Alright, real quick. Genome, or gene editing, is when targeted changes are made, like insertions and deletions, right in an organism's genome. Over the past decade, the CRISPR-Cas9 system has become a very popular method of genome editing because it's fast, cheap, precise, and relatively easy to use. The way it works is that researchers create a piece of RNA with a guide sequence, which is complementary to the DNA of the human genome. to a targeted bit of DNA in the host's genome.
In other words, if the DNA has a sequence that reads 5'GGCTAT 3', then the RNA guide sequence is exactly the opposite and reads 3'CCGAUA 5'. And remember, it's a U for uracil instead of a T for thymidine because the guide sequence is made of RNA and not DNA. The Cas9 protein then attaches to the RNA and the whole thing binds to the target DNA sequence in the host genome. The Cas9 RNA complex then makes a double strand cut in the genomic DNA, and an alternative piece of DNA can be spliced in right at that spot.
CRISPR-Cas9 technology works in a variety of cell types and organisms, and it's been used to study diseases and generate tissues from stem cells like heart muscle tissue and neuronal tissue. Now, it's also possible to treat a whole multicellular organism with genome editing. For example, a mouse with a liver disease due to a genetic defect was treated with a CRISPR-Cas9-mediated genetic change, and it improved the mouse's symptoms.
One important point to note about this mouse example, however, is that the change was made to somatic cells rather than germline cells, meaning these genomic modifications aren't passed to the next generation. That said, CRISPR-Cas9 technology is able to alter the DNA in germline cells, and if that's done, then the engineered changes can be transmitted across generations. And this has been done in several organisms, including mice, monkeys, and most recently in humans. Last month, Chinese scientist He Zhangquai claimed at a conference that he has edited the genes of twin girls using CRISPR-Cas9 technology. He specifically altered the CCR5 gene, making the carrier resistant to some strains of HIV.
He made the change in two embryos which were then implanted in a woman and carried to term. Reportedly, the unidentified twins were born and are healthy and at home with their parents. This announcement has led to an international moral outcry.
First off, Zhang Kuai acted in secrecy and didn't even tell his institution about his experiments. Second, it's not clear if there was proper informed consent from parents before the procedure was done. Third, the procedure didn't have a known medical need since neither of the infants had an HIV infection, and the procedure inactivated a healthy gene.
Fourth, genome editing technology is still in its infancy, and there's no global consensus about using gene editing technology on human subjects. In addition, The risks of doing this to human embryos remains difficult to predict. For example, according to John Quay's presentation, not every cell in the embryos was edited by CRISPR-Cas9.
One possible outcome of this is that the twins may be heterozygous for the edited gene, meaning each cell has a normal and an edited copy of the CCR5 genes, in which case they may not even be resistant to HIV. Another outcome is that the twins may be mosaic, meaning that some of their cells have two copies of the normal gene while the other cells have two copies of the edited gene, meaning that they'd only be resistant to HIV if the immune cells end up with the edited gene. In addition, the mutation made to the CCR5 gene didn't exactly match the known variation of the gene that confers HIV resistance. This means it's not clear if these untested changes will provide the same benefits, or if they'll introduce new unintended consequences.
All this said, while the research appears to be rushed, and the fallout from it may have consequences that impact the scientific community for years, there is support for the idea that germline genome editing of human embryos could someday be used ethically to prevent devastating genetic diseases like cystic fibrosis or muscular dystrophy. The general consensus is that we're just not there yet, and that the scientists have moved quicker. than the ethicists.