As basic research into Huntington's Disease continues, more and more is being learned about what goes wrong in the cell and the brain. The challenge is to understand which pathologies are both significant and targetable for treatment.
Researchers at the University of Southern California have discovered that the RCAN1-1l protein is dramatically reduced - by 70 percent - in the brains of HD patients after death as compared to controls who died of other causes.
This is not a surprising finding. It is known that the HD protein causes the dysregulation of gene transcription with the result that some proteins are downregulated while others are upregulated. However, the researchers feel that the downregulation of this particular gene may play a major role in HD pathology.
In a cell model of HD, they used a viral vector to add the RCAN1-1l gene. The cells were rescued from toxicity by this overexpression of the gene, leading to the hope that further research might support the idea that RCAN1-1l overexpression could be a treatment.
The researchers suggested that the mechanism by which upregulating RCAN1-1l exerts its positive effect is through inhibiting calcineuron. The gene is known to down regulate calcineurin, an enzyme which modifies proteins by removing phosphates. Earlier research has shown that phosphorylating the HD protein at a particular serine is neuroprotective. The decrease of RCAN1-1l may allow calcineurin to operate unchecked and make the HD protein more toxic.
However, other researchers found that inhibiting calcineurin actually accelerated the disease in an R6/2 mouse. What accounts for these different findings? It may be that what works in a cell model does not work in a living organism or the findings might be reconcilable in that either too much or too little calcineurin is damaging. Clearly more work needs to be done but the current findings are intriguing and need to be explored further in the HD mice.
David Hernandez-Espinosa and A. Jennifer Morton. "Calcineurin inhibitors cause an acceleration of the neurological phenotype ina mouse transgenic for the human Huntington's disease mutation." Brain Research Bulletin 2006 May 31;69(6):669-79.
University of California at Davis press release
-- Marsha L. Miller, Ph.D.
Kelvin J. A. Davies, Ph.D., D.Sc.
James E. Birren Professor of Gerontology,
and Professor of Molecular & Computational Biology.
Regulator of calcineurin (RCAN1-1L) is deficient in Huntington disease and protective against mutant huntingtin toxicity in vitro
Gennady Ermak, Karl J. Hench, Kevin T. Chang, Sean Sachdev, and Kelvin J. A. Davies
New hope for treatment of neurodegenerative disorder:
USC researchers uncover clues about cause of Huntington's disease
Los Angeles – Researchers from the University of Southern California have taken an important first step toward protecting against Huntington disease using gene therapy.
Huntington Disease is an incurable neurological disorder characterized by uncontrolled movements, emotional instability and loss of intellectual faculties. It affects about 30,000 people in the United States, and children of parents with the disease have a 50 percent chance of inheriting it themselves.
"Our findings allow for the possibility that controlled over-expression of RCAN1-1L might in the future be a viable avenue for therapeutic intervention in Huntington disease patients," said Kelvin J. A. Davies, professor of gerontology in the USC Davis School of Gerontology and professor of biological sciences in the USC College of Letters, Arts and Sciences.
In a paper in the June 2009 issue of Journal of Biological Chemistry, now available online, Davies and his coauthors use cell culture findings to show that a form of the gene RCAN1, known as RCAN1-1L, is dramatically decreased in human brains affected by Huntington disease. RCAN1-1L was first discovered in Davies' lab.
The investigators also show that increasing levels of RCAN1-1L rescues cells from the toxic effects of Huntington disease, a result that could someday lead to new avenues of treatment, according to Davies.
"Our discovery offers real hope and may even have wide-ranging implications for a variety of other important CAG repeat-related diseases," Davies said.
While the Huntington gene, which makes the normal Huntington protein, is an essential component to healthy nerve cells, the mutant Huntington gene makes a toxic mutant Huntington protein. Mutant Huntington contains increased levels of the amino acid glutamine, which is generated by a repetition of the DNA triplet CAG.
A normal Huntington gene has a sequence of between six and 34 CAG repeats. Any strand of DNA possessing more than 40 CAG repeats indicates the carrier will develop Huntington disease, according to the researchers.
Indeed, the more repeats of CAG, the earlier the disease manifests itself and the more devastating the disease becomes. Currently available drugs do little more than help control erratic movements associated with the condition.
"It is important to keep in mind that these protective findings are in-vitro, meaning in cell cultures. Further proof of protection by RCAN1-1L will be required in-vivo, or in actual Huntington disease patients," said lead author Gennady Ermak, research associate professor at the USC Davis School of Gerontology.
Previous in-vitro research has revealed that adding the phosphate PO4, an inorganic chemical, to the mutant Huntington protein can protect against the mutant gene. This process is called phosphorylation, and can be achieved by either inhibiting an enzyme (calcineurin) or by activating an enzyme (Akt).
"Our findings point to increased phosphorylation of mutant Huntington through calcineurin inhibition as the likely mechanism by which RCAN1-1L may be protective against the mutant Huntington," Ermak said.
As Davies explained: "RCAN1-1L may actually play a role in the cause of Huntington disease."
"The gene is required to down-regulate the activity of calcineurin. We have previously linked too much RCAN1-1L expression to Alzheimer's disease," Davies said. "Thus, Alzheimer's disease and Huntington disease appear to involve opposite problems with RCAN1 expression and calcineurin activity."
In cases of Huntington disease, too little RCAN1-1L may allow calcineurin to act unopposed and remove too many phosphates from the mutant Huntington protein.
"We observed complete protection against the mutant Huntington by RCAN1-1L," Ermak said, but he reiterated the need for further research with Huntington disease patients.
The results offer a new direction for further research, Davies added.
Our work suggests an important new link between the RCAN1 gene and Huntington disease. Huntington disease is caused by expansion of glutamine repeats in the huntingtin protein. How the huntingtin protein with expanded polyglutamines (mutant huntingtin) causes the disease is still unclear, but phosphorylation of huntingtin appears to be protective. Increased huntingtin phosphorylation can be produced either by inhibition of the phosphatase calcineurin or by activation of the Akt kinase. The RCAN1 gene encodes regulators of calcineurin, and we now demonstrate, for the first time, that RCAN1-1L is depressed in Huntington disease. We also show that RCAN1-1L overexpression can protect against mutant huntingtin toxicity in an ST14A cell culture model of Huntington disease and that increased phosphorylation of huntingtin via calcineurin inhibition, rather than via Akt induction or activation, is the likely mechanism by which RCAN1-1L may be protective against mutant huntingtin. These findings suggest that RCAN1-1L "deficiency" may actually play a role in the etiology of Huntington disease. In addition, our results allow for the possibility that controlled overexpression of RCAN1-1L in the striatal region of the brain might be a viable avenue for therapeutic intervention in Huntington disease patients (and perhaps other polyglutamine expansion disorders).
Source: Journal of Biological Chemistry 2009 Mar 6. [Epub ahead of print]