Submitted by Marsha Miller Ph.D. on Mon, 12/08/2008
Researchers in the Netherlands found that Nrf2 responsive genes are activated in a cell model of Huntington’s Disease. This new study adds to our knowledge of how gene transcription is altered in Huntington’s Disease.
One critical area in which basic research is continuing is gene transcription. As identified by Dr. Jang-Ho Cha, the dysregulation of gene transcription is a major pathology in Huntington’s Disease. DNA is the blueprint for life, a set of instructions for making an organism. The DNA blueprint has to be ‘read’ (gene transcription) and then transcribed (proteins made). The presence of the HD protein causes some genes to be abnormally up-regulated (more of the protein is expressed than is normal) and others to be down-regulated.
Studies of gene expression have been done in mouse models and human brain tissue. The value of the cell model however, is that it’s possible to look at the earliest changes as well as changes over time. The researchers used an inducible PC-12 cell model derived from rats with exon 1 of the HD gene (the sequence where the polyglutamine expansion occurs). In this model, the HD gene is activated by the administration of doxycycline.
Early on, before the aggregates that are a hallmark of HD appeared in the cell, most of the genes with changed levels of expression were up-regulated. Over time, more changes in gene expression occurred and after the aggregates appeared, as much down-regulation occurred as up-regulation.
The researchers found that Nrf2 responsive genes were up-regulated. Nrf2 is a protein, which, if added to the cell, stimulates sets of genes that boost the cell’s neuroprotective responses to both oxidative stress and the presence of toxins. These genes were activated, leading the authors to hypothesize that this is a compensatory measure given that oxidative stress is known to be a problem in the disease.
This reminds us that the changes that are found in the Huntington’s Disease brain through basic research need to be evaluated for their role in the disease. Some changes may be compensatory, some may be pathological, and some may have no effect either way. The activation of the Nrf2 responsive genes may be compensatory response rather than a consequence of the pathological dysregulation known to occur.
Other changes in gene expression were found which were dysfunctional, however. They found changes that are likely to increase oxidative stress. In addition, genes that control the biosynthesis of dopamine were down-regulated.
HDAC inhibitors that partially ameliorate gene transcription problems are already in the pipeline, but the more we learn about gene transcription in HD, the more likely we are to discover or develop a highly effective drug with minimal side effects.
The authors suggest that Nrf2-Antioxidant Response Element pathway might be a good target for the development of a treatment.
Huntington's disease is a progressive autosomal dominant neurodegenerative disorder that is caused by a CAG repeat expansion in the HD or Huntington's disease gene. Although micro array studies on patient and animal tissue provide valuable information, the primary effect of mutant huntingtin will inevitably be masked by secondary processes in advanced stages of the disease. Thus, cell models are instrumental to study early, direct effects of mutant huntingtin. mRNA changes were studied in an inducible PC12 model of Huntington's disease before and after aggregates became visible to identify groups of genes that could play a role in the early pathology of Huntington's disease.
Results
Before aggregation, up-regulation of gene expression predominated, while after aggregates became visible down-regulation and up-regulation occurred to the same extent. After aggregates became visible there was a down-regulation of dopamine biosynthesis genes accompanied by down-regulation of dopamine levels in culture, indicating the utility of this model to identify functionally relevant pathways. Furthermore, genes of the anti-oxidant Nrf2-ARE pathway were up-regulated, possibly as a protective mechanism. In parallel, we discovered alterations in genes which may result in increased oxidative stress and damage.
Conclusions
Up-regulation of gene expression may be more important in HD pathology than previously appreciated. In addition, given the pathogenic impact of oxidative stress and neuroinflammation, the Nrf2-ARE signaling pathway constitutes a new attractive therapeutic target for HD.