Table 2.

Evidence supporting adverse outcome pathway (AOP) network for DNA methylation (Figure 1A).

Key event	Event description	Key event relationship	Reference
Exposure
IE	Arsenic Exposure	Sources of human exposure to inorganic arsenic (iAs) include drinking water, diet, air, and soils (which can contain naturally occurring arsenic or contamination from anthropogenic sources).	(ATSDR 2007)
Molecular
KE1	Decreased DNA methylation	S-adenosyl methionine (SAM) is a methyl group source used during DNA methylation and is also consumed during the detoxification of iAs to mono- and dimethylated metabolites (MMA and DMA, respectively). The consumption of SAM during As metabolism may limit the availability of methyl donors.	(Bailey et al. 2013; Vahter and Marafante 1988).
KE2	Increased DNA methylation of TP53 promoter	In a cohort from Zamapan, Mexico, exposed to iAs (average $110\text{ppb}$) in drinking water, a positive association between hypermethylation of a TP53 network and arsenic-induced skin lesions was identified.	(Chanda et al. 2006; Smeester et al. 2011)
KE3	Decreased TP53 gene expression	TP53 is a tumor suppressor and its loss of activity propagates DNA damage, the accumulation of mutations, and cancer.	(Levine and Oren 2009)
Cellular/Tissue
KE4	Increased DNA damage	Loss of TP53 activity propagates DNA damage.	(Levine and Oren 2009)
KE5	Increased DNA mutation	DNA damage may lead to the accumulation of mutations.	(Levine and Oren 2009).
Organ
KE6	Increased tumors	Alterations in the activity of tumor suppressor genes is positively correlated with tumor progression.	(Yokota 2000)
Individual
AO1	Diabetes	In a cohort from Zamapan, Mexico, exposed to iAs (average $110\text{ppb}$) in drinking water, urinary MMA and DMA levels were positively associated with peripheral blood leukocyte DNA hypomethylation in the promoters of type 1 and type 2 diabetes mellitus target genes.	(Bailey et al. 2013).
AO2	Cancer	Increased incidence of skin cancer was observed in populations drinking water with high iAs concentration.	(Tseng et al. 1968; Tseng 1977)

Note: AO, adverse outcome; DMA, dimethylarsinic acid; IE, initiating event; KE, key event; MMA, monomethylarsonic acid.