DNA Repair Archives |

Elongating expansions in HD and SCA1

Written by Dr. Marija Cvetanovic Edited by Dr. Larissa Nitschke

Expanded CAG repeats are the cause of Huntington’s disease (HD) and several spinocerebellar ataxias (SCAs). Longer inherited CAG expansions correlate with the earlier disease onset and worse symptoms. We know from past research that these expansions are unstable and become longer from one generation to the next.
This study by Mouro Pinto and colleagues shows that repeat expansions also keep getting longer throughout life in patients affected with HD and SCA1 in many cells, including brain, muscle, and liver cells.

Expansion of CAG repeats in different human genes cause several neurodegenerative diseases. This includes Huntington’s disease (HD) and several spinocerebellar ataxias (SCAs). These long CAG repeats in disease genes tend to be unstable in the sperm and egg cells. This instability in sperm and egg cells can result in either longer repeat tracts (expansions) or shorter ones (contractions) in the children of affected patients. Unfortunately, CAG repeats more often expand than shrink. This results in a worse disease in the affected children, with earlier onset and more severe symptoms than their parents.

However, repeat instability and expansion of repeats are not confined to the sperm and egg cells. It can occur in many cells in a patient’s body. This ongoing expansion that occurs in other body cells is called somatic expansion.

Abstract background of DNA sequence — Long CAG repeats in disease genes can be unstable and expand. *Photo used under license by Enzozo/*Shutterstock.com.

As affected patients age, the ongoing somatic expansion, especially in the brain, may accelerate the onset of neuronal dysfunction and loss of neurons and. This may worsen the disease progression. This has been previously shown in mouse models and patients with HD. However, those studies examined expansion in only a few brain regions and tissues outside the brain (called peripheral tissues).

In this study lead by Dr. Vanessa C. Wheeler, the authors systematically examined repeat instability in 26 different regions of the brain, post-mortem cerebrospinal fluid (CSF) and nine peripheral tissues, including testis and ovaries from seven patients with HD and one patient with SCA1.

Failure to repair DNA damage may be linked to SCA3

Written by Dr. Ambika Tewari Edited by Dr. Maria do Carmo Costa

Mutations in Ataxin-3 protein prevent the normal functioning of a DNA repair enzyme leading to an accumulation of errors

Cells are bombarded by thousands of DNA damaging events each day from internal and external sources. Internal sources include routine processes that occur within cells that generate reactive byproducts, while external sources include ultraviolet radiation. This DNA damage can be detrimental to cells. But the coordination of many DNA repair proteins helps to maintain the integrity of the genome. This prevent the accumulation of mutations that can lead to cancer.

DNA repair proteins play very important roles in the nervous system. During development, cells are actively growing and dividing and can incur many errors during these processes. Therefore, it is not surprising that numerous DNA repair proteins are expressed in the mammalian brain to prevent the accumulation of DNA damage. To much DNA damage can produce devastating consequences.

Damaged DNA molecule — Ataxin-3 plays a role in a DNA repair pathway which fixes double-strand DNA break. If these breaks are not fixed, there are devastating consequences. Photo used under license by Rost9/Shutterstock.com.

In fact, DNA repair deficiencies usually result in profound nervous system dysfunction in humans. Examples include neurodegeneration, microcephaly and brain tumors. Altered DNA repair signaling has been implicated in neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease. This implicates DNA repair proteins in genome maintenance in the nervous system. There are many different types of DNA damage and DNA repair. Each repair process has its own proteins and sequence of events that lead to either repair or cell death.

Ataxin-3 is known for its role in Spinocerebellar ataxia type 3 (SCA3), an autosomal dominant disorder caused by a repeat expansion in the ATXN3 gene. Symptoms are progressive and include prominent ataxia, impaired balance, spasticity and eye abnormalities. These symptoms are primarily a result of cerebellum dysfunction, but brainstem and spinal cord regions also show abnormalities in SCA3 patients. Recent studies have shown that ataxin-3 is part of a complex of proteins that repair single-strand DNA breaks. A crucial member of this complex, polynucleotide kinase 3’-phosphatase (PNKP), is actively involved in not only repairing single-strand but also double-strand breaks. Since the activity of PNKP is dependent on ataxin-3, this group of researchers were eager to investigate whether ataxin-3 also functioned in the repair of double-strand DNA damage.

Spotlight: The Truant Lab

Principal Investigator: Dr. Ray Truant

Location: McMaster University, Hamilton, Ontario, Canada

Year Founded: 1999

What disease areas do you research?

SCA1
SCA7
Huntington’s Disease
Parkinson’s Disease

What models and techniques do you use?

Human cell biology
High content screening
Biophotonics
Microscopy

Research Focus

What is your research about?

We are looking into the role of oxidative DNA damage as a trigger to diseases like ataxia and neurodegeneration. We examine the roles of the disease proteins (ataxin-1, ataxin-7, etc,) and genes which modify or change disease that are involved with DNA damage repair.

Why do you do this research?

We are looking at what triggers the very first steps of disease. If we can understand this, we can design a treatment to stop it from happening in the first place.

Research team of 10 holding a sign which reads "We are Ataxia Aware" — Group picture of the Truant Laboratory celebrating International Ataxia Awareness Day 2019.

Fun Lab Fact

All our fridges in the laboratory are named after Game of Thrones characters! (We have several proud nerds in the lab)

For More Information, check out the Truant Lab Website!

We have an open lab notebook blog where our post-doctoral fellow Dr. Tam Maiuri post updates on her experiments in real-time! We plan to launch an ataxia open notebook in Winter 2021.

Written by Ray Truant, Edited by Celeste Suart

Snapshot: What is poly-ADP-ribose (PAR)?

