An Overview of Gene Drive

Basics of Cas9 Activity

Basics of Cas9 Activity

Gantz and Bier reflect on and consider the implications of Cas9-based genome editing on genetics in a recent Bioessays that is worth reading if you are trying to keep up on this rapidly evolving part of the genetics ecosystem.

Not long ago the authors created a gene drive system in Drosophila melanogaster that consisted of the Cas9 open reading frame regulated by germline active promoter and a guide RNA expressing transgene all flanked by sequences that were homologous to those flanking the genomic target site of the guide RNA expressed from the drive element.  The element behaves like a homing endonuclease gene and the authors called this Cas9-based homing endonuclease mediated gene conversion the mutagenic chain reaction.

In Bioessays they review their original work in Drosophila and imagine ways that active drive elements could be recalled and ways in which they could be exploited to do other things at the same time.

Synthetic gene drive system based on site specific DNA endonucleases (homing endonucleases, ZFN, TALENS, Cas9). Target sites and homology arms are chosen so that the endonuclease and associated genes (gRNA and other transgenes) are copied into homologous chromosomes.

Synthetic gene drive system based on site specific DNA endonucleases (homing endonucleases, ZFN, TALENS, Cas9). Target sites and homology arms are chosen so that the endonuclease and associated genes (gRNA and other transgenes) are copied into homologous chromosomes.

Recalling or killing gene drive systems is of some interest, particularly during these early days when our knowledge and experience involving planned releases or introductions is very limited.  The authors envision recalling a Cas9 drive element by introducing a second element   into the same genome in which there is a drive element and this second element would share homology arms but would not contain the Cas9 coding region.  It would instead contain two guide RNA-encoding transgenes that would result in existing Cas9-expressing drive elements being deleted from the chromosome with this second element being recombined into the corresponding site.  In essence the Cas9-expressing element would drive its own deletion from the genome.  It would be interesting to see this modeled.

The authors are very enthusiastic about the potential of Cas9-based technologies to enrich existing genetic systems and to facilitate genetic approaches in systems that have traditionally not be subject to extensive genetic analysis and rightfully so.

A selfish gene (blue) spreading through a mosquito population

A gene drive system (blue) spreading through a mosquito population

For example, the authors envision knock-in technology using homology-dependent repair (HDR) triggered by programmed endonucleases to be used instead of transposons in some cases.  Vectors with all of the bells and whistles present on transposon-based gene vectors would be present but integration would be driven by Cas9 and HDR.  They point so some advantages to this approach – namely, creating homozygotes could be more efficient and easier.  This is an interesting idea.

While the authors discuss a number of topics that are interesting and worth reading they briefly discuss some ideas about how gene drive technologies could have potential applications to human gene therapy to combat HIV or cancer.  For insect biologists thinking perhaps more about pest erradication, it was interesting to see these therapeutic analogs.

This is an interesting overview and prospectus.

Gantz, V. M., Bier, E., 2016 The dawn of active genetics. Bioessays 38: 50-63doi: 10.1002/bies.201500102.

 

 

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