The Molecular Basis of the Disease State

Huntington’s disease is caused by the elongation of the polyQ region of the protein huntingtin, resulting in mutant Huntingtin or mHtt. Current research indicates that the main contributions of mHtt in causation of the symptoms of Huntington’s Disease, are formation of protein aggregates and the alteration of Htt regulated transcription.

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Figure 1: Crystal Structure of Wild-type Huntingtin (Htt). The structure in green is the PolyQ region of Htt. Mutant Huntingtin is characterized by elongation of the protruding Poly Q region. PDB: 3IOR. Crystal structure obtained from www.rcsb.org

Aggregate Formation

The aggregates that form from mHTT, are caused by what Dr. Max Perutz described as a polar zipper. The polar zipper is formed when the N-terminal, containing the Poly Q region, is cleaved from the protein by a protease, the enzymes responsible for protein cleavage. The fragments formed from cleavage then hydrogen bond to each other to form the aggregate. The aggregate elicits toxicity to both the nucleus and the cytoplasm, causing some of the cellular damage associated with Hunitington’s Disease. (Perandones, Micheli, and Radrizzani 2010) While the mechanism for how the aggregates cause cellular damage is not completely understood, the leading hypothesis is that the formation of the truncated mutant huntingtin aggregates exceeds the cells ability to break them down using the proteosome or autophagosome. Thus the adult onset of Huntington’s Disease is attributed to slowing of cellular functions that are supposed to breakdown the toxic HD aggregates. (Martinez-Vicente et al. 2010)

Epigenetic Dysregulation

The other leading cause of cellular dysfunction is the dysregulation of transcription associated with mutant Huntingtin. Huntingtin regulates an extensive net of enzymes responsible for the epigenetics of the cell via mechnisms that methylate and acetylate DNA and histones as seen in Figure 2.

Figure 2: Summary of the disrupted epigenetic mechanisms in HD and the tested ameliorative strategies. Actions by mHtt: regular arrow, activation; blunt end, inhibition. Resulting effects in the relationship between transcription factors and chromatin-remodeling proteins and their downstream targets and related processes are represented by dashed (reduced activity/effect) and thick arrows(enhanced activity/effect). Image from: Valor, Luis M., and Deisy Guiretti. 2014. “What’s Wrong with Epigenetics in Huntington’s Disease?” Neuropharmacology, Neuroepigenetic Disorders, 80 (May): 103–14. doi:10.1016/j.neuropharm.2013.10.025

At the level of transcription there is evidence that mHtt effects the regulation of cystathione y-lyase (CSE) the enzyme responsible for the biosynthesis of the amino acid, cysteine. In the proposed pathway, mHtt inhibits the function of specificity protein 1, a known transcriptional activator of CSE. The subsequent inhibition of CSE, is implicated in diminished levels of cysteine, which increases oxidative stress and contributes to the disease state of striatal cells.  (Paul et al. 2014)

H3K4me3 (trimethylation of lysine 4 on histone 3) is a histone modification that signals active transcription start sites. The strength of the H3K4me3 signal is strongly correlated with the level of gene expression within a cell. Using ChiP seq, a method to analyze protein-DNA interaction, the H3K4me3 signal for active transcription was compared between Htt wildtype individuals and individual with Huntington’s Disease. Epigenetic dysregulation was evident in HD individuals, as H3K4me3 was reduced subsequently leading to the reduction of the mRNA of genes in close proximity to the H3K4me3 transcription start site. (Dong et al. 2015)

6 Replies to “The Molecular Basis of the Disease State”

  1. Hi – one of the things that I was curious about upon reading your article is the nature of the polar zipper. Is this something that is unique to mutant huntington protein or is it a general protein-protein interaction motif that occurs when there are many polar residues? Similar question, does the protease that cleave mHutt only cleave mutant protein, or does wildtype protein also get cleaved in the same spot (due to some signal sequence) and not form aggregates because there is not a high percentage of glutamine residues to mediate the tight interaction? Last question promise! – do other proteins also have similar polyQ regions (or other poly amino acid regions) that lead to a similar phenotype? (I guess I ask this because I’m curious why huntington is such a unique protein in this way. Thanks!!

    1. Great questions. The polar zipper is not unique to Huntingtin’s Poly Q region, Perutz also found polar zippers made by hydrogen bonding in Ascaris hemoglobin and several homeodomains in drosophilia. (Perutz 1994) The protease that cleaves mutant Huntingtin also cleaves wildtype huntingtin at the same spot, but the elongation of glutamines makes mHtt more prone to cleavage producing more fragments that contribute to the disease state. (Perandones, Micheli, and Radrizzani 2010) Fragments of wiltype Huntingtin are capable of becoming dimers, tetramers and larger oligimers, however mHtt fragments produce large oligimers because there are more fragments available due to the increased protease activity, and the increased gluatmines do make tighter interactions between the fragments, via increased hydrogen bonding (Bates, Tabrizi, Jones 2014) Yes, other proteins due have similar Poly Q regions, and PolyQ expansion in them causes misfolded proteins and leads to neurodegeneration and/or motor disorders. Spinobulbar muscular atrophy is caused by PolyQ expansion in Androgen receptor that causes motor disorders. (Williams, Paulson 2008)

  2. Great project Zach! You are very thorough in your presentation of the disease and are generally able to provide alternative information in areas where less is actually known about it. My question is in terms of the elongated polyQ region of mHtt. On the previous page you briefly mentioned that the elongation disrupts interactions with other proteins; however, I am curious as to which exact interactions are disrupted that lead to the formation of aggregates following protein cleavage? Is there any information in the literature itself that specifically explains how these protein interactions are disrupted in specific cases of the Huntington’s Disease that could possibly elucidate some other aspects of the mechanistic basis of the disease?

    1. Thanks for the question. The disrupted protein interactions do not have much of an impact on the fragmentation and aggregation of mHtt. The Protein interactions are disrupted because PolyQ expansion can cause some slight protein structure alterations that decrease its functionality. Reduced functionality of mutant Htt has been shown in studies of BDNF. The reduction of BDNF can be attributed to reduced function of mHtt or wiltype-Htt that is lost due to interactions with aggregates. (Cattaneo, Zuccato, and Tartari 2005)

  3. Zach, great explanation of the metabolic underpinnings. You described the role of huntingtin as an epigenetic regulator in a disease-free context, and go on to describe how patients with Huntington’s disease experience epigenetic dysregulation. Are patients with Huntington’s disease at a greater risk for cancer because of the dysregulation of their epigenetics? I was also wondering if there were an antibody that binds a poly-Q region, as this could disrupt the formation of mHtt aggregates by sterically blocking their aggregation.

    1. Thanks and interesting question. Studies have found that people with Huntington’s are statistically less likely to have certain cancers. (Truner et al. 2013) While looking through treatment options, I didn’t find any evidence of an antibody that binds the poly Q-region, but i do discuss this as a potential for future work.

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