Small Molecule Restores Mitochondrial Function in POLG Disorders

When mitochondria fail, the consequences can be devastating – particularly for children with genetic disorders that rob cells of their ability to produce energy.

Now, researchers at the University of Gothenburg have discovered a small molecule that restores function in a defective mitochondrial enzyme, offering a new therapeutic avenue for diseases caused by mutations in the POLG gene. The study, published in Nature, shows that the compound, PZL-A, can improve mitochondrial DNA replication and energy production in patient-derived cells.

Understanding POLG disorders and the need for new treatments

​Mitochondria are vital components within our cells. Often referred to as the cell's powerhouses, they generate the majority of cellular energy through a process known as oxidative phosphorylation. Energy production is critically dependent on mitochondrial DNA (mtDNA), which encodes essential components of the oxidative phosphorylation machinery.​

The POLG gene plays a role in maintaining mitochondrial function by encoding the catalytic subunit of DNA polymerase gamma (POLγ). POLγ is responsible for replicating and repairing mtDNA, ensuring the integrity and functionality of mitochondria. Mutations in the POLG gene can lead to a spectrum of mitochondrial disorders, as POLγ's ability to maintain mtDNA is compromised. Over 300 distinct mutations in POLG have been identified, each potentially resulting in various mitochondrial diseases. ​

POLG mutations recently gained public attention following the death of Prince Frederik of Nassau in Luxembourg, who passed away in March 2025 at just 22 years old due to complications linked to the disease.

POLG-related disorders can manifest at any age and present a wide range of symptoms. In infants and young children, mutations may lead to severe conditions such as Alpers-Huttenlocher syndrome, characterized by progressive neurological deterioration, liver failure and seizures.

The clinical manifestations of POLG-related diseases are diverse and can include liver failure, brain damage, epilepsy and muscle weakness. This variability often complicates diagnosis and management.

Currently, there are no effective therapies that address the underlying cause of these disorders. Gene therapy approaches face significant challenges due to the extensive heterogeneity of POLG mutations. While strategies aimed at increasing deoxynucleotide triphosphate (dNTP) levels have shown some promise in rescuing mtDNA depletion in POLG-deficient cells, these methods lack specificity and may pose safety concerns.

Given these challenges, there is an urgent need for mutation-independent therapeutic strategies that can restore POLγ function and improve mitochondrial health across the broad spectrum of POLG mutations. Developing such treatments would represent a substantial advancement in mitochondrial medicine, offering hope to individuals affected by these debilitating disorders.

A small molecule that repairs mtDNA replication

A team led by Professors Maria Falkenberg and Claes Gustafsson at the University of Gothenburg, in collaboration with biotech company Pretzel Therapeutics, set out to identify a small molecule that could restore the function of mutant POLγ.

They screened ~270,000 chemical compounds before identifying a promising candidate, which was chemically refined to produce PZL-A – a compound with potent and specific effects.

PZL-A binds at a site between the catalytic and accessory subunits of POLγ, a region not affected by most disease-causing mutations. The compound stabilizes the enzyme-DNA complex and improves two key aspects of polymerase function: catalytic efficiency and processivity (the ability to add many DNA building blocks in a single binding event). Importantly, PZL-A does this without disrupting the enzyme’s proofreading activity, which ensures replication accuracy.

To test its effects, the team used a wide range of methods: biochemical assays, cryo-electron microscopy to visualize the binding site and performed studies in both patient-derived fibroblasts and neural stem cells. The molecule was tested on cells carrying common POLG mutations, including A467T, W748S, G848S and R232H – mutations known to severely impair mtDNA replication.

“We demonstrate exactly where the molecule binds, between two separate chains of the enzyme. The binding site is extremely specific, which helps us understand how the enzyme works and how we can influence it,” said lead author Sebastian Valenzuela, a doctoral student at the University of Gothenburg.

In each case, PZL-A restored mtDNA synthesis and increased mitochondrial energy production, as measured by ATP output. These effects were observed not only in actively dividing cells but also in non-dividing, post-mitotic cells like neurons and muscle cells – cell types typically hit hardest by mitochondrial disease.

“We demonstrate that the molecule PZL-A can restore the function of mutated POLγ and improve the synthesis of mtDNA in cells from patients. This improves the ability of the mitochondria to provide the cell with energy,” said Falkenberg.

The compound also enhanced the performance of the full mitochondrial replisome, including TWINKLE helicase and mitochondrial single-stranded DNA-binding protein (mtSSB).

“This is a breakthrough as for the first time we can demonstrate that a small molecule can help improve the function of defective DNA polymerase,” said Gustafsson.

Implications for mitochondrial disease therapy

By restoring the activity of POLγ across a range of common mutations, PZL-A offers a potential foundation for a targeted therapy where none currently exists.

“Our results pave the way for a completely new treatment strategy,” said Gustafsson.

Because the compound acts on a site of the enzyme that is unaffected by most disease-causing mutations, it has the potential to be broadly effective, including in both early-onset, severe cases and later-onset, milder presentations. This approach could help overcome a key limitation in the field: the wide variety of POLG mutations that make gene-specific treatments difficult to design and apply in practice.

There may also be wider applications beyond POLG-related disorders. The research team notes that similar strategies could be relevant for conditions involving other components of the mitochondrial replication machinery, such as the TWINKLE helicase. In the longer term, this line of research may offer insights into how to support mitochondrial function in age-related and neurodegenerative diseases, where mitochondrial DNA depletion has been implicated.

Pretzel Therapeutics, which collaborated on the study, plans to begin Phase I clinical trials to assess the safety of a refined version of the compound in healthy volunteers. The team also intends to expand testing to additional POLG mutations and investigate whether the compound affects the formation of deleted mtDNA molecules – a characteristic of later-onset mitochondrial conditions.

Reference: Valenzuela S, Zhu X, Macao B, et al. Small molecules restore mutant mitochondrial DNA polymerase activity. Nature. 2025. doi: 10.1038/s41586-025-08856-9

This article is a rework of a press release issued by the University of Gothenburg. Material has been edited for length and content.