Abstract 2006: Genome-wide study of DNA methylation alterations in response to pesticide exposure in in vitro
Authors: Zhang Xiao , Wallace Andrew , Du Pan , Baccarelli Andrea , Jafari Nadereh , Lin Simon , Hou Lifang 
Published: July 2012

Pesticides are widely used in the US and worldwide, and are pervasive in our environment. All pesticides sold in the US have passed the Environmental Protection Agency (EPA) screening procedures for carcinogenicity based on their genotoxicity and mutagenicity. However exposure to pesticides among pesticide applicators and manufacturing workers has repeatedly been shown to increase cancer risk, suggesting that pesticides may cause cancer via alternative mechanisms, such as epigenetic changes. The purpose of the present study is to examine whether exposure to organophosphate pesticides (OPs), a group of the most commonly used pesticides in the US, induces DNA methylation alterations in in-vitro.

The K562 progenitor blood cell line was exposed to several OPs (i.e., chlorpyrifos, diazinon, fonofos, malathion, parathion, phorate, and terbufos) at different dosages and time periods. DNA was prepared from samples exposed to ethanol (control) and a range of pesticide concentrations similar to exposure levels experienced by the US licensed pesticide applicators. We conducted genomewide DNA methylation analysis using the Illumina Infinium HumanMethylation27 BeadChip that covers 27,578 individual promoter CpG sites in the entire genome. The relative level of methylation was calculated as the ratio of signal from a methylated probe relative to an unmethylated probe. Bayesian-adjusted t-tests were used to identify differentially methylated sites. A cut-off of False Discovery Rate (FDR)-adjusted p-value (q-value) &lt; 0.05 and fold change &gt; 2 was used to identify candidate CpG sites.

We observed significant differences in genomewide DNA methylation patterns in relation to exposure to three pesticides (i.e., fonofos, parathion, and terbufos) that have been associated with cancers in human studies. Out of all genes with differentially methylated CpG site(s) for each of the three pesticides, we identified 712 genes (625 were hypermethylated and 87 were hypomethylated) overlapped for these three pesticides. Gene ontology analysis showed that these hyper- or hypo-methylated genes are implicated in carcinogenesis and related biological process, such as tumor protein p53 inducible protein 11 (TP53I11) (4.0-fold for fonofos, 4.7-fold for parathion, 3.1-fold for terbufos, respectively), growth arrest and DNA-damage-inducible gamma (GADD45G) (25.2-fold for fonofos, 23.1-fold for parathion, 31.2-fold for terbufos, respectively), and interleukin-1 receptor (IL1R1) (−2.2-fold for fonofos, -2.1-fold for parathion, -2.2 fold for terbufos, respectively).

Our results provided direct experimental evidence that pesticides can modify DNA methylation in gene promoter CpG sites, which may play pathological role in cancer development. Further studies in other cell types and human samples are required before any firm conclusion could be reached on the significance of pesticide-induced methylation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2006. doi:10.1158/1538-7445.AM2011-2006