National Geographic : 2016 Aug
40 national geographic • August 2016 made a choice to do this—that unless there’s broad agreement, it is not going to happen.” “Scientists do not have standing to answer these questions,” Lander told me. “And I am not sure who does.” CRISPR-Cas9 has two components. The first is an enzyme—Cas9—that functions as a cellular scalpel to cut DNA. (In nature, bacteria use it to sever and disarm the genetic code of invading vi- ruses.) The other consists of an RNA guide that leads the scalpel to the precise nucleotides—the chemical letters of DNA—it has been sent to cut. (Researchers rarely include the term “Cas9” in conversation, or the inelegant terminology that CRISPR stands for: “clustered regularly inter- spaced short palindromic repeats.”) The guide’s accuracy is uncanny; scientists can dispatch a synthetic replacement part to any location in a genome made of billions of nucleotides. When it reaches its destination, the Cas9 enzyme snips out the unwanted DNA sequence. To patch the break, the cell inserts the chain of nucleotides that has been delivered in the CRISPR package. By the time the Zika outbreak in Puerto Rico comes to an end, the U.S. Centers for Disease Control and Prevention estimates that, based on patterns of other mosquito-borne illnesses, at least a quarter of the 3.5 million people in Puerto Rico may contract Zika. That means thousands of pregnant women are likely to become infected. Currently the only truly effective response to Zika would involve bathing the island in insec- ticide. James and others say that editing mos- quitoes with CRISPR—and using a gene drive to make those changes permanent—offers a far better approach. Gene drives have the power to override the traditional rules of inheritance. Ordinarily the progeny of any sexually reproductive animal receives one copy of a gene from each parent. Some genes, however, are “selfish”: Evolution has bestowed on them a better than 50 percent chance of being inherited. Theoretically, scien- tists could combine CRISPR with a gene drive to alter the genetic code of a species by attaching a correct major genetic flaws, including the muta- tions responsible for muscular dystrophy, cystic fibrosis, and one form of hepatitis. Recently sev- eral teams have deployed CRISPR in an attempt to eliminate HIV from the DNA of human cells. The results have been only partially successful, but many scientists remain convinced that the technology may contribute to a cure for AIDS. In experiments, scientists have also used CRISPR to rid pigs of the viruses that prevent their organs from being transplanted into hu- mans. Ecologists are exploring ways for the technology to help protect endangered spe- cies. Moreover, plant biologists, working with a wide variety of crops, have embarked on efforts to delete genes that attract pests. That way, by relying on biology rather than on chemicals, CRISPR could help reduce our dependence on toxic pesticides. No scientific discovery of the past century holds more promise—or raises more trou- bling ethical questions. Most provocatively, if CRISPR were used to edit a human embryo’s germ line—cells that contain genetic material that can be inherited by the next generation— either to correct a genetic flaw or to enhance a desired trait, the change would then pass to that person’s children, and their children, in perpe- tuity. The full implications of changes that pro- found are difficult, if not impossible, to foresee. “This is a remarkable technology, with many great uses. But if you are going to do anything as fateful as rewriting the germ line, you’d bet- ter be able to tell me there is a strong reason to do it,” said Eric Lander, who is director of the Broad Institute of Harvard and MIT and who served as leader of the Human Genome Proj- ect. “And you’d better be able to say that society No discovery of the past century holds more promise—or raises more troubling ethical questions.