Knowledge Shot: CRISPR-Cas9 and the age of gene-edited humans

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 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 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 Jiankui, 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.