With apologies to the brothers Grimm, molecular biologists may have found a way to treat 18 genetic diseases at one blow [ Cell vol. 170 pp. 899 – 912 ’17 ]. They use adeno-associated virus (AAV) packing a modified enzyme and an RNA to remove repeat expansions from RNA. The paper give a list of the 18, all but one of which are neurologic. They include such horrors as Huntington’s chorea, the most common form of familial ALS, 3 forms of spinocerebellar ataxia and 6 forms of spinocerebellar atrophy.
They use Cas9 from Streptococcus Pyogenes, part of the CRISPR system (https://en.wikipedia.org/wiki/CRISPR) bacteria use to defend themselves against viruses, with a single guide RNA. Even more interestingly, Cas9 is an enzyme which breaks up RNA, but the Cas9 they used is catalytically dead. They think that just binding to the aggregated RNA containing the repeats is enough to break up the aggregate. This is the way antiSense oligoNucleotides are thought to work.
The problem with getting a bacterial enzyme into a human cell is avoided here by using a virus to infect them (AAV). It did get rid of RNA aggregates in patients’ cells from 4 of the diseases (two myotonic dystrophies, and the familial ALS).
It is almost too fantastic to be true.
Why almost all of these repeat expansion diseases affect the nervous system is anyone’s guess. As you can image theories abound. So all we have to do is figure out how to get the therapy into the brain (hardly a small task).
Chemiotics II 
Comments
I’d say they stretch quite a bit claiming effectiveness of their approach for targeting r(GGGGCC), there are only two experiments with transfected cells, with quantitatively quite variable outcomes (cf. fig. 2F and fig. S2A)
Interestingly, the only relevant mouse model of r(GGGGCC)exp-associated ALS requires delivery of defected gene with the same AAV vehicle. Getting relevant mouse data under this circumstances will be challenging.
But anyway I can only salute using CRISPR/Cas9 for targeting RNA (not DNA), which I didn’t even known is possible!
The fascinating thing about the paper is that they use catalytically inactive Cas9. Just getting it, and the guide RNA near the aggregates appears to be enough