2) how would you guarantee that nearly 100% of the target cells get it. The disease is a dominant condition, and by virtue of its putative mechanism a minority of untreated cells could still trigger the problem (though presumably onset might be delayed)
3) I would worry that if you do the CRISPR cut in the wrong place, or at the wrong time, you could instead make the tandem repeats even longer which would make the progression of the disease faster.
4) Outright deletion of the gene is -- who knows. AFAIK all humans have some, low number of tandem repeats and the disease emerges when you have a lot of tandem repeats. IIRC the mouse tandem repeat KO gene model has no side effects but stuff like alzheimer's AB being protective of certain viral brain infections is coming out, so maybe there's some function there? And lab mice aren't really exposed to too many pathogens.
Not an expert, so experts please correct me if I'm wrong:
* CRISPR doesn't do all of the time what you want it to do. It is quite error prone. What's done usually is that you apply it to multiple cells, check each of them, and take the ones where it worked out. Usually only a fraction has the changes that you wanted.
* Many multicellular organisms like humans grow from a single cell called zygote. Furthermore, very early embryos consist of a ball of blastomeres that each, if you isolate them and put them into the right conditions, can form a full organism.
* So the most practical way right now of editing genome of mice or humans is to hijack that stage, to use either multiple zygotes or blastomeres (not an expert, not sure which of the two is actually used) and apply CRISPR to them, then take the successful results and gestate them.
* In mice as well as other animals like zebrafish, this kind of editing is pretty routine already. But in humans, it's never been done except for one Chinese scientist. We don't know at all whether there are any adverse effects of the therapy, something we overlooked, etc.
* Also, the gene can be identified prenatally in a sense that you sequence the genes of an embryo still in the womb but that is only possible at a much later developmental stage than the blastomere stage so your options of editing aren't better than the options you have in a living human.
* There is another option that doesn't involve CRISPR: If your parent has a healthy copy of the gene and a non-healthy one, one can perform IVF multiple times, take one blastomere each from the ball of blastomeres, test them for the bad gene, and gestate the embryos that have only healthy copies of the gene. If every at-risk patient gets such a therapy, this approach is enough to deal with Huntington's disease and many other genetic diseases but it limits you in animal or plant breeding to mendelian genetics, basically making your life harder than it could be if you want to combine multiple beneficial traits. Here, CRISPR based approaches give you a big benefit, as long as you can deal with the anti-GMO crowd.
dnautics|7 years ago
1) how would you get CRISPR into the brain?
2) how would you guarantee that nearly 100% of the target cells get it. The disease is a dominant condition, and by virtue of its putative mechanism a minority of untreated cells could still trigger the problem (though presumably onset might be delayed)
3) I would worry that if you do the CRISPR cut in the wrong place, or at the wrong time, you could instead make the tandem repeats even longer which would make the progression of the disease faster.
4) Outright deletion of the gene is -- who knows. AFAIK all humans have some, low number of tandem repeats and the disease emerges when you have a lot of tandem repeats. IIRC the mouse tandem repeat KO gene model has no side effects but stuff like alzheimer's AB being protective of certain viral brain infections is coming out, so maybe there's some function there? And lab mice aren't really exposed to too many pathogens.
dnautics|7 years ago
https://www.frontiersin.org/articles/10.3389/fnins.2018.0007...
and I correct myself, there is a phenotype to the deletion of the polglutamine track - the mice are dumber.
https://www.ncbi.nlm.nih.gov/pubmed/16403806/
est31|7 years ago
* CRISPR doesn't do all of the time what you want it to do. It is quite error prone. What's done usually is that you apply it to multiple cells, check each of them, and take the ones where it worked out. Usually only a fraction has the changes that you wanted.
* Many multicellular organisms like humans grow from a single cell called zygote. Furthermore, very early embryos consist of a ball of blastomeres that each, if you isolate them and put them into the right conditions, can form a full organism.
* So the most practical way right now of editing genome of mice or humans is to hijack that stage, to use either multiple zygotes or blastomeres (not an expert, not sure which of the two is actually used) and apply CRISPR to them, then take the successful results and gestate them.
* In mice as well as other animals like zebrafish, this kind of editing is pretty routine already. But in humans, it's never been done except for one Chinese scientist. We don't know at all whether there are any adverse effects of the therapy, something we overlooked, etc.
* Also, the gene can be identified prenatally in a sense that you sequence the genes of an embryo still in the womb but that is only possible at a much later developmental stage than the blastomere stage so your options of editing aren't better than the options you have in a living human.
* There is another option that doesn't involve CRISPR: If your parent has a healthy copy of the gene and a non-healthy one, one can perform IVF multiple times, take one blastomere each from the ball of blastomeres, test them for the bad gene, and gestate the embryos that have only healthy copies of the gene. If every at-risk patient gets such a therapy, this approach is enough to deal with Huntington's disease and many other genetic diseases but it limits you in animal or plant breeding to mendelian genetics, basically making your life harder than it could be if you want to combine multiple beneficial traits. Here, CRISPR based approaches give you a big benefit, as long as you can deal with the anti-GMO crowd.