yellow Direct Repeat (FYDR) transgene give rise to a fluorescent signal.…”