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Huntington's Researchers Get a New Mouse
Mice have symptoms of neurological disease; Huntington gene has 150 DNA repeats
  
By
Edward R. Winstead



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The mice begin to stagger some at about age one. Their gait progressively worsens, and abnormal clumps of protein form in the brain. The animals appear to have symptoms of Huntington's disease, and for good reason. They were born with a long stretch of repetitive DNA that expands the mouse Huntington gene, the same mutation that underlies the human disease. The hope among researchers is that these mice become ill for the same reason—and in the same way—as humans with the disease.


(A) Footprints of a normal mouse at age one. Grey represents hind paws; black represents front paws. (B) Footprints of a mutant with staggering gait at age one.

"We don't understand the molecular mechanism in Huntington's disease," says Peter J. Detloff, of the University of Alabama, in Birmingham, who led a five-year project to develop the new Huntington's mouse. His laboratory will use the mice to investigate how extra DNA and protein clumps lead to neurological problems and death. "The Huntington protein is essential for life, but we do not have a clear view of how the disease initiates and progresses," says Detloff.


Making a mutant mouse walk normally again is not so simple

Huntington's seems to be a single-gene disorder. Virtually everyone with the disease has the chemical letters 'CAG' repeated over and over in the Huntington gene. Affected individuals tend to have more than 40 repeats, with an upper limit of about 180 repeats (a few humans may have 200 repeats). Longer repeats are associated with an earlier onset of the disease. Huntington's can occur in childhood, but symptoms normally appear after middle age.

Detloff gave his animals 150 CAG repeats. "Our strategy was to develop a 'knock-in' mouse with a very long repeat," he says. "The longest we could get was 150." The researchers inserted the repeats directly into the mouse Huntington gene, making a precise replica of the human disease gene.

Mutant mice are most useful if they grow ill, and these do. The uncontrolled movements of Huntington's patients are seen in the adult mice. Neurologists monitor the progression of Huntington's by having patients walk heel-to-toe, and a similar test works for rodents. Detloff's team painted the paws of mice and let them walk down a 'runway' of white paper. Mice with the disease leave erratic trails of tiny paw prints.

"They do walk funny, but that doesn't mean they have mouse Huntington's," says Detloff, who admits that researchers are biased when it comes to diagnosing a human disease in an animal model they create. To gather more evidence of Huntington's-like symptoms, the researchers analyzed mouse brain tissue. They detected protein build-ups in the striatum region of the brain, a characteristic feature of CAG-repeat diseases. Furthermore, some mice developed seizures, which are associated with juvenile-onset Huntington's disease. The findings appeared last month in Human Molecular Genetics.


The mouse on the left is a chimeric mouse with 150 CAG repeats in the mouse Huntington gene. This chimera sired the mouse to the right, which also carries the mutation.

Detloff has shipped breeding pairs of the mice to Huntington's researchers around the world. One pair went to the laboratory of Gillian Bates, Professor of Neurogenetics at King's College London. In 1996, Bates created what became the most widely used mouse model in the field. She and colleagues discovered protein clumps, or inclusions, in the brains of these mice; the researchers subsequently used sophisticated imaging technology to locate inclusions in the brains of Huntington's patients.

A textbook published in 1979 has a plate showing inclusions in the human brain, but no one had reported this feature of Huntington's prior to the finding in mice. "We don't understand the relationship between these structures and disease," says Bates. Experiments in test tubes have shown, however, that clumps of the Huntington's disease protein form at a faster rate when the size of the repeat reaches a certain length. (Only a few extra repeats can be the difference between health and disease.)

The Bates mouse was constructed differently than Detloff's, and it gets a more severe disease at a younger age. While Detloff's team altered a mouse gene, Bates added a truncated human gene, creating a 'transgenic' mouse. Each strain has about the same length mutation, but Bates put the repeats in a shortened human Huntington gene that incorporates randomly in the mouse genome.

There are several Huntington's mouse models like Detloff's, in which the mutation is inserted in the correct gene. But those mice have shorter length repeats (about 110), and they appear to be free of obvious Huntington's symptoms. Some simply become more aggressive around other mice.


Image of inclusions in the striatum of a one-year-old mutant with 150 CAG repeats. The dark-immunostained spots are nuclear inclusions.

"Peter's mouse is very exciting," says Bates. "This is the first model in which researchers put the repeat into the mouse gene and have seen disease-related traits. This happened because he put in longer repeats."

Bates requested a pair of the mice after hearing about Detloff's project at a scientific meeting. "I think it's the best full-length model out there," she says. Her team will continue to use the transgenic mice in research, but they can now do follow-up experiments using Detloff's mice. The perfect Huntington's mouse, in her view, would be a knock-in like Detloff's that progresses to disease more quickly. "The pace of experiments could be greatly accelerated if symptoms occurred just a few months earlier in life," she says.

If a Huntington's mouse becomes ill, then researchers can attempt to cure its disease. And in some ways, the more complicated the biological problem the better. An aggressive mouse, for example, can be treated with sedative medications. But making a mouse walk normally again is not so simple. "To have a drug that fixes a movement disorder means you're probably fixing the neurons that carry out the complex motor behavior," says Detloff.

This was the second time Detloff put a long string of repeats in an experimental mouse. In 1997, he led a team that inserted 146 CAG repeats into a mouse gene not involved in Huntington's disease. Although the mutation occurred out of its usual context, it had severe consequences for the animals. The mice behaved like humans with a CAG-repeat disorder, and they died prematurely. The mice in the current study did better by comparison, indicating that the Huntington gene may actually protect against more severe disease.

Detloff's study was born unexpectedly at a human genetics meeting in the mid-nineties. Then a new assistant professor, Detloff had a brief conversation about CAG-repeat mouse models with a colleague. On his own, the colleague related the conversation to a friend at the Hereditary Disease Foundation (HDF), which supports research on CAG-repeat diseases.

A month later, Detloff received a phone call inviting him to a HDF workshop on Huntington's disease. At the workshop he met Nancy S. Wexler, the foundation's president. "She pulled me aside and said I should apply for one of the HDF grants to make the Huntington's mice," Detloff recalls. "For a young scientist scrapping for money, it was very encouraging to hear directly from the foundation that your efforts would be supported."

"The workshops and meetings supported by the HDF continue to be helpful," he says. When his project began, Detloff agreed to make the mice available to other researchers, and the HDF has paid for their distribution. "My view is that these mice belong to the Huntington's community," he says.

See related GNN article
»A Theme among Brain Diseases

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Lin, C.-H. et al. Neurological abnormalities in a knock-in mouse model of Huntington's disease. Hum Mol Genet 10, 137-144 (January 15, 2001).
 
Ordway, J.M. et al. Ectopically expressed CAG repeats cause intranuclear inclusions and a progressive late onset neurological phenotype in the mouse. Cell 91, 753-763 (December 12, 1997).
 
Mangiarini, L. et al. Exon 1 of the HD gene with an expanded CAG repeat is sufficient to cause a progressive neurological phenotype in transgenic mice. Cell 87, 493-506 (November 1, 1996).
 

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