Genetic Manipulation of Mitochondria: Is CRISPR the key?
Breaking down a study's recent advancements in mitochondrial disease gene therapy.
“Mitochondria are the powerhouse of the cell!”
Ah. That single phrase catapults you back to the good ol’ days when you were trying not to fall asleep in Mrs. Smith’s high school biology class.
Mitochondria are our cells’ power plant - they break down carbohydrates, mainly a sugar called glucose, and transform the sugar into a usable energy source called ATP (adenosine triphosphate). Our cells depend on ATP for countless processes, such as chemical reactions, cellular transport, and muscle contractions to name only a few.
That’s why when there’s an error in our mitochondria, the damage that results is catastrophic to our cells, and our bodies as a whole.
Mitochondria were not always organelles in our cells - over 2 billion years ago, mitochondrial ancestors were happily existing as fully independent aerobic (requires oxygen) prokaryotes (single-celled, microscopic organisms).
Scientists theorize that our eukaryotic cell precursor engulfed or “ate” an early mitochondria; kind of like Pac-Man. Some how, the early mitochondria survived the cell’s digestive process and became incorporated inside the cell. The eukaryotic cells and mitochondria formed a symbiotic relationship - the eukaryotic cell provided the mitochondria oxygen and the mitochondria provided the eukaryotic cell energy. It was a win-win for both parties.
Why do scientists think this occurred? Mitochondria have their own DNA.
Just like our nuclear (inside nucleus) DNA, mutations can occur in our mitochondrial DNA (mtDNA). These mutations can cause severe genetic diseases and disorders in tissues that require a lot of energy; for example, muscles, heart, liver, kidneys, the nervous system, eyes, … the list goes on.
According to the United Mitochondrial Disease Foundation (UMDF), “1,000 to 4,000 children in the U.S. each year are born with a mitochondrial disease.”
In past studies, researchers used older technology to genetically eliminate mtDNA mutations; however, this technology is difficult to engineer for the diverse kinds of mitochondrial mutations present. These studies also discovered a major setback to mitochondrial gene therapy research: double-stranded breaks in the mitochondrial DNA, a common method in genetic manipulation, sometimes cause subsequent deletion/insertion mutations in the DNA. These mutations may be due to a DNA repair mechanism.
A recent paper published to SCIENCE CHINA Life Sciences proposed a method that uses the CRISPR-Cas9 genome editing technology to make double-stranded breaks (DSBs) in human mtDNA in order to see whether the deletion/insertion mutations occur when using this technology.
Understanding if using CRISPR-Cas9 to edit mtDNA causes deletion/insertion mutations would increase researchers’ knowledge on mitochondrial DNA manipulation and also whether the CRISPR technology could potentially be used for mitochondrial disease research in the future.
So, what did they find?
Based on their data, these researchers discovered that insertion/deletion mutations occurred only if there was a double-stranded break (aka characteristic of CRISPR), not a single-stranded break (SSB) in the mtDNA. These results suggest that a DSB repair mechanism may cause these mutations, not a SSB repair mechanism.
So what?
Their findings help pinpoint which repair pathway induces mutations. Maybe a genetic engineering tool that performs single-stranded breaks instead of double-stranded could be used to repair small mutations? In other words, researchers are one step closer to developing a genetic tool to safely manipulate and repair mtDNA.
Problems?
There is some skepticism about this research group’s methodology. Payam Gammage, a Group Leader in the Mitochondrial Biology Unit at Cambridge University, is concerned that this research group did not specifically address how to target CRISPR guide RNA to mitochondria.
The implications of this research are promising, however, it is important to take into account the opinions of other professionals. One way research is deemed credible is though other professionals in the field reviewing it!
One thing is clear though - double-stranded break repair mechanisms in mitochondrial DNA seem to cause insertion/deletion mutations in the mitochondrial genome.
Scientists can use this knowledge and continue researching a future treatment or cure for mitochondrial disease.
Additional Reading:
mtDNA and Mitochondrial Diseases
CRISPR editing of mitochondria: Promising new biotech?
Understanding & Navigating Mitochondrial Disease
Very interesting article. You write so clearly. I think there is a real need to make scientific studies available and accessible to the non-professional. Our public needs to understand and think more about science just to live in this current age. Thank you so much for your writing.