Written by Dr. Ambika Tewari Edited by Dr. Mónica Bañez-Coronel

Mutations in the Huntingtin protein impair DNA repair causing significant DNA damage and altered gene expression

Our genome houses the entirety of our genetic material which contains the instructions for making the proteins that are essential for all processes in the body. Each cell within our body, from skin cells that provide a crucial protective barrier, immune cells that protect us from invading species and brain cells that allow us to perceive and communicate with the world contains genetic material. During early development in every mammalian species, there is a massive proliferation of cells that allows the development from a one-cell stage embryo to a functional body containing trillions of cells. For this process to occur efficiently and reliably, the instructions contained in our genetic material need to be precisely transmitted during cell division and its integrity maintained during the cell’s life-span to guarantee its proper functioning.

There are many obstacles that hamper the intricate and highly orchestrated sequence of events during development and aging, causing alterations that can lead to cell dysfunction and disease. Internal and external sources of DNA damage constantly bombard the genome. Examples of external sources include ultraviolet radiation and exposure to chemical agents, while internal sources include cell processes that can arise, for example, from the reactive byproducts of metabolism. Fortunately, nature has evolved a special group of proteins known as DNA damage and repair proteins that act as surveyors to detect erroneous messages. These specialized proteins ensure that damage to the DNA molecules that encode our genetic information is not passed to the new generation of cells during cell division or during the expression of our genes, ultimately protecting our genome. Many genetic disorders are caused by mutations in the genetic material. This leads to a dysfunctional RNA or protein with little or no function (loss of function) or an RNA or protein with an entirely new function (gain of function). Since DNA repair proteins play a crucial role in identifying and targeting mistakes made in the message, it stands to reason that impairment in the DNA repair process might lead to disease. In this study, Rui Gao and colleagues through an extensive collaboration sought to understand the connection between altered DNA repair and Huntington’s disease.

Huntington’s disease (HD) is caused by an expansion of glutamine residues in the huntingtin protein (HTT) that causes degeneration of neurons in specific areas of the brain, namely the striatum and cortex, and leads to progressive motor and cognitive decline. These polyglutamine expansions are also found in many Spinocerebellar ataxias (SCA) such as SCA1, SCA2, SCA3, and SCA6, just to name a few. Several studies investigating the mechanisms underlying HD have found that a common theme is the accumulation of damaged DNA, particularly breaks in the DNA strand. The burning questions that arise from this observation are whether the glutamine expansion in huntingtin causes DNA strand breaks and whether this DNA damage accumulation can lead to loss of neurons and ultimately to motor and cognitive decline.

The investigators took two different approaches to look at proteins that interact with the huntingtin protein (normal and mutant). For the biochemical approach, since huntingtin protein resides in a special compartment of the cell called the nucleus, they extracted the nucleus from cultured cells and used only this part of the cell for their studies. They also used an immunoassay that identifies the physical closeness of proteins to validate the biochemical results in cultured cells, HD mouse models and also from brain sections of HD patients and healthy controls. Both methods confirmed the interaction of both HTT and mutant HTT with several proteins.

To determine the role of the individual HTT interactors to the pathology of HD, Rui Gao and colleagues performed an elegant series of experiments. They used cultured striatal neurons, a type of cell that is severely affected in HD, and found that both normal and mutant HTT interact with the DNA repair enzyme, PNKP. However, in the presence of mutant HTT, there was reduced PNKP activity in striatal neurons, which caused an increase in DNA damage primarily seen as DNA strand breaks. The damage was rescued by restoring the expression of PNKP. Increased DNA damage and decreased levels of PNKP were also detected in an HD mouse model and in the cortex and striatum of HD patients. In contrast, HD cerebellum, where the effects of the disease are less severe, showed mild alterations in PNKP levels and DNA damage.

Moreover, HTT was found to be more abundant where genes are actively expressed. Therefore, in HD mice, the presence of mutant HTT caused a drastic decrease in gene expression in their cortex when compared to control mice.  ATXN3, well known for its role in SCA3, was also found to interact directly with both normal and mutant HTT. In its interaction with mutant HTT, ATXN3 activity decreased, which added to the impairment in gene expression.

Taken together, the data from this study identified HTT as a key player in a DNA repair complex and showed an important and causative link between polyglutamine expansion, DNA damage and repair, and dysregulation of gene expression. The researchers demonstrated that the expanded polyglutamine repeats in HD cause an elevation of DNA damage due to decreased DNA repair activity. However, additional studies will need to address whether these changes can drive the progressive neuronal degeneration and behavioral changes observed in HD patients and animal models. If yes, then further tests are warranted to determine whether restoration of the levels of DNA repair proteins affected by mutant HTT would constitute a therapeutic approach for HD and other polyglutamine diseases.

Key Terms

Mouse Model: A type of animal model with specific characteristics that allow for the study of various aspects of a human disease/condition.

Polyglutamine Expansion Disease / CAG-repeat diseases: A family of diseases caused by an expansion of glutamine amino acids in certain proteins. This includes SCA1, SCA2, SCA3, SCA6, Huntington’s disease, and others.

PNKP: A DNA repair protein involved in two types of DNA repair pathways: base excision repair (BER) and non-homologous end-joining (NHEJ). Its full name is “bifunctional polynucleotide phosphatase/kinase.”

Conflict of Interest Statement

The authors and editor declare no conflict of interest.

Citation of Article Reviewed

Gao R, Chakraborty A, Geater C, Pradhan S, Gordon KL, Snowden J, Yuan S, Dickey AS, Choudhary S, Ashizawa T, Ellerby LM, La Spada AR, Thompson LM, Hazra TK, Sarkar PS. Mutant huntingtin impairs PNKP and ATXN3, disrupting DNA repair and transcription. eLife, 2019; 8: e42988 DOI:10.7554/eLife.42988 https://www.ncbi.nlm.nih.gov/pubmed/30994454