New research suggests that the normal structure of the placenta resembles that of a tumour, and harbours many of the same genetic mutations found in childhood cancers.
In the first study of the genomic architecture of the human placenta, scientists have found the make-up of the placenta is different to any other human organ.
The study supports the theory of the placenta as a dumping ground for genetic defects, whereas the foetus corrects or avoids these errors, say researchers.
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In early pregnancy, the fertilised egg implants into the wall of the uterus and begins dividing from one cell into many.
Cells differentiate into various types of cell and some of them will form the placenta.
Around week 10 of pregnancy, the placenta begins to access the mother’s circulation, getting oxygen and nutrients for the foetus, removing waste products and regulating crucial hormones.
Researchers say it has long been known that the placenta is different from other human organs.
In 1 to 2 per cent of pregnancies, some placental cells have a different number of chromosomes to cells in the foetus – a genetic flaw that could be fatal to the foetus, but with which the placenta often functions reasonably normally.
Despite this, problems with the placenta are a major cause of harm to the mother and unborn child.
“Our study confirms for the first time that the placenta is organised differently to every other human organ, and in fact resembles a patchwork of tumours," said Professor Steve Charnock-Jones, a senior author of the study from the University of Cambridge.
“The rates and patterns of genetic mutations were also incredibly high compared to other healthy human tissues.”
This new study, published in Nature, is the first high-resolution survey of the genomic architecture of the human placenta.
Scientists at the Wellcome Sanger Institute and the University of Cambridge conducted whole genome sequencing of 86 biopsies and 106 microdissections from 42 placentas, with samples taken from different areas of each organ.
They found that each one of these biopsies was a genetically distinct ‘clonal expansion’ – a cell population descended from a single common ancestor.
This indicated a clear parallel between the formation of the human placenta and the development of a cancer.
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Analysis also identified specific patterns of mutation that are commonly found in childhood cancers, such as neuroblastoma and rhabdomyosarcoma, with an even higher number of these mutations in the placenta than in the cancers themselves.
Researchers say that now the link between genetic abnormalities in the placenta and birth outcomes has been established, further studies using larger sample sizes could help to uncover the causes of complications and diseases that arise during pregnancy.
“The placenta is akin to the ‘Wild West’ of the human genome, completely different in its structure from any other healthy human tissue," said Dr Sam Behjati, a senior author of the study from the Wellcome Sanger Institute.
“It helps to protect us from flaws in our genetic code, but equally there remains a high burden of disease associated with the placenta.
“Our findings provide a rationale for studying the association between genetic aberrations in the placenta and birth outcomes at the high resolution we deployed and at massive scale.”
Reader Q&A: How does radiation kill cancer if it causes cancer?
Asked by: Odysseus Ray Lopez, US
It’s rather like the way guns can be used to commit crime, or stop it. Radiation causes cancer because its high-energy photons can cause breaks in the DNA strands in your cells. Cells can repair this damage up to a point, but sometimes the repair isn’t perfect and leaves some genes defective.
If the break affects one of the many tumour-suppressing genes in your DNA, that cell can become cancerous. But cancer cells are also more vulnerable to radiation than ordinary cells. Part of what makes them cancer cells is their ability to divide rapidly and this normally means that some of the DNA ‘spellcheck’ mechanisms are turned off.
So when a cancer cell suffers a break in a DNA strand, it’s less likely to repair it correctly. Depending where the break occurs, it might either kill the cell outright, or make it reproduce more slowly.
Radiation therapy uses a focused beam that is aimed at just the part of the body with the tumour, and the dose is carefully calculated to cause the minimum collateral damage to healthy cells. Even so, radiation therapy does very slightly increase your chances of developing a second cancer.
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