2025 HDBuzz Prize: When the Repair Crew Breaks Down: How Expanded Huntingtin Disrupts DNA Repair

Every cell in our body is constantly fixing DNA damage that happens throughout our lifetime. Like a city sending out crews to mend roads and power lines, our cells rely on specialized proteins to keep our genetic code in a state of good repair. 

The huntingtin (HTT) protein has been a bit of a mystery in terms of precisely figuring out the many functions it participates in. However, there have been some clues about its link with DNA repair. In this study by Dr. Guo Min-Li’s group, researchers built on these clues to uncover more details about HTT: figuring out its involvement in the cell’s DNA repair crew, and how this crew alls apart when HTT is expanded in Huntington’s disease (HD). The result is unchecked DNA damage, activation of the cell’s internal immune alarms and ultimately cell death, suggesting HTT is a key player in DNA repair. Let’s get into the study. 

Workers of the Cell, Unite! 

HD is caused by an expansion of the DNA code in a repeating C-A-G letter stretch of the HTT gene. The expansion changes the HTT protein, producing a longer toxic version known as expanded HTT. 

While scientists have long known that the expanded version is harmful, researchers are still uncovering exactly how the expansion disrupts the many roles of HTT in cell function and what happens when those roles go unfulfilled. 

The expanded repeat isn’t stable, and can grow longer in certain types of cells, especially in the brain, through a process called somatic expansion. This means that even more expanded versions of the HTT protein are made in these cells too. 

Scientists have been homing in on DNA damage and how DNA repair functions in individuals with HD. One way that DNA can be damaged is called a double strand break – serious damage where both strands of DNA are severed, like when a falling tree takes out a power line or blocks a road. To fix these breaks, the cell recruits DNA repair proteins. To repair these breaks the team of proteins collaborate, with each protein having specific tasks and responsibilities. In this study, researchers focused on 3 members of this DNA repair team in HD:   

EXO1- a protein that trims broken DNA ends to get them ready for repair. Think of it as the excited rookie with a jackhammer or axe, great at shaping the site for a proper fix, but in need of guidance to avoid over-cutting. 

MLH1- works together with its PMS2 partner to help coordinate the DNA repair and rein in the DNA trimming performed by EXO1. MLH1 is like an experienced crew member who keeps the rookie in check and makes sure the repair project stays on track. 

HTT- the big boss themselves! HTT is at the DNA repair site according to this study. It keeps the whole crew running smoothly by giving orders and interacting directly with EXO1 to keep its activity in check. HTT also contacts MLH1 so the team can finish the job properly. 

When the repair crew falls apart

So, what happens when HTT is expanded in HD? In a healthy cell, HTT is like the big boss at a busy repair site, keeping the DNA repair crew working in harmony. To investigate how this changes in HD, the researchers used “co-immunoprecipitation” – a fancy way to say they yanked HTT out from cells, to see which other proteins come along for the ride. In mouse and human cells without the HD expansion, EXO1 and MLH1 were both found together with HTT indicating a tight knit crew working together. 

However, when they examined mouse brain cells as well as human cells with the HD expansion, the repair crew was nowhere to be found near expanded HTT. In these cells, expanded HTT seems to be a slacker and ignores the responsibilities of keeping EXO1 in check nor does it contact MLH1, leading the entire repair crew to collapse.  

Without the boss in charge, EXO1 trims DNA ends too aggressively leaving the repair site in poor shape. The researchers found that MLH1 drops off, a bit like it walking off the job, further weakening the entire repair operation. In the human and mouse cell models assessed by the researchers, this double blow left broken DNA ends flagged by a DNA damage marker called γ-H2AX and a pile of DNA “debris” scattered around the worksite. 

Ouch, That cGAS-STINGs 

To a cell, loose DNA “debris” is like finding suspicious material dumped in the middle of town; it sets off immune system alarms. With expanded HTT unable to coordinate the repair crew and the cell being full of broken DNA fragments, it triggers the cGAS-STING signalling pathway. This pathway is an in-built response system that normally detects foreign DNA, like that from viruses or bacteria. When triggered, it launches an inflammatory response to these “invaders”. In HD, the cell mistakes the DNA debris as foreign and the cGAS-STING induced response triggers the destruction and death of the cell.   

