George Huntington (1850-1916) described the nervous affliction, Huntington chorea, which bears his eponym. Chorea is an apt word (from the Latin "dance") for the jerky, dance-like movements that are salient hallmarks of the disease. Other visible features of the progressive, inherited malady include flicking limb movements, a lilting gait, difficult character changes and slowly advancing dementia, culminating inevitably in death.
Huntington disease (HD) becomes manifest, and diagnosable, in the late prime of life - 35 to 50 years of age. This delayed genetic time bomb gives its victims (men and women equally) time in early life to raise a family. All through their reproductive years, parents cannot know whether their offspring - with a 50-50 chance of inheriting HD - will be winners or losers in this lottery.
HD is unpredictable, untreatable and almost inscrutable. Neurologists assigned its cause to deterioration and death of nerve cells in the brain's striatum and cortex. Then in 1993 molecular geneticist James Gusella, at Massachusetts Institute of Technology in Cambridge, discovered HD's gene on the short arm of human chromosome 4. That huntingtin gene encodes a protein called huntingtin, which mutates weirdly to trigger and exacerbate the disease.
At first, huntingtin inserts a string of repeating neurotransmitters called polyglutamines into the doomed brain cells. These stuttering sequences consist of three nucleotides - cytosine-adenosine-guanosine (CAG for short), which repeat up to 36 times in symptom-free individuals. But as time goes by, that iterating CAG chain keeps lengthening, so that 72 repeats bring on full-blown Huntington symptoms.
Now, neuroscientists are digging deeper into the CAG-swollen cells of the body and have fixed their cross-hairs on the fraught organelles - the mitochondria. Biochemist and mitochondrologist Alexander Panov is lead author of a paper in Nature Neuroscience, released online July 1, 2002. Its title: "Early mitochondrial calcium defects in Huntington's disease are a direct effect of polyglutamines." Its senior author is neurologist J. Timothy Greenamyre, at Emory University in Atlanta.
"The major point of our paper," Panov told BioWorld Today, "relates to a ubiquitous phenomenon in the whole body. HD symptoms abound with specific damage to neurons in the striatum and cortex. But the protein itself, huntingtin, is expressed all over the body. So it probably has some important function. Otherwise, there would be no reason to express it everywhere. People didn't find any correlation between damage and expression of the protein," Panov continued, "But everybody agrees that mutated huntingtin has acquired some toxic functions. So the question is, Okay, if it has a toxic function that affects, say, mitochondria - the cells' electric power plant - then it must damage those organelles all over the body, not exactly in the brain only.'"
Calcium Sets Mitochondrial Energy Action
"When polyglutamine becomes too large," Panov pointed out, "it binds to mitochondria, which become more permeable for protons. So, they cannot maintain membrane potential efficiently, as normal mitochondria do. And when challenged with calcium, they depolarize, which causes a lot of disturbance in the cells - beginning with inefficient production of energy, ADP, and probably initiation of permeability transition. This is caused by calcium. When mitochondria accumulate calcium, at some point, suddenly, a large pore in the mitochondrial membrane opens and the organelles spill out everything they have."
For Panov's in vivo experiments, the Emory team obtained HD-mimicking transgenic mice from their creator, co-author Michael Hayden, at the University of British Columbia in Vancouver. "These transgenic mice are called YAC - for yeast artificial chromosome," Panov related. "They express full-length human HD genes with 72 polyglutamines. These transgenics have two phenotypes, low expressors and high expressors. The latter produce five times more huntingtin in tissues.
"Hayden's group also created mutant transgenics with 18 non-symptomatic polyglutamine CAG repeats," Panov added, "which we use as controls and wild-type animals. Normal genuine mouse huntingtin contains only seven polyglutamine CAGs. So these animals treat even 18 CAGs as toxic. But when we protected them with a specific inhibitor of mitochondria's permeability transition, they consumed much more calcium than unprotected mitochondria. Then YAC 18 behaved exactly like normal wild types. The inhibitor," Panov explained, "is cyclosporine A - better known as an organ-transplant immunosuppressor. Mouse tissues, particularly brain, as well as rat brain - unlike other tissues, heart, skeletal muscle or liver - respond to cyclosporine alone very poorly. Only in the presence of adenosine diphosphate, a mitochondrial energy package, is cyclosporine several-fold more effective. So we used a combination of cyclosporine and ADP. The behavioral Huntington's mouse mitochondria were completely different from normal cells. First, they needed much less calcium to depolarize and secondly, they did not respond to cyclosporine until ADP was added."
Mouse, Human HD Models Jibed Neatly
Turning from mouse to human experiments, Panov went on: "When I compared mitochondria isolated from blood cells of normal individuals with Huntington disease patients, I concentrated on young persons because aging by itself causes mitochondrial damage, making it difficult to distinguish.
"We grew up human lymphocytes - white blood cells - from three patients and three controls. That is where we first observed the difference in the response to calcium. But when we started to study the specific function, the permeability transition, by gradually loading the mitochondria with calcium, we saw tremendous differences. We took these lymphocytes from their blood, then grew up large quantities of cells in vitro. We found a difference between normal people and HD patients. It turned out that Huntington-diseased mitochondria, when loaded with calcium, depolarized significantly at much lower calcium loads than in normal people. We found a similar defect," he continued, "in brain mitochondria from transgenic mice expressing full-length mutant huntingtin, and this defect preceded by months the onset of pathological or behavioral abnormalities.
"Then I isolated liver mitochondria and brain mitochondria from transgenic mice, which express full-length human Huntington disease genes with 72 polyglutamine CAG repeats. Again I found similar changes in the liver mitochondria. We located the place where huntingtin exerts its toxic action," Panov concluded. "Now we can look for therapeutic measures to protect those organelles from this toxicity, allowing cells to maintain their energy reserves."