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]. |