Designer Genes
THE FIRST STEP IS a no-brainer. Say you are one of the tens of thousands of people with Huntington’s disease, a terrible, hereditary neurodegenerative disorder caused by a mutation in the HTT gene. The diseased version of the gene makes an abnormally long protein that becomes toxic in your neurons, eventually killing them. Symptoms usually appear in your 30s or 40s as small twitching movements, a lack of coordination, and depression. In the coming years, the spasms will grow. It will be increasingly difficult to walk, talk, and think. Within 20 years, you will most likely be dead. You are resigned to your fate, but you and your partner would very much like to have kids, and you have no interest in passing this particular legacy to your child.
Today, your best bet is a procedure called preimplantation genetic diagnosis (PGD), in which embryos produced through in vitro fertilization (IVF) are screened for the unwanted gene. PGD has been a game changer for many couples, but it has limitations. You may be one of the unlucky few who has two copies of the bad HTT, in which case all your embryos are guaranteed to inherit one copy. Or you could be uncomfortable with the idea of producing and discarding multiple embryos.
Soon you may have another option. The invention of CRISPR—a tool that allows scientists to program a protein to change specific sequences of DNA within a cell—gives us the ability to edit the human genome. We could change the bad code in the HTT gene of an embryo back to a normal version. We could even fix the code in your sperm or eggs before conception, ensuring an embryo that’s free of Huntington’s.
And it’s not just Huntington’s. As much as 10 percent of the population carries genes that put them at risk for disorders such as sickle-cell anemia, Tay-Sachs, cystic fibrosis, ataxia, muscular dystrophy, hemophilia, and albinism. These diseases originally arose as random mutations in the distant
