Chromatin assembly and formaldehyde toxicity

PROJECT SUMMARY Formaldehyde is a known environmental and occupational chemical carcinogen. The molecular mechanisms of formaldehyde-induced carcinogenesis are largely undetermined. The DNA damage and associated mutagenesis induced by DNA adducts and DNA-protein crosslinks have been the focus of formaldehyde carcinogenicity research. However, recent studies showed that exogenous formaldehyde caused only a modest increase in DNA adducts above levels caused by endogenous formaldehyde. This suggested the possibility that epigenetic mechanisms might contribute to formaldehyde-induced carcinogenicity. We and others have demonstrated that formaldehyde reacts with lysine residues on histone proteins to form a labile Schiff base or the more stable N6-formyllysine, and that both Schiff bases and N6-formyllysine residues are refractory to acetylation. These findings indicated that formaldehyde-histone lysine adducts might interfere with important cellular process regulated by histone lysine acetylation. However, it remains unknown which process is affected by this mechanism. Our preliminary data from cellular fractionation analyses demonstrate that formaldehyde exposure dramatically decreases lysine acetylations of the cytosolic histones H3 and H4. This finding is very intriguing, given that these modifications are critical for histone nuclear import and chromatin assembly. Our preliminary results further show that formaldehyde reduces the total amount of histone in the chromatin fraction and the levels of histones at genomic loci, suggesting inhibition of chromatin assembly. Defective chromatin assembly causes dysregulation of gene expression and genomic instability and was directly linked to different types of cancers. Based on these observations, we hypothesize that the formation of formaldehyde-histone adducts compromises histone nuclear import and/or chromatin assembly, thereby contributing to formaldehyde-induced carcinogenesis. To test this hypothesis, in Aim 1, we will examine whether the nuclear import of newly synthesized histones and their assembly into chromatin are compromised following formaldehyde exposure and determine the underlying mechanisms. In Aim 2, we will determine the impact of aberrant chromatin assembly on formaldehyde-induced changes in gene expression, formation of DNA adducts and DNA-protein crosslinks, and chromosome instability. In Aim 3, we will investigate whether formaldehyde exposure compromises chromatin assembly in vivo. The proposed concept that a chemical carcinogen reacts with newly synthesized histones to regulate chromatin assembly is novel. This study has the potential to reveal a new mechanism for chemical carcinogenesis.