Scientists switch off Down’s syndrome chromosome

Scientists in the United States used a naturally occurring “off switch” to turn off the chromosome copy which is responsible for Down’s syndrome.

Working in stem cells grown in petri dishes, the discovery of the scientists at the University of Massachusetts Medical School paves the way for new research into Down’s syndrome and even holds out hope that, many years down the line, there could be therapies for the condition.

Humans have 23 pairs of chromosomes, including two sex chromosomes, making a total of 46 in each cell. Down’s syndrome is caused by having an extra copy of chromosome 21, and is typically characterised by having learning difficulties and congenital heart defects, although there are other aspects such as early-onset dementia.

It allows us to ask questions that we can’t ask at the moment. Professor Victor Tybulewicz

Turning off the chromosome responsible for Down’s syndrome in the cells in the laboratory, using a gene called XIST, caused them to behave like normal cells with just two copies of chromosome 21.

Jeanne Lawrence, the lead author of the study published in the journal Nature, said: “Our hope is that for individuals living with Down’s syndrome, this proof-of-principal opens up multiple exciting new avenues for studying the disorder now, and brings into the realm of consideration research on the concept of ‘chromosome therapy’ in future.”

In recent years, scientists have made major leaps in correcting single-gene disorders, dealing with problems caused by a single defective gene. But until now, the idea of switching off a whole chromosome seemed unachievable.

‘Exciting’ discovery

Professor Victor Tybulewicz of the Medical Research Council’s National Institute told Channel 4 News it was an “exciting” discovery.

“It’s the first time anyone’s been able to do this so it’s a technical tour de force in terms of being able to turn off this whole chromosome. It allows us to ask questions that we can’t ask at the moment,” he said.

Down’s syndrome affects around 1 in 1,000 babies, and is on the rise in the UK as people living with the condition live longer than they used to – often into their 60s, when they used to die in their 30s or 40s – and as more women have children later, when the risk of Down’s is higher, although pre-natal screening is offered to women aged over 35.

The next step would be testing whether the discovery can be replicated in mice with Down’s syndrome, and finding out whether it reverses the syndrome in adult animals. If that did work, the step after that would be exploring whether the same thing could be done in humans.

But Professor Tybulewicz cautions that there are lots of “ifs” in this process – including whether the chromosome could be turned off in all of the cells in the body either before birth, in childhood or adulthood, and also whether the symptoms of the condition, such as cognitive difficulty, could be reversed in adults.

“It’s important not to suggest that this is a therapy now. The hope is a much longer-term one – it’s a new tool that opens up new avenues of research, which is most exciting,” said Professor Tybulewicz.

“It’s not relevant to affected families today or tomorrow, but it will be relevant for families with Down’s kids ten or twenty years from now.”