Nessa Carey

 Epigenetics - The Whistlestop Tour

The Word

The key bit here is "epi".  It comes from a Greek root and means "on, in addition to, as well as" that sort of thing.  So "epigenetics" refers to situations where the genetic code is not enough to explain something, there is another feature acting in addition to the genetic code.

The Phenomenon

We've recognised epigenetic phenomena for a long time.  Essentially we just look for situations where two things are genetically identical and yet are not the same in appearance or behaviour.  Here's some examples.....

The 70,000,000,000,000 or so cells in a human body are almost all genetically identical and yet they form loads of different cell types, which remain the same for our whole lives.  Just as well, who wants teeth in their eyeballs?

Scientists can create mice which are genetically absolutely identical and can keep them in very standardised conditions.  But no matter how much we control the environment, the genetically identical mice have a range of body weights, and you can never get them closer than a variability of about 15% around the average weight.

A caterpillar, and the butterfly that it turns in to, have the same genetic code but it's hard to think of two things that look more different.

A queen bee is genetically identical to thousands of her hive mates and yet she is about twice their weight, has different body parts and can live 20 times longer.

Identical twins are genetically the same, but they are rarely truly identical in appearance and behaviour.  These differences become more pronounced as the twins age, and can even be as extreme as one twin developing a serious disease such as rheumatoid arthritis or schizophrenia, while the other remains completely healthy.

In all these cases, and in many more, something epigenetic - something in addition to the genetic code - is taking place.

The Mechanism

We're always tempted to look at the types of examples listed in The Phenomenon and say "That's the effect of the environment'.  And that's fine, but it's not really an explanation.  How does the environment have this effect?  There's got to be a mechanism, something that changes how genes are expressed, so that the same genetic code can be used to create different outcomes.

This is where the other description of epigenetics becomes useful.  We have recently begun to understand that patterns of gene expression can be controlled, in some cases for decades, by adding small chemical groups to our genetic material.  These don't change our genes, but they change how well the genes are expressed.  These epigenetic modifications - chemical groups that are "on, in addition to, as well as" the genetic code - can switch genes or off, keeping teeth out of our eyeballs or maintaining queen bees in their royal splendour.

Why Is Epigenetics Important?

The science is really amazing but epigenetics isn't just important from a curiosity viewpoint.  Epigenetics is very significant in human health and disease.  Abnormal patterns of epigenetic modifications on our genes lead to cells developing the wrong patterns and combinations of gene expression and this contributes to diseases. Scientists are investigating the role of epigenetics in chronic diseases such as rheumatoid arthritis and schizophrenia, and also in drug addiction and the long term effects of abusive or neglectful childhoods.  Cancer is another major field of human disease where we know epigenetics is important.  Drugs which target epigenetic changes are being successfully used to treat patients with certain cancers.  We're hoping to develop drugs using this new understanding for lots of other human illnesses.

The pictures below may look unconnected but they are all examples of epigenetics in action.  Male and female crocodiles are genetically identical but their gender is determined by the temperature at key points in development, switching sets of genes on or off for life.  The brain undergoes subtle epigenetic changes in response to events that may eventually point us towards new treatments for conditions such as post-traumatic stress disorder.  Almost all tortoiseshell cats are female because of a specific epigenetic event.  And ageing - one of the most controversial areas of all......