Skip to main content

Table 1 Disease Modifying Experimental Therapies for HTT

From: RNAi mechanisms in Huntington’s disease therapy: siRNA versus shRNA

Strategy Pro and Con Citations
Humanized synthetic ZFN-KRAB repressors + no risk of DSBs
- off target effects
- triggers innate immune responses
- temporary effects depending on protein turnover
Garriga-Canut, M. et al. (2012) Synthetic zinc finger repressors reduce mutant huntingtin expression in the brain of R6/2 mice. Proc. Natl Acad. Sci. [54].
CRISPR knockout of mHTT + permanent
- too large to fit in AAV
- requires PAM site near PolyQ tract
- bacterial origin of Cas9 elicits innate immune response
- CAG repeats within sgRNA form secondary structure, limiting efficiency
Malkki H. (2016) Selective deactivation of Huntington disease mutant allele by CRISPR–Cas9 gene editing. Nature Reviews Neurology.
Intrabodies - immunogenic when injected as naked protein
- Nucleic acid delivery requires a large vector such as lentivirus, which integrates genomically and can cause cancer
Cardinale, A et al. (2008). The potential of intracellular antibodies for therapeutic targeting of protein-misfolding diseases. Trends in Molecular Medicine [55].
siRNA/miRNA and Antisense Oligonucleotides (ASOs) + drug-like properties, more suited to regulation than gene therapy requiring viral vectors
+ can be easily customized for allele specificity
+ symptoms can improve for longer than the period of mRNA knockdown (“Huntingtin Holiday”) See Note 1.
- short acting effect, requires long-term continuous dosing
- renal and hepatic toxicity, non-trivial off target effects
- inflammatory when recognized by extracellular toll-like receptors
Kordasiewicz, H. B. et al. Sustained therapeutic reversal of Huntington’s disease by transient repression of huntingtin synthesis. Neuron.
Rao et al. (2009) siRNA vs. shRNA: Similarities and differences. J. Advanced Drug Delivery Reviews [56].
shRNA-based RNAi + longer lasting but not permanent (months to years in primates)
+ can fit inside an AAV, episomal plasmid in nucleus
+ shRNA is virally encapsulated and elicits less inflammation from toll-like receptors
+ constructs can be inserted into an artificial miRNA scaffold to mitigate neurotoxicity specifically
- overdose due to excessively strong promoters is common
- off target effects can occur
Davidson, B (2008). Artificial miRNAs mitigate shRNA-mediated toxicity in the brain: Implications for the therapeutic development of RNAi. PNAS [57]. See also [58, 59].
  1. Note 1: Symptoms are reversed for longer than the period of HTT knockdown [60], known as a ‘Huntingtin Holiday,’ theoretically enabling cellular repair to occur [61]
  2. Note 2: Designer RNAi possible based on SNPs in loci nearby to the polyQ tract, to prevent theoretical problems associated with WT HTT silencing