People coping with Huntington's disease often suffer from instability in every part of their lives: emotional imbalances including violent and angry outbursts; progressive jerky or uncontrollable movements that will eventually render them unable to bathe, clothe, and feed themselves; and precarious concerns about how the disease will affect relationships with family and friends.

At the heart of these symptoms lies another instability, a genetic one, as suspected by medical scientists and recently confirmed by a Mayo Clinic research team led by Cynthia McMurray, Ph.D., with fellow researchers Eugenia Trushina, Ph.D., and Irina Kovtun, Ph.D.

Quick-change Artist

Huntington's disease is part of a group of disorders that arise from a specific genetic mutation called guanine expansion. Expansion is the insertion of extra, repetitive nucleic acid building blocks – represented by the familiar letters -- in the DNA of specific genes. Huntington's disease arises from a mutated version of the huntingtin gene that carries an extra DNA segment that repeats the nucleotides cytosine (C), adenine (A) and guanine (G) many times over. DNA expansion is normal to a certain extent; healthy people may have anywhere from 6 and 20 repeats in a gene, but a person with Huntington's has 40 to 80 repeats or more.

Everyone who carries the mutated huntingtin gene will eventually develop the disease. If one of your parents carries the mutation, you have a 50 percent chance of carrying it as well. What's more, Huntington's disease generally occurs later in life, but the number of CAG repeats vary among affected people. The greater the number of repeats, researchers have found, the earlier in life symptoms appear.

"The human body is very good at suppressing genetic mutations," says Dr. McMurray. Evidence of this is seen in the fact that most people don't get cancer and, in general, live fairly long lives. "So the question stands: How does this specific genetic mutation constantly escape repair and change as it's passed to subsequent generations?"

Connecting the Dots

The human genome is under constant assault from oxidative stress, which is linked to a number of degenerative diseases including cancer, cardiovascular disease, and brain dysfunction. Oxidants are byproducts of normal metabolism, and they cause damage to DNA in the form of oxidative lesions. In healthy cells, DNA repair enzymes constantly remove and dispose of oxidative lesions. However, the number of lesions can be overwhelming, and the repair process can break down.

In patients with Huntington's disease, when the single strand of CAG repeats is damaged by oxidative lesions and repaired by enzymes, the mutant huntingtin protein continues to add even more extra replacement segments. When the cell later duplicates the DNA so it can divide, it also duplicates these unusual loops — leading to even more CAG repeats.

Through the cell division process, the uncorrected loops of DNA actually become incorporated into existing DNA — forming areas of expansion. As the process repeats, the expansion is repeated many times over: CAGCAGCAG… and so on, becoming ever more severe as subsequent rounds of DNA expansion occur. The expansion eventually becomes toxic.

Although scientists long suspected that oxidative lesions played a role in expansion of the extra DNA segment, nobody has connected the dots before now, according to Dr. McMurray.

To prove that the DNA segment enlarges with age — consistent with late-onset Huntington's disease — Mayo researchers engineered mice to carry a version of the human huntingtin gene with an inserted segment — one large enough to cause Huntington's disease in humans. Researchers noted that the repeated tracts of repair segments seemed stable until the mice reached middle age (about 4 months old). After that point, the segments began expanding.

The team also noted that the expansion of the tracts caused toxicity in cells that cannot expand, such as nerve cells. The result: cell death accelerates in direct proportion to the number of CAG repeats.

To see if the oxidative lesions played a role in the expansion of the extra DNA segment, the researchers next deleted a key enzyme in oxidative lesion repair. Without the enzyme, most of the lesions remained untouched, and the inserted huntingtin segment did not grow at all, or at least grew far less than in mice carrying a working version of the enzyme.

The results were groundbreaking. Now we know that while doing the job of removing oxidative lesions, DNA repair enzymes trigger a far more damaging effect: the DNA expansion associated with Huntington's disease.

Expanded Genes, Sticky Proteins

Once the expansion of CAG repeats begins, the mutant stretch of DNA grows larger and larger over time, eventually producing a faulty huntingtin protein with a destructive effect: it impairs the brain's normal functions, causing a progressive loss of motor control and cognitive ability.

One proposed mechanism of how the mutant huntingtin protein kills brain cells includes sequestration of normal huntingtin protein and other cellular components into bundles, called aggregates, thereby preventing their normal function, vesicle trafficking in particular.

In normally functioning neurons, a variety of important vesicles and organelles are moving along the neuritis ensuring proper cell functioning. One of the crucial neuronal functions is neurotransmitter signaling process that works by selectively packaging proteins as cargo into transport vesicles bound for a specific destination. During transport, the vesicles are moved down along the extensions of the neuron and are pushed out into the synapse, where they stimulate yet another neuron.

However, the mutant huntingtin protein derails the train: the neuron needs to bring the cargo out in vesicle bags, but the mutant huntingtin interferes and keeps the vesicles from reaching their destination.

When the research team measured vesicle trafficking in mouse models for Huntington's, they found that trafficking of mitochondria, the energy fabric of the cell, was also impaired, actually slowing to a stop in many cases. Thus, inability to provide sufficient supply of energy along with slowing of vesicular trafficking could eventually result in the neuronal damage that in HD patients is associated with physical and emotional distress that are the visible hallmarks of the disease.

Turning Huntington's Disease Off

Now that researchers have learned what turns Huntington's disease on, they're better positioned to find ways to turn it off. Some approaches may involve disrupting the clumped mutant protein, perhaps by developing molecules to prevent the mutant huntingtin from grabbing onto its cellular targets. Dr. McMurray and her fellow Mayo Clinic researchers are also screening for molecules that block enzyme function, in hopes of developing drugs that will prevent further DNA expansions in patients with existing huntingtin mutations.

In addition to impacting Huntington's, this ongoing research may hold promise for effectively treating of other neurodegenerative diseases including Alzheimer's and Parkinson's, as well various forms of cancer in which genetic oxidation is believed to play a role.

Huntington's Disease Timeline

• 1872: Dr. George Huntington presents the first accurate definition of the disease
• 1983: Researchers using DNA marking methods find the general location of the huntingtin gene
• 1992: Researcher find the trinucleotide repeats (CAG, CAG, CAG, etc.) that affect disease severity
• 1993: The Huntington's Disease Collaborative Research Group isolates the precise gene that causes the disease
• 1995: Mayo Clinic researchers led by Dr. McMurray show that secondary DNA structures could account for the disease features
• 2001: Mayo Clinic researchers led by Dr. McMurray find that failed DNA repair is a causative factor for Huntington's disease
• 2004: the Mayo team led by Dr. McMurray demonstrates that the expanded huntingtin causes derailment of trafficking vesicles
• 2005: the same Mayo team publishes a paper showing that DNA repair molecules are causing the expansion
• 2006: Mayo researchers led by Dr. McMurray pinpoint a key trafficking defect of expanded huntingtin that destroys the normal cholesterol balance
• 2007: the McMurray team publishes findings in Nature pinpointing mechanisms by which botched DNA repair is key to Huntington's disease