Energy is made in our cells in structures called mitochondria. Mitochondria were born when an ancient cell combined with a bacterium. A relationship formed and the cell held on to the bacteria to use it to make energy. Antibiotics are drugs that are used to treat bacteria. Mitochondria can also be affected by these drugs. Ailments are caused from damaged mitochondria resulting in diseases in the brain and muscles. These ailments are called mitochondrial diseases and currently have no therapies. Our lab discovered that antibiotic drugs can treat these ailments in cells and mice. In this application we propose to explore exactly how these drugs work in the cell and in the brain. We will do so by performing experiments to test hypotheses based on the observations made by our lab and others in the field. This proposal will identify how these drugs rescue mitochondrial diseases. The experiments will identify new ways that cells survive and provide new ways to treat these diseases. This will open up a new field in mitochondrial biology and will impact mitochondrial disease. The drugs used in this application are already used in the clinic and approval for mitochondrial disease will be fast. This proposal will impact our understanding of the disease and how it harms cells and tissues and will also provide a new way to treat these diseases in the clinic.

Human cells have two types of DNA, nuclear and mitochondrial DNA. The latter controls energy production for the body which is essential for life. Each cell has many mitochondria to generate sufficient energy for our organs, and inside each mitochondrion, there are many copies of mitochondrial DNA. In patients with certain types of mitochondrial diseases, the mitochondrial DNA copies are not identical, resulting in a mixture of defective and normal copies. The defective copies lead to poor mitochondrial function and disease, so the more defective vs. normal copies a cell has, the worse the disease symptoms. Our goal is to develop a method to selectively reduce the number of defective mitochondrial DNA copies, leaving mostly normal ones. Gene therapy strategies that have worked for nuclear DNA defects have not reached clinical trials for mitochondrial genes because the mitochondrion is very selective as to what can or cannot enter the organelle. We propose a different approach taking advantage of the cell’s natural communication between the nucleus and mitochondria. Previous work has shown that intense mitochondrial dysfunction, such as the one present in cells with mostly defective mitochondrial DNA, produces modifications to nuclear DNA (not in sequence, but in function) that perpetuate the survival of such unhealthy cells and sustain disease symptoms. Consequently, we plan to modify these nuclear DNA modifications that exist in dysfunctional cells to “select” cells with the less defective mitochondrial function. This nuclear DNA modulation technique is already used to treat other illnesses, which increases the possibility for it to reach clinical trials in less time.

Mutations in mitochondrial DNA result in an array of disease which can be found in nearly all tissue types of the human body. Furthermore, these mutations can exist in varying states of prevalence due to a rare phenomenon known as heteroplasmy. Heteroplasmy is the percentage of mutant mitochondrial DNA within a cell, tissue, or organ system.  The level of heteroplasmy or the percentage of mutation directly correlates with disease and cellular dysfunction. 

Our objective is to determine how internal factors, such as development and cell specification cues, as well as external stimulus, oxygen levels, energy substrates, and drug compounds influence mitochondria heteroplasmy. Our preliminary assessments suggest that cell-type development and cell division influence heteroplasmy in developing tissues, additionally our work on cell conditions, and small molecules has yielded promising preliminary results for possible therapeutics. This body of work serves as a template for discovering compounds that reduce mitochondrial heteroplasmy and thus disease burden in patients.