Abnormal DNA Repair Mechanism May Be Therapeutic Target
The interaction between FAN1 and MLH1 — two DNA repair proteins known to be genetic modifiers of Huntington’s disease — protects against further expansion of disease-causing CAG repeats, according to a study using human and mouse models of the disease.
Specifically, this interaction prevents MLH1’s recruitment to a DNA repair protein complex that, when bound to MLH1, abnormally promotes CAG repeat expansion. FAN1 itself also was found to contribute to repeat stabilization.
“Evidence for DNA repair genes modifying Huntington’s disease has been mounting for years,” Robert Goold, PhD, and Joseph Hamilton, PhD, the study’s co-lead authors, both from the University College London (UCL), said in a UCL press release. “We show that new mechanisms are still waiting to be discovered, which is good news for patients.”
They also suggest that boosting FAN1-mediated suppression of this DNA repair mechanism may be a promising therapeutic approach for Huntington’s and other repeat expansion disorders, the researchers noted.
“We are now working with key pharma partners to try and develop therapies that target this mechanism and might one day reach the clinic,” said Sarah Tabrizi, MD, PhD. Tabrizi is the study’s co-senior author and director of the UCL Huntington’s Disease Centre, UCL Queen Square Institute of Neurology and UK Dementia Research Institute.
The study reporting the findings, “FAN1 controls mismatch repair complex assembly via MLH1 retention to stabilize CAG repeat expansion in Huntington’s disease,” was published in the journal Cell Reports.
A repeat expansion disorder, Huntington’s is caused by excessive repeats of a portion of DNA, called CAG triplets, within the HTT gene. This results in the production of an abnormal and toxic version of the huntingtin protein (mHTT).
Healthy people normally have between 10 and 35 CAG repeats, but those with Huntington’s may have 36 to 120 repeats, with longer regions tied to early disease onset.
Research suggests these CAG repeats are expanding due to the formation of repeat-associated DNA structures and their subsequent abnormal repair through a mechanism called DNA mismatch repair (MMR).
Previous studies have identified several DNA repair genes, such as FAN1 and MHL1, as genetic modifiers of Huntington’s, influencing its onset and progression. Genetic modifiers are genes or genetic variants that can increase or reduce the severity of a condition without necessarily causing the disease themselves.
FAN1 provides the instructions to produce an enzyme of the same name that is involved in DNA repair by cutting DNA strands at specific sites. MHL1 codes for the MHL1 protein, a member of the MMR pathway that forms a protein complex with other members to exert their DNA repair actions.
Notably, mutations preventing the production of a functional FAN1 enzyme have been associated with earlier onset Huntington’s, and either increased FAN1 levels or reduced MHL1 have been linked with fewer CAG repeats, or CAG repeats stabilization.
However, “the molecular relationship between MMR and FAN1 is not well understood,” the researchers wrote.
To address this, Tabrizi and her team, along with colleagues in the U.K., Switzerland, and the U.S., assessed the interaction between these two proteins and their role in CAG repeat expansion/stability in lab-grown human and patient-derived cells, as well as in mouse models of the disease.
The researchers first found that FAN1 directly binds to MLH1 in multiple human and mouse models of Huntington’s.
Using techniques that can read DNA repeat expansions, they also discovered that this interaction stops CAG repeat expansion by preventing the formation of a functional MMR protein complex that would otherwise promote CAG repeat expansion.
This protective effect was associated with FAN1’s direct competition with MSH3, another member of the MMR pathway, for MLH1, thereby reducing its recruitment to the MMR protein complex, leaving it non-functional.
Notably, analyses of healthy human brain samples showed similar levels of both FAN1 and MLH1, but higher than those of MSH3, in the cortex and striatum — two brain regions mainly affected in Huntington’s.
These findings suggest that “FAN1 suppresses MMR activity by sequestering MLH1 away from MSH3, thus preventing error-prone repair and CAG repeat expansion,” the researchers wrote.
Further analyses also revealed that FAN1’s DNA-cutting activity “plays an active role in suppressing expansion,” the team wrote.
Overall, the data support a model where FAN1 stabilizes CAG repeats via two distinct mechanisms: the prevention of functional MMR complex assembly through its binding to MLH1, and the promotion of accurate repair through its DNA-cutting activity.
This pivotal role of FAN1 highlights a potential therapeutic avenue for Huntington’s.
“Our next step is to determine how important this interaction is in more physiological models and examine if it is therapeutically tractable,” Tabrizi said.
Treatments that can mimic or boost FAN1-mediated blocking of MMR may have the potential to alter the course of the disease. The team now is working with Adrestia Therapeutics to translate these discoveries into potential therapies.
“There are currently more than fifty CAG repeat expansion disorders that are incurable,” Gabriel Balmus, PhD, the other co-senior author of the study, said. If this approach is found viable, “the field suggests that resulting therapies could be applied not only to Huntington’s disease but to all the other repeat expansion disorders,” Balmus added.
Steve Jackson, PhD, Adrestia’s founder, chief scientific officer, and interim CEO, noted: “My colleagues and I are delighted to be working with Professor Tabrizi, Dr. Balmus and the UK Dementia Research Institute to seek ways to translate their exciting science towards new medicines for Huntington’s disease and potentially also other DNA-repeat expansion disorders.”
The study was funded by the CHDI Foundation and UK Dementia Research Institute.