I got excited when I read this article. The research seems to be the start of something that could have a lot of applications and implications for pregnancies and cancer treatment and understanding placental and embryonic development. And it’s just fascinating since I’m pregnant.
I wondered what biologists here thought.
The study painted the clearest picture yet of the genomic landscape of the placenta — and it’s unlike that of any other human tissue ever seen by Behjati, who calls it the “wild west of the human genome.” When they sequenced the DNA of 86 samples from 37 placentas, each set of cells was found to be genetically distinct and chock-full of genetic aberrations typically seen only in aggressive childhood cancers.
But during those crucial first weeks, when a single genetic defect could derail the pregnancy, the placenta may also act as a “dumping ground” for aberrations. During early development, when some of the dividing cells randomly develop genetic abnormalities, they might get earmarked for the placenta instead of the fetus, Behjati reasoned. His team found evidence for this theory: In one of the biopsies, the researchers observed placental cells with three copies of chromosome 10 — two from the mother and one from the father. But cells in the rest of the placenta and fetus had two copies of the chromosome (both from the mother), which suggested that the error started in the fertilized egg but was later corrected.
Patches of the placenta continue to carry on these early mutations — a living archive of genetic defects from the first days of pregnancy — while the fetus remains unharmed. But that’s no problem for the placenta, Wildman hypothesizes, because “it’s not constrained by the necessity to successfully produce an organ that’s going to live for 85 years.” The placenta may not have the same genetic checks and balances that other human cells do because of its inherent transience, he said.
Another possible explanation for these mutations, Behjati said, is that the placenta must outpace the growth of the fetus for the first 16 weeks of human pregnancy, so it may be worth racking up mutations as it balloons inside the uterus. It can “live fast and die young,” as Behjati put it.
Wendy Robinson, a medical geneticist at the University of British Columbia who studies early human development, said that it’s an interesting theory, but she disagreed with the notion that the placenta is merely the genetic garbage pail for the fetus. “There’s very rapid cell divisions that occur early in pregnancy, and that probably imposes a strong selection against cells that just can’t keep up, and so only the good cells will contribute [to the fetus],” she said. “So, it’s not that you’re shunting the bad cells to the placenta — and I know it’s semantics — but it’s that you’re selecting for the good cells in the baby and leaving everything else behind.”