New research explains why older women more likely to have babies with Down syndrome

From The Guardian in the UK:

Scientists have made a breakthrough in understanding why older women become less fertile, suffer a miscarriage or have a baby with Down's syndrome.

The discovery could ultimately lead to treatments that would increase the chances of a successful pregnancy for growing numbers of would-be mothers in their late 30s and early 40s.

Researchers led by Dr Mary Herbert, an expert in reproductive biology at Newcastle University's Institute for Ageing and Health, have identified why some older women produce abnormal eggs, according to findings published in the journal Current Biology.

It has been known for a long time that would-be mothers who are nearing the end of their fertility are at higher risk than usual of having eggs that are affected by chromosomal abnormalities, but the underlying cause has been unclear.

The new study has identified problems arising from a woman's declining stock of proteins called Cohesins, which act as binding agents to hold chromosomes together by keeping them inside a ring. They are vital to ensure that chromosomes split evenly when cells divide.

Women's supplies of Cohesins fall as they age, Herbert and her colleagues discovered. Tests on eggs taken from both young and old mice indicated that the amount of Cohesins in women's bodies declines after their mid-30s.

When that happens it means that chromosomes are less tightly held together and they are therefore more likely to result in defective eggs, which can cause problems such as miscarriage and Down's syndrome.

Every cell in the human body, apart from eggs and sperm, contains two copies of each of the body's 23 chromosomes. Sperm and eggs must lose one copy each as they prepare for fertilisation. That process involves a complicated form of cell division.

This problem is compounded with eggs, because the attachments that hold chromosomes together have to be maintained by Cohesins until the egg divides just before ovulation.

When Herbert's team studied chromosomes during division in the egg, they found that the lower levels of Cohesin in eggs in older females led to some chromosomes becoming trapped and unable to divide properly.

"Reproductive fitness in women declines dramatically from the mid-30s onwards. Our findings point to Cohesin being a major culprit in this", said Herbert. More work was needed to understand why Cohesin declines over women's reproductive years, and such knowledge could lead to ways being developed to stop that loss from occurring.

Dr Peter Bowen-Simpkins, the medical director of the London Women's Clinic network of private fertility clinics and spokesman for the Royal College of Obstetricians and Gynaecologists, said the study was "very exciting" and could lead to real improvements in older women's chances of having children.

"This breakthrough could mean the difference between success and failure – them having a baby or not – for the fast-growing number of women who are trying to conceive after their late 30s," he added.