In Mice, CRISPR-based Alzheimer's Therapies Inch Forward
What will it take before CRISPR/Cas gene editing technology might treat Alzheimer’s disease? Much more work, to be sure, but researchers are devising approaches, and grappling with the challenges of translating the technology from cells to animals, and some day to people. The prospect of correcting disease-causing mutations seems most straightforward for early onset, autosomal-dominant forms of AD (ADAD), and toward that goal, Martin Ingelsson, Uppsala University, Sweden, and colleagues published the first instance of using CRISPR/Cas9 to neutralize the Swedish APP mutant in patient cells and in mice. In the April 18 in Molecular Therapy: Nucleic Acids, the scientists describe how they knocked out the mutant allele, diminishing the production of Aβ in patient cells, while leaving the wild type allele intact. Using the same strategy, they also modified the genome in transgenic mice bearing the human APPswe allele, but don’t know yet if that will affect AD pathology.
- CRISPR knockout of Swedish mutation in patient cells reduces Aβ
- A separate CRISPR approach to blunt Aβ production could be tried in sporadic AD
“This manuscript elegantly demonstrates the potential for correcting genetic defects both in cell models and in an animal model of AD based on the Swedish mutation. These findings move the field one step closer to being able to genetically correct harmful mutations, such as dominantly inherited AD,” said Randall Bateman, Washington University, St. Louis, Missouri. “If and how genetic modification could treat AD caused by mutations, and even sporadic AD, will be closely watched,” Bateman said.
In another new study, Subhojit Roy’s group at the University of Wisconsin, Madison, took on the challenge of sporadic AD. In a paper posted April 28 on the BioRxiv preprint server, the group report how they used CRISPR to frameshift the C-terminus of APP. This introduced an early stop codon, truncated the protein and dramatically decreased Aβ production. This edit worked in wild-type mice, too, but whether it affects amyloid production in vivo remains to be seen.
CRISPR/Cas9 technology allows researchers to rewrite the genome practically at will. The technique has been used to create cell models of ADAD by introducing presenilin and APP mutations into human induced pluripotent stem cells (iPSCs); neurons derived from those cells showed disease-related changes in Aβ production (May 2016 news). Other groups have used CRISPR to correct familial AD mutations in the presenilin gene in patient-derived iPSCs (Pires et al., 2016, Poon et al., 2016). In another study, researchers corrected a presenilin 2 mutation in iPSC-derived basal forebrain cholinergic neurons from patients with autosomal-dominant AD, and showed this normalized the cells’ electrophysiological function and Aβ secretion (Ortiz-Virumbrales et al., 2017).
In contrast, Ingelsson’s idea was not to correct, but to destroy APP harboring the Swedish mutation. This double point mutation in exon 16 just upstream of the BACE cleavage site promotes the amyloidogenic cleavage of APP, increasing production and secretion of Aβ40 and Aβ42 in brain and peripheral tissues. Simply knocking out the mutant allele, Ingelsson reasoned, should forestall Aβ accumulation and disease. While on sabbatical in co-author Xandra Breakefield’s lab at Massachusetts General Hospital in Boston, Ingelsson worked with co-first authors Bence György and Camilla Lööv to devise a way to slay the foe with CRISPR.
The Swedish mutation is an attractive target. Two mismatches between mutant and wild-type sequence should ensure better specificity of guide RNAs used in CRISPR than the one mismatch found in other APP and PS FAD mutations. Besides, a TGG trinucleotide right next to the mutated bases is a preferred binding motif for the Cas9 nuclease. “It was a fortunate combination that made it straightforward to design a system to target the mutation with high precision,” Ingelsson told Alzforum.
First, the researchers tested guide RNAs, transfecting them along with Cas9 into patient–derived fibroblasts from three Swedish mutation carriers and two non-affected family members. Guide RNAs complementary to the mutated allele caused Cas9 to make small deletions, resulting in frame shifts in APP that wiped out the BACE cleavage site in the mutant, but not in the wild type allele. The most effective guide, dubbed SW1, left APP protein levels in the cells unchanged but reduced Aβ40 and Aβ42 production by 50 to 60 percent in three independently generated cell lines.
Therapeutic targeting: A guide RNA (blue) binds exon 16 of human APP, spanning the 2-base Swedish mutation (yellow) directly adjacent to a TGG Cas9-binding protospacer adjacent motif (pink) and near the β-secretase cleavage site (arrow). [From György et al., 2018.]
To see if this would work in vivo, the authors used Tg2576 mice /research-models/tg2576, which carry multiple copies of the human APP Swe transgene. In a preliminary study, they packaged DNA encoding both Cas9 and guide RNAs in adeno-associated virus 9 (AAV) vectors, and infected cultured primary cortical neurons from Tg2576 embryos. After three weeks, they detected disrupted AAP Swe alleles, mainly in the form of 1 base pair insertions. Then they tested the vectors in adult mice, injecting them into the hippocampus. All the mice showed some disruption of the APP Swe gene, again mostly from single base pair insertions.
