Already set to revolutionise the food industry, CRISPR gene editing technology is also being intensively investigated as a way to fix human DNA errors associated with diseases such as cystic fibrosis. Aside from the challenges of introducing new 'correct' DNA sequences into the right location, there are also concerns that the Cas9 enzyme that CRISPR uses to cleave DNA at a specific location could potentially introduce cuts where it’s not intended to, potentially causing cancer. Despite these concerns, researchers are focusing on the long-term knowing that true breakthroughs do not happen overnight.
But when CRISPR is used to correct a gene using a strand of DNA that scientists supply to cells, not just to snip out some DNA, it doesn’t work very well. That’s because the cells must edit the DNA using a process called homology-directed repair, or HDR, that is only active in dividing cells. And unfortunately, most cells in the body—liver, neuron, muscle, eye, blood stem cells—are not normally dividing. For this reason, “knocking out a gene is a lot simpler than knocking in a gene and correcting a mutation,” says Cynthia Dunbar, president-elect of ASGCT and a gene therapy researcher at the National Heart, Lung, and Blood Institute in Bethesda, Maryland.