Thursday, February 26, 2009


NYTimes | One of the enduring mysteries of biology is that a variety of specialized cells collaborate in building a body, yet all have an identical genome. Somehow each of the 200 different kinds of cells in the human body — in the brain, liver, bone, heart and many other structures — must be reading off a different set of the hereditary instructions written into the DNA.

The system is something like a play in which all the actors have the same script but are assigned different parts and blocked from even seeing anyone else’s lines. The fertilized egg possesses the first copy of the script; as it divides repeatedly into the 10 trillion cells of the human body, the cells assign themselves to the different roles they will play throughout an individual’s lifetime.

How does this assignment process work? The answer, researchers are finding, is that a second layer of information is embedded in the special proteins that package the DNA of the genome. This second layer, known as the epigenome, controls access to the genes, allowing each cell type to activate its own special genes but blocking off most of the rest. A person has one genome but many epigenomes. And the epigenome is involved not just in defining what genes are accessible in each type of cell, but also in controlling when the accessible genes may be activated.

In the wake of the decoding of the human genome in 2003, understanding the epigenome has become a major frontier of research.

Since the settings on the epigenome control which genes are on or off, any derangement of its behavior is likely to have severe effects on the cell.

There is much evidence that changes in the epigenome contribute to cancer and other diseases. The epigenome alters with age — identical twins often look and behave a little differently as they grow older because of accumulated changes to their epigenomes. Understanding such changes could help address or retard some of the symptoms of aging. And the epigenome may hold the key to the dream of regenerative medicine, that of deriving safe and efficient replacement tissues from a patient’s own cells.