toxics Article Early Pregnancy Exposure to Ambient Air Pollution among Late-Onset Preeclamptic Cases Is Associated with Placental DNA Hypomethylation of Specific Genes and Slower Placental Maturation Karin Engström 1, * ,† , Yumjirmaa Mandakh 2,† , Lana Garmire 3 , Zahra Masoumi 4 , Christina Isaxon 5 , Ebba Malmqvist 2 , Lena Erlandsson 4,‡ and Stefan R. Hansson 4,6,‡   Citation: Engström, K.; Mandakh, Y.; Garmire, L.; Masoumi, Z.; Isaxon, C.; Malmqvist, E.; Erlandsson, L.; Hansson, S.R. Early Pregnancy Exposure to Ambient Air Pollution among Late-Onset Preeclamptic Cases Is Associated with Placental DNA Hypomethylation of Specific Genes and Slower Placental Maturation. Toxics 2021, 9, 338. https://doi.org/10.3390/ toxics9120338 Academic Editor: Soterios A. Kyrtopoulos Received: 12 November 2021 Accepted: 29 November 2021 Published: 6 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 EPI@LUND, Division of Occupational and Environmental Medicine, Department of Laboratory Medicine, Lund University, SE-22362 Lund, Sweden 2 Environment Society and Health, Division of Occupational and Environmental Medicine, Department of Laboratory Medicine, Lund University, SE-22362 Lund, Sweden; Yumjirmaa.Mandakh@med.lu.se (Y.M.); Ebba.Malmqvist@med.lu.se (E.M.) 3 Department of Computational Medicine and Bioinformatics, University of Michigan, Ann Arbor, MI 48104, USA; lgarmire@umich.edu 4 Division of Obstetrics and Gynecology, Department of Clinical Sciences Lund, Lund University, SE-22185 Lund, Sweden; Zahra.Masoumi@med.lu.se (Z.M.); Lena.Erlandsson@med.lu.se (L.E.); Stefan.Hansson@med.lu.se (S.R.H.) 5 Ergonomics and Aerosol Technology, Department of Design Sciences, Lund University, SE-22362 Lund, Sweden; Christina.Isaxon@design.lth.se 6 Obstetrics and Gynecology, Skåne University Hospital, SE-22184 Lund, Sweden * Correspondence: Karin.Engström@med.lu.se † These authors have contributed equally to this work and share first authorship. ‡ These authors have contributed equally to this work and share last authorship. Abstract: Exposure to ambient air pollution during pregnancy has been associated with an increased risk of preeclampsia (PE). Some suggested mechanisms behind this association are changes in placental DNA methylation and gene expression. The objective of this study was to identify how early pregnancy exposure to ambient nitrogen oxides (NO x ) among PE cases and normotensive controls influence DNA methylation (EPIC array) and gene expression (RNA-seq). The study included placentas from 111 women (29 PE cases/82 controls) in Scania, Sweden. First-trimester NO x exposure was assessed at the participants’ residence using a dispersion model and categorized via median split into high or low NO x . Placental gestational epigenetic age was derived from the DNA methylation data. We identified six differentially methylated positions (DMPs, q < 0.05) comparing controls with low NO x vs. cases with high NO x and 14 DMPs comparing cases and controls with high NO x . Placentas with female fetuses showed more DMPs (N = 309) than male-derived placentas (N = 1). Placentas from PE cases with high NO x demonstrated gestational age deceleration compared to controls with low NO x (p = 0.034). No differentially expressed genes (DEGs, q < 0.05) were found. In conclusion, early pregnancy exposure to NO x affected placental DNA methylation in PE, resulting in placental immaturity and showing sexual dimorphism. Keywords: ambient air pollution; preeclampsia; global DNA methylation; RNA sequencing; epige- netic age; placenta 1. Introduction Preeclampsia (PE) affects, in average, 0.2%–9% of pregnancies worldwide [1,2] and has been associated with increased risk of maternal mortality [3,4] and maternal cardiovascular disease (CVD) in later life. In addition, offspring born to PE mothers are at high risk of low birth weight at birth and CVD later in life [5,6]. Preeclampsia presents with new onset Toxics 2021, 9, 338. https://doi.org/10.3390/toxics9120338 https://www.mdpi.com/journal/toxics