Overview of Enzerna’s technology: ASREs are chimeric proteins consisting of an RNA endonuclease domain and RNA binding domains called PUF, that specifically recognize different 8-nucleotide RNA sequences. By changing 2-3 amino acids in each repeat, ASRES can be modified to bind and cleave any 8-nt RNA target.
Huntington’s disease (HD) is a major polyglutamine autosomal dominant disorder that affects about 1/10,000 people in the US, EU and Japan. The polyglutamine repeat expansion within the Huntingtin protein (HTT) is associated with the depletion of neurons and an increased number of glial cells in the region of the brain critical for movement, memory, and decision-making. The polyglutamine protein aggregates formed are the main cause of neuronal cell death, although recent results have suggested that the RNA repeats itself may also be directly responsible for neurotoxicity.
There are currently no FDA approved disease modifying drugs for Huntington's disease. Standard of care includes supportive and palliative care, including physical therapy, nutritional management, and emotional support. Therapeutic interventions include neuroleptics and benzodiazepines to manage chorea in Huntington's disease, and certain antidepressant to treat the psychological symptoms of the disease.
Enzerna Biosciences is commercializing a novel approach to specifically cleave (and thus inactivate) any RNA using Artificial Site-Specific RNA Endonucleases (ASREs) engineered with customized sequence specificity. Combined with gene delivery vectors or direct neural delivery approaches, ASREs provide a new strategy for selective degradation of pathogenic transcripts associated with triplet nucleotide disorders.
More on HD:
All HTT proteins have the polyglutamine repeats, but the number of repeats influences the onset, progression, and severity of the disease. Normal individuals have between 7-34 CAG repeats, while Individuals with ≥40 repeats develop HD. Since HD patients have one normal and one mutated HTT allele with long CAG repeats, an attractive therapeutic strategy would be to selectively degrade the product of mutated allele. Therapeutic strategies directly targeting mutated HTT mRNA, such as antisense oligonucleotides (ASO), have produced promising results. However, there are difficulties in ASO delivery and the effects of ASO on structured CAG repeats are difficult to predict. Moreover, if selective reduction of mutant HTT expression is not accomplished, it is still unclear whether elimination of the normal Htt transcript would lead to adverse consequences. A study using inducible conditional knock-out of mouse Htt expression suggested that reduction of Htt expression in the adult was tolerated, but a more recent study came to the opposite conclusion: loss of Htt expression in the adult brain resulted in progressive neurological phenotypes.
ASRE Technology: High Specific and Highly Differentiated
Targeting the pathogenic RNA or protein with ASRE-Technology offers an avenue for curative therapies. Antisense oligonucleotide (ASO) and RNA interference (RNAi)- based therapies have been shown to be effective in isolated cells. However, these therapies are limited by the need for lifelong administration, poor delivery across the blood brain barrier, and passive delivery to target cells in vivo. While antisense RNAs could be delivered via gene therapy, to date, targeting efficiency remains unacceptably low.
For many diseases, gene edited, most notable using CRISPR/Cas DNA editing technology offers an opportunity to correct mutant alleles. Unfortunately, given the mechanism of the gene editing process, CRISPR/Cas does not offer a viable therapeutic approach. Provided the editing machinery can be delivered to the appropriate cells, for each cell that undergoes a DNA editing event (intiated by a single double stranded break), one or more of several outcomes are possible:
(a) the mutant and/or normal allele will be targeted by the DNA editing enzyme where a random in frame deletion will occur to decrease the repeat length (desired event);
(b) the mutant and/or normal allele will be targeted by the DNA editing enzyme where a random in frame expansion will occur to increase the repeat length (adverse event);
(c) the mutant and/or normal allele will be targeted by the DNA editing enzyme where a random out of frame deletion or expansion will occur (adverse event);
(d) no change in mutant and/or normal allele. Since, each of the events will occur randomly (i.e., cannot be controlled) in each cell that is transduced with the editing machinery, CRISPR/Cas does not offer a viable therapeutic at this time.