Scientists discover 'selfish' chromosome causes miscarriages

Scientists have found that miscarriages are often caused by the wrong number of chromosomes, known as 'selfish chromosomes', in embryos.

The study, led by scientists at the Milner Center for Evolution at the University of Bath, is published in the journal PLoS Biology.

Professor Laurence Hurst, director of the Milner Center for Evolution, said normally a successfully fertilized egg would have a total of 46 chromosomes, 23 from the mother's egg and 23 from the father's sperm. When the number of chromosomes is too few or too many (also called aneuploidies), the embryo develops abnormally and the mother miscarries.

"A lot of embryos have the wrong number of chromosomes, usually 45 or 47. Nearly all fetuses die in the womb," he explains.

It is estimated that more than 70% of human oocytes are heterozygous. The abnormality arises during the first stage of egg production, also known as meiosis I, the scientists say.

At this stage, half of the chromosomes are selected to pass into the egg, the other half is removed. However, Hurst said some of the discarded chromosomes can become "selfish", sneaking into the young egg through a process called centromere, thereby creating an aneuploidy.

"Recent molecular studies have shown that if chromosomes find themselves removed, they will intentionally change to stop this process, causing loss or gain in chromosome numbers, leading to to embryonic death," explains Professor Hurst.

An aneuploidy can cause a woman to miscarry.

By damaging the embryo, the "selfish" chromosome ensures the egg and other embryos have a better chance of survival. Among the new embryos and eggs that may contain a copy of that chromosome, the chance of its genetic code being passed on to offspring is also increased.

This "reproductive compensation" occurs only in mammals, which carry their young to develop in the womb and nurture them continuously until birth. For example, in litter-producing species, when aneuploidy kills some embryos, the remaining young get more nutrition, thus having a better chance of survival.

However, humans usually carry a single baby. After a miscarriage, the mother has to wait a full 9 months before insemination. This means that the "selfish" chromosome immediately has another chance to pass into the embryo.