Commonly known as the genetic ‘glue cutter’, or molecular scissors, the CRISPR-Cas9 technique was born promising to revolutionize biomedicine, especially to detect and eliminate damaged genetic material. Capable of editing a single letter in DNA base pairs and RNA base pairs, it can correct mutations in the genome of living cells , potentially ending up with serious alterations and diseases such as cancer. And, what is more important: modifying embryos to correct genetic characteristics can be done in order to improve health, or to create babies ‘à la carte’) which provokes an intense debate that has already been on the table for years.
Bioethical debates aside, CRISPR-Cas9 is an absolutely promising technique that has been making headlines for years. Now, it has made it possible to unveil the first genetically edited reptiles: they are albino lizards , and they are about the size of your index finger.
In their study, published on August 27 by the journal Cell Reports , the researchers were also able to show that these modified lizards are also capable of transmitting this modification to their descendants ; that is, albino lizards were able to successfully transmit gene-edited alleles for albinism to their offspring.
Usually, the way in which gene editing is done in animal models is by injecting CRISPR-Cas9 gene editing reagents into newly fertilized lizard eggs, or also called single-celled zygotes (just before the fertilized egg begins cell division , and for this they must be very precise with the times).
Reptiles have an extra difficulty in using gene editing
And if CRISPR-Cas9 has already shown its magic in other animal models, this time the experiment has an extra difficulty : this technique cannot be used in reptiles, since lizards have internal fertilization, and therefore it cannot be predicted when, what is the exact moment when fertilization occurs.
On the other hand, a single-celled embryo isolated from a female lizard also cannot be easily transferred, making it almost impossible to manipulate it outside of the lizard.
But amid these difficulties, the team realized that the transparent membrane over the ovary revealed developing eggs, including eggs that were about to be fertilized. Still, they first decided to inject the CRISPR reagents into the unfertilized eggs inside the ovaries, to see if CRISPR worked.
For this reason, the logical thing would be to think that gene editing would only be possible in alleles inherited from the mother, since the birth DNA is not in those unfertilized oocytes . And the researchers waited 3 months for the lizards to hatch to see: “It’s a bit like slow-motion gene editing,” according to one of the authors and associate professor at the University of Georgia, Doug Menke.
Three months later, the surprise occurred:
“It turns out that about half of the ‘mutant’ lizards we generated had gene editing events on the maternal allele and the paternal allele .” How is that possible?
This suggests that the CRISPR components remain active for several days, or even weeks, within the unfertilized eggs. After examining the offspring, the researchers found that around 6% to 9% of the oocytes, depending on their size, produced offspring with gene editing events.
“No one has been able to do these kinds of manipulations on any reptile before,” says Menke. He adds: “There is not a large community of developmental geneticists studying reptiles, so we hope to tap into exciting functional biology that has yet to be explored.”
Why albino lizards?
The team had two reasons for editing the lizards to make them albinos, rather than editing other traits. First, because the technique had few risks for the lizards themselves; when the tyrosinase albinism gene is removed, a loss of pigmentation occurs without being lethal to the animal . Second, since humans with albinism often have vision problems, the researchers hope to use lizards as a model to study how the loss of this gene affects retinal development.
Humans and other primates have a feature in the eye called the fovea, which is a hole-like structure in the retina, key to sharp vision. The fovea is present in anole lizards, and they rely on it to hunt insects.
Fountain:
Cell Reports , Rasys et al .: “CRISPR-Cas9 Gene Editing in Lizards Through Microinjection of Unfertilized Oocytes” https://www.cell.com/cell-reports/fulltext/S2211-1247(19)31005-8
