Though cancer is a common condition, affecting around one-third of people, it can sometimes show up in very unusual ways. Doctors in the UK have just documented an incredibly rare—and ultimately tragic—case of cancer shared between identical twins in the womb.
Scientists at the Wellcome Sanger Institute and others detailed the case in a report last week. One twin developed the cancer, which then spread to the other, they found. Unfortunately, neither child survived, but the lessons learned from the case just might teach us something new about the nature of twins, the researchers say.
“Overall, as well as mapping tumor spread in utero, our work delineates a powerful approach to reconstructing the earliest steps that lead from the zygote to the genetic segregation of twins,” they wrote in their paper, published in the journal Nature Communications.
“A rare event”
According to the report, doctors first noticed something was wrong when a fetal imaging scan detected a mass on one of the twins’ faces. As a result, they decided to deliver the children early, at 34 weeks of gestation, via a caesarean section. Both twins had noticeable tumors throughout their bodies, and both sadly died soon after birth.
With the permission of the family, the researchers then conducted a deep genetic analysis. They sequenced the entire genome of cells taken from the newborns and their tumors. Though identical twins do share the same genetic make-up, it doesn’t take long for unique mutations in each to accumulate. This allowed the researchers to trace back the genetic path these children and their cancers took.
Cancers that start in the womb are very rare, and cases involving identical twins are even rarer. These cases can result from a shared inherited mutation that causes the same kind of cancer to form independently in each twin, or from one twin’s cancer spreading to the other. In this instance, the cancer began in one twin (described as twin A in the paper) from the facial mass, spread throughout twin A’s body, and was only then transmitted to the other child (twin B) via their shared blood supply in the placenta, the researchers concluded.
The transmission probably happened late in gestation, however, and likely only occurred once. This suggests, the researchers wrote, that the tumor’s spread “was a rare event and therefore not a foregone conclusion.”
Lessons about twinning
Aside from solving the origins of this cancer case, the researchers were also able to take a close genetic look at the twins themselves. And they found something unexpected: The twins’ placenta was almost entirely comprised of cells from twin B, and it seems likely that twin A was formed from only a single cell early on in the development of the zygote (the fertilized egg) as it became an embryo and then split into two distinct embryos.
This chain of events doesn’t make sense under the classic model of twinning, the researchers say, which would posit that half of the embryo’s cells before the split would have become twin A and the other half twin B and that the placenta would be a roughly even mix of twin A and B’s cells.
“If you go to placenta researchers and say, ‘Here’s this pregnancy where one twin made all the placenta,’ they’re going to say, ‘Oh, that’s interesting,’” study author Sam Behjati, a researcher at the Wellcome Sanger Institute in Cambridge, told New Scientist Wednesday.
Right now, this is just a curious finding, but Behjati and his team plan to continue studying the genetics of twins to find out whether the phenomenon is more common than assumed. Hopefully, an otherwise tragic incident might help scientists someday understand twins all the better.
“It’s an incredibly awful thing to have happened, but their mother was very clear that we had to learn something from it, and has made this amazing gift to science,” Behjati said. “It’s meant that we’ve been able to provide a little bit of an answer [to] what happened to these babies, but also gleaned a fundamental insight into twinning.”
