Caspase 6 Resistant HD Mice Do Not Develop Huntington's Disease

HD Lighthouse Contributing Editor's Comment: This landmark study by long time HD researcher Michael Hayden and colleagues points us in a new direction for the development of a treatment.

The study was done with the YAC128 mouse. This mouse model more closely resembles Huntington's Disease than the R6/2 mouse which gets sick very early. The YAC128 mouse model develops the disease later with cognitive dysfunction preceding movement disorder followed by neurodegeneration in the striatum.

In the last few years, researchers have learned a great deal about what goes wrong in HD -- defects in energy metabolism, dysregulation of gene transcription, impaired protein folding and clearance, etc -- but the key to developing effective treatments is to discover which are the most important pathologies and target them for drug development. This research supports the theory that the production of toxic fragments of the HD protein by caspase cleavage is a key pathology and identifies the inhibition of caspase 6 as a good target for drug development.

The toxic fragments theory of HD is that a key event in the development of the disease is the cleavage of the HD protein into fragments by caspases. The huntingtin's protein is a cytoplasm protein. However, the fragments enter the nucleus of the cell where they cause dysfunction in multiple ways.

Further clues about the role of fragmentation were provided by the development of the shortstop mouse in work by Hayden and colleagues published in 2005. (Slow et al 2005). This mouse expresses an N-terminal HD protein fragments. Fragments did enter the nucleus of the cell and did form aggregates and yet there was no evidence of disease in the mouse's development or behavior and no neurodegeneration occurred. Therefore, the problem could not be fragmentation itself but would have to be the generation of specific toxic fragments.

The word caspase comes from cysteine-aspartic-acid-proteases. Caspases are enzymes which are used in apoptosis, programmed cell death. There are various caspases which initiate the process, which cleave proteins and which actually 'execute' the cell. Apoptosis is a necessary process in development and also in destroying tumors. Unfortunately apoptosis is also implicated in neurodegenerative disorders. Apoptosis is also triggered by cellular stress, especially mitochondrial stress, and this is known to occur with Huntington's and the various other neurodegenerative disorders. Caspase 3 and caspase 6 cleave the HD protein.

The current experiment targeted two sites where fragmentation might occur, the site or part of the HD protein which cleaves in response to caspase 3 and the part which cleaves in response to caspase 6. The researchers engineered two types of HD mice. They found that caspase 3 resistant mice developed the disease but the caspace 6 resistant mice did not. The caspase 6 resistant mice did not appear to have cognitive defects or behave abnormally and neurodegeneration was absent. They did experience aggregation later than usual but the aggregation was not associated with pathology, just as in the shortstop mouse. The study therefore supports the idea that it is specific toxic fragments which are a major pathology in Huntington's disease, those generated by caspase 6 cleavage and not those generated by other caspases or processes.

Another positive finding was that the caspase 6 resistant HD mice were protected from excitotoxic stress and neurotoxins.

This is an exciting study because it identifies a target which can be addressed through drug development. It is important to note, however, that the mice were engineered to be resistant to caspase 6. They were not 'cured' because they never developed the disease, it was prevented. Since they were not treated with a drug (which would be our goal for HD patients), a next step will be to test a caspase 6 inhibitor in the mice.

More good news is that CHDI, the nonprofit drug development organization for Huntington's Disease, has been proactive about a potential treatment. Efforts are already underway to develop a safe, effective caspase 6 inhibitor. According to Dr. Robert Pacifici (Chief Scientific Officer at CHDI) they hope to have a caspase 6 inhibitor available for testing in mice by the end of the year. These compounds would need further development to make sure that they crossed the blood brain barrier and were safe for long term use in people, but if the concept works in animals, even if there is toxicity, more work can be done.

”We would like to congratulate Dr. Hayden and his colleagues on the publication of these very intriguing results.”, Dr. Pacifici said on learning of the Cell paper’s release. “CHDI is leveraging its considerable drug discovery and development resources to identify suitable inhibitors for caspace 6. We look forward to using these molecules to collaboratively test whether we can prevent or slow the progression of HD by intervening at this target and stopping the production of toxic fragments.”

To learn more about mouse models, read the Hereditary Disease Workshop report, "Behavioral Assessment of Mouse Models of Huntington’s Disease." Cardiff, Wales, July 20–21, 2002. [HDF Workshop Report]

To learn about the shortstop mouse, read this article: E.J. Slow, et al "Absence of behavioral abnormalities and neurodegeneration in vivo despite widespread neuronal huntingtin inclusions." Proceedings of the National Academy of Sciences of the United States of America. 005 Aug 9;102(32):11402-7. [National Academy Report]

To learn more about the toxic fragment theory of HD pathology, read the Hereditary Disease Workshop report, "The Role of Proteolysis in the Pathogenesis of Huntington's Disease." Novato, California, December 4-5, 2001. [HDF Workshop Report]

-- Marsha L. Miller, Ph.D.
Posted to the HDL: 16 Jun 2006

The Press Release

VANCOUVER, B.C. – June 16, 2006: Researchers at the Child and Family Research Institute’s Centre for Molecular Medicine and Therapeutics (CMMT) have provided ground-breaking evidence for a cure for Huntington disease in a mouse offering hope that this disease can be relieved in humans.