DNA repair is an important topic when talking about of neurodegenerative disorders. The amount of biochemical stress the brain experiences increases naturally as we age. Some connections have been made between the amount of stresses on the brain and the age people develop neurodegenerative disorders.

Many of these natural stresses can damage DNA. For this reason, many researchers are trying to find ways of helping or fixing DNA repair. Chemicals that effect DNA repair could be used as new drugs. Here, we will focus on just one part of the DNA damage response that has been a great success in cancer drug discovery.

PAR is like a net that pulls in proteins that repair DNA

Poly-ADP-ribose, also called PAR, are long molecules in the cell. They are made of of the same building blocks cells use to store enegry. PARylation is when these long chains of PAR are made and attached to different parts of the cell. This happens in response to many different types of stress. For example, a stress could be if a cell’s DNA is damaged or it is infected with a virus.

When DNA damage happens, PAR molecules are attached on the surface of proteins and can act as a basket to trap other proteins. PAR is made and woven together by PAR polymerase proteins (called PARPs). PARPs add PAR chains all around a site of damage to let other parts of the cell know that damage has happened. This attracts DNA repair proteins to DNA damage by binding to PAR and performing their role to fix the damage.

a lage black fishing net on a white background, it is worn in some placed. — PAR can act like a fishing net that “catches” and pulls in proteins to help fix DNA damage. Image of a fishing net by Nikodem Nijaki on Wikimedia.

To much PAR causes cells to run out of energy

Even though PAR does a good job of signalling that DNA damage has happened, it takes a lot of energy to make. If the damage can not be fixed, the cell will keep trying to make PAR until runs out of energy. This can lead to PAR molecules causing cell death. This effect of too much PAR can be seen in multiple types of neurodegenerative diseases.

A type of cerebellar ataxia called AOA-XRCC1 is known for having higher levels of PAR due to DNA damage. When researchers reduced the amount of PAR in a mouse model of AOA-XRCC1, the mouse had fewer ataxia symptoms and lost fewer neurons. This type of ataxia is caused by a mutation in a protein called XRCC1, which normally helps fix DNA and binds to PAR chains. But in the disease, the XRCC1 gets stuck at DNA along with the long chains of PAR.

These findings may be applicable to other types of ataxia and neurodegenerative disorders because of their link to higher levels of DNA damage. A lot more work to be done on PARylation and its role in neurodegeneration. But a lot of research has been done on PAR in cancer. Many drugs have been FDA approved for cancer patients as safe and effective. Cancer and ataxia are very different diseases. But all the work that has previously been done has laid the groundwork for new research in neurodegeneration.

If you would like to learn more about poly-ADP-ribose , take a look at these resources by the National Cancer Institute and Cancer Research UK.

Snapshot written by Carlos Barba-Bazan and edited by Dr. Ray Truant

Snapshot: What is Non-Homologous End Joining?

What is DNA damage?

As we go through life, our DNA undergoes a lot of stress, which can ultimately lead to DNA damage. The different stressors that can cause DNA damage are environmental factors such as UV light, radiation, and certain toxins. Additionally, DNA damage can be caused by metabolic processes that occur naturally in our body. DNA damage can manifest in multiple forms. One form of DNA damage are DNA breaks, which arise when either one or both of the DNA strands break, creating a physical separation between the DNA bases. Every day, each cell will experience about 50,000 single-stranded breaks, and every cell division will lead to about ten double-stranded breaks.

A blue DNA molecule that is broken in half, like it has been cut with a pair of scissors — An artist’s drawing of a double-stranded DNA break. Image part of the public domain.

How is the DNA repaired?

Thankfully, although DNA damage is a common occurrence in our cells, there are many ways our body can repair the damage. The type of repair is thereby determined by the type of DNA damage and the growth stage of the cell. A commonly used repair pathway to fix double-strand breaks is non-homologous end joining (NHEJ).

How NHEJ can repair DNA. DNA damage cuses either a single stranded or double stranded break. If it is a double stranded break, then NHEJ can "correct" the damage, but dome genetic information will be lost. — How NHEJ can repair DNA. Damage to DNA can cause Single Strand (SS) or Double Strand (DS) breaks. Non-Homologous End Joining can repair double stranded break, but also causes small deletions of DNA. Image by Eder Xhako, Created with BioRender

NHEJ occurs mainly when the cell is actively growing. Double-stranded breaks are very harmful to the cell. If left unrepaired, the break can make the whole DNA chromosome unstable, and result in death of the cell. In order to minimize the damage caused by the breaks, NHEJ is a very fast, although often imprecise, method of DNA repair. When DNA undergoes a double-stranded break, NHEJ will act like glue and stick the two broken DNA ends together. Depending on where the break occurred and how much damage happened, the repair can then either lead to a complete repair or the loss of some DNA base pairs at the junctions resulting in permanent deletions. It is more common for NHEJ to lead to small deletions rather than a precise repair. Depending on the location of the deletion, it can have either no consequence or be potentially damaging to a gene

How do scientists use NHEJ to their advantage?

Outside of the context of natural DNA damage, scientists have learned how to utilize the naturally occurring NHEJ repair pathway to inactivate genes of interest. As such, scientists have developed methods, such as the CRISPR-Cas9 system, to artificially introduce double-stranded breaks in essential and functional parts of the gene. The double-strand break then causes the cell to repair the double-stranded break using NHEJ. As NHEJ often leads to small deletions, which will damage the essential part of the gene, the gene will often be rendered non-functional. In this way, scientists can study what happens in the absence of the gene.

If you would like to learn more about Non-Homologous End Joining, take a look at this video by Oxford University Press.

Snapshot written by Eder Xhako and edited by Larissa Nitschke