The researchers tested this in several models. They used mouse cells in a dish engineered with the HD expansion, human cells from HD patients and neuron-like cells from mice. In every case, cells with expanded HTT had higher levels of DNA “debris” and thus cGAS-STING pathway activation which led to cell death. When they removed cGAS or STING from the cell, the alarms stayed quiet, the inflammation dropped and cell survival improved! Similar observations occurred when EXO1 was removed with the added benefit of less DNA “debris”. 

These findings show that expanded HTT is unable to coordinate EXO1 and MLH1. This failure not only leaves DNA damage unrepaired but also actively triggers the cGAS-STING pathway which can result in cell death.

Loose ends at the repair site for HTT

So where does that leave us and what do these findings mean for future work? This study offers a new explanation for how expanded HTT contributes to harm cells in HD. Expanded HTT fails to do a critical job when it comes to DNA repair, but much still remains unknown. While expanded HTT throws the DNA repair crew into disarray at double stranded DNA breaks, we don’t yet know if or how this problem might feed into somatic expansion, a process thought to drive HD progression. 

Another open question is when this breakdown in DNA repair occurs. Does the failure of expanded HTT in coordinating repair happen from birth, slowly adding stress to the cells of people with HD or does it emerge suddenly after a certain disease stage or trigger? We still don’t know whether this disruption due to expanded HTT is a gradual process or one that accelerates at specific points during HD.

From a therapeutic standpoint the findings open up some intriguing questions and possibilities. Could targeting cGAS-STING pathway activation help prevent harmful immune activation and cell death in HD? Additionally, what do these findings mean for HTT-lowering approaches currently being tested in the clinic? 

The challenge ahead is curbing the damage caused by expanded HTT while preserving the normal HTT protein’s essential jobs.  By uncovering a potential role of HTT in DNA repair, this study underscores how critical it is to unravel the basic biology of HTT. Each new insight builds the foundation that will ultimately pave the way for possible therapies capable of changing the course of HD. 

TL;DR: The major takeaways

• The problem: In HD, the expanded HTT protein loses some of its normal functions. One newly identified role of HTT is supervising DNA repair and keeping DNA repair proteins working together to fix DNA breaks safely. Without HTT and its oversight, DNA repair goes awry. 
• The insight: Expanded HTT can’t keep EXO1 in check or stabilize MLH1, leading to over-trimming of DNA ends during DNA repair and breakdown of repair coordination. This creates stray DNA fragments or “debris” in the cell, which the cell mistakes for an infection. This DNA “debris” triggers the cGAS-STING immune pathway, causing harmful inflammation and cell death.
• The breakthrough: Researchers showed that HTT is a direct player in a particular DNA damage repair pathway called double strand break repair while interacting with MLH1 and EXO1. 
• In the lab: This chain of events was seen in multiple systems including mouse and human cell lines as well as mouse neuron like cells. In all cases, mHTT led to higher DNA damage, more DNA “debris” and stronger immune activation. 
• Turning it off: Knocking out cGAS, STING or EXO1 reduced DNA fragments, quietened the immune response and improved cell survival. 
• Why it matters: This work links faulty DNA repair to immune activation in HD and points to the critical involvement of HTT in DNA repair. Therapies lowering HTT must find the sweet spot of balance between preserving function and therapeutic benefit. 

Learn More:

“Mutant huntingtin protein induces MLH1 degradation, DNA hyperexcision, and cGAS-STING-dependent apoptosis”, (open access).

Meet this 2025 HDBuzz Writing Competition Winner

Mustafa Mehkary is a PhD candidate at the University of Toronto studying the biology of Huntington’s disease, with a focus on targeting DNA repair and somatic expansion for therapeutic benefit. Mustafa is also the founder of the Huntington’s Disease Society of Pakistan, working to provide support and resources for HD families in Pakistan.

This year, the HDBuzz Prize is brought to you by the Hereditary Disease Foundation (HDF), who are sponsoring this year’s competition.