Effects of Halobenzoquinone Water Disinfection By-Products on Human Neural Stem Cells

Abstract

Water disinfection inactivates microbiological pathogens in drinking water but also unintentionally produces disinfection byproducts (DBPs). Epidemiological studies have observed potential correlations between the consumption of chlorinated water with an increased risk of developing bladder cancer and have found inconsistent correlations with adverse reproductive effects. Public health organizations, such as the World Health Organization, United States Environmental Protection Agency and Health Canada, have placed regulations on a small number of DBPs. These DBPs include total trihalomethanes (TTHMs) and five haloacetic acids (HAA5). Halobenzoquinones (HBQs), an emerging class of DBPs, are capable of causing damage to cellular DNA and proteins in T24 and CHO cells. Little is known about the neurotoxicity of DBPs in in vitro and in vivo systems. Human neural stem cells (hNSCs) are a potentially useful model to test the effects of chemical exposure, such as DBPs, on developmental neurotoxicity. The aim of this thesis is to study the effects of DBP exposure on the differentiation of hNSCs into neurons. Two HBQs, 2,6-dibromobenzoquinone (2,6-DBBQ) and 2,6-dichlorobenzoquinone (2,6-DCBQ), were selected because of their frequent and widespread occurrence in drinking water. To understand the significance of HBQ neurotoxicity, I also included two regulated HAAs, bromoacetic acid (BAA) and chloroacetic acid (CAA), in these studies. First, I used qualitative imaging methods to observe the physical characteristics and changes in protein expression of differentiated and undifferentiated hNSCs. Next, I analyzed the impact of HBQ and HAA exposure on the growth and cell cycle of hNSCs. Finally, I assessed the effects of HBQ and HAA exposure on the differentiation of hNSCs into neurons and measured axon length. I observed that hNSCs lost the expression of stem cell differentiation markers nestin and Sox2 after five passages. Differentiation was induced at passage three or four in subsequent experiments. Flow cytometry analysis showed that hNSCs exposed to 0.5 uM 2,6-DBBQ and 1 uM 2,6-DCBQ for 96 hours resulted in higher proportions of cells in S-phase. This result suggests cell cycle arrest in the S-phase, the phase at which DNA replication occurs. Additionally, the ratio of mature neurons to immature neurons was lower in the cells exposed to 0.5 uM BAA and 0.5 uM CAA for 12 days compared to negative controls, indicating that the tested DBPs can inhibit the cell maturation process. Overall, these results suggest that hNSCs are an appropriate model to test the in vitro developmental neurotoxicity of DBPs. Further research on DBP neurotoxicity will contribute to the understanding of the potential developmental effects of DBPs.