Gene fault increases blood clot risk in breast cancer patients taking tamoxifen

In collaboration with Adfero

Women who develop blood clots while taking tamoxifen for early-stage breast cancer are more likely to carry a faulty version of a gene involved in clotting than those who do not develop blood clots, a US study has found.

The genetic mutation Factor V Leiden (FVL) is already known to increase the risk of blood clots and is the most common inherited clotting factor mutation.

Tamoxifen is a hormone treatment that is widely used to treat women with hormone receptor-positive breast cancer, and researchers are also investigating if it can prevent the disease in healthy women who are at a higher risk. The drug is known to increase the risk of blood clots.

The latest study, published in the Journal of the National Cancer Institute, suggests that a woman's risk of developing a blood clot while being treated for breast cancer with tamoxifen may be influenced by her genes.

Scientists at the Dana-Farber Cancer Institute and the US National Cancer Institute's Cancer and Leukaemia Group B studied 412 women, all of whom had received tamoxifen while being treated for early-stage breast cancer.

Of these, 141 patients had developed blood clots while the remaining 271 had not.

The researchers found that women who developed blood clots were nearly five times more likely to carry the FVL mutation than those who did not develop blood clots. They were also more likely to be smokers or have a personal history of clots.

But previous research has shown that healthy women with the FVL mutation who were taking tamoxifen to prevent breast cancer had no increased risk of blood clots.

Professor Jack Cuzick, from the Wolfson Institute of Preventive Medicine at Queen Mary University of London, said that the reasons for these conflicting results are "a mystery".

Writing in an editorial accompanying the study, he said: "Continued follow-up and research on the relationship between the FVL mutation and tamoxifen, especially in the prevention setting, will be the only way to clarify these apparently contradictory findings."

Nell Barrie, Cancer Research UK's science information officer, commented: "We know that tamoxifen slightly increases the risk of blood clots, and this small study suggests that a woman's genetic make-up can increase this further. However, it's intriguing that the increase in risk is not seen in healthy women who are taking the drug to help prevent breast cancer.

"More research is needed to understand these confusing results and to find out if it would be helpful to test women for the FVL gene mutation before they start taking tamoxifen for preventing or treating the disease."