In the brain of a baby, developing in her mother’s womb, a horde of DNA is on the move. They copy themselves and paste the duplicates into different parts of the genome. They are legion. They have been released from the shackles that normally bind them. And in a year’s time, the baby that they’re running amok in will develop the classic symptoms of the debilitating brain disorder known as Rett syndrome.
Children with Rett syndrome – they’re almost all girls – appear normal for about a year before their development is spectacularly derailed. The neurons in their brain fail to develop properly. They lose control of their hands. Most will never speak and at least half cannot walk on their own. Digestive problems, breathing difficulties and seizure are common. They will depend on their loved ones for the rest of their lives.
In most cases, this panoply of problems are all caused by faults in a single gene called MECP2, nestled within the X chromosome. MECP2 is a genetic gag – it silences other genes in a way that’s essential for producing healthy, mature neurons. But Alysson Muotri and Maria Carol Marchetto – a husband and wife team – have found that MECP2 also has another role. It acts like a warden, restraining a mafia of mobile genes called LINE-1 sequences or L1.