Huntington's disease (HD) is a neurodegenerative disorder that follows an autosomal-dominant inheritance pattern. The pathogenesis of the disease depends on the degree of expansion of triplet (CAG) repeats located in the first exon on the gene. An expanded polyglutamine tract within the protein huntingtin (Htt) enables a gain-of-function phenotype that is often exhibited by a dysfunctional oligomerization process and the formation of protein aggregates. How this process leads to neurodegeneration remains undefined. We report that expression of a Htt-fragment containing an expanded glutamine tract induces DNA damage and activates the DNA damage response pathway. Both single-strand and double-strand breaks are observed as the mutant protein accumulates in the cell; these breaks precede the appearance of detectable protein aggregates containing mutant Htt. We also observe activation of H2AX, ATM, and p53 in cells expressing mutant Htt, a predictable response in cells containing chromosomal breakage. Expression of wild-type Htt does not affect the integrity of DNA, nor does it activate the same pathway. Furthermore, DNA damage and activated H2AX are present in HD transgenic mice before the formation of mutant Htt aggregates and HD pathogenesis. Taken together, our data suggest that the expression of mutant Htt causes an accumulation of DNA breaks that activates the DNA damage response pathway, a process that can disable cell function. Because these events can lead to apoptosis, it is possible that the DNA damage response pathway activated by single- and double-strand breaks that we found contributes to neurodegeneration.

Background: Huntington's Disease (HD) is an inherited autosomal dominant genetic disorder in which neuronal tissue degenerates. The pathogenesis of the disease appears to center on the development of protein aggregates that arise initially from the misfolding of the mutant HD protein.Mutant huntingtin (Htt) is produced by HD genes that contain an increased number of glutamine codons within the first exon and this expansion leads to the production of a protein that misfolds. Recent studies suggest that mutant Htt can nucleate protein aggregation and interfere with a multitude of normal cellular functions.

Huntington's disease (HD) is a progressive neurodegenerative disorder that is inherited in an autosomal dominant fashion. The disease is the result of an expanded CAG repeat in exon 1 of the HD gene, which encodes an elongated polyglutamine tract in the mutant form of the protein, huntingtin. Disease pathogenesis is linked to intracellular aggregates that form because of the tendency of the mutant protein to misfold. The role of huntingtin aggregates in disease pathology is unclear; it has been proposed that the aggregates themselves are toxic because of their ability to sequester intracellular proteins and disrupt normal cellular function. In addition, the mechanistic steps that lead to aggregate formation appear to be central to HD pathology. We have previously reported that guanosine-rich oligonucleotides with the ability to fold into a G-quartet are effective inhibitors of the aggregation process of a huntingtin protein fragment with an elongated polyglutamine tract, Htn 1-171(Q58). The most active molecule is composed of 20 guanosine residues, which adopt a G-wire conformation. Here we establish that G20 inhibits protein aggregation as judged by native gel electrophoresis, an agarose gel electrophoresis for resolving aggregates (AGERA) assay, and an immunoblotting assay. We also visualize the G20-Htn1-171(Q58) protein complex by using a streptavidin-biotin pull-down assay as well as atomic force microscopy (AFM). The G20 molecule also interacts with Htn1-171(Q23), a fusion protein that contains 23 glutamine residues instead of 58 (Q58), but in a more degenerate and nonspecific fashion. Taken together, our data support the notion that G20 exhibits some selectivity in binding to specific protein species that assemble along the aggregation pathway.