How Do Your Genes “Interact” with the Environment?
This is a guest post by Alycia Halladay, Ph.D. Dr. Halladay is Autism Speaks’ Director, Research for Enivronmental Services.
Instead of focusing on just genetics or just environmental factors, autism researchers have been studying gene-environment interactions as possible risk factors of the disorder. A next series of posts will begin to try and explain why this is an important concept, and how it is changing the way scientists think about causes.
Why is this concept important?
First, in the context of risk factors, if only the separate contributions of genetics and environmental influences is calculated without considering the interaction, the proportion of the disorder that is attributable to both is underestimated. For example, environmental factors may play an important role in the development of some diseases. However, in others, the effect is only seen in susceptible individuals. Studies that examine gene-environment interactions can do the following (taken from Hunter, 2005)
- Obtain a better estimate of the risk associated with genetic and environmental risk factors
- Strengthen the association between environmental risk factors and disease
- Help researchers understand the biological mechanism of disease
- Determine which environmental factors produce risk
- Lead to new prevention and therapeutic strategies
What does it mean?
As most people know, genetics typically refers to the stable sequence of nucleotides on DNA strands in every cell of the human body. The nucleotides are translated to amino acids, which in turn create proteins. The amino acid sequence determines how proteins are configured, which may affect their function. Put in an oversimplistic fashion, these proteins are what affect cell function. Some of the genetic code is inherited from both parents, and will be conferred to their children; another, more recently studied type of genetics, called epigenetics, refers to a change in protein synthesis that is not due to alterations in the DNA code. In other words, the DNA code stays the same but the way it is expressed changes. These concepts will also be discussed in a later chapter. With regards to the term “environment”, this is a term that can refer to many “non-genetic” influences on biology and behavior. Typically when they hear “environment” people think of one of the hundreds of thousands of potential chemicals and toxins that are present in food, air and water. However, environment can also include some demographic characteristics like socioeconomic status, nutritional status and education, as well as medical procedures and illnesses, and exposure to vitamins, pharmaceuticals and/or alternative medicines. It can even refer to exposures that we may not be thinking about every day, like UV sunlight, cosmetics, food additives, and ventilation in the home. While most people think of gene-environment interactions as an environmental risk factor producing more profound effects in a susceptible individuals, some genes may offer protection against deleterious environmental effects. Other genes may promote healthy development and their effects stifled, or even enhanced, in different environments. These concepts will be explored further in a different chapter.
How are these interactions determined and studied?
The best way to determine whether an interaction exists in a human population is an epidemiologic study. One of the biggest challenges is the need for large samples, or many individuals to enroll and participate. Typically, self-report measures are obtained from all participants and family members, and DNA and other biologicals are included to study DNA/RNA and level of exposure. If other measures are available such as medical records, these are also collected throughout the study. Genetic and environmental factors, and their interaction, can be studied retrospectively (after the disease has developed) or prospectively (prior to when the disease appears). Each design has strengths and weaknesses, and in many cases both approaches are taken to identify and then replicate findings. Other study designs include case-control vs. case-case. Case-control refers to studying both individuals with and without the disease. Case-case refers to studying cases (in this case individuals with autism only) both with and without different exposure levels and/or genotypes.