Association of air pollution and homocysteine with global DNA methylation: A population-based study from North India

Abstract

Background: Anthropogenic air pollution has been implicated in aberrant changes of DNA methylation and homocysteine increase (>15μM/L). Folate (<3 ng/mL) and vitamin B12 (<220 pg/mL) deficiencies also reduce global DNA methylation via homocysteine increase. Although B-vitamin supplements can attenuate epigenetic effects of air pollution but such understanding in population-specific studies are lacking. Hence, the present study aims to understand the role of air pollution, homocysteine, and nutritional deficiencies on methylation.

Methods: We examined cross-sectionally, homocysteine, folate, vitamin B12 (chemiluminescence) and global DNA methylation (colorimetric ELISA Assay) among 274 and 270 individuals from low- and high- polluted areas, respectively, from a single Mendelian population. Global DNA methylation results were obtained on 254 and 258 samples from low- and high- polluted areas, respectively.

Results: Significant decline in median global DNA methylation was seen as a result of air pollution [high-0.84 (0.37-1.97) vs. low-0.96 (0.45-2.75), p = 0.01]. High homocysteine in combination with air pollution significantly reduced global DNA methylation [high-0.71 (0.34-1.90) vs. low-0.93 (0.45-3.00), p = 0.003]. Folate deficient individuals in high polluted areas [high-0.70 (0.37-1.29) vs. low-1.21 (0.45-3.65)] showed significantly reduced global methylation levels (p = 0.007). In low polluted areas, despite folate deficiency, if normal vitamin B12 levels were maintained, global DNA methylation levels improved significantly [2.03 (0.60-5.24), p = 0.007]. Conversely, in high polluted areas despite vitamin B12 deficiency, if normal folate status was maintained, global DNA methylation status improved significantly [0.91 (0.36-1.63)] compared to vitamin B12 normal individuals [0.54 (0.26-1.13), p = 0.04].

Conclusions: High homocysteine may aggravate the effects of air pollution on DNA methylation. Vitamin B12 in low-polluted and folate in high-polluted areas may be strong determinants for changes in DNA methylation levels. The effect of air pollution on methylation levels may be reduced through inclusion of dietary or supplemented B-vitamins. This may serve as public level approach in natural settings to prevent metabolic adversities at community level.

Fig 1. Distribution of median levels of global DNA methylation in low and high polluted areas.

Fig 2. Distribution of median levels of global DNA methylation with respect to high homocysteine in low and high polluted areas.

Fig 3. Distribution of median levels of global DNA methylation with respect to folate deficiency in low and high polluted areas.

Fig 4. Distribution of median levels of global DNA methylation levels with respect to vitamin B₁₂ deficiency in low and high polluted areas.

Fig 5

A. The link between air pollution, homocysteine, micronutrient deficiencies and global DNA methylation. Air pollution as well as micronutrient (folate and vitamin B₁₂) deficiencies lead to increase in homocysteine levels and further cause global DNA methylation. Both folate and vitamin B₁₂ seem to play differential role in the causation of global DNA hypomethylation in the present study. B. The proposed mechanisms for global DNA hypomethylation due to air pollution could be through the key precursor homocysteine. The inhalation of air pollutants (PM_2.5, PM₁₀) triggers local inflammation (in lung epithelia) and systemic inflammation (in blood). This leads to increase in proinflammation due to release of cytokines (IL-1β, IL-6, IL-8 etc.) thereby increasing oxidative stress. Oxidative stress and homocysteine have a cause-effect relation and it results into increased homocysteine. On the other hand, continuous inflammatory state leads to an increased folate demand, which when not compensated through dietary supply leads to homocysteine increase. Dietary deficiency of vitamin B₁₂ also leads to homocysteine increase due to disturbances in one-carbon metabolic pathway. Air pollutants (PM_2.5, PM₁₀) also lead to inactivation of homocysteine remethylation enzyme (methionine synthase) due to which conversion of methionine is blocked and ultimately leads to increased homocysteine. Increased homocysteine alters the plasticity of genomic DNA methylation and leads to genomic instability and accumulation of mutations. This further makes an individual prone to other metabolic disorders such as cardiovascular diseases, cancers, neurological problems.