Huntington’s Disease Discovery Opens Door to a New Class of Treatments

New insights from cell studies

McMurray and Polyzos, with colleagues from Berkeley Lab and the Harvard T.H. Chan School of Public Health, began studying energy uptake in HD neurons ten years ago, after research by others showed that metabolic changes occur in the brains of HD patients before symptoms begin. Using a mouse model of the disease, the team saw that the support cells for neurons in the striatum, the brain region most severely affected by HD, reduced their uptake of glucose - the standard fuel for the brain - and switched instead to using fatty acids to generate ATP for themselves and their dependent neurons. Mice have the same Huntingtin gene as humans, which when edited to have the hallmark mutation of HD, also leads to a late onset neurodegenerative condition in the animals. When mitochondria inside cells break down fatty molecules for fuel, byproducts called reactive oxygen species (ROS) are generated. ROS are hazardous to cells and tissues because they are highly reactive and attach to most biomolecules. ROS are known to be particularly destructive to DNA, since oxidized DNA interferes with the role of genes and can lead to breakage of the DNA strands. So the team started looking at the integrity of the genomes in these cells.

They discovered a surprising accumulation of double-stranded DNA breaks (DSBs). These are the most severe type of breakage, wherein the double-helix of the DNA is broken. The DSBs appear in cells throughout the body with age, but in HD they accumulate significantly in neurons of the striatum. The continual damage causes cell dysfunction and death, giving rise to disease symptoms and death of the individual before other areas are deeply affected.

Organisms across the tree of life have evolved cellular processes to mediate damage to DNA caused by ROS, UV exposure, and toxins. The discovery of excessive DSBs meant something interferes with these safeguards in people with HD. The team later found that the normal huntingtin protein binds to DNA repair enzymes that fix these breaks. The protein's role in healthy DNA repair remains unknown, but when the mutant huntingtin interacts with these enzymes, their activity is suppressed. The team believes this aspect of the disease was not discovered by earlier investigations because suppression is much harder to detect in genome studies than complete inhibition.

They also discovered that the suppression in DSB repair is separate from the central CAG expansion in somatic cells (all the cells in the body except reproductive cells like eggs and sperm) that occur with age. This discovery was key, as it illustrated the disease unfolds on two parallel paths - and scientists working on drug R&D had only been targeting the mutation pathway.

"We clearly saw the repeats could expand unchecked during life, but it did not necessarily give rise to neuronal death," said McMurray. In the Nature Communications paper , she and her colleagues engineered two lineages of mice with the HD gene; in one, the expansion proceeded as normal during the mouse's lifespan, in the other, the expansion was artificially blocked. Both groups of mice developed DSBs in their striatum, experienced symptoms, and died of the disease. "We connected the dots to show this is a two-stage process. The mutation is the driver of the disease because it generates a faulty protein, which is suppressing the ability to repair DSBs. But the huntingtin protein itself doesn't kill cells."

Armed with this key breakthrough, the team began tests with a synthetic antioxidant compound designed to mitigate ROS from mitochondria.

The purpose of antioxidants is to safely neutralize ROS to prevent cellular damage, but very few natural or synthetic antioxidants can cross the blood-brain barrier to reach neuron support cells. Peter Wipf, a distinguished professor of chemistry, pharmaceutical sciences, and bioengineering at the University of Pittsburgh, recently developed a compound, called XJB-5-131, that is able to enter the brain and concentrate at mitochondria. "We realized that this compound might be what we're looking for, a tool to delineate the role of DSBs in Huntington's disease progression," said McMurray.

She and Polyzos gave XJB-5-131, administered as a daily infusion, to mice with HD, and were shocked by the efficacy.

The mice showed a reduction of double-stranded breaks, a lack of motor function deficits, and reduced inflammation in the brain.

"It basically attenuated the disease," said McMurray.

Next steps

The promising results of this study have already garnered enthusiasm from other HD researchers. Scientists around the world are now curious to see what happens when antioxidants are administered to real patients. The first step is to establish that the same disease mechanism that was curable in the mouse also occurs in humans. Polyzos is leading a study using induced pluripotent stem cells taken from HD patients, which will be coaxed to differentiate into neurons. The team can use these to confirm the disease-induced DNA breakage results in neuronal death in a human context, and further investigate how the disease suppresses DNA repair.

Polyzos and McMurray are optimistic that the results will translate, as the cellular processes involved are known to be identical across the species.

Despite the breakthrough proof-of-concept, it's unclear whether antioxidant therapy alone will be sufficient for a long-term treatment of the disease in humans, as it doesn't deal with the mutated protein itself. Polyzos speculates that in the future, a cure that allows genetic carriers to have a normal life expectancy without symptoms might involve a compound like XJB-5-131, to prevent double-stranded breaks, alongside a gene-modifying therapy to fix the mutation at the root of the disease.

This research project is supported by the National Institutes of Health.

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