I set a high bar for writing about mouse studies. I don’t include them in my textbooks or news articles, and only rarely blog about them. But when experiments in mice shine a glimmer of hope on a horrific illness with a long history of failed treatments, I pay attention. That happened last week for a report on editing out of mice the human version of the mutant Htt gene that causes Huntington disease (HD), published in the Journal of Clinical Investigation.
HD affects about 30,000 people in the US, and more than 200,000 family members are “at-risk,” possibly having inherited the mutation. The disease arises from a repeat of the DNA triplet CAG beyond the 35 or fewer copies that most of us have, at the start of the gene that encodes the protein huntingtin. CAG specifies the amino acid glutamine, and the extra stretch of it clogs certain neurons in the striatum in the brain, affecting movement, cognition, and behavior.
Symptoms typically begin in adulthood, but 10 percent of cases are juvenile. Karli Mukka developed symptoms at age 5, and died within weeks of her father Karl, she just 14 years old, he 43, in 2010. Karli’s huntingtin gene did a loop-de-loop upon itself, giving her 99 CAG repeats to her father’s 47. DNA Science told her story here.
ONLY ONE TREATMENT, FOR ONE SYMPTOM
An expanding triplet repeat presents a thorny drug-targeting challenge. Countering it isn’t as simple as supplying a missing enzyme, depleting a biochemical buildup, unfolding and refolding an errant protein, or even introducing a functional gene with gene therapy.
Unlike other genes in which mutations remove a normal function, abnormal huntingtin protein confers a “toxic gain of function.” Having two mutations is no worse than having just one, which means that lacking the normal (wild type) allele has no effect, at least after birth – that’s important.
The only FDA-approved treatment for HD is tetrabenazine, a repurposed schizophrenia drug used in other nations for decades before its FDA approval to treat the movement (chorea) part of HD in 2008. An altered version (deutetrabenazine) became available in April 2017: “heavy” hydrogen atoms (deuterium) keep the drug circulating longer.
Researchers have for years thrown every tool imaginable at the formidable expanded Htt gene:
• Deploying small molecules to target RNA loops or metabolites.
• Manipulating growth factor levels.
• Implanting stem cells to replace neurons.
• Dampening expression of the mutant gene using RNAi or antisense nucleic acids.
New biomarkers, prediction studies, scans, and induced pluripotent stem cells track the onset of the disease, with the hope of eventual early intervention.
I was once a fly-on-the-wall at private meetings of HD researchers, writing reports for a funding organization. I learned a great deal about the mouse models, the treatment modalities, and ways of detecting the disease early and tracking its progression. Progress was slow. That gig ended in 2010 – before the debut of CRISPR/Cas9 gene editing. And now it’s beginning to sound like a whole different ballgame.
CRISPR EDITING OUT MUTANT HTT
The peculiarities of HD make gene editing, which can add, replace, or remove a gene, the most logical therapeutic strategy. HD requires DNA to be jettisoned, not augmented.
While RNAi and antisense oligonucleotides can dampen expression of the extended gene, the effect isn’t permanent in the way that snipping out the repeat or even the entire gene would be. And a one-time or few-times editing out is preferable to a regular need for treatment, especially given the unsettling healthcare situation in the US.
Xiao-Jiang Li, MD, PhD, distinguished professor of human genetics at the Emory University School of Medicine, with colleagues there and at the Chinese Academy of Sciences, used CRISPR/Cas9 gene editing on mice that have the first exon (protein-encoding part) from the human Htt gene, including 140 CAG repeats – it’s called an HD140Q knockin (“Q” stands for polyglutamine). Specks of the toxic protein appear when the mice are 4 to 6 months old, aggregating by 9-10 months. The timetable is like that in people, because mice live about 2 years.
Technical details (jargon alert): CRISPR/Cas9 was delivered to the striata of two dozen 9-month-old mice in two batches of adeno-associated viruses: guide RNAs targeting exon 1 and the Cas9 enzyme that cleaves both DNA strands, removing the gene and triggering repair of the breaks. The guide RNA part included instructions for red fluorescent protein, and both batches were under different promoter (control) sequences, so that the researchers could compare delivery of the dual intervention to either alone. Both are required: find the target and cut it out.
Source: PLOS BLOGS