Corrupt DNA Might Be Good for You

Our bodies are a genetic patchwork, possessing variation from cell to cell. Is that a good thing?

Even healthy brains harbor genetic diversity, though scientists disagree over the extent. Credit:Olena Shmahalo for Quanta Magazine

From Quanta Magazine (find original story here).

Your DNA is supposed to be your blueprint, your unique master code, identical in every one of your tens of trillions of cells. It is why you are you, indivisible and whole, consistent from tip to toe.

But thats really just a biological fairy tale. In reality, you are an assemblage of genetically distinctive cells, some of which have radically different operating instructions. This fact has only become clear in the last decade. Even though each of your cells supposedly contains a replica of the DNA in the fertilized egg that began your life, mutations, copying errors and editing mistakes began modifying that code as soon as your zygote self began to divide. In your adult body, your DNA is peppered by pinpoint mutations, riddled with repeated or rearranged or missing information, even lacking huge chromosome-sized chunks. Your data is hopelessly corrupt.

Most genome scientists assume that this DNA diversity, called somatic mutation or structural variation, is bad. Mutations and other genetic changes can alter the function of the cell, usually for the worse. Disorderly DNA is a hallmark of cancers, and genomic variation can cause a suite of brain disorders and malformations. It makes sense: Cells working off garbled information probably dont function very well.

Most research to date has focused on how aberrant DNA drives disease, but even healthy bodies harbor genetic disorder. In the last few years, some researchers report that anywhere from 10 to 40 percent of brain cells and between 30 and 90 percent of human liver cells are aneuploid, meaning that one entire chromosome is either missing or duplicated. Copy number variations, in which chunks of DNA between 100 and a few million letters in length are multiplied or eliminated, also seem to be widespread in healthy people.

The exact extent of cell-to-cell diversity is still unclear and a matter of some debate. Its only in the last two years that scientists have been able to look carefully at just one genome at a time, with the advent of new methods of single-cell DNA sequencing. (Earlier methods averaged the results of thousands or millions of cells and could only detect huge aberrations or relatively common ones.) Because this work is so new, each study includes surprises: A single-cell genome sequencing study of 97 neurons from healthy brains, published today by Christopher Walsh, a neurologist at Boston Childrens Hospital and Howard Hughes Medical Institute, and the postdoctoral researcher Xuyu Cai found few that were aneuploid less than 5 percent. But most had at least one good-sized copy number variation.

Walshs findings and others mark a third phase in human genomics. When the complete DNA of one human being was first sequenced in 2000, it was considered to be the human genome. Soon after, researchers began to explore the differences between individuals, launching the era of the personal genome. Now science is entering the age of the microgenome, in which research begins to explore the worlds within us, examining our inherent imperfections and contradictions, the multitudes we contain.

With that third phase comes a deeper question. What do our genetic contradictions mean? Do they play an important role in our biology? At this point, just about every genome scientist has a slightly different take. One surprising theory suggests that DNA diversity might be good for you. Its a feature, not a bug.

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Corrupt DNA Might Be Good for You