The Link Between Environmental Toxins and Disease

by Bill Chameides | June 11th, 2008
posted by Wendy Graber (Researcher)

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You are what your mother eats…or can be! Mothers fed a diet rich in nutrients versus a normal diet produced offspring that were remarkably different in looks and disease resistance. Photo provided by Dr. Randy Jirtle.

Duke’s Medical School is a very happening sort of place. I’m never quite sure what new topic I will explore when I’m there–I just know it will be fascinating. A recent meeting with Dr. Randy Jirtle is a case in point. Jirtle’s lab studies epigenetics. A field that may just hold the key to understanding how environmental toxins cause serious, and often life-threatening diseases such as, obesity, diabetes, and cancer.

Have you ever wondered why is it that identical twins, with exactly the same genetic makeup, can take on different physical traits and have different susceptibilities to disease as they age? Epigenetics or the study of the epigenome holds the answers to these questions.

This is complicated stuff (and I’m not an expert in this) so let’s start with the basics. We all know that our genes and the DNA contained within them hold the basic information of what we are (animal, vegetable) and what we are like (short, tall). The total composite of all those genes is our genome. The human genome is estimated to be comprised of some 25,000 genes.

So If That’s the Genome. What’s the Epigenome?

The prefix “epi” means outer, above, or on. So the epigenome refers to the outer or super genome. This outer genome provides instructions for the genes in our genome; telling them what to do and when and where to do it. For example, a person may have a gene that makes him or her susceptible to diabetes. The epigenome can cancel out that susceptibility by effectively turning that gene “off” so that the diabetes-susceptible trait is not expressed. No change in the genes or the DNA, just a change in activation of a particular gene.

Randy Jirtle describes the difference between the genome and epigenome as the difference between the hardware and software of a computer. The hardware determines the basic properties of the computer, but the software largely determines how the computer’s capabilities are used; which features you activate and which you do not.

In the same way that making software changes is often quicker and easier than changing your computer, changes in the epigenome allows a species to effect rapid changes, say, in response to environmental conditions, without having to resort to the slower process of mutation. However, the potential for rapid change also leads the door open to bad outcomes, and that may be what is happening to us today. We are exposing ourselves to a growing array of synthetic chemicals and these chemicals may be playing havoc with our epigenome.

Changing Colors Without Changing Genes

The power of the epigenome was demonstrated in Randy Jirtle’s lab by what will surely be recognized as one of the seminal experiments in modern genetics.

By changing the diet of pregnant agouti mice, his group was able to dramatically alter the traits of the offspring. Typically agouti mice have yellow, sickly offspring that tend toward obesity. With a change in the mother’s diet, the offspring were brown healthy mice. This difference was not because of a difference in the genetic makeup of the mice but because of an epigenetic difference.

Jirtle argues that because the mothers of the brown healthy mice were fed a diet rich in methyl groups, this diet led to a methylation or “turning off” of the so-called agouti gene that causes mice to be yellow and sickly.

Conversely, Jirtle’s group was also able to increase the likelihood that offspring would be yellow obese mice by feeding the mothers a common environmental toxin bisphenol-A or BPA. (BPA is found in many household plastics, such as baby bottles and the lining of beer and food cans. But more on BPA in a later post.)

Do Environmental Toxins Redirect the Epigenome?

For quite some time scientists have been trying to determine how exposure to environmental toxins can result in serious disease years or even decades later. Epigenetics may provide the mechanism. An exposure to an environmental toxin at one point in a person’s life (and most critically during gestation) can trigger the epigenome to turn on or turn off a key gene. Years later, because of that epigenetic change, a disease may appear.

For example, our genetic make-up includes so-called proto-oncogenes that promote cell growth and tumor suppressor genes that do the opposite. A fetus exposed to a chemical during gestation that either turns on a growth promoter gene or turns off a growth suppressor could lead to the adult contracting cancer decades later.

All this begs the question of whether the ever-growing list of chemicals we are introducing into our environment, putting in our foods and adding to our drinking water are messing with our epigenome? We don’t know yet, but scientists at Duke and elsewhere are trying to find out.

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