B.N. Krishnan
North Creek High School,
United States
Keywords: Huntington's disease, small molecules, drug design
Summary:
Huntington’s disease (HD) is a terrible neurodegenerative 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 CYS263 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 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 critical t-value of -4.42 for Gibbs free energy and a critical t-value of 3.804 for pKd, 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. RMSD (Root Mean Square Deviation) was calculated for the protein complexes. The inhibitor for the arginine 260 residue was the most unstable. The diffusion coefficient was calculated for the final four inhibitors, indicating a diffusion coefficient of above 4.00 x 10^-10 for all the inhibitors. Residues from the mHtt protein were more unstable than residues from the Rhes protein. Retrosynthetic pathways for the inhibitors were also calculated using IBM RXN, which indicated an average of less than 10 steps for all inhibitors and high confidence, showing non-difficult synthesis. The percent yield for the arginine 260 inhibitor was 27.4%, the percent yield for the cysteine 263 residue was 33.03%, the percent yield for the lysine 191 residue was 26.68%, and the percent yield for the tyrosine 184 residue was 57.21%. The off-target effects of the inhibitors were also calculated using SwissTargetPrediction, and all of the inhibitors had minimal off-target effects. Arginine 260 had no off-target effects, cysteine 263 had 100 off-target effects with a low probability of 0.115737, and lysine 191 had 2 off-target effects with a low probability of 0.118883. 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.