Huntington's disease (HD) is a rare inherited neurological disorder caused by a defect in a single gene. Discovery of the gene, called huntingtin, in 1993
Studies using animals have helped to understand more about how the huntingtin gene defect causes brain damage
However, mice are unable to fully replicate the range of symptoms and changes to the brain seen in humans with Huntington’s disease. This has led researchers to develop primate models of the disease. In 2008, the first primates with Huntington’s disease were developed . This was the first time primates had been genetically engineered to replicate a mutation that caused a human disease. Of the five macaques that were born, three had multiple copies of the mutated Huntingtin gene and had to be killed within a month as their symptoms were so severe. One had a healthy version of the gene, as the instability of the gene during its integration caused the disease-causing additions to be removed. The remaining macaque had a single copy of the mutated gene and showed mild symptoms within a week of birth.
Although the researchers could not learn much from studying these transgenic macaques, this feat marked the start of great change in the technology behind genetic engineering. New techniques, including CRISPR and zinc finger nucleases, can allow much more specific targeting for inserting genes and mutating multiple genes at once . These techniques were developed and demonstrated using mice, and in January 2014 researchers presented the first primates to have been genetically modified at specific target genes .
Researchers are also working to develop other animal models of the disease, including sheep. Sheep have similar sized brains to macaques, which are more complex and developed than in mice. As the sheep can be more comfortably housed with several companions in large paddocks, there are fewer ethical concerns than research using monkeys. Although a genetic model for Huntingdon’s disease has been developed in sheep , research is still ongoing to validate tests for the sheep’s cognition .
Much research has gone into studying the potential for brain cell implants using foetal tissue. At one stage this showed great promise, having been shown to work in rats and monkeys
The researchers believe that the discrepancy with the animal models was due to the way that the symptoms had been brought about. In the macaque models, the area of the brain targeted, the striatum, was chemically destroyed as there were no genetic models. This meant that simply replacing the damaged tissue with the foetal donor tissue gave ready improvements to the symptoms. By contrast, in patients with Huntington’s disease, wider areas of the brain are damaged and simply being in that toxic environment caused the donor tissue to become damaged, even though it was originally healthy.
Researchers have also been looking for alternatives to using foetal tissue, which is in short supply and poses ethical problems. One approach is to use stem cells, which have the potential to develop into, and replace, the missing or damaged brain cells. Two independent teams of researchers have succeeded in coaxing human embryonic stem cells to differentiate into primitive brain cells that, when transplanted into mouse brains, developed further into nerve and other types of brain cells . Additionally, skin cells taken from patients can be coaxed back into an embryonic-like state and become induced pluripotent stem cells. This can be combined with other genetic techniques to fix the Huntington’s mutation, making them healthy cells that avoid rejection by the immune system because they match the patient. These cells have been made and injected into mouse models, where they formed neurons in the damaged areas of the brain . Further research is needed to show that this can improve symptoms and begin trials in patients.
A possible treatment for HD has been developed using hamster kidney cells genetically engineered to produce a naturally occurring nerve growth factor that protects brain cells. These were injected into the brains of monkeys and found to protect them from brain damage in the areas affected by HD. Further animal trials are needed to weigh up the risks and benefits.
Research in mice and cell cultures have indicated that the htt fragments could block the activity of the protein Sp1 , which in turn reduces the amount of CSE in the brain . CSE is a protein that is responsible for producing the amino acid cysteine. Researchers found that giving cysteine supplements to cell cultures and the HD mice reversed symptoms.
Targeting Rhes protein
The huntingtin protein, htt, is found throughout the body so it has been unclear why Huntington’s disease only appears to affect the striatum in the brain. Researchers have found a protein called Rhes, which is highly specific to the striatum and binds tightly to the mutated form of htt . Combining Rhes and mutated htt in mice brain cells caused half of the cells to die within 48 hours. It is thought that Rhes breaks down aggregated clumps of htt, allowing it to spread around the cells and cause greater damage.
Genetically modified mice with the Rhes gene deleted were protected from the effects of the chemical 3-nitropropionic acid (3-NP) . This chemical is used to create HD models in mice as it causes loss of neurons in the striatum, although the mechanism for this was not known. This study suggests that Rhes plays a role in both Huntingdon’s disease and the damage caused by 3-NP.
Huntington's disease is linked to enzymes called caspases that are active in patients' brains and cause brain cells to die . Scientists blocked caspases in mice carrying the Huntington gene defect, with the result that they developed HD symptoms about 10% later in life and lived about 20% longer than HD mice in which the enzyme wasn't blocked. Such therapy can't be used in humans, but it has since proved possible to block caspases in these mice using a common antibiotic, minocycline , one of the tetracycline family.
Further research has shown that caspase-6 in particular is responsible for triggering the degeneration that leads to Huntingdon’s disease . Companies and academic researchers are now searching for suitable drugs to selectively block the caspase-6 from working. Much of this work is being conducted in mouse and rat models of the disease ; if this approach can be shown to work in these animals, then this strategy can pursued for patients.
Another feature of the disease is the build-up of protein aggregates in the brain because the faulty gene produces a mutated form of the protein htt which aggregates readily and is regulated by a substance called arfaptin. In tissue culture arfaptin induces the formation of aggregates containing the htt protein, and is overproduced in the brains of HD mice
The htt protein contains excessive amounts of the amino acid glutamine and the brain attempts to cope with it by producing neuroprotective proteins. An enzyme called transglutaminase helps these proteins stick together, and a compound called cystamine can prevent this. So researchers injected mice with Huntington's with cystamine, which reduced tremors and abnormal movements and increased lifespan. To the researcher's surprise, the protein aggregates remained unchanged but the quantity of neuroprotective proteins increased, suggesting a new treatment strategy .
Scientists have discovered how htt, generated by the Huntingtin gene, poisons nerve cells in the hypothalamus, part of the midbrain of mice. They demonstrated that the mutant proteins interfere with the function of another protein abundant in the hypothalamus – huntingtin-associated protein-1 (HAP1) .
Mice genetically engineered to develop Huntington’s disease had fewer symptoms and declined at a slower rate when given gene therapy so that their brains produced a chemical called ciliary neurotrophic factor. Other researchers are now working to produce medication that has a similar effect. The research is further evidence that the lack of certain neurotrophic factors are key to the progressive symptoms of Huntington's
Another protein being investigated is brain-derived neurotrophic factor (BDNF). It is known to help protect neurons and this is apparent in cell cultures, however it is difficult to get enough of it into the right areas of the brain. To get around this, researchers have developed a drug, called LM22A-4, that mimics the beneficial effects of BDNF . This has been shown to improve symptoms of HD in mice, although it has had no effect on survival times.
Researchers have successfully rid mice of a condition similar to Huntington's disease using a laboratory dye that dissolves protein clumps. Although the dye does not cross the blood brain barrier and therefore is unlikely to end up as a treatment for humans, it may open up new lines of research .
As in many neurological diseases, there is a chain of cell death. In rats treated with a bile acid called TUCDA the amount of cell death was halved. TUCDA crosses the blood-brain barrier, and it is being developed as a possible treatment for Huntington's disease .
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