Monday, November 26, 2012

Scientists find that different cells contain different genomes

Basic, high-school level biology currently teaches that every cell in the human body contains the entire human genome, and that this genome is identical. The difference between a kidney cell and a brain cell has so far been believed to be in the expression of genes, and not the amino acid code itself.

Some scientists, though, have questioned the current dogma by positing that the copying of DNA from mother cells to daughter cells during human development is not 100% faithful, and that deletions, duplications and sequence changes occur.

Recently, a study conducted by Yale and Stanford scientists has helped give this hypothesis strong support. Using stem cells, the researchers have shown that "humans are made up of a mosaic of cells with different genomes", thereby debasing the longstanding belief that every cell contains the same genome.

The scientists used whole-genome sequencing to study stem cells (called iPS cells) that they developed from mature differentiated skin cells (i.e. matured daughter skin cells) of the inner upper arm area of two human families.

They compared these cells to the skin cells from which they originated (i.e. mother skin cells), and found that the iPS cell genomes closely resembled their mother cell genomes. However, there were deletions or duplications involving fairly large chunks of DNA (up to a thousand base pairs). Upon further inspection of where these differences first occurred, it was found that up to half these differences "pre-existed", i.e. already found among the mother skin cells and were not a result of deletions/duplications/changes during the copying of mother cell genome to daughter cells.

As it turns out, "mosaicism is extensive" in the cells of the skin. 30% of skin cells contain copy number variations (CNVs), meaning segments of DNA that are deleted or duplicated (without a change in sequence). These CNVs were previously only thought to occur in association with diseases such as cancer. These findings have huge implications because up till now, genetic analyses have only use blood samples. Evidently, blood cell genomes might be different from those of the cells of other parts of the body, and all work that has involved DNA (e.g. developing vaccines/medicine) may be missing mutations that exist outside of blood cells.

See the Yale article here
See another article here

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