Table 1.

Summary of studies that tested associations between toxicant exposures and 5-hmC levels in the brain

Study Author (citation)	Exposure	Model organism	Tissue	Sex	Method to measure 5-hnrC	Direction of differential 5-hnrC	Genomic scale of differential 5-hnrC	Differentially hydroxynrethylated gene IDs
Byun et al. 2015 [40]	BDE-47	Wistar rats	Frontal lobe	Male and female	ELISA	No significant change	Global	N/A
Pierce et al. 2016 [41]	Pyridostigmine bromide, DEET, pemrethrin, and mild stress (GWI)	Sprague-Dawley rats	Cerebellum, cortex, and hippocampus	Male	ELISA	Increased (cerebellum), decreased (cortex)	Global	N/A
Inrpey et al. 2016 [42]	proton irradiation	C57B16/J mice	Hippocampus	Male	hMeDIP-sequencing	Bidirectional (increased and decreased)	Genonre-wide	Thousands of genes with hyper- and hypo-DhMRs; exact number unclear in manuscript
Inrpey et al. 2017 [43]	Proton irradiation	C57B16/J mice	Hippocampus	Male	hMeDIP-sequencing	Bidirectional (increased and decreased)	Genonre-wide	1709 and 1628 genes with hyper- and hypo-DhMRs; 677 genes with bidirectional DhMRs.
Acharya et al. 2017 [44]	28Si particle irradiation	C57B16/J mice	Hippocampus	Male	Immunofluorescence	Increased	Global	N/A
Ozturk et al. 2017 [45]	Ethanol	C57BL/6 mouse embryo (El7)	Frontal cortex (subventricular zone and ventricular zone)	N/A	Immunocytochemistry	Decreased	Global	N/A
Du et al. 2018 [46]	Arsenic trioxide	Sprague Dawley rats	Cortex and hippocampus	Male	Liquid chronratography-mas s spectrometry	Decreased	Global	N/A
Bordoni et al. 2019 [47]	Pemrethrin	Wistar rats	Substantia nigra pars compacta and striatum nucleus	Male and female	ELISA	Increased (male), decreased (female)	Global	N/A
Malloy et al. 2019 [48]	Bisphenol A	Avy (viable yellow agouti) mice	Cortex and midbrain	Male and female	oxBS-pyrosequencing (Kcnql gene)	No significant change	CpG-level	N/A

In the last 5 years, a small number of studies have investigated the effects of toxicant exposures on 5-hmC in the brain using animal/cell models. Here, we summarize the results of these studies, and also provide information regarding the specific exposure, model organism, tissue, sex, and methods used to measure 5-hmC. Most of the available data relies on global measures of 5-hmC, which do not provide the specificity necessary to truly understand whether gene-level changes in 5-hmC modify disease risk. Only one study by Malloy et al. [48] measured 5-hmC at the CpG-level