The aggregation of mutant huntingtin (mHTT) protein is a central pathogenic mechanism in [Huntington's disease](/diseases/huntingtons) (HD). This pathway describes the molecular cascade from CAG repeat expansion in the [HTT](/genes/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.
Pathway Diagram
```mermaid flowchart TD A["CAG Repeat Expansion<br/>HTT Gene<br/>>36 CAG Repeats"] --> B["Expanded PolyQ Tract<br/>mHTT Protein"] B --> C["Conformational Change<br/>alpha-helix -> beta-sheet"] C --> D["Nucleation<br/>Oligomeric Nucleus Formation"] D --> E["Soluble Oligomers<br/>Toxic Intermediates"] E --> F["Protofibrils"] F --> G["Mature Fibrils"] G --> H["Inclusion Bodies<br/>Neuronal Inclusions"]
E --> I["Cellular Clearance<br/>Autophagy/UPS"] H --> I
C --> J["Post-Translational Modifications<br/>Phosphorylation/SUMOylation/Acetylation"] J --> K["Modulated Aggregation<br/>Increased or Decreased"] K --> I
I --> L["Normal Proteostasis<br/>or<br/>Proteostasis Failure"] L --> M{"Cellular Outcome"} M --> N["Neuronal Survival"] M --> O["Neuronal Dysfunction and Death"]
...
Mutant Huntingtin Aggregation Pathway
Overview
The aggregation of mutant huntingtin (mHTT) protein is a central pathogenic mechanism in [Huntington's disease](/diseases/huntingtons) (HD). This pathway describes the molecular cascade from CAG repeat expansion in the [HTT](/genes/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.
Pathway Diagram
Mermaid diagram (expand to render)
Molecular Mechanism
Step 1: Polyglutamine Expansion
The CAG trinucleotide repeat expansion in exon 1 of the [HTT](/genes/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.
Step 2: Conformational Transition
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:
Cellular environment: pH, ionic strength, and molecular crowding affect the transition
Post-translational modifications: Phosphorylation, SUMOylation, and acetylation can modulate the propensity for misfolding
Step 3: Nucleation and Oligomerization
The aggregation follows a nucleated polymerization mechanism:
Lag phase: Monomeric mHTT undergoes conformational changes to form unstable oligomeric nuclei
Growth phase: These nuclei recruit additional monomers to form β-sheet-rich structures
Transition to fibrils: Oligomers associate to form protofibrils and eventually mature fibrils
Step 4: Formation of Toxic Oligomers
Soluble oligomeric intermediates are now recognized as the primary toxic species in HD, rather than mature inclusions:[@miller2010]
Membrane disruption: Oligomers can form pore-like structures that disrupt cellular membranes
Synaptic dysfunction: Oligomers impair synaptic vesicle trafficking and neurotransmitter release
Mitochondrial dysfunction: Oligomers localize to mitochondria and disrupt energy production
Transcriptional dysregulation: Oligomers sequester transcription factors and co-activators
Step 5: Inclusion Body Formation
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:[@difiglia1997]
Nuclear inclusions: Formed when mHTT translocates to the nucleus
Cytoplasmic inclusions: Accumulate in neuronal soma and dendrites
Distribution: Striatal medium spiny neurons and cortical pyramidal neurons are most affected
While TDP-43 pathology is not a primary feature of HD, there are mechanistic intersections:
RNA metabolism: Both mHTT and TDP-43 disrupt RNA processing and splicing
Stress granules: Both proteins can be sequestered into stress granules
Proteostasis failure: Shared impairment of autophagy and UPS
Nuclear transport defects: Both proteins can disrupt nucleocytoplasmic transport
See [Amyotrophic Lateral Sclerosis](/diseases/amyotrophic-lateral-sclerosis) and [RNA Metabolism](/mechanisms/rna-metabolism) for more details on TDP-43 mechanisms.
Therapeutic Implications
Aggregation Inhibitors
Small molecules: Target polyQ conformation to prevent β-sheet formation
Disaggregases: Promote conversion of aggregates to non-toxic forms