Genes or environment? The relative impact of each on the development and health of plants and animals has long been debated. However, the two are interrelated in that the environment can influence how genes work.
How can the environment influence genes?
Scientists have known for some time that environmental factors can alter the function of genes. Epigenetics is the term used to describe changes in the activity of our genes in response to the environment.
Epigenetic changes are not due to difference in the genetic code, and occur despite the DNA sequence remaining intact. Epi comes from the Greek meaning ‘on top of’ and it is used in the genetics context to signify factors that operate on top of the information provided by the DNA sequence.
An example of epigenetic changes in plants is the use of prolonged exposure to cold to make them flower. This is thought to occur because the cold induces changes to proteins that coat the plant’s DNA. These ‘coating proteins’ then influence the function of the flowering genes.
In humans, epigenetic changes are illustrated by identical twins. Although identical twins have exactly the same DNA make up, they can have different physical features or have different likelihoods of developing a certain disease. For example, the risk of developing osteoporosis can be influenced by genes but studies on twins have shown that birth weight (that is, nutrition in utero) is also important. [1]
Epigenetic changes are one way that cells use to turn genes ‘on’ and ‘off’. Not every gene needs to be turned on in every cell all the time, a fact particularly relevant in a developing embryo.
Not surprisingly, epigenetics is considered to be a crucial factor in understanding how we develop and how we might develop new innovative treatments.
What does epigenetics involve?
The most widely investigated epigenetic mechanism is a chemical change in the DNA called DNA methylation. Methylation refers to the addition of a chemical (methyl) group that happens ‘on top of’ a person’s DNA sequence.
Once methylated, a gene is turned off, despite having the same DNA sequence as a gene that remains turned on and continues to function. Other epigenetic mechanisms include how tightly the DNA is packaged inside the cell and what proteins surround the DNA. These mechanisms can influence the working of genes.
Epigenetic changes are not usually permanent, that is, they are reset from one generation to the next. While some diseases in humans are known to be associated with epigenetic changes, it has not yet been proven that these changes are passed on in humans from one generation to the next. [2]
The strongest evidence that these changes can be inherited (thus providing a way in which the environment can have long-term generational effects on the expression of genes) is currently in mice. [3]
Are there diseases where epigenetics is known to be a factor?
The first example of epigenetics in human disease was cancer, where cells overgrow and can develop into a tumour. Scientists showed that DNA methylation was lost in colon cancer cells compared to normal cells from the same patient. This epigenetic change is the opposite of normal development where DNA methylation would turn off genes in some cells. [3]
Conclusions
Research in epigenetics is still at an early stage. However, an understanding of how genes interact with the environment is likely to be important in developing approaches to the early diagnosis, prevention and treatment of a number of human diseases and conditions.
More information
- A conversation on genes and the environment [Podcast with Professor John Hopper]
References
- Gluckman PD et al, Effect of in utero and early-life conditions on adult health and disease, N Eng J Med 2008; 359: 61-73.
- Morgan DK and Whitelaw E, The case for transgenerational epigenetic effects in humans, Mamm Genome 2008; 19: 394-397.
- Esteller M, Epigenetics in cancer, N Eng J Med 2008; 358:1148-59.