Emerging field of epigenetics sheds new light on mechanisms of inheritance
When it comes to genetics the message that most people have been getting is that if something is inherited nothing can be done about it, except maybe taking a drug for life. It’s easy to suspect that the pharmaceutical industry has had a hand in spreading this notion, but in reality the idea that inheritance sets a predetermined outcome is untrue in most cases.
In addition to true genetic defects, which are relatively rare, there are so-called “polymorphisms” – a term often abbreviated to SNPs (Single Nucleotide Polymorphisms). These are minor genetic mutations that make certain genes less efficient at doing some of their jobs but often more efficient in other ways. SNPs do not cause disease, but they may be associated with an increased susceptibility to certain illnesses given contributing environmental factors. In reality we all have our share of SNPs. They’re probably there by design, as part of the evolutionary process.
A classic example of a SNP is the so-called “thrifty gene” that makes individuals who are descended from certain American Indian tribes highly vulnerable to diabetes and obesity
when they switch to a modern diet, although they are capable of surviving in conditions of extreme scarcity.
Researchers looking at these genetic variants and trying to correlate them with diseases such as cancer or even autism have found themselves opening a Pandora’s box with no clear end point in sight. It’s not that they didn’t find genes associated with these conditions, it’s that they found too many of them and that the correlation was often too vague to draw any clear conclusions.
According to Texas A&M biochemistry professor Wallace McKeehan, “there are just a mind-boggling number of mutations associated with cancer.” This is leading some researchers to redirect their focus towards a newly emerging field known as epigenetics. See more on research on cancer and genetics at http://www.chron.com/cs/CDA/ssistory.mpl/metropolitan/3241933.
Epigenetics looks at the interaction between genes and the environment. As it turns out, environmental factors – including diet or chemicals in food, water or air – interact with genes by affecting a process called methylation, whereby “switches” on genes can be turned on or off by adding or taking away tiny compounds known as “methyl groups.”
It now appears that the interaction between genes and the environment has a much stronger impact in determining health events than genetic factors alone.
Much of our knowledge of epigenetics originated from research on identical twins who are born with exactly the same genes, but as time goes by develop growing differences and may end up becoming susceptible to different diseases later in life. To read more about epigenetic studies in twins go to http://www.nytimes.com/2005/07/05/health/05gene.html (registration required).
The difference between the old notion that genes predetermined health and the new understanding of genes interacting with the environment is that when it comes to environmental factors there is a great deal we can control. For example, diet becomes a factor that can impact expression of certain genes eventually leading to – or preventing – genetically associated illnesses (as in the example of the Indians above).
Likewise, detoxification programs can compensate for a genetic susceptibility to accumulate toxins. The reason why, for example, so many children with autism improve with detoxification may be that they are genetically more vulnerable than others to toxins and that genetic switches are turned back “on” once the toxins are released.
Whatís most shocking to me about epigenetic research is the finding that detrimental effects of environmental toxins can be inherited for multiple generations. For example, researchers exposed a group of pregnant rats to a pesticide known to cause reduced fertility in males. Predictably, their male offspring suffered low fertility rates. However, their female offspring were fine and care was taken to ensure that they experienced no further exposure to pesticides.
Later on, these female rats whose mothers had been exposed to pesticides were mated to male rats with no history of pesticide exposure. Surprisingly, their male offspring experienced low fertility and this scenario was repeated one more time in third-generation offspring. The fourth generation of male rats finally reverted back to normal fertility, proving that the genes themselves had not been altered (Science, Vol 308, June 3, 2005, pgs 1466-1469).
An analogy to help us understand this study might be a theoretical case of a woman developing breast cancer because her great-grandmother was exposed to a cancer-causing chemical when pregnant.
Although this study was performed on rats, it probably applies to humans as well. It also correlates with studies showing that women whose mothers had been smokers have children with an increased rate of asthma even if they never smoked themselves. For more information, visit http://pubs.acs.org/subscribe/journals/esthag-w/2005/jun/science/pt_toxins.html.
The implications of this type of research could be momentous considering the ever-growing number of potentially toxic chemicals to which we are exposed. The next study Iíd like to see (but that will probably never happen) is one that looks at whether detoxification, vitamin supplementation, or diet change can help erase these environmental insults, thus halting the inheritance mechanism.
