The impact of air pollution on sperm DNA methylation

A number of environmental factors have been shown to impact the sperm epigenome. Air pollution is one of the largest health and environmental hazards in the world today and has been implicated in many modern diseases. Recently, air pollution has been shown to alter methylation signatures in some body tissues, indicating that air pollution may also affect the sperm epigenome. The present experiment was conducted to analyze how seasonal air pollution in the Salt Lake Valley may impact DNA methylation patterns in sperm and to establish a relationship between air pollution and sperm epigenetic health as measured by DNA methylation. Sperm DNA methylation patterns were assessed in 74 individuals, who presented at the University of Utah Andrology Clinic for semen analysis, using the Illumina Human MethylationEPIC BeadChip array. Each semen sample collected, as per the fifth edition of WHO reference values for human semen characterization, was deemed normal. Two sample groups from the Salt Lake Valley, Urban Winter (UW, n = 20), Urban Summer (US, n = 21), and two sample groups east of the Wasatch mountains, Rural Winter (RW, n = 19) and Rural Summer (RS, n = 14), were compared to assess the effect of air pollution on sperm DNA methylation patterns. Due to seasonal inversions, urban winters are characterized by increased air pollution compared to summer months. Therefore, the UW sample group was designated as treatment and the three remaining groups (US, RW, RS) were designated as control. We conducted multiple differential methylation analyses using a sliding window approach which utilized the USeq software package. A sliding window analysis of UW versus US was conducted first, followed by a confirmatory analysis comparing UW versus RW and RS. Outputs from the USeq analysis were assessed using several tools including the Stanford GREAT analysis and an analysis of methylation instability at key promoter regions in sperm. The sliding window analysis identified six differentially methylated regions (DMRs) between the UW and US groups (Wilcoxon FDR ≥ 40, corresponding p-value of ∼0.0001). Three of these six regions were confirmed with the second confirmatory analysis of UW versus RS/RW (Wilcoxon FDR ≥ 20, p-value<0.01). According to a GREAT analysis, each of the identified regions exhibited multiple gene ontology associations. Air pollution subtly alters DNA methylation in sperm, indicating that certain regions of the sperm epigenome may be susceptible to air pollution-induced modification with possible implications for reproductive and offspring health.