Study confirms human placenta resembles a tumour
Scientists at the Wellcome Sanger Institute, the University of Cambridge and collaborators in the UK have confirmed that the placenta is unlike any other human organ and instead resembles that of a tumour. According to the scientists, the placenta harboursmany of the same genetic mutations found in childhood cancers – a “dumping ground” for genetic mutations which the foetus may correct or avoid.
Dr. Sam Behjati, from the Wellcome Sanger Institute, said: “The placenta is akin to the ‘wild west’ of the human genome, completely different in its structure from any other healthy human tissue. It helps to protect us from flaws in our genetic code, but equally there remains a high burden of disease associated with the placenta.”
Moving forward, this new study provides an in-depth rationale, and is the first high-resolution survey of the genomic architecture of the human placenta, using whole genome sequencing of some 86 biopsies and 106 microdissections from 42 placentas, with samples taken from different areas of each organ.
The scientists discovered that each one of these biopsies was a genetically distinct ‘clonal expansion’ – a cell population descended from a single common ancestor — indicating a clear parallel between the formation of the human placenta and the development of a cancer.
Moreover, analysis 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.
The placenta is also able to tolerate major genetic flaws: in one biopsy, the researchers observed three copies of chromosome 10 in each cell, two from the mother and one from the father, instead of the usual one copy from each parent. But other biopsies from the same placenta and from the foetus carried two copies of chromosome 10, both from the mother. A chromosomal copy number error such as this in any other tissue would be a major genetic flaw.
Professor Gordon Smith, from the University of Cambridge, said: “It was fascinating to observe how such a serious genetic flaw as a chromosomal copy number error was ironed out by the baby but not by the placenta.
“This error would have been present in the fertilised egg; yet derivative cell populations, and most importantly those that went on to form the child, had the correct number of copies of chromosome 10, whereas parts of the placenta failed to make this correction. The placenta also provided a clue that the baby had inherited both copies of the chromosome from one parent, which can itself be associated with problems.”
Further studies using larger sample sizes may help to uncover the causes of complications and diseases that arise during pregnancy, and even avoid premature or stillbirths.
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