Published today in Cell journal, Dr. Michael Hayden and colleagues discovered that by preventing the cleavage of the mutant huntingtin protein responsible for Huntington disease (HD) in a mouse model, the degenerative symptoms underlying the illness do not appear and the mouse displays normal brain function. This is the first time that a cure for HD in mice has been successfully achieved.

“Ten years ago, we discovered that huntingtin is cleaved by ‘molecular scissors’ which led to the hypothesis that cleavage of huntingtin may play a key role in causing Huntington disease”, said Dr. Michael Hayden, Director and Senior Scientist at the University of British Columbia’s Centre for Molecular Medicine and Therapeutics.

Now a decade later, this hypothesis has resulted in a landmark discovery. “This is a monumental effort that provides the most compelling evidence of this hypothesis to date”, said Dr. Marian DiFiglia, Professor in Neurology, Massachusetts General Hospital, Harvard Medical School and one of the world’s leading experts on Huntington disease. “Dr. Hayden and his team have shown in convincing fashion that many of the changes seen in HD patients can be erased in HD mice simply by engineering a mutation into the disease gene that prevents the protein from getting cleaved at a specific site”.

To explore the role of cleavage, Dr. Hayden’s team established an animal model of HD that replicated the key disease features seen in patients. A unique aspect of this particular animal model is that it embodied the human HD gene in exactly the same way seen in patients. This replication allowed researchers to examine the progression of HD symptoms including the inevitable cleavage of the mutant huntingtin protein. In the study, researchers confirmed that the deadly cleavage is caused by a key enzyme called caspase-6. By blocking the action of this target, they showed that the mouse did not develop any symptoms of Huntington disease.

Hayden's team is now trying to test this model of prevention in a mouse using drug inhibitors and then ultimately in humans. “Our findings are important because they tell us exactly what we need to do next”, said Dr. Rona Graham, Post Doctoral Fellow at the CMMT and lead author in the study.

This work is also pivotal for the individuals and families affected by Huntington disease. “Patients of this disease should know that this is a research milestone for all and that this work brings the field closer to finding effective treatment for a devastating disorder”, said Dr. DiFiglia.

The Huntington Society of Canada (HSC), a national network of volunteers and professionals united in the fight against HD, echoed this sentiment. “This ground-breaking research provides great hope for the Huntington community”, said Don Lamont, the Society’s CEO and Executive Director. “This research brings us closer to treatment and ultimately a cure”.

Huntington disease is a degenerative brain disease that affects one in every 10,000 Canadians. One in 1,000 is touched by HD — for example, as a person with HD, a family member, a person at risk, caregiver or friend. The disease results from degeneration of neurons in certain areas of the brain causing uncontrolled movements, loss of intellectual faculties, and emotional disturbances. Currently, there is no treatment to delay or prevent HD in patients.

This research was funded by Canadian Institutes of Health Research, Hereditary Disease Foundation, Huntington Disease Society of America, Michael Smith Foundation for Health Research, High Q Foundation, Merck Frosst, Child and Family Research Institute of BC.

The Abstract

Cleavage of huntingtin (htt) has been characterized in vitro, and accumulation of caspase cleavage fragments represents an early pathological change in brains of Huntington's disease (HD) patients. However, the relationship between htt proteolysis and the pathogenesis of HD is unknown. To determine whether caspase cleavage of htt is a key event in the neuronal dysfunction and selective neurodegeneration in HD, we generated YAC mice expressing caspase-3- and caspase-6-resistant mutant htt. Mice expressing mutant htt, resistant to cleavage by caspase-6 but not caspase-3, maintain normal neuronal function and do not develop striatal neurodegeneration. Furthermore, caspase-6-resistant mutant htt mice are protected against neurotoxicity induced by multiple stressors including NMDA, quinolinic acid (QA), and staurosporine. These results are consistent with proteolysis of htt at the caspase-6 cleavage site being an important event in mediating neuronal dysfunction and neurodegeneration and highlight the significant role of htt proteolysis and excitotoxicity in HD.

Source: Cell, Vol 125, 1179-1191, 15 June 2006

printer friendly version

	Read the HDAC/HDLighthouse Forum. Post your comments