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HD InsightsTesting Potential Huntington’s Disease Therapeutics In Vivo

Immunotherapy targeting extracellular huntingtin

More potential drugs are being discovered as more is uncovered on cellular mechanisms that are altered in Huntington’s disease, the normal function of the huntingtin (Htt) protein, and what goes wrong at the cellular level when huntingtin is mutated (mtHtt).

Two promising therapeutics have moved from in vitro to in vivo testing, and here we describe the mechanism of disease they are targeting.

It is well-understood that mtHtt affects the body systemically, not just the brain. Interestingly, Htt protein is found not only intracellularly, but extracellularly, and it has been shown that the Htt protein can spread by traveling between cells. How this extracellular Htt protein contributes to the HD clinical phenotype is unknown. To explore this, a group has recently developed an antibody to bind a known exposed region of extracellular Htt protein. Once this occurs, it is hypothesized that Htt can no longer travel peripherally or extracellularly.

The group showed that the antibody they created, mAB C6-17, could stop mtHtt from traveling between cells. In their recent manuscript, they tested if this had any therapeutic effects, utilizing the YAC128 mouse model of HD.1 YAC128 mice were immunized with the antibody and a reduction in peripheral mtHtt protein was found in the blood of treated mice. Interestingly, the authors found that the mAB could cross the blood-brain barrier and, therefore, could lower the number of mtHtt aggregates in the brain. When assessing behaviors of the mice, the mAB treatment had some mild effects on the motor symptoms. Based on this in vivo data, the authors suggest that clearing extracellular Htt may have benefits in delaying disease onset and could be used alongside other treatments that affect what is happening inside the cell to optimize treatment benefit.

Antisense oligonucleotides to change the huntingtin sequence without changing expression levels

The main therapeutic in clinical trials for HD is the use of antisense oligonucleotides (AONs or ASOs) to conduct either a non-specific or mutant-specific knockdown of Htt. While trials using AONs are underway, at this point none have yet shown full clinical success. One problem with knocking down Htt is that the protein is expressed in every cell of the body and has necessary cellular functions that it won’t be able to perform if knocked down.

Recently, a research group came up with a method for using an AON to alter the sequence of Htt without stopping it from being expressed.2 Part of the pathophysiology of Htt is that once it’s expressed as a protein, it gets broken into smaller pieces that aggregate and cause some of the pathology in HD.

The researchers are using an AON called AON12.1 to remove the site in the Htt protein that is prone to cleavage in HD. This site is called the caspase-6 cleavage site, located at amino acid 586. When this amino acid is removed from mtHtt in YAC128 mice, the mice do not get sick, making this a good target for an AON. The group performed a course of treatments by injecting AON12.1 into the brain of the YAC128 mice and found that AON12.1 successfully changed the sequence of the Htt gene. Approximately 40% of mtHtt in the treated brain area could not be cleaved by Caspase-6, but the overall levels of Htt was unchanged.

The mice were then assessed for mtHtt-induced phenotypes, and they found that body weight was improved as well as some motor phenotypes. The genetic expression profile after treatment showed that this treatment had impacts on metabolic pathways as well as rescuing genes that are dysregulated in the YAC128 HD model. It is not unexpected to find an incomplete rescue, as caspase-6 cleavage is likely not the entire cause of HD pathology. The authors suggest that AON12.1 needs to be tested in additional models. Finding new ways to deliver this AON to achieve a higher than 40% change is likely required to appreciate the complete suite of benefits this treatment could have.

1. Bartl, S. et.al. (2023) Reducing huntingtin by immunotherapy delays disease progression in a mouse model of Huntington disease. Neurobiology of Disease: 190 https://doi.org/10.1016/j.nbd.2023.106376

2. Kuijper, E.C. et.al. (2024) Antisense oligonucleotide-mediated disruption of HTT caspase-6 cleavage site ameliorates the phenotype of YAC128 Huntington disease mice :Neurobiology of Disease:
190 doi: https://doi.org/10.1016/j.nbd.2023.106368