Locking the SUMO Switch: Inhibiting Rhes and mHTT Interactions Using De Novo Design To Prevent The Spread of Huntington’s Disease

Huntington’s disease (HD) is a deadly neurodegener-ative disorder, claiming the lives of 2.27 million people annually. It is essential to create therapies and drugs that target the cause of the disease. HD is caused by an increase in the soluble and mutant forms of the huntingtin protein (mHTT). SUMOylation of mHTT by Rhes (ras homolog enriched in striatum) through the E3 ligase domain triggers the solubility of mHTT. The SUMOylation of mHTT by Rhes is essential for its toxicity. The spread of mHTT through the brain is facilitated by its toxicity and tunneling nanotubes (TNTs). Inhibition of the cysteine 263 residue on Rhes prevents the formation of TNTs and the spread of mHTT. A model of Rhes and mHTT was constructed using constrained docking. Anchor residues in the protein complex were identified using the web server PocketQuery, which was used to construct small-molecule inhibitors using the web server LEA3D. Eight inhibitors were found through PocketQuery and LEA3D, and an inhibitor for cysteine 263 was created. In order to predict the activity of the inhibitors, a QSAR model was constructed using open-access data on E3 ligase inhibitors. Calculations for the pKd and Gibbs free energy yielded a two-tailed P value of 0.0408 for ${p K d}$ and 0.0409 for Gibbs free energy, indicating that the Rhes- mHTT inhibitors are more efficient than the controls. Blood brain barrier permeability was tested, and five out of the nine inhibitors were predicted as blood brain barrier permeable. Molecular dynamics simulations indicated that the best SUMO E3 ligase inhibitor (ARG260) and the cysteine 263 inhibitor were stable as they had an RMSD of under $2 \mathrm{~A}^{\circ}$. Retrosynthetic pathways for the inhibitors were also calculated to facilitate wet-lab synthesis using IBM RXN, which indicated high confidence, showing non- difficult synthesis. The inhibitors had low toxicity, with an Acute Oral Toxicity Class of 4 for both inhibitors. The created drugs can also be used as a prophylactic therapy to prevent symptom onset. HD is terrible, claiming millions of lives and tearing apart families. In addition, key interacting residues of the Rhes-mHTT complex were discovered to guide further drug discovery. It is essential to create new therapies to target the SUMOylation and spread of mHTT, such as small-molecule therapies, which this research aims to address.