By Ela Selin Akgün
Air pollution is often seen as a threat to human genetics and health as well as a thread to environment. The components of air pollution such as particulate matter (PM), black carbon (BC), ozone (O3), nitrogen oxides (NOx), and polyaromatic hydrocarbons (PAHs) have been linked with several health outcomes. The studies conducted on two different age groups, 70 and infants, show that with air pollution DNA methylation and other epigenetic marks have decreased and therefore caused a weaker immune system and resulted with autoimmune diseases and other diseases affecting respiratory system mostly. In the first trimester, effects of air pollution to DNA methylation increases, therefore it results with fatal diseases and neonatal disorders, also increases the chances of miscarriages and infant mortality. Additionally, during the first trimester, vulnerability to changes in DNA methylation and histone acetylation increases, thus increases the chances of miscarriages and infant mortality.
Introduction
Air pollution is one of the major problems in today's world. It affects most of the cities in the world especially in the industrial zones. Almost 94 percent of world's population live in the areas that are considered and unsafe where air quality does not meet World Health Organization's guidelines. Air pollution can be a huge threat to human health and epigenetics, changing the epigenetics marks such as DNA methylation and histone acetylation present in the DNA of a cell. The changes in DNA ethylation are observed throughout individual’s whole life from the embryonic period to the old age. DNA methylation is also a great way to understand effects of air pollution on gene activity and therefore human health.
DNA methylation can be described as attachment of methyl (CH3) groups to cytosine and sometimes adenine bases of an individual’s DNA. These methyl groups lead to the formation of 5-methylcytosine (5-mC). In methylated genes, tRNA cannot bind to the promoter region which results with no expression of that gene and no protein synthesis, making the gene “switched off”.
Changes in epigenetic markers can result in several unwanted health problems. Regulation of the systems in human body can be altered by these changes. For example, decreases in DNA methylation affects the immune system, making the individual more open to diseases. One of the problems that air pollution causes is neonatal disorders. According to UNEP’s air pollution note, %7 of deaths caused by air pollution is neonatal disorders. Air pollution is often related to uncommon and risky factors that risk babies’ health, such as low birth weight and preterm birth. If babies are underdeveloped, this might have led to serious health problems like brain damage and inflammation, blood disorders, and jaundice which can be fatal.
Similarly, changes in histone acetylation can lead to the misregulation of gene expression, triggering chronic diseases like cardiovascular diseases, asthma and in some cases cancer.
Moreover, the changes in epigenetic patterns can be inherited through generations. During meiosis, 99 percent of the epigenetic tags are removed from the DNA, however, the ones which are not removed may be passed through generations, creating a vulnerability to pollution-linked diseases.
Methodology
Experiment 1
100 healthy pregnant women are enrolled in prenatal clinics Tahran, Iran. All of the women were pregnant before the 14th week of pregnancy. According to calculated levels of particulates/particulate matters of the areas that women had been living, two groups of interest were created and women were separated accordingly, numbers being 50/50. In cases of any changes like replacement, employment and constant travelling, women were removed from the study.
At the time of birth, placental tissue was taken individually for DNA methylation and gene expression tests. Also other birth factors such as new-born’s sex, weight, height, head and chest circumferences and level of needed neonatal care were recorded.
To analyse the DNA methylation, Genomic DNA is extracted from the placental tissue. The DNA is hydrolysed with several steps and RNA is removed, until the global DNA methylation can be expressed as the percentage of 5-methyldeoxycytidine divided by the sum of 5-mdC and deoxycytidine (dC): [5-mdC/(5-mdC + dC)]%.
To determine the amount of gene expression, RNA is extracted from the placental tissue. Qulity of RNA is assessed through agarose gel electrophoresis. Later on gene expression is analysed by using real-time PCR technique after cDNA synthesis. To measure the expression, six different primer sequences are used and mean was obtained for further calculations.
Experiment 2
776 elderly men (≥ 65 years of age) living in Boston, USA were enrolled in the Normative Aging Study. Four clinical examinations between the years 1999 and 2009 were done and at each visit medication use, pulmonary disorders, and smoking history were collected.
To measure DNA methylation, 7 ml blood samples were taken from each individual and DNA is extracted. Methylation levels of nine specific genes related to cardiorespiratory health, inflammation, and oxidative stress mechanisms measured in real time PCR. Methylation analysis was repeated for each sample, and the results were averaged to minimize variability. Methylation levels are expressed as the percentage of 5-methyldeoxycytidine divided by the sum of 5-mdC and deoxycytidine (dC): [5-mdC/(5-mdC + dC)]%.
