Yeast enzyme enables human cells to copy DNA without functional mitochondria

Human cells depend on their mitochondria for more than energy. In a dependency that has long complicated the study and treatment of mitochondrial diseases, normal cellular growth also requires working mitochondria for nucleotide synthesis - the production of the building blocks that make up DNA and RNA. When mitochondria fail, cells cannot replicate properly. The consequence, across dozens of rare mitochondrial diseases and several types of cancer, is an inability to proliferate under normal conditions.

A study published in Nature Metabolism shows this dependency is not absolute. An international team led by Jose Antonio Enriquez of the Centro Nacional de Investigaciones Cardiovasculares Carlos III (CNIC) in Spain transferred a yeast enzyme called ScURA into human cells carrying severe mitochondrial disease mutations - and found that the cells could grow normally even when their mitochondrial respiratory chains were completely blocked.

How the yeast enzyme works differently

In animals, a key enzyme in the nucleotide synthesis pathway is physically attached to the inner mitochondrial membrane and uses oxygen as its electron acceptor - making it functionally inseparable from mitochondrial respiration. When mitochondria stop working, this enzyme stops working, and nucleotide production halts.

Yeast evolved differently. Some yeast species, including Saccharomyces cerevisiae, can survive without oxygen and have alternative metabolic pathways. The yeast version of the equivalent enzyme - ScURA - sits in the cytosol (the cell's main fluid compartment, outside the mitochondria) and uses fumarate, a common metabolic byproduct, as its electron acceptor instead of oxygen. This functional independence means ScURA can run even when mitochondria are not.

When the CNIC team extracted the ScURA gene from yeast and inserted it into human cells with mitochondrial mutations, the cells began growing under standard laboratory conditions - the same conditions under which they had previously required expensive supplementation with uridine and other nucleotide precursors to survive at all.

What happened inside the cells

The results were examined carefully across different experimental models, including cells derived from patients with several types of mitochondrial disease mutations, as well as cells engineered to lack functional respiratory chain complexes. In all cases, ScURA-expressing cells showed restored nucleotide synthesis and normal proliferation without disruption to other cellular functions.

One of the study's notable findings was that ScURA helped cells use nutrients more efficiently without detectable negative effects on other cellular processes - an important consideration for any approach that might eventually be tested in patients. The tool also proved useful for separating the effects of mitochondrial failure specifically on nucleotide synthesis from other metabolic consequences of dysfunctional mitochondria, which had been difficult to disentangle previously.

"Mitochondria not only produce energy; they also shape fundamental processes such as DNA synthesis," said lead author Enriquez. "Our work shows that if we provide a cell with an alternative route to make nucleotides, we can sustain cell proliferation even when mitochondrial respiration fails."

Implications for disease research

Mitochondrial diseases are among the most severe and least treatable rare conditions. The current study does not constitute a therapy - ScURA is a research tool, and inserting a gene from yeast into patient cells involves gene therapy techniques that are not yet clinically ready for mitochondrial disease applications. The team plans to expand their findings to other disease models and optimize this approach for preclinical research, which means years of additional work before clinical translation could begin.

The tool also has potential relevance for cancer research. Altered mitochondrial metabolism is a feature of many cancers, and ScURA provides a precise way to decouple nucleotide synthesis from mitochondrial function - a capability that may help researchers identify which aspects of mitochondrial dysfunction drive cancer proliferation versus resistance to therapy.

The study was funded by the Spanish Ministry of Science and Innovation, the Human Frontier Science Program, and the Leducq Foundation.