Scientists find what causes older mothers to have babies with Down's

Published on Thursday 2 September 2010 18:36

SCIENTISTS made a major step towards understanding why older women are more likely to produce abnormal eggs, increasing the risk of conditions such as Down's syndrome, it was announced yesterday.

They believe the research could produce tests and potential interventions to help give women in their late thirties and forties a better chance to produce healthy babies.

However, a Scottish Down's syndrome charity has voiced concern about the research, saying that it could constitute another step towards the creation of "designer babies".

While researchers have long known that women having babies later in life posed an increased risk of disability due to eggs containing the wrong number of chromosomes, the underlying cause has not been known.

Research by Newcastle University, published in the journal Current Biology, has now shed new light on why this happens.

The key is declining levels of proteins called cohesins, which hold chromosomes together by entrapping them in a ring. This is essential for chromosomes to split evenly when cells divide.

All the cells in the body, except for sperm and eggs, contain two copies of each chromosome. Sperm and eggs must lose exactly one copy in preparation for fertilisation.

This halving of chromosome number requires a complex form of cell division. In eggs the problem is compounded by the fact that the physical attachments that hold chromosomes together are established before birth and must be maintained by cohesins until the egg divides just before ovulation.

In a study led by Dr Mary Herbert, and funded primarily by Newlife Foundation for Disabled Children, Infertility Research Trust, and the MRC, researchers at Newcastle University and Newcastle Fertility Centre, used eggs from young and old mice to show that cohesin levels decline gradually as females get older.

This results in weakened cohesion between chromosomes and failure to divide equally during the halving of chromosome number in eggs of older females.

By tracking chromosomes during division in the egg, the Newcastle team found that the reduced cohesin in eggs from older females resulted in some chromosomes becoming trapped and being unable to divide properly.

Eggs that are defective in this way may fail to develop resulting in infertility, or they may give rise to a pregnancy with a high risk of miscarriage, or to the birth of a baby with Down's syndrome.

Dr Herbert, a reader in reproductive biology at the Institute of Ageing and Health, cautioned that while the research was positive, it was still in its early stages: "The first thing to stress is we're nowhere near a treatment we've just managed to identify what goes wrong, we understand yet what's causing that to go wrong.

"So we have some way to go before we know if something can be done to prevent the loss of Cohesin. It is possible that at some stage that we may be able to use these findings to predict what's going to happen, but again it's somewhere down the line."

She said the best advice was still to have babies sooner rather than later. "Women do really need to be aware that once these changes happen, there may be no way back."

However, Pandora Summerfield, chief executive of Down's Syndrome Scotland, said the research sat "uncomfortably" with them. "As a society we need to consider if we want to eradicate everything that is different. Just because science can do something, is it desirable that it should?"

She said it could be construed as giving women "another box to check in designing your baby".

"We talk to some of our young people with Down's syndrome about issues like this. Imagine what it's like to read a [newspaper] piece that says they are effectively able to eradicate what you've got. How are we devaluing their lives and their contribution to society?"