The Song of the Cell: An Exploration of Medicine and the New Human

by Siddhartha Mukherjee

In 1543, he published his anatomical works in seven volumes entitled De Humani Corporis Fabrica (The Fabric of the Human Body). The word Fabric in the title was a clue to its texture and purpose: this was the human body treated like physical material, not mystery; made of fabric, not spirit. It was part medical textbook, with nearly seven hundred illustrations, and part scientific treatise, with maps and diagrams that would lay the foundation for human anatomical studies for centuries to come.

Coincidentally, it was published the same year that the Polish astronomer Nicolaus Copernicus would put out his “anatomy of the heavens,” the monumental book The Revolutions (On the Revolutions of Heavenly Spheres), which featured a map of the heliocentric solar system that placed the Earth in orbit and the sun firmly at its center. Vesalius had put human anatomy at the center of medicine.

The final phase of Rudolf Virchow’s life bore testimony not only to his theories about the cooperative social organization of the body—cells working with cells—but also a belief in the cooperative social organization of the state: humans working with humans. Immersed within a society that was becoming progressively racist and anti-Semitic, he argued vehemently for equality among citizens. Illness was an equalizer; medicine was not designed to discriminate. “Admission to a hospital must be open to every ill person who stands in need of it,” he wrote, “whether he has money or not, whether he is Jewish or heathen.”

Gene editing—making directed, deliberate, and specific changes in a genome—can be deployed through multiple strategies, but the most commonly used form relies on a bacterial protein called Cas9. This protein can be introduced into human cells and then “guided,” or directed, to a specific part of a cell’s genome to make a deliberate alteration: typically a cut in the genome that usually disables the targeted gene. Bacteria use this system to chop up the genes of invading viruses, thereby inactivating the invader. Pioneers of gene editing, including Jennifer Doudna, Emmanuelle Charpentier, Feng Zhang, and George Church, among others, have adapted this bacterial defense system and turned it into a way to make deliberate edits in the human genome.

Cas9, when combined with a piece of RNA to guide it, can be directed to make a deliberate change in the human genome. You can analogize it to finding and erasing one word in one sentence on one page in one volume of that eighty-thousand-book library. It errs occasionally and erases an unintended word as well, but its general fidelity is remarkable. More recently, the system has been modified not just to erase words but also to implement a vast array of potential changes in a gene, such as adding new information or making more subtle alterations. Cas9 is a search-and-destroy eraser. To continue the analogy, it can change Verbal to Herbal in the preface to volume one of Samuel Pepys’ Diary in a college library containing eighty thousand books. Every other word, in every other sentence, in every other book in the library is, for the most part, left alone.

The phenomenon in which some cells of an organism have been genetically altered while others have not been is called mosaicism.