Advanced motor dysfunction in a mouse model of Huntington disease (HD) disappeared after infusions of a ganglioside vital to neuronal signaling, raising hopes that the first effective treatment for humans may be within reach.
Canadian researchers injected the ganglioside GM1 directly into the right ventricle of YAC128 mice, which contain a gene for human huntingtin protein with a 128 CAG repeat expansion — large enough to produce the dyskinesias, striatal loss, cognitive dysfunction, and other HD symptoms.
Infusions began when the mice were five months old and already showing severe motor impairment. After two weeks of GM1 treatment, the balance, walking ability and coordination of the HD mice returned to normal. Their improvement lasted for two weeks after infusions were stopped, the investigators reported in the Feb. 29 edition of the Proceedings of the National Academy of Sciences.
“This paper is the first demonstration in vivo that administering GM1 might be therapeutic for HD,” said Steven M. Hersch, MD, PhD, professor of neurology at Massachusetts General Hospital and Harvard Medical School, and part of the team that recently identified a biomarker that may help measure the effectiveness of potential treatments for HD. “The approach is particularly exciting because it appears to modulate huntingtin phosphorylation, an upstream target of the disease, which makes it potentially highly significant.” Dr. Hersch was not involved with the current study.
GM1, a membrane lipid crucial for neuronal signaling, declines in mouse models of HD. Levels have also been found to be depressed in skin cells taken from people with the disease. This caused the Canadian researchers to wonder what would happen if normal levels were restored.
“About two years ago we observed that GM1 was decreased in models of HD,” said lead study author Simonetta Sipione, PhD, assistant professor of pharmacology, a Canada Research Chair in Neurobiology of Huntington disease, and an Alberta Innovates Health Solutions Scholar at the University of Alberta in British Columbia. “We were not really looking at GM1. We were looking at signaling, and GM1 is an important component of membrane microdomains called lipid rafts, which can modulate signaling.”
But as they reported in 2010 in the Journal of Neuroscience, decreased GM1 levels appeared to contribute to making cells more susceptible to apoptosis, suggesting that a decline in GM1 might be one of the key events leading to the progression of HD.
“Even relatively small reductions of GM1 — 25 to 30 percent — contributed to their susceptibility to apoptosis,” said Dr. Sipione. “If synthesis of gangliosides is partially blocked in normal cells, these cells become as highly susceptible to apoptosis and stressful stimuli as HD cells. So this was already an indication that GM1 modulated the response of neurons to stress and apoptosis.”
Administering GM1 to cells in vitro promoted activation of pro-survival signaling pathways and phosphorylation of mutant huntingtin protein, which reduced toxicity and increased cell survival, suggesting that GM1 might be an effective treatment for HD.
But does GM1 also prevent the neurodegeneration that often accompanies HD?
Since YAC128 mice show no signs of brain atrophy until they're nearly a year old, Dr. Sipione and her colleagues are also administering GM1 to the R6/2 mouse model of HD, which starts to show signs of neurodegeneration in 10 to 12 weeks. They plan to perform cognitive tests on both strains in an effort to detect cognitive impairment.
“We also are going to look at the size of ventricles, which usually correlates with striatal atrophy, and at the number of neurons in both R6/2 and YAC128 mice,” Dr. Sipione said. “It will be more difficult in the YAC128 mice because, in spite of early neuronal dysfunction, these mice have been shown to have neurodegeneration — a decreased number of neurons in the striatum in particular — only much later during the course of the disease. But we're hoping that with the two models of the disease we'll get answers.”
Since GM1 has already been used in clinical trials for Parkinson disease, “it could, in theory, be relatively easy to bring GM1 to clinical trials (for HD),” according to Dr. Sipione, “but it might still take two or three years to get safety data in place.”
Administering GM1 to humans has been known to stimulate the production of anti-GM1 antibodies, which can lead to Guillain-Barré syndrome, but this is an extremely rare complication that should not offset the benefit of using GM1 in patients, according to Dr. Sipione. “Considering that this reaction to GM1 is relatively rare, the risk/benefit ratio could be very positive and could support use of this molecule as a therapy for Huntington disease,” she said.
