Promotion of shorter HTT protein production shows promise in mice

ProQR . treatmentsThe experimental QRX-704 treatment significantly lowered levels of the toxic huntingtin protein blocks fed with Huntington’s disease A study in a mouse model of the disease demonstrated.

treatment, and Anti-allergic oligonucleotide (ASO) designed to promote the production of a shorter but working copy of the HTT protein, also resulted in signs of improved neuronal function. However, no significant differences were observed in the behavior of the animals. These may require a longer follow-up, the researchers said.

These results suggest that approaches designed to increase production of the shorter HTT protein, which is resistant to cleavage responsible for the formation of its toxic fragments, may have therapeutic potential for Huntington’s.

the study, “Huntingtin isoform resistant to pathogenic proteolysis by antisense oligonucleotide preserves huntingtin function.“in JCI Insight.

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Huntington’s disease is it causes by mutations in HTT The gene, which causes the HTT protein to be produced longer than normal. The mutated protein (mHTT) is cleaved by caspase-6 into toxic fragments that stick together and accumulate inside neurons in the brain, ultimately causing them to die.

Suppressing HTT cleavage may be a way to prevent these toxic fragments from forming and slow the progression of Huntington’s. However, since the absence of a working HTT protein exacerbates the disease, these efforts should also serve to preserve normal functions of the full-length HTT.

The HTT The gene can generate alternative forms of HTT through a process called alternative splicing. As in the recipe, adding or removing some key components – in this case, bits of genetic information – from the messenger RNA (mRNA) gene can alter the resulting protein. Of note, mRNA is the DNA-derived molecule that directs protein production.

One protein, called HTT delta 12, lacks a region near the cutoff site associated with the disease, suggesting that this shorter version may be resistant to caspase-6-induced cleavage.

The short HTT protein is similar to the full length HTT protein

Researchers from Korea, Sweden and France, in collaboration with ProQR Therapeutics, evaluated whether this shorter version preserves the functions of the full-length HTT protein and if boosting its production attenuates signs of disease in a mouse model.

The researchers first confirmed that HTT delta 12 was resistant to cleavage by caspase 6, in contrast to the full-length HTT protein. They then discovered that the shorter version retained structural and biochemical properties similar to those of the full-length HTT, indicating that it could exert similar functions.

Because the HTT protein is key to embryonic brain development, the team tested whether mice genetically engineered to produce HTT delta 12 similar to naturally developed humans.

No brain-related or behavioral differences were found between mice producing delta 12 HTT and those generating full-length HTT. Neurons grown in vitro from mice with the shorter HTT transcript did not show any impairment in the trafficking of proteins across neuronal fibers, including brain-derived neurotrophic factor (BDNF).

HTT is known to be involved in this transduction process, and the progressive loss of BDNF, a factor that helps neurons grow and survive, is associated with Huntington’s progression in mouse models.

Other experiments were performed in lab-grown connective tissue cells from these mice to evaluate changes in two other HTT functions: maintaining the structure of the cell’s protein-sorting factory (called the Golgi complex) and the cells’ ability to form cilia, finger-like projections that help move fluids through tissues, including those surrounding the brain and spinal cord.

No differences in these functions were observed between mice producing HTT delta 12 and those generating full-length HTT. These results indicate that the cleavage-resistant HTT protein retained the neuroprotective functions of the full-length HTT protein.

Effects of QRX-704 Experimental Therapy

Finally, the team evaluated the therapeutic potential to enhance HTT delta 12 production in a mouse model of Huntington’s disease by administering QRX-704 directly into the animals’ brains.

QRX-704 from ProQR is an experimental ASO designed to activate a specific binding site in HTTmRNA that promotes HTT delta 12 mRNA production. The results showed that QRX-704 was well tolerated, but resulted in different levels of HTT delta 12 mRNA across treated mice.

When looking at mice that produced high levels of HTT delta 12 mRNA, the researchers found that they showed significantly lower levels of toxic HTT fragments and clumps. In these mice, the number of dendritic spines – neuron structures involved in signal transduction and reduced in Huntington – was also significantly increased.

Despite this, QRX-704 was not associated with significant behavioral changes, “which may require long-term monitoring,” the researchers wrote.

Overall, “We have demonstrated a pharmacologically feasible ASO-based therapeutic approach, which has resulted in the establishment of an alternative HTT. [form]which is structurally, biochemically and functionally intact in neuroprotective HTT functions but resistant to caspase-6 cleavage,” the researchers write, adding that the approach may treat Huntington’s disease by increasing levels of protective full-length HTT and preventing toxic HTT fragments from forming.