Kate Johnson

February 23, 2013

SAN ANTONIO, Texas — Exposure to airborne polycyclic aromatic hydrocarbons is linked to epigenetic changes associated with childhood asthma and allergy as well as higher rates of asthma diagnosis in the highly polluted city of Fresno, California, investigators report.

"We've shown that the gene being changed is directly associated with asthma and severity of the asthma," Kari Nadeau, MD, told Medscape Medical News, adding that it was "also associated with a higher likelihood of elevated immunoglobulin, which is also associated with allergies." Dr. Nadeau is from the Department of Pediatric Allergy and Immunology at Stanford University in California.

"Even people in Fresno who don't have asthma have this gene change and we will be watching them over time to see if they have an increased rate of asthma," she said. "Unfortunately, this gene was changed over time and it was sustained — it doesn't seem to be reversible."

The research, presented here at the American Academy of Allergy, Asthma & Immunology (AAAAI) 2013 Annual Meeting, provides sobering insight into the persistently high rates of asthma and allergy in Fresno, which ranks among the most highly polluted areas of the United States.

Highly Polluted

"In Fresno, not only do they have to deal with a lot of pollution, but the children have higher rates of asthma as well as allergic rhinitis and other allergies, including food allergies and atopic dermatitis," said Dr. Nadeau.

Compared with the usual rates of asthma (12%) and allergies (30%) in California, the rates are 22% and up to 70% in Fresno, she explained.

The study recruited 302 children (median age, 14 years) from the Children's Environmental Health Study.

All children had lived in Fresno for at least 9 years, explained the presenter, Arunima Kohli, one of Dr. Nadeau's colleagues.

All subjects filled out questionnaires and underwent spirometry as well as blood and urine collection.

Polycyclic aromatic hydrocarbon exposure was estimated based on urinary metabolites as well ambient air quality and meteorologic data, which were collected from 7 air sampling sites in Fresno.

"Individual airborne polycyclic aromatic hydrocarbons exposure...was estimated based on land-use regression analyses," said Ms. Kohli. The estimates were based on various time durations ranging from 1 week to 12 months, she added.

"We chose to look at these different time intervals to see if airborne polycyclic aromatic hydrocarbons exposure over longer time periods resulted in stronger association with immune and clinical outcomes," she explained.

In a subset of 153 subjects, the researchers analyzed immune function, measured total immunoglobulin (IgE) levels, and performed functional studies of regulatory T (Treg) cells, which were purified from peripheral blood mononuclear cells. They used epigenetic analysis to determine methylation of the FOXP3 gene.

"FOXP3 is crucial for the function of T-regulatory cells — a major class of immunosuppressive cell," said Ms. Kohli. Increased methylation of FOXP3 leading to decreased expression of the gene has in turn been associated with decreased Treg function leading to an increase in asthma occurrence. The analyses showed "in exquisite detail how pollution could be affecting cells at the genetic level," said Dr. Nadeau.

"We found that pollution modified the DNA in regulatory T cells that are sensitive to pollution.... These regulatory T cells have a gene called FOXP3 and this gene was what we found to be most affected," she said, specifying that methylation of this gene was increased in subjects exposed to higher amounts of polycyclic aromatic hydrocarbon pollution.

Specifically, 3-month cumulative exposure prior to blood sampling was significantly associated with increased methylation of FOXP3 (P < .05), decreased Treg function (P < .05), and increased levels of total IgE (P < .05).

These associations were also significant at both 6 and 12 months of cumulative exposure.

Clinically, 3-month cumulative polycyclic aromatic hydrocarbon exposure was associated with an increased probability of asthma, and there was a significant association between increased total IgE levels and asthma diagnosis.

Table. Outcome After 3-Month Polycyclic Aromatic Hydrocarbon Exposure

Outcome P Value
FOXP3 methylation <.05
Decreased Treg function <.05
Increase of total IgE <.05
Probability of asthma diagnosis <.01

"What we have the most unique data on is to be able to trace a molecular level change at the DNA level to a cell function change, which was then associated with a health outcome such as asthma," said Dr. Nadeau, emphasizing that "the changes in the DNA are not at the sequence level, but rather at the methylation level."

She said while these preliminary data show no sign of reversal in these changes, longer follow-up is needed, including data on subjects who move to less polluted areas.

Asked by Medscape Medical News to comment on this study, Jonathan Bernstein, MD, editor-in-chief of the Journal of Asthma, said he is doubtful that the genetic changes are irreversible.

"These are dynamic changes that occur as a result of methylation — it's a dynamic process and can change over time," said Dr. Bernstein, from the University of Cincinnati in Ohio.

He said the study highlights the advantages of new technology for documenting personal environmental pollutant exposures. "Now with land-regression analysis, which is basically like GPS, they can actually look at exposures in people living in specific areas.

"If we are able to develop better technologies, better methods for measuring these exposures, which improves our phenotype," he said, "eventually we will be able to translate this to endotypes that have functional relevance. In terms of studying these patients, if you have a better, well-characterized endotype, then you can develop better strategies that are tuned to those specific individuals as opposed to a one-size-fits-all approach."

Dr. Nadeau, Ms. Kohli, and Dr. Bernstein have reported no relevant financial relationships.

American Academy of Allergy, Asthma & Immunology (AAAAI) 2013 Annual Meeting: Abstract 197. Presented February 23, 2013.