The effects of air pollution to lung functionality is later formulized as
Yit = β0 + ui + β1 Air pollutantit + β2 X2it + … + βpXpit + εit,
In the equation, Yit represents the log-transformed lung function measurement for participant i at visit t. β0 is the intercept, ui is the random effect, β1 is the effect of the air pollutant on lung function, and X2it to Xpit are the covariates. εit represents the within-participant error.
Results
Experiment 1
At the end of the experiment, 8 women were excluded from the experiment due to replacement. 44 out of 92 women lived in the non-polluted area, 48 lived in the polluted. According to post-birth data, babies that were born in non-pollutant area were more developed in terms height, weight and circumferences. However, there was no correlation between birth outcomes and DNA methylation, because similar results were seen between two regions.
Another remarkable data comparison is there was no significant difference observed in global DNA methylation percentages between two different regions, non-polluted being 2.44 (±0.86) and polluted being 2.59 (±0.70). Additionally, in different trimesters, there was a significant correlation between DNA methylation and amount of particulate matter. Also, there were no significant correlation found between gene expression and amount of particulate matter.
However, there were significant negative correlations between placental global DNA methylation and gene expressions of SAMe and DNMT-1α genes. (Maghbooli, Zhila et al. 2018)
Experiment 2
At the end of the experiment, it can be said that different particulate matters have different effects on the functionality of the lungs. BC and NO2 had the largest estimated effects on lung function parameters (Lepeule, Johanna et al. 2014).
Also, higher methylation is observed in the participants who were exposed the air pollution more. With more methylation, expression of respiratory genes is blocked, therefore results with poor functioning in lungs and respiratory system.
Genes GCR, F3, IL6, and TLR2 were methylated more, meanwhile, TLR2 gene had less methylation. Also, there was found no correlation between the methylation of NO2, FEV1 and LINE-1 and exposure to particulate matters.
Discussion
It can be seen that air pollution affects the epigenetics and health of an individual throughout their life. Evidences from experiment one done by Maghbooli, Zhila et al. and experiment 2 done by Lepeule, Johanna et al. suggest that air pollution affect the amount of DNA methylation in an individual’s DNA.
In the experiment one, it can be understood that exposure to air pollution in the first trimester (zygotic stage) could influence the DNA methylation and therefore gene expression. Considering the first trimester is the most important stage of a pregnancy, when the systems in the baby’s body are developing. These changes in epigenetic marks can later result with fatal diseases post-birth and even can result with infant mortality due to decrease in immune system and lack of properly functioning proteins.
Similarly, in the experiment 2, in a complete different age group, similar results can be observed. Exposure to air pollution unbalance the working rates of the genes related to respiratory system, causing poor functioning in lungs and lead to several respiratory system diseases like asthma and lung cancer.
Considering 94 percent of the world’s population gets exposed to air pollution every day, air pollution remains as one of the world’s biggest factors in health problems. Environment plays a huge role in epigenetic changes, diet, exercise and even stress can be one of the factors that cause a change in these epigenetic markings. Air pollution is made up of several toxic gases and heavy metals which can cause huge problems to the ones who got exposed to.
Conclusion
Exposure to air pollution of an individual throughout any time of their life can alter their epigenetics and therefore their health. Keeping in mind the evidences from experiment 1 and 2, it can be suggested that air pollution can decrease or increase the DNA methylation in an unwanted way. It posses a huge threat to human health and epigenetics, because the changes in the epigenetic markings can result with a weaker immune system, therefore an individual more vulnerable to diseases, especially exposure related ones.
References
Lepeule, Johanna et al. “Epigenetic influences on associations between air pollutants and lung function in elderly men: the normative aging study.” Environmental health perspectives vol. 122,6 (2014): 566-72. doi:10.1289/ehp.1206458
Maghbooli, Zhila et al. “Air pollution during pregnancy and placental adaptation in the levels of global DNA methylation.” PloS one vol. 13,7 e0199772. 6 Jul. 2018, doi:10.1371/journal.pone.0199772
“The First Trimester.” Johns Hopkins Medicine, 8 Aug. 2021, www.hopkinsmedicine.org/health/wellness-and-prevention/the-first-trimester.
IQAir. “World Most Polluted Cities in 2019 - PM2.5 Ranking | AirVisual.” Www.airvisual.com, 2020, www.iqair.com/world-most-polluted-cities.
UNEP. “Air Pollution Note – Data You Need to Know.” Www.unep.org, 7 Sept. 2021, www.unep.org/interactives/air-pollution-note/.
Shridhar, Nini, and Walker. “Gene-Environment Interactions and Epigenetics.” Collaborative for Health & Environment, edited by Nancy Hepp, 2016, www.healthandenvironment.org/environmental-health/social-context/gene-environment-interactions.
Rider, Christopher Francis and Christopher Carlsten. “Air pollution and DNA methylation: effects of exposure in humans.” Clinical Epigenetics 11 (2019): n. pag.
