The aggregation of mutant huntingtin (mHTT) protein is a central pathogenic mechanism in Huntington's disease (HD). This pathway describes the molecular cascade from CAG repeat expansion in the HTT gene through polyglutamine (polyQ) tract expansion, protein misfolding, oligomerization, and ultimately the formation of inclusion bodies. Understanding this aggregation pathway is crucial for developing therapeutic interventions that target protein clearance, stabilization, or prevention of toxic oligomer formation.
The CAG trinucleotide repeat expansion in exon 1 of the HTT gene translates to an expanded polyglutamine tract in the huntingtin protein. Normal huntingtin contains 10-35 glutamine repeats, while disease-causing alleles have 36 or more repeats. The polyQ expansion threshold correlates with disease onset - longer repeats cause earlier onset and more rapid progression.
The expanded polyQ tract undergoes a structural transition from random coil/α-helical conformation to β-sheet-rich structure. This conformational change is a prerequisite for aggregation and is influenced by:
The aggregation follows a nucleated polymerization mechanism:
Soluble oligomeric intermediates are now recognized as the primary toxic species in HD, rather than mature inclusions:[1]
Mature fibrils accumulate as visible inclusion bodies in neuronal nuclei and cytoplasm. While inclusions were initially thought to be the primary toxic species, current evidence suggests they may represent a protective mechanism that sequesters toxic oligomers:[2]
Multiple post-translational modifications regulate mHTT aggregation:
The autophagy-lysosome pathway is a major clearance mechanism for mHTT aggregates:[7]
The aggregation of mHTT shares mechanistic similarities with other protein aggregation disorders:
| Feature | Huntington's Disease | Alzheimer's Disease | Parkinson's Disease |
|---|---|---|---|
| Aggregating Protein | mHTT (polyQ) | Aβ, Tau | α-Synuclein |
| Oligomer Toxicity | Soluble oligomers | Soluble oligomers | Soluble oligomers |
| Template Seeding | Not prion-like | Limited | Prion-like spread |
| Cellular Clearance | Autophagy, UPS | Autophagy, UPS | Autophagy |
While TDP-43 pathology is not a primary feature of HD, there are mechanistic intersections:
See Amyotrophic Lateral Sclerosis and RNA Metabolism for more details on TDP-43 mechanisms.
Miller J, Arrasate M, Shaby BA, et al. Quantitative measurement of soluble huntingtin oligomers. Nat Methods. 2010. ↩︎
DiFiglia M, Sapp E, Chase KO, et al. Aggregation of huntingtin in Huntington disease. Science. 1997. ↩︎
Wellington CL, Singaraja R, Ellerby L, et al. Caspase cleavage of huntingtin generates aggregation-prone fragments. J Biol Chem. 2000. ↩︎
Humbert S, Bryson EA, Cordelieres FP, et al. Phosphorylation of huntingtin at Ser421 regulates neuroprotection. Dev Cell. 2002. ↩︎
Steffan JS, Agrawal N, Pallos J, et al. SUMO modification of huntingtin reduces aggregation. Science. 2004. ↩︎
Jeong H, Then F, Melia TJ Jr, et al. Acetylation of mutant huntingtin at Lys444 promotes clearance. Cell. 2009. ↩︎
Ravikumar B, Vacher C, Berger Z, et al. Inhibition of mTOR induces autophagy and reduces mutant huntingtin. Nat Genet. 2004. ↩︎
Hipp MS, Park SH, Hartl FU. Proteostasis of mutant huntingtin. Trends Cell Biol. 2014. ↩︎