DNA damage findings could point to a new treatment route for Huntington’s disease after an antioxidant reduced neuronal damage and symptoms in mice.
Researchers found a marked increase in breaks across DNA strands in a mouse model of the inherited neurodegenerative condition.
Treatment with an investigational antioxidant reduced these breaks, prevented motor deficits and reduced inflammation in the brain, although it is not yet known whether the approach will work in humans.
The research was led by scientists at Lawrence Berkeley National Laboratory, known as Berkeley Lab, alongside colleagues from the Harvard T.H. Chan School of Public Health.
Huntington’s disease is caused by a mutated copy of a protein-coding gene containing many extra nucleotides in a repeating sequence. It causes progressive death of neurons in parts of the brain, leading to cognitive and physical decline and eventual death.
These repeats can continue to expand during a person’s lifetime. Greater numbers of repeats are associated with earlier disease onset and more severe symptoms.
The researchers identified another process that appeared to contribute to neurodegeneration: an accumulation of double-stranded DNA breaks, where the DNA double helix is broken.
Aris Polyzos, a biochemist and research scientist in Berkeley Lab’s Biosciences Area, said: “Despite years of work worldwide, there’s no cure for Huntington’s, and only limited experimental treatments. We’re excited to add another piece to the puzzle for this disease, which has proven to be frustratingly complex for a condition caused by a single gene mutation.”
“We show that symptoms are preceded by DNA damage, and that this can be reversed using an investigational antioxidant compound, which also protects against neurodegeneration. This alleviation occurs even without altering or blocking the gene, or stopping the expansion of the mutation, which are the approaches that past and ongoing investigational treatments have taken.”
Polyzos co-led the work with Cynthia McMurray, a retiree affiliate in Berkeley Lab’s Biosciences Area who has studied the genetic and cellular changes underlying Huntington’s disease for decades.
McMurray said: “I believe we’re opening the door to a new way to treat Huntington’s patients.”
“Clinical agents already exist for humans that are known to change these breaks. We love that these could be easily tested and lead to a therapeutic strategy more quickly. And the simplicity of the approach is beautiful.”
“Past approaches have tried to edit the gene, shorten the repeats or block the gene’s expression; those are complicated interventions, and none of them have translated into efficacy for real patients. The question is, will ours work in humans? The next step is to show that our findings apply to human cells and that we can protect neurons, which would be a precursor leading to clinical trials.”
The work developed from research begun about 10 years ago into energy uptake by neurons affected by Huntington’s disease.
Using mice with the Huntington’s mutation, researchers found that support cells for neurons in the striatum, the brain region most severely affected by the disease, took up less glucose and instead used fatty acids to generate energy for themselves and the neurons they support.
Breaking down fatty molecules inside mitochondria, the structures that produce energy for cells, generates byproducts known as reactive oxygen species, or ROS.
ROS are highly reactive molecules that can damage cellular components, including DNA.
The team found an accumulation of double-stranded DNA breaks in neurons in the striatum. Such breaks occur with ageing, but accumulated significantly in this region in the Huntington’s disease model.
Researchers also found that normal huntingtin protein interacts with DNA repair enzymes responsible for fixing these breaks.
When the mutated form of huntingtin interacted with the enzymes, however, their activity was suppressed.
The team found that this impaired DNA repair was separate from the continued expansion of CAG repeats in somatic cells, meaning cells in the body other than reproductive cells, during ageing.
Researchers tested this by engineering two groups of mice carrying the Huntington’s gene. Repeat expansion continued normally in one group but was artificially blocked in the other.
Both groups developed double-stranded DNA breaks in the striatum, experienced symptoms and died from the disease.
McMurray said: “We clearly saw the repeats could expand unchecked during life, but it did not necessarily give rise to neuronal death.”
“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.”
Researchers then tested XJB-5-131, a synthetic antioxidant developed by Peter Wipf at the University of Pittsburgh.
Unlike many antioxidants, the compound can cross the blood-brain barrier, the protective boundary controlling which substances can move from the bloodstream into the brain, and concentrate in mitochondria.
McMurray said: “We realised that this compound might be what we’re looking for, a tool to delineate the role of DSBs in Huntington’s disease progression.”
Mice with Huntington’s disease received XJB-5-131 through a daily infusion.
The treatment reduced double-stranded DNA breaks, and the mice showed no motor-function deficits and had reduced brain inflammation.
McMurray said: “It basically attenuated the disease.”
Researchers are now investigating whether the same mechanism occurs in human cells.
Polyzos is leading work using induced pluripotent stem cells from people with Huntington’s disease. These are cells that can be reprogrammed and developed into specialised cell types, including neurons.
The team plans to use them to examine whether disease-related DNA breaks contribute to neuronal death in human cells and to investigate further how Huntington’s disease interferes with DNA repair.
It remains unclear whether antioxidant treatment alone could provide a long-term treatment in people because it does not address the mutated protein itself.
Polyzos suggested that a future treatment might combine an approach aimed at preventing double-stranded DNA breaks with a gene-modifying therapy targeting the underlying mutation.
