Human Mitochondrial Mutations Repaired by New Technique

For the first time, researchers have identified a generic approach to correct mutations in human mitochondrial DNA by targeting corrective RNAs, a discovery with the potential for treating a variety of mitochondrial diseases.

Mutations in the human mitochondrial genome are implicated in neuromuscular diseases, aging, and metabolic defects. There are currently no methods to effectively repair or compensate for these mutations, according to study cosenior author Dr. Michael Teitell, a professor of pathology and laboratory medicine and a researcher with the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at the University of California, Los Angeles (UCLA; USA).

Between 1,000 and 4,000 children per year in the United States are born with a mitochondrial disease and up to one in 4,000 children in the United States will develop a mitochondrial disease by the age of 10, according to Mito Action, a nonprofit organization supporting research into mitochondrial diseases. In adults, many aging disorders have been associated with defects of mitochondrial function, including diabetes, Parkinson’s disease, cancer, heart disease, stroke, and Alzheimer’s disease.

“I think this is a finding that could change the field,” Dr. Teitell said. “We’ve been looking to do this for a long time and we had a very reasoned approach, but some key steps were missing. Now we have developed this method and the next step is to show that what we can do in human cell lines with mutant mitochondria can translate into animal models and, ultimately, into humans.”

The study was published March 12, 2012, in the journal Proceedings of the [US] National Academy of Sciences. The study builds on earlier research published in 2010 in the peer-reviewed journal Cell, in which Drs. Teitell, Carla Koehler, a professor of chemistry and biochemistry, and their team revealed a role for an essential protein that acts to shuttle RNA into the mitochondria.

In addition to supplying energy, mitochondria also are involved in a wide range of other cellular processes including signaling, differentiation, death, control of the cell cycle, and growth. The introduction of nucleus-encoded small RNAs into mitochondria is critical for the replication, transcription, and translation of the mitochondrial genome, but the mechanisms that deliver RNA into mitochondria have remained poorly understood.

The study defined a new role for a protein called polynucleotide phosphorylase (PNPASE) in regulating the import of RNA into mitochondria. Reducing the expression--or output--of PNPASE decreased RNA import, which impaired the processing of mitochondrial genome-encoded RNAs. Reduced RNA processing inhibited the translation of proteins required to maintain the mitochondrial electron transport chain that consumes oxygen during cell respiration to produce energy. With reduced PNPASE, unprocessed mitochondrial-encoded RNAs accumulated, protein translation was inhibited, and energy production was compromised, leading to stalled cell growth.

The findings from this study provide a form of gene therapy for mitochondria by compensating for mutations that cause a wide range of diseases, said study cosenior author Dr. Koehler. “This opens up new avenues to understand and develop therapies for mitochondrial diseases,” Dr. Koehler said. “This has the potential to have a really big impact. We just have to get it to the next step.”

Gene therapy is frequently employed to express proteins that can treat the cause of a variety of diseases. In this case, post-doctoral fellow Geng Wang developed a strategy to target and import specific RNA molecules encoded in the nucleus into the mitochondria and, once there, to express proteins needed to repair mitochondrial gene mutations.

First, the researchers had to find a way to stabilize the reparative RNA so that it was moved out of the nucleus and then localized to the mitochondrial outer membrane. This was accomplished by modifying an export sequence to direct the RNA to the mitochondrion. Once the RNA was in the area of the transport machinery on the mitochondrial surface, then a second transport sequence was required to direct the RNA into the targeted organelle. With these two modifications, a wide range of RNAs were targeted to and imported into the mitochondria, where they worked to repair defects in mitochondrial respiration and energy production in two different cell line models of human mitochondrial disease.

“This study indicates that a wide range of RNAs can be targeted to mitochondria by appending a targeting sequence that interacts with PNPASE, with or without a mitochondrial localization sequence, to provide an exciting, general approach for overcoming mitochondrial genetic disorders,” the study authors stated.

In the next step, the scientists will evaluate their new method in small animal models to determine whether they can repair a mitochondrial defect as it occurs in a whole organism. One possible use for the new approach would also be to repair mitochondrial defects in reprogrammed, embryonic or adult-type stem cells for use in regenerative medicine therapies.

Related Links:
Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA