The huntingtin protein, whose mutated version causes Huntington’s disease. The condition emerges when patients are between 30 and 50: it causes involuntary movements, cognitive impairments and ultimately, death. Image credit: Jawahar Swaminathan and MSD staff at the European Bioinformatics Institute (CC0)
Our DNA encodes the instructions to make proteins, which then go on to perform many crucial roles in the cell. Breakages and damage to DNA occur over time, and if uncorrected, they can make the instructions illegible or incorrect. A build-up of damages can be harmful – for example, DNA damage from excessive UV light exposure can cause skin cancer. Luckily, cells contain DNA repair complexes, protein machines that surveil DNA and correct errors or breakages.
An accumulation of DNA breakages is thought to contribute to the development of Huntington’s disease, a devastating and currently incurable condition where brain cells slowly die. The immediate cause of Huntington’s disease is well known: Huntington’s patients have an abnormal, mutant version of a protein called huntingtin. However, it is still unclear how the mutant huntingtin causes the symptoms of the disease and participates in cell death.
Gao et al. carefully studied the proteins that huntingtin physically interacts with. The experiments revealed that huntingtin is part of a newly identified DNA repair complex that fixes breakages in DNA as the molecule is ‘read’ by the cell. The presence of the normal huntingtin protein promoted DNA repair. However, when the healthy huntingtin was replaced with the mutant version found in Huntington’s disease, the activity of the DNA repair complex was greatly reduced. This resulted in a build-up of DNA errors, triggering a series of events that ultimately led to cell death. In addition, in mice engineered to produce the mutant version of huntingtin, the accumulation of DNA damage was particularly important in two brain regions that are severely damaged in patients with Huntington’s disease.
There is currently no effective treatment for Huntington’s disease. However, understanding how the mutant huntingtin damages brain cells may provide new targets for future therapies. More broadly, several other brain disorders share similarities with Huntington’s disease, and it remains to be seen whether the same mechanisms could be at work in all these conditions.
