How a 'bad' gene may be good for you

With the exception of identical twins, everyone has a different genetic makeup. Some genes are associated with a tendency to disease, but such genes are usually rare. Some �bad� genes, however, are quite common, and it is a mystery how they have survived, because most disease-associated genes disappear from the population or occur in a very small proportion of people. One such �bad� gene is the factor-V-Leiden (FVL) mutation that is present in about 6% of white people and is associated with an increased incidence of deep-vein thrombosis (blood clots), miscarriages in the second three months of pregnancy, and premature birth. The FVL mutation is thought to have arisen about 21-34,000 years ago, and because it is associated with vein thrombosis (which can cause death) and reduced reproductive success, should probably have died out by now. It is reasonable to assume that FVL mutation has not disappeared because as well as having bad effects it may have advantages to people who have it.

In this week�s issue of THE LANCET, Wolfgang G�pel and colleagues from L�beck, Germany, explored the suggestion that women with the FVL mutation have a lining to their wombs that is more receptive to a fertilised egg (implantation). They examined the records of women who had had a fertility treatment (intracytoplasmic sperm injection) where a woman�s egg is taken from her ovary, fertilised with a sperm outside of her body and then injected into her womb. Whether a pregnancy results from this procedure depends on how receptive the woman�s womb is.

In 9 of 10 women (90%) where the mother or baby had the FVL mutation, implantation was successful, whereas in 92 mothers and babies without the mutation, only 45 achieved implantation (49%).

The authors conclude, �Our results accord with the hypothesis that the thrombotic tendency of carriers of the FVL mutation has some advantage in fetal implantation. This would be an important benefit of the FVL mutation and a possible explanation of the high frequency of the mutation in certain populations.�

Contact : Dr Wolfgang G�pel, Universit�tsklinikum L�beck, Neonatal and Intensive Care Unit, Ratzeburger Allee 160, D 23538 L�beck, Germany; T) +49 451 500 2589; F) +49 451 500 4124; E) [email protected]