Cancer can pass from mother to unborn baby

The placenta is an unborn baby's lifeline. Attached to the inside of the womb and connected to the baby by the umbilical cord, it is the trading post between mother and baby: oxygen and nutrients in the mother's blood pass through it to the baby, and waste and carbon dioxide pass the other way. The placenta also helps protect the baby from infections and harmful substances, but it isn't a perfect filter: alcohol, drugs and cigarette smoke can cross it and cause birth defects, drug addiction and fetal alcohol syndrome in newborns. Scientists have long suspected that cancer could cross the placenta and spread from mother to baby, but had no proof.

Researchers at the Institute of Cancer Research and University College London, with colleagues in Japan, studied the case of a 28-year-old Japanese woman who gave birth to a healthy baby girl after a normal, uneventful pregnancy. A month later the mother had vaginal bleeding and was diagnosed with acute lymphoblastic leukaemia. She began chemotherapy, but developed brain inflammation and died 19 days later.

The baby showed no signs of illness until she was 11 months old, when she was taken to hospital with a swollen right cheek. Tests showed a tumour in her jaw, and the cancer had spread to her lungs.

Comparing the DNA of mother and daughter, the researchers found that their cancer cells had the same gene mutation (BCR-ABL1), but the girl had not inherited it. Later tests on a blood sample taken from the baby at birth confirmed that the cancer had spread before birth.

How could this happen? Normally, if cells cross the placenta, the baby's immune system recognises them as foreign and destroys them, which is why mother-to-baby cancer is so rare. But in this case, the cancer cells were missing a vital piece of DNA that plays a key role in the immune system's ability to recognise them; without it, they could pass undetected.

"We assume that in other cases where cancer passed from the mother, the cancer cells crossed the placenta and succeeded because they were invisible to the immune system," says Professor Mel Greaves. "We're pleased to have solved this long-standing mystery, but we stress that passing cancer from mother to baby is extremely rare. The chance of a pregnant woman passing her cancer to her baby is remote."

Records going back to 1866 include 17 cases where mother and baby had the same cancer, usually leukaemia or melanoma. The most likely explanation is that the disease spread to the baby during pregnancy, although other scenarios are theoretically possible. "People have thought this was happening for some time," says William Chambers, director of scientific programmes at the American Cancer Society.

"The important message is that leukaemia cells can be destroyed by the immune system," said Dr David Grant, scientific director of Leukaemia Research, which partly funded the study. Knowing this will help scientists find ways to harness the immune system, first to treat and then to protect patients from leukaemia.

The report was published online on 12 October in the Proceedings of the National Academy of Sciences.