That has now changed, as Joonil Jung and Nancy Bonini of the University of Pennsylvania in Philadelphia report the successful induction of human-like repeat instability in a fruit fly model of polyQ disease. In the 1 March Science, they demonstrate that transcription of genes in germ line cells is behind the changes in repeat length. Using flies to probe the mechanism of the changes (which consisted mostly of expansions), they find that inhibiting DNA repair systems and, in particular, the CREB-binding protein (CBP), increases repeat instability. The results lead to the surprising conclusion that some toxic polyQ proteins, including huntingtin, might themselves promote repeat expansion through their well-established ability to sequester CBP and inhibit its activity.

This apparently circular logic of expansion suggests that treatments aimed at reducing polyQ protein toxicity could have the fringe benefit of preventing further expansion, as well. That would help affected families, who often have to watch their children and grandchildren develop symptoms at successively earlier ages. Besides the inherited expansion, this change can also occur during a person’s lifetime in somatic brain cells, possibly through similar mechanisms (Kennedy et al., 2003). One way to reduce polyQ toxicity would be to promote degradation of the offending proteins, and the second new paper, from Ole Isacson and colleagues at the McLean Hospital in Belmont, Massachusetts, shows that boosting the activity of the ubiquitin-dependent proteasome may offer a way to do that.

But first, the flies. It was in them that other scientists had previously demonstrated the link between CBP and the toxicity of mutant huntingtin protein (see ARF related news story). They showed that huntingtin inhibits CBP’s histone acetyltransferase activity, possibly explaining why the protein causes derangements in gene expression. Later work established that lowering CBP enhanced polyQ toxicity (Taylor et al., 2003). These studies have led to a promising therapeutic approach: Histone deacetylase inhibitors (which boost overall levels of acetylation) proved to suppress toxicity of polyQ proteins in fly or mouse models and are now in clinical development for HD (see review by Butler and Bates, 2006).

Jung and Bonini’s present work demonstrates that repeat expansion is part of the same story. In their work, flies with lower CBP levels showed decreased repeat stability of a polyQ-expanded ataxin transgene, whereas CBP overexpression stabilized the repeats. And treating the flies with the deacetylase inhibitor trichostatin A shut off the instability while restoring histone acetylation levels. For their mechanistic studies, Jung and Bonini used their fly model for the human disease spinocerebellar ataxia type 3, which expresses a mutated ataxin protein. However, the results are likely applicable to other polyQ diseases. The authors note that most triplet repeat disease genes are expressed in germ cells, and many polyQ proteins interact with CBP or other histone acetyl transferases. For example, flies with polyQ-expanded huntingtin also showed increased instability that was linked to germ line transcription or reduced CBP levels. Likewise, in a fragile X model, where the expansion occurs in noncoding regions, germ line transcription enhanced instability.

“Trinucleotide repeat instability has been viewed largely as a matter of DNA metabolism; however, our data suggest that repeat instability may be influenced by aspects of polyQ protein toxicity. Thus, treatments to curb polyQ protein pathology may also be effective means to help clamp repeated instability,” the authors conclude.

Enter the proteasome work from Isacson’s lab. It follows a previous study showing that the activity of the ubiquitin-dependent proteasome is reduced in cells from HD patients (Seo et al., 2004). In the February 28 PLoS ONE, first author Hyemyung Seo demonstrates that overexpressing a proteasome activator subunit (PA28γ) in fibroblasts from HD patients reverses compromised proteasome function. To test the strategy in a cell type affected by HD, they moved to rat striatal neurons expressing mutant huntingtin. The same proteasome activator protected those cells from neurotoxic stressors, including quinolinic acid or a proteasome inhibitor. “Although proteasome dysfunction is probably only one of multiple factors involved in the dynamic and progressive disease process of HD, our data at least demonstrate that proteasome actuators are relevant candidates for future comprehensive and effective treatment approaches to HD,” the authors write.—Pat McCaffrey.

References:
Jung J, Bonini N. CREB-binding protein modulates repeat instability in a Drosophila model for PolyQ disease. Science Express. 2007 March 1. Abstract

Seo H, Sonntag KC, Kim W, Cattaneo E, Isacson O. Proteasome Activator Enhances Survival of Huntington's Disease Neuronal Model Cells. PLoS ONE. 2007 Feb 28;2:e238. Abstract