19 November 2012

Mosaic Man

Biologists have discovered a genetic "kaleidoscope" in human skin cells

RIA NewsAmerican geneticists analyzed the DNA structure of skin cells and found out that almost all of them contain their own unique genomes, which turns a person into a "mosaic" of cells with different genetic material, according to an article published in the journal Nature (Abyzov et al., Somatic copy number mosaicism in human skin revealed by induced pluripotent stem cells; Skin cells reveal DNA's genetic mosaic press release can be read on the Yale University - VM website).

"We found out that a person is a "mosaic" of cells with different genomes. During the experiments, we realized that about 30% of skin cells differ from each other in the number of copies of different DNA fragments. Previously, we believed that such variations occur only in the case of diseases such as cancer. The genetic kaleidoscope that we found in the skin can also be seen in the blood, in the brain and in other parts of the human body," said Flora Vaccarino, head of the biologists group from the Center for the Study of Children at Yale University (USA).

Vaccarino and her colleagues came to this conclusion by studying cultures of reprogrammed (iPSC) stem cells derived from human skin cells.

During two years of continuous experiments, the authors of the article tried to understand why some iPSC cells acquire harmful mutations and turn into cancer cells. To do this, the scientists obtained skin samples from seven volunteers and used each of them to grow 20 lines of reprogrammed stem cells.

After obtaining the required number of iPSC cells, biologists extracted DNA from them and sequenced their genomes. Having received virtual "copies" of the genetic material, Vaccarino and her colleagues compared them to each other, trying to find traces of mutations. To the surprise of biologists, the genomes of even healthy stem cells differed markedly from each other.

According to the researchers' calculations, the genomes of about 30% of iPSC cells contained small mutations - they contained additional copies of individual DNA sections or one of the existing sections was deleted. Scientists have checked whether they did not arise in the process of "reprogramming" by comparing the genomes of stem cells with the DNA of their progenitors.

It turned out that the original skin cells contained about half of the mutations found in the stem cell lines. Such a result contradicts modern ideas about the structure of the genome of all multicellular organisms. It is believed that all cells of the body, including the skin, contain the same copy of the genome. As a rule, changes in its structure lead to the appearance of a cancerous tumor or the inclusion of a genome protection system that causes a cell with erroneous DNA to commit "suicide".

The authors of the article believe that similar variations exist in other types of cells in the human body. In particular, the genomes of blood and brain cells studied by biologists differed markedly from each other.

According to Vaccarino and her colleagues, the results of their work indicate the need to revise modern methods of genome analysis. As a rule, biologists use only blood cells to sequence the genome, which may not be enough to study inherited diseases and search for genes associated with a tendency to obesity and diabetes.

"The discovery of a genetic "mosaic" in non-sexual cells of the body has serious consequences for genetic analysis. For example, genetic material extracted from blood cells is not a full-fledged copy of the genome in other tissues of the body, including the brain. Their DNA may contain mutations that we don't know exist," Vaccarino concludes.

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