Structural characterization of formaldehyde-induced cross-links between amino acids and deoxynucleosides and their oligomers

Exposure to formaldehyde results in the formation of DNA-protein cross-links (DPCs) as a primary genotoxic effect. Although DPCs are biologically important and eight amino acids have been reported to form stable adducts with formaldehyde, the structures of these cross-links have not yet been elucidated. We have characterized formaldehyde-induced cross-links of Lys, Cys, His, and Trp with dG, dA, and dC. dT formed no cross-links, nor did Arg, Gln, Tyr, or Asn. Reaction of formaldehyde with Lys and dG gave the highest yield of cross-linked products, followed by reaction with Cys and dG. Yields from the other coupling reactions were lower by a factor of 10 or more. Detailed structural examination by NMR and mass spectrometry established that the cross-links between amino acids and single nucleosides involve a formaldehyde-derived methylene bridge. Lys yielded two additional products with dG in which the linking structure is a 1,N(2)-fused triazino ring. The Lys cross-linked products were unstable at ambient temperature. Reactions between the reactive N(alpha)-Boc-protected amino acids and the trinucleotides d(T(1)B(2)T(3)) where B(2) is the target base G, A, or C and reactions between dG, dA and dC and 8-mer peptides containing a single reactive target residue at position 5 yielded cross-linked products with structures inferred from high resolution mass spectrometry and fragmentation patterns that are consistent with those between N(alpha)-Boc-protected amino acids and single nucleotides rigorously determined by NMR studies. These structures will provide a basis for investigation of the characteristics and properties of DPCs formed in vivo and will be helpful in identifying biomarkers for the evaluation of formaldehyde exposure both at the site of contact and at distant sites.

Formaldehyde-induced genome instability is suppressed by an XPF-dependent pathway

Formaldehyde is a reactive chemical that is commonly used in the production of industrial, laboratory, household, and cosmetic products. The causal association between formaldehyde exposure and increased incidence of cancer led the International Agency for Research on Cancer to classify formaldehyde as a carcinogen. Formaldehyde-induced DNA-protein crosslinks (DPCs) elicit responses involving nucleotide excision repair (NER) and homologous recombination (HR) repair pathways; however, little is known about the cellular and genetic changes that subsequently lead to formaldehyde-induced genotoxic and cytotoxic effects. Herein, investigations of genes that modulate the cytotoxic effects of formaldehyde exposure revealed that of five NER-deficient Chinese Hamster Ovary (CHO) cell lines tested, XPF- and ERCC1-deficient cells were most sensitive to formaldehyde treatment as compared to wild-type cells. Cell cycle analyses revealed that formaldehyde-treated XPF-deficient cells exhibited an immediate G2/M arrest that was associated with altered cell ploidy and apoptosis. Additionally, an elevated number of DNA double-strand breaks (DSBs), chromosomal breaks and radial formation were also observed in XPF-deficient cells following formaldehyde treatment. Formaldehyde-induced DSBs occurred in a replication-dependent, but an XPF-independent manner. However, delayed DSB repair was observed in the absence of XPF function. Collectively, our findings highlight the role of an XPF-dependent pathway in mitigating the sensitivity to formaldehyde-induced DNA damage as evidenced by the increased genomic instability and reduced cell viability in an XPF-deficient background. In addition, centrosome and microtubule abnormalities, as well as enlarged nuclei, caused by formaldehyde exposure are demonstrated in a repair-proficient cell line.