The biggest challenge facing therapeutic use of GM1 is getting it past the blood-brain barrier. When injected subcutaneously, only a tiny amount would cross the blood-brain barrier, according to Dr. Sipione, which means a large dose of GM1 would have to be administered.
“However, in Parkinson disease and potentially in Huntington disease too, patients often have a modest impairment of the blood-brain barrier,” Dr. Sipione said. “This would allow more drug to get in. So something that is potentially negative could be exploited for therapeutic effect, and actually work to their advantage.”
Also, given the severity of HD, injecting GM1 directly in the human brain remains an option. “We are looking into this possibility,” Dr. Sipione said. “This is performed in other diseases. For example, intrathecal administration of baclofen is performed in MS for spasticity. So (infusion directly into the brain) is possible, although not optimal.”
NEUROPATHOLOGY DATA NEEDED
The researchers have not yet performed an analysis to verify that GM1 counteracts neurodegeneration and if so, to what extent, and this constitutes a limitation of the work, according to Dr. Hersch.
“For preclinical validation it's important to demonstrate that the treatment not only improves motor function, but is also neuroprotective,” he said. “Next steps should be to obtain quantitative neuropathology data and to try GM1 in additional HD models such as knock-in mice, which also would be more accurate.”
Overall, the paper has aroused great enthusiasm among HD researchers. Michael Hayden, MD, PhD, a leading investigator in HD, considers the work extremely promising.
“I am excited by the Sipione paper,” said Dr. Hayden, director and senior scientist at the Centre for Molecular Medicine and Therapeutics, and University Killam Professor in the department of medical genetics at the University of British Columbia. “I think this model has shown an excellent restoration of motor behavior, and also has provided us with potential mechanisms for improvement. I think what really excites me is, here's an intervention that restores phenotype and offers prospects for human therapy. That's very translational and very applied and very nice.”
ANOTHER MECHANISM, POSSIBLE TARGET FOR HD
Researchers at the National University of Ireland in Galway are investigating another potential way to curb the symptoms of Huntington disease. As they reported in the February issue of PLoS Biology, histone deacetylase complexes (HDACs) appear to promote the CAG expansions in people who carry the gene that causes HD.
HDACs are enzymes that remove acetyl groups on the histone, allowing transcription of DNA to take place. Since the symptoms of HD develop from a CAG repeat on the gene that transcribes huntingtin protein, inhibiting HDAC presumably would help to slow the progression of the disease symptoms.
However, since HDAC is crucial for all gene transcription, widespread inhibition produces significant side-effects.
“The first-generation HDAC inhibitors are not very specific,” said the lead study author Robert S. Lahue, PhD, professor of molecular genetics at the National University of Ireland. “They affect a pretty wide range of HDACs, so the doses needed for efficacy produced toxic effects that overcame the beneficial aspects.”
However, the HDAC inhibitor 4b, described by Joel M. Gottesfeld, PhD, professor of molecular biology at Scripps Research Institute in La Jolla, CA, and colleagues in a 2008 article in the Proceedings of the National Academy of Sciences, is highly specific for HDAC3, with some activity against HDAC1. Oral administration of HDACi 4b after the onset of motor symptoms of HD significantly improved motor performance, overall appearance, and body weight of R6/2 transgenic mice, which carry the human gene that produces Huntington disease. The inhibitor also reduced the rate of striatal atrophy and overall decline in brain size.
Dr. Gottesfeld supplied some HDAC inhibitor 4b to Dr. Lahue and his colleagues, who found that it suppressed CAG repeats in HD mice in a dose-dependent manner, resulting in up to 77 percent fewer expansions. HDACi 4b is licensed to Repligen for testing against Friedreich's ataxia, and if it can be tolerated by humans without unacceptable side effects, it could presumably become a treatment for HD as well.
“The serendipity of our discovery is that HDAC3 is the key HDAC in our system for expansions,” said Dr. Lahue.