However, the approach hit only a small fraction of transgenes. In cultured and in vivo Tg2576 neurons, 2.3 and 1.3 percent of transgenes, respectively, were disrupted. That’s low, but unsurprising, Ingelsson said. “This is a model with roughly 100 copies of the transgene per neuron, so it seems the CRISPR system got overwhelmed,” he said. The AAV delivery and CRISPR activity appeared efficient, as a guide RNA targeting a control endogenous gene modified 36 percent of target alleles in cultured Tg2576 neurons.
Because of the low modification rate in the overexpression model, it made no sense to look at plaque or other downstream effects in the mice, Ingelsson said. He plans to do that using a knock-in model. Ingelsson does not know what percentage of genes would need to be modified to reduce plaque formation in vivo, but thinks effects could add up over time.
In the second study, first author Jichao Sun in Roy’s group wanted a CRISPR therapy that would work on any case of AD, familial or sporadic. Previously, the group found that clipping off the C-terminal, cytoplasmic portion of APP prevents the protein from trafficking into endosomes, where it meets up with BACE and undergoes its first cleavage on the road to Aβ (Aug 2013 news, Dec 2015 news). In the new work, Sun took advantage of a protospacer adjacent motif (PAM) trinucleotide in the far C-terminal sequence of APP to target CRISPR cleavage and truncated the gene. Whether in mouse cells or in human iPSC-derived neurons, this strategy led to a dramatic drop in Aβ40/42 secretion, and an increase in non-amyloidogenic α-secretase products.
Roy’s group also tried to disrupt APP in mice. When they injected AAV9 expressing a guide RNA and Cas9 into the hippocampus, they detected less binding of the APP C-terminal antibody Y188, indicating the gene had been truncated. They were able to induce brain-wide expression of truncated APP after injecting AAV into the ventricles of newborn mice.
One problem with CRISPR gene editing is that if the guide RNAs are not 100 percent faithful, they can induce mutations in other genes, too. To look for off-target effects, Sun analyzed the top five most likely non-APP targets, based on the RNA guide sequence, and the two APP homologs APLP1 and 2, finding no evidence of disruption in them. The researchers detected no obvious side effects of CRISPR in primary hippocampal neurons, in other words, truncating APP did not alter neurite outgrowth, spine number, synapse density, or electrophysiological activity.
“The work from Roy’s lab represents another interesting approach of making therapeutic use of CRISPRs in the AD setting,” said Ingelsson. “It is more generally applicable compared to our approach, which has to be tailored to a specific disease-causing mutation,” he said. “The CRISPR/Cas technology certainly offers us a fascinating repertoire of strategies to disrupt or correct alleles that are involved in AD and other neurodegenerative diseases.”
What Lies Ahead
CRISPR has shown promise in animal models of Huntington’s and amyotrophic lateral sclerosis (Yang et al., 2017; Monteys et al., 2017; Dec 2017 news) even as new tools are expanding editing capabilities from DNA to RNA. Recently, one group directed CRISPR to RNA to correct aberrant tau splicing in neurons derived from people with frontotemporal dementia (Mar 2018 news). The next goal for AD researchers will be to show that CRISPR can curb amyloid deposition, and improve function, in an AD mouse model.
Even after crossing this hurdle, obstacles remain before CRISPR will be ready to use in the clinic. It’s likely that any therapeutic agent would need to be expressed widely throughout the brain, which rules out local injection as a means of delivery. Ingelsson thinks that brain-wide delivery is achievable with the latest AAV9-based vectors, which broadly transduce cells in the CNS after peripheral injection (Dec 2008 news, Sep 2014 news, and Dashkoff et al., 2016), However, whether they will drive sufficiently robust expression remains to be determined.
Off-target effects also cause concern. “The main challenge with CRISPR is that while you can watch and control as it repairs the mutation you want to eliminate, the enzyme can create new mutations elsewhere. To some extent you can find those with whole-genome sequencing, but if you miss one, that could be disaster,” wrote Sam Gandy, Mount Sinai School of Medicine in New York, in an email to Alzforum. Gandy’s group has used the technology to correct PS2 mutations in patient cells.
Ingelsson agrees. “We don’t have a clear picture yet of what off-target effects we may get. People are trying to come up with ways of assessing this for a particular treatment, and those systems will have to be further developed before we can run the risk of exposing patients to this type of treatment.”
Human trials of CRISPR anti-cancer therapies started in China in 2015, and have been approved to begin at the University of Pennsylvania. CRISPR Therapeutics, of Cambridge, MA, is pursuing trials of gene editing to treat hemoglobin disorders, including sickle cell anemia and β-thalassemia. All these efforts involve the removal of blood cells, ex vivo gene knockout, and reinfusion. Genetic editing in situ will be more complicated, say researchers. “I would like to see it perfected in blood diseases before jumping into the brain,” said Gandy.—Pat McCaffrey.
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CRISPR/Cas9-mediated gene editing ameliorates neurotoxicity in mouse model of Huntington’s disease.
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Published at Thu, 03 May 2018 23:05:57 +0000