She Has Her Mother's Laugh by Carl Zimmer

Author:Carl Zimmer
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2018-05-29T04:00:00+00:00

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The search for the causes of diseases has uncovered a number of cases of mosaicism. But scientists have also discovered some people in which mosaicism can heal.

A team of Dutch dermatologists and geneticists described the first case of mosaic healing in 1997. They examined a twenty-eight-year-old woman whose skin was so fragile that even a gentle rubbing would raise blisters. This painful condition is caused by a mutation to a gene called COL17A1. Normally, skin cells use this gene to make a type of collagen that makes them stretchy.

Both of the woman’s parents were carriers. They each carried a mutation on one copy of their COL17A1 gene. (They had different mutations in different locations—a detail that will turn out to matter tremendously in a little while.) Because each parent also had a normal copy of the COL17A1 gene, they could still make enough collagen to keep their own skin healthy.

The woman had the bad luck to inherit each parent’s bad copy of the gene. Those defective copies were present when she was still a fertilized egg. They were passed down to every cell that zygote gave rise to. When she developed skin, her skin cells needed to switch on her COL17A1 gene to make collagen. The gene failed at its job, and she was left with skin that couldn’t stretch.

Remarkably, however, the woman’s doctors noticed that she had a few patches of normal skin on her arms and hands. They didn’t blister when they were rubbed. The woman had been aware of some of the patches for as long as she could remember. Others had emerged more recently and were expanding. When the doctors looked at the molecular makeup of her healthy patches, they found healthy collagen.

Looking closely at the DNA in her cells, the geneticists figured out how these patches had developed. Each arose from a single faulty skin cell. Before it divided, the cell duplicated its DNA. And during that duplication, it mutated in a peculiar way: It swapped a section of the COL17A1 gene between its chromosomes.

When the two daughter cells pulled away from each other, one cell no longer carried the woman’s mutation from her mother. It had been replaced by the working portion of her father’s COL17A1 gene. Now altered, the cell could make collagen again. And when it divided, its daughter cells inherited a working version of the gene as well. The woman’s mosaics had repaired her defective genes.

Since that initial discovery, scientists have found more genetic diseases partially cured by mosaics. Their list now includes hereditary forms of other skin diseases, along with anemia, liver disorders, and muscular dystrophy. The growing inventory of mosaicism—causing diseases or healing them—raised the question of just how mosaic humans are in general. The definitive answer would come from breaking down people into their 37 trillion cells and sequencing every base of DNA in each one. For now, scientists are only carrying out rough surveys. But even these preliminary studies have come to one clear conclusion: We are all mosaics, and we have been so pretty much since our beginnings.

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