A team of researchers led by North Carolina State University biologists Cathrine Hoyo and Randy Jirtle have found links between lead exposure in children and epigenetic alterations in regulatory regions of genes that are imprinted and known to be critical in growth regulation and brain development. These alterations seem to persist into adulthood, with more profound effects in males. Their study sheds more light on the long-term effects of early lead exposure on DNA and may help to develop therapies to treat or reverse the damage.
Along with colleagues Kim Dietrich of the University of Cincinnati, Yue Li of Duke University, and NC State’s David Skaar, the team looked at data collected from 105 participants in the Cincinnati lead study, which measured lead in children from birth to age 6 and a half. They followed up with the now-adult participants and took blood samples, which were sequenced for DNA methylation – data spanning 36 years.
“We now have the first human evidence for an association between early lead exposure and three aberrantly methylated regulatory regions of imprinted genes,” says Hoyo. “But from a public health perspective, the results are very exciting because we can begin to think about identifying potential biological markers for early exposure to lead and other toxins in the environment.”
The team spent several years pinpointing regulatory regions within DNA that may link early lead exposure to specific diseases, characterizing 22 of these regions to date. With this study, researchers looked at the 22 regions to see if lead exposure affected DNA methylation, the process that controls how a gene is expressed, essentially determining whether or not it is switched on or off. When methylation is altered, genes are either turned off (or silenced) or they are more active than they normally would be.
The team found three imprinted genes whose expression was affected in adulthood by lead exposure from birth to 6 and a half years of age: PEG3, IGF2/H19 and PLAGL1/HYMAI. For PEG3 and IGF2/HI9, methylation decreased. The effects were sex-specific: decreased methylation for PEG3, which is associated with fetal development, affected males more than females, while the opposite was true for IGF2/H19. PLAGL1/HYMAI methylation, which increased, was not sex-specific.
Additionally, they found that increased blood levels of lead later in postnatal development did not seem to have any other effects on the regulatory regions – the methylation changes occurred during the first 12 months, even as lead exposure continued to increase over the study period.
“Genes are like computers with both hardware and software,” says Jirtle. “Most scientists have been studying the hardware, which is the genetic sequence, without looking at the software, which is the regulatory layer that alters how that gene is expressed. This study gives us a first look at how the software may be affected by early exposure to lead.”
Hoyo and her team plan to continue investigating possible connections between lead exposure, gene expression and disease. “The dysregulated genes we identified in this study seem to be highly malleable, especially during prenatal development and early childhood. This raises the possibility that we could nudge them back toward normal if we could therapeutically target them at the right stage of development.”
The research appears online in Environmental Health Perspectives. The study was funded by the National Institutes of Health (grants R01ES016772, R01DK085173, R01ES015559 and 8 UL1 TR000077), and the Department of Energy (grants DE-FG02-10ER64931, R21ES020048). Susan Murphy and Adriana Vidal of Duke University; Monica Nye of the University of North Carolina at Chapel Hill; and Kim Cecil, Kim Dietrich and Alvaro Puga from the University of Cincinnati College of Medicine also contributed to the work.
Note to editors: Abstract follows
“Lead Exposure during Early Human Development and DNA Methylation of Imprinted Gene Regulatory Elements in Adulthood”
Authors: Catherine Hoyo, David Skaar, Randy Jirtle, North Carolina State University; Yue Li, Susan Murphy, Adriana Vidal, Duke University Medical Center; Changchun Xie, University of Cincinnati; Monica Nye, University of North Carolina Chapel Hill; Kim Dietrich, Alvaro Puga, University of Cincinnati College of Medicine
Published: Online in Environmental Health Perspectives
Abstract: Lead exposure during early development causes neurodevelopmental disorders by unknown mechanisms. Epidemiologic studies have focused recently on determining associations between lead exposure and global DNA methylation; however, such approaches preclude the identification of loci that may alter human disease risk.
The objective of this study was to determine if maternal, postnatal and early childhood lead exposure alter the differentially methylated regions (DMRs) that control the monoallelic expression of imprinted genes involved in metabolism, growth and development.
Questionnaire data and serial blood lead levels were obtained from 105 participants (64 females, 41 males) of the Cincinnati Lead Study from birth to 78 months. During adulthood, peripheral blood DNA was used to quantify CpG methylation in peripheral blood leukocytes at DMRs of 22 human imprinted genes using Sequenom EpiTYPER assays. Statistical analyses were conducted using linear regression.
Mean blood lead concentration from birth to 78 months was associated with a significant decrease inPEG3 DMR methylation, (β=-0.0014, 95% CI:-0.0023, -0.0005, p=0.002), stronger in males, (β=-0.0024, 95% CI:-0.0038, -0.0009, p=0.003) than females (β=-0.0009, 95% CI:-0.0020, 0.0003, p=0.1). Elevated mean childhood blood lead concentration was also associated with a significant decrease in IGF2/H19 (β=-0.0013, 95% CI:-0.0023, -0.0003, p=0.01) DMR methylation, but primarily in females, (β=-0.0017, 95% CI:-0.0029, -0.0006, p=0.005) than males, (β=-0.0004, 95% CI:-0.0023, 0.0015, p=0.7). Elevated blood lead concentration during the neonatal period was associated with higher PLAGL1/HYMAI DMR methylation regardless of sex, (β=0.0075, 95% CI:0.0018, 0.0132, p=0.01). The magnitude of associations between cumulative lead exposure and CpG methylation remained unaltered from 30 to 78 months.
Our findings provide evidence for early childhood lead exposure resulting in sex-dependent and gene-specific DNA methylation differences in the DMRs of PEG3, IGF2/H19 and PLAGL1/HYMAI in adulthood.